Temperature more strongly influences Douglas-fir radial growth than precipitation under European temperate conditions

Abstract Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) has been introduced in Europe due to its high productivity. Although it is considered a relatively drought tolerant species, its ability to cope with climate change remains uncertain. This study aims to investigate the growth response of Douglas-fir to climate under European temperate conditions. A total of 720 bore cores were taken from 360 trees in 24 mature Douglas-fir stands in the Belgian Ardennes, spanning a range of annual climatic water balance (368–700 mm), estimated maximal soil extractable water reserve, and stand basal area (30–53 m2/ha). Climate-growth relationships over the period 1976–2020 were investigated by calculating Bootstrapped Correlation Coefficients (BCCs) on detrended chronologies. Principal Component Analysis was computed on the BCCs to explore how growth response might be modulated according to the site and stand characteristics mentioned above. The results revealed (i) a consistent positive effect of higher February temperatures and precipitation on radial growth; (ii) a consistent negative effect of higher maximum temperatures in October of the current year; and (iii) a consistent positive effect of prior-year October minimum temperatures on radial growth, with these last two relationships becoming stronger in recent decades. These findings suggest that in these environments, Douglas-fir likely photosynthesizes through the winter period, and its growing season extends into October. The general lack of significant correlations between growth and precipitation except in February also suggests that temperatures are a more important driver than precipitation in influencing Douglas-fir productivity in this region. The site and stand conditions studied did not significantly influence these climate-growth relationships.