Responses of ground beetles (Coleoptera: Carabidae) to variation in woody debris supply in boreal

northeastern Ontario

by

Paul Wojciech Piascik

A thesis submitted in conformity with the requirements

for the degree of Master of Science in Forestry

Faculty of Forestry University of Toronto

© Copyright by Paul Piascik 2013

ii

Responses of ground beetles (Coleoptera: Carabidae) to variation in woody debris supply in boreal northeastern

Ontario

Paul Piascik

Master of Science in Forestry

Faculty of Forestry

University of Toronto

2013

Piascik, Paul. 2013. Responses of ground beetles (Coleoptera: Carabidae) to variation in woody

debris supply in boreal northeastern Ontario. Master of Science in Forestry, Faculty of Forestry,

University of Toronto.

Abstract

The maintenance of downed woody debris supplies is increasingly being recognized as an

integral part of forest management. In order to better manage this resource, it is important to

assess its role in supporting biodiversity. In this thesis, I investigate the responses of carabid

communities to variation in woody debris availability in an experimental manipulation of woody

debris volume in closed-canopy forests and following a biomass harvest in a clearcut. Within

closed-canopy forests, total carabid abundance and the abundances of eight species increased

significantly with increasing volumes of various types of woody debris, particularly

large-diameter, late-decay conifer wood. Similarly, a strong affinity with woody debris was

observed in the clearcut. These findings suggest that reductions in woody debris will have

negative consequences for carabids and indicate the need to ensure a diverse and abundant

supply of woody debris during stand development.

iii

Acknowledgements

I would like to thank my advisors, Dr. Jay Malcolm and Dr. Sandy Smith, for the opportunity to

take on this study and for their continuous support throughout its duration. I thank my committee

member Dr. Chris Darling for his guidance and valuable inputs.

For invaluable help in the initial stages of learning carabid identification, I would like to thank

Kathleen Ryan and Nurul Islam. I thank Henri Goulet at the Canadian National Collection for his

hospitality and for helping to enhance my identification skills. Thanks to Brad Hubley at the

Royal Ontario Museum for providing me access to study the carabid collection.

Thanks to the Kapuskasing field crews for their efforts, enthusiasm, and for making each season

very enjoyable and memorable. I am very grateful to all those who spent long hours in the lab

tediously processing my samples.

I would like to thank everyone in the Wildlife Ecology and Forest Entomology labs for their

support, encouragement, and insightful discussions. I would also like to thank my friends and

colleagues in the faculty for a great atmosphere and continued inspiration that was an integral

part of my experience.

Funding for this project has been provided by the Sustainable Forest Management Network,

Natural Science and Engineering Research Council of Canada, Ivey Foundation, Tembec,

Canadian Forest Service, Ontario Ministry of Natural Resources, and the Faculty of Forestry. A

special thanks to Ian Thompson and Dave Morris for logistic support.

iv

Table of Contents Abstract .......................................................................................................................................... ii

Acknowledgements ...................................................................................................................... iii

List of Tables ................................................................................................................................. v

List of Figures ............................................................................................................................... vi

List of Appendices ....................................................................................................................... vii

General Introduction .................................................................................................................... 1

Chapter 1: The response of ground beetles (Coleoptera: Carabidae) to a large-scale

downed woody debris manipulation in boreal northeastern Ontario ...................................... 7

Introduction ................................................................................................................................. 7

Methods ....................................................................................................................................... 9

Study Sites ................................................................................................................................ 9

Experimental Design ............................................................................................................. 10

Carabid Sampling .................................................................................................................. 11

Downed Woody Debris Sampling .......................................................................................... 13

Statistical Analyses ................................................................................................................ 15

Results ....................................................................................................................................... 17

Discussion ................................................................................................................................. 26

Chapter 2: The importance of slash for ground beetles (Coleoptera: Carabidae) in a

biomass clearcut .......................................................................................................................... 35

Introduction ............................................................................................................................... 35

Methods ..................................................................................................................................... 38

Study Site ............................................................................................................................... 38

Experimental Design ............................................................................................................. 41

Carabid Sampling .................................................................................................................. 41

Statistical Analyses ................................................................................................................ 42

Results ....................................................................................................................................... 43

Discussion ................................................................................................................................. 50

General Conclusions ................................................................................................................... 58

Literature Cited .......................................................................................................................... 65

Appendices ................................................................................................................................... 77

v

List of Tables

Table 1.1. P values of downed woody debris (DWD) volume effects from mixed-model

ANCOVAs on total carabid abundance, species richness, and the abundances of the 20 most

abundant carabid species as a function of variation in nine DWD variables. Carabids were

collected between 2010 and 2011 on a DWD manipulation experiment in northeastern Ontario

(see text for details) (vtot represents total volume of DWD, in all other cases the first letter of the

acronym represents size [s = small-diameter, l = large-diameter], the second letter the taxon [c =

conifer, d = deciduous], and the last two numbers the decay class [12=early, 35=late]).

Table 2.1. Mean abundance (standardized to 100 bucket-nights), total abundance, and species

richness of carabids collected in 2010 and 2011 in a biomass clearcut in northeastern Ontario

near (83 m away) from slash piles at three distances from forest edge

(Near = 34-40 m, Medium = 66-84 m, Far = 181-268 m). Each mean represents abundances from

two pitfall arrays; means are over three collection periods (August of 2010 and June and August

of 2011).

vi

List of Figures

Figure 1.1. Most significant positive relationships between abundances of carabids and volume

of various downed woody debris types in a downed woody debris manipulation experiment in

mature mixedwood forests of northeastern Ontario.

Figure 1.2. Most significant negative relationships between abundances of carabids and volume

of various downed woody debris types in a downed woody debris manipulation experiment in

mature mixedwood forests of northeastern Ontario.

Figure 1.3. Individual-based rarefaction on total carabid captures in 2010 and 2011 from study

plots divided into three classes of downed woody debris volumes (low = white, medium = grey,

and high = black) for each of nine downed woody debris variables measured in a woody-debris

manipulation experiment in northeastern Ontario. Shown are 95% confidence intervals.

Figure 2.1. Map of the biomass clearcut in northeastern Ontario sampled for carabid beetles in

2010 and 2011. Pitfall trap arrays were close to slash piles ( 83 m; squares). White = forest; grey with black border = clearcut; solid black lines = roads

and major skid trails; dashed black lines = transects sampled for downed woody debris.

Figure 2.2. First two axes from a Principle Component Analysis on the covariance matrix of

carabid species ln-transformed abundances in a biomass clearcut in boreal northeastern Ontario.

Carabid species acronyms consist of the first four letters of the genus and the first four letters of

the species. Total carabid abundance and species richness were passive variables (totabun and

Richness, respectively). Symbol shapes and colours represent proximity to slash piles and to

forest edge (circles represent samples near slash piles [83 m away]; white indicates samples 34-40 m from forest edge; grey

indicates samples 66-84 m from forest edge; black indicates samples 181-268 m from forest

edge).

Figure 2.3. Relationship between total carabid abundance and distance from forest edge using A.

samples located near slash (< 5 m away; circles) and B. samples located away from slash (> 83

m away; squares) in a biomass clearcut in northeastern Ontario. Each regression based on six

pitfall samples collected in 2010 and 2011.

Figure 2.4. Individual-based rarefaction of carabid species for A. samples near slash piles

(black) and away from slash piles (white) (83 m away, respectively), B. samples at

three distances from forest edge (white = 34-40 m, gray = 66-84 m, and black = 181-268 m), and

C. samples near (83 m away; squares) from slash piles at three

distances from forest edge (white = 34-40 m, gray = 66-84 m, and black = 181-268 m) in a

biomass clearcut in northeastern Ontario. Error bars indicate 95% confidence intervals.

vii

List of Appendices

Appendix 1. Effective length of drift fence arms for carabid captures in a three-arm drift fence

pitfall array

Appendix 2. Principal component analysis (PCA) on the correlation matrix of volumes of

downed woody debris in the five decay classes [acronym definition: vdc = volume of decay

class; 1-5 = decay class 1-5]

Appendix 3. Schematic plot showing the method of partialling out site effects without removing

wood volume effects. In this example, the two sites (represented by white and black circles) have

both wood volume effects (i.e., each has a positive slope) and extraneous effects (i.e., differences

in regression elevations). As shown by the arrows, extraneous effects were removed by

partialling out the least square means (i.e., the differences in elevation)

Appendix 4. Non-standardized number of carabid beetle individuals by species collected in June

and August sampling sessions in 2010 and 2011 from experimentally-manipulated boreal

mixedwood stands in northeastern Ontario. In total, 27 plots were sampled, each with three

pitfall arrays (see text for details)

Appendix 5. Rank abundance curve for carabid species collected in closed-canopy boreal

mixedwood stands in northeastern Ontario

Appendix 6. Rank abundance curve for carabid species collected in a biomass clearcut that was

formerly a boreal mixedwood stand in northeastern Ontario

1

General Introduction

Conserving biodiversity, especially through ensuring critical habitat, is of central

importance to sustainable forest management (Attiwill 1994, Graham et al. 1994, Freedman et

al. 1996, Lindenmayer et al. 2000). In recognition of this goal, a major target of boreal forest

management in Ontario is emulating natural disturbance through clearcutting (OMNR 2001).

