APPROVED: Kevin Stevens, Major Professor Barney Venables, Committee Member Aaron Roberts, Committee Member Arthur J. Goven, Chair of the Department of
Biological Sciences James D. Meernik, Acting Dean of the
Toulouse Graduate School
INTEGRATING SELECTIVE HERBICIDE AND NATIVE PLANT RESTORATION
TO CONTROL Alternanthera philoxeroides (ALLIGATOR WEED)
Justin Adams, B.S.
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
December 2011
Adams, Justin. Integrating Selective Herbicide and Native Plant Restoration to Control
Alternanthera philoxeroides (Alligator Weed). Master of Science (Environmental Science),
December 2011, 39 pp., 7 tables, 19 figures, bibliography , 67 titles.
Exotic invasive aquatic weeds such as alligator weed (Alternanthera philoxeroides)
threaten native ecosystems by interfering with native plant communities, disrupting hydrology,
and diminishing water quality. Development of new tools to combat invaders is important for the
well being of these sensitive areas. Integrated pest management offers managers an approach that
combines multiple control methods for better control than any one method used exclusively. In a
greenhouse and field study, we tested the effects of selective herbicide application frequency,
native competitor plant introduction, and their integration on alligator weed. In the greenhouse
study, alligator weed shoot, root, and total biomass were reduced with one herbicide application,
and further reduced with two. Alligator weed cumulative stem length and shoot/root ratio was
only reduced after two applications. In the greenhouse, introduction of competitors did not affect
alligator weed biomass, but did affect shoot/root ratio. The interaction of competitor introduction
and herbicide did not significantly influence alligator weed growth in the greenhouse study. In
the field, alligator weed cover was reduced after one herbicide application, but not significantly
more after a second. Introduction of competitor species had no effect on alligator weed cover,
nor did the interaction of competitor species and herbicide application. This study demonstrates
that triclopyr amine herbicide can reduce alligator weed biomass and cover, and that two
applications are more effective than one. To integrate selective herbicides and native plant
introduction successfully for alligator weed control, more research is needed on the influence
competition can potentially have on alligator weed growth, and the timing of herbicide
application and subsequent introduction of plants.
ii
Copyright 2011
by
Justin Adams
iii
TABLE OF CONTENTS
Page
LIST OF TABLES ......................................................................................................................... iv
LIST OF FIGURES .........................................................................................................................v
Chapters
INTRODUCTION ...........................................................................................................................1
METHODOLOGY ..........................................................................................................................8
RESULTS ......................................................................................................................................19
DISCUSSION ................................................................................................................................29
WORKS CITED ............................................................................................................................35
iv
LIST OF TABLES
Page
Table 1. Greenhouse Experiment Table of Treatments ...................................................................8
Table 2. Field Experiment Table of Treatments ...........................................................................14
Table 3. Cover Classes of Daubenmire Method ........................................................................... 17
Table 4. Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide
Applications and Four Competitor Species Introductions on A. philoxeroides .................20
Table 5. Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide
Applications and Competitive Species (Monocots and Dicots Grouped) Introduction on
A. philoxeroides .................................................................................................................21
Table 6. Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide
Applications and Competitor Introduction on A. philoxeroides Cover .............................26
Table 7. Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide
Applications and Competitor Introduction (Monocots and Dicots Grouped) on A.
philoxeroides Cover ...........................................................................................................27
v
LIST OF FIGURES
Page
Figure 1. A. philoxeroides in treatment containers prior to herbicide treatments or introduction of
other plants. ..........................................................................................................................9
Figure 2. Treatment containers randomly selected for the first application of triclopyr herbicide. ..
............................................................................................................................................10
Figure 3. Plant species used for competition treatments. ..............................................................10
Figure 4. Introduction of a competitor species into pots containing herbicide treated A.
philoxeroides. .....................................................................................................................11
Figure 5. Satellite image of field study site in Grand Prairie, Texas. ...........................................13
Figure 6. Example of one row of a block at the Grand Prairie field site. .....................................15
Figure 7. Planting of Carex hyalinolepis following the first herbicide application. .....................15
Figure 8. The first triclopyr application on A. philoxeroides at the Grand Prairie field site. .......16
Figure 9. Carex brevior and A. philoxeroides in a greenhouse treatment container that received
two herbicide applications. ...............................................................................................19
Figure 10. Greenhouse treatment containers on day of harvest. ....................................................22
Figure 11. Justicia american roots (top), A. philoxeroides roots (bottom). ...................................23
Figure 12. Effect of herbicide application frequency on A. philoxeroides total, root, and shoot dry
weight. n = 15 for each treatment. .....................................................................................23
Figure 13. Effect of herbicide frequency on A. philoxeroides cumulative stem heights. n = 15 for
each treatment.. ..................................................................................................................24
Figure 14. Effect of herbicide frequency on A. philoxeroides shoot/root ratio. n = 15 for each
treatment.. ..........................................................................................................................25
vi
Figure 15. Effect of competitor introduction on A. philoxeroides shoot/root ratio. n = 9 for each
treatment.. ..........................................................................................................................25
Figure 16. 1m x 1m PVC quadrat to aid in estimating A. philoxeroides cover within field
treatment plots. ..................................................................................................................26
Figure 17. Mean ± standard error A. philoxeroides cover exposed to three herbicide application
frequencies. n = 25 for each treatment. ..............................................................................27
Figure 18. Field experiment block four. A. philoxeroides after three levels of herbicide
application. .........................................................................................................................28
Figure 19. Carex hyalinolepis and A. philoxeroides after two herbicide applications. ................28
1
INTRODUCTION
Spread and invasion by non-indigenous species is an expanding global problem. In the
U.S. alone, expensive prevention and eradication techniques costing hundreds of billions of
dollars annually are employed to stop the environmental, agricultural, and health damages these
invaders can cause (Bertness 1984, Vitousek et al. 1987, Mack et al. 2000, Pimentel et al. 2005,
Lodge et al. 2006). Invasive non-indigenous aquatic plants are particularly difficult to control
due to their ability to move easily through aquatic and riparian corridors, invade and dominate
disturbed habitat, and reproduce vegetatively from fragments carried downstream (Hobbs and
Huenneke 1992, Madsen and Smith 1997, Boose and Holt 1999, Hood and Naiman 2000). Alien
aquatic weeds often out-compete native species, consequently altering important ecosystem
processes (Madsen et al. 1991, Cheruvelil et al. 2001, Urgenson et al. 2009). As the movement to
protect wetlands and other aquatic ecosystems continues to grow, so too does the need to
establish new tools for combating invasion of non-indigenous aquatic plants.
