Chapter 4 - Fire frequency and vegetation
±site20
site3 Rainfall site50 Elevation
site-17+ Slope sitel 1 Shortest inter-fire interval site8v sitel Number of fires site19 site9 sitel2 sitel 3 site29 0 0 sitel 0 site Aspect Temperature site28 site18 site22
site2l Time since fire Topographic index
r-I -1.0 +1.0
Figure 4.13. DCCA of sites based on ground cover data in Tablelands sites. Symbols used are: HSS=circles, MSS=diamonds, MLS=crosses, LLL=down-triangles.
117
Chapter 4 - Fire frequency and vegetation
Blechnum canilagineum • Centella sp. •
Lomandra maltiflora Kennedia rubicunda Eustrephus latifollius Rainfall Juncus usitatus• Hydrocotyle• laxiflora Hibbertia scandens Picris hieracioides • • Elevation Bra cteantha bracteata Stylidium graminifolium • Soknogyne sp. Calotis dentexslope Hypocharis radicata •9411/ Gonocarpus tetragynus Entolasia stricta • Poa sieberiana • • Smilax australis Geranium solanderi Euchitsp• Praha purpurascens or Lomandra longifolia Number of fires iiianeila revoluta Desmodium van .9 • . Shortest inter-fire interval flardenbergia violaceae •• lichrysurn rutidolepts Leptdosperma laterale Gaiium propmquum. • Bossiaea rhombifolia sp. % 4_.___..------Polygala jaimnic • a ,Mentha ti Aspect Echinopogon caespitosus Entolasia sp. Ilb Oxalis sp. Temperature Poaceae sp. gEuchiton spp. Hibbertia obtusifolia Pterostylis sp. • Gahnia aspera Hydrocotyle pedicellosa • Time since fire Cissus antarctica. Rubus rosifolius Topographic index Unica incisa Rubus
-1. 0 +1.0
Figure 4.14. DCCA of Tablelands ground cover data. Due to the high number of species, labels were only added where possible.
The mean cover of the ground cover species aggregated into growth forms (Figure 4.15) showed a shift from high grass cover and few sedge and rush species in the sites of high NOF to more bare ground and leaf litter and higher cover of sedge and rush species in the sites of low NOF. Four main grasses, Themeda australis, Poa sieberiana, Sorghum leiocladum and Imperata cylindrica dominated the high NOF sites. There was little bare ground due to the thick grass cover. In contrast, the sites with low NOF had fewer grass species and were dominated by species such as Calochlaena dubia, Gahnia aspera and Lomandra longifolia. The forbs and orchids had a relatively consistent occurrence over all of the fire categories.
118 Chapter 4 - Fire frequency and vegetation
■ Grass ■ Sedge and rush 0 Forb 0 Orchid Bare lifter
MSS MLS LLL Fire category
Figure 4.15. Average cover per site by growth form type in the ground cover for the Tablelands vegetation survey. Fire category abbreviations in Appendix 1.1.
4.3.4 Gorge woody tree and shrub composition
The Gorge study sampled 99 species (including 13 identified only to genus, three to family and three unknowns). Many species (61%) occurred in only one fire category (Table 4.18) and of these the majority were in the MLL fire category (37%) followed by the MLS and MSS with 10% respectively. The MLL fire category was characterised by a diverse composition of species including those associated with wet sclerophyll forest such as Trochocarpa laurina, Cryptocarya rigida, Persoonia media, Rhodamnia rubescens, Psychotria daphnoides, Dysoxylum rufum and Phyllanthus virgatus.
Eight species were common to all of the fire categories (Table 4.19). These species (Lespedeza juncea, Eucalyptus eugenoides, Lotus australis, Exocarpos cupressiformis, Angophora subvelutina, Jacksonia scoparia, Allocasuarina torulosa and Eucalyptus tereticornis) were persistence through a range of fire and environmental conditions in the Gorge. These species were dominant in the common vegetation communities of the dry sclerophyll forest in the Gorge.
Nearly a third of the species sampled (31%) occurred in 2-4 of the fire categories (Table 4.20). Those that occurred over four fire categories were spread over all categories with only Eucalyptus biturbinata not occurring in the lowest (MSS) fire category. Eucalyptus species
119 Chapter 4 - Fire frequency and vegetation dominated those that occurred over three fire categories, and except for E. crebra, all occurred in the MSS fire category. The majority of species occurring in just two of the fire categories had higher abundance in the fire categories associated with shorter SIFI and shorter TSLF.
Table 4.18. Species sampled in the Gorge vegetation survey only occurring in one of the fire categories. Mean and SE sorted by decreasing density in the fire categories LLL = Low number of fires, long shortest inter-fire interval and long time since last fire, MLS .--z-. moderate number of fires, long shortest inter-fire interval and short time since last fire, MSS = moderate number of fires, short shortest inter-fire interval and short time since last fire and IISS = high number of fires, short shortest inter-fire interval and short time since last fire. Fire category LLL MLL MLS MSL MSS Species Mean SE Mean SE Mean SE Mean SE Mean SE Pittosporum 32.00 0.00 revolutum Kunzea sp. 11.00 0.00 Acacia sp.1. 6.00 0.00 Epacridaceae sp. 5.00 0.00 Phyllanthus sp. 4.00 0.00 Acacia sp.2. 1.00 0.00 Trochocarpa laurina 113.50 8.54 Cryptocarya rigida 86.00 28.46 Geijera salicifolia 64.00 0.00 Notelea sp. A 40.00 17.71 Prostanthera 30.00 0.00 lasianthos Eucalyptus 27.50 2.85 cameronii Myoporum 26.00 0.00 acuminatum Solanum 24.00 0.00 densevestitum Acacia irrorata 20.00 0.00 Zieria sp. 20.00 0.00 Elaeocarpus 17.00 0.63 reticulatus Persoonia media 17.00 3.16 Acacia falciformis 16.00 0.00 Rhodamnia 15.00 0.00 rub esc ens Atalaya silicifolia 13.00 0.00 Xanthorrhoea sp. 10.00 0.00 Eucalyptus conica 9.00 0.00 Eucalyptus 9.00 0.00 quadrangulata Pimelea latifolia 9.00 0.00 Psychotria 5.00 0.00 daphnoides Dysoxylum rufum 4.00 0.00 Phyllanthus virgatus 3.00 0.00 Goodia lotifolia 327.50 167.72
120 Chapter 4 - Fire frequency and vegetation
Indigofera 238.00 0.00 adesmiifolia Acacia obtusifolia 103.00 0.00 Eucalyptus viminalis 79.00 0.00 Acacia diphylla 55.00 0.00 Senecio 54.00 0.00 amygdalifolius Myoporum sp. 40.00 0.00 Ozothamnus 34.00 0.00 diosmifolius Eucalyptus brunnea 10.00 0.00 Persoonia oleoides 10.00 0.00 Acacia nerifolia 7.00 2.31 Swainsona 5.00 0.00 galegifolia Santalum 3.00 0.00 obtusifolium Hovea sp. 2.00 0.00 Eucalyptus fastigata 1.00 0.00 Eucalyptus 13.00 0.00 sideroxylon Pimelea sp. 6.00 0.00 Eucalyptus albens 106.00 0.00 Eucalyptus 65.00 18.34 sp.(ironbark) Alxyia rucifolia 30.00 0.00 Acacia falcata 28.50 4.74 Restionaceae sp. 23.00 0.00 Eucalyptus fibrosa 12.00 0.00 Fabaceae sp. 11.00 0.00 Podolobium 11.00 0.00 ilicifolium Acacia ulicifolia 9.00 0.00 Lantana camara 7.00 0.00 Unknown sp.1. 3.00 0.00 Eucalyptus nobilis 2.00 0.00 Brachychiton populneus 1.00 0.00 subsp. populneus Persoonia sp. 1.00 0.00 Unknown sp.2. 1.00 0.00
