Drought resilience of three coniferous species from Belgian arboreta highlights them as promising alternatives for future forests in Western Europe

Abstract

Searching for drought tolerant species is one of the adaptative management measures that could be implemented to improve forest resilience in the context of more intense and frequent droughts brought be climate change. Western European forests are already suffering from drought-induced tree mortality, in native as well as in well-established non-native species. Novel non-native species (i.e., non-native species that have not yet undergone thorough operational testing or previously been grown at forestry scale) with high drought resistance in their original geographic range could be an alternative. To this end, the ecology and drought response of these species need to be investigated in their area of introduction. We applied a dendroecological approach on trees of Abies nordmanniana (Steven) Spach (AN), Chamaecyparis lawsoniana (A.Murray bis) (CL), and Thuja plicata Donn ex. D.Don. (TP) from Belgian arboreta (Western Europe). First, we identified the main climatic drivers of species radial growth, using Bootstrapped Correlation Coefficients between tree ring indices and climate indicators related to drought, heat, and cold stresses. Second, we assessed the species growth response to exceptional drought events, using resistance, recovery and resilience indices and an integrated index comparing the actual resilience to a theoretical full resilience. We investigated the effects of species and drought timing on these indices using linear mixed models. The radial growth of the three species was negatively influenced by the water deficit during the previous growing season (especially in fall and summer), lower precipitation in the previous October, colder temperatures in late winter-early spring, and lower minimal temperatures in May. TP is the most sensitive species to previous summer conditions as it was negatively affected by the number of days with a mean temperature above 30°C, while AN was the least sensitive species with no significant Bootstrapped Correlation Coefficients for previous summer precipitation and temperatures. AN and TP differed from CL in being negatively affected in spring by higher maximal temperatures versus lower precipitation respectively. The effects of species and drought timing on resistance and recovery were significant. Overall, early and whole growing season droughts had a stronger negative effect than late droughts. AN and CL were more resistant to early and late droughts than TP, while the opposite was observed for recovery. The species showed less pronounced differences in resilience. CL was the species the closest to the theoretical full resilience, followed by AN: these two species appear to be good candidates for improving drought resistance of Western European forests. However, one must not forget that introducing novel species is associated with ecological risks and a thorough assessment of these risks must be carried out before promoting these species in forestry. Future research could focus on a comparison of novel non-native species with already well-established species in Western Europe such as Norway spruce and Douglas fir and explore how AN, CL and TP interact with native species in mixed stands.