The underlying idea behind this strategy is to mimic the post-disturbance conditions and

landscape mosaics that boreal biota are adapted to. Included in the guidelines set out for

achieving this objective is the recognition of the importance of maintaining sufficient woody

debris supplies (OMNR 2001). In Ontario, the dominant natural disturbance events that occur in

boreal forests are fire and insect outbreaks (Bergeron et al. 2001, Bergeron et al. 2007). These

events typically result in large influxes of woody debris to the landscape (Pearce et al. 2005,

Brassard and Chen 2008). A key difference that typically separates managed boreal landscapes

from those shaped by natural disturbance is the reduced amount of woody debris (e.g., Siitonen

2001, Pedlar et al. 2002). As boreal forests become increasingly managed, it is important to have

a sound understanding of the ecological role of woody debris and how management-related

reductions in availability may affect local biodiversity.

Growing concerns over climate change have led to an increased interest in the

development of renewable energy sources, including use of forest biomass as a fuel. Sources for

this biomass include logging residues and stems of low commercial value. Development of this

resource has the potential to result in even more removal of fibre from forests during harvesting,

and a further reduction in woody debris supplies in managed forests. In many forest regions,

such management-associated reductions in woody debris are considered a main threat to

2

biodiversity (Siitonen 2001, Grove 2002). For example, in Europe, where most of the forested

area has been heavily managed for long periods of time, many organisms are threatened by the

reductions in woody debris (Berg et al. 1994, Esseen et al. 1997, Martikainen et al. 2000).

Understanding the response of forest biota to variation in woody debris availability provides the

opportunity to minimize these negative ecological consequences in Canadian boreal forests

where the logging industry is younger and where management-related disturbances have been

relatively less-intense.

Woody debris is an important structural feature of forest ecosystems and comprises a

relatively large proportion of dead organic matter (Freedman et al. 1996). Decaying wood plays

an ecological role in nutrient cycling by providing a slow-release, long-term source of nutrients

(Hagan and Grove 1999) and as an eventual component of forest soils (Siitonen 2001). In

addition to its functional role within forest ecosystems, woody debris is recognised as a critical

habitat feature for many forest organisms (Harmon et al. 1986, Graham et al. 1994, Freedman et

al. 1996, Hagan and Grove 1999, Carey and Harrington 2001, Ehnstrom 2001) and an important

characteristic of old-growth forests (Niemelä 1997). A variety of forest biota are associated with

woody debris, including mosses, lichens, fungi, herbaceous and woody plants, and many

vertebrate and invertebrate animals (reviewed in Harmon et al. 1986). The maintenance of

woody debris is considered a principle strategy to sustain wildlife habitat and ecological function

in forest management (Graham et al. 1994, Hagan and Grove 1999).

In addition to the amount of woody debris available in an ecosystem, qualitative features

of the wood also are an important consideration (Esseen et al. 1997, Sturtevant et al. 1997,

Langor et al. 2008, Nieto and Alexander 2010). Many different microhabitats exist in decaying

wood and are highly variable based on wood characteristics such as tree species, size, state of

3

decay, and the types of fungi colonizing the wood (Jonsson 2000, Siitonen 2001). Studies in

western Canadian boreal stands reveal a distinct succession of saproxylic species as woody

debris changes physically, chemically, and biologically through the decomposition process

(Hammond et al. 2004, Jacobs et al. 2007).

The quantity and diversity of woody debris within managed ecosystems are generally

different from those that have not been managed (Siitonen 2001, Pedlar et al. 2002, Brassard and

Chen 2008) and the amount of woody debris present will vary with the intensity of the harvest

(e.g., Green and Peterken 1997). Sources of woody debris in unmanaged stands include tree

mortality and self-thinning as well as natural disturbance such as wind-throw, snow breakage,

and fire (Hansen et al. 1991, Lee et al. 1997, Siitonen 2001). Volumes of woody debris within an

unmanaged forest are in a continual flux (Graham et al. 1994) with the highest inputs attributed

to stand senescence and natural disturbance (Lee et al. 1997, Sturtevant et al. 1997). In

disturbance-driven ecosystems, woody debris dynamics have been described to follow a

“U-shaped” pattern (Spies et al. 1988, Sturtevant et al. 1997, Clark et al. 1998) consisting of an

initial influx of wood after disturbance followed by decreasing levels resulting from the decay of

the initial input and, as the stand matures, a subsequent increase in woody debris levels.

Managed boreal stands generally follow a similar pattern; however, they have lower overall

volumes of woody debris relative to unmanaged stands, especially of large-diameter woody

debris (Sturtevant et al. 1997, Fridman and Walheim 2000, Siitonen 2001, Rouvinen et al. 2002,

Ekbom et al. 2006). This is particularly true after clearcutting as a result of harvesting large

diameter trees and the relatively low volume of post-harvest residues as compared with natural

disturbance (Rouvinen et al. 2002). The amount of highly decayed wood is particularly limited

4

as it is largely destroyed by machinery or exposed to drying conditions following logging

operations (Hautala et al. 2004).

Because it is difficult to determine the volume of woody debris required to support all of

the woody debris-dependant organisms in an ecosystem, use of indicator taxa is a valuable

management tool. The use of indicator taxa can be an effective method for measuring the

impacts of habitat change by providing an efficient and economical approach to assessing the

effects of disturbance on an ecosystem (Pearce and Venier 2006). In North America, carabids

(Coleoptera: Carabidae) are a commonly used insect group to study environmental change in

terrestrial ecosystems (e.g., Beaudry et al. 1997, Burke and Goulet 1998, Pearce et al. 2003,

Moore et al. 2004, Pearce et al. 2005, Cobb et al. 2007). They exhibit many qualities of an

effective indicator group, such as high abundance and species diversity, varying habitat demands

among species, wide distributions, well-known taxonomy, and high sensitivity and rapid

responses to habitat change (Lindroth 1961, 1963, 1966, 1968, 1969a, 1969b, Thiele 1977,

Niemelä et al. 1988, Rainio and Niemelä 2003, Pearce and Venier 2006). As a largely predatory

group, carabids play an important role in ecosystem dynamics and trophic interactions between

plants and other ground-dwelling organisms such as spiders and springtails (Snyder and Wise

2001, Larochelle and Lariviere 2003). Changes in microhabitat conditions, such that may be

associated with structural features such as woody debris, are considered to be among the most

important factors in determining carabid assemblages (Thiele 1977, Lövei and Sunderland 1996).

Carabids are also relatively easy to sample with pitfall traps, a common method for sampling

ground-active invertebrates (Krebs 1999).

Woody debris is recognized to be among the most important substrates for sustaining

arthropod diversity in boreal forests (Ehnstrom 2001). Numerous studies have found the

5

abundance of many arthropod taxa to increase near woody debris (Evans et al. 2003, Jabin et al.

2004, Ulyshen et al. 2004, Ulyshen and Hanula 2009a). Indeed, this association with woody

debris is widely recognized specifically for carabids in both clearcuts and closed-canopy forests

(Carcamo and Parkinson 1999, Pearce et al. 2003, Work et al. 2004, Latty et al. 2006, Cobb et

al. 2007, Ulyshen and Hanula 2009b). Carabids have been reported to utilize woody debris as

sites for oviposition, larval development, shelter, and overwintering (Goulet 1974, Larochelle

and Lariviere 2003, Bousquet 2010).

Unfortunately carabids are relatively understudied from a woody debris perspective in

several Canadian forest types, including mixedwood forests, and specific associations with

various quantities and types of wood are poorly understood. Current knowledge on associations

with woody debris is typically based on correlations and has not been examined through

experimental manipulations that could detect causal relationships. Relationships of Canadian

carabid assemblages with woody debris in clearcuts are similarly understudied and have not been

examined for slash piles, a dominant structural feature of clearcuts. A number of European

studies have found carabids to be positively associated with slash piles in clearcuts (Koivula and

Niemelä 2003, Nittérus and Gunnarsson 2006, Nittérus et al. 2007); however, they were based

largely on conifer-dominated stands that contain relatively lower volumes of post-harvest woody

debris than mixedwood stands (e.g., Pedlar et al. 2002). Although positive correlations with

woody debris have been observed for several carabid species in Canadian mixedwood clearcuts

(Pearce et al. 2003), specific associations with slash piles have not been examined.

