Drivers of Riparian Tree Invasion on a Desert Stream

RIVER RESEARCH AND APPLICATIONS River Res. Applic. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.2619 DRIVERS OF RIPARIAN TREE INVASION ON A DESERT STREAM L. V. REYNOLDSa,d*, D. J. COOPERa,d and N. T. HOBBSb,c,d a Department of Forest and Rangeland Stewardship b Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado USA c Department of Ecosystem Science and Sustainability, Colorado State University, Fort Collins, Colorado USA d Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado USA ABSTRACT Understanding mechanisms of exotic species’ invasions is essential to managing riparian landscapes throughout the world. In the southwestern USA, the two most dominant invaders of riparian habitats are the exotic tree species tamarisk (Tamarix ramosissima, Tamarix chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). These plants were introduced around 1900, and their success may be facilitated by river regulation, river channel changes, and precipitation patterns. We hypothesized that riparian invasion in Canyon de Chelly, Arizona, was initiated by a change point event such as plantings, dam construction, or channel incision and that establishment near a change point was tied to flood events. We aged tamarisk, Russian olive, and native cottonwood trees from studysitesinCanyondeChellyandusedtreeringanalysis to determine the year of establishment and the elevation of the germination point relative to the channel. We used Bayesian Poisson regression and information theoretics to identify change points and precipitation variables driving annual tree establishment. We found that most tamarisk and Russian olive trees established in the late 1980s, and most cottonwoods established in 1930-1950 and 1980-2000. Regression models indicated that change points occurred in 1983 for Russian olive and 1988 for tamarisk, and precipitation was important for establishment. Although plantings and river regulation probably played a role in tree invasion, our results suggest that these species required precipitation and stream channel change for widespread establishment in Canyon de Chelly. The factors driving riparian invasions may not be those often associated with degraded rivers, such as altered hydrographs and land management changes, thus requiring analyses of the full range of ecological and physical processes. Copyright © 2012 John Wiley & Sons, Ltd. key words: Russian olive; tamarisk; floodplains; exotic plant species; invasive species; Bayesian change point model Received 16 March 2012; Revised 14 August 2012; Accepted 3 October 2012 INTRODUCTION (Populus spp.) and willows (Salix spp.) or had relatively low plant cover compared with modern floodplains (Webb and The ecological, economic, and social impacts of exotic, in- Leake, 2006). The ability of tamarisk and Russian olive to vasive plants on native ecosystems such as rivers and ripar- invade the southwestern USA may be explained by differ- ian areas have stimulated broad concern among researchers, ences in life history strategies between these exotic species land managers, and the general public (Stohlgren et al., and native ones (Di Tomaso, 1998; Reynolds and Cooper, 1998; Zavaleta, 2000; Richardson et al., 2007). Exotic inva- 2010). Seeds of cottonwood and willow disperse aerially in sive plants often exclude native plants and alter physical and late spring and early summer, are viable for 4 to 6 weeks to ecological processes (Simberloff, 2005). Riparian areas are coincide with spring peak river discharge, and germinate on of particular concern because they support critical biodiver- moist substrate (Cooper et al., 2003; Rood et al., 2003). In sity of both plants and animals, despite covering a small contrast, tamarisk and Russian olive seeds have longer periods percentage of the landscape (Knopf et al., 1988; Sabo of seed dispersal and viability than cottonwood and willow et al., 2005). Riparian areas have also been invaded by (Cooper et al., 1999; Katz and Shafroth, 2003). Mature exotic species to a greater extent than other habitats world- tamarisk and Russian olive plants can tolerate long periods wide (Stohlgren et al., 1998; Hood and Naiman, 2000). without available ground water, whereas native cottonwood In the southwestern USA, the dominant riparian and willow cannot (Brotherson and Winkel, 1986; Katz and plant invaders are the woody species tamarisk (Tamarix Shafroth, 2003). The formation of interspecifichybridsof ramosissima Ledebour, Tamarix chinensis Loureiro, and Tamarix spp. may also have facilitated its spread in North their hybrids) and Russian olive (Elaeagnus angustifolia L.) America (Friedman et al., 2008; Gaskin et al., 2011). (Friedman et al., 2005a). Historically, southwestern