|Huxman, Travis - UNIVERSITY OF ARIZONA|
|Wilcox, Bradford - TEXAS A&M|
|Small, Eric - UNIVERSITY OF COLORADO|
|Hultine, Kevin - UNIVERSITY OF ARIZONA|
|Pockman, William - UNIVERSITY OF NEW MEXICO|
|Jackson, Rob - DUKE UNIVERSITY|
Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 6, 2004
Publication Date: February 1, 2005
Citation: Huxman, T.E., Wilcox, B.P., Breshears, D.D., Scott R.L., Snyder, K.A., Small, E.E., Hultine, K., Pockman, W.T., Jackson, R.B. 2005. Ecohydrological implications of woody plant encroachment. Ecology. 86(2):308-319. Interpretive Summary: A commonly held view is that woody plant encroachment has lead to reductions in streamflow in semiarid and arid regions. Therefore, removal of woody shrubs is thought to result in increased water yields. However, there is little empirical evidence for this in water-limited regions. This paper presents a conceptual model for understanding and predicting the effects of woody plants on the hydrologic cycle. The goal is to develop a suitable framework for understanding where on the landscape woody shrub removal will increase water yields and where it may not have the desired effect. The paper also reviews the current literature on studies that partition evapotranspiration into its component parts, plant transpiration and soil evaporation. Although they are generally measured as one term, transpiration and evaporation drive fundamentally different processes with transpiration being proportional to plant production and evaporation driving soil process. Lastly, this paper presents a second conceptual model that addresses how evaporation and transpiration may change with woody shrub encroachment across an aridity gradient.
Technical Abstract: Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important implications for hydrology, ecology, and society. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into E (evaporation) and T (transpiration). On the basis of this framework, we suggest that streamflow and evaporative processes are affected by woody plant encroachment in different ways, depending on the degree of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Similarly, encroachment of woody plants can be expected to produce relatively large shifts in the ratio of E to T in semiarid ecosystems, whereas such shifts will be small or negligible in both subhumid and xeric ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change,such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased T and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils can be predicted. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides additional information on the consequences of woody plant encroachment.