Blunt-lobed woodsia (Woodsia obtusa) COSEWIC assessment and status report: chapter 6
Biology
Most of the general biological knowledge for this species comes from Brown’s “A monographic study of the fern genus Woodsia” (1964). More recently, research on habitat characteristics and available habitat has been undertaken in Canadian populations. Results from these studies have been reported in Wild (2003) and Wild and Gagnon (2005). Current research is underway on several other aspects of the species’ biology such as spore production, spore viability, spore dispersal potential, gametophyte recruitment and sporophyte survival. These studies are part of a Ph.D. thesis in preparation by M. Wild.
Life cycle and reproduction
Blunt-lobed woodsia is a perennial homosporous fern species. Individual lifespan is unknown, but suspected to be in the order of several decades. Spore production occurs annually towards the end of summer and beginning of fall. This fern is known to be capable of producing high quantities of spores; Peck et al. (1990) estimated 60 million spores per individual in Iowa. Studies are currently underway to actually quantify spore production and viability in Canadian populations, but large numbers of plants have already been produced from single fronds, and from spores contained in the soil, thereby indicating at least some level of viability. In culture, spores take from six to 15 days to germinate and the production of sporophytes takes from three to five months (Brown 1964). This species is not known to reproduce asexually, although (as is the case for most homosporous pteridophytes), bisexual gametophytes can be produced and self-fertilize, which results in a sporophyte that is 100% homozygous. Another important factor for blunt-lobed woodsia is its polyploidy. Polyploids are reputed to have increased vigour, increased ability to tolerate cold and an enhanced capacity for colonization relative to diploids (Rabe and Haufler 1992). Soltis and Soltis (2000) present the results from several studies on the potential for intragametophytic selfing of polyploid ferns, concluding that the available evidence for ferns, while limited, suggests reduced inbreeding depression in polyploids. Although it is highly likely that the isolated populations ofblunt-lobed woodsia in Canada are each the result of single spore arrival, implying complete homozygosity, it appears quite possible that the negative components of inbreeding, such as inbreeding depression and the related effects of deleterious recessive genes, are countered by the polyploid nature of blunt-lobed woodsia occurring in these populations, as suggested by Soltis and Soltis (2000).
Herbivory
It has been suggested that significantly fewer phytophagous insects (only 465 documented species) attack ferns compared to flowering plants (over 357000 species). This equates to a ratio of approximately 1 species of insect for every 19 species of fern as opposed to more than 1 species of insect per flowering plant (Hendrix 1980). As for grazing herbivores, ferns appear to be unpalatable to herbivores (Page 1979), and in Canadian populations, no evidence of browsing or grazing has been observed on blunt-lobed woodsia individuals, even in areas of high deer density (M. Wild pers. obs.).
Physiology
Although there is evidence that biologically the species may have weedy characteristics (Brown 1964), it is quite possible that water limitation plays a large role in blunt-lobed woodsia populations in Canada. Bryan and O’Kelley (1967) showed that an absence of calcium in their growing substrate precluded the formation of archegonia, antheridia and sporophytes, thereby demonstrating the necessity for soil with high calcium content.
Dispersal/migration
For ferns in general, the small size and large numbers of spores produced suggests that dispersal is rarely a limiting factor. Recent studies (Wild, unpubl. data) have shown that even at relatively large distances (50 m) from source plants, blunt-lobed woodsia spores can be found in large quantities in the soil (>100 spores/10 ml soil). This capacity to disperse added to the fact that spores from many fern species are known to remain viable for years in the soil suggests that, theoretically, dispersal is not a limiting factor in the specific case of W. obtusa. However, even if suitable habitat is located in proximity to extant populations, the xeric nature of this habitat possibly limits the establishment of young plants, thereby reducing functional dispersal.
Interspecific interactions
Interspecific competition can affect both fern sporophyte and gametophyte generations (Page 1979), although in a species-poor habitat, such as the calcareous rock habitat studied here, interspecific competition is expected to have little effect. Indeed, data collected in Canadianblunt-lobed woodsia populations show a low richness and cover of both vascular and non-vascular plant species in the immediate vicinity of W. obtusa individuals (Wild 2003, Wild and Gagnon 2005).
Adaptability
An ongoing experiment (Wild, unpubl. data) has shown that individuals can be grown from spores in a greenhouse for six months and then be subsequently transplanted in situ and have relatively high survival rates (approx. 30% after 2 years).
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