- Understand how an east slope versus a west slope restoration site responds to drought
- Compare the impact of cheatgrass on restoration at an east slope vs. a west slope site
- Understand the interaction between drought and cheatgrass, and whether that differs at an east slope vs a west slope site.
- Assess the usefulness of a water-conserving soil amendment, super-absorbent polymer (SAP), at aiding establishment of native species, and whether the effectiveness of SAP depends on site, drought, and/or cheatgrass.
Plant communities are impacted by the timing of precipitation as well as the amount. On the western slope of Colorado, most precipitation comes in the fall, winter, and early spring, while on the eastern slope, spring and summer rains comprise the majority of total yearly precipitation.
Winter-annual invasive species such as cheatgrass (Bromus tectorum L.) have historically been more dominant on the western slope as their typical life history pattern of fall germination and spring seed set matches the timing of precipitation in this region. However, cheatgrass is also expanding on the eastern slope and models predict that invasion could be accelerated under lower and/or more variable summer precipitation. Overlaid on these general patterns are yearly fluctuations in rainfall amount, which can make or break restoration efforts in either area.
Uncertain precipitation and cheatgrass invasion make restoring disturbed habitats challenging. With our research, we sought to better understand effects of drought and cheatgrass on restoration success at an eastern slope vs. a western slope site, and to assess whether a water-conversing soil amendment, super absorbent polymer, could aid native species establishment.
Super-absorbent polymers (SAP) absorb water when soil moisture is high, and then gradually release moisture as soils dry. In horticultural settings SAPs have been shown to reduce water stress and promote growth of many types of plants. Since cheatgrass is most competitive under variable and low precipitation, we wanted to know if SAP could increase soil moisture, help native plants establish, and help them overcome the competitive effects of cheatgrass.
Our west slope site was located at Dry Creek Basin State Wildlife Area in San Miguel County, and our east slope site was at Waverly Ranch in Larimer County. In both locations, we set up drought, cheatgrass, and SAP treatments in areas in need of restoration. We induced drought by intercepting about 66% of ambient rainfall (April – September). All treatments (drought, SAP, cheatgrass) were ‘crossed’, which means that we were able to look at the effects of the treatments in different combinations with each other. Sites were set up 2013-2014 and monitored through 2017.
Drought had opposite effects on cheatgrass establishment at the two locations: on the western slope, drought lessened cheatgrass cover, while on the eastern slope, drought increased cheatgrass cover. At the western slope site, we found that cheatgrass greatly decreased soil moisture; early in the study, plots with cheatgrass and ambient precipitation had lower deep soil moisture than plots where 66% of rainfall had been diverted. Drought lowered seeded species cover at both sites, as did cheatgrass. SAP helped the establishment of seeded grasses, but only at the eastern slope site, and only with ambient precipitation.