Research documents role of herbicides and herbivory on young plant communities

Herbicides are an important tool for forest managers, including for establishment of new stands. A number of studies have examined response of plant and wildlife communities in young forests to herbicide use. In the Pacific Northwest, it has been assumed that elk and deer foraging is a result of herbicide applications alone; however, no previous studies were able to clearly establish if this is the case or, alternatively, if herbicide use and herbivory by deer and elk work together to affect plant community structure in regeneration Douglas-fir stands.

Recently, researchers with NCASI, Oregon State University, and the U.S. Geological Survey (USGS) conducted a study to examine this relationship between herbivory and herbicide applications on plant communities in young Douglas-fir stands. Results of this study were published in a paper in Ecological Applications, authored by Thomas Stokely (OSU), Jake Verschuyl (NCASI), Joan Hagar (USGS), and Mathew Betts (OSU).

This was the first study to include both a control (no herbicide application) and 225 m2 (269 yards2) exclosures, to eliminate foraging by deer and elk, within each 10-19 ha (25-48 acre) plot. The combination of an untreated control and ungulate exclosures allowed assessment of combined and separate effects of herbicide and herbivory on broadleaf vegetation and Douglas fir crop trees. Results from this effort indicate that herbicides and herbivory work in combination to drive plant community structure and Douglas-fir seedling development. Specifically, in very intensive herbicide treatments (multiple applications across years), deer and elk had little influence on plant species composition. The same was true in controls where plant cover and richness was highest. However, in the intermediate level herbicide treatments, which are those most commonly applied in the Oregon Coast Range, effects of deer and elk herbivory on plant community composition and structure were evident. In these treatments, elk and deer were consuming shrubs that may have helped to release Douglas-fir seedlings, thus working with light to moderate herbicide treatments to increase stand growth. Importantly, the authors also failed to document negative effects of herbivory on stand establishment.

The abstract for the paper follows:

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 large herbivores compound the effects of intensive forest management 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-herbicide control, light herbicide, moderate herbicide, and intensive herbicide treatments), replicated at the scale of whole harvest units (10–19 ha). Vegetation structure, composition, and crop-tree responses to herbivory varied across the gradient of herbicide application during the first two years of stand establishment, with herbivory effects most evident at light and moderate herbicide treatments. In the moderate herbicide treatment, which approximates management applied to >2.5 million hectares in the Pacific Northwest, United States, foraging by deer and elk resulted in simplified, low-cover plant communities more closely resembling the intensive herbicide treatment. Herbivory further suppressed the growth of competing vegetation in the light herbicide treatment, improving crop-tree survival, and providing early evidence of an ecosystem service. By changing community composition and vegetation structure, intensive forest management alters foraging selectivity and subsequent plant–herbivore interactions; initial shifts in early seral communities are likely to influence understory plant communities and tree growth in later stages of forest development.

Reference 

Stokely, T. D., J. Verschuyl, J. C. Hagar, and M. G. Betts. 2018. Herbicide and herbivory interact to drive plant community and crop-tree establishment. Ecological Applications. 28(8):2011-2023. https://doi.org/10.1002/eap.1777.