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Historic water-use of Populus trees along a diverted mountain stream in the eastern Sierra Nevada.
Alstad, Karrin*,1,2, Hart, Stephen1,2, 1 School of Forestry, College of Ecosystem Science and Mgt., Flagstaff, AZ2 Merriam Powell Center for Environmental Research, Flagstaff, AZ
ABSTRACT- Stream flow diversions for hydropower generation and water supply to desert cities have greatly altered western riparian ecosystems. Thus, understanding the degree of vegetation dependence on instream flows is critical for setting minimum flow regulations. We assessed the historic water-use of Populus trees located along an extensively diverted stream in the eastern Sierra Nevada by quantifying the deuterium content in tree ring cellulose (Dcell) as an index of the water sources used, and by determining the carbon-13 content of tree ring cellulose (13Ccell) to estimate annual water use efficiency. Five streamside trees were sampled at two elevations along Bishop Creek, CA: P trichocarpa at 2465 m and P. fremontii at 1400 m. Cellulose was extracted from annual tree rings representative of high and low stream flow, and Dcell and 13Ccell were determined. Across all years, the correlation between Dcell and 13Ccell was only significant for the low elevation site (r = 0.30). This correlation between these two isotopes at the low but not the high elevation site suggests lower inter-annual variation in water-source use for individual trees at the lower site. Stream flow and the standard deviation of Dcell were well correlated at the high elevation site (r = 0.89) suggesting increased variation in water sources utilized by these trees in high-flow years. The confined nature of the stream reach at the high elevation site likely results in a more responsive water table to changes in stream flow. The non-linear relationship of seasonal stream flow to Dcell at both sites suggests ground water dependence (mean Dcell = -130‰) in the lowest stream flow (<250 m3/sec), and surface and soil water use (-123‰) in mid-flow years (300 to 1300 m3/sec). Depleted Dcellat high flows (-129‰ when >1300 to 3000 m3/sec) are likely influenced by depleted snow-pack runoff.
KEY WORDS: Populus, cellulose, Dcell, 13Ccell