Warmer springs cause trees to leaf out earlier. In Swiss forests, bud break has been advancing by several days per decade. The intuitive prediction: more growing season means more growth. The trees have more time with leaves on, more time photosynthesizing, more time building wood. Annual stem growth should increase.
Bose and colleagues at the Swiss Federal Institute for Forest, Snow and Landscape Research measured 228 trees across 48 forest sites from 2012 to 2022. Despite the earlier starts, annual stem growth declined. The trees are active for longer but producing less wood.
The mechanism is the summer. Heat waves and drought events compress the critical mid-season growth window. The extra days in spring are low-quality days — cool, sometimes with late frosts, with suboptimal light angles. The mid-summer days that used to be the productive core of the growing season are increasingly interrupted by heat stress, when trees close their stomata to conserve water and photosynthesis shuts down.
The species responses diverge. Silver fir, beech, and spruce all declined. Oak and pine held steady — both are more drought-tolerant, with deeper root systems and more conservative water-use strategies. The forest composition will shift as the species that can't handle the new summer regime lose ground to those that can.
Total season length is the wrong metric. The distribution of favorable hours within the season is what determines growth. An early start is irrelevant if July and August are hotter than the trees can tolerate. The calendar expanded. The usable calendar contracted. More time, less productivity — the same pattern visible in any system where the average improves but the variance kills.