Multi-temporal Terrestrial Lidar for Estimating Individual Tree Dimensions and Biomass Change

Abstract

Accurate measures of forest structural parameters are essential to forest inventory and growth models, managing wildfires, and modeling of carbon cycle. Terrestrial laser scanning (TLS) provides accurate understory information rapidly through non-destructive methods. This study developed algorithms to extract individual tree height, diameter at breast height (DBH), and crown width in plots at Ecosystem Science and Management (ESSM) research area and Huntsville, Texas. Further, the influence of scan settings and processing choices on the accuracy of deriving tree measurements was also investigated. The study also developed models to estimate aboveground biomass (AGB) and investigate different conceptual approaches to study tree level growth in forest structural parameters and AGB using multi-temporal TLS datasets.

DBH was retrieved by cylinder fitting at different height bins. Individual trees were extracted from the TLS point cloud to determine tree heights and crown widths. The R-squared value ranged from 0.91 to 0.97 when field measured DBH was validated against TLS derived DBH using different methods. An accuracy of 92% was obtained for predicting tree heights. The R-squared value was 0.84 and RMSE was 1.08 m when TLS derived crown widths were validated using field measured crown widths. Examples of underestimations of field measured forest structural parameters due to tree shadowing have also been discussed in this study. Correction factors should be applied or multiple high resolution scans should be conducted to reduce the errors in estimation of forest structural parameters.

TLS geometric and statistical parameters were derived for individual trees and used as explanatory variables to estimate AGB. An extensive literature review reveals that this is the first study to model the change in AGB using different innovative and conceptual approaches with multi-temporal TLS data. Tree level AGB growth was studied over a period of three years using three different approaches. Results showed that TLS derived geometric parameters were better correlated to field measured AGB. Promising results for AGB change were obtained using the direct modeling approach; hence forest growth could be studied independent of any field measurements when biomass models are available. However, the models could be improved by incorporating more trees with a wide range of DBH and tree heights. The results from this study will benefit foresters, planners, and other remote sensing studies from airborne and spaceborne platforms, for map upscaling, data fusion, or calibration purposes.