Relationships between depth to ground water and southwestern riparian tree physiological condition (Populus fremontii, Salix gooddingii, Tamarix chinensis)

I investigated the mechanistic relationships between ground water availability and the physiological condition and growth of two native, Populus fremontii and Salix gooddingii, and one exotic, Tamarix chinensis, riparian tree species. Four independent but complimentary studies comprised my dissertation. The first study compared physiological responses of these three species to variations in ground water availability between two river systems; one is dam-regulated (Bill Williams River) and the other is free-flowing (Hassayampa River). I found that the native species are more sensitive to decreases in ground water availability than T. chinensis. Also, in some desert riparian systems, river regulation may ameliorate water stress in mature riparian trees by ensuring a more reliable water source. The second study compared the physiological and growth responses of these three species at a free-flowing river system (Hassayampa River) between years that differed climatically. In 1997, the spring and summer were dry and monsoon precipitation began late in July creating drought conditions, while in 1998, a strong el niño event caused surface flow to be greater and ground water to be shallower than in 1997. All species experienced water stress in 1997, however, this stress resulted in more canopy dieback and mortality in the native species than in T. chinensis. All species responded favorably to increased water availability in 1998, but this response was greatest in T. chinensis. The ability to utilize greater water availability and to persist when water is less available are characteristics that help T. chinensis invade and dominate southwestern riparian ecosystems. The third study used multiple linear regression and boundary-line analyses of data pooled from both rivers and both years to elucidate and compare species responses to environmental and physiological variables. Compared to the native species, leaf gas exchange of T. chinensis was more tolerant of atmospheric and soil water stress, giving T. chinensis a competitive advantage over the native species in some desert riparian ecosystems. The fourth study examined the effects of different rates of water table decline on the growth and survival of S. gooddingii and T. chinensis seedlings. Greater root elongation and higher drought tolerance allowed T. chinensis seedlings to persist in dry soils caused by high rates of water table decline where S. gooddingii seedlings died.