Integrated pest management (IPM) strategies began to develop in the 1950’s in response
to concerns over “undisciplined” and sometimes “unrestrained” use of chemical pesticides in
agriculture (Castle et al. 2009). Modern IPM combines multiple control methods and integration
of pest biology knowledge in an attempt to more efficiently manage target species (Buhler et al.
2000). Many IPM strategies are employed to increase effectiveness of natural enemies, therefore
reducing dependence on chemical control. For example, modern insecticides tailored to specific
insect pests that do not adversely affect natural predators can be used to keep natural predation
viable (Gentz et al. 2010). Another strategy involves installation of “banker plants” amongst
crops that, through carrying a non-pest food source, aid in attracting, retaining, or dispersal of
natural enemies for biological control (Pineda et al. 2008). Some IPM strategies integrate
2
cultural methods. Examples include practices that interfere with pest lifecycles, such as crop
rotation or tillage (Meissle et al. 2011). Increasingly, crops are being grown with genetic
enhancements, such as introduction of genetic material from the natural insecticide producing
bacterium Bacillus thuringiensis (Meissle et al. 2011).
IPM management programs uniquely tuned for weeds have grown as attention to weed
biology and ecology has increased (Appleby 2005). As with insect pests, IPM for weeds attempts
to improve control without solely relying on chemical herbicides. Agricultural examples of
cultural IPM practices for weeds include; using alternative cultivars, adjusting fertilizer rates,
crop rotations, and smothering of weeds through the use of “cover crops” (Valenti and Wicks
1992, Wyse 1994). As with insect pest, biological control of weeds is possible. Introducing
herbivory from exotic insects or bolstered native populations are common tools (Cofrancesco
1998).
Integrated management techniques for aquatic weeds do exist, but integration of
available tools is less intensely researched then their terrestrial counterparts. Aquatic herbicides
are available for chemical control of aquatic weeds, but have a special set of problems associated
with them. Available herbicides are limited because, due to the sensitive nature of aquatic
habitats, many terrestrial herbicides cannot clear regulatory hurdles to cross over into aquatics.
Herbicides that can be used are often limited by application restrictions. Applications of aquatic
herbicides can result in dissolved oxygen crashes and subsequent fish and invertebrate mortality
if too much vegetation is eliminated at once. Control of invasive aquatic weeds with traditional
herbicide applications can lead to further invasions and impede progress if native plant
community restoration is the intended goal. Herbicides can create a disturbance that may rapidly
fill with new invasive species or be re-invaded by the original target weed if high efficacy is not
3
achieved (Morrison 2002, Ogden and Rejmanek 2005). Development of new IPM strategies for
aquatic weeds that replace, reduce, or improve effectiveness of herbicides is of particular
importance due to a federal regulatory environment that may soon favor tighter controls on
aquatic herbicide application practices (Nat’l Cotton Council v. EPA, 553 F.3d 927 6th Cir.
2009).
Biological and cultural control options for aquatics do exist. Well known insect
biocontrol agents for invasive aquatics include the giant salvinia weevil (Flores and Carlson
2006), alligator weed flea-beetle (Sainty et al. 1998) and waterhyacinth weevils (Haag and
Habeck 1991). Fish as well as insects are used in aquatics plant management. Examples include
introduction of tilapia for giant salvinia control (McIntosh et al. 2008), and triploid grass carp for
hydrilla control (Cuda et al 2008). Current cultural methods for aquatic weed control involve
watershed level techniques to reduce nutrient loads and improve water quality. Examples include
wetland reestablishment, installation of vegetated buffer strips between agricultural and
drainages, phosphate free detergents, and efficient agricultural fertilizer plans (Jeppesen et al.