In this thesis, I explore associations of carabid beetles with woody debris in mixedwood

forests of boreal northeastern Ontario, including forests at two ages in the cutting cycle: shortly

after harvest and closed-canopy forests some 36-68 years old. In chapter 1, I examine the

6

response of carabid beetles to variation in the availability of downed woody debris in

closed-canopy stands through a large-scale manipulation experiment. I specifically look at

changes in carabid community composition, richness, and abundance as a function of variation in

quantity, size, species, and decay states of downed woody debris. In chapter 2, I examine

whether remnant slash piles contribute to maintaining carabid populations in a clearcut where, in

addition to traditional harvesting, much of the logging residues and non-commercial trees were

removed from the site for biofuel. I specifically look at differences in local abundances and

species richness of carabids near and far from slash piles and as a function of distance from

forest edge.

7

Chapter 1: The response of ground beetles (Coleoptera:

Carabidae) to a large-scale downed woody debris

manipulation in boreal northeastern Ontario

Introduction

Downed woody debris (DWD) is increasingly being recognized as an important structural

feature of forest ecosystems that provides critical habitat for numerous species (Harmon et al.

1986, Freedman et al. 1996, Esseen et al. 1997, Hagan and Grove 1999, Ekbom et al. 2006). In

addition to the quantity of DWD, its qualitative features such as the size, species, and state of

decay have been identified as key functional characteristics providing diverse microhabitats

(Harmon et al. 1986, Økland et al. 1996, Siitonen 2001, Simila et al. 2003, Hammond et al.

2004, Langor and Spence 2006, Jacobs et al. 2007, Lassauce et al. 2012). A higher volume and

qualitative variety of DWD will provide a wider variety of niches and therefore is expected to

support greater numbers of saproxylic species (Siitonen 2001).

Forest management can have significant impacts on the availability of DWD in an

ecosystem (Sturtevant et al. 1997, Siitonen 2001) and in many forest regions, management-

associated reductions in woody debris are considered a main threat to biodiversity (Grove 2002).

In Europe, where most of the forested area has been intensively managed for the past 100 – 150

years, many organisms are threatened by the associated reductions in DWD, including saproxylic

insects (Berg et al. 1994, Esseen et al. 1997, Martikainen et al. 2000, Siitonen 2001). In contrast,

forest harvesting in the Canadian boreal region is a relatively young industry and there is an

opportunity to minimize negative ecological impacts associated with forest management. In

8

order to assess the potential impacts associated with reductions in DWD in Canadian boreal

forests, it is important to understand the response of forest biota to variation in DWD

availability. Of particular value are manipulation studies that extricate causation from correlation

(Thompson 2006).

Carabids (Coleoptera: Carabidae) are commonly used to evaluate changes associated with

disturbances in forest ecosystems as they are ubiquitous, diverse, abundant, well-known

taxonomically, and highly sensitive to environmental change; they also respond rapidly to habitat

change (Lindroth 1969a, Niemelä et al. 1988, Rainio and Niemelä 2003, Pearce and Venier

2006). They also are an integral part of trophic interactions on the forest floor in that they are

largely predatory, feeding on a variety of different prey (Lövei and Sunderland 1996) and

simultaneously provide an abundant source of food for other predators. Microhabitat conditions

are considered to be among the most important factors in determining carabid assemblages

(Thiele 1977) and therefore structural features, such as DWD that provide relatively stable

microclimates, are presumably important habitat features. Downed woody debris contributes to

habitat heterogeneity and structural complexity within an ecosystem and may also be an

important factor in carabid prey distributions. The abundance of numerous arthropod taxa have

been found to increase near woody debris (e.g., Evans et al. 2003, Jabin et al. 2004, Ulyshen and

Hanula 2009). A number of studies in a variety of habitats have reported correlations between

carabids abundances and variation in woody debris supplies (Pearce et al. 2003, Latty et al.

2006, Cobb et al. 2007). Downed woody debris is thought to serve as sites for oviposition,

diurnal shelter, and overwintering (Goulet 1974, Bousquet 2010). Unfortunately, however,

carabids are relatively understudied from a DWD perspective in Canada, especially in

closed-canopy mixedwood forests. Although recognized as an important habitat component for

9

carabids (eg. Work et al. 2004), habitat associations with DWD are poorly understood and

responses to experimental manipulations of DWD supplies have not been examined. Examining

the response of carabids to variation in the availability of DWD has the potential to provide

valuable insights into the importance of ensuring a sustainable supply of DWD during forest

management.

Here, I report on the effects of a large-scale DWD manipulation experiment on carabid

communities in closed-canopy, boreal mixedwood stands. My specific objective was to examine

changes in carabid communities as a function of variation in quantity, size, species, and decay

states of DWD.

Methods

Study Sites

The study was conducted in northeastern Ontario, Canada, in the Gorden Cosens Forest

Management Unit within 80 km of Kapuskasing (49°25’0” N, 82°26’0” W). Nine study sites

were established in the oldest, post-clearcut forests available in the study area, including 1)

forests clearcut mechanically using skidders between 1967 and 1975 (n = 4 sites) and 2) forests

clearcut using horses between 1943 and 1959 (n = 5 sites). A range of stand ages were used in

the experiment because it allowed indirect investigation of qualitative variation in DWD

supplies. The younger, mechanically-logged sites had relatively large quantities of highly

decayed DWD remnant from logging operations, but relatively lower quantities of recent DWD

inputs from stand development (Fischer et al. in press). By contrast, in the older, horse-logged

stands, DWD resulting from the logging operations had largely disappeared, but recent DWD

was relatively common (Fischer et al. in press) due to higher inputs associated with more mature

10

stands such as self-thinning (Sturtevant et al. 1997). All sites were closed-canopy mixedwoods;

that is, they had a mixture of both deciduous and coniferous tree species. Typically, such forests

are dominated by poplars, balsam fir, and white spruce. Based on 16 prism sweeps per

experimental plot (see below) undertaken in 2006 and 2007, the plots on average had 52%

deciduous composition by basal area and 48% conifer composition by basal area. All had at least

some Populus (mean = 40%, range = 6-78%) and at least some Picea (mean = 17%, range = 1-

48%).

Experimental Design

Within each site, which consisted of either a single stand (n = 6) or two nearby stands of

the same age and species composition (n = 3) according to Forest Resource Inventory maps

(Ontario Ministry of Natural Resouces, unplublished), we established three 2.25 ha plots (150 x

150 m) separated by at least 150 m and located at least 100 m from roads. In the central portion

of each plot, we established a 90 × 90 m grid with 15-m spacing to serve as a focus of sampling.

Within each site, three treatments were assigned, one at random to each plot. In the "full-

removal" treatment, all DWD ≥ 7 cm diameter was removed; in the "half-removal" treatment,

one-half of the DWD was removed; and in the control treatment, no DWD was removed. Three

sites were treated in 2006; the others were treated in 2007. In each plot, three parallel skid trails

separated by approximately 50 m were established to facilitate removal of the DWD and to

permit skid distances ≤ 25 m. In order to minimize site damage, skid trails were laid out to

minimize tree cutting and to avoid low-lying areas. Wood removal was undertaken using a small

(cable) skidder, with the exception of wood too decayed to skid, which was instead broken apart

with chainsaws and axes (it was completely removed from the full-removal plots at 4 years

post-treatment). In the full removal treatment plots, all new DWD accumulation was removed at

11

two year intervals. All removed wood was piled at least 75 m away from the plot edge. Skid

trails in control grids were lightly bladed to emulate the repeated travel on skid trails in the

removal plots. Wood removals occurred in August-October.

Detailed sampling of DWD was undertaken prior to and following the manipulation

(respectively, at 1.0 year pre-manipulation on average [range: 0.2 - 2.2 years] and at 1.1 years

post-manipulation on average [range: 0.6 - 2.0 years]). Full details on methods are provided

below in the section on additional DWD sampling undertaken in support of the carabid research.

This before-and-after sampling (1680 m of line intersect per plot) revealed the expected

reduction in DWD volumes as well as the expected change in DWD correlations before and after

the manipulation. DWD volume remained more-or-less constant in the control grids before and

after the manipulation (49 and 45 m3/ha on average, respectively) and the correlation between

the before and after measurements across these plots was high (R2 = 84%, p = 0.0004). In the

half-removals, DWD volume was reduced by 45% (from 61 to 33 m3/ha on average) and in the

full-removals, by 81% (67 to 13 m3/ha on average). As expected, due to the manipulation, the

correlation between pre- and post-manipulation DWD volume across the 27 sites was not

significant (R2 = 6%, p = 0.22).

