Plant functional groups and species contribute to ecological resilience a decade after woodland expansion treatments

Authors: FREUND, STEPHANIE - University Of Reno; Newingham, Beth; CHAMBERS, JEANNE - Us Forest Service (FS); URZA, ALEZANDRA - Us Forest Service (FS); ROUNDY, BRUCE - Brigham Young University; CUSHMAN, HALL - University Of Nevada

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2020
Publication Date: 1/1/2021
Citation: Freund, S.M., Newingham, B.A., Chambers, J.C., Urza, A.K., Roundy, B.A., Cushman, H.J. 2021. Plant functional groups and species contribute to ecological resilience a decade after woodland expansion treatments. Ecosphere. 12(1):1-24. https://doi.org/10.1002/ecs2.3325.
DOI: https://doi.org/10.1002/ecs2.3325

Interpretive Summary: Tree removal is commonly conducted in the Great Basin region to address fire risk and habitat quality issues resulting from pinyon-juniper woodland expansion into sagebrush plant communities. To date, few studies have examined how tree-removal treatments affect sagebrush communities over long timescales or large geographic areas. We used 10 years of data from the SageSTEP network, a project designed to evaluate the outcomes of prescribed fire and cut-and-leave treatments in sagebrush sites spread across the Great Basin region that are experiencing woodland expansion. We found that responses of major plant life-forms (sprouting shrubs, non-sprouting shrubs, perennial grasses, and annual grasses) were related to the life-history traits of dominant species, and depended on pre-treatment tree density and on soil climate characteristics of the site (i.e. soil temperature and moisture regimes). In prescribed fire treatments, sites with warm and dry soils did not recover desirable plant communities regardless of tree density, while sites with cool and moist soils did show favorable responses, particularly at low to medium tree density (early woodland expansion). Cut-and-leave treatments sustained or increased desirable understory plants and experienced smaller increases in invasive annual plants in both soil climate types at all tree densities. Selecting appropriate tree-removal treatments to achieve desired long-term outcomes requires considering the relative abundance and life-history traits of the dominant species in addition to the degree of tree expansion and site environmental conditions.

Technical Abstract: Woody plant expansions are altering ecosystem structure and function, as well as fire regimes, around the globe. Tree-reduction treatments are widely implemented in expanding woodlands to reduce fuel loads, increase ecological resilience, and improve habitat, but few studies have measured treatment outcomes over long timescales or large geographic areas. The Sagebrush Treatment Evaluation Project (SageSTEP) evaluated the ecological effects of prescribed fire and cut-and-leave treatments in sagebrush communities experiencing tree expansion in North American cold desert shrublands. We used 10 yr of data from the SageSTEP network to test how treatments interacted with pre-treatment tree dominance, soil climate, and time since treatment to affect plant functional groups and dominant species. Non-sprouting shrub (Artemisia spp.), sprouting shrub, perennial graminoid, and annual grass responses depended on tree dominance and soil climate, and responses were related to the dominant species’ life-history traits. Sites with warm and dry soils showed increased perennial graminoid but reduced Artemisia shrub cover across the tree dominance gradient after prescribed burning, while sites with cool and moist soils showed favorable post-burn responses for both functional types, particularly at low to moderate tree dominance. Cut-and-leave treatments sustained or increased native perennial plant functional groups and experienced smaller increases in exotic annual plants in both soil climates across the tree dominance gradient. Both treatments reduced biocrust cover. Selecting appropriate tree-reduction treatments to achieve desired longterm outcomes requires consideration of dominant species, site environmental conditions, and the degree of woodland expansion. Careful selection of management treatments will reduce the likelihood of undesirable consequences to the ecosystem.