The Effects of Tree Size, Stand Density, and Tree-Species Mixing on Stand Level and Tree Level Light Absorption and Light-Use Efficiency: A Review

Purpose of Review Tree species mixtures are often more productive than monocultures. One possible reason for this is higher absorption of photosynthetically active radiation (APAR) and improved light use efficiency (LUE) in mixtures. Here, we identified the processes influencing APAR and LUE in forests, examined how APAR and LUE are influenced by mixing species or reducing stand density, how these effects vary along site gradients, and implications for modelling of forest growth. Recent Findings Eight of 18 cases had 4 to 86% (mean 27%) higher stand APAR in mixtures than the most productive monoculture, four found 13 to 49% (mean 25%) higher APAR compared to the average of the monocultures, and three found lower APAR in mixtures than in the monoculture with lowest-APAR. Following the same sequence of comparisons for LUE in mixtures vs. monocultures, the counts were ten, one and four cases, respectively. Reductions in stand density reduced stand APAR, and either increased or did not influence LUE. While a common set of interactions and structural characteristics influenced APAR and LUE, their importance varied among forest types, sites, and ages, pointing to the value of using models to understand these processes. At nutrient and water rich sites, where leaf areas and competition for light are high, increased APAR in mixtures typically leads to increased productivity. Summary In mixtures, stand-level APAR and LUE can be greater than in monocultures, but this is not always the case, and the causes vary between forest types and sites. Increases in APAR or LUE do not necessarily increase growth, which is more likely on sites with higher soil resources and favourable climatic conditions. Forest growth models are available that summarise this information in a form that can be used by forest practitioners.