Droughts and the ecological future of tropical savanna vegetation

Abstract

Climate change is expected to lead to more frequent, intense and longer droughts in the future, with major implications for ecosystem processes and human livelihoods. The impacts of such droughts are already evident, with vegetation dieback reported from a range of ecosystems, including savannas, in recent years. Most of our insights into the mechanisms governing vegetation drought responses have come from forests and temperate grasslands, while responses of savannas have received less attention. Because the two life forms that dominate savannas—C3 trees and C4 grasses—respond differently to the same environmental controls, savanna responses to droughts can differ from those of forests and grasslands. Drought‐driven mortality of savanna vegetation is not readily predicted by just plant drought‐tolerance traits alone, but is the net outcome of multiple factors, including drought‐avoidance strategies, landscape and neighborhood context, and impacts of past and current stressors including fire, herbivory and inter‐life form competition. Many savannas currently appear to have the capacity to recover from moderate to severe short‐term droughts, although recovery times can be substantial. Factors facilitating recovery include the resprouting ability of vegetation, enhanced flowering and seeding and post‐drought amelioration of herbivory and fire. Future increases in drought severity, length and frequency can interrupt recovery trajectories and lead to compositional shifts, and thus pose substantial threats, particularly to arid and semi‐arid savannas. Synthesis. Our understanding of, and ability to predict, savanna drought responses is currently limited by availability of relevant data, and there is an urgent need for campaigns quantifying drought‐survival traits across diverse savannas. Importantly, these campaigns must move beyond reliance on a limited set of plant functional traits to identifying suites of physiological, morphological, anatomical and structural traits or “syndromes” that encapsulate both avoidance and tolerance strategies. There is also a critical need for a global network of long‐term savanna monitoring sites as these can provide key insights into factors influencing both resistance and resilience of different savannas to droughts. Such efforts, coupled with site‐specific rainfall manipulation experiments that characterize plant trait–drought response relationships, and modelling efforts, will enable a more comprehensive understanding of savanna drought responses.

Assessing the Response of Ecosystem Water Use Efficiency to Drought During and after Drought Events across Central Asia

“…In this study, a continuous drying period was found from 2000 to 2008, and then the drought trend eased in 2009 ( Figure 2); however, the WUE was still very low in 2009 and 2010. This low WUE was due to the drought legacy effect that was found in a large body of previous studies, which reported that the effects of drought on ecosystem WUE exhibit a legacy effect [6,24,56]. However, most of these studies compared the relationship between WUE and drought in the previous year.…”

“…This finding indicates that the WUE was affected by the drought in the previous one and two years in most vegetation areas during the drought periods. Previous studies suggested that arid plants respond to a moisture restriction environment by reducing aboveground biomass or even hibernating in consecutive dry years, and the biomass increases rapidly when the moisture is sufficient [56]; however, serious declines in soil moisture cause successive years of drought, and one year of recovery is not sufficient to provide the moisture to meet the basic physiological needs for vegetation growth. This phenomenon may explain the fact that the WUE was still very low in 2009 and 2010, although the drought ended in 2008.…”

Below-surface water mediates the response of African forests to reduced rainfall

“…Plants exhibit different strategies in the face of dry conditions. For example, anisohydric species are at risk of hydraulic failure as a result of xylem cavitation while isohydric species regulate stomata and thereby maintain plant water potentials to avoid cavitation, but are at risk of carbon starvation from depletion of carbohydrate reserves (Sala et al 2012, Klein 2014, Sankaran 2019). Here, we focused on periods where annual rainfall was lower than its annual average and not specifically on drought.…”

Rapid recovery of ecosystem function following extreme drought in a South African savanna grassland

“…Empirical data and model projections suggest that regional‐scale droughts are increasing in frequency, duration, and magnitude across the globe (Dai , Trenberth , Cook et al ). These more severe and prolonged droughts have the potential for large and long‐lasting effects on ecosystem structure and function (Du et al , Sankaran and Staver ). To maintain function in response to extreme drought, ecosystems must exhibit either high resistance during drought (e.g .…”