Biomass Estimation as Function of Vertical Forest Structure and Forest Height. Potential and Limitations for Remote Sensing (radar and Lidar)

Forest biomass stock and spatial distribution are still unknown parameters for many regions of the world. Today’s information is largely based on ground measurements on a plot basis without remote regions coverage. Thus, a method capable of quantifying biomass by means of Remote Sensing could help to reduce these uncertainties and contribute to a better understanding of the carbon cycle (Houghton et al., 2009). In Mette et al. (2004) a biomass estimation using the power law allometric relationship between biomass and forest height from ground inventories has been proposed: B= 1.66 H where B is the biomass and H forest top height. Thus, this height to biomass allometry allows biomass estimations from remote sensing systems capable to resolve forest height (LIDAR and polarimetric SAR interferometry (Pol-InSAR)). However, this approach meets its limitations for forest ecosystems under changing conditions in density and structure. Thus, to improve biomass estimation accuracy additional parameters need to be measured. Pol-InSAR and LIDAR allow getting, besides forest height, vertical backscattering profiles which are connected to forest vertical structure (Cloude, 2007; Lefsky, 1999). The main objective of the present study is to investigate whether vertical forest structure, is linked to total above-ground biomass (AGB) and to develop a methodology for the estimation of AGB from forest structure parameters. Later these structure parameters will be related to vertical backscattering profiles obtained from LIDAR and radar systems. Therefore inventory data from three different test sites situated in the South of Germany which represent different management systems are investigated: Traunstein, National Park “Bayerischer Wald” and Ebersberger Forst. The Traunstein test site is a temperate mountainous forest, the National Park “Bayerischer Wald” a close natural forest without management and Ebersberger Forst an intensively managed forest site. Forest vertical structure can be characterized by forest vertical biomass distribution. The AGB of a single tree can be divided into two main components: crown and stem. Defining the form of stems and crowns as well as amount of biomass per compartment for every tree (Pretzsch, 2001; Zianis et al., 2005), a vertical biomass distribution can be reconstructed. The sum of the biomass distribution for all trees within a certain area (measurement plot) results in characteristic biomass profiles, as shown in Figure 1-left. The best characterization of forest vertical structure is obtained using the Legendre polynomials. Biomass profiles can be then characterized by the decomposition into a set of basis functions. The Legendre decomposition is based on the Legendre series B(z) described as: