Regional strategies for restoring invaded prairies

Background and approach: Invasive plants, especially non-native perennial
grasses, are a critical threat to remnant prairies and oak savannas in the Pacific
Northwest. We evaluated the effectiveness of restoration treatments designed to 1)
reduce target exotic weeds with minimal non-target impacts and 2) increase native
species diversity and abundance. In cooperation with numerous partners, the
Institute for Applied Ecology and The Nature Conservancy conducted a 5-year study
at 10 sites along a 500-km latitudinal gradient from the Willamette Valley, OR to
Vancouver Island, BC. Our manager-recommended treatment combinations included
the following components: summer and fall mowing, grass-specific and broadspectrum
herbicide, and fall burning. All treatment combinations were crossed with
native seed addition. Each combination was created to target factors likely to limit
restoration success, including extreme exotic grass cover, litter accumulation, and
native seed limitation. Where possible, we also applied what appeared to be the
most promising treatment combination over a large area (100 x 100 m) to assess the
scalability of results. Results from small-scale studies may differ substantially when
applied at large scales due to edge effects in small plots (e.g., seed inputs from
untreated areas), community level effects (e.g., impacts from herbivores), or spatial
heterogeneity.
Results: After 5 years, we found that the most disturbance-intensive
treatment combination (sethoxydim, burning, and post-fire glyphosate) led to
reduced abundance of exotic grasses and forbs without causing a decline in native
species. Sethoxydim combined with fall mowing reduced exotic grasses and
increased native plant abundance. In all cases, disturbance treatments reduced
exotic cover to varying degrees but had no positive impact on native diversity; only
seed addition increased native species richness. Results from the large treatment
areas were complicated due to difficulties with treatment application and timing, but
generally reflect the results from the small-scale experiments. Our results show that
restoration of degraded grasslands is most successful when it employs a variety of
strategies applied in combination over several years, and where the type, timing, and
number of treatments are carefully chosen based on a thorough understanding of
limiting conditions, species biology, and grassland ecology.
Management recommendations: As expected, we found there was no ‘silver
bullet.’ While some treatment combinations led to large improvements in weed
control and native diversity and abundance, the degree of success varied across
sites. Where invasive grasses are the most pressing problem, we recommend the
use of grass-specific herbicides as highly effective with minimal non-target effects on
native forbs and some native grasses. Fire is a useful tool for preparing a site for
seeding, but may need to be followed closely with a broad spectrum herbicide to
control rapidly resprouting weeds. Careful timing of post-fire herbicide avoids latersprouting
natives. At all sites, we recommend seed addition to enhance native
diversity and abundance, as our data show even relatively high quality sites show
strong seed limitation. Mowing is ineffective at reducing weed abundance, and can
negatively impact some natives depending on timing. While mowing did reduce
thatch and increase light penetration, it did not increase bare soil leading to low
seedling success. If fire is not an option, we recommend testing mowing in
combination with treatments to reduce moss and increase bare soil.