1999). Reservoir level cultural techniques include temporary manipulation of water level to
suppress vegetation (Poovey and Kay 1998), or reduction of available light through aquatic dyes
or floating shade mats (Schooler 2008). Manipulating the availability of the resources required
for weed growth at the reservoir level is a possible management tool uniquely tuned for weeds
(Buhler et al. 2000). The availability of necessary resources to any particular plant is often
limited by competition for the same resources from another plant (Harris 1967). Re-introduction
of native plants to compete with aquatic weeds is a possible management tool, and has been
explored with some success in terrestrial applications. In a prairie restoration project, Bakker and
Wilson (2004) found that plots restored with native grasses reduced invasion of A. cristatum by
4
one-third as compared to un-restored plots. In situations where re-introduction alone is not
enough to stop an invasion, manipulation of the competitive arena before re-introduction of
native species may be necessary to give native competitors a head start or competitive edge
(Perry and Galatowitsch 2003). Integrating herbicide treatments targeting an invasive weed prior
to, and after native plant reintroduction is a management option that may result in more effective
control than either technique used exclusively. Hubbard (1975) reported a 30% increase in native
grass dry matter yield in knapweed infested plots treated with herbicide previous to seeding as
compared to plots exposed to seeding only. Little information is available for applying this type
of IPM technique to invasive aquatic weeds.
Alligator weed, Alternanthera philoxeroides, is a stoloniferous and rhizomatous perennial
weed native to South America (Sainty et al. 1998). The plant can grow under aquatic, semi-
aquatic, and terrestrial conditions. In water, hollow stems allow the plant to extend across deep
water to form dense floating mats (Julien et al. 1995). A. philoxeroides was first reported in the
United States in 1894, and although it may have reached the maximum extent of its range in the
US, A. philoxeroides has invaded many waterways, wetlands, irrigation canals, and low-lying
areas in California and much of the southeastern US (Maddox et al. 1971, Coulson 1977). A.
philoxeroides is classified as a noxious plant in seven states including Texas (USDA, NRCS
2010). In the United States, A. philoxeroides does not produce viable seeds; reproduction is
achieved vegetatively (Quimby and Kay 1977). Sections of stem containing a node are capable
of establishing a new colony (Vogt et al. 1992), and fragmentation readily occurs as a result of
mechanical or environmental disturbances such as harvest attempts or flooding. A. philoxeroides
invasions can reduce the economic and ecological value of a waterway. A. philoxeroides can
impede pump intakes for irrigation, restrict flow in small waterways leading to blockages and
5
sedimentation, and impede navigation and recreation (Holm et al. 1997). Like many exotic
invasive weeds, A. philoxeroides is often able to out-compete and replace native plant
communities. Vogt et al. (1992) reported un-contested spread of A. philoxeroides mats in the
southern Mississippi valley prior to introduction of biocontrol agents. Floating mats of A.
philoxeroides can restrict light penetration to the water below, effectively starving other
submerged plants and reducing oxygen levels (Quimby and Kay 1976).
Effective management options for A. philoxeroides invasions are limited. Mechanical
harvesting or tilling can be used for temporary control, but the plant is capable of rapid re-growth
after disturbance, and dislodged fragments may increase the extent of invasion if allowed to
disperse and colonize new territory (Vogt et al. 1992, Wilson et al. 2007). Biological control
options exist, but effectiveness is dependent on target area climate and A. philoxeroides growth
form. The alligator weed flea-beetle, Agasicles hygrophila, and the alligator weed stem borer,
Vogtia mallo, were first released in the southern United States by the USDA in 1964 and 1971
respectively (Sainty et al. 1998). These insects can cause significant damage to floating A.
philoxeroides mats, but do not populate terrestrial infestations and are not tolerant of the cooler
climate associated with the northern range of A. philoxeroides in the United States (Julien et al.
1995). Research into other possible biological control candidates is ongoing (Julien et al. 2004).
A. philoxeroides is a difficult plant to control chemically; numerous attempts at control
with various herbicides, application frequencies, and applications timings have been attempted.
Typically, only partial control is achieved. Leaves and stems are damaged, but re-growth
generally occurs. Bowmer et al. (1993) reported that only 7% of glyphosate herbicide applied to
A. philoxeroides via foliar treatment reached underground organs. Tucker et al. (1994) found
better herbicide transport to roots with imazapyr than glyphosate, but field studies involving
6
imzapyr lack evidence for complete control (Allen et al. 2007, Schooler et al. 2008). Kay (1992)
found increased resistance to quinclorac herbicide in a broad stemmed A. philoxeroides biotype
compared to a slender stemmed biotype. Kay (1992) was able to reduce A. philoxeroides biomass
after foliar applications applied to both biotypes, but was unable to completely kill the plant.
Recent field experiments involving relatively new selective aquatic herbicides offer
promising results that can be applied to the development of new integrated A. philoxeroides
control techniques. In a field study at a managed marsh in southeastern Alabama, Allen et al.
(2007) altered A. philoxeroides above ground biomass after applications and interactions of two
herbicides (selective and non-selective), two application dates, and three application rates. Allen
et al. (2007) included effects on native species into the design and found, in addition to a
decrease in A. philoxeroides over controls, an increase of native plant biomass within a year of
July application using selective herbicides over non-selective herbicides. Schooler et al. (2008)
reported presence of A. philoxeroides in all study plots seven months after treatment with
glyphosate, metsulfuron and triclopyr at four applications each and at the highest label
recommended rate. Although not directly tested, Schooler et al. (2008) credits increased
competition from monocots for elimination of A. philoxeroides from 29% of plots sampled one
year after treatment with dicot-selective herbicides (metsulfuron and triclopyr) versus 0%
elimination with non selective herbicide (glyphosate). These studies hint at the possibility of
integrating native plant community involvement with the use of selective herbicides for A.
philoxeroides control.
