TEMPORAL VARIATION IN ACTUAL EVAPOTRANSPIRATION OF TERRESTRIAL ECOSYSTEMS - PATTERNS AND ECOLOGICAL IMPLICATIONS

We compared the water balance among the earth's major terrestrial ecosystems. Potential evapotranspiration, actual evapotranspiration (AET) and deficit were derived for each month and year from 19-25-yr climate records at ninety-four sites around the world representing eleven biomes. For each variable, we determined mean annual values. Our focus, however, was to examine temporal variation in AET, which previously has been correlated with large-scale patterns of ecosystem structure and function. Standard deviation of annual AET, an absolute measure of interannual variability, was highest for grasslands and lowest for tundra and taiga. Coefficient of variation of annual AET, a relative measure of variability, was negatively related to mean AET, and was higher for non-forested than for forested ecosystems. Monthly variation, an index of seasonality, and interannual variation of AET were positively related for forested ecosystems and negatively related for non-forested ecosystems. There was a positive relationship between interannual variability and variation among sites within a biome. Results indicate (1) time-scale dependent differences in climatic variability among biomes that may help predict functional properties and explain structural patterns among terrestrial ecosystems; and (2) a link between temporal variability and spatial heterogeneity among biomes.