Decreasing water availability reduces productivity in Swiss forests along an altitudinal gradient

Abstract Forests are one of the most important terrestrial carbon sinks, but are increasingly under pressure due to drought, heat and the occurrence of extreme events. There are opposing longer term trends for European forest growth reported, and severe drought and disturbance events additionally impact forest ecosystems, so that the overall trend of forest productivity is uncertain. Thirty years of harmonized forest monitoring at 18 forest sites along an altitudinal gradient in Switzerland provides a good basis for assessing the effects of climate change on forest conditions. We found a decreasing trend of forest productivity (basal area index and net carbon uptake by growth), particularly pronounced since 2015 across all altitudinal ranges, age classes and species, which could not solely be attributed to stand density and ageing of the forest, but also to soil water availability and nitrogen deposition. The growth rate of trees, as well as the ingrowth rate, were hereby the most important factors explaining the overall forest productivity. At a given stand density, forest productivity was lower in recent years compared to earlier decades. Overall, our results indicate a decreasing stand‐level growth trend irrespective of site conditions and stand structure. This 30‐year declining trend can be partly attributed to water and nitrogen availability, and points to a decreasing growth capacity of the forest sites that is the long‐term potential of a site to sustain tree growth. The pivotal role of water availability for sustainable forest production and the long‐term effect of drought years on forest vitality urges us to rethink the adaptability of forests in view of the increasing frequency of drought and heat periods predicted for the future. Read the free Plain Language Summary for this article on the Journal blog.