Persistent shifts in riparian vegetation associated with the invasion of introducedElaeagnus angustifolia and native nonriparian Juniperus virginiana have been reported in the U.S. Great Plains, with significant impacts on ecosystem services. In Nebraska, these species have been expanding into the native Populus deltoides riparian forests along the Republican River. Using dendrochronological data and stable isotope ratios, we examined the annual growth and acclimation strategies of these three species to climatic and streamflow variability. We hypothesized that the ability of invasive species to grow under a wide range of environmental conditions favour vegetation shifts at the expense of native species. Streamflow was the strongest predictor for performance. When accompanied with above-average annual precipitation, streamflow resulted in peak tree-ring widths in P. deltoides. J. virginiana showed less sensitivity, and E. angustifolia showed no response. δ 18 O values did not differ among species, which indicates that all species compete for the same water source. δ 13 C ratios and WUEi were higher in J. virginiana than in P. deltoides and E. angustifolia and increased in all species over time. E. angustifolia and P. deltoides exhibit a strong stomatal control and response to relative humidity relative to J. virginiana, which showed anisohydric stomatal behaviour. Our results show that once established, J. virginiana and E. angustifolia thrive in the understory of P. deltoides using different adaptive and acclimation strategies. Based on current water flow management, which prevents highflood pulses through the riparian zone, these species will continue to establish and spread throughout the Republican River watershed.

During the summer of 1976, samples of the algal communities from Stony Brook Pond, Norfolk, Ma. and Norton Reservoir, Norton, Ma. were exposed in laboratory culture to various concentrations of Aroclor 1232 and 1254. Although the algal communities of the two systems were quite different in composition, the Aroclors exerted some similar effects upon both. Aroclor 1232 caused a significantly greater reduction in primary productivity than 1254 and the reduction, with both Aroclor fractions, was dose related. Algal populations were differentially affected by Aroclor exposure.