Ecosystem response to removal of exotic riparian shrubs and a transition to upland vegetation

Abstract

Understanding plant community change over time is essential for managing important ecosystems such as riparian areas. This study analyzed historic vegetation using soil seed banks and the effects of riparian shrub removal treatments and channel incision on ecosystem and plant community dynamics in Canyon de Chelly National Monument, Arizona. We focused on how seeds, nutrients, and ground water influence the floristic composition of post-treatment vegetation and addressed three questions: (1) How does pre-treatment soil seed bank composition reflect post-treatment vegetation composition? (2) How does shrub removal affect posttreatment riparian vegetation composition, seed rain inputs, and ground water dynamics? and (3) Is available soil nitrogen increased near dead Russian olive plants following removal and does this influence post-treatment vegetation? We analyzed seed bank composition across the study area, analyzed differences in vegetation, ground water levels, and seed rain between control, cut-stump and whole-plant removal areas, and compared soil nitrogen and vegetation near removed Russian olive to areas lacking Russian olive. The soil seed bank contained more riparian plants, more native and fewer exotic plants than the extant vegetation. Both shrub removal methods decreased exotic plant cover, decreased tamarisk and Russian olive seed inputs, and increased native plant cover after 2 years. Neither method increased ground water levels. Soil near dead Russian olive trees indicated a short-term increase in soil nitrogen following plant removal but did not influence vegetation composition compared to areas without Russian olive. Following tamarisk and Russian olive removal, our study sites were colonized by upland plant species. Many western North American rivers have tamarisk and Russian olive on floodplains abandoned by channel incision, river regulation or both. Our results are widely applicable to sites where drying has occurred and vegetation establishment following shrub removal is likely to be by upland species.

Russian Olive (Elaeagnus angustifolia) Removal in the Western United States: Multi-Site Findings and Considerations for Future Research

“…The latter are native species usually found in the more xeric, upland habitats adjacent to mesic riparian habitats. Future research should address the efficacy of restoring to mesic, riparian, native vegetation communities versus xeric, native, upland vegetation communities [12]. With reduced flooding to promote disturbance-adapted early succession species, the mechanisms to maintain riparian vegetation communities are absent.…”

“…and thus most of the literature from lower latitudes addresses multi-species stands and monotypic Tamarix spp. [12,19,20]. We identified sites by contacting land managers of private and public land (approximately 30) who had engaged in E. angustifolia removal.…”

“…Due to changes in stream flow and geomorphology, some riparian corridors lose their dynamic disturbance patterns and shift from an early to late successional overstory. They eventually become more like the adjacent upland habitats, which are predominantly agricultural wastelands and/or xeric steppe where exotic species coverage is more abundant than native species coverage [10][11][12].…”

“…removal [17,18]. Still others report on the restoration of mixed native and invasive stands [12,19,20], but none have synthesized vegetation response across sites after removal of E. angustifolia alone.…”

Effectiveness of Exotic Plant Treatments on National Park Service Lands in the United States

“…Reynolds and Cooper (2011) Canyon de Chelly National Monument Lake Mead National Recreation Area Nevada, Arizona Desert Tamarix spp. Abella et al (2009) Lake Mead National Recreation Area Nevada, Arizona Desert Nicotiana glauca, Peganum harmala Abella et al (2013b) Lake Mead National Recreation Area Nevada, Arizona Desert Abbreviations: NP, national park; B, burning; C, cutting; G, girdling; H, herbicide; M, mowing; P, pulling; PN, planting natives; T, temporal comparisons of treatments.…”