Foothill sedge (Carex tumulicola) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Given our current limited understanding of Carex tumulicola habitat requirements and population dynamics, it is unclear what factors have contributed to its current restricted abundance and distribution in Canada. Its rarity may be the result of intrinsic factors such as low competitive ability, limited dispersal ability, high habitat specificity, or inherently low recruitment and survival; extrinsic factors such as geographic barriers or past climatic changes; recent land-use history (e.g., habitat conversion, grazing, fire suppression); or a combination of any or all of these. None of the seven threat factors identified below has been shown empirically to be responsible for declines (either current or historical) in the abundance or range of C. tumulicola within Canada. This may simply be because the species has not been monitored long enough in its native Canadian habitat for deleterious impacts stemming from such threats to be documented. In the absence of any direct evidence to the contrary, all threats are considered here to be clearly imminent but their impacts uncertain.

1. Invasive species encroachment

The impact of invasive species is second only to that of habitat loss as a cause of species declines throughout the world (D’Antonio and Vitousek 1992, Myers and Bazely 2003).On eastern Vancouver Island, Garry oak and associated ecosystems have been invaded by non-native plants to such a degree that exotic species now comprise 59-82 % of the total herbaceous cover (Roemer 1995 in Fuchs 2001, Erickson 1996). Furthermore, the proportion of introduced species in Garry oak meadows increased from an estimated 25 % of the total in 1972 to40-76 % of the total in 1995 (Roemer 1995 in Fuchs 2001), suggesting that the rate of invasion is accelerating rather than slowing. Some of these species represent accidental introductions but a large number have been intentionally introduced into the area for livestock forage, erosion control, or ornamental purposes.

Several authors have identified the types of processes that may be modified by non-indigenous plant species. In general, it is thought that invasive plants have the ability to pre-empt safe sites (places where seeds can germinate) and otherwise suppress recruitment of native plants; alter vegetation stand structure; increase soil moisture deficits; alter soils and micro-climates through litter deposition; increase the probability, extent and severity of fires through increased fuel loading; alter soil nutrient levels and distribution; and compete directly with native species for space, water, light, and nutrients (Bergelson 1990, Facelli and Pickett 1991, D’Antonio and Vitousek 1992, Smith 1994, Gordon 1998, Brown and Rice 2000, MacDougall 2002, Myers and Bazely 2003). In cases where introduced species have higher evapotranspiration rates than those of the native flora, hydrologic regimes may be permanently altered and water tables lowered, thereby altering the distribution of native species (Gordon 1998).

All ten extant populations of Carex tumulicola occur in habitats that have been severely degraded by the presence of invasive exotic plants. Exotic grasses (e.g., orchard grass (Dactylis glomerata),common velvet-grass (Holcus lanatus), barren brome (Bromus sterilis),sweet vernalgrass (Anthoxanthum odoratum), hedgehog dogtail grass (Cynosurus echinatus), and English ryegrass (Lolium perenne) dominate the herbaceous flora at most sites, with introduced shrubs comprising the main component of the understory at several locations.Although most intrusive where the ground has been disturbed, non-native species are also invading even relatively undisturbed sites. Scotch broom is the most widespread of the non-native shrubs, and alsorepresents one of the most pernicious threats to C. tumulicola and its habitat. Introduced to the region as a garden ornamental in 1850, it has since become a dominant component of the plant community on eastern Vancouver Island. In many areas this leguminous shrub forms monospecific stands that have completely overtaken the native vegetation (Roemer 1972, Fuchs 2001). A nitrogen-fixer, Scotch broom has the potential to increase soil nitrogen levels, thereby changing the supply of this resource to the ecosystem (Parker and Haubensak 2004). It also generates large amounts of woody fuel that can support high intensity fires and in this way alter the natural disturbance regime. Gorse (Ulex europaeus) is another frequently associated shrub with similar life history characteristics and apparently similar destructive potential. Other introduced shrubs threatening to overgrow C. tumulicolaat several sites include English hawthorn (Crataegus monogyna),Himalayan blackberry (Rubus discolor), English ivy (Hedera helix), and leather-leaved daphne (Daphne laureola).

