Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2006
Publication Date: 4/17/2006
Citation: Gazal, R.M., Scott, R.L., Goodrich, D.C., Williams, D.G. 2006. Controls on transpiration in a desert riparian cottonwood forest. Ag. & Forest Meteorology. 137:56-67. Interpretive Summary: Cottonwood forests are located along many streams and rivers of western U.S. Unfortunately, little is known about how much water they use and the factors that control this amount. This study found cottonwood water use along the San Pedro River in southeastern Arizona was much higher at a site located near a portion of river that had water in it all year than at a site near a river reach that dried out for part of the year. Trees along the occasionally dry reach were more water-stressed due to the deeper water table, and this resulted in decreased water use that was due to a reduction in tree sap flow and in the amount of green leaves. Since cottonwoods depend upon the water table being near the land surface, their existence is threatened in western U.S. basins where water tables are decreasing. These measurements of cottonwood water use improve our understanding of the role of riparian vegetation in the water cycle of many semiarid basins and help to improve models that are used to manage the allocation of water resources in the West.
Technical Abstract: Cottonwood (Populus spp.) forests are conspicuous and functionally important elements of riparian vegetation throughout much of the western U.S. Understanding how transpiration of this vegetation type responds to environmental forcing is important for determining the water balance dynamics of riparian ecosystems threatened by groundwater depletion. Transpiration was measured in semiarid riparian cottonwood (Populus fremontii) stands along a perennial and an intermittent reach of the San Pedro River in southeastern Arizona. Sap flow was measured using thermal dissipation probes and scaled to the stand level to investigate its water use in relation to canopy structure, depth to groundwater and atmospheric forcing. The cottonwood stand located at the perennial stream site had higher leaf area-to-sapwood area ratio (0.31 ± 0.04 m2 cm-2), leaf area index (2.75) and shallower groundwater depth (1.1 to 1.8 m) than the stand at the intermittent stream site (0.21 ± 0.04 m2 cm-2, 1.75 and 3.1 to 3.9 m, respectively). Moreover, total annual water use was higher at the perennial stream site (966 mm) than at the intermittent stream site (484 mm). The significant positive and linear correlation between transpiration and vapor pressure deficit indicated high hydraulic conductance along the root-shoot pathway of cottonwood trees at the perennial stream site. During the peak dry period prior to the summer rainy season, the trees at the intermittent stream site exhibited greater water stress as transpiration did not increase beyond its mid-morning peak with increasing vapor pressure deficit, which was likely due to leaf stomatal closure. However, this stress was alleviated after significant monsoonal rains and runoff events had recharged soil moisture and raised groundwater levels. Riparian cottonwood forests can be exposed to extreme fluctuations in water resources and water demand throughout the growing season and their accessibility to shallow groundwater sources dictates their structural and physiological responses to drought. Changes in water table depth through time makes them more susceptible to drought stress that threatens their productivity and existence along the river systems throughout much of western U.S.