Study documents response of elk forage to forest herbicides and herbivory

Young forests are important for many wildlife species, including ungulates in many parts of North America, because they provide abundant forage. Timber harvest has been a major factor creating young forests in many regions of the nation.

Herbicides are often used on private lands to manage vegetation structure and composition in young forests. However, concerns have sometimes been expressed about the potential effects of herbicide applications on forage resources for elk and black-tailed deer in the Pacific Northwest, and possible population-level responses.

In response to these concerns, scientists with the University of Alberta and NCASI recently investigated effects of operational herbicide application and herbivory on nutritional resources for elk in young forest stands on commercial timberlands around Mount St. Helens, Washington.

A paper describing results from the study was recently published in Forest Ecology and Management.  It was authored by A.B. Geary and E.H. Merrill of the University of Alberta and J.G. Cook, R.C. Cook, and L.L. Irwin (retired) of NCASI. 

The authors found that herbicides reduced biomass of plants preferred as forage by elk for only two years following application and reduced biomass of species avoided by elk much longer. In contrast to the short-term effects of applying herbicides, herbivory resulted in a strong and consistent reduction in biomass and digestible energy of accepted plant species, but had little effect on avoided species. 

Under either treatment, more nutritional resources existed in young forests than in older forests. Thus, the authors suggest that widespread declines in the rate of timber harvest in the Pacific Northwest may exert a greater effect on the nutritional adequacy of landscapes in the region for elk than operational use of herbicides at stand initiation. The abstract for the paper follows.

“Concern exists about the effects that silvicultural herbicides, past herbivory, and forest succession may have on the carrying capacity for ungulates in the vicinity of Mount St. Helens in southwest Washington. We independently evaluated the effects of both operational herbicides and ungulate herbivory on biomass and available digestible energy of forages in a chronosequence of early-succession (ES) forest stands using a retrospective, paired-site design. We distinguished between trends in biomass by classifying species as avoided (used less than available) or accepted (used equal or more than available) forages for elk based on recent research involving foraging trials of hand-raised, tractable elk in western Oregon and Washington. Herbicide application reduced biomass of accepted forages (kg/ha) for only two years, whereas the initial reduction in avoided biomass persisted throughout the 13-year ES period that we evaluated. The reduction in avoided species was not associated with an increase in accepted species, which may be related to herbivory. We observed that forest canopy closure even at 10–13 years was similar in stands treated and untreated with herbicides, suggesting herbicides did not shorten the seral window when palatable forages were abundant. Because dry matter digestibility of accepted species was generally higher than avoided species, digestible energy (DE, kcal/ha) available to elk was similar to trends in accepted and avoided biomass. An initial 2-year increase in modelled estimates of dietary DE in herbicide-treated stands in years 1–2 resulted from a rapid recovery of plants with high digestibility. In contrast to herbicide treatments, effects of ungulate herbivory on ES communities increased with stand age with a reduction of deciduous shrub height but not densities, reduced biomass of accepted but not avoided species, and reduced standing DE of accepted species outside exclosures. Despite the influence of herbicide applications, herbivory, or their interactions, nutritional resources for elk were equal or more available in ES stands than in mid- and late-succession stands, highlighting the importance of maintaining ES stands for elk in this region.”  

Reference 

Geary, A.B., E.H. Merrill, J.G. Cook, R.C. Cook, and L.L. Irwin. 2017. Elk nutritional resources: Herbicides, herbivory and forest succession at Mount St. Helens. Forest Ecology and Management 401:242-254. https://doi.org/10.1016/j.foreco.2017.06.028