Climate change and deforestation have increased the risk of drought-induced forest-to-savanna transitions across the tropics and subtropics. However, the present understanding of forest-savanna transitions is generally focused on the influence of rainfall and fire regime changes, but does not take into account the adaptability of vegetation to droughts by utilizing subsoil moisture in a quantifiable metric. Using rootzone storage capacity (Sr), which is a novel metric to represent the vegetation's ability to utilize subsoil moisture storage and tree cover (TC), we analyze and quantify the occurrence of these forest-savanna transitions along transects in South America and Africa. We found forest-savanna transition thresholds to occur around a Sr of 550–750 mm for South America and 400–600 mm for Africa in the range of 30%–40% TC. Analysis of empirical and statistical patterns allowed us to classify the ecosystem's adaptability to droughts into four classes of drought coping strategies: lowly water-stressed forest (shallow roots, high TC), moderately water-stressed forest (investing in Sr, high TC), highly water-stressed forest (trade-off between investments in Sr and TC) and savanna-grassland regime (competitive rooting strategy, low TC). The insights from this study are useful for improved understanding of tropical eco-hydrological adaptation, drought coping strategies, and forest ecosystem regime shifts under future climate change.

Forest and savanna ecosystems naturally exist as alternative stable states. The maximum capacity of these ecosystems to absorb perturbations without transitioning to the other alternative stable state is referred to as ‘resilience’. Previous studies have determined the resilience of terrestrial ecosystems to hydroclimatic changes predominantly based on space-for-time substitution. This substitution assumes that the contemporary spatial frequency distribution of ecosystems’ tree cover structure holds across time. However, this assumption is problematic since ecosystem adaptation over time is ignored. Here we empirically study tropical forests’ stability and hydroclimatic adaptation dynamics by examining remotely sensed tree cover change (ΔTC; aboveground ecosystem structural change) and root zone storage capacity (S_r; buffer capacity towards water-stress) over the last two decades. We find that ecosystems at high (>75%) and low (<10%) tree cover adapt by instigating considerable subsoil investment, and therefore experience limited ΔTC—signifying stability. In contrast, unstable ecosystems at intermediate (30%–60%) tree cover are unable to exploit the same level of adaptation as stable ecosystems, thus showing considerable ΔTC. Ignoring this adaptive mechanism can underestimate the resilience of the forest ecosystems, which we find is largely underestimated in the case of the Congo rainforests. The results from this study emphasise the importance of the ecosystem's temporal dynamics and adaptation in inferring and assessing the risk of forest-savannah transitions under rapid hydroclimatic change. 

Tropical rainforests invest in their root systems to store soil moisture from water-rich periods for use in water-scarce periods. An inadequate root-zone soil moisture storage predisposes or forces these forest ecosystems to transition to a savanna-like state, devoid of their native structure and functions. Yet changes in soil moisture storage and its influence on the rainforest ecosystems under future climate change remain uncertain. Using the empirical understanding of root zone storage capacity, we assess the future state of the rainforests and the forest-savanna transition risk in South America and Africa under four different shared socioeconomic pathway scenarios. We find that by the end of the 21st century, nearly one-third of the total forest area will be influenced by climate change. Furthermore, beyond 1.5-2⁰C warming, ecosystem recovery reduces gradually, whereas the forest-savanna transition risk increases several folds. For Amazon, this risk can grow by about 1.5-6 times compared to its immediate lower warming scenario, whereas for Congo, this risk growth is not substantial (0.7-1.65 times). The insight from this study underscores the urgent need to limit global surface temperatures below the Paris agreement.

Moisture originating (i.e., evaporation) from the Amazon basin contributes to the rainfall precipitating over the forest and human-influenced land systems in South America. However, the alarming rate of land use change by landholders in the Amazon – mostly due to agricultural expansion – poses serious threats to regional water cycling. On the one hand, this moisture loss over forests reduces their resilience to future hydroclimatic perturbations (e.g., droughts). Loss of moisture over human-influenced land systems, on the other, threatens agricultural yields. However, the leverage these landholders have over the downwind rainfall is uncertain. Understanding their influence will help us realise the potential of land use change impact on the regional water cycle. In this study, we analyse landholders’ leverage over atmospheric moisture flows and the resilience of forest ecosystems in South America. Using remote-sensing datasets and a process-based moisture tracking model, we track moisture flows from different spatial explicit landholder-dominated regions over to the natural and anthropogenic land systems. We find that of all the moisture originating from small (3.0×103 km3 yr-1), medium (0.6×103 km3 yr-1) and large (4.6×103 km3 yr-1) landholders, nearly 43-56% contributes to the rainfall over the forests. Furthermore, nearly 50% of this evaporated moisture originates from the forests within these landholder-dominated regions. We also find that all landholders equally influence the rainfall precipitating over nearby regions (including their own) and those over the downwind remote actors. Among them, smallholders have a disproportionately larger influence over forests’ rainfall (19-39% more than other landholders’). Despite this, large landholders strongly influence forest resilience in South America, along with their disproportionately larger influence over the agricultural land systems (53-116% more than other landholders’). The results from this study emphasise the need for more stringent forest policies to factor in the influence of deforestation on downwind actors and the need for more effective ecosystem stewardship.