Amazonian forest-savanna bistability and human impact

Abstract

A bimodal distribution of tropical tree cover at intermediate precipitation levels has been presented as evidence of fire-induced bistability. Here we subdivide satellite vegetation data into those from human-unaffected areas and those from regions close to human-cultivated zones. Bimodality is found to be almost absent in the unaffected regions, whereas it is significantly enhanced close to cultivated zones. Assuming higher logging rates closer to cultivated zones and spatial diffusion of fire, our spatiotemporal mathematical model reproduces these patterns. Given a gradient of climatic and edaphic factors, rather than bistability there is a predictable spatial boundary, a Maxwell point, that separates regions where forest and savanna states are naturally selected. While bimodality can hence be explained by anthropogenic edge effects and natural spatial heterogeneity, a narrow range of bimodality remaining in the human-unaffected data indicates that there is still bistability, although on smaller scales than claimed previously.

Resilience of tropical forest and savanna: bridging theory and observation

“…This is consistent with the observation that grass growth is largely suppressed when tree canopy density exceeds a critical value (roughly a Leaf Area Index of three (Hoffmann et al, 2012a)). Moreover, at much larger scales across both African and South American landscapes, it has been noted that the observed burned area is very small in landscapes with more than 40% tree cover (Archibald et al, 2009;Wuyts et al, 2017). Such observations resonate with the idea of a positive feedback in which trees can prevent fire, thus stabilizing a forest state versus a landscape that is maintained open through fire (Staver et al, 2011b;Murphy & Bowman, 2012;Lasslop et al, 2016).…”

“…The model occupies a promising niche that could stimulate new insights in tropical fire-vegetation dynamics because it bridges a gap between observations and theory. On the observational side the work on tropical tree cover that indicates alternative stable states has so far been mostly based on static patterns instead of dynamic ones (Hirota et al, 2011;Staver et al, 2011b;Holmgren et al, 2013;Bucini et al, 2017; but see Wuyts et al, 2017), which hampers the inference of temporal dynamics.…”

“…Beckage et al, 2011;Accatino & De Michele, 2013;Van Nes et al, 2014;Schertzer et al, 2015;Wuyts et al, 2017) that we do provide, and/or do not account for the spatial character of fire-vegetation dynamics (e.g. Van Nes et al, 2014;Lasslop et al, 2016;.…”

“…Various modeling approaches have been developed for this, ranging from simple (Van Nes et al, 2014; to more complex models (Scheiter & Higgins, 2007;Accatino et al, 2010;Lasslop et al, 2016;Wuyts et al, 2017). Here we design a very simple model (Fig.…”

“…Building a convincing case for hypotheses on such large-scale phenomena therefore has to rely on a combination of remotely sensed observations and constrained field experiments with a mechanistic understanding of key processes, brought together to analyze the coherence between these different lines of evidence (Scheffer & Carpenter, 2003). The central hypothesis proposed to explain bistability of savanna and forest states is the existence of a strong feedback between tree cover and fire risk Staver et al, 2011b;Hoffmann et al, 2012b;Murphy & Bowman, 2012;Dantas et al, 2016;Wuyts et al, 2017). The idea is that if tree cover becomes sufficiently dense, it precludes the growth of grasses that serve as an easily ignitable fuel for wildfires (Hoffmann et al, 2012b).…”