GLOBMAP LAI: A long-term global Leaf Area Index (1982-2014) With Seperation of forest overstory and understory

Abstract:

Leaf area index (LAI) is a critical parameter for modeling vegetation’s water, carbon and energy exchange with the atmosphere. Several global LAI datasets have been generated from satellite observations. However, they are limited in temporal coverage and representation of canopy structure. In this presentation, we generate a long time series (1982-2014) of global LAI products and separately estimate global forest overstory and understory LAIs by fusing AVHRR, MODIS and MISR data.

A long-term consistent LAI was generated from AVHRR and MODIS data by establishing an AVHRR Simple Ratio (SR)-MODIS LAI relationship pixel by pixel. Firstly, A MODIS LAI series was derived from MODIS data based on the GLOBCARBON LAI algorithm. Then, the pixel-based relationships between AVHRR SR and MODIS LAI were constructed using these two datasets for the overlapping period. This relationship was applied to historical AVHRR data to derive LAI back to 1981. Since the MODIS LAI is used to constrain AVHRR retrieval, it ensures that the LAI from these two sensors are consistent. Then, the global forest overstory and understory LAIs were estimated separately by fusing MISR and MODIS observations. Monthly forest understory LAI was retrieved from the forest understory reflectivity estimated using MISR data at a spatial resolution of 1 km (Jiao et al., 2014, JGR). The forest overstory LAI was estimated from the MODIS land surface reflectance, after correcting for the background contribution using monthly mean forest understory reflectivities.

The comparison during the overlapped period show that the LAIs from the two different sensors are good consistency, with LAI differences are within 0.6 over 99.0% vegetated pixels. Forest understory vegetation is mainly distributed in the boreal forests zones at northern latitudes (40°N to 70°N). Higher understory LAI values are found in needleleaf forests than in broadleaf forests. The results are also compared with other products and field measurements.