Multi-century changes in plant diversity,

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Multi-century changes in plant diversity, composition and vegetation structure after fire in gimlet woodlandsCarl Gosper, Suzanne Prober and Colin Yates

22 July 2014

LAND & WATER FLAGSHIP

The Great Western

Woodlands• World’s largest extant Mediterranean-climate

woodland (16 M ha)

• Largely intact, diverse

• Mosaic of woodlands, mallee, shrublands, ironstone and greenstone ranges & salt lakes

• Woodlands at as low as 220mm MAR

• Fire is a key process driving vegetation composition, structure and recruitment

• Poor understanding of historic fire regimes, including fire intervals

• Woodland communities fire sensitive

• Canopy fires

Fire in GWW

Photo: L. McCaw

• Recent decades have seen many large fires

• Impacts on lives and infrastructure

• Increasing fire a concern, especially with climate change

Fire in GWW

1. Establish a series of permanent plots along a time-since-fire gradient

2. Develop a method to estimate time since fire of long-unburnt stands

3. Estimate the current stand age structure of Eucalyptus woodlands

4. Assess how plant diversity and composition change with time since fire

5. Measure how vegetation structure and flammable fuel change with time since fire

Aims:

since fire

•Serotinous non-resprouter

•Recruit after fire in even-aged stands, very slow growing

•Sparse understorey of sclerophyllous shrubs, chenopods, grasses and annuals: fire resistant

Gimlet (E. salubris) woodlands

• 72 plots stratified by time-since-fire using Landsatimagery covering the period 1972 to 2010

• 3 broad areas of similar climate and sufficient fire-age diversity

Aim 1: Establishing permanent plots

Fire layer developed from Landsat

Time since

fire (years)

# plots

2-10 17

11-38 6

38-60 13

>60 36

Hollowed mature gimlet

Aim 2: Estimating time since fire

1. post-1972 fires dated from Landsat imagery

2. growth ring counts to estimate post-1910 fires

3. Use growth ring counts to develop size-age relationships to estimate age in stands with hollowed trees (pre-1910)

Tim

e si

nce

fire

(yea

rs) 30

40

Time since fire = -0.890 + 0.9695(growth rings)95% Confidence Band Expected relationship (y = x)

(Adj. r2 = 0.992, F1,55 = 7158, P < 0.0001)

• Strong positive relationship between time since fire from Landsat and growth rings

• Slight tendency to overestimate time since fire

Aim 2: Estimating time since fire

Mean growth rings per plant

0 10 20 30 40

Tim

e si

nce

fire

(yea

rs)

0

10

20

• Strong positive relationships between growth rings and plant size (diameter at base and/or tree height)

• Uncertainty over best model form

beyond limits of growth ring record

• Estimated maximum stand times

since fire of 370 years (constant

Aim 2: Estimating time since fire

Tim

e si

nce

fire/

grow

th r

ings

200

Extrapolation

assuming decline in

growth rate with age

R2=0.882

▲ = trunks dated from growth ring counts

since fire of 370 years (constant

growth rate) or 1460 years

(declining growth rate)

Diameter at base (cm)

0 5 10 15 20 25

Tim

e si

nce

fire/

grow

th r

ings

0

50

100

150

Extrapolation

assuming constant

growth rate

R2=0.877

95% prediction interval

?

Key findings:

• Gimlet tree rings and size can be used to estimate stand time since fire

• Extends time since fire record well beyond that able to be discerned from Landsat imagery

• Allows the investigation of time since fire effects on ecological processes, using relative age after 100 years

Aim 2: Estimating time since fire

Aim 3: Age-structure in GWW woodlands

Per

cent

age

of w

oodl

and

area

20

40

60

80Satellite image analysis • Understanding the age structure of GWW woodlands could provide

clues as to whether recent levels of fire are unprecedented

• To provide a crude assessment of the age structure of woodlands, we extrapolated the results of Landsat fire mapping to the regional scale, by assuming that the distribution in age classes older than 60 years is proportional to our random samples

Age class (years since fire)

00-60 61+

Area mapped for fire age

GWW boundary

Landsat-derived age-class distribution

Key findings:

• Estimated age-class distributions can be compared to theoretical distributions to determine fire management interventions.

