Fire variability, as well as frequency, can explain coexistence between seeder and resprouter life histories.
Abstract
Studies in fire-prone Mediterranean ecosystems have repeatedly shown that the mean values of fire regimes (particularly frequency, but also size and intensity) are important for managing sensitive species and maintaining diversity. However, recent studies suggest that invariant fire regimes - that is, those with no variation about the mean value - may not be sufficient to maintain the coexistence mechanisms which could help explain the high levels of species diversity. However, there has been a little examination of the potential mechanisms by which variability in fire regimes might foster coexistence. In these species-diverse ecosystems, fluctuations in fire regimes promote the coexistence of competitively unequal species, thus providing a potential mechanism of coexistence. We examine the role of variability in the length of the interfire interval and ask whether this variability can allow a fluctuation-dependent mechanism, namely the storage effect, to promote the coexistence of species. We focus on dominant trade-offs in fire regeneration strategies (i.e. obligate resprouting vs. obligate seeding) common among Mediterranean plant species and use simulations to explore the inter-relationship between variability in the time between fires and the coexistence of species. Several empirical studies have found that variability in the length of the interfire interval improved diversity - our simulations suggested one mechanism that could explain this result. Variability can greatly increase the regions over which coexistence between two species - a fire obligate seeder and a resprouter - occurs. Synthesis and applications. Mediterranean ecosystems tend to have high plant diversity, and yet the mechanisms maintaining this diversity are often incompletely understood, and thus management actions that aim to promote coexistence may be relying on imprecise information. In general, fire events drive the evolution and maintenance of diversity and are an important management tool. It is high likely that fluctuations or variability in fire are also important, and this suggests that invariant regimes of prescribed burning or fire suppression could be detrimental to the mechanisms that play a role in the maintenance of diversity in these Mediterranean ecosystems. As a result, attention should be paid to historical fire regimes and the variation in fire return times they displayed when developing prescribed burning regimes.