Terrestrial laser scanning measurements to characterise temporal changes in forest canopies

Light detection and ranging (lidar) systems are active sensors capable of creating apermanent three-dimensional (3D) record of forest canopy structure. This 3Dcharacterisation can provide increased accuracy for aboveground biomass estimates inhigh-biomass ecosystems, where passive optical sensors only provide a two-dimensional(2D) perspective. The aim of this study was to test a quantitative, accurate, andrepeatable method to obtain estimates of canopy biophysical properties and monitorseasonal variations in forests by using multi-temporal terrestrial laser scanner (TLS) data.This research is one of the first detailed multi-temporal terrestrial lidar studies undertakenanywhere in the world. The study site chosen for this research was Delamere Forest,located in Cheshire, Northwest England. TLS data on vegetation structure were acquiredfor seven sampling plots, comprising two broad-leaf and five conifer stands, betweenMarch 2008 and April 2009. Canopy directional gap fractions were derived from the TLSdatasets collected and compared with estimates derived from coincident hemisphericalphotographs. The comparison showed that TLS gap fractions estimates were consistentlylower than those estimated from hemispherical photographs. To examine this apparentdifference further the potential information available from the intensity values recordedby TLS were investigated. The use of this information in the computation of gap fractionsled to a better agreement between estimates derived from both sources, as well as a betterunderstanding of how intensity values are activated within forest canopies. Estimates ofother biophysical properties were also computed from the TLS data, including leaf areaindex, average leaf angle distributions, and clumping index. The analysis of theseestimates highlighted the repeatability and consistency of the TLS measurements incomparison with corresponding results derived from the hemispherical photographs.Analysis of the TLS datasets was conducted in order to improve the understanding of theinteraction between lasers and vegetation canopies. The novelty of this research is inapplying a ground-based lidar sensor to characterise the structure of a range of treecanopies using intensity corrected data, and assessing the utility of estimates ofbiophysical properties for monitoring temporal variations in forest canopies