121 Chapter 4 - Fire frequency and vegetation
Table 4.19. Species sampled in the Gorge vegetation survey occurring in all fire categories. Mean and SE sorted by decreasing density in the fire categories. See Table 4.17. for fire category definitions. Fire category LLL MLL MLS MSL MSS Species Mean SE Mean SE Mean SE Mean SE Mean SE Allocasuarina 131.00 44.48 51.00 12.40 23.60 14.58 83.00 29.91 29.25 15.16 torulosa Jacksonia scoparia 98.00 35.27 349.00 53.76 288.25 146.84 135.00 0.00 138.20 44.82 Angophora 40.40 31.70 5.67 2.88 4.00 0.00 12.33 5.36 29.50 0.95 subvelutina Exocarpos 34.67 11.03 19.67 11.16 28.60 9.89 44.00 4.62 40.67 30.34 cupressiformis Lotus australis 32.00 0.00 5.00 0.00 34.50 17.61 5.00 0.00 18.00 0.00 Eucalyptus 28.75 11.61 51.00 0.00 43.25 13.59 30.50 11.84 9.67 2.98 tereticornis Eucalyptus 20.33 5.41 6.00 0.00 23.75 7.05 19.00 11.43 17.00 6.15 eugenioides Lespedeza juncea 12.00 1.26 9.00 0.00 55.00 27.14 17.00 0.00 8.00 0.00
Table 4.20. Species sampled in the Gorge vegetation survey occurring in 2 - 4 fire categories. Mean and SE sorted by decreasing density in the fire categories. See Table 4.17 for fire category definitions. Fire category LLL MLL MLS MSL MSS Species in 4 fire Mean SE Mean SE Mean SE Mean SE Mean SE categories Maytenus silvestris 254.00 0.00 50.00 21.94 20.00 3.27 20.00 0.00 Rapanea variabilis 237.00 0.00 29.00 5.77 7.00 0.00 6.00 0.63 Eucalyptus carnea 93.00 0.00 8.00 0.00 117.00 0.00 21.00 0.00 Eucalyptus crebra 29.00 5.69 21.00 0.00 11.33 5.63 2.00 0.00 Melichrus urceolatus 11.00 0.00 1.00 0.00 4.00 0.00 2.00 0.00 Persoonia sericea 8.00 3.79 24.00 0.00 8.00 0.00 7.00 0.00 Eucalyptus melliodora 7.00 0.63 4.00 0.00 7.00 0.00 12.00 0.00 Acacia melanoxylon 4.00 0.00 17.00 0.00 29.33 9.10 12.00 1.26 Hibbertia obtusifolia 4.00 0.00 12.00 2.53 11.50 3.75 13.50 6.01 Eucalyptus biturbinata 2.50 0.95 11.00 5.05 30.50 13.57 10.00 4.08 Acacia implexa 2.50 0.32 9.00 0.00 1.00 0.00 3.00 0.00
Species in 3 fire categories Eucalyptus sp.(gum) 9.00 0.00 6.00 1.26 39.00 5.06 Eucalyptus microcorys 30.00 1.90 85.00 0.00 5.50 2.21 Breynia oblongifolia 9.00 0.00 27.50 7.79 13.50 1.58 Eucalyptus amplifolia 8.00 0.00 15.00 0.00 36.50 21.19 Eucalyptus caliginosa 8.00 0.00 8.00 0.00 4.00 0.00
122 Chapter 4 - Fire frequency and vegetation
Species in 2 fire categories Eucalyptus caleyi 33.00 0.00 51.00 0.00 subsp. caleyi Eucalyptus laevopinea 28.50 12.02 32.00 0.00 Polyscias 48.00 2.53 1.00 0.00 sambuccifolia Leucopogon 22.00 8.28 5.00 0.00 lanceolatus Eucalyptus blakelyi 20.00 0.00 4.00 0.00 Correa reflexa 16.00 0.00 301.23 0.00 Lomatia silaifolia 16.00 0.00 2.00 0.00 Lophostemon confertus 13.00 6.77 9.00 0.00 Eucalyptus 7.50 1.58 18.00 0.00 campanulata Eremophila debilis 4.00 0.63 1.00 0.00 Eucalyptus mollucana 2.00 0.63 4.00 0.00 Indigofera australis 580.00 0.00 90.00 0.00 Solanum sp. 79.00 0.00 1.00 0.00 Unknown sp.3. 2.00 0.00 1.00 0.00 Pimelea linifolia 1.00 0.00 1.00 0.00
Analysis of the 15 most common species in the Gorge found nine species were significantly related to the environmental variables (Table 4.21). Four were found to be significantly related to at least one of the fire variables in the final regression equation. Angophora subvelutina, Eucalyptus microcorys and E. tereticornis were significant for the SIFT and Breynia oblongifolia for TSLF. Breynia oblongifolia was found to have increased density with increasing SIFI and decreasing TSLF. Eucalyptus microcorys and E. tereticornis were all found to have increasing density with increasing SIFI and increasing TSLF. Angophora subvelutina, and Allocasuarina torulosa all had increasing density with decreasing SIFI and increasing TSLF.
Table 4.21. Significant results from the multiple regressions of common species in the Gorge. Significant species (P-value < 0.05) with **. Species are: Angophora subvelutina (Ans), Allocasuarina torulosa (Ator), Breynia oblongifolia (Brob), Exocarpos cupressiforrnis (Ec), Eucalyptus microcorys (Eumi), E. tereticornis (Eutc), Hibbertia obtusifolia (Hiob) and Jacksonia scoparia (Js). Species Ans Ator Brob Ec Eumi Eutc Hiob Js
P-value Number of fires Shortest inter-fire 0.047** 0.021** 0.007** interval Time since last 0.022** fire Elevation 0.260 0.020** 0.260 0.301 Slope 0.001** 0.001** 0.008** 0.008** Aspect 0.169 0.134 0.018** Rainfall 0.096 0.005** 0.004** Topographic index 0.046** 0.065
123 Chapter 4 - Fire frequency and vegetation
Regression result Degrees of 2,17 4,15 4,15 3,11 1,18 3,7 1,18 2,10 freedom F-value 9.7553 18.4090 4.6954 14.1540 6.3739 46.8050 6.7637 54.6860 P-value 0.002** 0.001** 0.012** 0.001** 0.022** 0.001** 0.018** 0.001** R2 0.5344 0.8308 0.5560 0.7942 0.2615 0.9525 0.2731 0.9572
Overall there was no significant difference between the fire categories and woody plant richness. Figure 4.16 shows species richness with each fire category, including the LLL category that was removed from analyses, but retained in these figures for comparison with other sites. Richness in the Gorge was highest in the MLL and MSS fire categories and lowest in the intermediate fire categories of MLS and MSL.
20
18
16
14
12
10
8 CA 6
4
2
0 LLL MLL MLS MSL MSS Fire category
Figure 4.16. Mean woody species richness in the Gorge by fire category (mean ± SE).
Multivariate analysis
Observations of scorch height and charcoal at the site indicated that most of the Gorge LLL fire category sites had been burnt in the recent past and were incorrectly interpreted in the satellite data as long unburnt. These sites were predominantly in areas affected by shadow or other terrain influences. For these reasons, the LLL fire category was removed from the analysis in the Gorge. To focus on the smallest, most significant number of explanatory variables, temperature was also removed due to its correlation with elevation. 124 Chapter 4 - Fire frequency and vegetation
The first gradient length of the detrended correspondence analysis was 5.195, demonstrating a strong binomial trend in the Gorge density data. To avoid the arch effect as discussed in the methods (Section 4.2.6), a detrended canonic correspondence analysis (DCCA) was undertaken. The variance explained by the two axes was high with the first eigenvector equalling 0.834 and the second 0.298. The DCCA accounted for 57% of the variance in the data. The first and second eigenvalues accounted for a high proportion of the variance (eigenl = 0.736 and eigen2 = 0.337). Of the total variance explained, the fire variables accounted for 35% of the overall variance. This reflected the higher influence of the environmental variables as the intraset correlation coefficients showed that axis 1 was primarily separating species due to rainfall (Table 4.22) and axis 2 was primarily separating species along the gradient of SIFT.
