Unravelling the Effects of Tree-Size and Canopy Structural Complexity on Forest Productivity

Abstract

Late-successional, unmanaged forests are important terrestrial carbon sinks. Their structurally complex features have been hypothesized to strongly affect forest productivity. However, the mechanisms through which structural complexity influences forest productivity remain unresolved. Here, we report a set of idealised simulations in boreal, sub-boreal, cool-temperate, warm-temperate and tropical forests using a dynamic model of atmosphere and vegetation interactions (Multilayered Integrated Numerical Model of Surface Physics–Growing Plants Interaction, MINoSGI) to elucidate the effects of tree–size and canopy structural complexity on stand-scale gross primary productivity (GPP). We show that, for a given biome-specific leaf area index (LAI) and individual foliage shape (η), the difference in GPP between forests with more and less structural complex canopies is in the range of 14–131 gC m -2 year -1 , equivalent to approximately 10% of total GPP. Our simulation results indicate that greater tree–size and canopy structural complexity is associated with a broader vertical distribution of foliage, supporting the localisation of leaves in multiple layers. This increases the efficacy of light usage, particularly for shaded leaves within the canopy. As a consequence, for all forest biomes, an increase in the diffuse light fraction increases GPP in forests with complex tree-size and canopy structures due to an enhancement of light use of shaded leaves. Overall, we unravel the mechanism underlying the effects of tree-size and canopy structural complexity on forest productivity. Individual foliage shape and tree-size and canopy structures in each forest biome can be factors that cause uncertainties (about 10%) when predicting GPP based on simulation models. Therefore, precise values of the factors associated with biome-specific tree-size and canopy structures should be employed into simulation models of global carbon budgets.

Keywords: Old-growth forest, In-canopy microclimate, Canopy photosynthesis, Sunlit and shaded leaves, Diffuse light, Atmosphere-vegetation dynamics model (MINoSGI)