Understanding the Impact of Multi-Year Droughts on Vegetation: An Observational and Model Approach

Multi-year droughts (MYDs), droughts lasting more than a year, have devastating effects on vegetation. Due to climate change, MYDs are expected to become more frequent and intense, making it crucial to assess their impact on vegetation accurately.

In this study, we combined MODIS satellite observations, ERA5 meteorological reanalysis data, and the LPJmL dynamic vegetation model to evaluate the sensitivity of vegetation to droughts and to quantify the impact of MYDs on seven types of vegetation in six different regions across the globe during the 21st century. To measure the response of vegetation to drought, we used the standardized Enhanced Vegetation Index (EVI) and compared this to EVI climatology.

As anticipated, the overall impact of MYDs on vegetation was negative, but our findings revealed significant spatial and temporal variations with areas showing significant greening during MYDs (around 35% of the world). In general, shrublands experienced the largest decrease in greenness, while trees flourished. The natural water availability of a region is the primary factor influencing vegetation response to MYDs.  Vegetation in water-limited areas tends to suffer during MYDs, whereas vegetation in energy-limited areas thrives as long as sufficient water is available. Compared to normal droughts (NDs), MYDs generally caused stronger negative EVI anomalies.

To address the limitations of the short observational record and the relatively low number of MYDs during the 21st century, we extended the analysis back to 1901 using the LPJmL-5 dynamic global vegetation model. Simulating vegetation dynamics over this 120-year period allowed us to increase the number of MYDs available for study, improving the statistical robustness of our results.