Measures of stand structural complexity
As the structure of the canopy influences heterogeneity of light
conditions at the forest floor (and hence spatial-temporal light
availability for understory plants), it is important to find measures
which best represent the structural complexity of the canopy. Three
measures were used in this study to quantify canopy structural
complexity, which is hypothesized to affect resource heterogeneity and
thus understory plant species diversity. First, we calculated the
standard deviation of diameter at breast height (DBHsd) for all trees
with DBH > 7 cm from the full inventory of each plot
(Storch et. al 2020).
Second, we used a public dataset of aerial image flights with 20 cm
pixel resolution and 60% forward and 30% sideward image overlaps to
generate a digital surface model (DSM) using a structure from motion
workflow (equivalent to
Zielewska-Büttner
et al. 2016) using Agisoft Photoscan commercial software, v. 1.3.4,
AgiSoft, St Petersburg, 2017). From the resulting DSM with a resolution
of 40 cm, we computed the Terrain Ruggedness Index (TRI,
Wilson
et al. 2007), as a measure of the geometric complexity of the crown
surface.
Third, to measure the geometric complexity of the distribution of plant
material within the stand, we used the data from terrestrial laser scans
to compute the index of stand structural complexity (SSCI) following the
approach suggested
by
Ehbrecht et al.
(2017).