Forest succession and habitat management for Leadbeater's possum in the State of Victoria, Australia

Forest Ecology and Management, 49 (1992) 311-332

311

Elsevier Science Publishers B.V., Amsterdam

Forest succession and habitat management for Leadbeater's possum in the State of Victoria, Australia A.P. S m i t h a n d D.B. L i n d e n m a y e r

Departmentof Ecosystem Management, Universityof New England, Armidale, NSW 2351, Australia (Accepted 9 April 1991 )

ABSTRACT Smith, A.P. and Lindenmayer, D.B., 1992. Forest successionand habitat management for Leadbeater's possum in the State of Victoria, Australia. For. Ecol. Manage., 49:311-332. The aim of the study was to develop models which predict habitat availability for a rare and endangered marsupial, Leadbeater's possum (Gymnobelideusleadbeateri), in space and time. Measuresof forest age, structure and floristics were correlated with the density of Leadbeater's possum at 32 sites in montane ash forests of the Victorian Highlands of south-east Australia. The density of Leadbeater's possum was significantlycorrelated with the number of trees with hollows (PNTs), the number and biomass of Acacia spp., canopy and understory closure, and forest age. Peak densities occurred in regrowth forests ( 15-50 years), with abundant tree hollows (greater than 6 PNTs per 3 ha), and a high biomass of acacias (20-50% of stand basal area). Leadbeater's possums also occurred at low frequenciesand densities in old-growth-regrowth forest ecotones, and in mature and multi-aged forests with an understory of eucalypts and acacias. Models of trends in habitat availability predict a massivedeclineover the next 30 years, followedby a population bottleneck lasting until the year 2075. Survival of Leadbeater's possum during the bottleneck will depend on protection of refuge habitats, particularly patches of mature, multi-age and old-growthforest. Recovery after the bottleneck is expected to depend on the extent to which current silvicultural practices can be modified to guarantee recruitment of PNTs within timber production forests.

INTR(3DUCTION T h e a i m o f t h i s s t u d y was t o d e v e l o p m o d e l s w h i c h p r e d i c t h a b i t a t a v a i l a b i l i t y for t h e r a r e a n d e n d a n g e r e d L e a d b e a t e r ' s p o s s u m ( G y m n o b e l i d e u s leadbeateri) i n space a n d t i m e . L e a d b e a t e r ' s p o s s u m has a r e s t r i c t e d d i s t r i b u t i o n i n m o n t a n e ash forests o f t h e V i c t o r i a n C e n t r a l H i g h l a n d s o f s o u t h e a s t e r n A u s t r a l i a ( L i n d e n m a y e r et al., 1 9 8 9 ) , w h e r e 75% o f its k n o w n r a n g e falls w i t h i n state forests s u b j e c t t o clear-felling for w o o d p r o d u c t i o n a n d 20% falls Correspondence to: A.P. Smith, Department of Ecosystem Management, Uaiversity of New England, Armidale, N.S.W. 2351, Australia.

© 1992 Elsevier Science Publishers B.V. All fights reserved 0378-1127/92/$05.00

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in water catchments where logging may occur in the future. Less than 3% falls in flora and fauna reserves (Macfarlane and Seebeck, 1990). Recent forest policies in Victoria have placed increased emphasis on ecologically sustainable utilization of native forests for both timber and non-timber values, including the protection of threatened species (Victorian Government, 1986, 1987). One of the most significant challenges facing the forest industry is the development of new silvicultural practices which will ensure the continued existence of Leadbeater's possum in timber production forests. Leadbeater's possum was presumed extinct earlier this century until re-discovered in 1961 (Wilkinson, 1961 ). Early sightings of Leadbeater's possum were in structurally dense regrowth forests which developed after intense wildfires in 1939 (Hodgson, 1968 ). This led to speculation that the preferred habitat of Leadbeater's possum was a forest of 20-40 years of age, and that forest clear-felling on short rotations for timber production would be essential for the species survival (Department of Agriculture, Forestry and Timber Bureau, 1975). This conclusion was not supported by subsequent sightings (1968-1977) of Leadbeater's possum in or adjacent to old-growth forests with a dense tangled understory and an abundance of old trees with hollows (Rawlinson and Brown, 1977; Tunks, 1977). An autoecological study of Leadbeater's possum at a single location (Smith, 1980) identified its habitat requirements to include the following: (a) large dead or living trees with hollows suitable for constructing nests; (b) structurally dense vegetation with an interlocking canopy to facilitate food harvesting and movement; (c) an understory of certain Acacia spp. which produce edible gum exudates; (d) an overstory of certain smooth barked (montane ash ) Eucalyptus spp. which provide harbor for insect prey. This combination of resources is comparatively rare, developing only after wildfire in old-growth forest. Fire promotes regeneration of montane ash eucalypts and acacias, and creates a dense young forest or forest understory with a closed canopy (Ashton, 1976; Adams and Attiwill, 1984). Emergent oldgrowth eucalypts ( > 190 years), which survive or remain standing after fire, provide hollows for Leadbeater's possums in regrowth and multi-age habitaL Thus Smith (1982) concluded, that in theory Leadbeater's possum conservation could be achieved in timber production forests if silvicultural practices were modified to mimic natural, ash forest dynamics. During 1983-1985, a survey of vegetation structure, floristics and tree hollow availability was conducted at 32 sites throughout the known range of Leadbeater's possum and results were correlated with concurrent measures of possum density (Smith et al., 1985). The aim of the survey was to define Leadbeater's possum habitat, clarify the effects of secondary succession after wildfire and logging on habitat availability, and develop models for predicting and mapping habitat availability in space and time, as a basis for recommending changes to forest management. These data have been evaluated to

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determine the importance of tree hollows for Leadbeater's possum (Smith and Lindenmayer, 1988), and results have been used to establish prescriptions for tree hollow management for conservation of Leadbeater's possum and other hollow-dependent arboreal mammals in logged forests. In this complementary study we define the structural and floristic components of Leadbeater's possum habitat in areas with abundant tree hollows, and evaluate the effects of fire and logging on habitat availability and Leadbeater's possum succession and dynamics. We present models which predict the availability of Leadbeater's possum habitat availability in space and time, and discuss the implications of these predictions for forest management. METHODS

