Tamarisk and River Restoration Along the San Pedro and Gila Rivers Juliet Stromberg and Sharon Lite School of Life Science, Arizona State University, Tempe AZ Charles Paradzick Arizona Game and Fish Department, Phoenix, AZ Abstract—The abundance of tamarisk (Tamarix ramosissima and related species) along the San Pedro and Gila River flood plains varies with differences in stream flow regimes. Tamarisk abundance, relative to Fremont cottonwood and Goodding willow, is greater at sites with more intermittent stream flows and deeper and more fluctuating ground-water levels. Tamarisk abun- dance is further increased below Coolidge Dam, where both flood and low flow patterns have been altered. Shifts from cottonwood-willow to tamarisk parallel other changes in the riparian community: as rivers are dewatered and flood regimes altered, species diversity and landscape heterogeneity decline. Tamarisk dominance can be seen as an indicator, rather than a cause, of riparian degradation, reflecting changes in the physical processes that shape riparian plant communities. Restoration strategies should focus on identifying and alleviating these underlying environmental stressors to produce long-lasting results. Correlations observed between tamarisk abundance, low Introduction biodiversity, and low abundance of cottonwoods-willows have The biota and physical environment of an ecosystem un- fostered this species-removal approach to river restoration. dergo continuous change. There has been a particularly high However, it can be difficult to distinguish between alterations to rate of influx of new species into riverine habitats, partly due an ecosystem caused by a new species (autogenic effects) and to their intensive use as agriculture lands, urban areas, and those caused by changes to physical processes that facilitated transportation routes, and partly due to their intrinsic nature the shift in species composition (allogenic effects) (Stromberg as high-disturbance, linear corridors, whose community com- and Chew 2002). A plant species may be a cause of change or position is largely driven by immigration processes (Brown it may be a symptom of change; distinguishing between the and Peet 2003). The millions of neotropical migrant birds two is critical for designing restoration plans. flying north and south along the San Pedro riparian corridor, Hydrology and geomorphology are key drivers of vegetation for example, transport plant seeds. At the same time, frequent dynamics in riparian ecosystems, and as these physical factors flooding provides for nearly constant flux in resource avail- change, so does the biota (Poff et al. 1997). Shifts in hydrologic ability and thus provides frequent opportunities for species regimes can alter competitive hierarchies and cause shifts in replacement (Davis et al. 2000). Thus, many immigrant plant species composition (Tickner et al. 2001). Thus, consideration species have become integrated into riparian landscapes of hydrologic regimes and fluvial processes is essential for river (McLaughlin 2004). restoration projects. Our objectives in this paper are to discuss Tamarisk (Tamarix chinensis and T. ramosissima) was (1) relationships between hydrologic factors and cottonwood, introduced to the United States from Asia in the 1800s for willow, and tamarisk distribution along the San Pedro River soil erosion control and landscaping purposes. Tamarisk now and middle Gila River; (2) ecosystem changes associated with forms a minor component of the riparian landscape mosaic on shifts from cottonwood-willow to tamarisk, and (3) relation- some western rivers, but on others it has become the dominant ships between tamarisk management and riparian ecosystem vegetation type. Contemporaneous with tamarisk increase restoration. has been decline of Fremont cottonwood-Goodding willow (Populus fremontii-Salix gooddingii) associations on some (but certainly not all; Robert Webb, unpublished data) rivers Stream Flow Regimes of the Southwest. Given perceptions of tamarisk’s high water The flood flow regime of the undammed San Pedro River use and role in altering ecosystems, there are considerable ef- reflects the climatic signal. Floods occur mainly in winter, forts to control its spread and dominance in western riparian late summer, and fall; peak flows of >10,000 cfs occur fairly ecosystems. Methods such as burning, herbicide application, frequently. The low flow regime ranges from perennial to mechanical removal, and biologic control via insect herbivores intermittent to ephemeral. The location of perennial reaches are frequently employed (Quimby et al. 2003). varies depending on proximity to major tributaries, sub-surface 302 USDA Forest Service Proceedings RMRS-P-36. 