Light absorption and light-use efficiency of juvenile white spruce trees in natural stands and plantations

Aboveground tree growth is influenced by light availability, light capture, and the efficiency captured light is converted into growth. All three factors are influenced by neighborhood species composition and stand structure and can be modified with silvicultural treatments. The objective was to examine the absorption of photosynthetically active radiation (APAR), light-use efficiency (LUE), and aboveground biomass growth of juvenile white spruce (Picea glauca (Moench) Voss) trees in plantations and planted in naturally regenerated stands with diverse species composition. Trees were sampled across a range of sizes, and measurements of white spruce and neighborhood trees were collected at ages seven and eight. Light absorption of individual trees was modeled with MAESTRA accounting for species-specific crown structures of neighboring trees. APAR increased linearly with leaf area in both treatments. The correlation was more influenced by neighborhood competition than self-shading, since self-shading was minimized by crown spread and height growth. Biomass growth increased with increasing APAR in both treatments, but growth for a given unit of APAR was greater in the natural stands, possibly due to greater belowground biomass allocation in plantations due to lower stand densities. LUE declined slightly across the range of tree size, which contrasts most other studies. The decline could be due to juvenile age of the trees and inherent slow growth of spruce species.