Table 4.22. Intraset correlation coefficients for axes 1 and 2 for the Gorge detrended canonic correspondence analysis. Principal correlates indicated with ** Variable name Axis 1 Axis 2 Number of fires -0.2872 0.364 Shortest inter-fire interval 0.4679 -0.455 ** Time since last fire 0.3999 0.3077 Elevation 0.7464 0.1763 Slope -0.3093 0.1289 Aspect -0.0148 -0.3357 Rainfall 0.8907 ** -0.2665 Topographic index -0.224 -0.4323
Forward selection showed that none of the fire variables were significant at the P < 0.05 level. Only the SIFI came close where P = 0.055. However, other environmental variables, rainfall and elevation, were significant (Table 4.23).
Table 4,23, Forward selection results for Gorge variables. Significant variables (P < 0.05) indicated with **, data sorted on descending values of LambdaA. Variable name LambdaA F-value P-value Rainfall 0.64 3.75 0.005** Elevation 0.32 1.92 0.020** Slope 0.22 1.41 0.110 Time since last fire 0.25 1.63 0.055 Shortest inter-fire interval 0.16 1.08 0.315 Number of fires 0.18 1.23 0.255 Aspect 0.15 0.98 0.485 Topographic index 0.10 0.67 0.845
The distribution of sites along the ordination axes 1 and 2 is shown in Figure 4.17. With regard to the fire variables, the sites were predominantly separated along the axes of NOF and
125 Chapter 4 - Fire frequency and vegetation
SIFI that were almost aligned but in opposing directions. Orthogonal to this trend was the axis of TSLF with the majority of sites occurring in the region of longer intervals. The MSS and MSL sites were characterised by higher NOF, slope and low rainfall. The separation between these two fire categories, from the other categories, was primarily due to low rainfall and high slope. The topographic index then separated MLS (high topographic index) from MSL (low) sites. The most divergent sites were those in the MLS fire category separated from the other fire categories by high rainfall and longer TSLF. There were some outlier sites. The largest being site 37 in the MSL fire category and sites 39 and 41 in the MLS category. These sites all had very high abundance values due to massive regrowth following a fire in 1996. Sites 39 and 41 had an abundance of species not commonly found in other sites and were in an area of higher rainfall when compared to the other sites in this fire category.
s37 D
Time since fire q s35 s47 Number of fires 0 s49 s34 s38
Slope v-, s40 Elevation s2
s25 s45 q s48
s24
s30
Rainfall s31 Aspect
Topographic index -4- s39 s41 Shortest inter-fire interval
O
rH –1.0 +1.0 Figure 4.17. The DCCA biplot for the Gorge sites (diamonds = MSS, right triangles = MSL, crosses = MLS, box = MLL). The 's' is abbreviated for 'site', indicating the site number.
126 Chapter 4 - Fire frequency and vegetation
The species ordination reflected the trends in the site ordination and showed the propensity of species for sites associated with certain fire categories. The distribution of the species along the DCCA ordination axes of 1 and 2 is shown in Figure 4.18. The majority of species were clustered in the centre-left of the plot associated with higher NOF, shorter SIFT and lower elevation, reflecting the clustering of moderate NOF sites. Many of the species common throughout the Gorge and found in a number of fire categories (Table 4.18) were represented in the mid-range cluster of species with shorter SIFI and lower rainfall. These occurred in the moderate NOF and short SIFT sites and included species such as Maytenus silvestris, Allocasuarina torulosa, Acacia implexa, Persoonia sericea, Jacksonia scoparia and Eucalyptus tereticornis. Species identified as being unique to the MLL fire category such as Trochocarpa laurina, Persoonia media and Cryptocarya rigida, were also apparent in the ordination characterised by longer fire intervals and higher rainfall. Goodia lotifolia, occurring in the region of high SIFT and exposure (high topographic index) was abundant at the outlier sites (39 and 41). The Eucalyptus sp. that occurred as a large outlier was common only in site 37.
127
Chapter 4 - Fire frequency and vegetation
Eucalyptus sp. •
Eucalyptus cal iginosa Time since fire 0
Number of fires Lomatia silaifolia Angophora subvelutina 0 E. laevopinea Maytenus silyestris 0 Lotus australis 0 0 Polyscias sarnbuccifolia Allocasuarina torulosa Melichrus umeeolatus E. campanuiata 0 0 Notelea sp. A Acacia implexa 0 / • E. biturbinata 0 % Cryptocarya rigida Slope / • Persoonia media Persoonia serieea E. mollucana Elevation Jacksonia scoparia E. teretieomis 0 0 • Lophostemon confertus Eremophila debilis • cH ibbertia obtusifolia E. microcorys E. creber 0 • \ Rapanea variabilis p E. camea\ Acacia neritb/ia• Ci Indigofera australis Rainfall Aspect 0 Correa reflects Topographic index O Goodia lotifolia Shortest inter-fire interval
-1.0 +1.0
Figure 4.18. The Gorge DCCA analysis showing canonic axes with species density. Where possible species names have been added.
4.3.5 Gorge woody tree and shrub plant structure
The multiple regressions of the common Gorge species on mean height (Table 4.24) showed eight species to be significant. At least one of the fire variables was significant in the final regression for six of the species. These were Acacia melanoxylon and Exocarpos cupressiformis for TSLF, Breynia oblongifolia and Eucalyptus microcorys for SIFI and Hibbertia obtusifolia and Lotus australis for NOF. Broad trends from the multiple regressions showed Acacia melanoxylon, Angophora subvelutina, Allocasuarina torulosa and Exocarpos cupressiformis were decreasing in height with increasing SIFI. Breynia oblongifolia, Eucalyptus microcorys, Hibbertia obtusifolia and Lotus australis were
128 Chapter 4 - Fire frequency and vegetation increasing in height with increasing SIFI. For TSLF, Acacia melanoxylon, Angophora subvelutina, Allocasuarina torulosa, Breynia oblongifolia, Exocarpos cupressiformis, Eucalyptus microcorys and Lotus australis all had increasing height with increasing TSLF. Only Hibbertia obtusifolia showed decreasing height with increasing TSLF.
Table 4.24. Significant species from the multiple regression of mean height of the most frequent Gorge species with fire and environmental variables. Significant variables (P < 0.05) indicated with **. Abbreviations used are: Acacia melanoxylon (Acmel), Angophora subvelutina (Ans), Allocasuarina torulosa (Ator), Breynia oblongifolia (Brob), Exocarpos cupressiformis (Ec), Eucalyptus microcorys (Eumi), Hibbertia obtusifolia (Hiob) and Lotus australis (Laus). Species name Variable name Acmel Ans Ator Brob Ec Eumi Hiob Laus P-value Number of fires 0.160 0.153 0.083 0.094 0.031** 0.028** Shortest inter-fire 0.070 0.033** 0.141 0.012** interval Time since last fire 0.001** 0.382 0.013** Elevation 0.001** 0.134 0.197 Slope 0.069 0.008** 0.027** Aspect 0.123 0.203 Rainfall 0.016** 0.001** 0.047** Topographic index
Regression results Degrees of freedom 2,17 4,15 2,17 1,18 5,9 5,14 4,15 1,4 F-value 9.1893 3.2586 28.573 5.2997 4.0557 8.3417 5.8921 11.36 P-value 0.002** 0.041** 0.001** 0.033** 0.033** 0.001** 0.005** 0.028** R2 0.519 0.465 0.771 0.227 0.693 0.749 0.611 0.740
The multiple regression results for mean stem diameter (Table 4.25) had the addition of Eucalyptus tereticornis being significant in the regression equation. At least one of the fire variables was significant in the final regression for five of the species. These were Acacia melanoxylon and Eucalyptus tereticornis for TSLF, Eucalyptus microcorys for SIFI and Hibbertia obtusifolia and Lotus australis for NOF. Therefore in comparison with height, for Exocarpos cupressiformis TSLF was significant for height but not diameter, for Breynia oblongifolia SIFT was significant for height and not diameter and for Eucalyptus tereticornis, diameter was significant but not height.
