Determining optimum seeding densities of a native plant mixture on degraded sites

Establishing vegetation to control erosion, rebuild the soil and improve the visual appearance of degraded sites is an important aspect of ecosystem restoration. However, the maintenance of biodiversity and ecosystem function, wildlife management and aesthetic appeal are also important factors . The use of native species for purposes of revegetation is an important consideration in addressing all of these issues, but there is little information regarding their use. An experiment was designed using native plant species to test seeding densities, fertilizer use, season of seeding and their interaction on six degraded sites in the northwestern interior of British Columbia. In the fall of 1999 and the spring of 2000, a single seed mixture consisting of a fixed proportion of20% Achi//ea millefolium, 20% Festuca occidentalis, 20% Elymus glaucus, 20% Carex aenea, 16% Geum macrophyllum and 4% Lupinus polyphyllu was applied at six different densities at all replicate locations. A total of 24 treatment combinations were applied in 2.5 m by 2.5 m rototilled test plots at each location. Seed densities tested were 0, 375, 750, 1500, 3000 and 6000 pure live seed (PLS)/m2 in fall- and spring-sown plots. At all sites, commercial fertilizer (18-18-18 N-P-K) was applied in one plot of each density treatment; a corresponding density treatment plot was left untreated. Plots were monitored for plant density and cover for two years, at the end of each growing season. The highest mean cover (62%) was achieved after two growing seasons in the fertilized spring plots sown at 6000 PLS/m2. However, statistical tests revealed that there was no significant difference in cover between densities of 3000 and 6000 PLS/m2 in the first year. By the second year, there was no significant difference in cover between densities of 1500, 3000 and 6000 PLS/m2. There was no significant difference between fall seeding and spring seeding, and there were few treatment interactions. Results revealed that exotic cover declined significantly in Year 2, especially in plots sown at densities of 3000 and 6000 PLS/m2. There was also significantly less cover of exotic species in the unfertilized plots in Year 1. By Year 2, this trend was no longer significant and the cover of exotics was similar in both the fertilized and unfertilized plots. Based on the results of this experiment, it is concluded that native plant seed can be successfully used to restore vegetation cover on degraded sites. A revised seeding mix consisting of Achillea millefolium, Festuca occidentalis, Elymus glaucus and Lupinus polyphyllus is recommended for use with fertilizer to achieve the best cover. If rapid cover establishment in the first growing season is required, seeding densities of at least 3000 PLS/m2are recommended. Densities of 1500 PLS/m2 are acceptable if rapid establishment of cover is not essential. Seeding densities of 3000 to 6000 PLS/m2 are recommended for exotic species control where the cover of exotic plants is expected to be greater than >3.3%.