Above-treeline ecosystems facing drought: lessons from the European 2022 summer heatwave
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.
Status: open (until 21 Jun 2023)
- RC1: 'Comment on bg-2023-74', Anonymous Referee #1, 22 May 2023 reply
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This is an excellent article on a very timely issue. In fact, effects of climate change on alpine vegetation have so far almost exclusively been analysed with respect to the impact or warmer temperatures. The parallel effect of drought, triggered by higher demand at higher temperatures even with stable precipitation sums, has largely been neglected. In his paper, Choler convincingly shows that this might become a major driver of plant productivity, especially when warm and dry years / years with little snow fall coincide – a coincidence that will likely occur more frequently in the future. I think the article can and will contribute to re-focusing the interesting of alpine ecologists on these interactions between warming and drought, not only in terms of productivity, but also in terms of species distribution and diversity.
I found the analysis very well done and the paper very well written. The discussion is thoughtful and does not overstretch the findings. I have just one minor comment