Abstract
The structure of forests is important to observe for understanding coupling to global dynamics of ecosystems, biodiversity, and management aspects. In this paper, the sensitivity of X-band to boreal forest stem volume and to vertical and horizontal structure in the form of forest height and horizontal vegetation density is studied using TanDEM-X satellite observations from two study sites in Sweden: Remningstorp and Krycklan. The forest was analyzed with the Interferometric Water Cloud Model (IWCM), without the use of local data for model training, and compared with measurements by Airborne Lidar Scanning (ALS). On one hand, a large number of stands were studied, and in addition, plots with different types of changes between 2010 and 2014 were also studied. It is shown that the TanDEM-X phase height is, under certain conditions, equal to the product of the ALS quantities for height and density. Therefore, the sensitivity of phase height to relative changes in height and density is the same. For stands with a phase height >5 m we obtained an root-mean-square error, RMSE, of 8% and 10% for tree height in Remningstorp and Krycklan, respectively, and for vegetation density an RMSE of 13% for both. Furthermore, we obtained an RMSE of 17% for estimation of above ground biomass at stand level in Remningstorp and in Krycklan. The forest changes estimated with TanDEM-X/IWCM and ALS are small for all plots except clear cuts but show similar trends. Plots without forest management changes show a mean estimated height growth of 2.7% with TanDEM-X/IWCM versus 2.1% with ALS and a biomass growth of 4.3% versus 4.2% per year. The agreement between the estimates from TanDEM-X/IWCM and ALS is in general good, except for stands with low phase height.
Highlights
Remote sensing methods to determine forest properties such as above ground biomass (AGB) and forest structure such as height and vegetation density are important, cf. [1] (In this paper, the word vegetation density will be used to describe the horizontal vegetation density as measured either by airborne lidar scanning, Airborne Lidar Scanning (ALS), or TanDEM-X.) AGB has been identified as an Essential Climate Variable, needed to reduce uncertainties in our knowledge of the climate system [2] and the structure is important for management aspects and biodiversity
For the 4 × 12 plots with specific changes, we find that the TanDEM-X/Interferometric Water Cloud Model (IWCM) height is in good agreement with ALS H95 but with some overestimation for lower phase heights
The goal of this paper was to estimate height, density, and stem volume/AGB from bistatic TanDEM-X data and a digital terrain model (DTM), by means of IWCM solved without the need for local field data to train the model or for regression between properties
Summary
Remote sensing methods to determine forest properties such as above ground biomass (AGB) and forest structure such as height and vegetation density are important, cf. [1] (In this paper, the word vegetation density will be used to describe the horizontal vegetation density as measured either by airborne lidar scanning, ALS, or TanDEM-X.) AGB has been identified as an Essential Climate Variable, needed to reduce uncertainties in our knowledge of the climate system [2] and the structure is important for management aspects and biodiversity. Low frequency has been seen as the promising alternative due to its high penetration into the forest and sensitivity to the more important part of the forest contributing to AGB such as branches and stems. This conclusion goes back to observations of radar backscatter investigations, e.g., [19,20,21].
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