Carabid Sampling

Carabids were sampled using pitfall buckets in combination with drift-fences formed into

“Y”-shaped arrays. The 90 × 90 m grid was divided into four 45 × 45 m quadrants, and an array

was placed in the centre of three of the four quadrants. Drift fences were built of polypropylene

geotextile ("green line") with one pitfall bucket placed at the end of each of the three arms and

one in the centre of the array (i.e., 4 buckets per array and 12 buckets per grid). Each arm was

0.5 m high and 3.65 m long, with the bottom 10 cm of the geotextile buried in the soil. Buckets

12

were 15 cm in diameter by 15 cm deep (with the exception of one sampling session where a

number of smaller buckets were used; see below) and were buried flush with the ground surface

and the edge(s) of the pitfall arms. During trapping, the buckets were filled with 5 cm of a 5%

saline solution to which a small amount of soap had been added to break surface tension.

Carabids were sampled twice per year in 2010 and 2011; 4 – 12 June and 24 August – 1

September in 2010 and 12 – 19 June and 15 – 22 August in 2011. During each sampling session,

traps were active for five consecutive nights; buckets were covered with lids and leaf litter at

other times. Sampling among the various plots was conducted as near to simultaneously as

possible in each sampling session (i.e., all traps were set within 3 days of one another and the

three plots per site were set simultaneously). Pitfall samples were collected at the end of each

five-night session and preserved in 70% ethanol. Carabids were identified to species using keys

from Lindroth (1961, 1963, 1966, 1968, 1969a, 1969b) and Bousquet (2010). Nomenclature

follows that of Bousquet (2010). A voucher collection was authenticated by H. Goulet at the

Canadian National Collection, Ottawa, and is located at the Faculty of Forestry, University of

Toronto, Ontario.

Prior to statistical analysis, carabid abundances were standardized to account for missing

effort by calculating the number of individuals per 100 bucket-nights. Missing effort (which was

due to bears and other vagaries of field sampling) was relatively rare: 1.65 % of end buckets and

0 % of centre buckets in 2010, and 7.41 % of end buckets and 3.70 % of centre buckets in 2011.

In order to calculate effort, I accounted for the fact that the centre and end buckets did not

represent the same sampling effort: centre buckets can be expected to catch relatively more

carabids than end buckets because they are fed by three drift fence arms instead of one. To

determine the appropriate conversion factor, I counted the number of carabids in centre and end

13

buckets for 219 arrays over both sessions in 2010. On average, centre buckets captured 2.236

times as many individuals as end buckets, which was therefore used in calculating bucket-nights

and in standardizing for missing effort (specifically, one array set for 5 nights with no missing

effort was assumed to represent [2.236 · 1 centre bucket + 3 end buckets] · 5 nights = 26.18

bucket-nights). Interestingly, this conversion factor can be used to estimate the length of an array

arm that was effective in directing carabids into a bucket (Appendix 1).

An additional complication arose during the June 2010 trapping session in that 69 of the

end buckets across 37 pitfall arrays had slightly smaller buckets (11 cm diameter × 13.3 cm high

instead of 15 × 15 cm). To calculate the appropriate conversion factor, I counted the number of

carabids in small versus large buckets for 28 arrays, and found that a small bucket captured 60.1

% of the number of individuals as a large one. This was not significantly different from

expectations based on the ratio of perimeters of the small:large buckets (0.733; one-sample t-test

p > 0.05) and is consistent with the findings of Lange et al. (2011) who found that the number of

carabid individuals captured increased with the diameter of the pitfall trap.

Downed Woody Debris Sampling

Detailed surveys of DWD were carried out in 2010 in each plot using the line-intersect

method (Van Wagner 1968; 3 years after the manipulation on average [range: 2.6 - 3.8 years]).

Fourteen transects, each 120 m long (extending 15 m beyond the 90 × 90 m grid) were sampled

in each plot for a total of 1680 m per plot. Transects were spaced 15 m apart with seven transects

spanning each dimension of the square grid (and thereby intersecting 49 times). For every piece

of downed wood ≥ 7 cm diameter intercepted on the transect, the diameter, state of decay

(“early” or “late”), and whether it was coniferous or deciduous was recorded. Early decay wood

had firm outer layers, was largely intact, and could not be kicked apart; those in a late stage of

14

decay had soft to substantially decayed or missing outer layers and could be kicked apart with

either some or little effort. These groupings approximately correspond to decomposition classes

I-II and III-V respectively as described by Maser et al. (1979). A principle component analysis

(PCA) on the correlation matrix of volumes in decay classes I-V revealed that volume of

decomposition class III was approximately equally correlated with both classes I-II and IV-V

(Appendix 2); based on the substantial decay of sapwood in decay class III logs, I grouped it

with the “late decay” category. From these data, I calculated wood volumes for nine variables: 1)

total downed wood volume, 2) small-diameter, early-decay conifer, 3) small-diameter, late-decay

conifer, 4) large-diameter, early-decay conifer, 5) large-diameter, late-decay conifer, 6)

small-diameter, early-decay deciduous, 7) small-diameter, late-decay deciduous, 8)

large-diameter, early-decay deciduous, and 9) large-diameter, late-decay deciduous.

Classification as either "small-diameter" or "large-diameter was based on the median diameter

(small = ≤ 12 cm diameter, large = > 12 cm diameter). When downed wood could not be

identified as either deciduous or coniferous with certainty, I assumed that it was distributed

between the two taxonomic groupings in the same ratio that was obtained for identified pieces of

the same size and decay class. The mean volumes across the 27 sites for the various DWD

variables were: 1) total downed wood volume = 43.5 m3/ha [range:6.2 - 133.4 m

3/ha], 2)

small-diameter, early-decay conifer = 1.4 m3/ha [range: 0.1 - 4.6 m

3/ha], 3) small-diameter,

late-decay conifer = 2 m3/ha [range: 0.2 – 9.1 m

3/ha], 4) large-diameter, early-decay conifer =

2.5 m3/ha [range: 0.2 – 8.8 m

3/ha], 5) large-diameter, late-decay conifer = 5.4 m

3/ha [range:

0 - 19 m3/ha], 6) small-diameter, early-decay deciduous = 3.3 m

3/ha [range: 0.6 – 9.7 m

3/ha], 7)

small-diameter, late-decay deciduous = 2.3 m3/ha [range: 0.6 – 8.4 m

3/ha], 8) large-diameter,

15

early-decay deciduous = 6.8 m3/ha [range: 0 – 37.5 m

3/ha], and 9) large-diameter, late-decay

deciduous = 19.7 m3/ha [range: 1.4 – 67.4 m

3/ha].

Statistical Analyses

The wood removal treatments in this study were designed to result in a net reduction in

wood volume among the sites with two rationales: 1) species DWD relationships were more

likely to be more evident when wood volume was limiting and 2) such relationships would

reflect causation rather than correlation; that is, effects due to variation in DWD volumes per se

rather than other factors that might have originally been correlated with variation in DWD

volumes. Because the reductions in wood volumes were relative rather than absolute, I took a

regression approach rather than an analysis of variance approach (that is, the volumes of DWD in

the plots as measured in 2010 were was used as a continuous independent variable rather than

classifying grids as being from one or another of the removal treatments). I did this because

half-removal treatments were relative to what was originally there; for example, after treatment,

a half removal plot may have a higher volume of DWD than a control plot. In addition, control

plots were assigned at random, so it is possible that a control plot might not only have less DWD

than a half removal plot, but it might even have less DWD than a full removal plot (given that

steady, albeit slight, additions of DWD over time occurred in the full removal plots through

natural events such as snag falls and windthrow).

For those species that strongly varied in abundance between the June and August

sessions, I reasoned that from a statistical standpoint, the period of peak abundance would best

reveal DWD relationships because sampling error at other times would play a larger role in

determining whether or not a trap contained individuals. Accordingly, if > 80% of the

standardized captures for a given species were in either June or August, I analyzed standardized

16

abundance from only their most abundant month (except for rare species; see below).

Standardized species abundances were ln(x + 1) transformed to better meet the assumptions of

normality and homogeneity, which were evaluated from plots of residuals in species-specific

tests (see below). The 20 most abundant species (species occurring in ≥ 50% of samples) and

rare species were examined separately for species-specific responses to variation in DWD

volume.

Prior to species-specific analyses, a permutation test using redundancy analyses (RDA;

Lepš and Šmilauer 2003) on the 20 abundant species was performed to assess whether the nine

DWD variables explained significant variation in the correlation matrix of carabid standardized

abundances.

