Conclusions

In the boreal zone, summer rainfall frequency and quantity decline significantly from south to north, except in maritime Scandinavia. Correspondingly, the vegetation changes from forest to tundra with wetland being relatively ubiquitous. Average rainfall rate and soil water storage capacity range from ca. 0.5-2.0 mm day-1 and 8-13 mm water per 0.1 m soil, respectively. Snowmelt generally ensures a relatively full store of soil water at the beginning of summer. Wetland can receive a supplemental supply of water that accounts for its evaporation (averaging 2.6 mm day-1), sometimes exceeding rainfall. Tundra and forest evaporation (averaging 1.5 and 2.2 mm day-1, respectively) and rainfall rates are almost equal, illustrating the dominant effect of summer precipitation on terrestrial ecosystem evaporation rate. For wetland, tundra, and broad-leaved deciduous forest, seasonal average evaporation obtains the theoretically expected equilibrium rate. Given patchiness of the boreal landscape and entrainment of dry air into the con-vective boundary layer on fine summer days, we do not consider these surfaces to be completely devoid of influence on E. However, a variety of sometimes unrelated factors apparently compensates over the course of a summer. For deciduous and evergreen needle-leaved forests, evaporation is about 70 and 50%, respectively, of the equilibrium rate, indicating an overwhelming degree of surface control. Among the three tree life-forms found in the boreal zone, forest evaporation rate is physiologically related to over-storey leaf habit, xylem anatomy, and especially successional position following disturbances such as fire. For the forests compared, this determines leaf nitrogen content and in turn the maximum stomatal and surface conductances and the leaf area index via the effects on photosynthetic and growth rates. The leaf area index affects understorey evaporation rate which is half the total in the needle-leaved forests, and is governed largely by rainfall frequency. These conclusions about tree life-form are supported by data on leaf carbon isotope discrimination reflecting close linkages between nutrients, water use, and carbon gain in boreal forests.

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