Transpiration (water that passes through vascular plants to the atmosphere) is an important parameter in wetland plant studies because it represents the interaction between a wetland's hydrologic regime and its vegetation. Transpiration is the only component of the water budget that is dependent entirely upon plants. Estimates of transpiration are often combined with evaporation (water that vaporizes directly from the water or soil); this measure is known as evapotranspiration (ET). When water supplies are not limiting, meteorological factors tend to control rates of ET. The rate of evapotranspiration is affected by solar radiation, wind speed and turbulence, available soil moisture, and relative humidity. Rates vary with the difference in vapor pressure at the water surface or leaf surface and the vapor pressure of the atmosphere. As the vapor pressure of the water or leaf surface increases relative to the atmosphere (due to solar energy or increases in temperature, for example), ET rates increase. When differences in vapor pressure decrease, for example when humidity increases or wind speeds decrease, ET rates decrease in response (Mitsch and Gosselink 2000).
On an ecosystem level, water outputs due to ET are largely controlled by vegetation (both the species present and their areal extent) and the supply of water (Lafleur 1990a, b; Gilman 1994). In many cases, ET is the largest loss term in the water balance equation (Hollis et al. 1993; Gilman 1994; Owen 1998). For example, Verhoeven and others (1988), working in mesotrophic and eutrophic fens, found ET rates of 482 mm yr-1. This accounted for 60% of total annual precipitation.
Additionally, soil porosity may affect ET by limiting or facilitating the movement of water in the soil to roots or to the soil surface. Mann and Wetzel (1999) demonstrated this in a mesocosm study using Juncus effusus (soft rush). When grown in clay, J. effusus did not cause a decrease in soil water levels. However, in more porous sandy soils, where water movement in the soil is relatively quick, J. effusus caused a decline in the water level. Table 3.2 summarizes the results of some studies comparing the rates of ET from different vegetation stands.
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