Species-specific analyses for the 20 most abundant species and analyses of total

abundance and species richness were performed using analysis of covariance in the SAS Mixed

procedure (SAS v. 9.2) with trapping session as a repeated measure, site as a random effect, and

DWD volume as a continuous variable. DWD variables included in the model were checked for

possible curvilinear relationships via binomial regression. In order to present significant

relationships graphically, I removed date and site effects and plotted mean standardized

abundance over all sessions against the relevant DWD variable. A complication that may arise in

this situation is that site effects might consist of both DWD volume effects (i.e., the possibility

that some sites might have more DWD on average than others) and extraneous effects (i.e., the

possibility that some sites might have more individuals of a certain species than others,

irrespective of DWD volumes). Accordingly, to partial out the latter, but retain the former, I

used Analysis of Covariance to remove elevation (intercept) effects (see Appendix 3). This

ensured that DWD volume effects among sites remained intact, but that extraneous site-related

17

variation was removed. For rarer species, I classified abundances as above or below the

species-specific median, and undertook contingency table analyses on counts in the two

categories. In these analyses, standardized total species abundance across all four sampling

sessions was used and each plot was categorized as having either “low” or “high” volumes of

each DWD variable.

I also undertook rarefaction analyses to test whether species richness corrected for

abundance varied as a function of variation in DWD volume. Rarefaction curves were generated

(before standardization for sampling effort) for each DWD variable by categorizing each plot as

having either "low" "medium" or "high" volumes of that variable. In this way, I could examine if

species accumulation varied between the three wood categories. Wood categories were

determined using tri-tiles; i.e., approximately one-third of plots were in each class. Rarefaction

estimates were calculated using the formula for individual-based rarefaction (mean and variance)

from Coleman et al. (1982).

Results

A total of 11,604 individuals from 22 genera and 43 species were collected over the four

trapping sessions in 2010 and 2011 (Appendix 4; Rank abundance curve: Appendix 5). The 20

most abundant species (each represented in at least 50% of the samples) were Agonum gratiosum

(Mannerheim), Agonum retractum LeConte, Agonum sordens Kirby, Bradycellus lugubris

(LeConte), Calathus ingratus Dejean, Harpalus fulvilabris Mannerheim, Loricera pilicornis

(Fabricius), Patrobus foveocollis (Eschscholtz), Platynus decens (Say), Platynus mannerheimii

(Dejean), Pterostichus adstrictus Eschscholtz, Pterostichus coracinus (Newman), Pterostichus

melanarius (Illiger), Pterostichus pensylvanicus LeConte, Pterostichus punctatissimus (Randall),

Scaphinotus bilobus (Say), Sphaeroderus nitidicollis Guerin-Meneville, Sphaeroderus

18

stenostomus Dejean, Synuchus impunctatus (Say), and Trechus apicalis Motschulsky.

Abundances varied strongly between sampling years and months; 2011 represented 72% of the

total captures, and June 2011 itself represented 60% of the total captures. Of the 20 abundant

species, 13 were much more abundant in June than in August (Agonum gratiosum [95% of

captures in June], Agonum retractum [98% of captures in June], Agonum sordens [97% of

captures in June], Bradycellus lugubris [99% of captures in June], Calathus ingratus [87% of

captures in June], Harpalus fulvilabris [80% of captures in June], Loricera pilicornis [96% of

captures in June], Patrobus foveocollis [98% of captures in June], Platynus decens [96% of

captures in June], Platynus mannerheimii [96% of captures in June], Pterostichus adstrictus

[94% of captures in June], Pterostichus pensylvanicus [95% of captures in June], and

Pterostichus punctatissimus [97% of captures in June]); two were more abundant in August than

June (Sphaeroderus nitidicollis [87% of captures in August], and Synuchus impunctatus [100%

of captures in August]); and five did not show strong seasonal variation (Pterostichus coracinus

[63% of captures in June, 37% in August], Pterostichus melanarius [58% of captures in June,

42% in August], Scaphinotus bilobus [50% of captures in June, 50% in August], Sphaeroderus

stenostomus [46% of captures in June, 54% in August], and Trechus apicalis [70% of captures in

June, 30% in August ]).

Redundancy analysis indicated that 40.8 % of the variance in the abundances of the 20

species was explained by the nine DWD variables (p = 0.0127). Species-specific tests revealed

that 12 of the 20 abundant species were significantly correlated (p < 0.05) with variation in one

or more of the nine DWD variables (Table 1.1). Of these, six showed significant relationships

with multiple DWD variables, and all except one showed at least one relationship that was highly

significant (p < 0.01). In all cases, the sign of the relationships for a given species were either

19

always positive or always negative. Agonum sordens was positively associated with small and

large-diameter early-decay deciduous DWD (p = 0.0102 and 0.0251, respectively) and both

coniferous and deciduous small-diameter late-decay DWD (p = 0.0201 and 0.0016, respectively).

Bradycellus lugubris was negatively associated with total DWD volume (p = 0.0038), small-

diameter early-decay conifer DWD (p = 0.0052), and both small and large-diameter late-decay

deciduous DWD (p = 0.0408 and 0.0021, respectively). Loricera pilicornis was positively

associated with both early and late-decay small-diameter coniferous DWD (p = 0.0146 and

0.0260, respectively) and small-diameter, late-decay deciduous DWD (p = 0.0093). Pterostichus

adstrictus was negatively associated with total DWD volume (p = 0.0324), both small and large-

diameter late-decay deciduous DWD (p = 0.0318 and 0.0351, respectively), and large-diameter,

early-decay deciduous DWD (p = 0.0048). Scaphinotus bilobus was positively associated with

total DWD volume (p = 0.0028), small and large-diameter early-decay deciduous DWD (p =

0.0181 and

20

Table 1.1. P values of downed woody debris (DWD) volume effects from mixed-model ANCOVAs on total carabid abundance, species richness, and the

abundances of the 20 most abundant carabid species as a function of variation in nine DWD variables (significant values in boldface type (α = 0.05)).

Carabids were collected between 2010 and 2011 on a DWD manipulation experiment in northeastern Ontario (see text for details) (vtot represents total

volume of DWD, in all other cases the first letter of the acronym represents size [s = small-diameter, l = large-diameter], the second letter the taxon [c =

conifer, d = deciduous], and the last two numbers the decay class [12=early, 35=late]).

Woody debris variableb

Response variablea vtot sc_12 sd_12 sc_35 sd_35 lc_12 ld_12 lc_35 ld_35

Total abundance 0.8611

0.7632

0.7345

0.8100

0.5256

0.1794

0.7817

0.0004 (+) 0.2033

Species richness 0.7582

0.6544

0.9003

0.8614

0.1968

0.4268

0.4795

0.1825

0.6733

Agonum gratiosum2 0.7760

0.3396

0.4977

0.8652

0.6613

0.8680

0.5179

0.6333

0.2085

Agonum retractum2 0.3329

0.0804

0.5485

0.6133

0.6873

0.6062

0.4111

0.1567

0.4812 Agonum sordens2 0.3005

0.6465

0.0102 (+) 0.0201 (+) 0.0016 (+) 0.8864

0.0251 (+) 0.6966

0.7155

Bradycellus lugubris2 0.0038 (-) 0.0052 (-) 0.2187

0.0644

0.0408 (-) 0.0559

0.0975

0.8165

0.0021 (-)

Calathus ingratus2 0.2016

0.7618

0.6786

0.6899

0.8163

0.1262

0.9588

0.0344 (+) 0.1189

Harpalus fulvilabris2 0.4190

0.4141

0.2316

0.8871

0.2597

0.3832

0.2582

0.5436

0.6041 Loricera pilicornis2 0.5409

0.0146 (+) 0.2571

0.0260 (+) 0.0093 (+) 0.4704

0.2625

0.7092

0.0624

Patrobus foveocollis2 0.4360

0.9023

0.3897

0.9349

0.2818

0.6163

0.3410

0.0941

0.9613 Platynus decens2 0.1924

0.1858

0.3316

0.2523

0.1098

0.8816

0.4923

0.0002 (+) 0.7135

Platynus mannerheimii2 0.3827

0.0716

0.1490

0.3207

0.1943

0.4345

0.5261

0.2538

0.5553 Pterostichus adstrictus2 0.0324 (-) 0.2972

0.2231

0.1389

0.0318 (-) 0.8657

0.0048 (-) 0.8004

0.0351 (-)

Pterostichus coracinus1 0.5956

0.7493

0.3371

0.9303

0.6676

0.0796

0.4240

0.0465 (+) 0.1174 Pterostichus melanarius1 0.8721

0.6491

0.2815

0.5299

0.5443

0.4424

0.2699

21

In terms of the number of significant relationships with the various DWD variables, each

variable had two or more significant relationships with one or another of the twelve carabid

species, with the exception of large-diameter early-decay conifer, which had one. Deciduous and

conifer DWD had approximately equal numbers of significant relationships (12 and 13,

respectively); however, deciduous DWD had more highly significant relationships than conifer

DWD (6 and 4, respectively). Concerning the signs of the significant relationships, eighteen were

positive and eleven were negative. Of these, seven of the positive relationships were highly

significant and four of the negative relationships were highly significant. Of the most significant

DWD relationships found for each carabid species, eight of twelve were with large-diameter

logs, and ten of twelve were with late-decay logs. Of these, six of twelve were with large-

diameter, late-decay logs of which five were conifers.

