Abstract
Medium resolution leaf area index (LAI) products have usually been validated only at the peak of the growing season for boreal forest areas. In this study, we evaluated seasonal variations in MODIS Collection 5 (C5) LAI products for an evergreen coniferous forest area in southern Finland. A time series of field measurements of LAI and nine fine resolution satellite images (SPOT4 HRVIR, EO-1 Hyperion) for the snow free period in 2010 were used for developing empirical regression models for LAI estimation. We used two different reference LAI: the first one corresponded to an effective overstory LAI, and the second one was a clumping-corrected LAI that included both over- and understory vegetation. According to the results, MODIS LAI retrievals were closer to the effective LAI in the spring and autumn, when saturation of MODIS LAI main algorithm did not occur. On the other hand, in the summer, when saturation occurred, MODIS LAI was closer to the total LAI. Therefore, as a result of high LAI retrieved under the condition of saturation, the seasonal course of LAI was exaggerated in comparison to the reference LAI time series. Furthermore, MODIS LAI retrievals were unrealistically small before the leaf burst if considering the large proportion of evergreen coniferous forest in the study area. The results emphasize the need for multi-seasonal validation data sets in order to better understand the performance of LAI retrieval algorithms for evergreen coniferous forests throughout the snow free period.
Original language | English |
---|---|
Pages (from-to) | 104-115 |
Number of pages | 12 |
Journal | Remote Sensing of Environment |
Volume | 126 |
DOIs | |
Publication status | Published - 1 Nov 2012 |
MoE publication type | A1 Journal article-refereed |
Funding
We thank Anu Akujärvi for field work and Petri Keronen (Department of Physical Sciences, University of Helsinki) for providing the field observations on snow depth for our study area. We also thank the editor and three anonymous reviewers for their helpful comments on the manuscript. Our work has been supported by the Academy of Finland , the Emil Aaltonen Foundation and the University of Helsinki Research and Postdoctoral Funds . The Hyperion data is provided courtesy of the USGS and the SPOT imagery was obtained from SPOT data/ISIS Programme (©CNES (2010). Distribution Spot Image SA).
Keywords
- Conifer
- Evergreen needleleaf
- Leaf area index
- Vegetation index