Drought timing, intensity, and consecutiveness have more influence on Douglas fir growth response than site conditions and stand density in European temperate climate

Abstract

Douglas fir is a largely introduced species in Europe and is often presented as a promising alternative to more drought-sensitive species. However, the observed and predicted increase in drought frequency and intensity could undermine its ability to cope with drought. This study aims to investigate the radial growth response of Douglas fir to drought in its climatic optimum in Europe, considering a number of drought characteristics (in terms of timing during the growing season, intensity, and consecutive occurrence), site conditions (average climatic water balance, maximal extractable soil water), and stand densities. Using growth data from 360 trees sampled across 24 sites in Wallonia (Belgium), we fitted linear mixed models to investigate the influence of drought, site, and stand characteristics on three commonly used resilience indices, as well as on an integrated index comparing observed resilience with a theoretical full resilience reference. On average, radial growth was reduced during droughts regardless of drought characteristics and site conditions. Trees always recovered to some extent, but not always to full resilience. Drought characteristics had a stronger influence on drought response than site and stand characteristics. Under the most intense droughts, trees were less resistant and less resilient to early droughts than to late droughts. Higher intensity and consecutive droughts increased the negative impact of a drought on resistance and resilience. Resistance was slightly higher on sites that were wetter and had higher maximal extractable soil water. In contrast, resilience to non-consecutive droughts was higher on drier sites. Finally, we did not detect any significant impact of stand density on Douglas fir growth response. The integrated index showed that Douglas fir has the best chance to be completely resilient to droughts when droughts are later, not very intense, and not consecutive and when trees are located on more water-limited sites. In conclusion, Douglas fir radial growth may not be as drought-resilient as expected. It is therefore advisable to avoid an overreliance on this species, and increase species diversity to strenghten forest stability. In this context, future research avenues could involve the comparison of drought response of Douglas fir with other species.