Nonnative species influence vegetative response to ecological restoration: Two forests with divergent restoration outcomes

Changes in the vegetative structure and diversity of ponderosa pine forests have generated interest in conducting ecological restoration projects to improve the overall forest health of these ecosystems. Ecological restoration prescriptions often consist of thinning trees to emulate pre-1870s forest structure followed by prescribed burning. Disturbances associated with ecological restoration can, however, promote invasion by nonnative species. We compared two northern Arizona ponderosa pine forests treated for ecological restoration, one at the Fort Valley Experimental Forest and one at Mt. Trumbull on the Grand Canyon-Parashant National Monument. We examined the response of native and nonnative plant species, as well as all species combined, to treatments at the two forests. Both study sites showed a significant increase in native and nonnative species cover and richness by the fifth year post-treatment that remained significant by the tenth year post-treatment. Despite these general trends in native and nonnative community development, the understory vegetation at the two sites followed diverging successional patterns after treatment. By the tenth year post-treatment Fort Valley was dominated by native species and Mt. Trumbull was dominated by a single nonnative species, cheatgrass. The differences in post-treatment understory recovery are likely due to pretreatment forest conditions. At Fort Valley, nonnatives were present, but accounted for only 0.11% of the pretreatment cover. At Mt. Trumbull, nonnatives accounted for 5.26% of the pretreatment understory cover, with cheatgrass accounting for approximately 4% of the understory cover. Additionally, the soil seedbank at Fort Valley had greater overall species richness and greater native perennial grass richness than Mt. Trumbull. We propose that the application of ecological restoration treatments should be targeted to sites with low abundance of nonnatives prior to treatment. Sites containing high abundance of nonnatives prior to treatment should be managed for nonnative species mitigation before initiating any ecological restoration projects.