Given the potential economic and ecological damages that can ensue from the spread of
A. philoxeroides and the paucity of information on effective control measures for this plant, the
objective of this study is to determine the effects of integrating selective herbicide applications
7
and native plant restoration on A. philoxeroides growth. I will test the hypothesis that
introduction of native monocot species between dicot-selective herbicide treatment intervals may
provide stronger control than selective herbicide treatments or native plant restoration alone. The
results of this study will lead to the availability of an effective tool and management option for A.
philoxeroides control.
8
METHODOLOGY
A greenhouse component and a field component were included in this project.
Similar treatment regimes were applied to both components.
Greenhouse Experiment
The greenhouse component of this project took place at the Water Research Field Station
greenhouse at the University of North Texas. The experiment utilized a 3X5 factorial design with
three replicates per treatment level (Table 1). Treatments applied were “herbicide application
frequency” and “competitor species.” Effects of herbicide application frequency were tested
using three levels; application frequencies of zero, one, and two. Effects of competitor species
were tested using five levels; no competitors, two native monocot species, and two native dicot
species. The four plant species were selected based on data from previous field surveys and seed
bank surveys taken within the field study site.
Table 1
Greenhouse Experiment Table of Treatments
Competitor Species
No Competitor
Justica
americana
Polygonum
densiflorum
Sagittaria
platyphylla Carex brevior
Herbicide
Application
Frequency
0 n=3 n=3 n=3 n=3 n=3
1 n=3 n=3 n=3 n=3 n=3
2 n=3 n=3 n=3 n=3 n=3
Note. Three replicates per treatment were used.
Greenhouse Experiment Materials and Methods
A. philoxeroides was propagated vegetatively from similarly sized, single node, clippings
collected from greenhouse stock plants originally collected from a retention pond at Six Flags
9
over Texas in Arlington, Texas. On August 6th 2010, clippings were planted in 45- 6.59L plastic
treatment containers filled with 4.0L of a (Premier Tech Horticulture Ltd, Quakertown, PA)
50/50 mixture of sand and PRO-MIX™ potting soil (Figure 1). Holes drilled near the top of each
container allowed irrigation water to drain, creating a shallow layer of surface water (< 1.0 cm)
for plants to grow in. Before A. philoxeroides clippings were planted, empty 0.95L pots were
placed in the soil of 36 randomly selected treatment containers to allow space for introduction of
competitor species with minimal A. philoxeroides root disturbance (Figure 4). Two competitor
dicot species; Justica americana and Polygonum densiflorum, and two competitor monocot
species; Sagittaria platyphylla and Carex brevior, were propagated at the University of North
Texas aquatic plant nursery in 0.95L pots containing a 50/50 mixture of PRO-MIX™ (Premier
Tech Horticulture Ltd, Quakertown, PA), and sand (Figure 3). All plants were drip irrigated with
1.89 L/Day filtered well water.
Figure 3. A. philoxeroides in treatment containers prior to herbicide treatments or introduction of
other plants.
10
Figure 4. Treatment containers randomly selected for the first application of triclopyr herbicide.
Figure 3. Plant species used for competition treatments. Left to Right: Juncus sp. (not used),
Carex brevior, Sagittaria platyphylla, Justica americana, Polygonum densiflorum.
11
Figure 4. Introduction of a competitor species into pots containing herbicide treated A.
philoxeroides.
Greenhouse temperature was maintained at 22°C. Sufficient lighting to keep plants from entering
senescence was not available until January 10th 2011, after which plants were grown under a
14/10 light/dark cycle. Triclopyr amine, a dicot-selective herbicide under the trade name
Renovate®3 (Renovate, SePRO, Carmel, IN 46032) was used for herbicide treatments. Triclopyr
was applied at a rate of 3.42 kg ae ha¯¹ using a Solo® 425® 15.14L pressurized back-pack
sprayer. AQUA-KING®Plus (Estes Inc., Wichita Falls, TX 76301) a non-ionic surfactant was
added at a rate of 0.25% of spray solution. Forty-three days after A. philoxeroides was planted,
the first round of herbicide application was administered. To prevent cross contamination from
herbicide drift, treatment containers randomly selected for herbicide treatment were removed
from the greenhouse and sprayed outside (Figure 2). Spraying took approximately 15 minutes.
To eliminate the possibility of herbicide transferring to neighboring plants through plant to plant
contact, treated plants remained outside for one hour to allow the spray solution to evaporate.
12
Competitor species were introduced into randomly selected treatment and control containers
after 54 days. On day 85, a subset of previously sprayed treatment containers was randomly
selected for a second spray application. After 239 days, all vegetation was removed from
treatment containers and separated into root and shoot sections. Fresh cumulative stem heights
were obtained to the nearest millimeter during processing. Above ground, below ground, and
total dry biomass was to the nearest mg obtained after plant material was placed in drying ovens
at 40°C for five days.
Statistical Analysis
After harvesting, a two-way analysis of variance (ANOVA)(SAS Institute 2003) was
used to test effects of competitor species, herbicide application frequency, and all interactions on
total A. philoxeroides dry biomass, above ground dry biomass, below ground dry biomass, fresh
cumulative stem length, and dry shoot/root ratio. A duplicate analysis was run with individual
species combined as monocots or dicots. Data met requirements for normality and
homoscedasticity. Means were considered significant at p < 0.05. When significant main effects
or interaction effects were detected, multiple comparisons were conducted using the Bonferonni
correction (Zar 2009).
