Assessment of the MODIS Leaf Area Index Product (MOD15) in Alaska

The Moderate Resolution Imaging Spectroradiometer (MODIS) Leaf Area Index (LAI) Product (MOD15A2) was evaluated for the growing seasons of 2000 through 2004 in Alaska. The LAI estimate may be affected by three factors not directly related to canopy leaf area: snow melt, cloud contamination and conifer forest versus broadleaf shrub canopy type. Increases in MODIS LAI values occurred during the snowmelt period, prior to leaf flush along an elevation gradient in central Alaska. This false LAI increase prior to broadleaf budburst could lead to an overestimate of growing season length based on the MODIS LAI product. During the spring greenup period, there were temporal dips in MODIS LAI estimates for up to 57% of the pixels. This decrease in MODIS LAI value was likely due to cloud contamination, despite use of the MODIS quality control information to select pixels that were cloud-free. The MODIS LAI algorithm may be sensitive to variation in near-infrared reflectance due to canopy type rather than leaf area. For example, coniferous boreal forests typically have a higher LAI than shrub tundra. However, the maximum seasonal LAI estimate from the MODIS product was consistently higher from shrub tundra areas compared to coniferous boreal forest areas. There was a strong correlation of the MODIS LAI estimate with MODIS near-infrared reflectance among conifer and broadleaf shrub frames. This could lead to overestimates of LAI in areas where coniferous forest is replaced by broadleaf shrub following wildfire in boreal forest regions.