2005. geology, and the extent of ground water pumpage. In perennial to establish without competition from an existing overstory. reaches, surface flows remain in the channel even during dry Receding flows in spring moisten the bare sediments during the seasons, sustained by ground-water inflow from the stream al- brief window when viable seed of the cottonwoods and willows luvium and regional aquifers. In intermittent areas, surface flow are present. Continued flow recession during late spring and ceases during the months (e.g., May, June; October, November) summer creates opportunities for the later-seeding tamarisk to between rainy seasons. The depth to ground water (averaged establish. However, where cottonwood and willow seedlings across the flood plain) at the intermittent sites can be more are present, they can reduce the growth rate of intermixed than five meters with inter-annual fluctuation of more than one tamarisk seedlings. Field and pot studies have shown that as meter. At the driest sites, flow is present only for a few days a seedling, tamarisk, a stress-tolerant species, is not a strong each year following run-off events. Intermittent-flow reaches competitor against co-establishing cottonwoods (Sher and are prevalent in the Lower Basin downstream from areas of Marshall 2003). Often, a negative relationship exists between ground-water pumping for agricultural and copper mining uses, the ability of plants to tolerate abiotic stress and the ability to and also occur in the Upper Basin, such as below the St. David compete (Grime 1979). diversion structure. Cottonwood and willow are the dominant tree species in Flows in the middle Gila River (San Carlos Lake—Ashurst/ the flood plains of perennial and wet-intermittent reaches of Hayden Diversion Dam) are influenced by the operation of the San Pedro River, given the presence of suitable hydro- Coolidge Dam. In the below-dam reach, river flow is maintained logic conditions for their establishment and survival (Lite during most years by scheduled releases for downstream agri- and Stromberg, in press). In these wet reaches, rates of flood culture users. Steady releases for irrigation can cause alterations water recession and water table decline during recruitment in channel morphology (Graf et al. 2002), and the Gila River years remain within tolerance ranges (Shafroth et al. 1998). through this reach appears to have narrowed and deepened to Over time, the flood-plain surfaces vegetated by cottonwood carry higher spring and summer base flows (Paradzick unpubl. and willow aggrade during depositional flood events. Depth- data). Inflow from the Lower San Pedro River, about 25 km to-ground water under the mature forests typically remains less downstream of the dam, contributes some water. However, than about three meters, with less than one meter inter-annual when water levels in the San Carlos reservoir are low, due to fluctuation, which prevents mortality from drought in these drought and high levels of diversion and pumping by upstream drought-sensitive species (Horton et al. 2001). At these peren- water users, flow releases from Coolidge Dam can cease. The nial river sites, tamarisk occurs as a slow-growing understory flood regime also has been greatly altered. Prior to dam closure component of the cottonwood-willow forests (and see Lesica in 1928, annual flood peaks frequently exceeded 10,000 cfs and and Miles 2001). It also forms discrete patches in the flood sometimes exceeded 100,000 cfs. Since that time, other than a plain, typically on surfaces that have slightly deeper ground large release in 1993 (29,300 cfs), peak flows have been small water than those supporting cottonwoods and willows. (typically <1,000 cfs) immediately downstream of the dam. The As flows become increasingly intermittent and ground water disturbance affects of flood flows entering from the San Pedro deepens, conditions become less suitable for cottonwoods and River apparently have been damped (i.e., reduced flood-plain willows and more suitable for tamarisk. Lite and Stromberg (in scouring and inundation) by channel adjustments described press) have defined hydrologic thresholds at which dominance above (Paradzick unpublished data). shifts from cottonwood-willow to tamarisk along the San Pedro River. Cottonwood and willow were dominant at sites where Abundance Patterns of surface flow was present more than 74% of the time, inter-an- nual ground-water fluctuation was less than 0.5 m, and average Tamarisk, Cottonwood, maximum depth to ground water was less than 2.7 m, during a two-year period of hydrology data collection. Tamarisk was and Willow dominant, with cottonwood and willow absent or sparse, where Tamarisk distribution and abundance should be considered flow permanence was less than 46%. The three species were within the context of the underlying physical