Integrating ground observations and remote sensing to assess Swiss forest growth over the past 25 years

Drought- and heat-induced tree mortality has been increasingly observed and is expected to intensify under ongoing climate change, raising urgent concerns about forest vulnerability across Europe. Identifying the forest ecosystems most susceptible to climate extremes and understanding the mechanisms underlying their responses are therefore critical. Here, we integrate multiple hydrological and satellite-based proxies of large-scale forest growth with long-term ground-based forest monitoring data across Switzerland. From 2000 to 2024, hydrological stress, quantified by the Standardized Precipitation Evapotranspiration Index (SPEI), has increased consistently across the country. In contrast, vegetation structure (represented by Normalized Difference Vegetation Index, NDVI) and carbon uptake (represented by gross primary productivity, GPP) exhibit coherent but spatially contrasting trends, with pronounced declines in northwestern regions and increasing trends in the southeastern Alpine areas. Ground-based observations corroborate these patterns, showing higher crown defoliation rates, stronger declines in net primary productivity, and reduced tree growth in areas characterized by decreasing NDVI and GPP, while tree mortality rates remain comparable across regions. Species-specific responses were also evident, with European beech exhibiting increasing growth trends, whereas other dominant Swiss tree species show overall growth declines in recent decades. By jointly analyzing these patterns with environmental drivers, including meteorological factors and soil conditions, we aim to identify the dominant forcing mechanisms driving forest growth stress and to develop models for predicting forest GPP in Switzerland. We further quantify how interacting environmental stressors, such as vapor pressure deficit and soil water availability, jointly regulate forest productivity dynamics, providing an integrated assessment of forest vulnerability to climate extremes.