When the relationship with the most significant DWD variables was plotted, ten of the

twelve abundant carabid species showed linear responses (Fig. 1.1 and Fig. 1.2). Only Agonum

sordens and Pterostichus melanarius displayed a significant binomial relationship, in these cases

with small-diameter, late-decay deciduous DWD and large-diameter, late-decay coniferous

DWD, respectively (Fig. 1.1).

Species-specific tests on rare species revealed four species with significant (p < 0.05)

relationships with variation in the nine DWD variables, three of these with multiple variables. In

all cases, the sign of the relationships for a given species were either always positive or always

negative. Amara lunicollis was positively associated with small-diameter, late-decay conifer (p =

0.0407). Bembidion wingatei was negatively associated with both early and late-decay

small-diameter conifer (ps = 0.0183). Carabus maeander was positively associated with small

22

Total carabid abundance

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

ln(a

bu

ndan

ce +

1)

3.9

4.2

4.5

4.8

5.1

5.4

p = 0.0004 r2 = 0.3575

y = 0.0336x + 4.3465

Calathus ingratus

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

0.0

0.5

1.0

1.5

2.0

2.5

p = 0.0344 r2 = 0.3062

y = 0.0603x + 0.9648

Platynus decens

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

0

2

4

6

p = 0.0002 r2 = 0.4249

y = 0.0709x + 3.3662

Pterostichus coracinus

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

0

1

2

3

4

5

p = 0.0465 r2 = 0.1480

y = 0.0557x + 2.2651

Pterostichus melanarius

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

0

1

2

3

4

p = < 0.0001 r2 = 0.6528

y = 0.1108x + 0.1786

Agonum sordens

Small-diameter, late-decay deciduous (m3)

0 2 4 6 8 10

ln(a

bu

ndan

ce +

1)

-0.5

0.5

1.5

2.5

3.5

p = 0.0016 r2 = 0.3431

y = 0.2172x + 0.5858

Loricera pilicornis

Small-diameter, late-decay deciduous (m3

)

0 2 4 6 8 10

-0.4

0.0

0.4

0.8

1.2

1.6

p = 0.0093 r2 = 0.4118

y = 0.1277x + 0.1264

Trechus apicalis

Small-diameter, late-decay deciduous (m3

)

0 2 4 6 8 10

0.0

0.3

0.6

0.9

p = 0.0314 r2 = 0.3322

y = 0.0637x + 0.0844

Scaphinotus bilobus

Large-diameter, early-decay deciduous (m3)

0 10 20 30 40

0.0

0.5

1.0

1.5

2.0

p < 0.0001 r2 = 0.6402

y = 0.0345x + 0.4754

Figure 1.1. Most significant positive relationships between abundances of carabids and volume

of various downed woody debris types in a downed woody debris manipulation experiment in

mature mixedwood forests of northeastern Ontario.

23

Pterostichus adstrictus

Large-diameter, early-decay deciduous (m3)

0 10 20 30 40

ln(a

bu

ndan

ce +

1)

0

1

2

3

4p = 0.0048 r

2 = 0.3808

y = -0.0371x + 2.7761

Bradycellus lugubris

Large-diameter, late-decay deciduous (m3)

0 20 40 60 80

ln(a

bu

ndan

ce +

1)

-1

0

1

2

3p = 0.0021 r

2 = 0.5849

y = -0.0312x + 1.4289

Synuchus impunctatus

Small-diameter, late-decay conifer (m3)

0 2 4 6 8 10

ln(a

bu

ndan

ce +

1)

0

1

2

3p = 0.0267 r

2 = 0.2162

y = -0.1246x + 1.7267

Sphaeroderus stenostomus

Large-diameter, late-decay conifer (m3)

0 5 10 15 20

ln(a

bu

ndan

ce +

1)

0.3

0.6

0.9

1.2

1.5

1.8p = 0.0275 r

2 = 0.2028

y = -0.0246x + 1.0484

Figure 1.2. Most significant negative relationships between abundances of carabids and volume

of various downed woody debris types in a downed woody debris manipulation experiment in

mature mixedwood forests of northeastern Ontario.

and large-diameter early-decay conifer as well as small-diameter late-decay conifer (ps =

0.0159). Pterostichus luctuosus was negatively associated with total DWD volume,

small-diameter early-decay deciduous DWD, and large-diameter late-decay conifer (ps =

0.0407).

Total carabid abundance showed a highly significant (p < 0.001) positive relationship

with volume of large-diameter, late-decay conifer (Table 1.1, Fig. 1.1), but no significant

24

relationships with any of the other wood variables. Species richness was not significant for any

of the nine wood volume variables (Table 1.1).

The asymptotic nature of the individual-based rarefaction curves indicated that the

carabid communities were relatively well sampled in that most of the species in the sampling

area were captured (Fig. 1.3). Rarefaction revealed no overarching pattern of variation in species

richness among the three classes of wood volumes. Generally, considerable overlap of standard

error bars were shown among the three DWD classes, with the possible exceptions of total DWD

volume and large-diameter, early-decay deciduous DWD, where sites with the lowest volumes

had the highest relative species richness. There was little consistency in the patterns shown

among the nine DWD variables, however. Sites with the greatest wood volume had the highest

relative species richness for only 1 of the 9 variables, whereas sites with low or medium wood

volumes had the highest relative species richness in 8 of 18 cases. This difference in proportions

was not significant, however (Fisher's Exact Test, p = 0.19).

25

Total woody debris volume

Nu

mb

er

of sp

ecie

s

0

10

20

30

40

Small-diameter, early-decay conifer Small-diameter, early-decay deciduous

Small-diameter, late-decay conifer

Nu

mb

er

of sp

ecie

s

0

10

20

30

40

Small-diameter, late-decay deciduous Large-diameter, early-decay conifer

Large-diameter, early-decay deciduous

Number of individuals

0 1000 2000 3000 4000 5000

Nu

mb

er

of sp

ecie

s

0

10

20

30

40

Large-diameter, late-decay conifer

Number of individuals

0 1000 2000 3000 4000 5000

Large-diameter, late-decay deciduous

Number of individuals

0 1000 2000 3000 4000 5000

Figure 1.3. Individual-based rarefaction on total carabid captures in 2010 and 2011 from study plots divided into three

classes of downed woody debris volumes (low = white, medium = grey, and high = black) for each of nine downed woody

debris variables measured in a woody-debris manipulation experiment in northeastern Ontario. Shown are 95% confidence

intervals.

26

Discussion

Annual and seasonal population fluctuations among carabids similar to those found in

this study have been reported elsewhere (Barlow 1970, Jones 1979, Willand et al. 2011). These

fluctuations have been suggested to be associated with variation in temperature (den Boer 1981),

seasonal activity periods coinciding with reproductive period (typically either spring or autumn)

(Lövei and Sunderland 1996, Langor et al. 2008), and changes in prey abundance (Symondson et

al. 2002). In the current study I did not measure factors that might be affecting carabid

population fluctuations; however, there was no significant difference in mean temperature across

trapping months between years (p > 0.15; paired t-test with day as the replicate; Environment

Canada 2012) and so the higher carabid captures in June 2011 are not likely an effect of variation

in temperature. The period of peak abundance for the 20 abundant species I analyzed coincided

relatively well with their respective breeding periods as suggested by Lövei and Sunderland

(1996) and Langor et al. (2008). Of the thirteen species with peak abundances in June samples,

ten are considered spring breeders and the two species with peak abundances in August are

considered autumn breeders. Of the five species that did not show seasonal variation, four are

considered autumn breeders.

Of the twelve abundant carabid species correlated with DWD, eight displayed positive

relationships (Agonum sordens, Calathus ingratus, Loricera pilicornis, Platynus decens,

Pterostichus coracinus, Pterostichus melanarius, and Scaphinotus bilobus) and four negative

(Bradycellus lugubris, Pterostichus adstrictus, Sphaeroderus stenostomus, and Synuchus

impunctatus). Within each species this relationship was consistent for all DWD variables.