129 Chapter 4 - Fire frequency and vegetation
Table 4.25. Significant species from the multiple regression of mean stem diameter (or base diameter for plants under 2 m) of the most frequent Gorge species with fire and environmental variables. Significant variables (P < 0.05) indicated with **. Small shrub species diameter taken at the base. Abbreviations used are: Acacia melanoxylon (Acmel), Angophora subvelutina (Ans), Allocasuarina torulosa (Ator), Breynia oblongifolia (Brob), Exocarpos cupressiformis (Ec), Eucalyptus microcorys (Eumi), E. tereticornis (Eutc), Hibbertia obtusifolia (Hiob) and Lotus australis (Laus). Species Acmel Ans Ator Brob Ec Eumi Eutc Hiob Laus Variables P-value Number of fires 0.177 0.049** 0.015** Shortest inter-fire 0.193 0.029** 0.210 interval Time since last 0.039** 0.113 0.073 0.324 0.046** fire Elevation 0.100 0.051 0.319 0.044** Slope 0.001** 0.002** 0.024** 0.001** Aspect 0.113 0.094 Rainfall 0.267 0.016** 0.002** Topographic 0.283 0.020** 0.050** 0.151 index Regression results Degrees of 4,15 3,16 3,16 3,16 1,13 6,13 3,7 3,16 2,3 freedom F-value 3.509 13.401 25.329 4.363 3.819 5.939 55.241 5.443 20.483 P-value 0.033** 0.001** 0.001** 0.020** 0.073** 0.004** 0.001** 0.009** 0.018** Rz 0.483 0.715 0.826 0.450 0.227 0.733 0.059 0.505 0.931
Vegetation community structure in the Gorge was not as markedly different between fire categories as in the Tablelands (Figure 4.19) although the variation in abundance between sites was high. Like the Tablelands, the highest abundance of stems was recorded in the intermediate fire category (MLS). The MSL sites had the lowest mean abundance. The abundance of the LLL sites was generally similar to the categories of higher NOF, suggesting these sites had experienced more fire than recorded.
130 Chapter 4 - Fire frequency and vegetation
1200
1000
800
.>, u) 600 c z)a) c co (1.) 400
200
LLL MLL MLS MSL MSS Fire category
Figure 4.19. Mean stems density in the Gorge by fire category (± SE).
The broad structural differences in the Gorge were most marked in the shrub stratum. The shrub height class less than 10 m and trees between 2 and 10 m showed a significant difference in the Gorge structure in relation to the fire (Figure 4.20). All other height classes were not significant (Table 4.26) when assessed against the fire categories.
131 Chapter 4 - Fire frequency and vegetation
A 200 180 160 u) 140
-0 120
„a:, 100 a. 80 cu 60
40 20 0 LLL MLL MLS MSL MSS Fire category
B 200 180 160 cr) 140
-61)1 120 100 o. 80
o 60 40 20 0 LLL MLL MLS MSL MSS Fire category
200 180 160 co 140
26 120 .9 100 80 60 40 20 0 LLL MLL MLS MSL MSS Fire category
Figure 4.20. Mean plant density (±SE) of tree (blue) and shrub (red) species in the height category of greater than 10 meters (F 4, 19 = 0.713, P < 0.594) (A), > 2-10 meters (F 4, 19 = 3.414, P < 0.029) (B) and less than or equal to 2 meters (F 4, 19 = 3.083, P < 0.041) (C). Maximum values for shrubs in graph C have been truncated for comparison - maximums for MLS = 666 and MSS = 212. 132 Chapter 4 - Fire frequency and vegetation
Table 4.26. ANOVA results for height classes of Gorge vegetation. The significant (P < 0.05) height categories are indicated with **. Growth form and height category F-value P-level Shrubs less than 2 m 3.083 0.041** Shrubs from 2 to 10 m 3.414 0.029** Shrubs greater than 10 m 0.713 0.594 Trees less than 2 m 0.490 0.743 Tree from 2 to 10 m 3.345 0.031** Trees greater than 10 m 0.386 0.816
The structural and compositional differences observed between the sites within the fire categories are shown pictorially in Figures 4.21 to 4.24.
Figure 4.21. Site 044 in the Gorge medium-short-short (MSS) fire category. The typical composition and structure is of grassy open forests.
133 Chapter 4 - Fire u ney and vegetation
Figure 4.22. Site 037 in the Gorge medium-short-long fire category. The typical composition and structure is of grassy forest and emergent understorey shrubs.
Figure 4.23. Site 034 in the Gorge medium-long-short fire category. The typical composition and structure is of grassy forests and a noticeable shrub layer and leaf litter.
134 Chapter 4 - Fire uency and vegetation
Figure 4.24. Site 038 in the Gorge medium-long-long fire category. The typical composition and structure is of multilayered forests.
4.3.6 Gorge ground cover
The vegetation survey of the Gorge ground cover sampled 187 ground cover species (31 to genus level and 8 to family level including 4 unknown grasses (Poaceae sp.), 3 unknown daisies (Asteraceae sp.) and 1 unknown plant (Restionaceae sp.)). This included 48 woody plant species that, like the Tablelands, will be discounted from any further discussion in this section as they have already been covered in detail. The mean and SE for each species within each fire category is given in Appendix 4.2 (woody plants excluded).
The more common species that occurred throughout all fire categories were the grasses including Dichelachne micranthus, Sorghum leocladum, Poa sieberiana, Themeda australis and Cymbopogon refractus. Themeda australis had the highest cover in the fire categories of long fire intervals and Poa sieberiana in the short fire intervals. Imperata cylindrica occurred in four out of the five fire categories, with highest mean cover in the sites with short SIFI. Over half the sedge and rush species occurred in one fire category. Those that occurred over all fire categories were dominated by Lomandra species (Lomandra longifolia, L. confertifolia and L. filiformis). Nearly half of the forb species were unique to one fire category, with MLL having the highest (34%) unique to only that fire category followed by the MSS category (25%). The common forbs occurring over all fire categories included a number of
135 Chapter 4 - Fire frequency and vegetation
Desmodium (Fabaceae) species (Desmodium varians, Desmodium rhytidophyllum and Desmodium brachypodum). Other species (Glycine clandestina, Vernonia cinerea, Dichondra repens, Viola betonicifolia and Phyla nodiflora) were common and abundant throughout the Gorge vegetation communities.
The mean ground cover declined as NOF decreased and SIFT increased. The variation between the growth forms showed the decline was primarily in the cover of leaf litter. Like the Tablelands, there was an increase in sedge and rush species in the fire categories with longer intervals. The grass cover was highest in the MSL category and then declined to the category with longer SIFT and TSLF. There was little overall difference in the forb cover. The amount of ground not covered by vegetation (including rock, woody and leaf litter) steadily declined as the fire interval become longer. This was a reflection of the increase in the sedges and rush species in the MLL fire category (Figure 4.25) resulting in a thicker ground cover.
