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Wetness and warmth are the principal factors that control global vegetation distribution. This paper investigates climate-vegetation relationships at a global scale using the normalized difference vegetation index (NDVI), warmth index (WAI), and wetness index (WEI).

The NDVI was derived from a global, 20-year Advanced Very High Resolution Radiometer (AVHRR) dataset with 4-min resolution. The WEI was defined as the ratio of precipitation to potential evaporation. The WAI was defined as the cumulative monthly mean temperature that exceeds 5 degrees C annually. Meteorological data from the International Satellite Land-Surface Climatology Project Initiative II (ISLSCP II) dataset were used to calculate the WEI and WAI. All analyses used annual values based on averages from 1986 to 1995 at 1 x 1 degree resolution over land. Relationships among NDVI, WEI, and WAI values were examined using a vegetation-climate diagram with the WEI and WAI as orthogonal coordinates.

The diagram shows that large NDVI values correspond to areas of tropical and temperate forests and large WEI and WAI values. Small WEI and WAI values are associated with small NDVI values that correspond to desert and tundra, respectively.

Two major regimes are revealed by the NDVI vegetation-climate diagram: wetness dominant and warmth dominant. Wetness dominates mid- and low latitudes. Warmth dominates high latitudes north of 60 degrees N or elevated land such as the Tibetan Plateau. The boundary between the two regimes roughly corresponds to the vegetation boundary between taiga forest and southern vegetation. Over northern Eurasia, the boundary occurs in areas where the NDVI is large and the maximum monthly temperature is around 18 degrees C. Copyright (C) 2006 Royal Meteorological Society.