@article {50566,
	title = {Ecohydrological implications of woody plant encroachment},
	journal = {Ecology},
	volume = {86},
	number = {2},
	year = {2005},
	month = {February 1, 2005},
	pages = {308-319},
	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.<a href="http://www.esajournals.org/pdfserv/i0012-9658-086-02-0308.pdf"> http://www.esajournals.org/pdfserv/i0012-9658-086-02-0308.pdf</a>},
	keywords = {arid, ecohydrological, ecosystems, evapotranspiration, semiarid, woody plant encroachment},
	url = {/files/bibliography/05-006.pdf},
	author = {T. Huxman and Wilcox, B.P. and R.L. Scott and K. A. Snyder and Small, E.E. and Hultine, K.R. and Pockman, W.T. and R. B. Jackson}
}
@article {50673,
	title = {Precipitation pulses and carbon fluxes in semiarid and arid ecosystems},
	journal = {Oecologia},
	volume = {141},
	number = {2},
	year = {2004},
	month = {October 1, 2004},
	pages = {254-268},
	abstract = {In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a {\textquoteright}pulse{\textquoteright} of water influences physiological activity in plants, soils, and ecosystems and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare- soil/beneath-canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these ecohydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.},
	keywords = {arid, biological activity, carbon fluxes, ecosystems, precipitation, pulses, semiarid},
	url = {http://www.springerlink.com/media/99a0unmhlkdv8ypgdmwk/contributions/j/c/f/4/jcf4lnwltcxt7wga.pdf},
	author = {T. Huxman and K. A. Snyder and Tissue, D. and Leffler, J. and Pockman, W.T. and Sandquist, D.R. and Potts, D.L. and Schwinning, S.}
}
@conference {50787,
	title = {Ecohydrology and woody plant encroachment: A conceptual framework for evaluating landscape consequences},
	booktitle = {2003 Fall Meeting, American Geophysical Union},
	year = {2003},
	month = {December 8, 2003},
	pages = {F320},
	address = {San Francisco, CA},
	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.  In these systems, shrub removal has been proposed as a mechanism for increasing watershed yield, but such increases are likely only in systems with shallow bedrock, riparian systems, or climatic systems where precipitation exceeds potential evapotranspiration for extended periods.  In semiarid and arid systems abiotic controls on evaporation overwhelm biotic control of streamflow.  In these systems, the predominant effects of woody plant encroachment that promote ecohydrological feedbacks are changes in: infiltration, depth of plant water removal, generation of transient overland flow and near-ground energy budgets that affect the ratio of evaporation (E) to transpiration (T).  In these more xeric areas, the ratio of E:T is critical because it indicates the biological use of water which influences landscape structure.  Increasing woody plant cover is associated with larger open interspaces; at the landscape scale the differential contribution of canopy versus interspaces may change landscape E:T.  Alterations of spatial structure and ecohydrology likely influence other biogeochemical processes, such as carbon cycling.  At the landscape scale, total respiration depends on the magnitude of canopy/interspace respiration flux scaled by the spatial extent of interspaces and canopy space.  Canopy respiration is strongly correlated to T, while interspace respiration is largely driven by soil water availability and, therefore, is driven by factors that control E.  Resolving the partitioning of E:T should help resolve the net effect of woody plant encroachment on other biogeochemical cycles at the landscape scale.  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.},
	author = {T. Huxman and K. A. Snyder and Wilcox, B.P. and R.L. Scott and David D. Breshears and Small, E.E. and R. B. Jackson and Pockman, W.T. and Hultine, K.R.}
}