Location of survey sites Eight survey regions were selected from throughout the present known range of Leadbeater's possum. These included the Toolangi, Alexandra, Marysville, Powelltown, and Neerim forestry districts. Within each region an average of four survey sites was selected to cover a range of habitat types. Sites were located to cover as wide a geographic range as possible to ensure that our conclusions would be applicable throughout the ash-dominated forests ofthe Central Highlands of Victoria. The rarity of Leadbeater's possum and the fire history of its habitat suggest that it has a patchy distribution, leaving some areas of potentially suitable habitat unoccupied. To ensure that possum density at each site reflected true carrying capacity we located each site within 2 km of known records of Leadbeater's possum, except at Toolangi where sites averaged 3 km (range 2-6 km) from known records. At each site a 3 ha (200 m × 150 m) rectangular grid was marked out in a patch of relatively homogeneous vegetation by pacing along compass bearings. This plot size was selected, on the basis of prior density estimates (Smith, 1984a), to give a maximum possum density of 10 animals per site. Each site contained more than four and less than 22 potential nest trees (PNTs), where PNTs were defined as those dead or living trees greater than 6 m height and greater than 0.5 m diameter at breast height (dbh). Sites were stratified over successional age and Eucalyptus spp. and Acacia spp. dominance. All but three sites were located in montane ash forest dominated by mountain ash (Eucalyptus regnans), alpine ash (Eucalyptus delegatensis), and shining gum (Eucalyptus nitens). Ninety-eight percent of the 167 reliable records of Leadbeater's possum collected since 1977 fall in, or within 100 m of, montane ash forests (Lindenmayer et al., 1989). Three additional sites were also located in low to mid-elevation stringy bark (Eucalyptus obliqua, Eucalyptus macrorhyncha) forest at the margins of montane ash forest.

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Sites were located in patches of homogeneous forest to minimize edge effects. Homogeneous patches of old-growth ( > 80 years) were rare. Most regions were dominated by post-1939 fire regrowth, recent clear-felling regrowth ( < 12 years), or regrowth resulting from salvage logging of old-growth forests which were burnt in the 1939 fires. Surviving old-growth forests in timber production forests were progressively logged after the 1939 fire (Noble, 1977) and remnants were generally small, isolated and multi-aged in structure, suggesting a history of exposure to infrequent, mild fires and/or low intensity (selection) logging.

Census of Leadbeater ~ possum Possum density at each site was determined by an absolute count method termed stagwatching. Stagwatching is the most effective and reliable method for estimating the abundance of Leadbeater's possum (Smith et al., 1989). It involves simultaneous counts of possums emerging from tree hollows at dusk, by large numbers of volunteers.

Measurement of habitat variables A total of 21 habitat variables was measured in six 10 m X 10 m plots. Plots were located representatively to take vegetation heterogeneity into account. Habitat variables were chosen to reflect the known foraging requirements of Leadbeater's possum (after Smith, 1980), as well as the successional age and disturbance history of the site. The dbh and height of all trees and shrubs greater than 2 m height were recorded in each plot. Each tree or shrub was classified into one of the following six diameter classes: 0-20, 21-40, 41-60, 61-80, 81 - 100, 100 + (cm); and one of the following seven height classes: 25, 6- l 0, 11-20, 21-30, 31-40, 41-60, 60 + (m). Height and dbh classes were determined initially by diameter tape and clinometer, but were later determined by visual estimate. The basal area of trees was determined by multiplying the number of stems in each dbh class by the basal area of plants in the mid point of each diameter class. The following variables were measured in each sample plot and, unless otherwise stated, totaled to produce a value for each site: ( l ) Slope; (2) Position on slope (ridge, topslope, midslope, bottom slope, gully, plateau ); ( 3 ) Stand Height (canopy height ); (4) Emergent Height (height of the tallest living or dead tree if present); (5) Stumps (the number of tree stumps greater than 0.5 m diameter, per 0.06 ha, remaining from prior selection logging); (6) Tree Ferns (the number of Cyathea australis and Dicksonia spp. );

PREDICTING LEADBEATER'SPOSSUM HABITATAVAILABILITY

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(7) Shrubs (the total number of shrub stems); ( 8 ) Bark Index (an index of decorticating Eucalyptus bark volume, determined by classifying the size of bark clumps on a scale of one to three and summing for all trees in all plots); (9) Canopy Connectance (a measure of upper canopy closure, determined by summing the maximum distance of continuous canopy along a vertical projection of the horizontal line delineating the plot boundary and summed to give a score between 0 and 40); (10) Understory Conneetance (a measure of shrub understory connectante using the above procedure, for sites with a distinct understory stratum); ( 11 ) Canopy and Understory Connectance (the sum of the preceding two variables); (12) Total Connectance (the average connectance values for all vegetation strata, for a maximum of three strata); (13) Eucalypt Numbers (the total number of eucalypt stems greater than 2 m height per 0.06 ha); (14) Ash B.A. (basal area of ash eucalypt species: E. nitens, E. regnansand E. delegatensis in m 2 h a - ~); (15) Ash B.A. ex culls (basal area of ash eucalypts excluding over-mature trees greater than 1 m dbh, in m 2 h a - ~); ( 16 ) Wedge Area (estimate of tree basal area using an optical wedge ); (17) Acacia Numbers (the number of Acacia dealbata, Acacia obliquinervia and Acaciafrigescens stems greater than 2 m height per 0.06 ha); (18) Acacia B.A. (basal area ofA. dealbata, A. obliquinervia and A. frigescens stems in m 2 h a - ~); (19) Percentage Acacia (basal area of gum producing acacias expressed as a percentage of the total basal area of all trees greater than 2 m height); (20) Elevation (m). Two additional variables measured on each site were: (21) PNT Numbers (the total number of PNT, trees greater than 0.5 m dbh and greater than 6 m height, on the site); (22) PNT Spacing (the number of 0.25 ha blocks on each site with one or more PNT).

