Terrestrial lidar scanning reveals fine-scale linkages between microstructure and photosynthetic functioning of small-stature spruce trees at the forest-tundra ecotone

Abstract The forest- tundra ecotone(FTE) is exhibiting myriad responses to rapid environmental change. Microstructural variability(at cm to m length scales) of vegetation canopies and geomorphic features may modulate the response of FTE vegetation to regional climate changes. Understanding the influence of microstructureon tree function at the FTE is particularly relevant during vulnerable early growth stages. During these stages, individual trees are tightly coupled to conditions of the surface boundary layer, which can be more conducive to growth than the conditions above the boundary layer. Until recently, however, it has been difficult to characterize microstructurein a replicable, transferable manner. This study builds upon substantial research on ecological responses of trees at the FTE to growth environment conditions by integrating high-resolution terrestrial lidar scanning (TLS) to characterize microstructure. Our main goal was to use TLS technology to understand the effects of microstructureon photosynthetic functioning (i.e., chlorophyll fluorescence) of small-stature white spruce ( Picea glauca (Moench) Voss) trees at the FTE. Our specific objectives were to: 1) determine how much variance in photosynthetic functioning is explained by microstructure; 2) identify microstructuralmetrics that most strongly control variance in photosynthetic functioning; and 3) determine the scales at which microstructuralmetrics most strongly drive variance in photosynthetic functioning. Random Forest modeling demonstrated that 28% of variance in photosynthetic functioning can be explained through variation in fine-scale environmental conditions that are modulated by microstructurealone. Insolation and canopy roughness were the most important predictors of photosynthetic functioning, and the sensitivity of photosynthetic functioning to canopy roughness was scale-dependent. This suggests that microstructureaffects spatial heterogeneity in the boundary layer that may influence carbon assimilation of small-stature spruce trees. This research emphasizes the importance of quantifying microstructurein study systems where fine-scale heterogeneity of the growth environment may modulate plant responses to regional climate change.