Effects of Stream Flow Patterns on Riparian Vegetation of a Semiarid River: Implications for a Changing Climate

As global climate change affects recharge and runoff processes, stream flow regimes are being altered. In the American Southwest, increasing aridity is predicted to cause declines in stream base flows and water tables. Another potential outcome of climate change is increased flood intensity. Changes in these stream flow conditions may independently affect vegetation or may have synergistic effects. Our goal was to extrapolate vegetation response to climate-linked stream flow changes, by taking advantage of the spatial variation in flow conditions over a 200 km length of the San Pedro River (Arizona). Riparian vegetation traits were contrasted between sites differing in low-flow hydrology (degree of stream intermittency) and flood intensity (stream power of the 10-year recurrence flood). Field data indicate that increased stream intermittency would cause the floodplain plant community to shift from hydric pioneer trees and shrubs (Populus, Salix, Baccharis) towards mesic species (Tamarix). This shift in functional type would produce changes in vegetation structure, with reduced canopy cover and shorter canopies at drier sites. Among herbaceous species, annuals would increase while perennials would decrease. If flood intensities increased, there would be shifts towards younger tree age, expansion of xeric pioneer shrubs (in response to flood-linked edaphic changes), and replacement of herbaceous perennials by annuals. Woody stem density would increase and basal area would decrease, reflecting shifts towards younger forests. Some effects would be compounded: Annuals were most prevalent, and tree canopies shortest, at sites that were dry and intensely flooded. Vegetational changes would feedback onto hydrologic and geomorphic processes, of importance for modeling. Increased flood intensity would have positive feedback on disturbance processes, by shifting plant communities towards species with less ability to stabilize sediments. Feedbacks between riparian vegetation and stream lowflow changes would be homeostatic, with reduced evapotranspiration rates ameliorating declines in base flows arising from increased aridity. Copyright # 2009 John Wiley & Sons, Ltd. key words: climate change; riparian vegetation; semiarid; flood disturbance; stream intermittency; Populus; Tamarix Received 5 September 2008; Revised 13 March 2009; Accepted 8 April 2009