Agonum sordens, Loricera pilicornis, and Trechus apicalis displayed positive relationships to the

volume of small-diameter, late-decay, deciduous DWD. In the case of A. sordens this

27

relationship was curvilinear suggesting a threshold response. These largely nocturnal species are

known to utilize DWD for diurnal shelter, and in the case of A. sordens and L. pilicornis

overwinter in DWD (Larochelle and Lariviere 2003), and are often associated with wet habitats

(Larson et al. 1999, Bousquet 2010). Downed woody debris may, to some extent, provide the

necessary wet or humid conditions in these mixedwood stands, which are otherwise a drier

habitat than typically associated with these species. Calathus ingratus, Platynus decens,

Pterostichus coracinus, and Pterostichus melanarius were all positively correlated with large-

diameter, late-decay coniferous DWD. The former three species are considered to be forest

habitat generalists and are endemic to North America, while P. melanarius is an introduced

species that is now widespread and sometimes associated with moist habitats (Niemelä and

Spence 1991, Niemelä and Spence 1999, Bourassa et al. 2011). These species are known to

utilize woody debris for shelter and overwintering (Larochelle and Lariviere 2003). C. ingratus

and P. decens are often found under logs, loose bark of DWD, and generally in moist areas

(Larochelle and Lariviere 2003). However, Pearce et al. (2003) found contradictory results in

that P. decens was negatively associated with woody debris in mature (>100 years old)

deciduous boreal forests, although they did not undertake experimental manipulations.

Gastropods, which are an important prey for P. melanarius (Symondson et al. 1996, Symondson

et al. 2002, Oberholzer and Frank 2003), are often found in the moist environments under logs

and bark of woody debris (Savely 1939, Kappes et al. 2006). Scaphinotus bilobus was positively

correlated with six of the nine DWD variables and is usually found on moist and shady soils and

is known to utilize DWD for shelter (Larochelle and Lariviere 2003). S. bilobus has also been

associated with old growth forests containing high volumes of woody debris (Bertrand 2005,

Janssen et al. 2009). As a specialist gastropod predator, S. bilobus may also be drawn to DWD as

28

a substrate often associated with its prey. The results of this study are consistent with the existing

literature and provide additional evidence of the importance of DWD as a habitat feature for S.

bilobus.

The species and state of decay of DWD have been suggested to be important factors

structuring saproxylic beetle assemblages (Siitonen 2001, Grove 2002, Jacobs et al. 2007) and

associations with either early-decay or late-decay wood have been noted for insect communities

(Vanderwel et al. 2006). Different tree species have varying decay rates and chemical

composition and therefore will provide variable microhabitats for insects (Harmon et al. 1986,

Siitonen 2001). Based on a combination of rearing from bolts and window traps on snags of

Populus in boreal aspen stands, Hammond et al. (2004) found that beetle species richness

increased with wood decay whereas abundance was higher in early stages of decay. The

microclimatic conditions provided by DWD will vary with the state of decay of the wood, with

increasing humidity and cool conditions in later states of decay. Such variation in temperature

and humidity are known to influence the distribution of carabids within a stand (Thiele 1977).

Late-decay wood was the best predictor variable for total carabid abundance and seven of the

twelve abundant carabid species that were positively correlated with DWD, Agonum sordens,

Calathus ingratus, Loricera pilicornis, Platynus decens, Pterostichus coracinus, Pterostichus

melanarius, and Trechus apicalis; each of these species, with the exception of T. apicalis, is

known to utilize wood as an overwintering substrate (Larochelle and Lariviere 2003). Wood in

an advanced state of decay is softer and more accessible than early-decay wood for individuals

requiring access for overwintering or oviposition. For example, Pterostichus adstrictus

individuals are often found ovipositing under the bark of moist decayed wood, a substrate also

used by its three larval instars and pupae (Goulet 1974). These results suggest that wood in

29

advanced stages of decay are especially important for some carabid species. The range of

microhabitats available in DWD will increase with the diameter and therefore it is likely that the

size distribution of DWD is important to many saproxylic organisms (Martikainen et al. 2000). A

Canadian boreal forest study suggests that many beetle species depend directly on large-diameter

woody debris (Hammond et al. 2004). In my study, total carabid abundance and four of the eight

species positively correlated with DWD (Calathus ingratus, Platynus decens, Pterostichus

coracinus, and Pterostichus melanarius) were associated with only large-diameter, late-decay

coniferous DWD.

Interestingly, four of the abundant species in this study were negatively correlated with

DWD (most significant relationships Fig. 1.2). In other studies, three of these species have been

positively associated with DWD in several habitats including mature and post-fire forests and

clearcuts. Bradycellus lugubris, a species favouring wet environments, has been found to be

positively associated with high volumes of early-decay DWD in clearcut habitats (Pearce et al.

2003), but similar correlations have not been reported in closed-canopy forests. Pterostichus

adstrictus, a species known to utilize woody debris for egg and larval development,

overwintering, and shelter (Goulet 1974, Larochelle and Lariviere 2003), has been positively

associated with high volumes of woody debris in post-fire stands (Cobb et al. 2007) and woody

debris of moderate-decay in clearcuts (Pearce et al. 2003). However, P. adstrictus is a habitat

generalist with regionally varying habitat preferences and flexibility in oviposition substrates and

has displayed a fast rate of development in a range of temperatures and moisture levels (Goulet

1974). Pearce et al. (2003) also found P. adstrictus to be less abundant in wet microhabitats in

mixedwood boreal stands, which may explain the negative relationship with DWD, a feature

associated with humid microclimate (Harmon et al. 1986). Synuchus impunctatus, also a forest

30

generalist that utilizes woody debris for shelter (Larochelle and Lariviere 2003), has been found

to be positively associated with moderate to well decayed woody debris in clearcut habitats

(Pearce et al. 2003), however such results are not necessarily transferable to closed canopy

forests. It is likely that DWD may be important for some carabid species only when other habitat

features are limiting. For example, Pearce et al. (2003) found that woody debris was more

important for carabids in clearcuts than in closed forest. DWD may be less important in the

structurally complex habitats of mature forests. Indeed, my own work suggests this; for

example, I found that DWD, in the form of slash piles, strongly increased local abundances of

carabids in clearcuts (by 167% on average; Chapter 2), but only relatively weakly in the present

study (for example, as seen in Fig. 1.1, an increase in the volume of large-diameter, late-decay

conifer from 0 to 19 m3/ha increased total carabid abundance on average by 91%). Sphaeroderus

stenostomus, a species considered to be a forest specialist and is often found sheltering under

logs and overwintering in wood (Larochelle and Lariviere 2003, Bousquet 2010) was found to be

negatively correlated with large-diameter, late-decay coniferous DWD. This is a curious result

given that this substrate apparently provides ideal conditions for S. stenostomus and indeed was

found to be a positive correlate for three other species that in the literature show associations

with DWD similar to those reported for S. stenostomus. Furthermore, as described above, it is

likely that DWD may be more important for some carabid species only when other habitat

features are limiting. Species responding negatively to increasing volumes of DWD may also be

influenced by the presence and distribution of their competitors that could be associated with

DWD. For example, carabids were found to be negatively correlated with the presence of

Formica ants in two studies in southern boreal forests in Finland (Niemelä et al. 1992a, Heliola

31

et al. 2001). Until additional manipulative experiments are undertaken, it will be difficult to tease

apart such relationships.

Of the four rare species significantly correlated with DWD, two were positively

associated and two were negatively associated, and for each species this relationship was

consistent for all DWD variables. Each of these four species has been noted to utilize DWD for

shelter, and in the case of Carabus maeander and Pterostichus luctuosus, also as an

overwintering substrate (Larochelle and Lariviere 2003). Interestingly, C. maeander and P.

luctuosus displayed opposite associations (positive and negative, respectively) with three

different DWD variables each despite having very similar moist habitat preferences and

suggested affinities with DWD, however these species are typically associated with wet

environments (e.g., marshes and swamps) and are likely not in their preferred habitat in the

relatively drier mixedwood forests (Bousquet 2010). The positive association of C. maeander

with both small and large-diameter early-decay conifer and small-diameter, late-decay conifer is

consistent with a species utilizing DWD for shelter and overwintering. Amara lunicollis is

considered a generalist species that is typically associated with open habitats; however this

species positive association with small-diameter, late-decay, conifer DWD is consistent with its

reported DWD affinity (Larochelle and Lariviere 2003). Bembidion wingatei, a forest specialist

species typically associated with both conifer and mixedwood forests was negatively associated

with both early and late-decay, small-diameter conifer DWD. This species is generally only

collected in low abundances and no specific affinities with DWD have been reported aside from

observational data therefore it is difficult to speculate on this this result. In fact, with the

exception of P. luctuosus, all of these species are often reported in relatively low abundances in a

variety of forest types in North America (e.g., Werner and Raffa 2000, Pearce et al. 2003,

32

Klimaszewski et al. 2005, Pearce et al. 2005, Gandhi et al. 2008). In this study these

relationships are relatively weak as a result of the rarity of these species (standardized

abundances; A. lunicollis is represented by 9.07 individuals [present in 18.5% of sample plots],

B. wingatei is represented by 15.49 individuals [present in 22.2% of sample plots], C. maeander

is represented by 12.09 individuals [present in 18.5% of sample plots], and P. luctuosus is

represented by 11.83 individuals [present in 14.8 % of sample plots]).

