Restoration of semi-arid lands in the western U. S. is hindered by invasion of exotic species such as cheatgrass (Bromus tectorum) and unpredictable precipitation. With their ability to absorb moisture when soils are wet and slowly release it, superabsorbent polymers (SAPs) may aid in the establishment and survival of native species during periods of stress (low precipitation and high temperatures) when communities are most susceptible to invasion. In 2014, we implemented a study to address three questions: (1) what is the relative influence of precipitation amount and timing on establishment of invasive and native species in restoration, (2) do SAPs amplify and extend resource (soil water and nitrogen) availability during environmentally stressful periods, and (3) do SAPs alter the establishment of invasive and native species in restoration?
The experiment was established at two sites in Colorado: one on the Western Slope and one on the Front Range. Three treatments were fully crossed at both sites: (1) B. tectorum addition, (2) drought (66% reduction of ambient rainfall), and (3) SAPs incorporation. In the first growing season we collected plant density and soil moisture data at both locations and plant available nitrogen data at the Western Slope site.
Results/Conclusions
Drought treatments decreased soil volumetric water content (VWC) at both sites. The effects of SAPs on VWC were dependent on other treatments. At the Front Range site, VWC was surprisingly lower in plots with SAP under drought treatments but had no effect in ambient treatments. At the Western Slope site, the effect of SAPs depended on the level of B. tectorum. Soil nitrate concentrations at the Western Slope site were higher in drought treatments from May through July but lower from August through September. Soil nitrate concentrations were lower where B. tectorum was seeded.
At the Front Range site, drought treatments lowered seeded perennial densities but had no effect on seeded annuals. B. tectorum treatments had no effect on seeded perennials or annual species. At the Western Slope site, drought treatments had no effect on seeded annual or perennial densities. Seeding B. tectorum had no effect on densities of seeded perennials but decreased densities of seeded annuals. Considering these variable results, more research is needed to clarify the effects of treatment interactions on VWC and seedling responses. This understanding will provide new insights into the mechanisms driving early plant establishment in restoration and help restoration practitioners adapt to changing climates.