Forest management and cervid herbivory data from Western Oregon, USA, 2012

Land management practices often directly alter vegetation structure and composition, but the degree to which ecological processes such as herbivory interact with management to influence biodiversity is less well understood. We hypothesized that intensive forest management and large herbivores have compounding effects on early-seral plant communities and plantation establishment (i.e., tree survival and growth), and the degree of such effects is dependent on the intensity of management practices. We established 225 m2 wild ungulate (deer and elk) exclosures nested within a manipulated gradient of management intensity (no-spray Control, Light herbicide, Moderate herbicide and Intensive herbicide treatments), replicated at the scale of whole harvest units (10-19 ha). Herbivory and herbicide applications interacted to drive vegetation structure, composition and crop-tree establishment, with herbivory effects most evident at intermediate herbicide treatments. Control stands were too forage-rich and Intensive stands too forage-poor to be substantially affected by herbivory. However, with Moderate herbicide treatment – which approximates treatments applied to > 2.5 million hectares in Pacific Northwest U.S.A. – foraging by deer and elk exacerbated the effect of the herbicides, resulting in simplified, low-cover plant communities resembling the Intensive herbicide treatment. In the Light herbicide treatment, herbivory suppressed shrub growth following herbicide treatment, improving planted conifer seedling survival, likely via competitive release from shrubs. Minor reductions in management intensity from the Moderate to Light herbicide treatments therefore facilitated the capacity of wild ungulates to benefit seedling survival – which constitutes early evidence of an ecosystem service. However, this ‘service’ may be to the detriment of native early-seral plant communities. These results demonstrate that by changing community composition and vegetation structure, intensive forest management alters foraging selectivity and subsequent plant-herbivore interactions; such shifts in early-seral communities are likely to influence understory plant communities and tree growth in later stages of forest development.

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