Statistical analysis The effects of vegetation floristics and structure on Leadbeater's possum density were evaluated using the following general procedure: ( 1 ) Preliminary inspection of scatterplots of Leadbeater's possum density and each individual habitat variable, to identify any outliers and non-linear relationships requiring transformation;

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(2) Elimination of redundant habitat variables and those showing no significant, or near significant, relationship with Leadbeater's possum density, (3) Correlation and principal components analysis to identify patterns of confounding between those variables which were significantly correlated with possum density; (4) Calculation of partial correlation coefficients of all possible combinations of remaining variables, to rank them in order of importance as predictors of Leadbeater's possum density; (5) Construction of simple and multiple regression models for predicting Leadbeater's possum density. Preliminary examination of scatterplots, associations and correlations revealed one outlier (Site 32), three redundant variables (Canopy Connectance, Understory Connectance, Total Connectance), and seven variables which exhibited no significant correlation with Leadbeater's possum density, either singly or in combination with other variables (Elevation, Slope, Position on Slope, Tree Ferns, Ash B.A. ex culls, Emergent Height and Wedge Area). No relationships were significantly improved by transformation. A high density of Leadbeater's possum on Site 32 was attributed to edge effects. This site comprised a small patch of old-growth shining gum bordered on two sides by favorable regrowth habitat. A similar situation existed at Site 25, where possums were observed to leave tree hollows in old-growth forest and feed in adjacent regrowth forest, beyond the limits of the survey site. These two sites and all redundant and non-significant variables were eliminated from the data set prior to further analysis. The effects of vegetation succession on Leadbeater's possum density and habitat requirements were evaluated by correlating possum density and key habitat variables with forest age. Sites were classified i n t o a simulated age gradient using two different methods. The first procedure involved classification of sites according to the number of stems in each of six increasing diameter classes, using Ward's agglomerative, hierarchical clustering algorithm (Statistical Analysis Systems, 1982). This was followed by hand ranking clusters according to stem basal area in the'largest diameter class represented by more than a single tree in each survey plot ( > 17 trees ha- i ). The last three groups identified using this method were aggregated to improve interpretation. The second procedure involved hand ranking sites into six groups according to the stem diameter class which contributed the greatest stem basal area. RESULTS

Habitat requirements of Leadbeater's possum All significant correlations between Leadbeater's possum density and measured habitat variables are listed in Table 1. Leadbeater's possum density

PREDICTINGLEADBEATER'SPOSSUMHABITATAVAILABILITY

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TABLE 1 List of habitat variables significantly ( P < 0.05 ) linearly correlated with Leadbeater's possum density at all sites (n = 30 ), and at a subset of sites ( n = 21 ) where numbers of PNT are greater than 6 per site

Habitat variable

Allsites (r)

Siteswith morethan 6 PNT (r)

PNT Numbers PNT Spacing Stumps Canopy Connectance Understnry Connectance Canopy and Understory Connectance Total Connectance Ash B.A. Acacia Numbers Acacia B.A. Percentage Acacia

0.42 0.53 0.47 0.41 0.54 0.58 -

0.62 0.44 0.58 0.56 -0.51 0.76 0.63 0.79

3 h a - i was significantly correlated (P<0.05) with PNT Density and PNT Spacing, Acacia Numbers, Stumps, and Canopy and Understory Connectance. Inclusion of all variables in a stepwise multiple regression generated the following predictive model:

(31 nos. per 3 ha=0.588 (PNT Spacing)+0.141 (Canopy and Understory Connectance) +0.051 (Acacia Numbers)-6.7333 (adjusted r2=0.59). (G1 is Gymnobelideus leadbeateri ).

Effects of vegetation floristics and structure on Leadbeater's possum density Multiple regression models are additive and assume that high values o f one variable can compensate for low values of another. This assumption may not always be valid, particularly when 'essential resources' (those for which there is no substitute) are included in the equation. Tree hollows are an essential resource for Leadbeater's possum. At densities of less than 11 PNT per 3 ha, hollow availability sets an upper limit to Leadbeater's possum density irrespective of other habitat values (Smith and Lindenmayer, 1988). Therefore the tl~e effects of other habitat variables on Leadbeater's possum density can only be evaluated in sites with abundant tree hollows. We re-examined associations between Leadbeater's possum density and habitat variables from a subset of 21 sites with more than 6 PNT per 3 ha and a PNT spacing more than four per 3 ha. This resulted in an increase in the proportion of the variability in Leadbeater's possum density explained by habitat variables (Table 1).

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At these sites Leadbeater's possum density was significantly correlated with: Acacia Numbers, Acacia B.A., Stumps, Total Connectance, and Ash B.A. Examination of the correlation matrix revealed that all of these variables, except Ash B.A., were significantly intercorrelated, making the interpretation of possible causal associations difficult. Principal components analysis was used to classify these variables into a reduced number of independent groups (Table 2). Factor l, which had high loadings for Acacia Numbers (0.79), Canopy and Understory Connectance (0.63), and Stump density (0.86), was interpreted as a gradient of increasing acacia abundance associated with the intensity of selection and/or salvage logging. Factor 2, with high loadings for Stand Height (loading 0.86) and Ash B.A. (loading 0.81 ), was interpreted as a gradient of increasing successional age following wildfire. Factor 3 was interpreted as a gradient of increasing density of trees with hollows (loading 0.95 ). Factor 4 was interpreted as a gradient of increasing eucalypt stocking (loading 0.86). Factor 5 was interpreted as a gradient of increased non-acacia shrub density (loading 0.89). This analysis revealed that two important Leadbeater's possum resources, acacias and PNT numbers, were not confounded on a simple forest age gradient. Leadbeater's possum density was significantly correlated with Factor I (r 2= 0.28, P = 0.003) and Factor 3 (r 2= 0.15, P = 0.04), and not at all with Factors 2, 4 and 5. The most important habitat variables, from amongst the 'intercorrelated set, were identified by measurement of partial correlations between each variable and Leadbeater's possum density, once the effects of all possible combinations of remaining variables had previously been considered. Results of this analysis indicated that Percentage Acacia was the best predictor, followed TABLE 2 Factor loadings for ash forest habitat variables generated by principal components analysis Factor number (% variance explained) Habitat Variable PNT Numbers PNT Spacing Stand Height Stumps Shrubs Bark Index Canopy and Understory Connectance A s h B.A. A c a c i a B.A. Acacia Numbers Eucalypt Numbers

