Background/Question/Methods Tamarisk (Tamarix) species are among the most successful plant invaders in the western United States. Tamarisk has been cited as having enormous economic costs; local, state and federal agencies have undertaken considerable efforts to eradicate this invasive plant and restore riparian habitats to pre-invasion status. A new biological control, the saltcedar leaf beetle Diorhabda elongata, has been released along many watersheds in the western United States to reduce the spread of tamarisk, if not eradicate it. Recent releases along the upper Colorado River and its tributaries in southeastern Utah, and southwestern Colorado, USA, have had considerable impact on tamarisk. In fact in both 2007 and 2008, the beetle defoliated tamarisk along more than 200 river km in the region. We measured the physiological response and water relations of tamarisk before, during and after seasonal defoliation by the salt cedar leaf beetle at the University of Utah's, Entrada Field Station on the Dolores River, in southeastern Utah. Measurements of stem sap flux, leaf area, total non-structural carbohydrates, and leaf carbon isotope ratios were measured on 20 mature trees beginning in the spring of 2007, before the first defoliation event by the beetle. Defoliation initiated at the field station in July of 2007.

Results/Conclusions During both 2007, and 2008, transpiration, measured by stem sap flux, increased substantially during the period of defoliation (mid-July), before decreasing by several fold over a several week period. Increases in nighttime transpiration were particularly noticeable, as nighttime water use increased by several fold before decreasing to near zero. Transpiration and stand leaf area index returned to pre-defoliation values after trees constructed new leaves in late July to early August, in both 2007 and 2008, even though total non-structural carbohydrates were much lower in the stems than before initial leaf flush in the spring. Stem sap flux summed over the entire 2008 growing season was 13% lower than predicted by modeling sap flux with atmospheric vapor pressure deficit before defoliation commenced (May – June). These results improve our understanding of how defoliation will impact riparian ecohydrologic processes at multiple scales. Several years of data will be necessary in understanding how the salt cedar leaf beetle will serve as a successful bio-control agent of tamarisk on the Colorado Plateau.