Decoding hotter-drought impacts on canopy activity and tree growth to diagnose forest die-off

• Assessing drought impacts on canopy activity and growth improves die-off forecasts. • Vegetation indices and radial growth were measured in 45 die-off hotspots in Spain. • Hotter droughts were more frequent during the 21st century triggering die-off. • Drought constrained canopy activity and growth, which were more coupled in dry sites. • Severe reductions in greenness and growth occurred 6 years before die-off started. Decoding the long-term impacts of hotter droughts on canopy activity and growth provides information to forecast forest die-off leading to mortality hotspots. However, remotely sensed information often lacks field-based validation including long-term assessments of tree radial growth in those hotspots. To relieve this deficiency, we investigated climate data, canopy activity (NDVI, EVI) and radial growth in 45 die-off hotspots sampled across Spain and affecting six conifers (silver fir, Scots pine, Aleppo pine, maritime pine, stone pine, and Spanish juniper), and four broadleaves (European beech, pedunculate oak, downy oak, and holm oak). Hotter droughts, characterized by elevated evaporative water demand, are becoming more frequent in the study sites during the 21st century. Such warming and aridification started in the late 1990s and early 2000s and triggered several die-off hotspots, whose onset dates peaked in 1992, 2005, 2012, and 2017. In those sites, hotter droughts caused abrupt declines in canopy activity and radial growth. Year-to-year growth variability was more sensitive to drought than canopy greenness, and wetter sites showed the most negative growth trends. Drought constrained canopy activity and growth, which were more coupled in dry sites. The year of the strongest greenness loss and growth predated the observed die-off onset by six years, while abrupt growth reductions coincided with species-specific severe droughts such as 1986 (silver fir) or 2005 (Mediterranean pines). Both abrupt greenness loss and grow reductions were more common in recent years in drier sites as warming leads to more aridity. Forest mortality risk involves both site aridity and hotter-drought severity with tree-ring series representing promising data to forecast hotspot emergence.