flood- Beginning in the late 1800s, government agencies and plains were populated by stands of native cottonwood private landowners planted tamarisk and Russian olive for shelter belts and stream bank erosion control in the central *Correspondence to: L. V. Reynolds, Department of Biology, Colorado State University, Fort Collins, Colorado 80523, USA. USA (Di Tomaso, 1998; Katz and Shafroth, 2003). The E-mail: [email protected] US Department of Agriculture planted tamarisk in Arizona, Copyright © 2012 John Wiley & Sons, Ltd. L. V. REYNOLDS ET AL. New Mexico, and Nevada by 1910 (Carleton, 1914), and Floodplain dynamics and the formation of fluvial features Russian olive was introduced in western states by 1909 can also influence riparian plant establishment (Friedman (Christensen, 1963). In 1934, the US Soil Conservation et al., 1996). A stream that has frequent over-bank flooding Service implemented a program to control stream erosion in and wide but shallow channels usually has wide, open the Colorado River basin by planting tamarisk and Russian floodplains with little woody vegetation (Friedman and olive throughout the Navajo Indian Reservation (SCS, 1934). Lee, 2002; Webb and Leake, 2006). Conversely, streams Similar to riparian erosion control projects in other regions that rarely flood yet have shallow ground water often of the world that have introduced exotic plants (Henderson, support dense stands of riparian vegetation that establishes 1991; Holland-Clift et al., 2011), plantings in the southwestern after floods (Friedman and Lee, 2002). A stream that US have been widely reported to be the seed sources and changes from one type to another, for example, from a wide, catalysts of the tamarisk and Russian olive invasion in the braided channel to a narrow, incised channel, may incur western USA (Robinson, 1965; Katz and Shafroth, 2003). dramatic changes in its riparian vegetation (Johnson, 2002). In addition to plantings, humans have dramatically altered Furthermore, rivers that narrow and incise can allow riparian the flow regime of rivers throughout the southwestern USA vegetation to establish during the narrowing process (Merritt with dams and flow diversion structures (Graf, 1999; Carlisle and Cooper, 2000; Webb and Leake, 2006). et al., 2011). Southwestern rivers were historically influenced Changes in flood and sediment regimes, river regulation, by snowmelt and rain-driven floods that reworked river beds, land use, and climate have affected patterns of riparian vege- keeping them relatively free of vegetation (Webb and Leake, tation establishment in many areas (Friedman et al., 1998; 2006). River regulation has altered natural hydrologic Johnson, 2002; Webb and Leake, 2006; Merritt and Poff, processes, reducing snowmelt-driven flood peaks that occur 2010). In Canyon de Chelly National Monument, Arizona during the period of native plant seed dispersal (Stromberg, (Canyon de Chelly) tamarisk and Russian olive plantings, 2001). Dams are reported to favour exotic plant establishment dam construction, climate variation, and channel changes because tamarisk and Russian olive can more easily exploit have all occurred in recent history. Historic photos by Ansel altered flow regimes than native cottonwoods and willows Adams and others showed that a broad, sandy, and unvege- (Stromberg et al., 2007; Merritt and Poff, 2010). tated channel occurred in the early to middle 1900s, where Annual precipitation is highly variable in the southwestern valley bottoms are now dominated by dense stands of USA and directly influences river flood events and baseflows tamarisk and Russian olive interspersed with stands of (Hereford and Webb, 1992; Woodhouse et al., 2006). Winter native cottonwood (Figure 1; Cadol et al., 2011). However, snow melt and high spring stream flows create habitat for both it is unknown which processes or events changed the native and exotic plants (Cooper et al., 2003). Late summer landscape character and triggered tamarisk and Russian rain-driven floods can allow exotic plant establishment, olive invasion as well as native cottonwood establishment. whereas cottonwood and willow have no viable seeds at The goal of our study was to analyse the role of plantings, that time. Precipitation amount and timing, in addition to dams, precipitation, and channel change in facilitating plant plantings and dams, can influence the establishment and local establishment and landscape change in Canyon de Chelly. distribution of exotic and native riparian trees on floodplains We hypothesized that if invasion was initiated by a discrete (Levine and Stromberg, 2001; Katz et al., 2005; Birken and change point event such as plantings, dam construction, Cooper, 2006). or channel incision, the majority

Load more