University of Nevada, Reno Plant Community Invasibility in Riparian Landscapes: Role of Disturbance, Geomorphology, and Life History Traits. A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Ecology, Evolution, and Conservation Biology by Susan G. Mortenson Dr. Peter J. Weisberg / Dissertation Advisor December 2009 THE GRADUATE SCHOOL We recommend that the dissertation prepared under our supervision by SUSAN GRACE MORTENSON entitled Plant Community Invasibility in Riparian Landscapes: Role of Disturbance, Geomorphology, and Life History Traits be accepted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Peter J. Weisberg, Ph.D., Advisor Jeanne C. Chambers, Ph.D., Committee Member Elizabeth A. Leger, Ph.D., Committee Member Ashley D. Sparrow, Ph.D., Committee Member Thomas F. Bullard, Ph.D., Graduate School Representative Marsha H. Read, Ph. D., Associate Dean, Graduate School December, 2009 i ABSTRACT Riparian landscapes are highly susceptible to invasion by non-native plant species. High productivity and frequent disturbances associated with flooding contribute to invasibility of riverbanks and floodplains. The hydrogeomorphology of riparian landscapes is intricately tied to plant community composition and structure. Plant invasions often coincide with the period directly following regulation because novel riparian habitat is created. Additionally, river regulation in the form of dams, diversions, and inter-basin water transfers alter disturbance regimes (flood frequency, magnitude, timing, and duration). Native species lag behind introduced species in colonization of new riparian habitat due to life history constraints imposed by adaptation to the previous disturbance regime. Plant invasions often coincide with the period directly following river regulation, but non-native plant species also spread along rivers that have not been hydrologically altered. Tamarix spp. are invasive shrubs that have spread prolifically throughout riparian landscapes of the southwestern US. Tamarix has a long period of seed release and high salinity and drought tolerance relative to native pioneer shrubs and trees. These characteristics combined with the ability to form a thick litter layer contribute to competitive exclusion of native shrubs. The presence of large, dense stands of Tamarix along rivers also resists scour from floods, stabilizes the river channel, causes channel incision, and reduces overbank flooding which is required for establishment of native pioneers and overall ecosystem function (e.g., nutrient availability, decomposition). Tamarix control is currently conducted with a variety of methods (herbicide application, mechanical removal, biocontrol, burning, flooding). I researched the potential for ii prevention of Tamarix establishment using controlled flooding along one of the most regulated rivers in the US, the Colorado River through the Grand Canyon National Park. The high geomorphologic diversity of the Grand Canyon and long period of flow records make this an ideal study site for understanding the influence of hydrogeomorphology and other environmental factors on Tamarix recruitment. Although Tamarix has been present in the Grand Canyon since at least the 1930s, this species expanded its range extensively following post-dam flooding in 1965 and 1973. Through a tree-ring analysis I investigated influences of past hydrologic and climatic conditions on Tamarix establishment and persistence at different flow stage elevations from 1984 through 2008. Historic records and scarcity of pre-1983 Tamarix revealed high Tamarix mortality from the 1983 through 1985 floods, but these floods also initiated a large establishment event. Tamarix establishment was positively correlated with years of high summer flows and low precipitation the following year. The importance of precipitation for large recruitment episodes and evidence of Tamarix establishment in every year after 1983 suggests that controlled floods are unlikely to prevent future Tamarix establishment. Elevated levels of establishment caused by the 1983 through 1985 floods led us to conclude that controlled floods should not be conducted during Tamarix seed release (April through September). In addition to hydrologic and climatic regimes, bi-trophic interactions (e.g., herbivory, pathogens) also influence riparian plant community composition and structure. I investigated the potential for selective foraging by beavers to affect coarse-scale spatial patterns of riparian vegetation along the Colorado River in the Grand Canyon. Spatial associations of beaver occurrence and Salix and Tamarix cover were analyzed using iii multiple linear regression models after accounting for relationships with geomorphic variables (geomorphic reach, sinuosity, and rock resistivity). Beaver presence had a strong positive association with Tamarix cover and a slight, positive association with Salix cover. This suggests that Tamarix and beavers occupy similar habitats even at spatial scales less than 4.5 ha, beavers prefer habitats with high Tamarix cover, or that beavers promote Tamarix dominance through selective foraging of Salix . To explore the hypothesis that river regulation increases dominance of non-native species I conducted a survey of riparian shrubs and trees along 20 river segments across the southwestern US. I created an index of flow alteration based on hydrologic conditions prior to and after dam completion or prior to and after 1962 for non-dammed rivers. Regressions of dominance of native ( Salix exigua and Populus spp.) and non- native ( Tamarix spp. and Elaeagnus angustifolia ) woody species with the degree of flow alteration revealed a positive relationship between Tamarix and flow alteration, a negative relationship between Populus and flow alteration, and no significant relationship between S. exigua and E. angustifolia and flow alteration. Native and non-native species respond to hydrologic characteristics based on their life history strategies, not based on their native status. Therefore, river regulation does not necessarily increase cover of non- native woody plant species. I organized seven guidelines for river restorationists based on a comprehensive review of riparian research. These guidelines focus on restoration principles relevant to woody riparian vegetation and are particularly applicable for regulated rivers where flow regime can be altered for restoration purposes. I advocate formulation of alternative flood regimes based on knowledge of natural variability, consideration of effects of iv increased fire frequency in regulated systems and potential opportunities to use fire and floods to reinvigorate plant establishment and geomorphic processes, and use of functional groups and consideration of multi-trophic species interactions to predict effects of management scenarios. Watershed-scale and site-scale restoration approaches are often required for restoration of connectivity, variability, and geomorphic processes. The most promising strategy for riparian restoration planning, implementation, and monitoring remains adaptive management. However, the temporal scale of adaptive management must extend to incorporate climate change scenarios. v OVERVIEW The majority of this dissertation focuses on the ecology of non-native, invasive riparian plants, river regulation, and the potential for restoration. For chapters one and two, I researched different aspects of the Tamarix invasion along the Colorado River through the Grand Canyon. Tree-ring analyses and surveys for Tamarix seedling establishment and adult density were used to understand the influences of hydrologic, climatic, and geomorphic factors on Tamarix establishment and persistence. The second chapter examines the spatial association of beavers, Tamarix , and Salix and the hypothesis that beavers may contribute to Tamarix dominance through selective foraging of Salix . The third chapter describes a regional survey of river segments in the southwestern US. Through this survey I addressed relationships between flow regime, climatic factors, and changes in flow regime with dominance of native and non-native woody species. Chapter four provides seven principles for riparian restoration for restoration practitioners. This chapter is based on a comprehensive literature review of riparian research. Important questions addressed by this study include: 1) How has the regulated flow regime along the Colorado River in the Grand Canyon influenced Tamarix establishment and persistence? 2) Do bi-trophic interactions contribute to Tamarix dominance? 3) How does the degree of flow alteration affect dominance of non-native, invasive woody plants? vi ACKNOWLEDGEMENTS I am grateful to my future husband, Chris Kratt, who made many sacrifices so that I could pursue this degree. Chris spent weeks in the field with me digging up Tamarix , driving all over the southwestern US, and cutting Tamarix slabs. He also provided unending encouragement when I was frustrated with the magnitude of my Ph.D. project. Without Chris’s support, I do not know how I would have accomplished the tasks necessary to receive this degree. My mother, Julia Mortenson, also assisted with field work and has always encouraged me to pursue my goals. I thank her for a lifetime of support and excellent advice. The exciting nature of this Ph.D. project and constant enthusiasm of my advisor, Peter Weisberg, were integral to completion of this research. Peter played an active