Abstract
Both vegetation multi-angular and LiDAR (light detection and ranging) remote sensing data are indirectly and directly linked with 3D vegetation structure parameters, such as the tree height and vegetation gap fraction, which are critical elements in above-ground biomass and light profiles for photosynthesis estimation. LiDAR, particularly LiDAR using waveform data, provides accurate estimates of these structural parameters but suffers from not enough spatial samplings. Structural parameters retrieved from multiangular imaging data through the inversion of physical models have larger uncertainties. This study searches for an analytical approach to fuse LiDAR and multiangular data. We explore the relationships between vegetation structure parameters derived from airborne vegetation LiDAR data and multiangular data and present a new potential angle vegetation index to retrieve the tree height and gap fraction using multi-angular data in Howland Forest, Maine. The BRDF (bidirectional reflectance distribution factor) index named NDMM (normalized difference between the maximum and minimum reflectance) linearly increases with the tree height and decreases with the gap fraction. In addition, these relationships are also dependent on the wavelength, tree species, and stand density. The NDMM index performs better in conifer (R = 0.451 for tree height and R = 0.472 for the gap fraction using the near infrared band) than in deciduous and mixed forests. It is superior in sparse (R = 0.569 for tree height and R = 0.604 for the gap fraction using the near infrared band) compared to dense forest. Moreover, the NDMM index is more strongly related to tree height and the gap fraction at the near infrared band than at the three visible bands. This study sheds light on the possibility of using multiangular data to map vegetation’s structural parameters in larger regions for carbon cycle studies through the fusion of LiDAR and multiangular remote sensing data.
Highlights
Vertical and horizontal forest-structure information is essential for many aspects of global ecosystem studies
Our analysis demonstrates that the NDMM index from visible to near infrared wavelengths increases according to LiDAR-derived tree height and RH50 and decreases with a gap fraction in all three types of forests: conifer, deciduous, and mixed
We found better relationships between the LiDAR-derived tree height, gap fraction, and NDMM index at the near-infrared band for sparse conifer forests in Howland
Summary
Vertical and horizontal forest-structure information is essential for many aspects of global ecosystem studies. The tree height and vegetation gap fraction are the key parameters used to calculate the above ground biomass. Biomass represents the carbon store of forest cover. The accurate assessment of the carbon storage of forest cover over large areas is essential to the quantitative measurement and modeling of the global carbon ecosystem. Three-dimensional forest-structure data represent the different successive stages of forest ecosystems and are good indicators of ecosystem processes, such as natural and anthropogenic disturbances. Forest-structure data are important for many ecosystem process studies. Measuring the 3D canopy structure from the ground is difficult and time consuming. Remote estimation of vegetation-structure characteristics is an essential tool for advancing 3D vegetation-structure parameter estimation for many ecosystem modeling studies
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