Field Experiment
The field component of this project occurred within an oxbow of the Trinity River
adjacent to the city landfill in Grand Prairie, Texas (96°56'35.793"W 32°46'27.035"N) (Figure
5). The oxbow covers an approximate area of 7.5 hectares and is subject to periodic flooding
after heavy rains. A. philoxeroides was first reported within the oxbow area in 2008 and was
widespread by 2010. A. philoxeroides growth in the oxbow may best be described as rooted
emergent. As a follow up to the greenhouse component of this project, the field component used
13
a similar experimental design. The field experiment utilized a blocked 3X5 factorial design with
5 replicates per treatment level (Table 2). Treatments applied were “herbicide application
frequency” and “competitor species”. Effects of herbicide application frequency were tested
using three levels; frequencies of zero, one, and two applications. Effects of competitor species
were tested
Figure 5. Satellite image of field study site in Grand Prairie, Texas.
14
using five levels; no re-established native competitors, two re-established native monocot
species, and two re-established native dicot species. The four plant species were selected based
on data from previous field surveys and seed bank surveys taken within the oxbow area.
Table 2
Field Experiment Table of Treatments
Competitor Species
No
Competitor
Justica
americana
Polygonum
setaceum
Sagittaria
platyphylla
Carex
hyalinolepis
Herbicide
Application
Frequency
0 n=5 n=5 n=5 n=5 n=5
1 n=5 n=5 n=5 n=5 n=5
2 n=5 n=5 n=5 n=5 n=5
Note. Five replicates for each treatment combination were used.
Field Experiment Materials and Methods
Five experimental blocks were established at separate sites within the oxbow. Each block
exhibited relatively uniform hydrology and A. philoxeroides coverage, and was of sufficient size
to house subplots. Study blocks consisted of fifteen 1m² treatment sub-plots divided into three
columns (Figure 6). To simplify herbicide applications and reduce the risk of cross-plot herbicide
drift, entire columns received the same level of herbicide application frequency. Columns had a
2m buffer zone between them to minimize contamination from drift. Within each column, all
representatives of the “competitor” treatment were randomly assigned a subplot. Two competitor
dicot species; Justica americana and Polygonum setaceum, and two competitor monocot species;
Sagittaria platyphylla and Carex hyalinolepis, were propagated at the University of North Texas
15
Figure 6. Example of one row of a block at the Grand Prairie field site. A. philoxeroides pictured
has not been subjected to any treatments.
Figure 7. Planting of Carex hyalinolepis following the first herbicide application.
16
Figure 8. The first triclopyr application on A. philoxeroides at the Grand Prairie field site.
aquatic plant nursery in 0.95L pots containing a 50/50 mixture PRO-MIX™ (Premier Tech
Horticulture Ltd, Quakertown, PA) potting soil and sand. On October 1st 2010, two columns
from each study block were randomly selected to receive the initial herbicide application (Figure
8). Triclopyr amine, a dicot-selective herbicide under the trade name Renovate®3 (Renovate,
SePRO, Carmel, IN 46032) was used for herbicide treatments. Triclopyr was applied at a rate of
3.42 kg ae ha¯¹ using a Solo® 425® 15.14L pressurized back-pack sprayer. AQUA-KING®Plus
(Estes Inc., Wichita Falls, TX 76301) non-ionic surfactant was added at a rate of 0.25% of spray
solution. Thirteen days after initial spraying, competitor plant species were randomly distributed
amongst the subplots. To simulate a high density re-establishment, each subplot was issued four
17
individual plants of its assigned species. Individual plants were uniformly dispersed within
subplots using a PVC planting “jig” (Figure 7). The “jig” was placed at each right angle corner
of subplots and directed the planter to a specific planting location so plants were situated
identically across all treatment plots. On day 28, one of the previously sprayed columns in each
study block was randomly selected to receive the second herbicide treatment. Two-hundred
thirty-seven days after the initial herbicide application, A. philoxeroides coverage was evaluated
in the field with the Daubenmire method. The Daubenmire method allows a sampler to place
percent cover estimates into one of six cover classes (Table 3) (Dabenmire 1959). Digital photos
of each subplot were taken as well. This data has been archived for use in future projects where
comparisons of the historic plant community in the subplots may be appropriate (Figure 16).
Table 3
Cover Classes of Daubenmire Method
Daubenmire Cover Class Range of Coverage Midpoint of Range
1 < 5% 2.5%
2 5-25% 15.0%
3 25-50% 37.5%
4 50-75% 62.5%
5 75-90% 85.0%
6 95-100% 97.5%
Statistical Analysis
A two-way randomized complete block analysis of variance (ANOVA)(SAS Institute
2003) was used to test effects of competitor species, herbicide application frequency, and all
interactions on A. philoxeroides mean cover class midpoints. A duplicate analysis was run with
individual species combined as monocots or dicots. Data met requirements for normality and
18
homoscedasticity. Means were considered significant at p < 0.05. When significant main effects
or interaction effects were detected, multiple comparisons were conducted using the Bonferonni
correction (Zar 2009)
19
RESULTS
Greenhouse Experiment
Regardless of treatments, there was no complete eradication of A. philoxeroides from the
treatment containers (Figure 9). A. philoxeroides cumulative stem length, shoot, root, and total
dry weight were affected by herbicide application, but not significantly influenced by competitor
introduction or the interaction of competition and herbicide (Table 4). A. philoxeroides
shoot/root ratio was affected by herbicide application and competitor introduction, but not
influenced by their interaction (Table 4). Combining individual competitors into monocot or
dicot groups did not significantly alter results (Table 5).
