Rootzone storage capacity reveals drought coping strategies along rainforest-savanna transitions

Chandrakant Singh,Ruud Van Der Ent,Lan Wang-Erlandsson,Johan Rockström,Ingo Fetzer

doi:10.1088/1748-9326/abc377

Abstract

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 (S r), 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 S r 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 S r, high TC), highly water-stressed forest (trade-off between investments in S r 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.

Highlights

Rainforests, host vast biodiversity, but they are essential in stabilizing the Earth’s climate by sequestering carbon dioxide [1] and maintaining the global water cycle (e.g. [2])
Using multiple transects in South America and Africa, we analyzed the relationship between tree cover (TC) and storage capacity (Sr)
The currently lowly water-stressed forest areas with low Sr may need to start investing in their root system if a changing hydroclimate brings more frequent droughts, less rainfall, or larger rainfall variability, eventually changing into a moderately waterstressed forest

Summary

Introduction

Rainforests, host vast biodiversity, but they are essential in stabilizing the Earth’s climate by sequestering carbon dioxide [1] and maintaining the global water cycle (e.g. [2]). Global warming and deforestation are causing rising trends in drought frequency, severity, and duration These trends further threaten the rainforests’ ecological integrity and biodiversity [3, 4], increasing the risk of triggering self-amplified forest loss [5–7]. Understanding the coping strategies in the rainforest ecosystem to water-stress (defined here as a deficit in soil water availability inhibiting plant growth) and droughts are important for understanding forest-savanna transition risks [8]. To cope with this water deficit, forest and savanna ecosystems adopt an array of strategies [9], such as adjusting water demand [10], growth rates [8], hydraulic safety margins (stem hydraulics [11] and stomatal conductance [12]) and rooting strategies [13, 14] or combinations of these. On a continental scale, evidence of water-stress driven above- and below-ground forest dynamics based on observational data (including remote sensing) is still lacking

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