Beyond Vegetation Cover as a Measure of Restoration Success: Long Term Patterns on Removed Grassland Roads

Picture a restored road halfway overgrown with vegetation. Most of us involved in restoration would generally see this vegetative cover as a good thing. It is a sign that something is able to grow on the once disturbed and compacted soil. Erosion is held in check, minus some bare spots here and there. Wildlife are likely beginning to use some of the plants for cover and food. As for the plant community itself, we expect that successional processes will eventually result in a diverse array of desired plant species. However, if we take a closer look at the plant community, we may find more to the long-term story.

Grassland Road Removal
Curiosity about such a story led me to restoration ecology research in western North Dakota. Since about the 1950’s – when oil and natural gas exploration began in this part of the Great Plains – varying degrees of well site and access road removal have been attempted in an effort to reverse the impacts of drilling activities on native grasslands. Public lands have the tightest regulations, and currently road removal on those lands involves removing surface materials, recontouring the soil to match the surroundings, and planting a seed mix of 3-7 grassland species (USDI and USDA 2006).

Study Design
For my study, I took a closer look at the plant communities of 58 of these removed access roads in the oilfields of the Little Missouri National Grasslands (LMNG). The roads I selected were decommissioned 3 to 22 years ago, and my goal was to answer 3 main questions: Which species were planted during restoration? How do these species compare to the vegetation currently growing on the restored roadbed? And finally, how is the plant community on the restored roadbed similar to or different from undisturbed vegetation adjacent to the old roadway? The answers to these questions, along with more traditional vegetative cover assessments, can give us a better idea of whether these restoration practices are leading to long-term recovery. So to answer them, I trekked across the rugged and rolling prairie of the LMNG to sample plots that I set up on and along the 58 study roads, recording all plant species that I observed. I also did a fair amount of detective work to retrieve records of seeding for these roads.

Findings
My main finding was that, in general, the species that were planted on the removed roads were still the most abundant (Simmers 2006). When I incorporated time into the analysis, accounting for the length of time since restoration, I found that this pattern held. Even on the oldest restorations, which would have had the most time for surrounding species to colonize, the species observed were very much like the seed mixes.

This finding could be a positive one, particularly if the species that were planted are desired components of the restoration, i.e. native species also dominant in surrounding, undisturbed vegetation. However, I found that the seed mixtures used in the older restorations contained non-native forage species, transitioning to native grasses and forbs for the more recent decommissionings (Simmers 2006). Both old and new mixtures were low in diversity (averaging 5 species per mix) and mis-matched the adjacent plant community, even if composed of native species. Numerous species common in the surrounding vegetation were either absent or present infrequently and at low cover on the removed roads. The result: linear corridors of a very different species assemblage compared to the surrounding matrix of native grassland – and a pattern that remains even after 20 years.

Discussion
One explanation for this finding is simply that this system needs more time to recover from such a disturbance. After all, the climate of the Great Plains is harsh, with dramatic swings in precipitation and temperature within and among years. Other research indicates that both natural successional processes and revegetation after human disturbances can be slowed by such a climate (Burke et al. 1998, Bakker et al. 2003). However, my results suggest that restoration choices and practices could also be contributing to the lag in recovery. More specifically, recovery may be hindered due to characteristics of seeded species and/or due to insufficiently ameliorated soil conditions.

Evidence for the first of these explanations is found in the persistence and dominance of seeded species, whether non-native or native. Other work in the Great Plains has shown that many of the non-native species traditionally used for revegetation projects have competitive advantages over local native species and tend to spread from initial introductions (Wilson 1989, Bakker and Wilson 2001, Bakker and Wilson 2004). Not unexpectedly, I found evidence that several non-native, seeded species were spreading, such as crested wheatgrass (Agropyron cristatum), smooth brome grass (Bromus inermis), and yellow sweetclover (Melilotus officinalis). Yet the availability, dependability, and vigor of these species make them hard to pass up when soil stabilization and vegetative cover are needed quickly. Incorporating native species might be a solution to this problem. Indeed, as my study demonstrated, native cultivars can be just as competitive as their non-native counterparts if selected for traits like fast growth or high seed production.

