Restoring semi-arid lands with superabsorbent polymers under reduced precipitation and threat of Bromus tectorum invasion
Date
2016
Authors
Garbowski, Magda, author
Brown, Cynthia S., advisor
Johnston, Danielle B., advisor
Hardegree, Stuart P., committee member
Schipanski, Meagan E., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Restoration of arid ecosystems in the western United States (US) is often constrained by low and variable moisture and invasion by exotic species. After disturbance, variability in resources as well as inherent soil moisture and temperature regimes may influence the susceptibility of an ecosystem to exotic species invasion. The invasive winter annual grass, Bromus tectorum (B. tectorum), is particularly competitive in disturbed semi-arid areas, has invaded tens of millions of hectares throughout the western US, and its range is projected to expand under predicted climate scenarios. Increasing soil moisture and resources in restoration projects may decrease soil moisture variability and promote establishment of a native plant community that is resilient to disturbance and resistant to invasion of B. tectorum. With their ability to absorb moisture when it is abundant and slowly release it over time, superabsorbent polymers (SAP) may increase overall soil moisture and decrease soil moisture variability during restoration. In this study, we aimed to investigate the interactive effects of precipitation timing, drought, B. tectorum, and SAP on soil resources and developing restoration plant communities. The study was established in 2014 at two climatically distinct sites: one site was located on the Eastern Slope (Larimer County) and one on the Western Slope (San Miguel County) of Colorado. Both sites fall under the mesic soil temperature regime and ustic-aridic soil moisture regime but vary in their susceptibility to invasion largely due to differences in seasonal precipitation patterns. While the Eastern Slope receives most of its growing season moisture in the early spring and summer, the Western Slope site receives most of its growing season moisture in the late summer and early fall. Two levels of three treatments (drought: exclusion of 66% of ambient rainfall or ambient rainfall; B. tectorum presence: 465 seeds m-2 or none; SAP: 26 g m-2 or none) were fully crossed in three blocks at each site resulting in a complete factorial experiment. After one year of monitoring soil moisture, plant available nitrogen (at the Western Slope site), and plant community responses, we observed significant effects of exclusion at both sites on soil resources and the developing plant communities. Independent and interactive effects of drought and SAP at the Eastern Slope site and drought and B. tectorum at the Western Slope site influenced plant communities and soil resources. Overall B. tectorum establishment was low on the Eastern Slope and high on the Western Slope in the first year of the study. At the Eastern Slope site, drought limited seeded species recruitment late in the season and the positive effects of SAP on seeded species were apparent only under ambient precipitation conditions. Total and annual seedling densities were higher under SAP treatments at this site. At the Western Slope site, total seedling densities were lower in drought treatments, and native seedling densities were lower in drought treatments at the end of the growing season. The effects of B. tectorum on seeded annuals at the Western Slope site depended on date and precipitation treatment. Seeded annuals densities were highest in mid-summer in treatments without B. tectorum and ambient precipitation. Interestingly, at the Western Slope site, B. tectorum under ambient precipitation had a stronger negative impact on soil moisture at 30 cm depth than drought treatments regardless of level of B. tectorum or SAP. B. tectorum also decreased soil moisture at 5cm depth early in the season at the Western Slope site while exclusion treatments decreased soil moisture later in the growing season at both sites. Our results demonstrate that drought negatively impacts soil resource availability and native plant community development in restoration. Techniques that improved water and nutrient availability especially under drought conditions are needed to promote native species establishment. While SAP did not improve soil moisture, higher seedling densities were found in SAP treatments at one site, especially under ambient conditions. This suggests that incorporating SAP into the soil may improve plant establishment, but effectiveness is likely affected by antecedent soil moisture and precipitation patterns. In our study, B. tectorum negatively impacted soil moisture and native plant establishment at one site demonstrating the need for management of this species in restoration of semi-arid lands.