25 Grasses ■ Sedge and rush q Forbs 20 q Bare Litter
ZI3 15 I 0 C) a)
> 10
5
T
0 MSS MSL MLS MLL Fire category
Figure 4.25. Average ground cover in the Gorge for each growth form type by fire category.
Mean (± SE).
Multivariate analysis
The detrended correspondence analysis of the Gorge ground cover data showed long gradients (4.56) so a detrended canonic correspondence analysis was undertaken. The overall variance in the Gorge was higher (3.443) than the Tablelands and the environmental variables
136 Chapter 4 - Fire frequency and vegetation explained 47% of the total variance. The first two axes explained a large proportion of the data (eigenl= 0.480, eigen2 = 0.289). The intraset correlation coefficients showed the first axis to be primarily related to rainfall and the second axis to TSLF (Table 4.27).
The fire variables accounted for 33% of the explained variance; however, the significance of the fire variables was low in comparison to the environmental variables. Of all the environmental variables, only rainfall was statistically significant at the P < 0.05 level (Table 4.28). None of the fire variables were statistically significant in relation to species cover. This was found in the site ordination with the environmental variable of elevation and rainfall having the most significance. The MLL sites with longer SIFT were related to higher elevation and rainfall. Like the woody plant ordination, NOF and SIFI were of similar influence but in opposing directions, with TSLF orthogonal to this trend. The MLS sites were characterised by long SIFT, high slope and high topographic index. The MSL and MSS sites were better represented in the central values of the ordination. There was some overlap with the MLL sites in the mid-range of TSLF.
Table 4.27. Intraset correlation coefficients for the Gorge ground cover data. Principal correlates indicated with **. Variable name Axis 1 Axis 2 Number of fires -0.0957 -0.1481 Shortest inter-fire interval 0.2236 0.3671 Time since last fire 0.3613 M.5465** Elevation 0.6651 0.5066 Slope -0.0481 0.2836 Aspect -0.0606 0.0264 Rainfall 0.7719** 0.5460 Topographic index -0.1971 0.3080
Table 4.28. Forward selection of the Gorge ground cover data. Significant variables (P < 0.05) indicated with **, sorted by decreasing LambdaA Variable name LambdaA F-ratio P-value Rainfall 0.37 2.03 0.020** Slope 0.22 1.27 0.185 Time since last fire 0.25 1.41 0.095 Topographic index 0.19 1.13 0.320 Shortest inter-fire interval 0.17 0.94 0.585 Aspect 0.14 0.82 0.615 Elevation 0.16 0.88 0.595 Number of fires 0.12 0.69 0.740
137 Chapter 4 - Fire frequency and vegetation
The species ordination clustered the majority of species in the centre of the variables reflecting the site distribution and high influence of TSLF (Figure 4.26 and 4.27). Some of the more common species were found, such as Desmodium varians, Desmodium brachypodum and Microlaena stipoides, associated with short SIFI and lower rainfall. Increasing TSLF and rainfall resulted in species such as Pteridium esculentum, Hibbertia obtusifolia and Adiantum aethiopicum. Moderate SIFI, high slope and topographic index were found to relate to species such as Poranthera microphylla, Imperata cylindrica and Stylidium graminifolium
Rainfall Elevation
s41 + Shortest inter-fire interva
Topographic index Slope s23
s24+ s31 s38 s30 Aspect 547 s37 t> s49 s44 q s45 Number of fires <> s42 s40
s46 s48
Time since fire
-1.0 +1. 0
Figure 4.26. Site ordination plot from DCCA of Gorge ground cover vegetation. Symbols used: MLL=square, MLS=cross, MSL=right-triangle, MSS=diamond.
138 Chapter 4 - Fire frequency and vegetation
Rainfall Elevation
Shortest inter-fire interva Topographic index Slope Poranthera microphylia Imperata cylindri Glycine tabacina lb Stytidiurn grarnifiyiium Entolasia stricta Opercularia aspera Cheilanthes sieberi Poa sp_ 0 0 Hardenbergia violaceae Pteridium esculentum Vi la betonicifolia 0 Desmodium varians0 4its Loman longifolia la Smilax australis Glossogyne tannensis Asperula conferta custrephus latifolia. Geranium solanderi Microlaena stipoides .0 Desmodium brachypodum 0 Dianella sp. 0 • Hibbertia obtusifolia Dichelachne sp. 0 • elichrysum rutidolepis • Lepidosperma laterale Hypericurn japonicum WCalotis dentex Number of fires 10 diaramn aethiopicum Euchiton apiculatum V Hypochaeris radicata9% w Oxalis sp. Conzya bonariensis • Phylanthus\sp. Verbena bonariensise 0 Polygala japonica Panicum effusume
Time since fire
-1.0 +1.0
Figure 4.27. Species ordination plot from DCCA of species in Gorge ground cover vegetation.
4.3.7 Tablelands and Gorge combined woody composition
The ordination of the combined Tablelands and Gorge woody species data had a gradient length of 4.352, demonstrating binomial trends in the data, so a detrended canonic correspondence analysis was undertaken. The overall variance of the data was high (5.579) when compared to the Tablelands (2.455) and Gorge (3.56). The first two eigenvalues explained a high proportion of the variance (eigenvalue 1 = 0.754, eigenvalue 2 = 0.368). The constrained axes explained a total of 43% of the variance. Of the explained variance, 34% was explained by the fire variables.
The intraset correlation coefficients are given in Table 4.29. The separation of sites along the first axis was primarily due to physiography, demonstrating the major differences between the
139 Chapter 4 - Fire frequency and vegetation
Tablelands and Gorge vegetation. The second axis was due to NOF, demonstrating the influence of NOF over the whole study region.
Table 4.29. Intraset correlation coefficients for woody tree and shrub species from the Tablelands and Gorge. Principal correlates indicated with **. Variable name Axis 1 Axis 2 Number of fires -0.0783 0.9497** Shortest inter-fire interval -0.2862 -0.7515 Time since last fire -0.2789 -0.4946 Elevation -0.8000 -0.0318 Slope 0.8298 -0.1902 Aspect 0.1213 -0.2191 Rainfall -0.8219 -0.2007 Topographic index 0.1061 0.2594 Physiography 0.8768** -0.1751
The forward selection of each of the variables showed a number of the variables were significant (P < 0.05) these were physiography, rainfall, NOF, elevation and TSLF (Table 4.30).
Table 4.30. Forward selection of each of the environmental and fire variables for the combined Tablelands and Gorge data. Significant variables indicated with **, sorted by decreasing LambdaA. Variable name LambdaA F-ratio P-value Physiography 0.68 4.83 0.005** Rainfall 0.48 3.73 0.005** Number of fires 0.29 2.31 0.005** Elevation 0.25 2.04 0.005** Time since last fire 0.20 1.73 0.005** Slope 0.15 1.22 0.165 Shortest inter-fire interval 0.13 1.14 0.275 Topographic index 0.14 1.15 0.300 Aspect 0.08 0.73 0.870
The combined ordinations separated species characteristic of the Tablelands and Gorge along the first axis and by NOF along the second axis (Figures 4.28 and 4.29). There were similarities in the composition to genus, but a separation between the Tablelands and Gorge for species. For example, a common dry sclerophyll stringybark eucalypt in the Tablelands was Eucalyptus caliginosa. In the Gorge, a similar common stringybark was E. eugenoides. These two occurred in a similar region of the ordination for NOF, but were separated by
140 Chapter 4 - Fire frequency and vegetation physiographic region. Similarly, a common understorey shrub in the Tablelands was Persoonia oleoides. In the Gorge there was Persoonia sericea, again these had a similar result in the ordination for NOF but were separated by physiography. The species biplot also identified species associated with low NOF and long intervals for both physiographic regions. Species such as Cryptocarya rigida, Notelea sp. A, Lophostemon confertus and Elaeocarpus reticulates. Species such as Hibbertia obtusifolia and Melichrus urceolatus were found to occur in both physiographic regions and in association with moderate NOF and intervals.
s40 0 + s41 s38 0 ±s39
Rainfall Shortest inter-fire interval Physiography
Time since fire Slope Aspect s12 s21 s18 547 Elevation 513v7 0 s49 545 s46 s42 s23 ly34 s20 s44 Topographic index elf? s25 s24 s74-0s22 s3 s5 s19 s35 +530 s9 +s31 s8<> s17+ s11 s1 Q s48 s28 s29 C s10 Number of fires
-1.0 + 1.0
Figure 4.28. The output of the DCCA ordination of both Tablelands and Gorge (down- triangles = LLL, box = MLL, cross=MLS, right-triangle=MSL, diamond=MSS, circle=HSS).
