Uncovering Dryland Woody Dynamics Using Optical, Microwave, and Field Data—Prolonged Above-Average Rainfall Paradoxically Contributes to Woody Plant Die-Off in the Western Sahel

Paulo N Bernardino,Stéphanie Horion,Rasmus Fensholt,Ilié Storms,Wanda De Keersmaecker,Ben Somers,Martin Brandt,Jean-Pierre Wigneron,Jan Verbesselt

doi:10.3390/rs12142332

Abstract

Dryland ecosystems are frequently struck by droughts. Yet, woody vegetation is often able to recover from mortality events once precipitation returns to pre-drought conditions. Climate change, however, may impact woody vegetation resilience due to more extreme and frequent droughts. Thus, better understanding how woody vegetation responds to drought events is essential. We used a phenology-based remote sensing approach coupled with field data to estimate the severity and recovery rates of a large scale die-off event that occurred in 2014–2015 in Senegal. Novel low (L-band) and high-frequency (Ku-band) passive microwave vegetation optical depth (VOD), and optical MODIS data, were used to estimate woody vegetation dynamics. The relative importance of soil, human-pressure, and before-drought vegetation dynamics influencing the woody vegetation response to the drought were assessed. The die-off in 2014–2015 represented the highest dry season VOD drop for the studied period (1989–2017), even though the 2014 drought was not as severe as the droughts in the 1980s and 1990s. The spatially explicit Die-off Severity Index derived in this study, at 500 m resolution, highlights woody plants mortality in the study area. Soil physical characteristics highly affected die-off severity and post-disturbance recovery, but pre-drought biomass accumulation (i.e., in areas that benefited from above-normal rainfall conditions before the 2014 drought) was the most important variable in explaining die-off severity. This study provides new evidence supporting a better understanding of the “greening Sahel”, suggesting that a sudden increase in woody vegetation biomass does not necessarily imply a stable ecosystem recovery from the droughts in the 1980s. Instead, prolonged above-normal rainfall conditions prior to a drought may result in the accumulation of woody biomass, creating the basis for potentially large-scale woody vegetation die-off events due to even moderate dry spells.

Highlights

Dryland ecosystems support the livelihood of almost one-third of the human population and dominate inter-annual dynamics in the global land carbon sink [1,2,3]
Understanding how water availability and soil nutrients buffer woody vegetation against droughts is pivotal for ecosystems characterized by recurrent negative rainfall anomalies and, this suggests that attention has to be given to other variables than water availability to fully assess the resilience of dryland ecosystems
We used novel long-term passive microwave data to compare events of negative woody vegetation anomalies in Senegal from 1989 to 2017, and optical data together with a simultaneous autoregressive model to determine the drivers of distinct die-off severity and recovery spatially

Summary

Introduction

Dryland ecosystems support the livelihood of almost one-third of the human population and dominate inter-annual dynamics in the global land carbon sink [1,2,3]. The water dependency of the Sahelian vegetation [6,7] makes it susceptible to changes in rainfall patterns, with a direct impact on the livelihood of the local population [3]. This does apply for the herbaceous vegetation, which serves as fodder for livestock, and for the woody vegetation which serves, for example, as a source of fuelwood, construction material, fruits, shelter, and livestock fodder [3]. Given the crucial role of woody plant resources for the livelihoods of local populations [3], it is of great importance to understand how woody vegetation endures and recovers from drought events, and which factors are more important in modulating vegetation response

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