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

Abstract:

Dryland ecosystems have to deal with droughts frequently, but woody vegetation is often able to recover once precipitation returns to before-drought conditions. Climate change, however, may impact woody vegetation resilience as more extreme and frequent droughts are expected. Therefore, it is necessary to better understand how woody vegetation responds to drought events. We used a phenology-based remote sensing approach, coupled with field data, to estimate the severity of - and recovery from - a large scale drought-induced die-off event that occurred in 2014-2015 in Senegal. Novel low and high-frequency passive microwave vegetation optical depth (VOD), and optical MODIS data, were used to estimate woody vegetation dynamics. Next, we evaluated the influence of soil, human-pressure, and pre-drought vegetation dynamics over the woody vegetation mortality and recovery. Although the 2014 drought was not as strong as the ones in the 1980s and 1990s, the die-off in 2014-2015 caused the highest dry season VOD drop for the studied period (1989-2017). Soil physical characteristics highly affected die-off severity and post-disturbance recovery, and biomass accumulation prior to the drought (i.e., in areas that benefited from above-normal rainfall conditions before the 2014 drought) was the most important factor affecting die-off severity. Here, we provided new evidence supporting a better understanding of the “greening Sahel”, suggesting that increasing trends in woody vegetation biomass are not always a sign of stable ecosystem recovery. Instead, prolonged periods of favorable rainfall conditions prior to a drought, leading to the accumulation of woody biomass, may create the basis for potentially large-scale woody vegetation die-off events due to even moderate dry spells.