As Carex tumulicola has been tracked for only a few years on Vancouver Island, it is difficult to assess quantitatively the impact of these invasions on either its distribution or population dynamics. The fact that C. tumulicola has managed to persist in highly invaded habitats until now, in some instances directly beneath the canopy of invasive shrubs, suggests that, once established, plants of this species can withstand a certain level of competition and overtopping. However, the very low ramet abundances observed at most sites suggest that recruitment is occurring rarely, if at all. It is likely that many of these small patches are relictual and will disappear once the current established individuals die.

Population #1

The municipal park that supports this population is a remnant Garry oak woodland-meadow-vernal pool complex located in a residential area within a few kms of downtown Victoria. Here, Carex tumulicola is restricted primarily to mesic microsites within the shrubby, snowberry (Symphoricarpos albus)dominated ecotone that separates the low-lying wet meadows in the middle of the park from the surrounding upland forest. English hawthorn (which in many places forms dense, monospecific stands), Scotch broom, gorse, leather-leaved daphne, and Himalayan blackberry are all prominent components of this zone (Collier et al. 2004). European ash (Fraxinus excelsior) is an introduced, exotic tree dominant in a 0.5 hectare section of the park close to the central meadow where the majority of C. tumulicola plants occur. In addition to several seed-producing trees approximately 70 years old, there are hundreds of saplings established in the surrounding meadows and extending up to 150 metres to the east and west of the mature trees. These saplings are expected to begin producing seed within fifteen years (Collier et al. 2004). Ash seedlings, which are usually found in areas of at least 80 percent herb/grass cover with less than 10 percent shrub cover, can reach densities of between 5 and 25 per . If not controlled, this species will likely continue to spread into adjacent areas, competing with native vegetation for moisture, soil nutrients, and light (Collieret al. 2004).

Along with these aggressive woody species, introduced forbs such as field garlic (Allium vineale) and introduced perennial grasses such asorchard grass, sweet vernalgrass, Kentucky bluegrass (Poa pratensis), common velvet-grass, English ryegrass, creeping bentgrass (Agrostis stolonifera), and hedgehog dogtail grass have established in the same sites occupied by Carex tumulicola. Each of these grasses competes aggressively for water and nutrients and can form dense litter layers that block light and suppress the regeneration of native plants. Litter accumulation from these species also creates conditions for high-intensity fires (Garry Oak Ecosystems Recovery Team 2003). Orchard grass, which forms particularly extensive root systems and requires high nitrogen inputs, may pose the greatest imminent threat to C. tumulicola persistence. Originally introduced to coastal B.C. as a meadow forage crop, this species is still grown for hay and used in grass-seed mix to stabilize clearings and road cuts.

Scotch broom, gorse, leather-leaved daphne, English hawthorn, Himalayan blackberry, orchard grass, velvet-grass, sweet vernalgrass, andhedgehog dogtail grass have all been ranked amongthe top ten invasive plants on Vancouver Island in terms of the significance of their impact on Garry oak and associated ecosystems, their resistance to control or management, and the urgency associated with their control or management (Murray 2004).

Population #2

This population is scattered across several different habitats including a remnant Garry oak meadow, a disturbed road verge, and a second growth grand fir (Abies grandis) forest. The meadow microsite is dominated by the invasive grass barren brome (Bromus sterilis), while the surrounding habitat is heavily invaded by Scotch broom. At the road verge site, Carex tumulicola competes on the margins of a Himalayan blackberry-snowberry thicket with Kentucky bluegrass, quackgrass (Elymus repens), redtop (Agrostis gigantea), orchard grass, and English ryegrass. However, the most tenuous localities appear to be those associated with the third habitat, where the four extant C. tumulicola tussocks are found along a metre-wide strip between a walking trail and a forest understory dominated by English ivy, an evergreen climbing vine that has infested most of this site. Two of these C. tumulicola tussocks, which were only first discovered in 2004 (Table 1), have already been largely overgrown by English ivy. Introduced to North America as an ornamental, English ivy is a significant invader on Vancouver Island throughout the Garry oak range and beyond. Ivy’s dense growth and abundant leaves form a thick canopy just above the ground and prevent light from reaching other plants, crowding them out and preventing germination of their seeds. If left to spread, ivy can eventually exclude most plants on the forest floor (Garry Oak Ecosystems Recovery Team 2003). At the site in question, periodic mowing of the trailside appears to be the only factor preventing C. tumulicola from being completely overgrown by this species.

