Spatial and temporal patterns of early leaf discoloration in Switzerland’s European beech forests

Le résumé fourni par la source

The onset of leaf discoloration in deciduous trees is primarily triggered by seasonal declines in temperature and photoperiod. However, severe drought conditions can induce early leaf discoloration and shedding. Early discolored trees may be more susceptible to crown dieback and mortality following a drought. These persistent legacy effects of drought underlie efforts to understand the environmental drivers of early discoloration but also complicate ongoing discoloration monitoring. Here, field observations of early leaf discoloration in Switzerland’s European beech (Fagus sylvatica L.) forests and annually normalized Sentinel-2 vegetation index time series were used to model and isolate the signal of discoloration in both drought and post-drought years. To demonstrate model efficacy and examine empirical inter- and intra-annual trends in early leaf discoloration, the trained model was applied to predict monthly April to August early leaf discoloration across Switzerland at a 10 m resolution from 2017 to 2023. Results reveal a striking nine-times increase in early leaf discoloration during the drought-affected month of August 2018 as compared to non-drought affected August 2020 and a nearly three-times increase in August 2023 discoloration over August 2020. In 2018 and 2023, discoloration hotspots formed as early as June in Switzerland’s most severely affected northwestern regions and spread rapidly in the following weeks. Efforts here establish a robust framework for disentangling seasonal drought related discoloration from legacy effects of drought and forest management forming a baseline for further investigations of biotic and abiotic early leaf discoloration drivers.

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