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Woody encroachment affects the biohydrology of rangelands worldwide and can increase evapotranspiration by increasing plant rooting depth, increasing the duration of the growing season, or by initiating a process of hydrologic recovery in formerly overgrazed landscapes. Eastern redcedar (Juniperus virginiana) is encroaching rapidly into rangelands in the Southern Great Plains of the USA, and beyond, including Oklahoma. However, the degree to which increasing growing season duration causes higher evapotranspiration after encroachment is not known. Here we show that increasing the duration of the growing season in north-central Oklahoma’s water-limited climate from seven months (April–October) to 12 months increases modeled evapotranspiration only marginally, from 95% to 97% of precipitation. However, this increase in evapotranspiration with woody encroachment into grassland corresponded to a two-thirds reduction in deep drainage. This study’s estimate of the hydrologic effects of eastern redcedar encroachment is likely to be highly conservative because it does not take into account the runoff-inducing effects of livestock grazing. Comparing simulated hydrologic fluxes in the present study to past work measuring runoff from grazinglands suggests that eastern redcedar encroachment into overgrazed rangelands is likely to increase evapotranspiration significantly. Whether or not eastern redcedar encroachment effects on evapotranspiration are discernable at the watershed scale will depend on the extent of encroachment throughout the watershed. Further research is necessary to quantify how the hydrologic effects of eastern redcedar encroachment vary due to climatic gradient.
