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Does the application of glyphosate-based herbicides affect forest plant diversity?

Category: Environment and Wildlife

Yes. Like treating weeds in your garden, the point of herbicide application is to reduce the population of competitive plant species in a woodlot. Herbicide treatment is one of many tools used by forest managers to promote the growth of softwood trees early in their development. However, plant communities rapidly regrow, providing softwoods with only a temporary head start. While a temporary impact on plant diversity will occur in planted stands, the use of herbicides, such as glyphosate, does not impact diversity at the forest landscape level. Only 1% of New Brunswick’s forests are harvested in any year. Only one third of the area harvested in any year is planted, resulting in 0.3% of the New Brunswick forest being planted and treated with herbicide in any year.

In answering this question it is important to distinguish between diversity, richness and abundance of plants within a community. Species richness is simply the number of different species in a given community, whereas measures of diversity incorporate aspects of both richness and evenness (i.e. how equal the abundances of different species are). Glyphosate-based herbicides are most commonly applied to enhance conifer regeneration on recently harvested sites. Consistent with this objective, effective application of these products will temporarily reduce the abundance, cover and vigour of targeted competitive plant species (e.g. deciduous woody species, shrubs) such that growth of conifer seedlings is enhanced. Since glyphosate is strongly bound in soils, and not taken up appreciably by plants through roots, plant species that regenerate from seed will rapidly re-establish on treated sites. Manipulating the plant community dynamics to temporarily advantage conifers can be viewed as a means of shortening the early phase of a natural successional pathway and ensuring the replacement of conifer dominated stands on the landscape. Although abundance and vigour of targeted competitors is temporarily reduced, treatments with glyphosate-based herbicides typically do not result in significant reduction in richness of vascular plants on the site and do not generate plant monocultures.

As part of a broader review of glyphosate-based herbicide effects on plant and animal diversity, Sullivan and Sullivan (2003) reported that 10/12 studies showed that species richness and diversity of vascular plants was either unaffected or increased, particularly for herbaceous species, following glyphosate treatments. Of the two exceptional studies, Sullivan et al. (1988) observed a reduction in species richness of shrubs in the initial five years post-treatment and Santillo et al. (1989) observed lower species richness of shrubs and forbs in treated as compared to untreated sites. The silvicultural objective of herbicide treatments is to temporarily reduce competition from woody deciduous, shrub and other plants such that regeneration of desired crop tree species (typically conifers) is enhanced. In essence, the herbicide treatment provides conifers with a “jump-start” down the successional pathway toward a conifer dominated stand replacing that which was harvested. Largely because glyphosate is non-persistent and has no soil activity, the effect on the plant community is short-lived. Treatments with glyphosate-based herbicides are very effective at temporarily reducing the abundance and vigour of the targeted competing plant species (dominant woody deciduous, shrub and herbaceous species), but they do not generate single layer plant monocultures or reduce overall species richness (Freedman et al. 1994; Gagne et al. 1999; Bell and Newmaster 2002; Newmaster and Bell 2002). Results of the study by Bell and Newmaster (2002) may be considered typical in that they show herbicides had a relatively greater initial effect on plant community composition as compared to the two different mechanical vegetation control treatments, but that woody, herb, and grass layers showed substantial resilience to all treatments and recovered to pre-treatment levels within five years. Relatively greater effects were observed on ferns, mosses and lichens, but even these species groups recovered within the five year time frame. (Newmaster and Bell 2002).

Sullivan TP, Sullivan DS. Vegetation management and ecosystem disturbance: impact of glyphosate herbicide on plant and animal diversity in terrestrial systems. Environmental Review. 2003; 11:37-59.

Sullivan, T.P., Wagner, R.G., Pitt, D.G., Lautenschlager, R.A., and Chen, D.G. Changes in diversity of plant and small mammal communities after herbicide application in sub-boreal spruce forest. Can. J. For. Res. 1998; 28: 168–177.

Santillo, D.J., Leslie, D.M., Jr., and Brown, P.W. 1989a. Responses of small mammals and habitat to glyphosate application on clearcuts. J. Wildl. Manage. 53: 164–172.

Freedman B, Woodley S, Loo J. Forestry practices and biodiversity, with particular reference to the Maritime Provinces of eastern Canada. Environmental Review. 1994; 2:33-77.

Gagné N, Belanger L, Hout J. Comparative response of small mammals, vegetation and food sources to natural regeneration and conifer release in boreal balsam fir stands of Quebec. Can. J. For. Res. 1999; 29:1128-1140

Bell FW, Newmaster SG. The effects of silvicultural disturbances on the diversity of seed-producing plants in the boreal mixedwood forest. Can. J. Forest Res. 2002; 32:1180-91.

Newmaster SG, Bell FW. The effects of silvicultural disturbances on cryptogam diversity in the boreal-mixedwood forest. Can. J. Forest Res. 2002; 32:38-51.