141 Chapter 4 - Fire frequency and vegetation
Persoonia media Trochocarpa laurina Elaeocarpus reticulatusel Lophostemon confertus 0 Goodia lotifoha • Correa retlexa 0 Rainfall Cryptocarya rigida 0 Shortest inter-fire interval 0 Rapanea variabilis Notelea sp. A Cts Eucalyptus carnea Physiography Eucalyptus camcronii° 0 Indigofera australis 0 Slope Time since fire Eucalyptus microcorysAspectoBreynia oblongifolia 0 Polyscias sambuccifolia
Santaturn obtusifolium Banksia integrifolia0 Elevation Leptospermum galifolium 0 Eucalyptus biturbinata Leucopogon lanceolatus Q 0 Allocasuarina tondosa Eucalyptus obli Topographic index% Eucalyptus moluccana Olearia oppositifolia 0 0 O °Acacia falcata Eucalyptus brunnea Acacia melanoxylon 0 Lotus australis Acacia falcifonnis 0 0 I; ° 0 98 Jacksonia scoparia Acacia irrorata 00 Hibbertia obtusifolia Maytenus silvestris cptqlPersoonia sericea Eucalyptus caliginosa Cb ,ucalyptus euginoides Persoonia oleoidesi0 0 Eucalyptus tereticomis Lespedeza jw1cea Allocasuarina littoralis Mehchrus urceolatus 0 Eremophila debilis Ozothamnus diosmifoli us w Bursaria spinosa 0 Pimelea linifolia Acacia nenifolia Eucalyptus dalrympleana 0 0 Eucalyptus bridgesiana
N umber of fires
-1.0 +1.0
Figure 4.29. Species biplot from the DCCA ordination of the combined density of woody trees and shrubs from the Tablelands and Gorge.
A comparison of the ordinations of the three datasets of woody tree and shrub density (Table 4.31) show increasing variance in the data from the Tablelands, Gorge and combined data set. A higher percentage of the variance in the Gorge was explained by all variables, but a reduced amount of variance was explained by the fire variables. Of the variance explained, 66% of the Tablelands variation was attributed to NOF, SIFI and TSLF, with none of the other environmental variables proving to be significant. In the Gorge, none of the fire variables were significant when assessed with forward selection. Elevation and rainfall were the significant environmental variables. In the combined data set, the other environmental variables increased in influence. Physiography was highly significant, demonstrating the difference in species composition between the Gorge and Tablelands, and rainfall and
142 Chapter 4 - Fire frequency and vegetation elevation were also significant. Of the fire variables, NOF and TSLF were significant in the combined data.
Table 4.31. Comparison of the ordination results for woody trees and shrubs for each of the physiographic regions and then combined. Tablelands Gorge Tablelands Gorge Total variance 2.455 3.56 5.579 Variance explained by 1.541 2.023 2.402 all variables Total variance 63% 57% 43% explained by all variables (%) Variance explained by 33% 21% 16% the fire variables (%). Explained variance 53% 37% 37% due to fire variable (%). Significant variables in Number of fires Rainfall Number of fires forward selection Shortest inter-fire Elevation Time since last fire (P < 0.05) interval Physiography Time since last fire Rainfall Elevation
4.4 Discussion
This study revealed a relationship between vegetation composition and structure and the fire frequency categories in GI-RNP. Number of fires, shortest inter-fire interval and time since last fire were all found to influence plant species richness and abundance. This influence tended to differ qualitatively and quantitatively in the two physiographic regions.
4.4.1 Tablelands vegetation and fire frequency
Marked differences due to fire frequency were found in the environmentally similar area of the Tablelands. The LLL fire category representing the areas of least fire occurrence had greater species richness and many species unique only to that fire category. The ordination confirmed this with a significant separation of species, particularly in relation to NOF. The MLS fire category had high richness compared to the HSS and MSS fire categories, and the highest abundance of plants, particularly below 10 m suggesting this to be one of the most favourable fire categories to optimise species richness and composition in the Tablelands. The MSS fire category was more similar to the HSS category, but with a moderate increase in
143 Chapter 4 - Fire frequency and vegetation
shrubs and two species unique only to that category (Callistemon salignus and Acacia implexa).
Overall the vegetation of the Tablelands was characterised by open-forests, with a predominantly grassy understorey in the HSS fire categories, to a more multistorey forest with a developed midstorey in the LLL fire categories. The compositional trend was for a number of unique flora to occur in each of the fire categories (Table 4.6), with the highest number occurring in the LLL category. The high number of species unique to the LLL fire category was thought to reflect the fire regime in that the longer time between fires and the low occurrence of fires was providing more time for a greater variety of species to regenerate and mature. The low number of unique species and the marked lack of small shrubs in the HSS sites reflected the reduced diversity due to high fire occurrence. These sites were dominated by mature Eucalyptus trees, and had few shrubs in the midstorey apart from Pimelea linifolia, which was abundant in the HSS sites and correlated in the multiple regressions with high NOF. This small shrub showed a particular resilience in high NOF conditions. It regenerates from an underground lignotuber after fire and is common and abundant throughout the Tablelands. This contrasts with its response in the Southern Tablelands where Pimelea linifolia was fire-sensitive, with 96% of plants dead two years following a burn (Purdie, 1977a). Specific species responses to fire are investigated in Chapters 6 and 7.
The univariate analysis of common Tablelands species in relation to fire and environmental variables (Section 4.3.1) separated species by specific fire responses. This included those that were increasing in density with increasing NOF and decreasing SIFT, from those that were decreasing in density. The significant correlations between species such as Polyscias sambuccifolia and Olearia oppositifolia to long fire intervals indicated the need for longer intervals for some species to optimise regeneration after fire. These species are both small shrubs of the mid-stratum. They both require long periods (> 10 years) between fires for regeneration and their maximum abundance occurred in sites with these longer fire intervals. In contrast, the abundance of Hibbertia obtusifolia was negatively related to long fire intervals. This species was tolerant of short intervals and exhibited maximum abundance under such conditions. This analysis identified those species that were positively related to fire variables, those that were positively related to the environmental variables and those that were not related to any of the measured variables. Time since last fire was significant for Eucalyptus campanulata and E. saligna. The abundance of E. campanulata declined with increasing TSLF, reaching maximum abundance 4-5 years post-fire. However, there was no significant variation in height or DBH in relation to the fire categories. In contrast, E. saligna showed an increase in abundance, height and DBH with increasing TSLF.
144 Chapter 4 - Fire frequency and vegetation
These results, in relation to the Eucalyptus species that are long-lived, woody, resprouting species, and the relatively small changes in the fire categories, were throught to reflect the density of the juvenile Eucalyptus rather than the mature tall trees. The results reflected the high density of regrowth following fires in 1994, this being the 4-5 year post-fire peak observed for E. campanulata. This demonstrates the need for further individual species analysis, possibly including a stratification of density by size class, however, this was beyond the scope of this present study.
