Ecosystem resistance to concurrent and propagated soil moisture droughts

Soil moisture droughts are defined by the prolonged, abnormally dry conditions in the soil. These extreme dry events directly impact terrestrial ecosystem activities like photosynthesis, biomass production, rehydration, and carbon uptake, which can be monitored at regular intervals using satellite-retrieved different vegetation descriptors such as solar-induced fluorescence, leaf-area index, vegetation optical depth, and gross primary productivity, respectively. While previous studies have primarily focused on the vegetation anomalies during drought and its recovery time, very little attention was put on how the ecosystem resistance can vary in terms of different vegetation descriptors. The resistance is defined as the ratio between the maximum perturbance in vegetation and the time taken to reach it. It can infer which ecosystem activity is more vulnerable to drought events or can persist through such abnormal dry conditions.

Previous investigations have demonstrated that soil moisture droughts are frequently initiated by meteorological droughts at the sub-seasonal-to-seasonal scale. Therefore, the onset of a soil moisture drought event coincides with or succeeds by a meteorological drought. Alongside, a soil moisture drought can also arise due to a , primarily overserved over dryland regions, which depends on the evapotranspiration constraint imposed by the regional soil moisture deficiency. The state of land (dry-down rate) and atmosphere (specific humidity and vapour pressure deficit) differ across these conditions under which the soil moisture drought occurs. Due to this, the expected response of the terrestrial ecosystem under the three scenarios of soil moisture drought would also be different. The current work examines the effects in different vegetation descriptors due to soil moisture droughts occurring under three situations: 1) soil moisture and meteorological droughts co-occurring, 2) delayed onset of soil moisture drought from the onset of meteorological drought, and 3) self-propagating soil moisture droughts. While the concurrence of meteorological and soil moisture drought would be severely devastating for agricultural practices, self-propagated and meteorological drought-driven soil moisture drought could pose a continuous threat to the natural ecosystems. We hypothesise that the anomalies in land-atmosphere conditions during a drought govern the ecosystem resistance. Since the initiation of soil moisture drought could vary, as mentioned earlier, the subsequent resistance by an ecosystem would also differ. In addition, we checked how these associations vary for different vegetation descriptors for different plant-functional types (croplands, grasslands, shrublands, and evergreen, deciduous and mixed forests). Our study encompasses the soil moisture drought events that occurred at a seasonal scale during the period of 2003-2020, which were identified using the GLEAM data. The vegetation descriptors are acquired from multi-satellite sources.

This study will help to infer how land and atmosphere anomalies jointly attribute to the ecosystem state in subsequent times and how much these impacts vary in terms of different vegetation descriptors. A region-specific global-level analysis would also highlight the most vulnerable regions, where a seasonal early-warning system can be designed using the inferences drawn from the current study looking into the land and atmospheric anomalies during a drought event.