  Tree species have different shapes and forest stand dynamics. Light detection
and ranging (LiDAR) can measure the three dimensional position of reflective
material by sending laser pulses through the canopy. We examined the influence
of three forest stand attributes: species, crown closure, and age on the
vertical distribution of aerial LiDAR returns of forested stands. We studied
over five thousand 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
vertical distribution of LiDAR returns against the three attributes using a
functional glm and a novel nonparametric graphical test of significance.
Results indicate that aspen stands had the most uniform vertical distribution
of LiDAR returns; balsam fir and white birch distributions were similar and
centered around 50% of the stand height; black spruce and white spruce
distributions were skewed below 30% of stand height (p<0.001). An increase in
crown closure concentrated the distributions around 50% height. Increasing age
gradually shifted the distributions higher in the stand until 50-70 years,
where it plateaued and slowly declined at 90-120 years. The full model maximal
R-squared was 0.47 around 10% of stand height. Results suggest that the stands
vertical distributions of LiDAR returns depend on the three variables studied.
This understanding can be used for stand-level species classification and to
study the evolution of the forest structure over changing conditions.
