Natural Resources, School of


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A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Natural Resource Sciences, Under the Supervision of Professor Tala Awada. Lincoln, Nebraska: December 2012

Copyright (c) 2012 Kristen M. Skolaut


Forested riparian areas of the Northern Great Plains have historically been dominated by the native Populus deltoides L. vegetation type. Changes in climate, stream flow, groundwater, and management practices in the past few decades have promoted the expansion of the upland native woody species Juniperus virginiana L. and the invasion of the non-native Elaeagnus angustifolia L. into these riparian ecosystems. This study aims at using dendrochronology, or the study of tree rings, to assess the impacts of intra- and inter- annual climatic variability and stream flow over the past decades, on the annual tree ring growth, oxygen and carbon isotopic signatures, and performance of native and invasive woody species in a riparian ecosystem along the Republican River in Nebraska.

Tree ring analysis showed that P. deltoides and J. virginiana growth displayed a greater dependency on climatic factors than E. angustifolia. Populus deltoides growth was significantly dependent on previous year summer temperatures, and on annual stream flow, and Juniperus virginiana growth showed a significant dependency on the previous growing-season precipitation and on annual stream flow. On the other hand, E. angustifolia growth was negatively correlated with annual stream flow and the species displayed the least growth response to climate variability.

Oxygen and carbon isotopic ratios (δ13O and δ18C, respectively) of tree rings indicated that water use efficiency (WUE) in J. virginiana was significantly greater and showed the most response (i.e., increased) under stressful environmental conditions compared to the other two species. Water use efficiency in P. deltoides showed little dependency on climate variability and/or water availability. Results show that the three species followed different strategies to co-op with environmental stresses. The significance of these results and their consequence on the ecology of the ecosystem will be discussed.

Advisor: Tala Awada