Canopy spectral responses of temperate forests to late spring frost and hot drought events assessed with Sentinel-2 NDVI time series

Extreme climatic events (ECEs) are projected to increase due to climate change, but we still have limited understanding of how these events affect the functioning of forest ecosystems. Each species may react differently to ECEs, depending on their ecology, but we lack a regional perspective on these responses. Here we tracked intra-annual changes in the canopy greenness (i.e. NDVI from Sentinel-2 imagery) of 16 tree species growing within 3000 km² of forests of the Italian Alps. The study region was subject to a late spring frost event in May 2019, and a hot drought in July 2022, allowing us to quantify species responses to ECEs by comparison of seasonal trends in NDVI observed over the period 2018–2024. The effects of 2019 frost were very localized and mainly affected the canopy spectral response and phenology of Fagus sylvatica L. in areas around 1000 m a.s.l.. There, trees had developed buds and some juvenile leaves when frost occurred, resulting in the wilting or dropping of the earliest leaves, and slower green-up phase but no lasting impacts. The hot drought had its largest impact on Quercus ilex L. forests growing at low elevations: there was a clear decrease in canopy greenness from July onwards in 2022, but no residual impacts were observed the following years. At higher elevations, some species had unusually green canopies in response to the heatwave suggesting they benefitted from warmer conditions.