Comparing vegetation impacts of single vs. consecutive dry events

Dryness can negatively affect vegetation functioning, including both immediate effects as well as effects extending beyond the duration of the dryness. Such legacy effects are for example related to hydraulic damage of plants or a reduced amount of leaves. While the impact of singular dry events are relatively well-studied, the impact of consecutive events on ecosystem functioning is less understood. Our study hypothesis here is that legacy effects may be more likely after consecutive dry periods as the vegetation adaptation potential (e.g. through carbon reserves) is exhausted after a previous dry period. In particular, we follow a four-step approach: (1) identify dry events through dry soil moisture, using a reanalysis soil moisture dataset from 2000 to 2023; (2) determine single and consecutive events by examining the temporal proximity of other dry events to a primary event. A single event is defined as a dry period with no subsequent or preceding dry events occurring within a two-year window, whereas consecutive events are further classified based on when, during this two-year period, another event occurs; (3) assess vegetation responses to single versus consecutive dry events, and their recovery, using anomalies of the Normalised Difference Vegetation Index (NDVI); and (4) use generalized additive models (GAMs) to explore the aspects contributing to vegetation response and recovery. We consider dry event characteristics, including timing, magnitude, and hydrological conditions during the event, as well as static variables such as climate and vegetation type. Preliminary results indicate that severe previous dry events events can induce legacy effects on vegetation, with impacts comparable in magnitude to those driven by dry event characteristics and static environmental variables. Notably, these legacy effects manifest as both positive and negative responses. By understanding how ecosystems are shaped by recurring climatic extremes, this research aims to provide insights for ecosystem response and management in a changing climate with more frequent dry periods.