I (29)

2 (19)

3 (16)

4 (10)

5 (9)

- 8 5 - 22 86 -6 7 63

- 7 8 86 - 20 - 18 50 24

95 93 9 6 -4 - 23 - 7

9 - 16 - 20 9 9 64 - 56

- 2 2 - 26 - 13 89 3 26

- 26 54 79 - 10

8! -- 2 0 - 45 - 15

16 -- 22 9 2

6 - 29 - 12 83

3 -- 50 - 5 23

319

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Fig. 1. Relationshipsbetween Leadbeater'spossumdensity (numbersper 3 ha) and the number of acacia stems per 0.06 ha (a), and acacia basal area as a percentage of total stand basal area (b). by Acacia Numbers, Stumps, then Ash B.A. and Canopy and Understory Conncctance with equal rank. When the effect of either Percentage Acacia or Acacia Numbers was considered in stepwise multiple regression models no significant increase in the proportion of variability in Leadbeater's possum density was explained by any additional variables. The best models for predicting Leadbeater's possum density in ash forests with abundant tree hollows (greater than 6 P N T ) were: Density=0.191 (Percentage Acacia) + 0.4904

r2=0.62, P < 0 . 0 0 1

Density=0.1033 (Acacia Numbers) +0.3693

r2=0.57, P<0.001

where density is the number of Leadbeater's possums per 3 ha of forest. The models are intended to provide forest managers with a simple method for identifying potential Leadbeater's possum habitat, and incorporating its requirements into the coupe planing process. The first model will have great-

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est u t i l i t y for p r e d i c t i n g p o s s u m d e n s i t y i n late s u c c e s s i o n m a t u r e a n d m u l t i aged forests w i t h a low d e n s i t y o f large m a t u r e A. dealbata. T h e s e c o n d will h a v e greatest u t i l i W i n early s u c c e s s i o n r e g r o w t h forests w i t h a high d e n s i W of,4. dea[bata, A. obliquinervia a n d / o r A. ~igescens. Visual i n s p e c t i o n o f these m o d e l s (Fig. 1 ) i n d i c a t e s t h a t L e a d b e a t e r ' s p o s s u m d e n s i t y will b e greatest w h e n e i t h e r t h e s t o c k i n g o f g u m p r o d u c i n g acacias is i n t h e r a n g e o f 6 0 0 - 1 0 0 0 s t e m s h a - l or w h e n A c a c i a B.A. is 20-500/0 o f t h e total s t a n d b a s a l area. T h e s e m o d e l s n e e d to b e c o n s i d e r e d i n c o n j u n c t i o n w i t h p r e v i o u s m o d e l s w h i c h pred i e t h a b i t a t s u i t a b i l i t y solely o n t h e b a s i s o f t r e e - h o l l o w n u m b e r s ( S m i t h a n d Lindenmayer, 1988).

Forest succession S u r v e y sites were classified i n t o s u c c e s s i o n a l g r o u p s o n t h e basis o f s t e m d i s t r i b u t i o n across six d i a m e t e r classes. W a r d s a g g l o m e r a t i v e , h i e r a r c h i c a l

TABLE 3 Diameter distribution of eucalyptus stems in ash forest survey sites showingthe number of stems per hectare in six diameter classes. The final two columns show the allocation of each site to simulated age gradients, determined by classificationof sites (according to distribution across the six diameter classes) by Ward's algorithm, and by simple allocation to the diameter class with the largest basal area. Sites are ranked to show increasing dominance by the larger dbh classes Site

8 18 7 6 24 9 27 22 i 4 31 19 5 30 21 16 29 23 26 2

No. stems per hectare in diameter class 0-20

21-40

41-60

61-80

81-100

> 100

1183 100 83 200 0 533 150 183 100 350 250 33 167 200 100 50 316 217 83 50

0 100 150 250 500 517 933 366 283 316 283 100 217 600 550 417 450 866 283 33

17 0 0 0 0 0 17 66 67 34 133 33 50 67 33 216 100 50 50 0

0 17 0 0 0 0 0 0 0 0 0 17 34 33 67 83 100 0 117 0

0 0 0 0 0 0 0 0 0 17 0 0 0 0 17 0 51 0 0 0

3 0 0 0 2 0 0 0 0 0 0 2 0 0 3 0 2 28 33 16

Wards group

Largest dbh class

i 2 2 2 3 3 4 5 5 5 6 7 7 8 8 9 10 I1 12 13

I 1.5 2 2 2 2 2 2 2 2 3 2.5 2 2 2 3 5 6 6 6

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TABLE4 List of habitat variablessignificantlycorrelated (P< 0.05) with simulated succession/agegradients in ash forests Habitat variable

Leadbeater'spossumdensity Stand Height UnderstoryConnectance Ash B.A. AcaciaNumbers PercentageAcacia

Wards group

Largestdbh group

(r)

(r)

