Land-cover and management modulation of ecosystem resistance to drought stress

Drought events are projected to become more severe and frequent across many regions in the future, but their impacts will likely differ among ecosystems depending on the capability of ecosystem to maintain functioning during droughts, i.e., the ecosystem resistance. Different plant species have diverse strategies to cope with drought. As a result, responses of different vegetation types have been found to be divergent for similar levels of drought severity. However, it remains unclear whether such divergence is also caused by different drought duration, climatological settings, or co-occurring compound events, etc.

Here, we evaluate vegetation resistance using different proxies for vegetation condition, namely the Vegetation Optical Depth (SMOS L-VOD) data from ESA’s Soil Moisture and Ocean Salinity (SMOS) passive L-band mission and EVI and kNDVI from NASA MODIS. L-VOD has the advantage over more commonly used vegetation indices (such as kNDVI, EVI) in that it provides more information on vegetation structure and biomass and suffers from less saturation over dense forests compared (Wigneron et al., 2020). We apply a linear autoregressive model accounting for drought, temperature and memory effects to characterize ecosystem resistance by their sensitivity to drought duration and temperature anomalies. We analyze how ecosystem resistance varies with land cover across the globe and investigate the modulation effect of forest management and irrigation. Furthermore, estimates of ecosystem resistance obtained from a similar methodology are compared between L-VOD, kNDVI and EVI.

We find that regions with higher forest fraction show stronger ecosystem resistance to extreme droughts than cropland for all three vegetation proxies. L-VOD indicates that primary forests tend to be more resistant to drought events than secondary forests, but this phenomenon cannot be detected in EVI and kNDVI. This is possibly related to their saturation in dense forests. In tropical evergreen deciduous forests, old-growth trees tend to be more resistant to drought than young trees from L-VOD and kNDVI. Irrigation increases the drought resistance of cropland substantially.

These results suggest that ecosystem resistance can be better monitored using L-VOD in dense forests and highlight the role of forest cover, forest management and irrigation in determining ecosystem resistance to droughts.

 

Wigneron, J.-P., Fan, L., Ciais, P., Bastos, A., Brandt, M., Chave, J., Saatchi, S., Baccini, A., and Fensholt, R.: Tropical forests did not recover from the strong 2015–2016 El Niño event, Science Advances, 6, eaay4603, https://doi.org/10.1126/sciadv.aay4603, 2020.