Foothill sedge (Carex tumulicola) COSEWIC assessment and status report: chapter 6
Biology
There is little published information available on the biology of Carex tumulicola. The sections that follow present summary information gleaned from primary sources (floras), published literature on other Carex species, personal communications with Carex specialists in British Columbia and Oregon, and the authors’ own (unpublished) field observations.
Life cycle and reproduction
Carex tumulicola is a tufted perennial herb in the sedge family (Cyperaceae) that flowers in May or June. As with all sedges, the flowers are wind-pollinated. Fruits (achenes) mature during the summer. Seeds are released in the late summer or fall and are primarily gravity-dispersed. It is not known how long the seeds of C. tumulicola remain viable in the soil, although some Carex species have been shown to form persistent soil seed banks (Nariyasu et al. 2001). The timing of germination is also unknown, but presumably occurs in the spring, following the onset of winter rains.
The specific germination requirements of this species are unknown, but recruitment from seed may be infrequent under certain conditions. An attempt to sow native Carex tumulicolaseeds at Eugene, Oregon as part of a larger wetland mitigation project failed to yield any new recruits by the following year (City of Eugene 2002).
In addition to reproducing sexually via seed, Carex tumulicola spreads vegetatively from short rhizomes, and establishes readily from rhizome fragments. Consequently, it is difficult to estimate a generation time (defined by COSEWIC as the average age of parents in the population) for this species.
Herbivory
Many species of Carex are ranked equal to the best grasses for forage, as well as in the amount taken by grazing cattle and wild animals; other members of the genus are coarse and of little value as foraging plants (Booth 1950).
The palatability of Carex tumulicola to herbivores is not known. The species can presumably withstand occasional grazing, due to its deeply-set rhizomes and growing points and its ability to replenish roots through the growing season. Nevertheless, given its extreme rarity at the local scale, it is unlikely that herbivores are currently exploiting C. tumulicola in British Columbia to any great extent. Historically, livestock grazing during the early 20th century may have offset the impact of altered fire regimes at some sites (e.g. Uplands Park) by preventing the intrusion of woody species into open habitats (Fairbarns et al. 2003). It is possible that the continued persistence of C. tumulicola at these sites is partly related to the lingering effects of this earlier grazing activity. Although the site supporting Population #10 is heavily grazed by introduced fallow deer and a small population of native black-tailed deer, Carex tumulicola did not appear to be grazed in 2006 (M. Fairbarns, pers. comm. 2008).
The seeds of Carex species are, in general, rich in stored food and are occasionally eaten by wildfowl and other vertebrates (Booth 1950, Holt and van der Valk 2002). It is not known what impact seed predation may be having currently on the local population dynamics of C. tumulicola.
Physiology
There is no information on physiology relevant to the assigning of at-risk status in Canada.
Dispersal
There is no specific information available on dispersal patterns in Carex tumulicola. Carex seeds do not possess any innate dispersal mechanisms, although birds appear to be effective dispersal agents for Carex species in general. For example, waterfowl are estimated to be able to transport viable seeds of some wetland species up to 1,400 km following ingestion (Holt and van der Valk 2002). Seed dispersal by adhesion to feathers may be an important method of seed transport, and ants are thought to be key agents in the short-distance dispersal of someCarex species (Vellend et al. 2000). BecauseC. tumulicola is rhizomatous, dispersal may also depend to some extent on the passive transport of rhizome fragments (e.g. in soil, on vehicles, etc.), but this has not been established.
Interspecific interactions
There is no information on the interactions of Carex tumulicola with other species that is relevant to assigning at-risk status in Canada.
Adaptability
Carex tumulicola probably requires moist conditions for germination but, once established, appears tolerant of seasonal drought. The species’ rhizomatous habit presumably helps to protect it to some extent from (and perhaps allows it to exploit) surface disturbances such as ground fires, flooding, and trampling.
In the southern part of its range, Carex tumulicola has the reputation of being a hardy species that is resilient to disturbance and is able to withstand a range of growing conditions. For example, The Jepson Horticultural Database (Jepson Herbarium 1993) makes note of its “untested” potential in “stabilizing or restoring disturbed or degraded areas.” Likewise various California garden catalogues list it as a versatile plant that does well in sun or shade, in dry to boggy or regularly watered areas. These catalogues also state that it is stimulated by periodic cutting/mowing and is capable of reseeding once established. It is generally purchased as rootstocks or cuttings for transplanting. Until recently, some of these garden suppliers appear to have confused Carex tumulicola with the European sedge Carex divulsa. The statements made in the garden cataloques, above, likely refer to the European species.
The environmental conditions encountered by a species at the edge of its geographic range are often sub-optimal relative to other areas. In cases where ecological marginality coincides with range marginality, peripheral populations may possess reduced demographic latitude for responding to alterations in local conditions or to stochastic disturbances, compared with populations at the core of the range (Lesica and Allendorf 1995). In the case of Carex tumulicola, therefore, it would be imprudent to assume that because the species appears characteristically hardy and adaptable in some parts of its range, the same automatically holds true for those populations found at its northern range limit on Vancouver Island. Indeed, the extreme rarity of the species in British Columbia, both in terms of the number of populations and total area of occupancy, would seem to suggest otherwise.
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