Decreasing Stem Growth in Common European Tree Species Despite Earlier Growth Onset

Recent findings suggest that global warming is altering the timing of trees' phenological activities, including earlier emergence from winter dormancy. While early-season warming can boost carbon uptake, tree growth does not seem to benefit. The underlying mechanisms and the altered intra- and inter-annual growth dynamics, as well as their interaction with environmental factors, remain poorly understood. We analysed daily-resolved dendrometer data from 228 trees across 48 Swiss forest sites over 2012-2022 to examine stem radial growth timing, intra- and inter-annual dynamics, and environmental controls for five tree species. Our results showed that higher winter and spring temperatures induced an earlier accomplishment of annual growth-indicating earlier growth start-but did not increase the total annual growth in any of the species studied. In contrast, we found a significant negative growth trend for P. abies, A. alba, and F. sylvatica across a wide climatic gradient. The reduction in growth was associated with the decrease in the number of days with growth. The positive effect of higher temperatures in spring was canceled out by a negative effect towards the end of the growth period. Overall, such a negative effect of increased temperature at an annual scale was strongest in P. sylvestris and persisted over 2012-2022. The effect size of temperature was larger in explaining weekly growth compared to the effect sizes of precipitation or of soil water potential. This indicates that temperature, as an integrating indicator of growing conditions, can capture the variability of growth on a weekly time scale, which is less the case on a daily or hourly resolution. Overall, our results challenge the expectation that climate warming will offset carbon losses caused by heat and drought with a longer growing season. Our findings from five of the most common tree species in Central Europe do not support this assumption.