Figure 9. Carex brevior and A. philoxeroides in a greenhouse treatment container that received
two herbicide applications.
20
Table 4
Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide Applications
and Four Competitor Species Introductions on A. philoxeroides Biomass, Cumulative Stem
Length, and Shoot/Root Ratio
Response Variable
Competitive Species Herbicide Herbicide X Competitive Species
F p Value
F p Value
F p Value
Shoot Dry Weight 1.90 0.1372 35.29 <.0001 1.21 0.3292
Root Dry Weight
0.66 0.6233 38.25 <.0001 1.59 0.1692
Total Dry Weight 1.16 0.3478 43.12 <.0001 1.47 0.2090
Cumulative Stem
Length
1.51 0.2231 23.46 <.0001 1.04 0.4297
Shoot/Root Ratio 3.10 0.0301 25.52 <.0001 0.77 0.6329
21
Table 5
Summary Table of Two-way ANOVA Assessing the Effects of Triclopyr Herbicide Applications
and Competitive Species (Monocots and Dicots Grouped) Introduction on A. philoxeroides
Biomass, Cumulative Stem Length, and Shoot/Root Ratio
Response Variable
Competitive Species
(Monocots and Dicots
Combined )
Herbicide
Herbicide X Competitive Species
(Monocot and Dicot Combined)
F p Value
F p Value
F p Value
Shoot Dry Weight 2.02 0.1479 31.48 <.0001 1.43 0.2444
Root Dry Weight
0.54 0.5851 31.72 <.0001 1.38 0.2612
Total Dry Weight 1.09 0.3476 36.86 <.0001 1.52 0.2159
Cumulative Stem
Length
2.65 0.0841 22.44 <.0001 1.04 0.2069
Shoot/Root Ratio 5.87 0.0062 21.33 <.0001 0.41 0.8003
Mean A. philoxeroides shoot, root, and total dry weight were significantly reduced with
increasing herbicide application frequency (Figure 12). Triclopyr applications reduced mean
shoot weight below controls (3.722g ± 0.285) after one application (2.257g ± 0.285), and further
after two applications (0.341g ± 0.285). Triclopyr applications reduced mean root weight below
controls (8.004g ± 0.501) after one application (5.028g ± 0.501), and further after two
applications (1.744g ± 0.501). Triclopyr applications reduced mean total weight below controls
(11.726g ± 0.735) after one application (7.284g ± 0.735), and further after two applications
(2.085g ± 0.735). A. philoxeroides mean cumulative stem length was significantly lower at the
22
two herbicide application level (23.660cm ± 15.893), but did not differ significantly between
zero (175.67cm ± 15.893) and one application (120.85cm ± 15.893) (Figure 13). A.
philoxeroides mean shoot/root dry weight ratio was significantly lower at the two herbicide
application level (0.1486 ± 0.035), but did not differ significantly between zero (0.461 ± 0.035)
and one application (0.4522 ± 0.035) (Figure 14). Shoot/root dry weight ratio was the only
measured response affected by a treatment other than herbicide application (Figure 11).
Introduction of competitor species influenced A. philoxeroides mean shoot weight/ root dry
weight ratio (Table 4). Of introduced competitors, Justicia sp was the only plant that
significantly reduced A. philoxeroides shoot/root dry weight ratio lower than control (Figure 15).
Figure 10. Greenhouse treatment containers on day of harvest.
23
Figure 11. Justicia american roots (top), A. philoxeroides roots (bottom).
Figure 12. Effect of herbicide application frequency on A. philoxeroides total, root, and shoot
dry weight. n = 15 for each treatment. Means are presented ± standard error. Different letters
within each group of response variables indicate significantly different means (p < 0.05). 0 = no
herbicide application. 1 = 1 herbicide applications. 2 = 2 herbicide applications.
24
Figure
13. Effect of herbicide frequency on A. philoxeroides cumulative stem heights. n = 15 for each
treatment. Means are presented ± standard error. Different letters indicate significantly different
means (p < 0.05).
Figure 14. Effect of herbicide frequency on A. philoxeroides shoot/root ratio. n = 15 for each
treatment. Means are presented ± standard error. Different letters indicate significantly different
means (p < 0.05).
25
Figure 15. Effect of competitor introduction on A. philoxeroides shoot/root ratio. n = 9 for each
treatment. Means are presented ± standard error. Different letters indicate significantly different
means (p < 0.05). No_comp = no competitor introduction. Sag_pla = Sagittari platyphylla.
Car_bre = Carex brevior. Pol_den = Polygonum densiflorum. Jus_ame = Justicia american.
Field Experiment
A. philoxeroides cover was affected by herbicide application, but not significantly
influenced by competitor introduction or the interaction of competition and herbicide (Table 6)
(Figure 19). Combining individual competitors into monocot or dicot groups did not significantly
alter results (Table 7). Triclopyr reduced A. philoxeroides mean cover significantly over controls
(31.00 ± 6.935) after both one application (16.70 ± 6.935) and two applications (6.90 ± 6.935),
but there was no significant difference in cover between one and two herbicide applications
(Figure 17) (Figure 18). Eight months after initial treatments, A. philoxeroides above ground
parts were not present in six plots that received two applications, two plots that received one
application, and two plots that received zero applications.