Some of the species showed no significant response to any of the fire variables. These were thought to be responding to variables not measured in this study such as site rockiness or soil fertility. This group included two Eucalyptus species (E. caliginosa and E. nobilis) that were abundant and common throughout the Tablelands study area. Of the other species, Leptospermum polygalifolium retains its fruits in woody capsules and was relatively common and abundant in most sites on the Tablelands except areas of high NOF. This species was also observed regenerating vegetatively from lignotubers so it has the dual regeneration strategy of stored seed protected in woody fruits and vegetative resprouting. Similarly, two other species common and abundant on the Tablelands (Acacia melanoxylon and Ozothamnus diosmifolius) were not common in high NOF areas. Acacia melanoxylon was observed resprouting from the base after fire and regenerated from seed (Chapter 5). These species appeared to be tolerant of a range of conditions, but high NOF and short SIFI in the HSS fire category were not optimum for their survival. Maintaining a fire regime of MLS or less would be more suitable for these species. The individual species analysis showed potential to further analyse the community trends using individual species responses, but these analyses were beyond the scope of this study as outlined in the objectives and beyond the magnitude of the thesis. These trends in species response provided input for future management. Site-specific management plans can thus target particular species and the responses to fire in GFRNP.
The Tablelands ordination revealed that 66% of the floristic variation in woody tree and shrub abundance was related to the fire. This was similar to the variation found in the Sydney sandstone region, where approximately 60% of the floristic variation was due to TSLF and other derivations of the inter fire interval (Morrison et al., 1995a). Like Morrison et al. (1995a) the Tablelands results confirm the hypothesis of Nieuwenhuis (1987) and Bradfield (1981) that recent (< 30 year) fire frequency patterns affect the floristic composition of dry sclerophyll communities. In this study, however, the variation in the Tablelands vegetation was primarily attributed to NOF and SIFT, with the number of fires, as well as the occurrence of short intervals between fires, influencing woody plant abundance.
Fire was found to influence the shrub and juvenile tree structure (Figure 4.7) with the largest variation in height found in the stratum below 10 m. This was particularly evident on the
145 Chapter 4 - Fire frequency and vegetation
Tablelands where the number of shrubs below 10 m declined from the MLS category to the
MSS and HSS categories. The high NOF sites had few small ( 2 m) shrubs and trees, indicating a decline in the numbers surviving frequent fires with short intervals. In contrast, the tall trees (> 10 m) were largely unaffected by fire with the variation between fire categories being small. More woody shrubs were found in sites of lower fire occurrence and the abundance of shrub species was high in those sites where the SIFI were longer.
The structural differences in the sites, particularly in the MLS fire category may also be a function of fire intensity. These sites were burnt in the 1994 fire and visual evidence at the site suggests the fire was of a high intensity. This, in conjunction with a previously long period prior to 1994 of no fire, may have resulted in large amounts of regrowth and hence high numbers of small (< 10 m) tree and shrub regeneration. This is discussed below in relation to species abundance. Further work on the effects of the fire categories on individual species, rather than lumping all the stems together, would have allowed the analysis of particular species responses and the separation of the structural differences of dominant species in particular sites. Also, as the height of each species was measured, data are available to analyse other variables other than the mean height, which may be undertaken in the future.
A decrease in species abundance, particularly shrub abundance, from sites with a low occurrence of fire through to high, was particularly evident. The peak abundance in the MLS fire category was a function of long periods (10-25 years) between fires enabling the build-up of seed in the soil. The long fire interval before the previous fire seemed to have enabled high seed storage, so the recent fire produced the conditions for abundant seedling recruitment, growth and high species regeneration. This was observed throughout the Tablelands and Gorge surveys.
A further potential influence on plant abundance, not researched in this study, is fire intensity and patchiness. Two of the sites in the Tablelands and Gorge, in the MLS category and one in the MSS category were different to the other sites in that fire category. In the field, observations of scorch height on trees and amount of charcoal in the soil indicated that the previous fires in these sites had been recent and of at least moderate intensity. The species abundance in the two Gorge sites in the MLS category was very high especially of Acacia and Correa species. In the Tablelands the sites had a high abundance of Eucalyptus campanulata and Acacia falciformis. Acacia species have been found to have high abundance following high intensity fires (Shea et al., 1979) and this abundance was thought to be related to higher intensity fire. Likewise, this study was undertaken at a scale that the patchiness inherent in any fire was not captured and thus, some species within fire areas may not have burnt in all of the mapped fires. This highlights the need for further research on all aspects of the fire regime
146 Chapter 4 - Fire frequency and vegetation and particularly the interactions between all of the variables of frequency, intensity and season.
The influence of fire on the cover of ground cover species in the Tablelands was lower than on the density of woody trees and shrubs. Of the explained variance, only 22% was due to fire. Shortest inter-fire interval was the most significant of the fire variables in explaining the measured variance. The ordination plots showed large separation between sites in the low (LLL) and moderate (MLS) fire categories. The strength of the environmental variables was apparent in the ordination (Figure 4.14) and topographic index was a greater determinant of ground cover species abundance than any of the other variables except for SIFI. This demonstrated that short SIFI were highly significant for ground cover species and that intervals less than 1-2 years were reducing composition and abundance. Yet, it also demonstrated the strength of environmental variation on ground cover composition particularly topographic index. This measure, representing site exposure and site condition, reflected the strength of site conditions for ground cover species.
4.4.2 Gorge vegetation and fire frequency
There were weaker patterns of species composition and abundance due to the lower fire frequency in the Gorge where there were no high NOF sites. Species identified as being particularly significant in relation to fire in the multiple regression, were Angophora subvelutina, Breynia oblongifolia, Eucalyptus microcorys and E. tereticornis. Shortest inter- fire interval was the significant variable for the trees and TSLF for the shrub (Breynia oblongifolia). Short intervals have the potential to limit the number of tree species growing to a size that can survive subsequent fire. Eucalyptus microcorys and E. tereticornis were all found to have increasing density with increasing fire interval that indicated that short intervals were reducing the number of plants maintaining long-term persistence at sites with short intervals.
The highest diversity of species (Table 4.18) in the Gorge was in the MLL fire category. The MSL fire category had the lowest number of species and only two (Eucalyptus sideroxylon and Pimelea sp.) were unique to this fire category.
The plant and community structure in the Gorge varied with fire occurrence. The highest abundance of shrub species was in the MLS category for shrubs less than 2 m (Figure 4.20). The MSL fire category had the greatest numbers of shrub species under 2 m. In this shrub height class there were many more plants in the classes with short TSLF. This may be reflecting the initial pulse of regeneration in the years following fire. There was little difference in the numbers of small trees (< 2 m) between all of the moderate fire categories.
147 Chapter 4 - Fire frequency and vegetation
For the trees, the height category of 2-10 m showed variation between the fire categories with high tree abundance due to moderate frequency and long intervals. This was further evidence that fire interval had an influence on tree height with long periods since the last fire and longer intervals resulting in more trees in the 2-10 m class.
The Gorge ground cover was significantly influenced by environmental variation, especially rainfall. Of the environmental variables rainfall was the most significant (P < 0.05). The ordination analysis showed a weaker relation with fire when compared with the Tablelands. It was only TSLF that had an influence in the ordination analysis (Figure 4.26) although the variable was not significant. The Gorge ground cover was more highly correlated to rainfall when compared to the Tablelands ground cover that was more correlated to topographic index. However, both of these indicated that environmental variables, rather than fire variables were of significant influence. A study of understorey species in Victoria using multivariate techniques also found little floristic variation between sites (Bradfield, 1981), but structural differences were more evident due to different burning regimes. These were primarily the woody species in the understorey. Herbaceous species were more likely to be responding to the immediacy of environmental conditions in the previous month or season, rather than over longer time-periods. Due to the vast range of herbaceous species surveyed in this study, over such a range of vegetation communities, some of the detail on species responses may not have been explored. More detailed studies would be warranted on the individual responses to fire for specific ground cover species.
