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

Along rivers, native and invasive species may establish and persist on active channel
bedforms as part of channel narrowing. Using historical aerial photography and
dendrochronology, we quantified spatial and temporal patterns of narrowing and
vegetation expansion, including native Fremont cottonwood (Populus fremontii)
and non‐native Russian olive (Elaeagnus angustifolia), along the largely unregulated
Escalante River in south‐western United States. Russian olive establishment was
examined with respect to hydrologic and climate variables. Narrowing along the
Escalante River was initiated during a mid‐20th century drought. Cottonwood rapidly
colonized higher, bar surfaces between the 1950s and 1981. Small numbers of
Russian olive established in moist sites during this period as the channel narrowed
by nearly 80%. After 1981, there was no obvious cottonwood establishment but
low channel bars and banks were rapidly colonized by Russian olive. Hydroclimate
predictors were equivocal but exponential growth of this large‐seeded, shade‐tolerant
species lagged its introduction by 30 years, apparently because of delayed reproductive
maturity, limited seed availability, and widespread availability of favourable establishment
sites following initial channel narrowing. Sediment trapping, levee formation,
and modification of channel form by dense, channel‐edge bands of Russian olive
progressively limited new establishment sites and by 2000, recruitment declined
sharply. Our results have implications for management of non‐native tree invasions
along arid‐region rivers, including identification of low, moist, active channel bars
where the establishment and physical impacts of Russian olive appear to be most
pronounced and where focused management efforts are likely to be most effective.