Secondary invasion and re-invasion after Russian-olive removal and revegetation

Authors: Espeland, Erin; Muscha, Jennifer - Boyle; SCIANNA, JOSEPH - Natural Resources Conservation Service (NRCS, USDA); KILIAN, ROBERT - Natural Resources Conservation Service (NRCS, USDA); West, Natalie; Petersen, Mark

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2017
Publication Date: 12/14/2017
Citation: Espeland, E.K., Muscha, J.M., Scianna, J., Kilian, R., West, N.M., Petersen, M.K. 2017. Secondary invasion and re-invasion after Russian-olive removal and revegetation. Invasive Plant Science and Management. 10:340-349. https://doi.org/10.1017/inp.2017.36.
DOI: https://doi.org/10.1017/inp.2017.36

Interpretive Summary: Russian olive is a nitrogen-fixing tree currently invading land next to waterways in the Northern Great Plains. Because understory species that survive tree invasions may be only a small subset of available native species, revegetation may be required to return native species to a Russian olive removal site. Revegetation may also act to reduce subsequent invasion if revegetetation species competitively exclude invaders. This study that summarizes data collected 5 years after Russian olive removal and four years after revegetation planting. Our hypotheses were that revegetation increasingly excludes invasive species as it ages and increases native species as the planting matures compared to unrevegetated controls. We expected our transplanted tree and shrub species to vary in survivorship, with some simply unable to establish. We found that all transplanted species except narrowleaf cottonwood established, although at different rates. We also expected resprouting to dominate Russian olive stand regeneration early, with germination from the seed bank to vary among years; this appeared to be the case. Russian olive trees resprouted for two years post-removal, with no seed germination in the removal year, and seed germination resulted in stand regeneration rates ranging from 10% to 215% among years. All native understory species cover and diversity increased with time since removal. Invasive species diversity remained the same over the course of our study with cover increasing with time since removal, regardless if the plot was planted. Our restoration plantings succeeded: all planted species but two established, however the long-term nature of this study allowed seeded species to move beyond treatment boundaries, resulting in few treatment effects by the study’s end. The high level of disturbance resulting from tree removal appeared to drive understory vegetation dynamics. Our research shows that follow-up control of seedling establishment is critical to Russian olive management and that native species can establish quite well after removal.

Technical Abstract: Russian olive is a nitrogen-fixing tree invading riparian corridors in the Northern Great Plains. Native species establishment can be hampered by invasive plant soil legacies that may be particularly likely in the case of Russian olive, and understory species that survive the invasion may be only a subset of abailable native species. Therefore, revegetation may be required to return native species to a removal site. Revegetation may also act to reduce invasion through competitive exclusion. In this study that summarizes data collected 5 years after Russian olive removal and four years after restoration planting, our hypotheses were that revegetation increasingly excludes invasive species and increases native species as the planting matures compared to unrevegetated controls. We expected transplanted tree and shrub species vary in survivorship, with some simply unable to establish and evaluated this expectation descriptively, finding that all transplanted species except narrowleaf cottonwood established, although at different rates. We also expected resprouting to dominate Russian olive stand regeneration early, with germination from the seed bank to vary among years; this appeared to be the case. Trees resprouted for two years post-removal, with no seed germination in the removal year, and seed germination resulted in stand regeneration rates ranging from 10% to 215% among years. Our hypotheses regarding change in plant cover and diversity after removal and restoration were not supported: all native understory species cover and diversity increased with time since removal. Invasive species diversity remained the same over the course of our study with cover increasing with time since removal. Our restoration plantings succeeded: all planted species but two established, however the long-term nature if this study allowed seeded species to move beyond treatment boundaries, resulting in few treatment effects by the study’s end. The high level of disturbance resulting from tree removal appeared to drive understory vegetation dynamics. Our research shows that follow-up control of seedling establishment is critical to Russian olive management and that native species can establish quite well after removal.