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Understory physical structures reduce browsing damage to palatable shrubs in a dry conifer forest, western North America

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Abstract

High densities of cattle (Bos taurus) and wild ungulates (Rocky Mountain elk, Cervus canadensis; mule deer, Odocoileus hemionus) have impacted the abundance and population dynamics of palatable woody species in forests of the interior Pacific Northwest, USA. As a result, large shrubs are functionally absent from some forest stands. Physical structures can provide safe sites for browsed species to persist within forests. We evaluated the role of coarse woody debris and conifer trees in protecting a common, yet heavily browsed species, serviceberry (Amelanchier alnifolia) from elk, deer and cattle. We measured height and architecture of individuals and the degree to which they were protected by coarse woody debris and unpalatable conifers in areas exposed to ungulates and in ungulate exclosures. Multiple linear regression and quantile regression were used to test for the effects of protection and foliar overlap on plant height. Results showed that increased protection and foliar overlap resulted in a lower proportion of shrubs exhibiting arrested architecture. Shrub height increased as foliar overlap and protection increased, with effects primarily restricted to high levels of protection (> 300° surrounded by nearby conifer trees and/or coarse woody debris). Taller shrubs were most associated with intermediate levels of foliar overlap. Results support the hypothesis that understory structural elements can act as protective barriers. Forest management actions (e.g., prescribed fire, understory thinning) that reduce the prevalence of structures may unintentionally suppress browsed shrub species and contribute to the continued functional absence of tall deciduous species in dry conifer forests.

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Acknowledgements

We thank the Pacific Northwest Research Station, USDA Forest Service, particularly M. Wisdom, M. Rowland, and B. Naylor for their assistance and support. T. Clay, K. Coe, T. Dungannon, K. Dutto, R. Endress, K. Haliewicz, W. Powell and B. Watson assisted with data collection. We thank P. Kennedy, R. Mata-Gonzalez, B. Lachenbruch and M. Wisdom for comments on previous versions of this manuscript.

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Eastern Oregon Agricultural Research Center, Oregon State University.

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Authors and Affiliations

  1. Department of Animal and Rangeland Sciences, Eastern Oregon Agricultural Research Center – Union Station, Oregon State University, Union, OR, USA

    Dallas Hall Defrees, Joshua P. Averett & Bryan A. Endress

  2. Baker County Sage-Grouse Local Implementation Team Coordinator, Baker City, OR, USA

    Dallas Hall Defrees

  3. Eastern Oregon Agricultural and Natural Resource Program, Oregon State University, La Grande, OR, USA

    Bryan A. Endress

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  1. Dallas Hall Defrees
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Communicated by Leland Russell.

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Hall Defrees, D., Averett, J.P. & Endress, B.A. Understory physical structures reduce browsing damage to palatable shrubs in a dry conifer forest, western North America. Plant Ecol 222, 807–817 (2021). https://doi.org/10.1007/s11258-021-01144-2

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