Fire severity, feedback effects and resilience to alternative community states in forest assemblages
Alternative stable state theory is often used to explain the occurrence of flammable vegetation adjacent to less flammable vegetation where fire regimes mediate the shift between states. In 2002 an extreme landscape scale fire burnt extensive areas of forests in eastern Australia, including rainforests that are rarely severely burnt. This unique event allowed us to test long-held assumptions that predict landscape scale change after major disturbance. We tested three assumptions for detecting alternative community states; (1) that the scale of the event was large enough to remove canopy dominants, (2) fire feedbacks, both positive and negative are present, and (3) shifts in the floristic composition of communities are detected. We also examined whether high severity fires resulted in a community shift from less flammable to more flammable vegetation (e.g. from rainforest to wet sclerophyll vegetation), by examining floristic composition of vegetation communities (rainforest, wet sclerophyll forest, and dry sclerophyll forest) when burnt at different fire severities (high and low). Conversely, we tested whether there was a state shift from flammable to less flammable vegetation communities in sclerophyll forests long unburnt. In addition, we determined if there was any evidence that antecedent fire regimes and fire severity influenced flammability feedbacks. Severe fire caused significant ongoing disruption to forest canopies and fire effects were still detectable some 7 years after fire. Whilst some pyrogenic environmental feedbacks were detected from historic fire regimes, we found no shifts in the floristic composition or pyrogenic traits of forests burned at high severity. Medium-term (30 year) fire exclusion did not result in the sclerophyll forest becoming more pyrophobic although some fire-cued species senesced in the absence of fire. Contrary to expectation, we found no evidence that the floristic composition of less flammable vegetation burnt at high severity became more similar to flammable vegetation burnt at low severity or that were unburnt. Conversely, with more than 30 years of fire exclusion there was no evidence that the sclerophyllous communities became more floristically similar to rainforest. We have shown that species assemblages in warm-temperate rainforest were resilient to a catastrophic fire event and propose that these forests are unlikely to represent alternative community states driven by fire alone.
Fire, soil fertility and delayed seed release: a community analysis of the degree of serotiny
Delayed seed release (serotiny) is a convergent plant trait in fire-prone regions of the world but explaining the degree of serotiny has remained elusive because of the paucity of community data. Selective forces involving seed predators, fire and soil nutrients have been suggested as factors influencing serotiny. We tested whether protection of seeds and/or synchronized dispersal were associated with different levels of serotiny and if resprouting ability influences selection for strong serotiny. We compared the numbers and abundance of 146 woody species with delayed dispersal among five community types varying in combinations of fire severity, fire frequency, soil fertility and seed predators. The strength of the relationship between levels of serotiny and environmental factors was tested among community types ranging from rainforests to heathlands. Highest levels of serotiny were recorded in low nutrient shrublands with intermediate fire return intervals that burn at high severity, while the lowest were recorded in high nutrient, low flammability forests. Both protection of seeds and synchronized seed release were related to fire effects in nutrient-limited environments. Strong serotiny is prominent in species killed by fire whereas weak serotiny is more common in resprouting species. Recruitment failure in the inter-fire interval appears to drive selection for strong maternal care of seeds and synchronized seed dispersal in fire-prone environments. Weak serotiny is proposed as a bet-hedging strategy that relies on resprouting after fire for population persistence and higher probability of inter-fire recruitment. The spectrum of serotiny (weak to strong) in these communities is proposed to be driven by the interactive effect of both fire and soil nutrients on the selection for delayed seed dispersal.
Resprouting as a key functional trait: how buds, protection and resources drive persistence after fire
Resprouting as a response to disturbance is now widely recognized as a key functional trait among woody plants and as the basis for the persistence niche. However, the underlying mechanisms that define resprouting responses to disturbance are poorly conceptualized. Resprouting ability is constrained by the interaction of the disturbance regime that depletes the buds and resources needed to fund resprouting, and the environment that drives growth and resource allocation. We develop a buds-protection-resources (BPR) framework for understanding resprouting in fire-prone ecosystems, based on bud bank location, bud protection, and how buds are resourced. Using this framework we go beyond earlier emphases on basal resprouting and highlight the importance of apical, epicormic and below-ground resprouting to the persistence niche. The BPR framework provides insights into: resprouting typologies that include both fire resisters (i.e. survive fire but do not resprout) and fire resprouters; the methods by which buds escape fire effects, such as thick bark; and the predictability of community assembly of resprouting types in relation to site productivity, disturbance regime and competition. Furthermore, predicting the consequences of global change is enhanced by the BPR framework because it potentially forecasts the retention or loss of above-ground biomass.
Fire severity and nutrient availability do not constrain resprouting in forest shrubs
Plants often survive disturbances such as fire by resprouting which involves having both protection traits and carbohydrate storage capacity. Protection traits not only act directly to insulate meristems but also prevent combustion of carbohydrate stores. Rapid stem growth also allows replenishment of carbohydrate stores ensuring persistence through another event. Resource availability may, however, constrain the ability to develop resilience to high-severity fires through either nutrient limitation or light limitation. We tested whether fire severity influenced resprouting ability of woody plants in two contrasting environments, low nutrient dry sclerophyll forest and more fertile wet sclerophyll forest. We tested which fire protection and growth traits were associated with resprouting ability (27 species) and resprouting vigour (16 species). Fire severity did not limit the ability of most species to resprout in either forest type. There was no generalized protection syndrome for surviving top kill, but combinations of bud protection and growth together with storage capacity appear to drive resprouting ability. In nutrient-limited forests, low specific leaf area (SLA) may reduce stem growth in resprouters, causing more reliance on bud protection through bark thickness. Conversely, in the more fertile forests, where light becomes limiting with time-since-fire, high SLA appears to increase the capacity for rapid stem growth with less emphasis on developing thicker bark. These different syndromes appear to be adaptive as fire severity did not influence survival in either forest type.