  Light detection and ranging (LiDAR) provides information on the vertical
structure of forest stands enabling detailed and extensive ecosystem study. The
vertical structure is often summarized by scalar features and data-reduction
techniques that limit the interpretation of results. Instead, we quantified the
influence of three variables, species, crown cover, and age, on the vertical
distribution of airborne LiDAR returns from forest stands. We studied 5,428
regular, even-aged stands in Quebec (Canada) with five dominant species: balsam
fir (Abies balsamea (L.) Mill.), paper birch (Betula papyrifera Marsh), black
spruce (Picea mariana (Mill.) BSP), white spruce (Picea glauca Moench) and
aspen (Populus tremuloides Michx.). We modeled the vertical distribution
against the three variables using a functional general linear model and a novel
nonparametric graphical test of significance. Results indicate that LiDAR
returns from aspen stands had the most uniform vertical distribution. Balsam
fir and white birch distributions were similar and centered at around 50% of
the stand height, and black spruce and white spruce distributions were skewed
to below 30% of stand height (p<0.001). Increased crown cover concentrated the
distributions around 50% of stand height. Increasing age gradually shifted the
distributions higher in the stand for stands younger than 70-years, before
plateauing and slowly declining at 90-120 years. Results suggest that the
vertical distributions of LiDAR returns depend on the three variables studied.