- 0.46 0.74 0.86 - 0.43 -

- 0.48 0.80 - 0.48 0.78 - 0.43 -0.54

clustering algorithm produced 11 groups which were then hand ranked to reflect increasing stem numbers in the larger diameter classes (Table 3). Sites were also ranked into six groups, corresponding to the possible stem diameter classes, according to which class had the greatest total stem basal area (Table 3 ). With only a single exception the older groups ( 8-10) were multi-aged in structure. They had the same stocking in the small diameter classes as the younger regrowth forests. Significant correlations between habitat variables and derived forest successional age classes are listed in Table 4. Strong correlations with Stand Height and Ash B.A. provide some confirmation that the classification method reflects an age gradient following severe disturbance. Leadbeater's possum density was significantly inversely correlated with both gradients (Fig. 2), but the proportion of the variation in possum density explained by these regressions (r 2---0 . 2 - 0.23 ) was low relative to the proportion of variability explained by other habitat variables. Possums were detected in forests of all ages, but peak densities (2.0 animals h a - ~) were recorded in regrowth stands 15-50 years after fire and/or logging disturbance. This peak corresponds with the period of maximum Acacia basal area and maximum closure of the forest and understory canopy. Leadbeater's possums occurred in old-growth forests which were either multi-aged in structure; supported a mature A. dealbata understory; or were situated adjacent to regrowth forest. It is of some concern, however, that all colonies detected in old-growth forest were small (one to two animals). This identifies a need for further studies of colony viability in forests of mature age. The youngest site occupied had been logged 12 years previously, but was unusual in supporting a high density of retained emergent old-growth trees and was connected to nearby multi-aged forest by a retained streamside corridor of old-gro~,h trees.

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Fig. 2. Relationships between Leadbeater's possum density (numbers per 3 ha), and forest age, where forest age is determined by the tree size class which contributes the largest basal area ( a ) , and by Ward's clustering algorithm ( b ) .

Population dynamics Preliminary estimates of Leadbeater's possum population size can be made by multiplying possum densities in forests of different age, with the area of ash forests in different stand or age classes (after Forests Commission of Victoria, 1984), and correcting for the proportion of habitat with a suitable density of PNTs. Models of relationships between Leadbeater's possum density and PNT density (Smith and Lindenmayer, 1988) predict that 4 PNT per 3 ha arc required to maintain minimum population densities. Aerial photographs were used to map the actual density of PNTs in regrowth ash forests, in a sample region of high possum density surrounding Cambarville in the Victorian Central Highlands. In 1980 an estimated 73% of regrowth mountain ash forests and 38% ofregrowth alpine ash forests in the region supported sufficient hollows to maintain populations of Leadbeater's possum. Using these values as correction factors the carrying capacity of the sample region

PREDICTING LEADBEATER'S POSSUM HABITAT AVAILABILITY

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(Fig. 3) was estimated to be 1600 animals. Using a similar procedure and applying a conservative correction factor for PNT density, Smith et al. (1985) estimated the total population of Leadbeater's possum throughout its range to be approximately 7500 (+_2300) animals in 1980. This number is expected to decline dramatically in the near future due to a rapid reduction in the density of PNTs in regrowth forests, and the logging of refuge habitats !

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Fig. 3. Montane ash stand structure in the Cambarville region. Symbols: 1939 r, 1939 rcgrowth mountain ash; 1939 d, 1939 rcgrowth alpine ash; ur, mountain ash w,th abundant acacia; ud + gully, non-ash and gully vegetation; ms + sg, mixed species and snow gum; R, mature and/ or old-growth mountain ash; Rr, multi-aged mountain ash; D, mature and/or old-growth alpine ash; L, recent clear-felling. Numbers indicate sites surveyed during this study, and black dots indicate previous sightings of Leadbeater's possum.

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(Smith, 1982). For example, habitat carrying capacity in the Cambarville region (Fig. 3) is expected to decline by 90% or more over the next 75 years, even if logging ceases immediately, due to PNT collapse in regrowth forests. The rate of collapse of fire-filled PNTs has been monitored and used to develop a stochastic, model of tree fall (Lindenmayer et al., 1990) which predicts that few regrowth forests will support PNTs in 50 years time. By combining the predictions of this model with measures of the proportion of ash forest in different age classes, it is possible to estimate the percentage of ash forests which will support a minimum PNT density at different times in the future (Fig. 4). Results indicate an impending bottleneck in habitat availability commencing in the year 2020. Recovery from this bottleneck will depend on prevailing management regimes as well as the pattern of future wildfires. If clear-felling occurs throughout the species range, including water catchments, and forests are not managed for tree hollow recruitment, habitat availability could be reduced to such a limited extent that the species becomes extinct (line A, Fig. 4). If existing mature and old-growth forests in water catchments are protected from logging, and timber production forests are managed to ensure recruitment of tree hollows, moderate habitat recovery can be expected by the year 2100 (line B, Fig. 4 ). The maximum possible rate of recovery can be expected if logging of existing mature and old-growth forests ceases immediately throughout the species range, and silvicultural regimes are modified to ensure tree hollow replacement in all logged forests 8O

ql. A

-0- A •4- B

60

---C

= 40

~

~o

u_

•

o 900

1950

2000

2050

2100

YEAR

Fig. 4. Predicted percentage of montane ash forest in the Victorian Central Highlands with the minimum density of hollow-bearing trees (greater than 4 PNT per 3 ha) necessary to support Leadbeater's possum, under three different management regimes. Derived from data in Smith et al. ( 1985 ) and Lindenmayer et al. (1990). A represents clear-felling throughout the species range without PNT recruitment; B represents protection of existing water catchments from logging with PNT recruitment in timber production forests; C represents protection of existing water catchments and mature or multi-aged forest in timber production forests from logging, and recruitment of PNT in timber production forests.

