On the Potentials of PolInSAR Inversion for Monitoring Long-Term Forest Height Dynamics at L-band

Abstract

Forest height is one of the most important parameters for (forest) stand characterization, and it is closely related to biomass. It provides information on stand condition and site index, and it allows characterizing the successional state of the forest. At the same time, the distribution of forest heights within a stand may be used to assess the disturbance regime and to detect logging activities. In the characterization of dynamic forest processes, the knowledge of forest height changes is even more important than absolute height measurements itself. Indeed, forest height changes can be directly used to characterize forest growth, mortality and deforestation, and to conclude about the associated carbon fluxes independently from the successional status of the forest. Forest densdormetric variables are usually measured by airborne laser scanning by using LiDAR instruments that are very precise in characterizing the tree heights and the main related parameters. However, LiDAR technology is expensive and in many cases not affordable, especially in a multitemporal monitoring scenario. Low frequency synthetic aperture radar (SAR) sensors in a polarimetric-interferometric configuration represent a very promising alternative tool to LiDAR for estimating forest height and its dynamics as they can provide large continuous coverage and high spatial and temporal resolution with a limited cost. From the methodological point of view, polarimetric SAR interferometry (PolInSAR) combines the inherent sensitivity of the interferometric coherence to the vertical structure of volume scatterers combined with the potential of SAR polarimetry to interpret and characterise the individual scattering processes. Exploiting this combination, forest height estimations obtained in the last years from a pre-operational to an operational PolInSAR product that have been validated in the frame of several campaigns over a wide range of forests, terrains and environmental conditions. The overall obtained estimation error is in the order of 10% or smaller. The objective of this work is to assess qualitatively and quantitatively the capability of monitoring forest dynamics by using PolInSAR height maps. To this purpose, a multibaseline-multitemporal PolInSAR L-band airborne dataset over the temperate forest site of Traunstein (South of Germany) has been used. Data have been acquired since 2003 with the DLR’s E- and F-SAR radar platforms covering more than a decade, with temporal baselines ranging from hours to years. For each acquisition date, a multibaseline PolInSAR inversion with short temporal baseline (in the order of one hour) has been carried out. Accordingly, a time series of accurately estimated forest heights has been obtained without disturbances due to (long-term) temporal decorrelation. Depending on the time lag between two different height maps, several comparisons could be carried out in order to characterize height differences induced by scattering changes within the same stand and/or variations of the baseline distribution. Such scattering changes can in general be caused by different sources, like the water content of the vegetation (e.g. before and after a rain event), the presence of wind, the seasonal cycles, disturbances and management activities. These components could be observed in the data and their effects characterized. Extensive results of this analysis will be presented, and the capability of distinguishing between different sources of PolInSAR height changes will be discussed together with their potentials for identifying and monitoring ecosystem changes.