Drought-related mortality modifies mixing effects on light absorption and growth in mono-specific and mixed stands of Fagus sylvatica, Alnus glutinosa and Betula pendula.

Little is known about how drought-related mortality influences light absorption of surviving trees and consequent changes in tree species interactions. Here, we used the detailed tree-level light model (Maestra) in combination with measurements of tree dimensions, crown architectures, and stand structures to examine experimental mixing effects of Fagus sylvatica, Alnus glutinosa, and Betula pendula on light dynamics following a drought in Bangor, Wales. The experimental stands, planted in 2004, were composed of clusters with one to three species in different combinations. Droughts occurred in 2011 and 2014 during the growing seasons, and trees were measured in 2014 and 2015. Species mixing resulted, on average, in higher tree growth, absorption of photosynthetically active radiation (APAR), and light-use efficiency (LUE) compared with the mean of the monocultures. An exception was the monoculture of B. pendula, which was the most productive species and had higher growth, APAR, or LUE than some mixtures. Drought-related mortality reduced the stand basal area across all plots by an average of 8.3% and tree density by 11%. This moderate change in the structure did not result in significant increases in individual tree APAR, LUE, or growth. From a management perspective, mortality might need to reduce stand density more strongly than it did in this study before light absorption or LUE is altered.