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Colorado State University (2010)

Response of native phreatophytes to changes in precipitation regime in the San Luis Valley, Colorado

Kray, Julie A.

Titre : Response of native phreatophytes to changes in precipitation regime in the San Luis Valley, Colorado

Auteur : Kray, Julie A.

Université de soutenance : COLORADO STATE UNIVERSITY

Grade : Master of Science (MS) 2010

Résumé
Throughout western North America, semiarid and arid basins are likely to experience changes in the timing and amount of precipitation due to global climate change, which may alter regional water budgets. These hydrologic changes may exacerbate water limitations on agriculture, municipalities, and ecosystems in arid regions. Thus, accurate estimates of groundwater outflow from native plant evapotranspiration (ETg) are increasingly critical to managing water resources in basins with large, shallow aquifers. Phreatophytes can contribute significantly to total groundwater outflow on a watershed scale. Some phreatophytes can also acquire soil water recharged by precipitation, which may reduce or supplement their groundwater use. As a result, groundwater use by phreatophyte communities may vary both spatially and temporally in response to seasonal or long-term changes in growing season precipitation. I conducted a two-year rainfall manipulation experiment in the San Luis Valley, Colorado to investigate the responses of four common native phreatophyte species to ambient, increased, and decreased summer monsoon rainfall. Volumetric soil water content was measured in experimental plots to evaluate rainfall treatment effectiveness. I measured xylem pressure potentials (Ψ) to assess the effects of altered precipitation on plant water relations, and compared stable oxygen isotope signatures (δ18O) of plant xylem water to surface (0-15 cm) and sub-surface (15-30 cm) soil layers and groundwater to identify plant water acquisition patterns. The response of plant water relations and water acquisition patterns to changes in surface soil water availability differed by species. A decrease in rainfall had a larger influence on Ψ in the grasses Sporobolus airoides and Distichlis spicata than the more deeply rooted shrubs Sarcobatus vermiculatus and Ericameria nauseosa. S. airoides, D. spicata and S. vermiculatus had significantly lower Ψ when rainfall was naturally low or experimentally reduced, while Ψ of E. nauseosa did not respond to natural or experimental differences in available soil water. Plant xylem water δ18O indicated that S. airoides and D. spicata are almost entirely dependent on precipitation-recharged soil water, while E. nauseosa is almost entirely groundwater-dependent throughout the growing season. S. vermiculatus used groundwater during dry periods, but incorporated more precipitation from upper soil layers after heavy monsoon rainfall. These results suggest that changes in growing season precipitation are more likely to affect S. airoides and D. spicata, while E. nauseosa and to a lesser extent, S. vermiculatus, may be more affected by a decline in water table depth. Persistent changes in precipitation patterns may cause a shift in plant community composition that would alter basin-scale groundwater use by native plants. Results of this work could inform models for managing groundwater in the San Luis Valley, and may have implications for other water-limited regions.

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