ESTABLISHMENT OF NATIVE AND INVASIVE PLANTS ALONG A RANGELAND RIPARIAN GRADIENT

Authors: LECAIN, RONALD - MONTANA STATE UNIVERSITY; HOOK, PAUL - MONTANA STATE UNIVERSITY; Sheley, Roger

Submitted to: Ecological Restoration
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2006
Publication Date: 9/1/2006
Citation: Lecain, R.R., Hook, P.B., Sheley, R.L. 2006. Establishment of native and invasive plants along a rangeland riparian gradient. Ecological Restoration. 24(3):173-181.

Interpretive Summary: Riparian areas are vulnerable to undesired plant invasions and are often in need of restoration because water fluctuations create disturbances that may favor weed establishment and growth. Once weeds become established along the riparian area, they often spread to adjacent uplands. We tested the ability of spotted knapweed and Canada thistle to establish and grow along a riparian gradient from wet bottomland to relatively dry uplands. Both weeds had poor emergence, but the survivors grew vigorously in sub irrigated environments. In spite of low initial establishment, weedy species appeared to be fully successful. We also tested six desired species as to their ability to establish along the gradient. In the field, no species establishment was acceptable. In areas where, nutrient additions were added, weeds dominated, supporting caution toward practices that increase N availability to plants where invasive weeds are present.

Technical Abstract: Effects of environmental gradients on plant establishment are important to a wide range of problems including exotic weed management and ecological restoration, but they are not well studied in western North American riparian ecosystems. Previous research and field observations indicate that western riparian systems are more vulnerable to exotic plant invasion than surrounding rangelands, and conventional wisdom holds that riparian areas are easier to revegetate than adjacent, water-limited uplands. We used parallel field and greenhouse experiments to investigate the influence of hydrology and nitrogen availability on establishment of six native and invasive exotic species of upland and riparian areas. We hypothesized that (1) emergence and early growth would be higher in transitional, subirrigated than saturated or dry environments, (2) annuals and invasive exotics would respond more to variation in N availability than late-successional perennials, and (3) N effects would be greatest in subirrigated environments. In the greenhouse, seedling emergence was relatively uniform across dry to subirrigated pots for upland species, and across subirrigated to saturated but not flooded pots for riparian species; average seedling biomass was greatest in subirrigated environments. Contrary to expectations, seedling emergence in the field was lower – at or near zero for most species – in two transitional, subirrigated sites than an upland and a wetland site; consequently, biomass patterns generally could not be assessed. In the greenhouse, biomass consistently increased with N availability, and average response to N was greatest in subirrigated pots. In both greenhouse and field experiments, the upland exotic weed Spotted knapweed (Centaurea maculosa) responded more to N availability than did the native bunchgrass (Agropyron spicatum); such differences were not found for the riparian species, possibly due to low emergence. Possible explanations for low emergence in transitional, subirrigated sites in the field include residual effects of pre-existing vegetation via litter or allelopathy or, most likely, seed and seedling predation by small rodents, which were more active in the transition than upland or wetland areas; it may be necessary to address these factors, as well as physical environment, to assure success of some riparian revegetation efforts. Establishment of C. maculosa and Canada thistle (Cirsium arvense) in riparian sites appeared to be limited by poor germination and early survival, but once established, growth was good in subirrigated environments, suggesting that exotic weeds may invade riparian sites successfully in spite of low rates of initial establishment. Results concerning nutrient effects on relative performance of desirable and weedy plants were inconclusive but support caution towards practices that increase N availability.