Blunt-lobed woodsia (Woodsia obtusa) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

All Canadian populations are found in the Great Lakes-St.Lawrence Forest Region (Rowe 1972) and all are located on calcareous rock (marble, dolomite, limestone) with a southern aspect. The habitat of the species is generally forested, consisting of scattered, relatively small trees (10 to 15 m high; 10 to 20 cm diameter breast height). Canopy openness ranges from fairly closed (~7% in Gatineau Park, QC) to fairly open (~20% in Frontenac Park, ON). One population is atypically found in a very open site on a rock outcropping in an abandoned field. Shallow soils over bedrock and the overall xeric nature of the sites are the major cause for the small stature of the trees, although trees appear to be relatively young at several sites, perhaps indicating some recent disturbance (i.e. harvesting of stems, fire, etc.). Generally associated tree species are sugar maple (Acer saccharum), red oak (Quercus rubra), white oak (Q. alba), white ash (Fraxinus americana) and ironwood (Ostrya virginiana). The shrub layer has low cover on sites where sugar maple and red oak dominate (Quebec sites). Poison ivy (Rhus radicans), and prickly gooseberry (Ribes cynosbati) are the most frequent shrub species on these sites. In all Ontario sites (although to a lesser degree for the Frontenac Park population), red oak and white oak dominate, eastern red cedar (Juniperus virginiana) is a common tree, and shrubs such as fragrant sumac (Rhus aromatica) and downy arrowwood (Viburnum rafinesquianum) are abundant. The herb layer is very well developed and is most often dominated by Pennsylvania sedge (Carex pensylvanica). Other commonly found associated herbaceous species are bottlebrush grass (Elymus hystrix), bluestem goldenrod (Solidago caesia), marginal shield fern (Dryopteris marginalis), and herb-Robert (Geranium robertianum). A recent study on six Canadian sites shows an average soil depth of 3.5 cm (range: 0.5 - 9.5 cm) on an average slope of 43° (range: 26 - 88°) under an average 82% closed canopy (range: 40 - 96%) (Wild and Gagnon 2005). Recent soil data collected from Canadian blunt-lobed woodsia populations shows a pH range of 5.9 to 7.1, with an average of 6.6, and a high soil calcium content of almost 10000 ppm on average (Wild and Gagnon 2005).

Habitat trends

Although in some populations the habitat of blunt-lobed woodsia has shown signs of minimal disturbance over the past several years, no sites appear to have been dramatically modified. The sites are generally on steep rock faces, unsuitable for development or agriculture. Most sites are located within abandoned or rarely used areas that are not likely to suffer from direct human disturbance. The one exception possibly being the Westport – Sand Lake site where evidence of garbage dumping and tree removal/felling was reported in the original status report (Consaul 1994). Although no evidence of garbage dumping has been noticed over the past few years, there is recent evidence of tree felling in close proximity to extant plants. Although increased light levels resulting from tree removal could possibly be beneficial to blunt-lobed woodsia, the opening of the canopy appears to have caused an increase in the invasive exotic common buckthorn (Rhamnus cathartica). The future spread and increase of common buckthorn could severely impact this, the largest population of the fern in Canada.  Common buckthorn has been shown to modify ecosystem properties in woodlands (Heneghan et al. 2004), and could therefore have a detrimental effect on the blunt-lobed woodsia population.

Habitat protection/ownership

Five of the nine Canadian populations occur on land benefiting from some level of protection; two are on federal land in Quebec (Gatineau Park, owned by the National Capital Commission) and three are on provincially owned land in Ontario (Frontenac Park, Foley Mountain Conservation Area, and on crown land along the Rideau Trail). The four remaining populations (three in Quebec and one in Ontario) are located on privately owned land. Two of these (Quebec) are currently considered part of “exceptional forest ecosystems” (a Quebec Ministry of Natural Resources classification of conservation significance). Landowners in both provinces have been notified of the species’ presence.

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