Abstract
Fires and herbivores shape tropical vegetation structure, but their effects on the stability of tree cover in different climates remain elusive. Here, we integrate empirical and theoretical approaches to determine the effects of climate on fire- and herbivore-driven forest-savanna shifts. We analyzed time series of remotely sensed tree cover and fire observations with estimates of herbivore pressure across the tropics to quantify the fire-tree cover and herbivore-tree cover feedbacks along climatic gradients. From these empirical results, we developed a spatially explicit, stochastic fire-vegetation model that accounts for herbivore pressure. We find emergent alternative stable states in tree cover with hysteresis across rainfall conditions. Whereas the herbivore-tree cover feedback can maintain low tree cover below 1,100mm mean annual rainfall, the fire-tree cover feedback can maintain low tree cover at higher rainfall levels. Interestingly, the rainfall range where fire-driven alternative vegetation states can be found depends strongly on rainfall variability. Both higher seasonal and interannual variability in rainfall increase fire frequency, but only seasonality expands the distribution of fire-maintained savannas into wetter climates. The strength of the fire-tree cover feedback depends on the spatial configuration of tree cover: Landscapes with clustered low tree-cover areas are more susceptible to cross a tipping point of fire-driven forest loss than landscapes with scattered deforested patches. Our study shows how feedbacks involving fire, herbivores, and the spatial structure of tree cover explain the resilience of tree cover across climates.
Highlights
Large‐scale analyses of the frequency distributions of tropical tree cover indicate that forests and savannas can be alternative stable states (Hirota, Holmgren, van Nes, & Scheffer, 2011; Staver, Archibald, & Levin, 2011; Xu et al, 2016)
We quantified the full feedback between fire and tree cover and showed that it may generate alternative stable states of tree cover across rainfall levels in the tropics
Assessing the strength of the fire–tree cover feedback globally integrates pieces of evidence, ranging in scale from field studies (e.g., Dantas, Batalha, & Pausas, 2013) to pantropical remote sensing analyses (e.g., Hirota et al, 2011; Staver et al, 2011; Xu et al, 2016), that forests and savannas can be alternative stable states separated by tipping points
Summary
Large‐scale analyses of the frequency distributions of tropical tree cover indicate that forests and savannas can be alternative stable states (Hirota, Holmgren, van Nes, & Scheffer, 2011; Staver, Archibald, & Levin, 2011; Xu et al, 2016). They show that savannas become more common with decreasing average rainfall. If positive feedbacks are sufficiently strong, they can maintain alternative stable states. This implies hysteresis: For different initial conditions, the system may end up in different end states under the same environmental conditions. Away from tipping points, relatively large environmental changes may have small effects, but close to tipping points, even small changes in an environmental factor can produce a shift that is hard to reverse (Scheffer, Carpenter, Foley, Folke, & Walker, 2001)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
