Showing papers by "Christian Wirth published in 2001"

Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems

Abstract: Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

Evaporation in the boreal zone during summer - Physics and vegetation

Abstract: Publisher Summary The short but hot and dry summers in the boreal zone involve significant surface–atmosphere energy exchange. This chapter examines the regulation of this exchange in terms of evaporation from vegetation and soil by utilizing available field data, about half of which has been published in the past two years, with some new information from Siberia. For wetland, tundra, and broad-leaved deciduous forest, seasonal average evaporation obtains the theoretically expected equilibrium rate. 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 overstorey 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.