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).