Assessing the growth response to climate of Douglas fir under European temperate oceanic conditions

Abstract Douglas-fir has been largely introduced in Europe due to its high productivity. Although it is considered as a relatively drought tolerant species, its ability to cope with climate change remains uncertain. This study aims to investigate Douglas fir growth response to climate in its current climatic optimum in Europe. A total of 720 cores were taken from 360 trees in 24 adult 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 1976–2020 period were investigated through Bootstrapped Correlation Coefficients (BCCs) calculation on detrended chronologies. Principal Component Analysis was computed on the BCCs to explore how growth response could be modulated according to the three above-mentioned site and stand characteristics. Radial growth was favoured by mild and wet late winters-early springs; warmer temperatures at the end of the growing season and in winter; and wetter early summers. The studied site and stand conditions did not influenced these climate-growth relationships, indicating a strong tree species response to climate. BCCs computed on moving windows showed an increased Douglas fir sensitivity to summer water deficit over time, which could offset its ability to benefit from higher temperatures in the late growing season and in winter in the future.