Stoichiometric patterns in the responses of organisms and ecosystems to fire

Vegetation fire is an important driver of ecological and biological structure and function on the Australian continent. An enhanced understanding of fire’s role in this context will be essential as the frequency, severity and extent of fire increases with climate change. The effects of burning are complex and difficult to predict; however, fire regimes might affect the balance between biologically-essential elements, such as carbon (C), nitrogen (N) and phosphorus (P), in consistently predictable ways. The conceptual framework of ‘ecological stoichiometry’ is explicitly concerned with the balance of elements and its influence over organisms and ecosystems, and thus seems ideally-suited for fire ecology. However, there have been very few explicitly stoichiometric investigations into fire’s effects, and none that take a whole-ecosystem approach, incorporating soil, plants, micro-organisms and invertebrates. The following study tested the overarching hypothesis that different fire regimes produce distinctive stoichiometric balances between biologically-essential elements in soil, and that these balances influence the effects of fire regime on the properties of ecosystems and the organisms within them. A meta-analysis and four experiments were performed to test this hypothesis across varying degrees of ecological complexity. The primary questions addressed by these components were (1) are the effects of fire on the C:N:P stoichiometry of the soil consistent on a global scale; (2) does the stoichiometric signature of fire regime change the type or extent of nutrient limitation for plant growth; (3) how do fire-induced stoichiometry shifts in soil and litter influence the stoichiometry of microbes and litter invertebrates, and the community characteristics of litter invertebrates; (4) how are the effects of fire regime on litter decomposition, and the relative roles of micro-organisms and invertebrates in decomposition, influenced by fire-induced litter stoichiometry shifts; and (5) does the stoichiometric signature of fire in ecosystems, and its potential implications, extend to other biologically-essential elements (i.e. sodium [Na], potassium [K], magnesium [Mg] and sulfur [S])? The global-scale meta-analysis found that fire consistently reduces C:P and N:P ratios in soil and plant litter (Chapter 2), which set the scene for Chapters 3–6, as it showed that any stoichiometric mechanisms influencing fire’s effects could be applicable all around the world. Chapters 3–6 report the results of four original experiments which addressed research questions 2–5. These experiments were carried out using a long-term prescribed burning trial in a wet eucalypt forest at Peachester, south-east Queensland, Australia, which consisted of three replicated fire regime treatments: burned every two years since 1972 (2yB), burned every four years since 1972 (4yB) and no burning since 1969 (NB). Soil in the 2yB treatment tended to have lower total and soluble C:P and N:P ratios compared to the NB and 4yB treatments. However, this did not correspond to altered nutrient limitation status for the growth of Eucalyptus pilularis (the dominant tree species at Peachester) seedlings, which were co-limited by P and at least one other nutrient, regardless of fire history. Phosphorus cycling was clearly enhanced relative to N cycling in the 2yB treatment, compared to the NB and 4yB treatments, as indicated by the lower C:P and N:P ratios of litter in the 2yB treatment. This had important implications for soil and litter micro-organisms and particularly surface active invertebrate fauna (Chapter 4). The N:P ratios of leaf litter microbial biomass tended to be lower in the 2yB treatment relative to the NB treatment, while invertebrate community biomass C:P and N:P ratios were respectively 18.2% and 18.8% lower in the 2yB treatment than in the NB treatment. This coincided with clear differences in the community composition of invertebrates on the level of order, and of Coleoptera (beetles) on the level of morphospecies, which might suggest fire-driven re-assemblage of invertebrate and beetle communities along stoichiometric lines. Unexpectedly, there was some evidence of P-depletion in litter in the 4yB treatment, which might have been driven in part by leaky P cycling following fires. This generated a strong state of P-limitation for microbially-driven litter decomposition (Chapter 5). Invertebrate-driven decomposition was not sensitive to P-constraints, so the positive effect of invertebrates on decomposition increased as microbial P-constraints increased. Invertebrate-driven decomposition also seemed to be subtly linked to invertebrate community composition on the level of order. In Chapter 6, Na, K, Mg and S were incorporated into the investigation, and it was revealed that the stoichiometry of these elements in soil is affected by fire regime in a manner consistent with their potential to turn to gas during fires, much like C, N and P. Some of these effects were present in litter and in the Nitidulid beetle Thalycrodes pulchrum (Blackburn), which had higher P content and P:Na ratios and lower Na:K ratios in the 2yB treatment relative to the NB treatment. Further, T. pulchrum abundance was related in various ways to these ‘multi-element’ stoichiometric ratios, suggesting that Na, K, Mg and S should be considered in future studies of fire’s stoichiometric effects. However, this study indicated that P is a ‘master element’ at Peachester due to its scarcity relative to other elements, and it was fire’s ability to ease or exacerbate this imbalance that underpinned many of fire’s stoichiometric effects. Thus, while many factors likely contributed to fire’s ecological effects at Peachester, the results of this study support the overarching hypothesis. This study provides a novel stoichiometric framework for predicting and interpreting the ecological effects of changing fire regimes. Further, it reveals how the physico-chemical properties of elements on an atomic or molecular level can influence the effects of fire at multiple scales of biological organisation, from organisms to ecosystems, and over timeframes ranging from the moment of heat-induced mortality, combustion and volatilization, to post-fire regeneration, ecological succession and, perhaps, to the depths of evolutionary time.