Changes in abundance of eight sagebrush-steppe bunchgrass species 13 yr after coplanting

Authors: Hamerlynck, Erik; Davies, Kirk

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2018
Publication Date: 1/9/2019
Citation: Hamerlynck, E.P., Davies, K.W. 2019. Changes in abundance of eight sagebrush-steppe bunchgrass species 13 yr after coplanting. Rangeland Ecology and Management. 72(1):23-27. https://doi.org/10.1016/j.rama.2018.07.001.
DOI: https://doi.org/10.1016/j.rama.2018.07.001

Interpretive Summary: Self-sustaining perennial bunchgrass populations are important to the successful conservation and restoration of sagebrush steppe rangelands. However, there are very few long-term studies on the population dynamics of sagebrush steppe bunchgrasses. In 2011, we counted the number and species of grasses present in each cell of nine grids of 29x29 cells planted with a single adult grass individual in 1998. Cells were classified as a species hold (present in 1998 and 2011), gain (not present in 1998, present in 2011), or cede (present in 1998, missing in 2011). No matter the kind of change in population (growth, static, or declines), nearly 90% of all plants in 2011 were found in cells outside the original plantings, indicating that establishment from seed was the most important driver of population dynamics in this bunchgrass plant community. We also found that the number of native grasses declined dramatically after a threshold density of the exotic grass, crested wheatgrass. These results could help land managers modify seeding or grazing strategies with crested wheatgrass to maintain the diversity and productivity of these semi-arid rangelands.

Technical Abstract: Stable bunchgrass populations are essential to resilience and restoration of sagebrush steppe, yet few studies have assessed long-term variation in plant abundance from a known starting point in these important rangelands. We capitalized on a previous paddock study by re-establishing in 2011 nine replicate blocks consisting of 29x29 grid of cells, each planted in 1998 with a single individual of one of eight sagebrush steppe bunchgrasses, including the widely planted exotic, crested wheatgrass (Agropyron cristatum). Plant species and numbers were determined for each cell, which were classified as holds, gains, or cedes. We hypothesized the competitive crested wheatgrass would proportionally occur more in gained cells compared to native grasses. While crested wheatgrass did proportionally hold and gain the greatest number of cells, the relative number of plants within holds and gains was constant across all species, with most plants (80-87%) occurring outside cells originally planted with them. Crested wheatgrass had greater proportions of holds and gains where it was the only species within the cell, and showed even presence across all cells planted with other grass species in 1998, while native grasses were under-represented in 1998 crested wheatgrass cells, and frequently over-represented in other native species cells. The ratio of total plants of crested wheatgrass to native bunchgrasses followed a sigmoidal step increase with increasing crested wheatgrass density. These results show population change in sagebrush steppe bunchgrasses are strongly determined by seed production and emergent seedling survival, both of which are stronger in the exotic bunchgrass. This study also showed that native grasses can maintain a considerable presence via seed in areas depending on crested wheatgrass density, and this could inform management strategies that could capitalize on the utility of crested wheatgrass but still attain sustained levels of native grass productivity and diversity.