Light absorption and light-use efficiency in mixtures of Abies alba and Picea abies along a productivity gradient

•Growth of Picea abies and Abies alba was greater in mixed compared with monospecific neighbourhoods.•Light absorption and light-use efficiency increased in mixtures.•For A. alba, complementarity increased on sites with faster growing trees.•For P. abies, complementarity increased with increasing minimum temperatures.•Complementary effects were influenced by differences in crown architecture and canopy structure.

There is great interest in how species interactions change along spatial gradients in resource availability and climatic conditions. Many studies have shown that facilitation or complementary effects increase as growing conditions become harsher, however, several studies in forests have recently shown the opposite trend. Increasing complementary effects with improving growing conditions may result when species interactions influence light absorption or use. This hypothesis was tested in mixed-species forests of Abies alba Mill. and Picea abies (L.) Karst. in south-western Germany where complementary effects on growth increased as climatic conditions improved. The absorption of photosynthetically active radiation (APAR) by individual trees was predicted using a detailed light model. Light-use efficiency (LUE) of individual trees was estimated as basal area growth per unit APAR. APAR and LUE were modelled as functions of crown size, climatic variables and the species composition and density of the neighbourhood of individual trees. For a given tree size, APAR (for both species) and LUE (of A. alba trees) were greater for trees in mixed-species neighbourhoods than monospecific neighbourhoods and this complementary effect increased as climatic conditions improved and on sites with faster growing trees. Increases in APAR for A. alba probably resulted because shading from P. abies trees was less intense than that from other A. alba trees on the more productive site. The species composition of individual tree neighbourhoods did not influence relationships between stem diameter and crown diameter or height to the crown base, for either species. However, the height of each species, for a given stem diameter, increased as the proportion of a given tree’s neighbourhood basal area that was composed of the same species increased. This change in crown architecture could also have contributed to the complementarity effects in these stands. Increasing complementary effects with improving growing conditions is consistent with the stress-gradient hypothesis if competition for light is considered to be the stressor. This study shows that such a spatial pattern in species interactions could be associated with increasing differences in APAR or LUE between mixtures and monocultures as climatic conditions become more favourable.