02 May 2023
 | 02 May 2023
Status: this preprint is currently under review for the journal BG.

Above-treeline ecosystems facing drought: lessons from the European 2022 summer heatwave

Philippe Choler

Abstract. In 2022, Europe experienced an extremely dry and hot summer. In the Alps, this episode occurred after an unusually low snowfall winter, which aggravated the dryness of soils. This study examines the impact of this particular year on the canopy greenness of above-treeline ecosystems by comparison with previous heat waves that hit the Alps during the last two decades. Normalized Difference Vegetation Index (NDVI) time series derived from the MODIS satellite were processed to extract the temporal variability of yearly maximum NDVI (NDVImax). The responsiveness of NDVImax to snow cover duration and growing season weather conditions was evaluated in contrasting hydro-climate regions of the Alps using linear mixed effect models. The year 2022 was unique in that the summer heat wave led to a widespread negative anomaly of NDVImax. The magnitude of this anomaly was unprecedented in the southwestern, driest part of the Alps, where vegetation activity was found to be particularly responsive to snow cover duration and early summer precipitation. In the colder and wetter regions, all warm to very warm summers before 2022 had led to increased canopy greenness, but the combination of a reduced snow cover and low early summer precipitation counteracted this expected beneficial effect in 2022. This study provides evidence that the control of canopy greenness by temperature and water balance differs markedly across regions of the Alps and that the year 2022 bears witness to a shift toward an increasing importance of moisture availability for regulating plant growth at high elevation. This is viewed as a warning sign of what could become the new norm in the years ahead in the context of increasing frequency and intensity of extreme droughts throughout temperate mountain ecosystems.

Philippe Choler

Status: open (until 21 Jun 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-74', Anonymous Referee #1, 22 May 2023 reply

Philippe Choler


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Short summary
The year 2022 was unique in that the summer heat wave and drought led to a widespread reduction of vegetation growth at high elevation in the European Alps. This impact was unprecedented in the southwestern, warm and dry part of the Alps. Over the last two decades, water has become a co-dominant control of vegetation activity in areas that were, so far, primarily controlled by temperature, and the growth of mountain grasslands has become increasingly sensitive to moisture availability.