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[1] Reliable monitoring of the leaf area index (LAI) is required to further understand the carbon, water, and energy cycles of forests. In this study, we proposed a new satellite-based method to estimate the overstory LAI (LAI o ) separately from the understory LAI (LAI u ) for larch forests covering eastern Siberia. We modeled forest scenes representative of larch forest structure, with particular consideration of the typical clumped shoot structure of larch. Three-dimensional radiative transfer simulations were then conducted under various forest conditions to establish the relationships between LAI o and seasonal increases in the normalized difference water index after leaf appearance. Model-based sensitivity analyses indicated a maximum error of up to 26% under known noise levels. Averaged at the continental scale, total LAI from our estimates, the CYCLOPES version 3.1, and the Moderate Resolution Imaging Spectroradiometer MOD15 Collection 5 (main algorithm) showed similar ranges in summer. However, spatial pattern of LAI was slightly different, with smoother variability for CYCLOPES LAI. Our LAI and CYLOPES effective LAI reproduced a realistic seasonal variation with exact timing of spring increase in LAI o . The main drawbacks of MOD15 Collection 5 were unrealistically strong temporal variability, and the fact that LAI began to increase earlier than the overstory leaf appearance date. Overall, the results show that our new method is a good alternative to MOD15 Collection 5 and CYCLOPES, as it provides separate estimates of LAI o and LAI u and true LAI instead of effective LAI.Citation: Kobayashi, H., N. Delbart, R. Suzuki, and K. Kushida (2010), A satellite-based method for monitoring seasonality in the overstory leaf area index of Siberian larch forest,

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This study investigates the regional relationship between the satellite‐derived vegetation index (Normalized Difference Vegetation Index, NDVI) and climatological parameters (temperature and precipitation) over Siberia on a 5‐year (1986–1990) annual mean basis. The NDVI in Siberia shows a large value around the 60°N zone, and it gradually decreases toward the southern arid region and the low temperature polar region. This meridional profile (south–north regionality) of the NDVI was analysed in two meridional transects, along 75°E (arid–forest transect) and 110°E (forest–tundra transect). A modified warmth index (WI(0)) was utilized as a temperature index. In the 75°E transect, high positive (0.79) and negative (−0.58) correlations were found in the NDVI–precipitation and the NDVI–WI(0) meridional profiles, respectively. This fact implies that precipitation plays a role in providing water and dominates the vegetation distribution, while high temperature induces dryness and water shortages, i.e. the critical factor for the vegetation meridional profile is available water in the arid–forest transect. In the 110°E transect, a high positive (0.92) correlation was found, which suggests that the dominant factor for the NDVI profile is temperature in the forest–tundra transect, i.e. the critical factor for the vegetation meridional profile here may be temperature. Furthermore, these meridional profiles were scrutinized in terms of the station altitude and it was suggested that the NDVI tends to change depending on the WI(0) and precipitation at regional variations, which are basically the result of the altitude difference between the stations. The comprehensive relationship between the NDVI, WI(0) and precipitation regionalities is discussed in terms of six vegetation types. The result shows that high NDVI (>0.2) is observed when the WI(0) is over 40°C and precipitation is more than 2.5×WI(0)+50. Copyright © 2000 Royal Meteorological Society

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