Differences in xylem response to drought provide hints to future species selection

The long-term impacts of summer drought and high temperatures on xylem traits of conifers are scarcely investigated, even though such traits can play an important role in determining xylem functional performance and wood properties under the ongoing climate change. To investigate how different species are expected to react to climate change, especially summer drought, we studied the xylem plasticity of six non-native conifer species from a Danish common garden based on the hypothesis that wood anatomy variables may be particularly valuable for predicting drought resistance differences between species. We focused on two xylem functional traits (tracheid wall thickness and lumen area) and three wood properties relevant for utilisation of the material (density, microfibril angle and stiffness) and studied how they were affected by drought and temperature in the period 1975-2012. The results showed that xylem traits responded stronger to summer drought (July and August) than to temperature, especially for the part of the ring produced late in the season. The response was stronger for Larix kaempferi (Lamb.) Carr., Picea abies (L.) Karst., Picea sitchensis (Bong.) Carr. and Abies alba Mill., which formed latewood tracheids with smaller lumen and thicker walls, and wood characterised by higher density, lower microfibril angle and higher stiffness during drought years, than for Pseudotsuga menziesii (Mirb.) Franco. The latter, and to some extent Abies grandis Lindl., showed lower response of xylem traits to summer drought. P. menziesii also showed only minor effects on growth ring widths and was the conifer species showing least effects from summer drought. Retrospective comparison of xylem plasticity between different species from common gardens was thus found to provide useful hints to species selection for afforestation in light of future climate scenarios.