• Irrespective of model used, recently-burnt vegetation is over-represented on a fixed proportion basis but less so compared to a negative exponential function.

• We are working on improving these estimates

Aim 3: Age-structure in GWW woodlands

80

Untransformed growth rings predicted by diameter and location

80

Age class (years since fire)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60 predicted by diameter and locationNegative exponential theoretical age-class distributionFixed proportion theoretical age-class distribution

60

Age class (years since fire)

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 8000

20

40

60Square-root growth rings predicted by diameter and locationNegative exponential theoretical age-class distributionFixed proportion theoretical age-class distribution

751+60

Constant growth rate Decline in growth rate

Hypotheses:

• Initial floristic composition (IFC) hypothesis: declining diversity with time

• Intermediate disturbance hypothesis (IDH) : highest diversity at intermediate time since fire

Aim 4: Multi-century changes in plant diversity

Div

ersi

ty

IFC

at intermediate time since fire

Both hypotheses imply a need for recurrent fire to maintain diversity

Time since fire

IDH

•72 50 x 50 m plots sampled for species composition and cover

• diversity indices – richness, diversity, evenness

Aim 4: Multi-century changes in plant diversity

0 5 10 15 20 25

Sha

nnon

Div

ersi

ty

1.0

1.5

2.0

2.5

3.0

3.5

4.0y = 3.313 - 0.194x + 0.011x2

Adj. r2 = 0.304, F2,69 = 16.5, P < 0.0001

Results

Young Mature

Square-root (years since fire)

0 5 10 15 20 25

• globally rare ‘U’-shaped relationship between diversity and time since fire is likely to be driven by dominant trees and shrubs having maximum competitive influence at intermediate times since fire• did not match IFC or IDH

Results

Intermediate

• graph uses model for stand age assuming constant growth rate

• Traits used to define Plant Functional Types were selected on the basis of competition and recruitment over the period between fires being important processes driving vegetation change

• PFTs defined according to plant growth form (7 categories) and seed

Aim 4: Multi-century changes in plant diversity

Life forms

Aerial parasite (Aerial)

Phanerophytes – trees (> 8 m height) (Tree)

Phanerophytes – shrubs (1–8 m height) (Shrub)

Chamaephytes (shrubs 0.1–1 m height) (Low shrub)

Hemicryptophytes (persistent buds at soil surface) (Hemicryp)growth form (7 categories) and seed dispersal distance potential (2 categories) to form 12 PFTs

Geophytes (Geo)

Therophytes (annuals) (Annual)

Dispersal potential

Long (Endozoochory, Epizoochory, Anemochory) (L)

Short (Myrmecochory, Barochory) (S)

Results:• Richness and cover of many PFTs changed with time since fire.

35

40

45

50

Pe

rce

nta

ge

co

ve

r

Trees - short dispersal

Shrubs - short dispersal

bb

a

bb

a

bb

a

aa

8

10

12

Pe

rce

nta

ge

co

ve

r

Annuals - long dispersal

Perennial herbs & grasses - long dispersal

a

Aim 4: Multi-century changes in plant diversity

0

5

10

15

20

25

30

Young (< 18) Intermediate (38-120) Mature (> 140)

Pe

rce

nta

ge

co

ve

r

Time since fire age class (years)

bbb

a

0

2

4

6

8

Young (< 18) Intermediate (38-120) Mature (> 140)

Pe

rce

nta

ge

co

ve

r

Time since fire age class (years)

Perennial herbs & grasses - long dispersal

ab

b

b

b

a

Results:

• ground-layer PFTs had greatest cover in mature vegetation while tree and shrub-layer PFTs had peak cover at intermediate times since fire• ground-layer PFTs taking advantage of increasingly litter-free ground surface

• short dispersal potential PFTs decreased with increasing time since fire while long dispersal potential PFTs increased• long-dispersal potential PFTs have the capacity to disperse to suitable micro-sites for recruitment between fires

Aim 4: Multi-century changes in plant diversity

sites for recruitment between fires

Management implications

• Important changes in diversity occurred beyond times since fire able to dated by remote sensing

•No evidence for fire initiating community change

Aim 4: Multi-century changes in plant diversity

Photo: L. McCaw

Management implications

• As diversity was highest in mature woodlands, there is no support for gimlet woodlands requiring recurrent fire to maintain plant diversity

• Intense stand-replacing fires at intervals of < 200 yrs would have adverse implications for biodiversity conservation.

