Responses of riparian trees to interannual variation in ground water depthin a semi-arid river basin

We investigated the physiological and growth responses of native (Populus f remontii S. Wats. and Salix gooddingii Ball) and exotic (Tamarix chinensis Lour.) riparian trees to ground water availability at the free-flowing Hass ayampa River, Arizona, during dry (1997) and wet (1998) years. In the drier year, all species experienced considerable water stress, as evidenced by l ow shoot water potentials, low leaf gas exchange rates and large amounts of canopy dieback. These parameters were significantly related to depth of gr ound water (DGW) in the native species, but not in T. chinensis, in 1997. C anopy dieback was greater in the native species than in T. chinensis when g round water was deep in 1997, and dieback increased rapidly at DGW > 2.5-3. 0 m for the native species. Analysis of combined data from wet and dry year s for T. chinensis tentatively suggests a similar physiological sensitivity to water availability and a similar DGW threshold for canopy dieback. In 1 998, shoot water potential and leaf gas exchange rates were higher and cano py dieback was lower for all species because of increased water availabilit y. However, T. chinensis showed a much larger increase in leaf gas exchange rates in the wet year than the native species. High leaf gas exchange rate s, growth when water is abundant, drought tolerance and the maintenance of a viable canopy under dry conditions are characteristics that help explain the ability of T. chinensis to thrive in riparian ecosystems in the south-w estern United States.