LEAF AREA INDEX ESTIMATION OF BOREAL AND SUB-ARCTIC FOREST USING ENVISAT/ASAR DATA OF VARIOUS SWATHS Terhikki Manninen (1) , Pauline Stenberg (2) , Miina Rautiainen (2) , Pekka Voipio (3) Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland, Email: terhikki.manninen@fmi.fi Department of Forest Ecology, P.O. Box 27, FI-00014 University of Helsinki, Finland, Email: pauline.stenberg@helsinki.fi , miina.rautiainen@helsinki.fi Finnish Forest Research Institute, Suonenjoki Research Station, FI-77600, Finland, Email: pekka.voipio@metla.fi ABSTRACT Altogether 75 ENVISAT ASAR alternating polarization data were acquired over two VALERI test sites, boreal and sub-Arctic, in 2004. The swath range from IS1 to IS6 was studied. In both test sites a linear relationship between the VV/HH backscattering ratio and leaf area index (LAI) was found. In the boreal test site about 20 cases out of 30 the R 2 value exceeded 0.6 already for mean LAI values of 43 plots and corresponding truncated mean backscattering values of distributions obtained from 43 circles of 90 m radius. For the sub-Arctic test site high R 2 values required inclusion of a larger number of pixels in the mean VV/HH backscattering ratio. Also the regression parameter values were slightly different from those obtained at the boreal site. All swaths could be used for LAI estimation, but larger incidence angles are preferred due to the smaller pixel size. 1 INTRODUCTION The leaf area index (LAI) is one of the most important biophysical vegetation parameters, because it drives the exchanges of matter and energy flow between vegetation and the atmosphere. Because LAI is a dynamic parameter, which varies seasonally and responds rapidly to stress factors and (changes in) climatic conditions, the information is needed at sufficiently frequent intervals. Typically, estimates of LAI are derived using spectral vegetation indices (SVIs) calculated based on optical satellite data. The cloud cover, however, constitutes a big problem for optical LAI estimation of large areas. Therefore a microwave data based LAI estimation method would be a valuable alternative and/or addition. Promising results of LAI estimation using the VV/HH backscattering ratio of ENVISAT/ASAR were indeed obtained already at a previous Norway spruce (Picea abies) dominated test site in Suonenjoki, close to the Hirsikangas test site used in this study [1, 2]. Theoretical support for a linear relationship between VV/HH backscattering ratio and LAI was also found using DDA modelling [3]. The effect of the swath is studied here. The VAlidation of Land European Remote sensing Instruments (VALERI) is an international project funded by CNES and coordinated by INRA with an objective to evaluate the absolute accuracy of the biophysical products, such as LAI, derived from large swath sensors using a range of possible algorithms and to inter-compare the products derived from different sensors and algorithms [4]. For this purpose VALERI project develops a network of sites distributed over the Earth’s surface [4]. In Finland there were two VALERI sites in 2004: one representing boreal forest of Central Finland (Hirsikangas) and the other representing sub-Arctic forest at the polar circle (Rovaniemi). The VALERI network is concentrated on using optical instruments, but related microwave studies have been carried out in an ENVISAT AO project ID423. 2 MATERIAL 2.1 Test sites The boreal test site Hirsikangas (62°38.7N, 27°0.5E) was established in 2003. The site, according to VALERI standards, is a 3 km x 3 km area without any large water bodies [see Section 2.3]. Approximately 90% of the site is covered by forest with Scots pine (Pinus sylvestris) as the dominating species, the rest consisting of agricultural fields, peat production area and a few small ponds. The topography of the test site is relatively flat: the height variation is about 15 m, but again there are steep slopes (25° at 25 m resolution) and anisotropy is evident (aspect angle distribution is peaked at 232° and 30°). The 3 km x 3 km sub-Arctic test site Rovaniemi (66.47º N, 25.32º E, 120 meters above sea level), was established in 2004. It belongs both to the VALERI network and the NorSEN network [6]. About 75% of the forest site is dominated by Scots pine and the rest is dominated by Norway spruce. The topography of the test site is relatively flat: the height variation is about 80 m at the LAI measurement plots the slope is less than 10˚ at 25 m resolution and the aspect angle distribution is very flat. _____________________________________________________ Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)