A primary goal of restoration is to establish a stable community that is resilient and resistant to global change stressors by matching the composition of an intact or historic reference community. It is often assumed that seeding restoration sites with species found in reference communities will naturally lead to the desired composition over time. However, restoration trajectories can be highly unpredictable and little is known about how restoration age and initial restoration conditions such as seed mix richness affect similarity to reference community conditions, known as convergence. In this analysis, we leveraged an existing data set of species abundances in restored and intact reference sagebrush steppe communities from Grand Teton National Park spanning 2011 to 2019. Line-point-intercept transects were used to sample restored sites and randomized quadrats within permanent plots were used to sample reference sites. We evaluated proportions of grass, forbs, shrubs, and native and invasive species to determine whether restoration trajectories converge with reference communities. The average values for the reference sites were used as a baseline for comparison and linear models were used to assess how restoration age and seed mix diversity (richness) affected community composition.
Results/Conclusions
Restored sites had higher proportions of grasses and invasive species and lower proportions of forbs, shrubs, and native species compared to the reference sites. Grasses in the restored communities exceeded levels in the reference sites as early as the first year, peaked in year five, and declined thereafter (R2 = 0.38, P < 0.0001). Conversely, proportions of forbs and native species never reached levels found in the reference sites and decreased significantly with age (forb: R2 = 0.29, P < 0.0001, native: R2 = 0.14, P < 0.0001). Shrubs increased slowly with age, though the reference proportion was never reached (R2 = 0.18, P < 0.0001). More diverse seed mixes were associated with reduced proportions of grass and increased proportions of shrubs and forbs. Seed mix diversity was not a significant driver of the proportions of invasive and native species. The diversity of the seed mix impacted the resulting community composition independent of restoration age, suggesting that land managers could manipulate seed mixes to optimize restoration outcomes. Additional work will consider other potential drivers of restoration trajectories, such as climate of the seeding year, soil type of the site, and effects of inter-annual climatic variation.