Proposed principles governing how vegetation changes affect transpiration

Abstract

Understanding global variation in evapotranspiration (ET) is critical for accuracy of climate models, predictions used in water resources management, and assessment of land use change impacts on the water balance of ecosystems-yet we lack unifying principles to predict when transpiration (T) varies with land use. Plant T is a dynamic and often dominant component of ET, and is affected by a variety of processes controlled by land use changes superimposed onto edaphic conditions. We propose the following three principles that determine whether T will vary with changes in vegetation: variation will result if energy balance partitioning has been altered, if deeper or shallower active rooting depth has changed the amount of soil moisture accessible to plants, or if temporary changes in water use add up over longer time scales. Clearly these concepts are not new; however, they are often overlooked in favor of blanket assumptions that large changes in vegetation inevitably alter T. Not so. Our suggested framework incorporates both edaphic and plant traits that determine whether T will vary or not in response to altered land cover conditions. We suggest that this simple set of principles unifies results of wide-ranging studies of T following land use change and can explain underlying causes of T variation. These principles are illustrated through case studies from four different environments: Pacific Northwest forest, Texas mesquite brushland, Texas savanna and woodland, and Middle Rio Grande riparian forest. The proposed principles seem broadly applicable, should be further evaluated, and are directly relevant for land management.

Ecohydrology and the Critical Zone: Processes and Patterns Across Scales

“…Transpiration and carbon assimilation is driven by available energy and plant-available water in the Critical Zone (Moore and Heilman, 2011). The energy used to transpire water, that is, latent heat, is partitioned from net radiation and sensible heat by factors such as albedo, leaf area, thickness of the boundary layer, and atmospheric vapor pressure deficit.…”

An ecohydrological sketch of climate change impacts on water and natural ecosystems for the Netherlands: bridging the gap between science and society

“…Errors in the simulated actual evapotranspiration (ET) can be considerable (e.g. Liu and Yang, 2010;Moore and Heilman, 2011;Zhang et al, 2010) but are usually obscured because modelling is not validated on ET. Though the availability of measured actual ET, e.g.…”

“…In addition, hydrological models often do not account for the fact that plants may reduce their future transpiration rate because their necessary consumption of carbon through their stomata is fulfilled more easily at enhanced atmospheric CO 2 -levels (De Boer et al, 2011;Kruijt et al, 2008). Finally, in practice current transient models do not account for the fact that vegetation may respond to increased drought by changes in vegetation cover and in rooting depth (Knapp et al, 2008b;Moore and Heilman, 2011;Porporato et al, 2001;Wegehenkel, 2009). In the Netherlands, this adaptation of vegetation to climate change can especially be expected on elevated sandy soils with deep groundwater tables, such as Pleistocene hills and coastal dunes.…”

IRRIGATED AND RAINFED CROPS Zea mays L. (MAIZE) AND Glycine max (SOYBEAN) ACTING AS A SOURCE OR SINK FOR ATMOSPHERIC WARMING AT MEAD, NEBRASKA

“…More work needs to be done to increase understanding of the influence that vegetation type and irrigation have on other physical processes such as transpiration, cloud formation and precipitation at the local to global scale. The combined impact of irrigation and the promotion of crop cultivars with significantly higher albedos (Doughty et al, 2011) and crops with deeper active rooting systems (Moore and Heilman, 2011); may potentially offset atmospheric warming in temperate regions (latitudes>30°).…”

“…These results may be explained by the edaphic conditions (e.g. available soil water, soil water holding capacity) and active rooting depth of the crop (Moore and Heilman, 2011). In 2002 and 2004, irrigation of soybeans (IMS) mitigated 37 and 9 % of H released into the atmosphere by a neighboring rainfed soybean field (RMS).…”