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325

(line C, Fig. 4). Although the area of mature and old-growth forest (including multi-age forest) has been greatly depleted by logging, some substantial areas remain which may act as imp,,, rant refuge and recolonization sources for subsequent population expansion. DISCUSSION

Habitat requirements of Leadbeater's possum Prior to this study knowledge of Leadbeater's possum habitat was based on observations of diet, feeding behavior and den tree use at a single location (Smith, 1982; 1984a,b), and fioristic composition at a limited number of chance sighting locations (Tunks, 1977; Smith, 1982). This study established the occurrence of Leadbeater's possum at many new locations, identified and ranked the importance of vegetation floristic and structural variables associated with high densities of Leadbeater's possum throughout its known range, and generated models for predicting and mapping habitat availability in space and time. A multiple regression model incorporating measures of PNT spacing, Acacia numbers and canopy and understory closure, explained the greatest amount of variation in Leadbeater's possum density (59%) at all sites. Multiple regression models are additive, however, and assume that low values of one variable can be compensated for by high values of another. Present evidence indicates that a shortage of tree honows cannot be compensated for by high values of other habitat variables. A previous analysis has shown that possum density varies linearly with PNT density, from an average of zero on sites with less than 4.2 PNT per 3 ha to an average max.:mum of 11.3 animals per 3 ha on sites with more than 12 PNT per 3 ha (Smith and Lindenmayer, 1988). To avoid noise generated by sites with low PNT numbers we re-examined associations between Leadbeater's possum density and habitat variables at a subset of 21 sites with abundant PNTs (more than 6 per 3 ha). Possum density was then found to vary linearly with the abundance of gum producing Acacia spp., forest age, and understory and canopy closure. These associations are generally consistent with earlier predictions (Smith 1980, 1982). However, they suggest that site variation in key habitat variables, particularly the abundance of PNTs and certain Acacia spp., is more important than forest age in determining the distribution of Leadbeater's possum in space and time. On sites with abundant PNTs the effect of Acacia Numbers and/or Acacia B.A. was so great that it masked the effects of all other structural and floristic variables significantly correlated with Leadbeater's possum density. The apparent importance of Acacia spp. is consistent with their production of edible gums, which may contribute up to 80% of the possum's daily energy require-

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A.P. SMITH AND D.B. LINDENMAYER

ment (Smith, 1984b). Acacias also provide an interlocking canopy, which may facilitate foraging and territory defense and reduce the high activity costs characteristic of this species (Smith et al., 1982). Leadbeater's possum has been presumed to require structurally dense habitat because it moves through the canopy by running and jumping. It lacks the gliding mg/inbrane, characteristic of its nearest living relatives in the genus Petaurus, which inhabit more open forests. We found Leadbeater's possum density to correlate significantly with Canopy and Understory Connectance but these relationships were confounded with the effects of acacia abundance. Connectance caused no significant increase in Leadbeater's possum density once the effects of acacia abundance had been taken into account in partial regression analysis, whereas the reverse was not true; acacia abundance had a significant effect on possum density after the effect of connectance was taken into account. Hence present data make it impossible to confirm or deny the importance of structurally dense vegetation to Leadbeater's possum. Leadbeater's possum occured at maximum density in forests where gum producing acacias made up 20-50% of the combined Acacia and Eucalyptus basal area. However, it should not be inferred that possums prefer patches of 'unstocked forest' which have a unifArm or very high density of Acacia and very low density of Eucalyptus. It ~s likely that ash eucalypts are a limiting resource at low stocking densities and that Leadbeater's possums exploit patches of pure Acacia only along their edges. Dietary analyses and feeding observations have shown that the decorticating bark of ash eucalypts harbors important insect prey which provide a source of protein (Smith, 1984b). We found no significant association between the volume of decorticating bark clumps and the abundance of Leadbeater's possum. However, this result may simply reflect an inadequate sample size. Decorticating bark volume was negatively confounded with Acacia numbers and we had too few sites to examine the effects of bark volume on possum density in a subset of sites with uniform acacia stocking. An association was also found between the abundance of Leadbeater's possum and the number of large tree stumps left by selection and salvage logging operations conducted after the 1939 wildfire. This result can be explained by confounding between logging activity and Acacia regeneration. Stump abundance was sigaifi¢ antly correlated with acacia abundance, and did not explain any variation in Leadbeater's possum density once the effects of acacias had been taken into account. Prolific regeneration of Acacia spp. typically occurs after soil disturbance (Adams and Attiwill, 1984), which is concomitant with selective logging. Acacia dealbata was observed to have regenerated prolificly on areas of disturbed soil exposed during recent logging operations throughout the study region. Old-growth sites disturbed by low intensity selection logging during the previous 30-50 years could be therefore expected to sup-

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port a more suitable understory for Leadbeater's possum than undisturbed sites of a similar age. Forest succession Mountain ash, shining gum and to a lesser extent alpine ash, which dominate forests inhabited by Leadbeater's possum, are considered to be fire sensitive and shade intolerant. These species typically regenerate in even-age stands after intense wildfire, and multi-aged stands after low intensity wildfire (Ashton, 1976). Young regrowth forests have dense interlocking canopies (Ashton, 1976) and may support over 400 000 acacias ha -I (Adams and Attiwill, 1984). As secondary succession progresses the basal area of eucalypts increases, the tree canopy becomes more open and the number and diversity of acacias in the understory declines. A. dealbata may persist as a tall understory species as the forest ages, but the A. obliquinervia g e n ~ y declines and disappears (Adams and Attiwill, 1984). Regeneration of the acacia understory may occur in mature and old-growth stands, but only after disturbance by low intensity wildfire and/or selection logging. Leadbeater's possum density was found to decline significantly with forest age in sites with abundant PNTs. However, the relationship was not strong and Leadbeater's possum density varied considerably in forests of all ages. Peak density occurred in young stands 15-50 years after fire but possums were also present on the oldest sites surveyed. This result needs to be treated with some caution, however, because the majority of our older sites were multiaged in structure. It is likely that with a larger data set a clearer picture will emerge showing preference for only those old-growth forests which have been disturbed by fire or selection logging to produce a multi-age structure. At present more variation in Leadbeater's possum density is explained by individual habitat variables, including Acacia Numbers, PNT Spacing, and Canopy and Understory Connectance, than by forest age alone. This finding is consistent with the results of principal components analysis of forest structural variables, which separated Acacia abundance, PNT abundance, and forest age (as indicated by forest height and stand basal area) onto independent gradients. Leadbeater's possum density did not correlate significantly with the age gradient (Factor 2), but was correlated highly significantly with the Acacia, Canopy and Understory Connectance and Stumps gradient (Factor 1 ) and moderately significantly with the PNT gradient (Factor 3). Changes in acacia abundance, ash basal area, and eucalypt stocking observed in our simulated age gradient are generally consistent with established patterns of succession in ash forests. However, our results suggest that multiaged forests, which provide important refuge habitat for Leadbeater's possum, may be a more common natural feature of ash forests than previously supposed. Ash forests in the Central highlands are thought to be predomi-

