Thread-leaved sundew (Drosera filiformis) COSEWIC assessment and status report: chapter 5

Habitat Requirements

The Canadian habitats of thread-leaved sundew are all raised (or plateau)bogs (Zinck, 1991; Freedman et al., 1992). These are domed, ombrotrophic, highly infertile wetlands that have developed on relatively low, flat sites under the influence of a moist, mild, maritime-temperate climatic regime. The bogs have a hummock – hollow surface microtopography. The hollows are relatively moist, while the tops of the hummocks are drier. Interstitial water is highly acidic, with a pH of 3.1 to 3.3 (Freedman et al., 1992). The acidity is due to the ombrotrophic character of the raised bogs and the large concentrations of fulvic acids that give a dark-brown stain to the interstitial water (Freedman et al. 1989).The calcium concentration in interstitial water is 0.4-1.3 mg/L and similar to oligotrophic lakewaters in southwestern Nova Scotia (Freedman et al. 1989). Magnesium concentration is 0.8-2.0 mg/L, about twice as large as in those lakes. Depending on the time of year and recent rains, water may be present at the surface or somewhat lower in the peat.

The vegetation of the bogs is dominated by species of peat mosses (Sphagnum spp.) and low-growing shrubs. The habitat is well vegetated but open, consistent with the need of the rare sundew for weakly competitive conditions (Juniper et al., 1989; Freedman et al., 1992). The most consistently prominent indicator of the habitat of thread-leaved sundew is the short-statured, clumped deer-grass, Scirpus caespitosus (Freedman et al., 1992). On relatively drier and shrubbier microsites, the lichens Cladina mitis, C. terrae-novae, and Cladonia cervicornis are relatively prominent, as are the shrubs Aronia prunifolia, Gaylussacia baccata, G. dumosa, Juniperus communis, andKalmia angustifolia. On relatively wetter microsites with frequent exposures of poorly vegetated peat substrate, the peatmoss Sphagnum tenellum is prominent, as are Carex exilis, Rhynchospora alba, and various species of mosses and liverworts. The raised bogs are discontinuously surrounded by a moat (or lagg) of standing or slowly flowing water, with an inside band of taller heaths and other shrubs. Within these vegetation zones is a more extensive open area of shorter plants that provides habitat for the rare sundew.

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