The future distribution of the savannah biome: model-based and biogeographic contingency

Abstract

The extent of the savannah biome is expected to be profoundly altered by climatic change and increasing atmospheric CO 2 concentrations. Contrasting projections are given when using different modelling approaches to estimate future distributions. Furthermore, biogeographic variation within savannahs in plant function and structure is expected to lead to divergent responses to global change. Hence the use of a single model with a single savannah tree type will likely lead to biased projections. Here we compare and contrast projections of South American, African and Australian savannah distributions from the physiologically based Thornley transport resistance statistical distribution model (TTR-SDM)-and three versions of a dynamic vegetation model (DVM) designed and parametrized separately for specific continents. We show that attempting to extrapolate any continent-specific model globally biases projections. By 2070, all DVMs generally project a decrease in the extent of savannahs at their boundary with forests, whereas the TTR-SDM projects a decrease in savannahs at their boundary with aridlands and grasslands. This difference is driven by forest and woodland expansion in response to rising atmospheric CO 2 concentrations in DVMs, unaccounted for by the TTR-SDM. We suggest that the most suitable models of the savannah biome for future development are individual-based dynamic vegetation models designed for specific biogeographic regions.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

Reliability ensemble averaging of 21st century projections of terrestrial net primary productivity reduces global and regional uncertainties

“…Conversely, uncertainties on the sign of NPP remain large for all REA cases in semi-arid regions of southern Africa and Australia. It is a non-trivial result as the response of these ecosystems to climate events like El Niño and La Niña drives the inter-annual variability and the trend of the global terrestrial carbon sink (Bastos et al, 2013;Poulter et al, 2014;Ahlström et al, 2015), while projections indicate a gain of forest ecosystems over savannahs in the future (Moncrieff et al, 2016).…”

Tropical grassy biomes: linking ecology, human use and conservation

“…fire and mammalian herbivory) controls in structuring TGB vegetation has significantly improved our process understanding of the dynamics and limits of these systems [14,53]. But, it has also highlighted the degree to which contemporary dynamics of TGBs, from local to continental scales, are a function of historical contingencies [44,54]. In assessment of regional patterns and dynamics of vegetation, emergent patterns of woody cover can appear almost stochastic, due to the array of structural states possible for a given set of environmental conditions [29].…”

“…It is increasingly appreciated that the functional biogeography of TGBs has critical implications for our capacity to determine the sensitivity and resilience of TGB regions to global change (e.g. [44]), and yet our quantitative understanding of the functional biogeography of TGBs remains limited. This information is needed as our capacity to predict future change will rely on a quantitative representation of the aggregation of the traits that characterize these floras in influencing ecosystem dynamics and responding to environmental variation.…”

“…For the first time, analyses are presented on the biogeography and potential distributions of Asian savannahs [39]. Other significant steps forward in our understanding include: methods for characterizing ancient versus derived grassy biomes [40,41], comparative data on the species diversity of TGB regions across the globe [42], an improved understanding of the complex ecology of herbivory and fire [16,43] and the context-dependent response of vegetation to global change [44], and finally, tools to examine trade-offs in biodiversity, carbon and agriculture to aid land-use planning and policy [10].…”

Climate seasonality, fire and global patterns of tree cover

“…Broad-scale studies have often focused on tropical and subtropical savannas (Sankaran et al 2005, Hirota et al 2011, Lehmann et al 2011, Staver et al 2011, Lehmann et al 2014 to the exclusion of extratropical regions that also support savannas along with other open-canopy systems (Scheffer et al 2012, Ratajczak et al 2014. Studying the environmental relationships that characterize these systems globally is particularly relevant because of projected climate change and the likelihood of resultant shifts in ecosystem structure and distribution (Beckage et al 2006, van der Werf et al 2008, Bond and Midgley 2012, Moncrieff et al 2016.…”