PTERIDOPHYTES
Daniel F. Brunton
RECENT TRENDS
The Mosquito Fern (Azolla caroliniana) is the only pteridophyte possibly extirpated from the Mixedwood Plains Ecozone. It has not been confirmed here since the mid 19th century although it is apparently capable of remaining dormant and invisible for many years. Substantial populations have been found along the southern shore of Lake Ontario and a few kilometers beyond the ecozone in the Frontenac Axis north of Lake Ontario (Cody and Schueler 1988).
The loss of populations of common and rare taxa has been substantial as habitat degradation continues throughout the ecozone. Particularly significant elements of this recent trend include:
- fragmentation of natural habitat - reduction in the area and quality of natural landscape from industrial and agricultural development has resulted in significant population and habitat loss across the Mixedwood Plains Ecozone and particularly in the Carolinian Zone of southwestern Ontario. Few if any of the natural pteridophytes of the Mixedwood Plains Ecozone are unaffected. This fragmentation loss is particularly severe in isolated woodlots (Bricker and Reader 1990).
Case Study: Evergreen Woodfern (Dryopteris intermedia)
The Evergreen Woodfern (Dryopteris intermedia) is a common species of mesic and dry-mesic sites in hardwood forests across the ecozone (Marie-Victorin 1947, Raymond 1950, Britton and Soper 1966, Rousseau 1974, Carlson and Wagner 1982). In Essex County in southwestern Ontario where a mere 3% of the original forest cover remains (Botham 1981, Klinkenberg et al. 1990), it is a rare to locally common ground cover species. It is still moderately common in more forested areas elsewhere in southwestern Ontario (Oldham 1993). Similarly, in the St. Lawrence Seaway area of southeastern Ontario where less than 18% of the land is still forested (Ontario Ministry of Natural Resources 1983) this remains a common woodland species (Dore and Gillett 1955, Brunton 1995). The reduction in pteridophyte populations in these areas, particularly in southwestern Ontario, has been massive. The long term consequences of such losses in the ecozone are unknown.
- loss of specialized habitats - populations of uncommon and/or disjunct species of emergent shores (Isoetes, Selaginella) and cliffs and rock barrens (Pellaea, Woodsia, Asplenium, Dryopteris) continue to decline where previously intact areas of relict habitats along the Niagara Escarpment and the Ottawa and St. Lawrence Rivers are being destroyed by quarrying, infilling and environmental pollution.
Case Study: River Quillwort (Isoetes riparia) and Hart's-tongue Fern (Asplenium scolopendrium)
Although species of very different habitats, the emergent aquatic River Quillwort (Isoetes riparia) and the Hart's-tongue Fern (Asplenium scolopendrium) of limestone cliff- faces demonstrate similar population trends. The Hart's-tongue fern is confined to 'islands' of relatively unimpacted wooded cliff sites along the Niagara Escarpment, a number of which have been destroyed by quarrying, forest cover destruction and slope erosion (Soper 1954, Britton 1969, Cody and Britton 1989). The River Quillwort is primarily confined in the Mixedwood Plains Ecozone to isolated sections of emergent, gravelly shoreline along the Ottawa, Richelieu and lower St. Lawrence Rivers (Britton and Brunton 1989, Brunton and Di Labio 1989). Most of the original shoreline along these portions of the rivers has been severely impacted by infilling, pollution, flooding and agricultural, residential and industrial developments. The natural habitat for both species has been reduced in recent decades and populations have been eliminated in the process. Despite the detection of additional populations through recent, concentrated investigations, the overall decline in the populations of both taxa in the Mixedwood Plains Ecozone has undoubtedly been substantial.
- loss of mature forest populations - while concentrated field investigations have resulted in the discovery of new stands of uncommon to rare pteridophytes in mature (primary) forest, the majority of such old forest habitat has been eliminated and its reduction continues. Large populations of mature forest species such as the Broad Beech-fern (Phegopteris hexagonoptera) (Fig.PT-5), Goldies Fern (Dryopteris goldiana) and Glade Fern (Diplazium pycnocarpon) are accordingly being lost across the ecozone.
Case Study: Broad Beech-fern (Phegopteris hexagonoptera)
The Broad Beech-fern (Phegopteris hexagonoptera) occurs under rich, undisturbed, mature hardwood forest in mesic soil across the southern half of the Mixedwood Plains Ecozone. It is sensitive to direct physical impact and to microclimatic variation caused by disturbance to the forest canopy. It adapts poorly to younger, second growth forest cover. It is now rare in Canada (Argus and Pryer 1990). Over 60% of Broad Beech-fern stands in the ecozone are known from records 50 years old or older (Vincent 1981). Occurring within the most populated area of the ecozone, many Broad Beech-fern stands have been destroyed by urban and agricultural expansion. Others have been severely degraded or eliminated as a result of alterations to the forest ground and canopy vegetation by recreational and other similar human activities.