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A.P. SMITH AND D.B. LINDENMAYER

nantly even-aged (Ashton, 1976), but this may be as much an artefact of salvage logging as of natural ecological processes. The majority of sites encountered during this study which were not burnt in 1939, were multi-aged in structure. Many forests which would have developed a multi-age structure after the 1939 fire may have been eliminated by salvage logging. Only 24% of ash forests burnt in the recent (1983) Warburton wildfires, for example, were killed or severely damaged (Forests Commission of Victoria, 1984). Many of the remaining forests, which had the potential to develop into multi-aged stands, were harvested in salvage logging operations and converted to uniform-age regrowth. A combination of post fire, salvage logging and intensive harvesting of old-growth forest, has adjusted the natural pattern of age structure in ash forests in favor of relatively uniform-aged young regrowth. This can be expected to have a detrimental effect on long-term habitat availability for Leadbeater's possum in timber production forests.

Population dynamics Models of relationships between possum density and forest age determined in this study can be combined with previous models of relationships between PNT numbers and possum density (Smith and Lindenmayer, 1988), and models of PNT dynamics (Lindenmayer et al., 1990 ), to generate the following scenario of historic and future trends in Leadbeater's possum population size. Prior to 1939 Leadbeater's possum would have widely distributed at moderate to low densities throughout multi-aged forests, old-growth/regrowth forest ecotones, and occasional old-growth forests with an A. dealbata understory. Numerous tree hollows would have been present but regrowth forest with closed canopies and abundant acacias would have been scarce. After the 1939 wildfire the population would have been reduced by approximately the area burnt (65% of ash forests) and temporarily confined to fragmented refugia, patches of unburnt multi-aged forest. This would account for proximity of early sightings of Leadbeater's possum to multi-aged and oldgrowth forests (Rawlinson and Brown, 1977; Tunks, 1977). In the following 15-40 years the area of optimum regrowth habitat with abundant acacias and PNTs would have increased, permitting the population to expand and reach a maximum. An estimate of total population size for the year 1980, places this peak at approximately 7500 (+2300) animals (Smith et al., 1985). Around, or shortly before, this time the population would have begun to decline due to natural decay, active culling, and logging-inducedcollapse of PNTs in regrowth forests. This decline would have been exacerbated by harvesting of old-growth and multi-aged refugia. Harvesting of mature and old-growth forests was intense after the 1939 wildfire when post fire regrowth forests were too young to provide sawlogs. Clear-felling of 1939 regrowth forests intensi-

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fled in the 1980s, and is expected to continue on rotations of 40-150 years throughout timber production forests (Victorian Government, 1986). This activity is exacerbating the rate of collapse of PNTs in regrowth forests and the short rotation time is preventing the recruitment of replacement PNTs. A model of tree fall in regrowth ash forests (Lindenmayer et al., 1990) predicts that less than 6% of the ash forests in the Central highlands will have a suitable density of PNTs for Leadbeater's possum i~ 50 years time and less than 2% in 70 years time. The proportion of ash forests suitable for Leadbeater's possum is expected to decline rapidly over the next 30 years, reaching a bottleneck in about the year 2020 which will last at least until 2075. Whether or not the population recovers or declines to extinction after 2075 may depend on management strategies for conservation of Leadbeater's possum devised and implemented over the next few years.

Forest management for conservation of Leadbeater's possum Our results and those of previous studies indicate that management strategies for conservation of Leadbeater's possum should aim to: ensure the regeneration of gum producing acacias, and recruitment of PNTs in regrowth ash forests following timber harvesting; and protect long-term refuge habitats to maximize survival of possum populations during the expected population bottleneck. We found no evidence that current silvicultural practices adversely affected the regeneration of gum producing acacias, except in instances where pure stands of acacia were clear-felled and planted to eucalypt. Silvicultural methods which promote acacia regeneration may benefit timber production as well as Leadbeater's possum conservation. Acacias fix atmospheric nitrogen and are thought to be important for nutrient conservation, replacement and redistribution (Adams and Attiwill, 1984), as well as providing habitat for insectivorous birds and mammals which harvest forest canopy pests. Management strategies and requirements for recruitment of PNTs in timber production ash forests were discussed at length by Smith and Lindenmayer (1988). They recommended recruitment of 10-12 PNTs per 3 ha within logged coupes to sustain maximum densities of arboleal marsupials. This recommendation has not been incorporated into management practice because of the difficulty of protecting retained trees during regeneration bums. An option to use tractor clearing to promote regeneration has yet not been tried except on an experimental basis. Current management plans for Leadbeater's possum address tbe problem of PNT recruitment by recommending retention of trees in strips between and along the edge of logging coupes (Macfarlane and Seebeck, 1990). Recent surveys of retained strips suggest, however, that they are unsuitable for occupation by Leadbeater's possum (D. Lindenmayer, unpublished data, 1990). This finding casts doubt on the effectiveness of current strategies and raises the possibility that Leadbeater's pos-