The Gorge MSL sites were similar to the Tablelands MSS sites. They were both grassy open- forest or forest with emergent midstorey shrubs. In contrast, the Gorge MSS category had higher richness than the MLS and MSL categories. This was exposed in the ordination with the greater influence of rainfall and decreased influence of the other fire variables except for Sin Shortest inter-fire interval was separating species in the ordination in the Gorge, as found in the Tablelands. This was further evidence that short intervals were reducing opportunities for shrub regeneration and persistence.
The strong influence of the non-fire environmental variables was evident in the Gorge. Despite attempts to constrain the environmental variation, a greater range of variation was incorporated than in the Tablelands survey, due to the inherent nature of the Gorge study region. The variation in woody plant density due to fire was weaker (fire explaining 35% of the variation - Section 4.3.4) in the Gorge when compared to the Tablelands. The floristic composition and abundance were influenced by other environmental variables, of which only some were measured in this study. Bradfield (1981) also found low explained variance when analysing open eucalypt forest variation due to fire, using principal component analysis, and attributed it to the narrow range of edaphic and climate conditions reflected in the selection of
148 Chapter 4 - Fire frequency and vegetation sites. The goal in this study was to constrain site conditions in order to focus on the fire variation in the study region. Obviously there was a range of other site factors that could have influenced the richness and abundance of woody plant species: soil moisture and rockiness have been suggested in other studies (Fensham, 1990) and further work in this area may be required. There is potential for further work relating topographic position, fire interval and the dominance of particular vegetation types (Romme and Knight, 1981). This may be particularly relevant in the Gorge, because of the large topographic variation and the possible link between the fire interval and more exposed slopes that may favour certain vegetation types. However, this was beyond the scope of this study.
4.4.3 Tablelands and Gorge
The results from the combined analysis of the whole study region demonstrated the strong physiographic variation between the Tablelands and the Gorge. Species predominantly occurred in one or other physiographic area. For example, Allocasuarina littoralis occurred on the Tablelands and A. torulosa in the Gorge, possibly reflecting the other environmental variations such as soil fertility and soil moisture, between the regions. Second to the physiographic variation was the strong influence of NOF. It had a strong and highly significant influence on floristic composition over the whole study region (Figure 4.29). The ordination also demonstrated the strong influence of the environmental variables over the whole region. Variation between sites showed a reduction in complexity with increasing frequency of fire. The MSS Gorge sites showed some similarity to the Tablelands HSS sites but tended to have more understorey species especially Jacksonia scoparia. There was increasing shrub numbers as fire frequency declined.
Environmental variation will always be a factor in correlative vegetation studies. The best approach is long-term monitoring of the same sites, which is rarely implemented due to the high on-going costs. A second approach adopted for this study is to obtain fire history data and stratify sites to minimise the natural environmental variation. A third approach is to undertake a random vegetation survey and measure the environmental variation and fire intervals from indicators such as scorch height on trees. This approach does not take into account the effects of fire frequency. Micro scale site differences such as site seepage, nutrient variability and soil depth can affect vegetation, but could not be accounted for in the methods used for this study.
There are also correlations between fire and environmental variables. Areas of high exposure and low moisture are naturally more susceptible to fire ignitions and occurrence. The multivariate approach to vegetation analysis taken addressed some of this overlap.
149 Chapter 4 - Fire frequency and vegetation
4.4.4 Synthesis
Overall, the results demonstrate that increasing fire frequency results in the simplification (in species composition and structure) of vegetation communities in GFRNP. McDonald et al. (1990) outlined a rainforest classification system based on the work of Webb (1959) and Webb et al. (1976) that described complexity in the structural typology of forests. 'Simple' forests tended to be dominated at the canopy level by one or a few species and one or two growth forms, stems were uniform in size, and one or more discrete strata were distinguishable below the tallest stratum. 'Complex' forests contained a number of species in the tallest stratum, a large range of structural features, stems were usually uneven in size and discrete strata were not identifiable below the tallest stratum. The 'simple-complex' was an in-between type.
The vegetation communities in GFRNP can be broadly differentiated using this classification framework. The Tablelands sites with high NOF tended to have a simple structure. The canopies were dominated by two or three species, Eucalyptus dalrympleana, E. bridgesiana or E. caliginosa. There was a low abundance of shrubs and the understorey strata were dominated by grass species.
Vegetation in the areas of moderate NOF could be described as simple-complex. This group contained a larger number of species in the canopy and a greater variety of shrubs in the mid- storey, accompanied by more grasses, herbs and sedges in the ground cover than the sites with a high NOF. The types of shrub species also changed with NOF. Higher NOF sites included shrubs such as Allocasuarina sp., Jacksonia scoparia and Acacia melanoxylon. As NOF declined, sites tended to increase in serotinous species such as Banksia integrifolia and Hakea microcarpa, as well as other Acacia species (A. falciformis and A. irrorata). These species were tolerant of moderate fire-frequencies.
The 'complex' forests were characterised by multistorey forest with a diverse range of canopy species. There was little distinction between strata. In GFRNP the areas of low NOF were typically more complex. There were a range of species in a range of strata and the ground cover included Lomandra, Dianella and Gahnia species. These three structural types identified in GFRNP in this study are shown diagrammatically in Figure 4.30. The full range of vegetation communities in GFRNP includes dry rainforest at the 'complex' end of the spectrum and 'grasslands' at the 'simple' end of the spectrum. As these were not studied, they are not included in Figure 4.30.
These results concur with those of Noble and Slatyer (1981) and Catling (1991) who introduced the model of decreasing habitat complexity with increased fire frequency and the model of decreasing proportions of mixed forest with increasing fire frequency (Figure 1.2).
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Multi-layered, 'complex' forest - many species in many strata.
Three layered 'simple-complex' forest - high abundance with a Three layered 'simple-complex' range of species in all strata. forest - low abundance and reduced species numbers. `Simple' forest - grassy non understorey with few to no shrubs in mid-storey.
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Fire frequency Figure 4.30. Schematic diagram of the trend in vegetation community development with increasing fire frequency resulting in decreasing community complexity.
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4.5 Conclusions
The null hypothesis underpinning this study was that vegetation composition and structure were unaffected by fire frequency. This study has demonstrated the hypothesis can be rejected for the GFRNP, where increasing NOF is associated with a decline in both species richness and abundance. This was particularly evident on the Tablelands where fire effects were more easily separated from environmental variation.
The overall community comparisons in vegetation composition and structure suggested that increasing NOF and decreasing fire intervals (both SIFT and TSLF) resulted in a decline in total species composition and a decline in the abundance of some species. Structurally the largest variation occurred in the shrub layer. Once the eucalypt species are large enough to survive recurring fires, it is more likely the maximum potential height will be obtained, as there is little variation in the maximum height of the communities. However, the mid-stratum was strongly influenced by fire. Recurring fires removed the potential for many shrub species to regenerate, and shrub diversity and abundance decreased. Overall, when sites burn frequently and often, communities can shift towards similar structural and compositional types. With less fire and longer inter-fire intervals, variation between sites in floristic and structure increases.
The influence of fire over the whole study region was subtle and overlain on a backdrop of a long (longer than this study) history of frequent fires. The present vegetated landscape is a construct of a regime reflecting frequent and possibly low intensity fires. It is possible that the variation in vegetation that may have occurred in the past has now been isolated into small refugia (protected gullies). This was observed in the Gorge but not recorded in the vegetation survey possibly due to their small size. In concert with Cary and Morrison (1995) and Morrison et al. (1995a) I consider that the relationship between all components of the fire regime is complex and confounded by other factors such as site features. These results provide data on the present vegetation patterns. Fire management planning also needs to take into account the seed bank that is explored in Chapter 5.
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