Response of vegetation and vertebrate fauna to 23 years of fire exclusion in a tropical Eucalyptus open forest, Northern Territory, Australia

Abstract

This opportunistic study compares the vegetation, fuel loads and vertebrate fauna of part of a 120-ha block of tropical open forest protected from fire for 23 years, and an adjacent block burnt annually over this period. Total fuel loads did not differ significantly between the unburnt and annually burnt sites, but their composition was markedly different, with far less grassy fuel, but far more litter fuel, in the unburnt block. There were major differences between treatments in the composition of trees and shrubs, manifest particularly in the number of stems. There was no overall difference in plant species richness between the two treatments, but richness of woody species was far higher in the unburnt treatment, and of annual and perennial grasses, and perennial herbs in the annually burnt treatment. Change in plant species composition from annually burnt to unburnt treatment was directional, in that there was a far higher representation of rainforest-associated species (with the percentage of woody stems attributable to 'rainforest' species increasing from 24% of all species in the annually burnt treatment to 43% in the unburnt treatment, that of basal area from 9% to 30%, that of species richness from 8% to 17%, and that of cover from 12 to 47%). The vertebrate species composition varied significantly between treatments, but there was relatively little difference in species richness (other than for a slightly richer reptile fauna in the unburnt treatment). Again, there was a tendency for species that were more common in the unburnt treatment to be rainforest-associated species. The results from this study suggest that there is a sizeable and distinct set of species that are associated with relatively long-unburnt environments, and hence that are strongly disadvantaged under contemporary fire regimes. We suggest that such species need to be better accommodated by fire management through strategic reductions in the frequency of burning.

Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

“…Modelled using quantile regression of non-eucalypt basal area and species richness against eucalypt basal area and species richness (s = 0.90, 0.75, 0.50, 0.25, 0.10). For basal area, the relationship was significant for all quantiles examined; for species richness, the relationship was not significant for any of the quantiles examined variation among plot-averaged metrics); (3) there is a considerable time-lag in basal area response of the eucalypt component to environmental drivers, particularly fire (compare Williams et al 1999;Russell-Smith et al 2003;Woinarski et al 2004), so that a lag in mortality of mainly large eucalypts may give the appearance of relatively invariant eucalypt basal area across the fire gradient. The strength of our analysis and conclusions derives from using many plots (n = 127) that are evenly scattered across the fire and rainfall gradient.…”

“…For example, at three tropical savanna sites in northern Australia with similar high rainfall (Solar village, Munmarlary and Kapalga) but different fire frequencies, varying from complete exclusion to frequent burns, there was a concomitant decline in the density of the non-eucalypt component, while eucalypt basal area was either not significantly different or increased in the burnt relative to unburnt plots (Williams et al 1999;Russell-Smith et al 2003;Woinarski et al 2004). The increase in eucalypt basal area at Munmarlary was attributed to an increase in the average size of stems rather than to significant additions AIC Akaike's Information Criterion; AIC c second order AIC for large and small sample sizes; w I the Akaike weight, representing the probability of a model being the 'best' in the candidate set; BA basal area a For each model, DAIC c is the difference between the model's AIC c value and the minimum AIC c of all models in the candidate set.…”

“…By virtue of their dominance and their adaptation to important environmental stressors, such as fire (Gill 1997;Burrows 2002;Prior et al 2006Prior et al , 2009, the eucalypts have a pervasive influence on the tree dynamics of tropical savannas. Where fires are frequent and/or of high severity, eucalypts increase in relative abundance (Russell-Smith et al 1998Williams et al 1999;Woinarski et al 2004), largely because of the loss of noneucalypt elements to fire rather than their competitive exclusion ). Since most tree diversity is contained within the non-eucalypt tree component of these savannas, fire is a key management tool for maintaining this diversity .…”

Fire-Driven Decline of Endemic Allosyncarpia Monsoon Rainforests in Northern Australia

“…Similarly, many regional studies support the observation of recent monsoon rainforest expansion in a variety of regional, climatic and topographic contexts [5][6][7][30][31][32][33][34][35]. In various cases, the observed expansion of monsoon rainforests has occurred under relatively fire-prone conditions (e.g., [32,33,35]), indicating that enhanced growing conditions associated with increased regional rainfall especially from the 1950s [70,71], reduced evaporation from at least the 1970s [72], global CO 2 fertilisation [14,73], but see [74], and possibly grazing interactions through reducing fuel loads, are likely to have been contributory.…”

“…Likewise, many north Australian studies have reported expansion of typically small (<10 ha) patches or larger tracts of monsoon rainforest vegetation within the savanna matrix, particularly under higher rainfall conditions [5][6][7][30][31][32][33][34][35]. In different situations, such expansion has been attributed variously to reduced burning or browsing pressure, increased rainfall in recent decades, possible CO 2 fertilisation, and interactions between these.…”