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A.P. SMITH AND D.B. LINDENMAYER

sum could be totally eliminated from timber production forests under current management regimes. This situation needs to be addressed by modification of management guidelines outlined in the Code of Forest Practice (Department of Conservation, Forests and Lands, 1988) to require recruitment of a suitable minimum number and spacing of PNTs in at least a significant portion ( > 20%) of timber production forests. Survival of Leadbeater's possum over the coming century is expected to depend primarily on the identification and protection of refuge habitats. Refuge habitats include multi-aged forest with an old-growth canopy and regrowth understory, old-growth-regrowth ecotones, regrowth forests with scattered living or persistent dead emergent PNTs, and mature forests which will reach an age sufficient to generate new hollows during the next 50-100 years. The natural occurrence of refugia has been greatly reduced in timber production forests by harvesting of mature and old-growth forests, and their value may have been diminished by fragmentation. In the Central Gippsland region of the Central Highlands the area of mature and old-growth forest was less than 1800 ha in 1989 (Gruen et al., 1989). Government policies (Victorian Government, 1986, 1987) require the maintenance of a balance of age classes in timber production forests but fail to specify actual minimum areas or percentages of different age classes. The Code of Forest Practice again needs to be modified in order to guarantee the availability of an acceptable minimum area of refuge habitat for Leadbeater's possum and other mature treedependent wildlife in timber production forests. The Victorian Department of Conservation and Environment is currently mapping the distribution of PNTs and refuge habitat in timber production forests (M. Macfarlane, personal communication, 1990). Once these refuge areas have been mapped they should be surveyed to provide more precise estimates of Leadbeater's possum population size, and to evaluate present concerns that linear and isolated refuges will not support valuable possum populations. Until such studies have been completed it would appear desirable to protect all remaining known habitats and refuge areas from further logging. ACKNOWLEDGMENTS

This study was funded by a grant to the senior author from World Wildlife Fund Australia and the Fisheries and Wildlife Division of Victoria, for 'Project 5 l: Forest Management of Leadbeater's Possum'. The Forests Commission of Victoria provided office accommodation, administrative assistance (by G.C. Suckling), and access to maps, aerial photographs and unpublished records. Stagwatching and measurement of habitat variables was carried out with the assistance of numerous volunteers. For details see acknowledgments in Smith and Lindenmayer (1988).

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REFERENCES Adams, M.A. and Attiwill, P.M., 1984. Role of Acacia spp. in nutrient balance and cycling in regenerating Eucalyptus regnans F. Muell. forests. I Temporal changes in biomass and nutrient content. Aust. J. Bot., 32: 205-215. Ashton, D.H., 1976. The development of even aged stands of Eucalyptus regnans F. Muell. in Central Victoria. Aust. J. But., 24: 397-414. Department of Agriculture, Forestry and Timber Bureau, 1975. Multiple Use of Forest Resources. Aust. Govt. Publishing Service, Canberra, A.C.T. Department of Conservation, Forests and Lands, 1988. Code of Practice. Code of Forest Practices for Timber Production. DCFL, Melbourne, Vic., 48 pp. Forests Commission of Victoria, 1984. Division of Forest Management. Assessmer,t Section Inventory Report No. 71. Forests Commission of Victoria, Melbourne, Vie. Gruen, F.H., Leslie, A. and Smith, A., 1989. Inquiry into the proposed trial of the value adding utilization system, Central and East Gippsland forest management areas under the Environment Effects Act. Ministry for Planning and Environment, Melbourne, Vic., 67 pp. Hodgson, A., 1968. The role of fire in the ecology of the eucalypt. Victorian Nat., 85: 21-22. Lindenmayer, D.B., Smith, A.P., Craig, S.A. and Lumsden, L.F., 1989. A survey of Leadbeater's possum, Gymnobelideus leadbeateri, McCoy in the Central Highlands of Victoria. Victorian Nat., 106: 174-178. Lindenmayer, D.B., Cunningham, R.B., Tanton, M.T. and Smith, A.P., 1990. The conservation of arboreal marsupials in the montane ash forests of the Central Highlands of Victoria, southeast Australia. I 1. The loss of trees with hollows and its implications for the conservation of Leadbeater's possum, Gymnobelideus leadbeateri, McCoy ( Marsupialia Petauridae 1867). Biol. Conserv., 54: 133-145. Macfarlane, M.A. and Seebeck, J.H., 1990. Draft management plan for the conservation of Leadbeater's possum, Gymnobelideus leadbeateri, in Victoria. Arthur Rylah Institute for Environmental Research, Melbourne, Vic., 78 pp. Noble, W.S., 1977. Ordeal by Fire. The Week a State Burned up. Hawthorn Press, Melbourne, Vic., 85 pp. Rawlinson, P. and Brown, P., 1977. The fairy possum. Wildl. Aust., 14: 74-79. Smith, A.P., 1980. The diet and ecology of Leadbeater's possum and the sugar glider. Ph.D. Thesis, Monash University, Melbourne, 294 pp. Smith, A.P., 1982. Leadbeater's possum and its management. In: R.H. Groves and W.D.L. Ride (Editors), Species at Risk: Research in Australia. Australian Academy of Seience, Canberra, pp. 129-147. Smith, A.P., 1984a. Demographic consequences of reproduction, dispersal and social interaction in a population of Leadbeater's possum. In: A.P. Smith and I.D. Hume (Editors), Possums and Gliders. Surrey Beatty, Sydney, pp. 359-373. Smith, A.P., 1984b. Diet of Leadbeater's possum. Aust. WildL Res., I h 265-273. Smith, A.P. and Lindenmayer, D., 1988. Tree hollow requirements of Leadbeater's possum and other possums and gliders in timber production ash forests of the Victorian Highlands. Aus. Wildl. Res., 15: 347-362. Smith, A.P., Lindenmayer, D.B. and Suckling, G.C., 1985. The ecology and management of Leadbeater's possum. Research report to the World Wildlife Fund, University of New England, Armidale, NSW, 56 pp. Smith, A.P., Lindenmayer, D.B., Begg, R.J., Macfarlane, M.A., Seebeck, J.H. and Suckling, G.C., 1989. Evaluation of the stagwatching technique for census of possums and gliders in tall open forest. Aust. Wildl. Res., 16: 575-580. Statistical Analysis Systems, 1982. SAS User Guide: Statistics. SAS Institute, Cary, NC.

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Tunks, D., 1977. A habitat analysis of a forest dependent marsupial GymnobelideusleadbeaterL Honours Thesis, La Trobe University, Bundoora, Victoria. Victorian Government, 1986. Government Statement No. 9. Timber Industry Strategy. Government Printer, Melbourne. Victorian Government, 1987. Protecting the Environment. A Conservation Strategy for Victoria. Government Printer, Melbourne. Wilkinson, M.E., 1961. The rediscovery of Leadbeater's possum. Victorian Nat., 78: 97-102.