Transpiration: Whether it varies with land-use change or not, and why

Background/Question/Methods

Understanding global variation in evapotranspiration 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 and yet we lack unifying principles to predict when transpiration varies with land use. Plant transpiration (T) is a dynamic and often dominant component of evapotranspiration, 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 land use changes: 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, and 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. To illustrate these principles, four case studies are examined.

Results/Conclusions

In the Pacific Northwest, T was 3.3 times higher in a 40-yr-old forest compared with a 450-yr-old forest because of differences in water use per unit amount of vegetation, not differences in leaf area, which was similar for both forests. In a Texas mesquite brushland, T differed briefly after plowing, which temporarily increased groundwater recharge by 1.9 mm per year because soil depths were sufficient for brush roots to access deep moisture reserves. Despite striking differences in land cover between a Texas savanna and woodland, differences in ET between sites were small because of compensations in the energy balance, driven in part by differences in edaphic conditions. Likewise, T was not altered in a Middle Rio Grande riparian forest after removal of the Tamarix understory because the overstory compensated for it. We suggest this simple set of principles unify results of wide-ranging studies of T following land use change and can explain underlying causes of T variation.