Longer and faster: Intra-annual growth dynamics of Douglas fir outperform Norway spruce and silver fir over wide climatic gradients

Climate change is expected to have significant impacts on European forests, causing changes in the geographic distribution of species and ecosystem functioning. Douglas fir (Pseudotsuga menziesii) and silver fir (Abies alba) are considered potential alternatives to the drought endangered Norway spruce (Picea abies). However, still little is known about differences in their intra-annual growth dynamics, an important characteristic determining the adaptive capacity of each species. Here we make use of more than 5000 microcores from 132 trees of the three species distributed along three elevational transects (370-1125 m a.s.l.) spanning a temperature gradient of 5 degrees C in South-Western Germany to compare their intra-annual growth dynamics in a context of changing climate. Results indicate an earlier onset of cambial cell production of about 5.1 days per degrees C temperature increase for all tree species. Douglas fir produced the highest number of cells and exhibited the longest seasonal period of wood formation, starting two and four weeks earlier and ceasing about two and three weeks later than silver fir and Norway spruce, respectively. Additionally, Douglas fir displayed the highest maximum cell production rate and a 20% higher average cell production rate than Norway spruce and even 50% higher rate than silver fir. We found that soil moisture, but even more the number of produced cells were significantly correlated with the date of growth cessation, compared to a negligible correlation with mean annual temperature (MAT). The superior growth performance of Douglas fir resulting from a longer growth duration with higher rates of cambial cell division was consistent across our climatic gradient. These results corroborate that Douglas fir could be a highperformance alternative to the more climate-change-endangered Norway spruce.