Previous studies have found that carabid species richness and diversity increase with

structural complexity such as dead wood (e.g., Fuller et al. 2008). In a woody debris

manipulation study in approximately 50-year-old loblolly pine stands, for example, Ulyshen and

Hanula (2009) reported higher carabid species richness and diversity in sites with higher

volumes of woody debris. In the present study, I found little evidence of an increase in carabid

species richness with increasing volumes of DWD. Abundances, however, were consistently

higher in sites with high volumes of coniferous DWD both small and large-diameter and early

and late-decay, and were generally higher in sites with late-decay wood of various sizes and

species. Large-diameter, late-decay conifer was a particularly good predictor of total carabid

abundance. This is consistent with the species-level analysis in this study as seven of the twelve

abundant species positively correlated with DWD were found to be specifically correlated with

late-decay DWD and four of those seven species were specifically correlated with large-

diameter, late-decay coniferous DWD. The presence of coniferous DWD, especially well-

decayed large-diameter logs, appears to be an important substrate for carabids.

Unfortunately, not enough is known about individual carabid species life histories and

microhabitat requirements to fully understand habitat affinities with DWD. Detailed studies on

specific associations of carabids (autecology) with DWD would be very beneficial for further

33

interpretation of these results and in determining how critical DWD is for the survival of carabid

species. Study of carabid egg, pupal, and larval stages, the most vulnerable phases due to limited

mobility and sensitivity to extremes (Lövei and Sunderland 1996), could provide insight in to the

importance of DWD in maintaining viable populations.

The results of this study provide evidence that several species are affected by limited

availability of DWD, particularly large-diameter, late-decay logs. These results should be

interpreted with caution, however, as it is difficult to isolate the importance of a single habitat

feature and there may be other habitat factors that could explain variation in carabid abundances

which have not been measured in this study such as soil moisture and pH, vertical stand

stratification, litter composition and their potential interconnectivity (e.g., Paje and

Mossakowski 1984, Niemelä et al. 1992b, Antvogel and Bonn 2001). However this study was

designed to limit confounding variables by sampling in stands of similar tree species

composition and management history (the fact that they are all relatively mature post-logged

sites) and by manipulation of DWD. Standing dead wood and stumps were not considered in this

study because these forms of dead wood are unlikely to be important for the ground dwelling

carabids; at the same time, the latter do provide some woody debris, and hence may have made

the DWD relationships that I found weaker.

The results of this study generally support the increasingly accepted recognition of the

importance of woody debris in forest ecosystems as limitations in the availability of DWD

evidently have strong implications for some carabids in boreal mixedwood forests. Positive

correlations of carabid species with various DWD types, particularly large-diameter logs and

those in advanced stages of decay, indicate the need to maintain a diverse supply of DWD within

managed forests in order to ensure the conservation of these species. Interestingly, over a wide

34

range of volumes, I found that total abundance and abundances of most species increased more-

or-less linearly, indicating that management-associated reductions in DWD volumes can be

expected to decrease populations. In the context of clearcut harvesting, where the majority of

late-decay DWD is destroyed by machinery or exposed to drying conditions (Hautala et al.

2004), leaving aggregated retention patches that are representative of the given forest may serve

to protect some existing DWD and provide a source of new DWD through the early stages of

stand development. Maintaining larger patches will increase the long-term stability of residual

trees and provide larger source habitats. Clearly, the more intensive harvest operations become

with respect to features such as decreased rotation lengths and higher fibre removals, the greater

will be the reductions in the abundances of DWD-loving species, making the value of residual

leave areas, reserves, and protected areas even more important.

35

Chapter 2: The importance of slash for ground beetles

(Coleoptera: Carabidae) in a biomass clearcut

Introduction

Clearcutting is the dominant forest management strategy in Canadian boreal forests and is

largely aimed at mimicking natural disturbance (OMNR 2001). One key difference separating

clearcutting from natural disturbance, however, is the reduced amount of woody debris that

remains after the disturbance (Fridman and Walheim 2000, Siitonen 2001, Pedlar et al. 2002).

Growing concerns over climate change and diminishing petroleum supplies have led to an

increased interest in the development of renewable energy sources, including the use of logging

residues and stems of low commercial value as fuel. The development of this resource and the

associated intensification of biomass harvest may lead to a further reduction in woody debris

supplies and a loss of structural complexity in clearcuts. In parts of Europe, such intensive

management has resulted in reductions of woody debris by as much as 90 – 98 % relative to

levels found in unmanaged stands (Siitonen 2001).

Reductions in woody debris in intensively managed forests have been directly tied to

negative impacts on saproxylic flora and fauna (Siitonen and Martikainen 1994, Siitonen 2001).

In Sweden, slash harvesting has been found to result in lower species richness of ground-active

beetles in recently clearcut (< 1 year post-harvest) stands (Gunnarsson et al. 2004) and decreases

in arthropod abundances over the longer term (15 – 18 years post harvest; Bengtsson et al. 1997).

This is especially concerning in light of the high numbers of red-listed invertebrates (referring to

species considered extirpated, endangered, or threatened in an area) that are saproxylic, including

beetles, of which an estimated 85 % are red-listed in Sweden (Jonsell et al. 1998). The

harvesting of logging residues is a relatively new practice in Canada and therefore it may be

36

possible to minimize negative consequences similar to those observed in Europe. In order to

assess potential impacts associated with slash harvesting, it is important to understand its

ecological role in clearcuts.

Numerous studies have utilized carabid beetles (Coleoptera: Carabidae) to assess impacts

associated with forest management. As a group, they are highly sensitive to habitat conditions

and respond rapidly to habitat change (e.g., Niemelä et al. 1993a, Koivula et al. 2002). Structural

features, such as leaf litter and logging residues have been found to have a significant influence

on carabid abundances (Koivula et al. 1999, Koivula and Niemelä 2003). Slash is a dominant

structural feature within clearcuts and, because it is largely composed of woody debris, it is

likely an important habitat feature for many carabid species for which woody debris is an

important substrate (e.g., Pearce et al. 2003, Latty et al. 2006, Cobb et al. 2007). Pearce et al.

(2003) found strong associations between several carabid species and woody debris in clearcuts

of Ontario’s northwestern boreal region, although they did not examine biomass harvest-related

slash reductions specifically. Studies in Europe have shown that harvesting slash after

clearcutting results in changes in microclimatic conditions and vegetation (Proe et al. 2001,

Astrom et al. 2005, Dynesius et al. 2008) and microhabitat is considered to be among the most

important factors in determining carabid assemblages (Thiele 1977). A number of European

studies have examined associations of carabids with slash in clearcuts. In a slash-manipulation

experiment in boreal Sweden, carabids were found to be significantly more abundant near slash

than on bare ground (Nittérus and Gunnarsson 2006). In a comparison of Balsam fir (Picea

abies) -dominated boreal clearcuts with and without slash harvest, Nittérus et al. (2007) found

that sites with slash harvest had significantly fewer forest carabid species, a guild that is

particularly sensitive to clearcutting (Szyszko 1990, Niemelä et al. 1993a, Niemelä et al. 1993b,

37

Haila et al. 1994, Duchesne et al. 1999, Heliola et al. 2001). In a Finnish study, Koivula and

Niemelä (2003) found that the amount of slash on the ground significantly explained variation in

carabid communities. Finally, in a laboratory habitat-choice experiment in which total time spent

in either slash or bare ground was measured, the two carabid species used in the experiment

(Pterostichus oblongopunctatus and Carabus hortensis) spent significantly more time in slash

than away from slash (Nittérus et al. 2008).

Although carabids evidently respond positively to slash, current knowledge in the context

of biomass harvesting is limited to European studies in largely conifer-dominated stands that

contained very little post-harvest woody debris. Responses have not been examined for Canadian

carabid assemblages or from harvests of mixedwood stands with substantial deciduous

components. Clearcuts of mixedwoods in Canada contain comparatively higher volumes of

post-harvest woody debris, in part because of lower commercial values of deciduous trees and in

part because such cuts typically represent a first rotation of harvesting. For example, c.

5-year-old conifer-dominated clearcuts in Finland were found to contain 15.6 ± 25.1 m3/ha of

woody debris > 10 cm in diameter and >130 cm in length (32.7 ± 27.9 m3/ha when all woody

debris > 10 cm including stumps was included; Eräjää et al. 2010), whereas mixedwood

clearcuts of the same age in Canada were found to contain 111.97 ± 35.14 m3/ha of woody debris

> 10 cm in diameter and > 50 cm in length (including stumps, however stumps comprise a very

small portion of the volume; Pedlar et al. 2002). According to island biogeography theory

(MacArthur and Wilson 1967), associations of carabids with slash would presumably be weaker

in clearcuts where more woody debris is available compared to clearcuts where such resources

are in short supply.

38

The spatial distribution of habitat patches is considered to be of central importance in the

conser