Estimating leaf area organization (LAO) using Sentinel-2 and airborne LiDAR to assess silvicultural thinning effects on canopy structure in dryland forests 

Abstract

The leaf area organization (LAO) approach uses a set of metrics describing the spatiotemporal organization of the forest leaf surface interface. It is useful for reflecting the complexity of canopy structure in assessing forest functions, such as evapotranspiration and primary productivity. We developed the estimation of LAO parameters, including LAIEcosystem (the one-sided leaf area of the entire forest vegetation, per unit ground area), its separation into forest strata (LAIOverstory, LAIUnderstory), vertical arrangement (LAI profile), and forest growth status with respect to site potential (growing space occupancy: GSO=LAIEcosystem/LAImax) using the Sentinel-2 satellite and an airborne LiDAR system (LiDAR-ALS). The LAO parameters were examined during 2017–2022 in two dryland conifer forests along a climatic aridity gradient from arid (Yatir Forest) to semi-arid (Kedoshim Forest) and across silvicultural thinning treatments (clear-cut, 100, 200, 300 trees ha-1 and non-thinned control in 70×70 m plots). We found significant correlations across the thinning treatments between ground-based optical measurements and both the estimations of LAIEcosystem based on Sentinel-2 data of 11 vegetation indices and one texture parameter (R2=0.73–0.87), and the estimations of LAIOverstory, LAIUnderstory, and LAIEcosystem, based on the LiDAR-ALS (R2=0.82–0.90). Using the LiDAR-ALS estimations, we found that 10 years after thinning, LAIEcosystem decreased with thinning intensity in both sites but was higher in the wetter Kedoshim site (0.45–1.27 m2 m-2 vs. 0.04–0.31 m2 m-2 in the arid Yatir forest). However, the overall decrease in LAIEcosystem reflected the combined effects of a reduction of LAIOverstory, in both sites and an increase in LAIUnderstory in the wetter site but not in the dry site (increasing by 0.08–0.86 m2 m-2). Using Sentinel-2 to assess the inter-annual dynamics of forest structure in the 8–13 years since thinning indicated that in the wetter Kedoshim site, LAIEcosystem significantly correlated with rainfall (R2=0.75–0.85), in all thinning treatments, with an overall increasing trend in the more intense thinning treatments only. The GSO showed a positive temporal trend in the clear-cut (3–15%) and the more intense thinning treatments in both forest sites (45–51% in the wetter site and 30–34% in the arid site). Our research uses a novel approach to demonstrate complex, ecosystem-scale, long-term structural responses to thinning in dryland forests. Such effects of thinning treatment should be considered for forest management for facing climate change in these forests.