Abstract
The gap fractions of three mature hemi-boreal forest stands in Estonia were estimated using the LAI-2000 plant canopy analyzer ( LI-COR Biosciences, Lincoln, NE, USA), the TRAC instrument (Edgewall, Miami, FL, USA), Cajanus’ tube, hemispherical photos, as well as terrestrial (TLS) and airborne (ALS) laser scanners. ALS measurements with an 8-year interval confirmed that changes in the structure of mature forest stands are slow, and that measurements in the same season of different years should be well comparable. Gap fraction estimates varied considerably depending on the instruments and methods used. None of the methods considered for the estimation of gap fraction of forest canopies proved superior to others. The increasing spatial resolution of new ALS devices allows the canopy structure to be analyzed in more detail than was possible before. The high vertical resolution of point clouds improves the possibility of estimating the stand height, crown length, and clumping of foliage in the canopy. The clumping/regularity of the foliage in a forest canopy is correlated with tree height, crown length, and basal area. The method suggested herein for the estimation of foliage clumping allows the leaf area estimates of forest canopies to be improved.
Highlights
Radiative transfer and reflectance of vegetation canopies determine the energy budget, photosynthesis, and possibilities of the remote monitoring of vegetation canopies
The gap fractions of three mature hemi-boreal forest stands in Estonia were estimated using the LAI-2000 plant canopy analyzer ( LI-COR Biosciences, Lincoln, NE, USA), the TRAC instrument (Edgewall, Miami, FL, USA), Cajanus’ tube, hemispherical photos, as well as terrestrial (TLS) and airborne (ALS) laser scanners
While Airborne laser scanner (ALS) is used for the estimation of gap fraction near zenith and the TRAC instrument allows the estimation of gap fraction only at large zenith angles at hemiboreal latitudes, Terrestrial laser scanner (TLS) and hemispherical photos allow the analysis of the whole angular profile of the gap fraction
Summary
Radiative transfer and (directional) reflectance of vegetation canopies determine the energy budget, photosynthesis, and possibilities of the remote monitoring of vegetation canopies. In the radiative transfer in a vegetation canopy, transmittance has a key role in direct radiation and the chance of a photon to escape—the gap fraction in the view and/or Sun direction in the canopy. For the analysis of the structure of low vegetation covers, the point quadrat method was suggested a long time ago [1]. Sometimes records are made of only the first contact made by a point quadrat. [2,3] analyzed the point quadrat method and possible errors associated with the method. The method of point quadrats cannot be directly applied for the study of forest canopy. These include: 1. Cajanus tube—a simple handheld instrument equipped with a mirror for looking upwards
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