PS 78-153 - Assessing the impacts of ungulate grazing on biological soil crusts using long-term grazing exclosures

Friday, August 11, 2017
Exhibit Hall, Oregon Convention Center
Upekala C. Wijayratne and Sabine Mellmann-Brown, USFS Region 6 Northeast Oregon Ecology Program, Baker City, OR

Biological soil crusts are an important component of functioning semi-arid ecosystems. In sparsely vegetated communities, interspaces between vascular plants are often dominated by biological soil crusts. These crusts fix carbon and nitrogen, prevent soil erosion, maintain soil albedo, and increase water infiltration, and also may aid in seedling establishment of native perennial plants. Recent public interest regarding biological soil crusts underscores the importance of understanding how management practices in the Blue Mountains impact these ecologically significant organisms. We assessed the impact of ungulate grazing on biological soil crust morphological groups and vascular plant community composition across different sites using a network of long-term 3-way grazing exclosures (open, livestock exclusion only, complete ungulate exclusion) on the Malheur, Wallowa-Whitman and Umatilla National Forests. An ungulate grazing exclusion index was created to quantify treatment effect because of the varying age and maintenance of exclosures as well as varying livestock management in allotments where exclosures are located. Community composition and cover were measured using the line-point-intercept and cover frequency methods. Biological soil crust cover was analyzed using a mixed-effect model to isolate treatment effects from site effects. Vascular plant community composition was analyzed for each site separately using a multi-response permutation procedure.


Preliminary results indicate no effect of grazing exclusion on total biological soil crust cover but when analyzed by seral stage we found that there was an 8% relative increase in cover of early seral morphological groups (primarily short mosses) with a one unit increase in ungulate grazing exclusion. Pocket gophers and ground squirrels were very active within some of the complete ungulate exclosures and may explain lower biological soil crust cover in those exclosures. Very few late seral morphological groups (foliose and fruticose lichens) were present in the study sites overall. After decades of grazing exclusion, the vascular plant community inside exclosures at one site was very different from that found outside the exclosures resulting in higher fine fuels on the inside. When a wildfire burned through the area, burn severity was higher inside the exclosures than outside which differentially reduced biological soil crust cover. In conclusion, preliminary results show that grazing exclusion is weakly associated with higher biological soil crust cover at our sites; however, interactions with other types of disturbance also play a role in determining levels of biological soil crust cover.