Effects of silvicultural use of the herbicide glyphosate on breeding birds of regenerating clearcuts in Nova Scotia, Canada.
Abstract
The effects of habitat changes caused by the spraying of herbicides on the breeding birds of regenerating clearcut mixed woodland (comprising Picea rubens, Abies balsamea, Acer rubrum, Betula alleghaniensis, B. papyrifera and Tsuga canadensis) were studied for one pre-spray and four post-spray years. Aerial application of glyphosate caused large decreases in the abundance of vegetation. There was a substantial recovery of some plant taxa by the end of the second post-spray growing season, especially of Rubus spp. and various herbaceous angiosperms, and there was further recovery to the end of the fourth post-spray year. Between the pre-spray and first post-spray years, the densities of most common breeding species decreased on all treatment plots, including the reference. In the second post-spray year, bird abundance on the reference plot increased to about the pre-spray condition, while on spray plots abundance stayed similar to the first post-spray year, and was smaller than the pre-spray population. The two most-abundant species, white-throated sparrow [Zonotrichia albicolis] and common yellowthroat [Geothlypis trichas], decreased on all spray plots up to the second post-spray year, and then substantially recovered by the fourth post-spray year. Song sparrow [Melospiza melodia] and Lincoln's sparrow declined on the reference plot among the four study years, while on the spray plots these species were most abundant in the second and fourth post-spray years. As the reference plot continued its post-cutting successional development, it was colonized by black-and-white warbler [Mniotilta varia], red-eyed vireo [Vireo olivaceus], ruby-throated hummingbird and palm warbler [Dendroica palmarum]. This change in avifauna was inhibited on the spray plots, because the herbicide treatment set the vegetation back to an earlier successional state, and onto a different successional trajectory which was probably more conifer-dominated. In an analysis of the avifauna data by detrended correspondence analysis, all of the pre-spray plot-years ordinate together, indicating a similarity of species composition and abundance. The reference plot-years ordinate with chronologically increasing scores on axis 1, reflecting avifaunal changes during post-clear-cutting succession. Axis 2 reflects post-spraying changes in the avifauna of spray plots, lasting over a 2-year period, followed by a successional, post-herbicide application development of the avian community. Axis 2 is weaker than axis 1 (eigenvalues 0.151 and 0.065, respectively), so the avifaunal changes associated with post-clearcutting succession are more prominent in the ordination than changes associated with glyphosate spraying.