Population #3

This single patch also grows on the side of a footpath and is subject to mowing. It is partly overgrown by a stand of English hawthorn and native trembling aspen (Populus tremuloides). English hawthorn is an introduced ornamental plant, now widely naturalized in Garry oak and associated ecosystems. It replaces open grassland habitat with a dense shrub or small tree layer, dramatically altering the vertical structure and composition of the plant community. As noted above, this species has been ranked among the top ten invasive plants on Vancouver Island in terms of the significance of its impact on Garry oak and associated ecosystems (Murray 2004).

Populations #4-6

These populations all occur in disturbed meadows dominated by a suite of invasive grasses and forbs including Kentucky bluegrass, English ryegrass, redtop, and vetch. Much of the available Carex tumulicola habitat has become overgrown with either Scotch broom or gorse.

Population #7

A large English hawthorn bush grows next to the lone Carex tumulicola tussock at this site (A. Ceska, pers. comm. 2004).

Population #8

The shrub leather-leaved daphne now dominates the adjacent understory at this site and appears poised to overgrow the small opening where Carex tumulicola occurs. A relatively recent introduction to Vancouver Island, daphne has already become a major pest within Garry oak and associated ecosystems. It has the ability to replace native vegetation by producing a shrub layer where none existed before. Its dense canopy blocks light to the ground, eliminating herbaceous species and leaving the ground bare. Daphne is also thought to alter soil chemistry and acidity, thereby inhibiting the re-establishment of native plant species (Garry Oak Ecosystems Recovery Team 2003). As noted above, this species has been ranked among the top ten invasive plants on Vancouver Island in terms of the significance of its impact on Garry oak and associated ecosystems (Murray 2004).

Population #9

This site has Scotch broom that has been recently removed, but it is likely to regenerate. Other invasive species include common velvet grass, barren brome, hedgehog dogtail grass, smooth brome (Bromus hordeaceus) and orchard grass. All these species can impact Carex tumulicola by shading out or competing for spring moisture and thereby reducing the potential seed bed. Other subpopulations are threatened by cutleaf evergreen blackberry (Rubus laciniatus) or bull thistle (Cirsium vulgare).

Population #10

This population occurs in an abandoned pasture with a large component of invasive grasses and forbs.

2. Secondary succession due to altered fire regimes

Prior to European settlement of Vancouver Island, natural and human-initiated fires played an important role in the maintenance of the region’s dry Douglas-fir forests and Garry oak savannahs (Turner and Bell 1971, Roemer 1972, MacDougall et al. 2004). While the average fire return interval in the Coastal Douglas-fir zone is estimated to be between 100 and 300 years (Agee 1993), First Nations tribes used frequent, low-intensity fires to maintain good hunting conditions and an open stand structure favourable to important staple foods such as camas (Camassia spp.) and other wild root crops (Turner 1999, Fuchs 2001). Regular burning slowed the succession of native shrubs (e.g., snowberry, Saskatoon (Amelanchier alnifolia), Nootka rose) and conifers such as Douglas-fir (Pseudotsuga menziesii), while ensuring a continuous supply of safe sites for the germination and establishment of herbaceous meadow plants. First Nations fire management practices may have also played an important role in the development and fertility of soils, by ensuring the steady release of organic nutrients into the upper soil horizon. Over the last 150 years, fire suppression has led to encroachment of woody shrubs and Douglas-fir into many formerly open areas, dramatically altering community composition and structure (Fuchs 2001, Lea 2002, MacDougall et al. 2004).

