Category: Environment and Wildlife
The woody browse that deer like to eat is reduced on sites treated with herbicides, such as glyphosate. However, deer and moose are well adapted to locate and feed on the most nutritious food sources possible. A large scale study is underway to better understand how deer are using our forests, how suitable our forests are to support deer in summer and winter months, and the effects of glyphosate on availability of food for deer.
Both deer and moose consume significant amounts of non-woody foods during the growing season, including foliage, shoots and twigs from deciduous trees and shrubs (e.g. maples, pin cherry, birch, dogwood). These food sources are commonly referred to as "woody browse". These same species are often the target of conifer release treatments and their biomass is commonly reduced 50-70% by successful conifer release treatments. However, herbaceous species contribute significantly to deer diets and are eaten by moose, and they commonly remain, or quickly “rebound,” in treated areas. Moose, which feed more on woody browse, do reduce their use of treated areas for five to seven years after treatment. However, that is only important in the short-term and then only if the landscape is dominated by young released stands. When the landscape is not dominated by young released stands these animals simply find and use better habitat found in other parts of their range.
Large mammals range over large areas of forested landscapes and hence potential effects must be considered in relation to the dynamic mosaic of forest conditions that exist on that landscape (Lautenschlager and Sullivan 2002). On an annual basis, based on a national average, less than 1/3 of clear cut harvested areas are treated with glyphosate-based herbicides. There is no question that the abundance of deciduous woody browse species will be at least temporarily reduced on these treated areas. However, as many of these plant species regenerate from the soil seed bank, reduced browse effect, even on treated sites, will be transient as evidenced in the studies noted above. Moreover, copious amounts of browse material will typically occur on the 2/3rds of the forest cutover lands which are not treated with herbicide.
A specific area of concern is the potential for reduced winter browse availability for deer whose winter foraging range area may be limited by snow depth. The potential for such effects would depend on both the proportion of the foraging area around deer yards that is treated with glyphosate-based herbicides and the actual reduction in winter browse species typically used as winter forage by deer on those areas. Considering information from New Brunswick as an example, on the broader landscape, much of the clearcut area (66% as a long term average estimate on crown lands) is left to regenerate without the aide of glyphosate treatment. Assuming this is true on cut over sites close to deer wintering areas as well, suggests that 66% of the open areas in which deer might browse in winter would be unaffected by glyphosate-based herbicide treatment. Potential effects would also be further ameliorated given that:
As noted by Morrison et al. (2002), by identifying browse species selected by deer and by understanding their associations with forest stand types, managers will be better able to meet the dual objectives of providing appropriate conifer dominated cover and an adequate supply of browse to support deer through winter. In this context, although it is quite unlikely that glyphosate-based herbicide use is a significant factor influencing deer winter browse availability, detailed geographic analysis of the proportion of deer foraging areas actually receiving glyphosate-based herbicide treatments would provide useful information furthering our understanding on this specific aspect. Broader assessment of deer population viability must consider a number of other potentially influencing factors such as natural predation (e.g. by coyotes, bears and wolves) facilitated predation through predator use of of roads and trails, hunting pressure and poaching, disease and deleterious effects of artificial feeding.
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