Advancing satellite-driven monitoring of phenological responses to extreme climatic events in the Alpine space

Climate extremes driven by climate change, such as heatwaves and severe droughts, have significantly impacted the ecosystem of the European Alps. Climate projections indicate that the frequency and intensity of such extreme events in this region will continue to rise. However, it remains unclear how heatwaves and extreme droughts affect the long-term evolution of local vegetation. Long-term time-series of satellite-based vegetation indices and climate records were analyzed to catch effects of climate extremes on spatiotemporal trends of plant phenology. MODIS data from the Alpine space was processed for a 23-year range (2001 to 2023). Correlation between anomalies in the length of the growing season (LOS) and extreme droughts and heatwaves were analyzed. Results indicated that the impact of these climate extremes on plant phenology varies significantly across different phenological stages and regions. Heatwaves led to an earlier onset of SOS primarily in the western high-altitude areas (-0.23 to -0.41 days yr⁻¹), whereas in the lower eastern mountainous regions, they resulted in a later onset of SOS. The consistency analysis between climate extremes and LOS showed that negative LOS anomalies were primarily caused by extreme droughts, which account for 65.4% of cases. Additionally, in drought-dominated areas, approximately 78% of the LOS shortening can be attributed to earlier EOS. Regions under extreme drought stress were mainly located in the southwestern and eastern low-altitude areas of the Alpine space. Furthermore, compound drought-heatwave events significantly shorten LOS across different areas (coincidence rate = 0.68). The study highlighted how remote sensing can advance understanding of mixed effects of extreme droughts and heatwaves on plant phenology considering also the spatial context.