Temporal changes in Ecosystem Resistance and Resilience to Compound Drought and Heat Extremes

In recent decades, compound drought and heat-extreme (CDHE) events have garnered attention due to their amplified impacts on the food-energy-water nexus. In comparison to individual extremes, these short-duration compound events severely impede the terrestrial ecosystem, leading to perpetual damage, yield loss, and mortalities. During these events, plants experience xylem embolism, resulting in a reduced water transport capacity. Alongside, elevated heating intensifies the hydric stress resulting from soil dryness through cascading land-atmosphere interactions. This results in rising leaf-level evaporative demand and canopy temperature, driving stomatal closure, which in turn reduces carbon uptake and increases desiccation. However, the impact of these extreme events varies across plant-functional-types (PFTs), primarily due to differences in hydraulic and carbon-economy traits. On the other hand, plants can adjust their thermal tolerance, structure, and stomatal sensitivity by experiencing frequent perturbances; such acclimation alters their later responses. Therefore, there is not only a need to understand how terrestrial ecosystems varyingly respond to CDHE events, but it is also essential to investigate whether there are any temporal changes in their response.

In this study, we use rootzone soil moisture from GLEAM and near-surface air temperature from ERA-5 to identify CDHE events that persist for at least 5 consecutive days during the growing season during 2001-2021 globally. Then, we estimate the resistance and resilience of four distinct PFTs in the context of CDHE. These are: forests (woody), shrublands (non-forest-woody), grasslands (non-woody and natural), and croplands (non-woody and managed). We estimated resistance as the ratio between normalised loss (maximum perturbation in vegetation) and tolerance period (the time taken to reach maximum perturbation from its onset). Resilience is articulated as the recovery rate up to the pre-drought level following the tolerance period. For this purpose, we have acquired daily gridded datasets of gross primary productivity (GPP) and evapotranspiration (ET) from X-BASE and estimated the ecosystem water-use efficiency (WUE). It represents the coupled carbon-water exchange of vegetation at ecosystem-level. Since it is a flux ratio rather than a structural or radiometric index, it captures changes in plant function under environmental stress in ways that greenness metrics cannot. Under drought or heatwaves, ET declines faster than GPP in water-limited regions, resulting in momentary increases in WUE, followed by sharp decline as stress continues to increase. This bidirectional sensitivity is beneficial for analysing stomatal behaviour. Earlier studies have reported that WUE spontaneously responds to stomatal regulation and is also able to capture stress signals across woody and non-woody vegetation.

Outcomes of this study exhibit significant changes in ecosystem resistance and resilience during the last two decades; however, the magnitude of alterations varies across PFTs. During the period of tolerance and recovery, changes in WUE can result from physiological adjustments that alter photosynthesis per unit water loss, and changes in surface partitioning that alter the fraction of ET attributable to plants. Thus, we also evaluate the contribution of physiological coupling and hydrological partitioning (between vegetation and non-vegetative evaporation) in WUE alterations during tolerance and recovery periods, and found that these contributions also exhibit significant temporal changes.