Population #1

Aggressive fire suppression management in this residential park has led to a substantial reduction in the amount of Carex tumulicola habitat available locally, due to widespread encroachment of Douglas-fir and a dramatic increase over historical times in the cover of invasive native shrubs such as snowberry, Nootka rose, and Indian plum (Oemleriacerasiformis) (Collier et al. 2004). Even in wooded areas whereC. tumulicola may have once survived, snowberry and various species of exotic shrubs noted above now dominate the understory. The density of these species severely limits light penetration and thus growth of the herbaceous layer. Furthermore, the incursion of shrubs and trees into adjacent open habitats may have begun to affect local hydrologic and light regimes through alteration of drainage patterns, increased competition for water, increased shading, and thatch buildup from non-native grasses. If unchecked, this process could result in feedback loops that accelerate the overall rate of secondary succession.

Population #2

The municipal park that supports locality #2f, like the one supporting Population #1, has had a long history of fire suppression. The single tussock at this site occurs in an opening surrounded by thick stands of snowberry and Nootka rose. Locality #2a occurs at the edge of a remnant Garry oak meadow in a microsite largely overgrown by various shrubs including snowberry. The other localities occur in a heavily shaded second growth Douglas-fir-grand fir (Abies grandis) forest that, before European settlement, likely also had the character of a more open woodland (Lea 2002). Snowberry, and introduced English ivy are now the major understory components at this site.

Populations #4-6 and #9

The coastal headland that supports these populations has undergone extensive forest and shrub encroachment since European settlement. Shrub thickets occur adjacent to several of the Carex tumulicola patches, and thatch buildup is heavy at most localities, with unknown consequences forC. tumulicola germination as well as for soil structure, microclimate, and nutrient cycles.

Population #8

The vegetation history of this site is unclear. Currently a mixed conifer-arbutus-Garry oak forest, in pre-settlement times it presumably was also maintained in a more open state by periodic fires. In the winter of 2004, an ice storm knocked down several Douglas-fir saplings that had established in the small opening containing Carex tumulicola, causing them to land directly on top of the (then dormant) C. tumulicola patch. Only by sheer happenstance was this fact discovered, allowing the trees to be removed before any lasting damage to the site or plants could occur. Such events demonstrate the need for diligence in monitoring rare plant populations, even in instances where the habitat has not undergone any obvious recent human modification.

3. Habitat conversion (urbanization)

Currently, only one of the extant Carex tumulicolasites, Population # 7 near Nanaimo, appears to be directly threatened by habitat conversion. However, extirpation of this population would be a significant loss, as it is the most northerly occurrence (by nearly 100 km) in the species’ global range. Loss of this one population would also result in a 10-fold reduction in the species’ Canadian Extent of Occurrence, from ~1,700 km² to ~ 90 km². The site in question is a rocky upland meadow and seepage area on the outskirts of Nanaimo, and one of the last remaining tracks of undeveloped land within the Regional District of Nanaimo. In 2003, a Preliminary Layout Application (PLA) was filed for a proposed trailer park development on a section of this property. The application has since expired, although a new PLA could be filed by the landholder at any time (R. Lawrance, pers. comm. 2004). It is not known at present whether the proposed development would directly impact the only known extant C. tumulicola colony (A. Ceska, pers. comm. 2004). Regardless, residential development of this area would inevitably result in the elimination of potential survival and recovery habitat for the species.

4. All-terrain vehicle (ATV) traffic

At both sites #1 and #7, recreational all-terrain vehicle traffic has created deep and lasting ruts through vernal pools, swales and meadows. At the latter site, this activity has also resulted in extensive erosion of the thin topsoil in sloping upland areas (Donovan 2004, C. Thirkill, pers. comm. 2004). Despite efforts of the landowner to block trail access with boulder placements and ditches, recreational users of all-terrain vehicles, 4x4s and (to a lesser extent) dirt bikes continue to access the area on a regular basis (C. Thirkill, pers. comm. 2004). In addition to altering the local hydrologic regime, off-roading has disturbed and compacted the soil, facilitated the spread of invasive species, and directly endangered the survival of at least one nationally endangered plant, bog birds-foot trefoil (Lotus pinnatus), through crushing (Donovan 2004). In the case of Carex tumulicola, even one carelessly laid vehicle track could do irreparable damage to the extant population.