26
Figure 16. 1m x 1m PVC quadrat to aid in estimating A. philoxeroides cover within field
treatment plots.
Table 6
Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide Applications
and Competitor Introduction on A. philoxeroides Cover
Response Variable
Competition Herbicide Herbicide X Competition
F p Value
F p Value
F p Value
A. philoxeroides
cover
0.13 0.9697 9.03 0.0004 0.14 0.9966
27
Table 7
Summary Table of Two-Way ANOVA Assessing the Effects of Triclopyr Herbicide Applications and
Competitor Introduction (Monocots and Dicots Grouped) on A. philoxeroides Cover
Response Variable
Competition (Monocots
and Dicots Combined )
Herbicide Herbicide X Competition
F p Value
F p Value
F p Value
A. philoxeroides 0.20 0.8167 8.01 0.0008 0.21 0.9333
Figure 17. Mean ± standard error A. philoxeroides cover exposed to three herbicide application
frequencies. n = 25 for each treatment. Means are presented ± standard error. Different letters
indicate significantly different means (p < 0.05)
28
Figure 18. Field experiment block four. A. philoxeroides after two herbicide applications (left),
Zero applications (middle), one application (right).
Figure 19. Carex hyalinolepis and A. philoxeroides after two herbicide applications
Carex hyalinolepis
A. philoxeroides
29
DISCUSSION
Greenhouse Experiment
There was no evidence in the greenhouse experiment of enhanced control of A.
philoxeroides through integrating herbicide application and competitor introduction. When each
component is considered separately, it maybe surmised that the herbicide fulfilled its role, but the
competitive interaction needed was not present. The greenhouse experiment should have
provided the opportunity to run the experiment at any photoperiod desired. Prior to early January
2011, there was inadequate lighting available to keep the introduced plants from falling into their
seasonal senescence cycle. To take advantage of the window of competitive opportunity
provided by the herbicide applications, introduced plants may have benefited from more
vigorous growth associated with longer day lengths. Total biomass at harvest for most untreated
competitors was quite low in comparison to A. philoxeroides. For example, final mean biomass
for untreated Sagitaria was only 1.43g ± 0.42 as compared to 12.35g ± 2.86 for untreated A.
philoxeroides sharing the same treatment containers. Size certainly plays a role in a plants ability
to influence neighbors (Goldberg and Werner, 1983). In this experiment, competitors may not
have been given adequate conditions during a critical time to achieve the growth rate and size
required to take advantage of stressed A. philoxeroides.
A. philoxeroides biomass was reduced by triclopyr applications. This result was expected
and reported in the literature (Schooler et al. 2008, Allen et al. 2007). A second application six
weeks after the first did result in further biomass reduction than a single application. To our
knowledge, this has not been reported in A. philoxeroides experiments using triclopyr. Schooler
et al. (2008) reported no difference in A. philoxeroides biomass between three applications and
four applications, but did not test a single or double application in his design. Allen et al. (2007)
30
tested single triclopyr applications. Two herbicide applications were required to reduce
cumulative stem length beyond controls. After two applications, 50% of treatment pots had no
stems present, whereas after one treatment, 100% of pots had stems present. These results
suggest that although one application reduced stem density of above ground parts, elimination of
stems and leaves required two applications.
Literature points to poor herbicide translocation to roots as a reason for low herbicide
efficacy with A. philoxeroides treatments (Sainty et al. 1998, Julien et al. 2004). Bowmer et al.
(1993) reported that only 7% of glyphosate herbicide applied to A. philoxeroides via foliar
treatment reached the root system. We observed significant root reduction after both herbicide
applications frequencies, but this effect may not have been a direct result of root mortality due to
triclopyr. The shoot/root ratio is a measure of how a plant is allocating resources. The ‘functional
equilibrium’ theory suggests that a plant will shift resource allocation between roots and shoots
to aid in acquisition of resources that are most limiting (Brouwer, 1962). Experiments involving
pruning of roots and shoots have demonstrated that plants will grow toward a specific shoot/root
ratio over time after a disturbance (Alexander and Maggs 1971, Poorter and Nagel 2000). The
reduction of root biomass we observed could be attributed to a reallocation of plant resources
from roots to shoots due to disturbance of above ground parts via herbicide. Only a snapshot of
the plants' recovery and subsequent shoot/root partitioning was observed because the
experimental design called for a single harvest of all treatments. After one application of
triclopyr, the shoot/root ratio of treatment plants did not differ from controls. A. philoxeroides
that was treated with two applications had significantly lower shoot/root ratios than controls. The
plants exposed to a single treatment may have had enough time to reach functional equilibrium at
the cost of some total biomass, whereas the plants exposed to two treatments did not.
