Comparison of Stem Volume Retrieval Accuracy in Boreal Forest Using Multi-temporal Ers Coherence Images

The availability of multi-temporal ERS one-day coherence images and extensive in situ data from test sites located in Sweden, Finland and Siberia has allowed a first assessment on the consistency of stem volume retrieval at stand level within the boreal zone. The test sites are characterised by different forest structure and ground properties. The coherence data had baselines up to 374 m. The relationship between coherence and stem volume measurements and the spread of the coherence measurements depended on the environmental conditions at and between image acquisitions, specific features of the test site (e.g. forest homogeneity, ground dielectric properties) and ground data accuracy. To retrieve stem volume the semiempirical Interferometric Water Cloud Model, IWCM, was used and it was compared to a simple empirical model. The IWCM could describe the trend in the observations under all environmental and baseline conditions. The accuracy of stem volume retrieval depended on environmental conditions and forest stand structure as well as ground data. Frozen and stable weather conditions were most suitable for retrieval. Smaller errors were obtained at managed test sites. For natural forests the spread of the measurements affected the retrieval negatively. Multi-temporal combination of estimates from individual images could yield results comparable to in situ measurements for managed homogeneous forest stands.