5. Hydrologic alterations

Hydrologic alterations caused by development, deforestation, ditching, draining, paving, off road traffic, and agriculture can affect adjacent ecosystems through changes to the water table, increased annual runoff, frequency and duration of flood events, and disruption of surface and groundwater drainage patterns (Wardet al. 1998).

Population #1

This site has had an extensive history of draining and ditching dating back over a century, with major consequences for the vegetation communities in the park (Collier et al. 2004). Most of the early water diversions were carried out to improve pasture for grazing, but constructed drainage systems for roads and residential properties adjacent to the park have also had permanent impacts on the local hydrologic regime (Collier et al. 2004). A busy suburban thoroughfare bisects the park near its eastern end, effectively splitting the park in two. In past years, the municipality has deposited gravel in low-lying vernal swale areas in an attempt to repair tire ruts left by maintenance vehicles. There have also been calls from some area residents for increased ditching to reduce the amount of standing water on walking trails (R. Collier, pers. comm. 2004).

Population #3

This population occurs on the margins of Rithet’s Bog, a remnant wetland area that in the past 110 years has been grazed, ditched and drained for agricultural use. At one time, the bog’s fate was nearly sealed by the proposed development of a golf course. Wetland draining, combined with fire suppression, has allowed shrub thickets (e.g., willow, red-osier dogwood) to invade to the centre of the 38-hectare wetland, further altering its hydrology. More recently, urban development immediately adjacent to the bog has caused fluctuations in the supply of water flowing through it (Golinski 1996).

The site where Carex tumulicola grows is presently dominated by English hawthorn and trembling aspen (Populus tremuloides), a species not generally associated with C. tumulicola habitat. It is possible thatC. tumulicola was once more widespread at this site but has since been crowded out by the increasing canopy cover around the margins of the bog. As forest succession proceeds, it is expected that most of the remaining C. tumulicola habitat will disappear, placing the current population at immediate risk of extirpation and possibly preventing its future recovery at the site.

Wetland restoration work on Rithet’s Bog was initiated in 2001, via a partnership established between Ducks Unlimited, Rithet’s Bog Conservation Society, Fisheries and Oceans Canada (DFO), and the Municipality of Saanich. To date, however, there have been no initiatives specifically targeting the management ofCarex tumulicola in its native habitat.

Population #7

This population is located at the end of a vernal seep. Any major change in the pattern of surface and subsurface water flow, as might follow from new residential development or from damage to the soil caused by all-terrain vehicle traffic, would presumably put this population at risk.

6. Trampling and mowing

Three Carex tumulicola populations (#1, 2, & 3) occur at the edges of well used walking trails where trampling could pose a threat to plant survival and/or growth. Foot and dog traffic is especially heavy in the park supporting Population #1 (Collier et al. 2004). Here, winter rains result in large pools forming in the middle of most trails, sometimes forcing pedestrians onto the trail margins where C. tumulicola occurs. During the summer, many of these same trail sides are mowed. Portions of populations #2, 3, 4, & 5 are also subject to mowing. The long-term impact of these mechanical disturbances on the population dynamics of C. tumulicola is unknown. On the one hand, mowing likely has the beneficial effect of reducing competition from invasive shrubs and alien invasive grasses. On the other hand, mowing during summer, when C. tumulicola normally flowers and sets seed, must inevitably result in lost reproductive opportunity for the species. Carex tumulicolapopulations may be less resilient to such demographic perturbations on Vancouver Island, where the species reaches its northern range limit, than has been generally noted elsewhere.

7. Bank slumping

The shoreline bank supporting subpopulation #1m is gradually being eroded by wave action, which has resulted in segments of the bank slumping away. If this process continues unabated at the current rate, much of the bank--including the part of it presently supporting Carex tumulicola--could disappear within the next few years, placing this subpopulation at imminent risk of extirpation.

Page details

Date modified: