Effectiveness of the Canopy Closure Curve to Inform Management of Two-Storied Stands

Abstract

In this study we examined the validity of the canopy closure curve as a reference in relation to light conditions and demonstrated the applicability of the curve to different thinning types. An even-aged sugi (Cryptomeria japonica) plantation of 91 years was thinned heavily and transformed to a two-storied stand by underplanting with sugi and hinoki cypress (Chamaecyparis obtusa) seedlings. Three initial sample plots were selected in the stand before thinning. After the conversion, five permanent sample plots were established and both overstory and understory trees were measured up to the age of 118 and 28 years, respectively. The canopy closure curve was estimated from measurements collected in the initial sample plots and crown density (i.e. the ratio of overstory productivity to the maximum stand productivity) was calculated based on the canopy closure curve as a reference. To validate the curve, we derived three time points from the measurements collected in the permanent sample plots: one when the trajectory of stem density and stand volume of the overstory trees intersected the canopy closure curve, one when the relationships between the volume of individual overstory trees and their growth changed, and one point when the growth of understory trees dropped. The first of these time points coincided with the second and third time points. This means that dominant overstory trees suppressed smaller overstory trees and overstory trees constrained the growth of understory trees because of light reduction when the canopy closed. The canopy closure curve was found to be a valid way to assess light controlling the growth of understory trees. Furthermore, we generated virtual thinning data from the measurements taken in the initial sample plots for different thinning types. We obtained the canopy closure curve for these. We plotted the trajectory of stem density and stand volume in relation to the thinning as well as the canopy closure curve to demonstrate the applicability of the canopy closure curve. The thinning simulation showed sufficient effect to demonstrate that there was no need for additional accretion cutting.