Aim 4: Multi-century changes in plant diversity

Photo: L. McCaw

• After fire, changes occur in the composition and structure of vegetation affecting the mass, spatial arrangement and condition of fuels

• Flammability is often assumed to increase monotonically with time since fire

• Understanding changes in fuels are important for protection of human assets and biodiversity conservation

Aim 5: Changes in fuel with time since fire

Fla

mm

abili

ty

Theoretical relationships

A

and biodiversity conservation

• Sampled tree density and size, ground cover and the vertical distribution of vegetation

Time since fire

Years since fire

Pro

port

ion

Cov

er

0.0

0.2

0.4

0.6

0.8

1.0

95% Confidence Band

100

Litter(Adj. r2 = 0.34, F2,73 = 20.2, P < 0.0001)

0 25 400

• Canopy and litter cover peaked at intermediate

times since fire (~35 to 150-250 years)

• Discontinuous litter cover in young and mature

woodlands reduces flammability

•Connectivity of fuels from litter to the canopy

was greater in intermediate-aged woodlands

• Gaps in foliage increased with time since fire

Results:

Aim 5: Fuel changes with time since fire

Young MatureIntermediate

Management implications

• Important changes in fuel structure occurred beyond times since fire able to dated by remote sensing

• Non-monotonic changes in fuel

• Adjusted fire behaviour ratings

• Woodlands at an intermediate time since fire are likely to support fires under more

Aim 5: Fuel changes with time since fire

fire are likely to support fires under more benign weather conditions

• Community flammability is also influenced by seasonal conditions

Photo: L. McCaw

Management implications

• Replacement and fragmentation of mature woodlands with lower flammable fuel by higher-fuel intermediate time since fire woodlands potentially instigates a self-reinforcing fire regime shift favouring larger and/or more uniform fires.

• As a consequence of recent large widlfires, large areas of regenerating woodlands will soon be passing into a more flammable stage of post-fire development

Aim 5: Fuel changes with time since fire

• Evaluate time-since-fire impacts on ants and soil processes

• Improve estimates of woodland age structure

• Measure changes in carbon content of woodland with time since fire

Future Directions

Further information:

Parsons & Gosper (2011) International Journal of Wildland Fire 20, 184-194.

Prober et al. (2012) Climatic Change 110, 227-248.

Gosper et al. (2013) Australian Journal of Botany 61, 11-21.

Gosper et al. (2013) Journal of Applied Ecology 50, 1187-1196.

Gosper et al. (2013) Forest Ecology and Management 310, 102-109.

Gosper et al. (2014) International Journal of Wildland Fire 23, 385-393.

Science and Conservation Division Information Sheets 65, 68 and 72. http://www.dpaw.wa.gov.au/about-us/science-and-research/publications-resources/111-science-division-information-sheets

Thank youCarl Gosper, Suzanne Prober and Colin Yates

t (08) 9333 6442e carl.gosper@csiro.au

CSIRO LAND & WATER FLAGSHIP AND DEPARTMENT OF PARKS & WILDLIFE

Fire in gimlet woodlands. NatureMap: http://naturemap.dec.wa.gov.au/default.aspx

Thank you

Further information:

Parsons & Gosper (2011) International Journal of Wildland Fire 20, 184-194

Prober et al. (2012) Climatic Change 110, 227-248

Gosper et al. (in press) Australian Journal of Botany doi: 10.1071/BT12212

Thank youCarl Gosper, Suzanne Prober and Colin Yates

t (08) 9333 6442

e carl.gosper@csiro.au

CSIRO ECOSYSTEM SCIENCES AND DEPARTMENT OF ENVIRONMENT & CONSERVATION