Drivers of Riparian Tree Invasion on a Desert Stream

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 study sites in Canyon de Chelly and used tree ring analysis 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.

The role of a non‐native tree in riparian vegetation expansion and channel narrowing along a dryland river

“…However, a gap in hydrologic information prior to 1970 precluded a rigorous assessment of changes in these variables before and after the period of rapid population increase. Reynolds et al () reported rapid recruitment of Russian olive in Canyon de Chelly, AZ, beginning in 1983 and following a shift from a dry to a relatively wet period throughout the region that lasted from about 1978 to 1998 (Hereford, Webb, & Graham, ). Despite the lack of consistency in hydroclimate results between reaches in our study, the possible importance of regional climate shifts in Russian olive invasion patterns should not be discounted.…”

“…Throughout the south‐western United States, regional channel narrowing following both climate‐related drought and flow regulation has featured establishment and spread of the introduced riparian shrub tamarisk ( Tamarix ramosissima , Tsuga chinensis , and hybrids; Allred & Schmidt, ; Birken & Cooper, ; Hereford, ; Nagler, Glenn, Jarnevich, & Shafroth, ). More recently, Reynolds, Cooper, and Hobbs () document the invasion of tamarisk and Russian‐olive ( Elaeagnus angustifolia ), an introduced tree, along a south‐western U.S. stream with related channel narrowing. Generally speaking, whereas extensive research has been conducted on Tamarix spp.…”

“…More recently, Reynolds, Cooper, and Hobbs (2012) document the invasion of tamarisk and Russian-olive (Elaeagnus angustifolia), an introduced tree, along a south-western U.S. stream with related channel narrowing. Generally speaking, whereas extensive research has been conducted on Tamarix spp.…”

Spatially implemented Bayesian network model to assess environmental impacts of water management

“…Geomorphic hardening of channel landforms would lead to decreases in sediment transport and channel migration. Channel narrowing has also been shown to coincide with Tamarix establishment [ Birken and Cooper , ; Everitt , ], although the direct influence of Tamarix on channelization is still debated [ Reynolds et al ., ]. In addition, Tamarix colonization often produced greater evaporation‐transpiration (ET) water losses due to expansion of riparian areas [ Hultine and Bush , ].…”

Abundance of Invasive, Non-Native Riparian Herbs in Relation to River Regulation

“…By contrast, Russian-olive cover was unrelated to flow regulation or the degree to which flows were altered. Russian olive has established extensively along both regulated and largely unregulated river systems across the southwestern USA including Canyon de Chelly (Reynolds et al, 2014) and the Escalante River (Webb et al, 2007, M. Scott, unpublished data). These regional studies emphasize the notion that river regulation alone does little to explain the invasion of riparian ecosystems by non-native, woody species.…”