Seed limitation in the prairie grasslands of the Garry Oak Ecosystem

Background/Question/Methods   The open grasslands associated with the Garry Oak Ecosystem that once dominated the lowlands of the Puget Sound trough have all but disappeared due to fire exclusion, urban development, and other anthropogenic pressures.   The prairies that remain are among the most threatened ecosystems in North America and contain several at risk native plant species.  Invasive European grasses now further threaten these fragmented grasslands.  Invasive species lower native diversity, alter fire regimes, vegetation structure, soil characteristics, and affect the associated fauna.  Techniques for controlling many invasive species are often based on anecdotal information.  Technicians and land managers battling against invasion often find themselves making difficult restoration decisions in the absence of conclusive scientific evidence.  Within a larger region-wide replicated restoration experiment, we ask how commonly used restoration techniques (combinations of selective herbicide application, mowing, and burning) in combination with seed addition affect native seedling density.  Treatments, consisting of a combination of mowing, burning, selective herbicide application, and a control, were replicated four times at each site in 5x5m plots across ten prairie sites stretching from the southern tip of Vancouver Island, Canada to Corvallis, Oregon.  Half of each plot was seeded with six native species in the fall of 2006.  Seedlings were censused in the spring of 2007.   

Results/Conclusions Seed addition vastly increased the density of native seedlings, suggesting that seed limitation prevents native populations from expanding into suitable sites.  Treatment effects were site specific, but some generalizations emerged.  Several species showed increased germination in treatments with burns, presumably because seeds are able to come into direct contact with the soil.  The germination of other species responded more strongly to herbicide treatments, suggesting that competition with exotic grasses also affects seed germination.  As expected, different treatments maximized germination of different species, implying that the optimal restoration technique in these systems will depend on which native plant species are being targeted for restoration.