The long and short of it: a review of the timescales of how fire affects soils using the pulse-press framework

Authors: Pellegrini AFA, RB Jackson

Abstract

Understanding factors that regulate ecosystem responses to disturbances is a long-standing ecological goal and has been useful in predicting the effects of global change. The response of an ecosystem to a single perturbation does not necessarily predict its response to repeated perturbations—highlighting the ‘pulse’ vs. ‘press’ framework to predict ecosystem responses to single or recurring perturbations, respectively. This framework can help guide hypotheses around how ecosystems may, or may not, respond to changes in disturbance regimes. Here we apply the pulse-press framework to understand fire effects on soils and plant communities. Specifically, we combine a quantitative meta-analysis on the response of soil carbon and nutrients with a literature review of the response of microbial and plant communities. We argue that fire is a cumulative disturbance driving a ‘press’ response and can be both coupled and decoupled from the ‘pulse’ response to a single burn depending on whether the response variable is regulated by the direct effects of heating or the indirect effects of changing plant and microbial communities. Our meta-analysis demonstrates that carbon and nutrient content in mineral soils responded differently because carbon and nitrogen, which are volatilized to the atmosphere in a fire and turn over relatively slowly in soil, are regulated by the long-term losses of plant biomass inputs more so than heating-induced volatilization, and thus change relatively little in a single fire. Contrastingly, the elements calcium, potassium, and phosphorus, which are enriched in ash because of their resistance to volatilization, are regulated by the intermittent pulses into the soil following a fire. A literature review suggests microbial biomass and decomposition activity tended to decline after a single fire because of heat-induced mortality, which was consistent with the effects of repeated burning, likely due to combined effects of repeated heat-induced mortality and lower soil organic matter. Another important ‘press’ response was how nutrient losses shape the nutrient use strategies of plants (tissue stoichiometry and nutrient acquisition strategy) and how changes in plant strategies influence soil responses. Past studies have generally found that species with low nutrient concentrations in biomass, high nutrient recycling before tissue senescence, and a greater reliance on mycorrhizal symbioses tend to increase in abundance with frequent burning. Many of these traits likely lead to slower decomposition and nutrient availability, but few studies have rigorously linked fire-driven shifts in traits with carbon and nitrogen cycling. The processes we have identified are likely to be especially important in tropical ecosystems, where disturbance by fire is shifting rapidly, attributable to changes in land use and climate. Tropical savannas are some of the most responsive landscapes to altered fire regimes, with large changes in soil carbon and nutrients, suggesting that shifts in fire regimes may change the carbon balance of these ecosystems substantially. We propose that more fire-manipulation experiments are needed in tropical forests, especially in moist forests where plants are not as well adapted to fire.