Unresolved soil problems may also be a factor in this story. By taking several exploratory soil cores, I found evidence of compaction within the top 10 cm of the restored roadbeds. Mechanically ripping the roadbeds during the recontouring process is not routinely implemented during road removal in the LMNG. Compaction can physically affect the germination or root establishment of plants (McSweeney and Jansen 1984, Bell et al. 1994). I found another indication of soil problems: a greater abundance of salt-tolerant species on roads compared to adjacent prairie. This means that salts and carbonates are likely brought to the surface during recontouring and could affect the growth of salt-intolerant species.

Conclusion
As suggested by my findings, problems stemming from restoration choices can ultimately be detrimental to longer-term goals such as the re-assembly of the native plant community. The spread of non-native species can be prolonged rather than reversed, and the conservation of locally adapted native species can be affected by genetic contamination from restoration seeding. Soil problems could further delay recovery. Evaluating restorations by vegetative cover only would fail to detect issues such as these.

Because roads are so pervasive, poor choices during their restoration can further degrade otherwise intact ecosystems, resulting in more harm than good. My study indicates that several restoration details would be worth investing in if long term recovery of the ecosystem is desired: seeding with locally collected or locally produced native seeds (or non-aggressive native cultivars); broadening the number of species used in mixes when the adjacent plant community is slow to colonize; and adequately preparing the soil before seeding. It is not enough to assume that any restoration project will benefit wildlands and natural areas. We must continue to closely evaluate both positive and negative consequences of road removal practices and implement changes accordingly.

Sara recently received an M.S. degree in Conservation Biology from the University of Minnesota – Twin Cities. She is currently employed with Western Plains Consulting, Inc. in Bismarck, North Dakota as an Environmental Scientist/Ecologist.

References

Aubry, C., R. Shoal, and V. Erickson. 2005. Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest Service.

Bakker, J., and S. Wilson. 2001. Competitive abilities of introduced and native grasses. Plant Ecology 157:117-125.

Bakker, J. D., and S. D. Wilson. 2004. Using ecological restoration to constrain biological invasion. Journal of Applied Ecology 41:1058-1064.

Bakker, J. D., S. D. Wilson, J. M. Christian, X. Li, L. G. Ambrose, and J. Waddington. 2003. Contingency of grassland restoration on year, site, and competition from introduced grasses. Ecological Applications 13:137-153.

Bell, J. C., R. L. Cunningham, and C. T. Anthony. 1994. Morphological characteristics of reconstructed prime farmland soils in western Pennsylvania. Journal of Environmental Quality 23:515-520.

Burke, I. C., W. K. Lauenroth, M. A. Vinton, P. B. Hook, R. H. Kelly, H. E. Epstein, M. R. Aguiar, M. D. Robles, M. O. Aguilera, K. L. Murphy, and R. A. Gill. 1998. Plant-soil interactions in temperate grasslands. Biogeochemistry 42:121-143.

Hammermeister, A. M., M. A. Naeth, J. J. Schoenau, and V. O. Biederbeck. 2003. Soil and plant response to wellsite rehabilitation on native prairie in southeastern Alberta, Canada. Canadian Journal of Soil Science 83:507-519.

McSweeney, K., and I. J. Jansen. 1984. Soil structure and associated rooting behavior in minespoils. Soil Science Society of America Journal 48:607-612.

Rogers, D. L. 2004. Genetic erosion: no longer just an agricultural issue. Native Plants Journal 5:112-122.

Simmers, S. 2006. Recovery of semi-arid grassland on recontoured and revegetated oil access roads. MS Thesis. University of Minnesota, St. Paul, MN.

USDI, and USDA. 2006. US Department of the Interior and US Department of Agriculture. Surface Operating Standards and Guidelines for Oil and Gas Exploration and Development. BLM/WO/ST-06/021+3071. Bureau of Land Management. Denver, CO. 84 pp.
Wilson, S. D. 1989. The suppression of native prairie by alien species introduced for revegetation. Landscape and Urban Planning 17:113-119.