31
Competitive influence of introduced plants was limited to effects on A. philoxeroides
shoot/root ratio. Only one species, Justicia american, reduced A. philoxeroides shoot/root
significantly over controls. Competition between plants can affect biomass allocation. Gersani et
al. (2001) found lower shoot/root ratios among soybeans that experienced intraspecific
competition than those that did not. Shortly after transplanting at the end of September, existing
above ground parts of both sprayed and un-sprayed Justicia withered away to no observable
living tissue, before exhibiting minor regrowth at the time of harvest in April. Any competition
between A. philoxeroides and Justicia must have occurred below ground. Resource availability
in the absence of plant to plant interactions has the ability to effect plant biomass allocation
(Wilson, 1988). Plant species respond differently to competition, but these competitive responses
are also altered by abiotic factors, particularly nutrients (Wilson and Tilman, 1995). Introduction
of Justicia may have diminished nutrients in shared treatment pots enough to prompt a biomass
re-allocation response from A. philoxeroides. In addition to resource mediated competition,
plants can interfere with one another below ground through allelopathy (Mahall and Callaway,
1992).
Field Experiment
Results from the field component did not suggest any reduction in A. philoxeroides cover
after the integration of introduced competitor plants and triclopyr applications. The herbicide
applications alone resulted in reduction, but as in the greenhouse study, introduction of
competitors resulted in little effect. Alternative herbicide timing may have resulted in different
results. Allen et al. (2007) found that a high rate of herbicide applications in April for A.
philoxeroides resulted in increased native plant biomass within the treatment year over July
applications. Allen et al. (2007) proposes earlier applications provided a critical window of
32
opportunity for native plants to become established as A. philoxeroides cover diminished. In this
study, A. philoxeroides was sprayed at maximum rate in October. Late season applications of
herbicides tend to work better because plants tend to accumulate carbohydrates and other
nutrients into roots and other storage structures in preparation for winter senescence (Madsen
and Owens 1998). Because A. philoxeroides suppression through introduced native plants after
herbicide application was the intended goal of this study, early season applications and plant
introductions may have provided different results.
Herbicide applications effected A. philoxeroides cover in the field. There was no
significant difference between one and two applications. Late season applications in general are
more effective than early season applications. Allen et al. (2007) found improved reduction in A.
philoxeroides biomass with July triclopyr applications as compared to April applications. In this
experiment, the two triclopyr applications in the field occurred at the beginning and end of
October. The first application on October 1st may have resulted in maximum potential effect for
the remaining growing season. If the project had occurred earlier in the spring, a second
application may have provided improved control.
There was no evidence of introduced competition alone effecting A. philoxeroides cover
in the field. Due to availability of viable plants, different species of Polygonum and Carex as
competitors were substituted moving from the greenhouse to the field. This in effect broadens
the range of potential competitive influence of different plants on A. philoxeroides examined in
the context of this study by two species. Unlike the greenhouse component, native plants and
seeds were present in all treatment plots during the entirety of the field experiment. Any effect
caused by the introduced plants may have been overshadowed by effects of plants already
present. Below ground aspects of A. philoxeroides were not measured in the field, and therefore
33
there was no data to measure competitive influence on roots or biomass partitioning. There was
evidence in the greenhouse of influence by one competitor species on A. philoxeroides shoot/root
ratio. There may have been interactions underground that could not be quantified due to
constraints inherent by the chosen data collection method.
Comparing Greenhouse and Field Components
In general, results of the field experiment and greenhouse components were similar, with
a few exceptions. As in the greenhouse component of this study, A. philoxeroides was reduced
by triclopyr applications. In contrast to the greenhouse component, there was no difference
between a single application and two applications in above ground metrics recorded. There was a
reduction in cover after one application in the field, which contrast with no reduction in
cumulative stem length after one application in the greenhouse. We must be cautious about
making direct comparisons between the field and greenhouse components because of data
collection techniques used. For example, we found cumulative stem length was only reduced
after two applications in the greenhouse, but above ground biomass was reduced by one and
further by two. In effect, the stems were still there, but the ratio of mass to length was reduced.
We used human observers to place A. philoxeroides into cover classes in the field. Cover class
estimates could be somewhat inflated in comparison to above ground biomass simply because of
the influence of the presence of stems on sampling estimates, regardless of stem density.
Conclusions and Suggestions for Future Work
This study contributes to evidence that triclopyr is effective at reducing alligator biomass and
cover, but is not successful at completely eliminating the plant. Managers that wish to use
triclopyr to control A. philoxeroides can expect improved results after a second application.
Schooler et al. (2008) attributed improved A. philoxeroides control with selective herbicides over
34
broad spectrum herbicides to the competition from existing monocot species. After two
experiments, we found no advantage of integrating herbicide applications and introductions of
six species of plants to compete with A. philoxeroides. To fully explore this theoretical
integration of management tools completely, the individual components should be better
understood. It is well reported that herbicides can reduce A. philoxeroides biomass, but further
understanding of herbicide transport, or lack thereof, and effects throughout the entire plant are
needed. Because competitive interactions underground are so important, it may be important to
use herbicides that could directly damage A. philoxeroides roots to improve the magnitude of
competitive responses from other plants. Trials with a larger pool of potential competitors and
their subsequent influence on A. philoxeroides could have allowed for improved competitor plant
selection for this study and resulted in a different outcome. We knew that herbicides would most
likely damage A. philoxeroides, but we had little evidence of A. philoxeroides being directly
suppressed by native plants or how the plant responds physiologically to competition. Further
research is needed relating timing herbicide application and subsequent introduction of plants.
Herbicides chosen may control A. philoxeroides better after late season applications, but opening
an earlier window for response from introduced competitors may improve results over either
individual method. Because of the morphological plasticity of A. philoxeroides, any study
attempting to characterize competitive effects from other plants would certainly have to take
different hydrological regimes and the resulting A. philoxeroides growth forms into account.
35
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