Abstract
Terrestrial laser scanners (TLS) have the potential to revolutionise measurement of the three-dimensional structure of vegetation canopies for applications in ecology, hydrology and climate change. This potential has been the subject of recent research that has attempted to measure forest biophysical variables from TLS data, and make comparisons with two-dimensional data from hemispherical photography. This research presents a systematic comparison between forest canopy gap fraction estimates derived from TLS measurements and hemispherical photography. The TLS datasets used in the research were obtained between April 2008 and March 2009 at Delamere Forest, Cheshire, UK. The analysis of canopy gap fraction estimates derived from TLS data highlighted the repeatability and consistency of the measurements in comparison with those from coincident hemispherical photographs. The comparison also showed that estimates computed considering only the number of hits and misses registered in the TLS datasets were consistently lower than those estimated from hemispherical photographs. To examine this difference, the potential information available in the intensity values recorded by TLS was investigated and a new method developed to estimate canopy gap fraction proposed. The new approach produced gap fractions closer to those estimated from hemispherical photography, but the research also highlighted the limitations of single return TLS data for this application.
Highlights
The foliage element of a forest canopy represents the primary surface that controls mass, energy, and gas exchanges between photosynthetically active vegetation and the atmosphere [1]
We present estimates of canopy gap fractions derived from the Terrestrial laser scanners (TLS) measurements, calculated using both the point-based and intensity-based methods, and corresponding estimates obtained from hemispherical photography
The results presented indicate that gap fractions derived from the intensity-based method can be used to characterise temporal dynamics in forest canopies
Summary
The foliage element of a forest canopy represents the primary surface that controls mass, energy, and gas exchanges between photosynthetically active vegetation and the atmosphere [1]. Hemispherical photography has been widely and effectively used to characterise biophysical properties of forest canopies for several decades [6], since they provide a permanent two-dimensional (2D) record of the canopy structure and can be used to estimate canopy gap fraction distributions [3]. This method has been shown to be adequate for the quantification of canopy structure, Macfarlane et al [7] pointed out a range of potential error sources associated with estimates derived from the photographs that must be taken into consideration. Results derived from two different TLS methods were compared with estimates computed from hemispherical photographs in order to better understand the information recorded by TLS in the computation of canopy gap fraction
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