Water Cycles in Forests

Abstract

This chapter presents observational studies of water and energy balance, mainly of larch forest ecosystems, during the plant-growing season in eastern Siberia (Central Yakutia). Characteristic seasonal variation in surface energy and water balance reflects hydrothermal conditions of the freezing/thawing layer of the upper permafrost and plant phenology, as well as atmospheric variation. Interannual variation in forest evapotranspiration is smaller in this region than is variation in precipitation because the supply of soil water during the growing season consists of snowmelt and thawed water from the upper frozen soils, which is equivalent to soil water storage in the previous autumn, as well as rainfall during the current season. Forest evapotranspiration and its biophysical response, represented by surface conductance, covary with environmental variables such as atmospheric humidity deficit and soil water in daily, seasonal, and interannual scales. Differences in forest structure, species composition, and soil properties result in seasonal and interannual evapotranspiration that is both specific and common to several forests in eastern Siberia. Seasonal variation in the surface energy balance in thermokarst landscapes, which consist of lakes and surrounding grasslands, contrasts with that in adjacent forests. Finally, apart from the environmental variations typical of this region, an unexpectedly wet climate in the late 2000s had an influence on these forest ecosystems. Field observations of damaged larch forests and irrigation experiments have revealed the responses of larch forest ecosystems to excess soil water conditions.