Researching grassland restoration effects in House Rock Valley
Reseeding degraded areas with native species is an important step toward restoring grassland ecosystems, however, some uncertainty exists about the most effective methods for reseeding under extremely arid conditions such as those found in the House Rock Valley. We are working with researchers at Northern Arizona University to implement a small-scale experimental reseeding project, to compare candidate models and analyze the synergistic effects between cattle grazing, climate, and reseeding treatments on the success of grassland restoration in House Rock Valley.
The House Rock Valley (HRV) is seated between the Paria Plateau to the North, the Kaibab Plateau to the West, and drops into Marble Canyon at its eastern and southern boundaries. This landscape is typical of many western public rangelands in the arid west. In places it has been badly damaged by livestock grazing over the past century, as evidenced in places where invasive weeds, eroded washes, and grazing-tolerant plants have replaced more diverse native plant communities. Livestock grazing has been a major stressor to native communities in the HRV over the last century, and while livestock numbers are far reduced from what they were around the turn of the century, the legacy of overgrazing remains. An arid climate and increasing drought has stalled the recovery of this system, and vegetative cover and soil stability remain low while invasive species pose a major threat to native biodiversity.
The HRV provides important winter range for the livestock operation, but conditions are degraded over large areas and productivity is low in all but the wettest years. Expert opinion, historical accounts and the recent ecological assessment have led to the conclusion that reseeding will be necessary on the most degraded sites within the HRV to achieve our restoration goals. However, some uncertainty exists about the most effective methods for reseeding in these arid grassland landscapes. That being said, there is good consensus that cool-season grasses have been most negatively affected by historical overgrazing and drought and are good candidates for re-seeding efforts.
Through the initial baseline ecological assessment, we identified the HRV as an area of high exotic plant abundance and poor range quality; most likely as a result of heavy winter grazing over the years. Recognizing the climatic and ecological constraints that could potentially impede the success of large-scale restoration efforts, we employed a graduate student from Northern Arizona University to initiate a small-scale experimental reseeding project, where we could analyze the independent and crossed effects of cattle grazing, climate, and reseeding methods on the success of grassland restoration in HRV. In addition, experimenters measured the impacts of restoration treatments on plant community composition, invasive species, and soil stability in order to more fully evaluate the costs and benefits of common restoration practices.
Experimenters established (n=100) 3 m2 study plots in the center of HRV where soil and vegetation conditions are most representative of the valley as a whole. Using a fully randomized treatment design, 8 different restoration treatments and 2 controls, were conducted in the fall of 2006. Treatments included: drill seeding, broadcast seeding, broadcast seeding with cattle trampling, and cattle trampling alone. Through the use of supplemental irrigation, half of all plots were subjected to a simulated climate of the 90th percentile historic maximum precipitation recorded in HRV, while the other half that were not irrigated were subjected to 2007’s below-average precipitation conditions. This also applied to control plots.
Preliminary results from this work emphasize the difficult road ahead in restoring native grass species to HRV. First, drill seeding seemed to be critical to successful germination of native grasses. However, drill seeding caused significant amounts of soil disturbance, which led to a much higher than expected establishment of invasive weeds and reductions in soil stability. Second, timing and amount of big precipitation events appear to be crucial to restoration success. In the first year following treatments, seeded grasses only germinated well in the abnormally wet spring seasons that we simulated with irrigation treatments. Interestingly, in year two the germination of seeded grasses species nearly doubled compared to germination one year following seeding. While precipitation in the winter before this second year was well above average, experimenters did not control for winter precipitation and therefore cannot attribute the spike in germination two years after seeding to this factor. Still, according to the literature, the amount and seasonality of annual precipitation is a highly influential factor on plant germination and growth in arid regions.
Initial results appear to support the growing acknowledgment that there are unintended consequences associated with arid rangeland restoration. It is plausible that methods which appear to meet restoration objectives could actually be moving the system further away from ecological health. Practically, these results give us reason to be very cautious about pushing ahead with treatments without taking into consideration the highly influential and complex roles that soil disturbance and climate have on such restoration actions.
Due to the uncertainty associated with seeding efforts, and within the context of global climate change, restoration priorities in the HRV should focus on minimizing soil disturbance and controlling exotic species invasions. Actions that can target improvements in ecological function without depending on the reintroduction of specific ecosystem components, such as cool-season grasses, should be explored. Whether such actions exist and how to carry them out is an immediate research priority.