A main objective of revegetation treatments is to quickly establish vegetation in order to deter erosion (Orr 1970). Nonnative species are often favored because of the availability of relatively inexpensive seed of species that are thought to have rapid rates of growth and establishment. By seeding with nonnative species the potential for invasion into the surrounding landscape increases, potentially setting off a cascade of negative ecosystem effects. A collaborative project between the University of Montana, USFS Native Plant Materials Program (Region One) and Wildlands CPR began in 2007 to examine alternative revegetation strategies. This collaborative project has been investigating the short-term effects of road decommissioning on plant community composition: looking specifically at the effects of seed-mix origin (native vs. nonnative), species diversity, and seeding density on vegetative establishment.

Methods

This study was conducted on thirteen road segments, ten on the Kootenai National Forest’s Three Rivers Ranger District in northwestern Montana and three on the Clearwater National Forest’s Powell Ranger District in northeastern Idaho. Each of these sites is located in moist Pacific Maritime influenced vegetative communities. Experimental transects were established along roadbed segments and pre-treatment vegetation data was collected before roads were removed in the summer of 2007. In the autumn of 2007, after road decommissioning, experimental plots at each site were hand seeded with one of six seed-mix treatments or left as an unseeded control. Seed mixes varied by three factors, with two possible levels of each factor: seed origin (native versus nonnative), seed-mix diversity (three grass species versus six species [grasses, forb and shrubs]), and seeding density (low [15 lbs/ace] versus high [30 lbs/acre]). During the summer of 2008, one year after road removal, vegetation data was collected at each of the experimental sites.

Results / Discussion

Comparing pre- to one year post-treatment we found total vegetative cover declined by 60%, with nonnative plants showing the greatest magnitude of response (ca. 90%; Table 1).

Numerous studies have shown that both roads and disturbance can increase the presence of nonnative plant species. In contrast, our findings indicate that nonnative plants were present at less than 1% cover one year after treatment. Given the low levels of nonnative species immediately after road decommissioning, this time period may be crucial for establishing native vegetation on highly disturbed former roadbeds, before nonnatives have the opportunity to colonize. Although managers often justify the use of nonnative seed by the need for rapid vegetative establishment on disturbed sites, we did not find significant differences in percent cover of total vegetation between plots seeded with native versus nonnative species. Furthermore, cover of native species was significantly higher on plots seeded with natives compared to those of other treatments (12.3% vs. 7.8%, respectively; Figure 1). On treatments seeded with native species, 43% of total vegetative cover was due to cover of seeded species; in comparison, seeded species accounted for only 18% of total vegetative cover on nonnative treatments.

These findings suggest that native seed mixes may outperform nonnative ones in terms of vegetative establishment after road removal. Despite the fact that federal land managers often favor nonnative seed mixes (Grant et al. unpublished data) due to their alleged rapid establishment rates, our results indicate that native seed mixes may in fact result in faster vegetative establishment. This study was not effective at testing for the effect of seed-mix diversity on vegetative establishment. The two shrubs that were included in the high-diversity native seed mix, snowbrush ceanothus (Ceanothus velutinus) and kinnikinnik (Arctostaphylos uva-ursi), did not establish on any plots, one year after treatment. Fireweed (Epilobium angustifolium) was also included in our high-diversity native seed mix. In contrast to the shrub species, fireweed was present in all of our experimental plots. The lack of difference in cover between seeded and unseeded plots was likely due to two factors. First, it was present in greater than 25% of the roadbed plots prior to road removal, and many of the residual plants were able to persist after road decommissioning. Secondly, the large pre-treatment abundance of this species suggests an abundant soil seed bank. Consequently, seeds may have been available on plots where they were not seeded. The lack of establishment of these shrub species, coupled with the ubiquitous establishment of fireweed, reduced the effective diversity of the high-diversity treatment to that of the low-diversity treatment.

One year after road decommissioning, we found no difference in vegetative cover between high and low-density treatments (Figure 3). Although policies are in place to increase the use of native plants (USDA 2008), budget limitations continue to restrict their use (Robichaud et al. 2006). Our data suggest that it may be possible to reduce the cost of using native seed by decreasing seeding density. The high-density treatment that we tested utilized the standard seeding density employed by the national forests where our sites are located (USDA 2005); our low-density treatment used half the seed of the high-density application. Despite this large difference in seed application rate, seed density did not affect any vegetation response variable. Thus, cutting the current seeding density in half could halve the cost of seed for revegetation projects.

Conclusion

Our results present convincing evidence that native seed mixes can be more effective than nonnative ones in facilitating rapid establishment of vegetation one year after road decommissioning. These results, although short term, are significant given that the first year after disturbance is a critical time period for erosion control — a driving factor in many revegetation projects. The extent to which treatments vary with respect to longer-term vegetation responses remains to be seen with future years of monitoring on these and other sites. Longer-term observations are also needed to assess the extent to which seeded species may compete with or facilitate the establishment of colonizing native and nonnative species, the dispersal of seeded species into the surrounding landscape, and the resistance and resilience of treatments to climatic stressors.

— Ashley conducted this study for her Masters degree in the College of Forestry and Conservation at the University of Montana. Cara is Assistant Professor of Restoration Ecology at the University of Montana.

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