Yellow montane violet (Viola praemorsa) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Table 2. provides an assessment of the severity of the major types of threat at each extant population.

Table 2. Threat matrix for extant populations of yellow montane violet
Name Invasive Herbaceous Species Invasive Alien Shrubs Altered Fire Regime Trampling Stochastic Events
(imminent)
Herbivory
Victoria1 high medium low medium low low
Victoria2 high high low medium low low
Oak Bay 1 high high medium high high low
Saanich 1 high high medium medium medium low
Saanich 2 high medium low medium low low
Saanich 4 high high low high high low
Saanich 5 high high medium high medium low
Saanich 7 medium high low low high low
Duncan 1 high high medium medium medium low
Duncan 2 high high medium low medium low
Duncan 3 high high high low medium low
Duncan 4 medium low low medium high low
Saltspring 1 high medium low low high low
Saltspring 2 high medium low medium low high

Threats associated with invasive plants

Roemer (pers. comm. 2006) reported that in one site, 42% of the vascular flora was introduced and that non-native species accounted for 88% of the cover of the herb layer. While the identity of the introduced species varies among sites, this degree of invasion is typical of many areas with yellow montane violet. Major invaders at one or more sites include grasses (Agropyron repens, Anthoxanthum odoratum, Bromus hordeaceus, B. sterilis, Cynosurus echinatus, Dactylis glomerata, Lolium perenne, Poa pratensis andVulpia bromoides) and, to a lesser extent, forbs (Erodium cicutarium, Geranium molle, Lychnis coronaria, Trifolium repens and Vicia hirsuta). Livestock grazing likely played a major role in the dispersal of the most serious invasive grasses although the impacts were not fully realized until grazing pressure was removed and the invasive species were able to form a taller and denser sward. In formerly grazed areas, yellow montane violet is scarce or absent from microsites with a high cover of invasive shrubs and coarse grasses but this likely represents a decrease in niche breadth due to competition and it may once have been common on such sites.

Scotch broom (Cytisus scoparius), a highly invasive shrub, poses one of the greatest threats to yellow montane violet and its habitat. The invasive shrub Daphne laureola has not yet been observed in large numbers at any of the sites but it is expanding its range and abundance rapidly. It favours the Garry Oak woodland habitat types suited to yellow montane violet, where it may achieve cover values in excess of 75%.

Threat associated with altered fire regimes

Pre-European fire regimes in the dry coastal belt of southeast Vancouver Island are probably more complex than is generally reported. There is no doubt that First Nations in the area used fire extensively to stimulate the growth of food species – particularly camas bulbs that provided a storable form of starch. Fire may also have been used to improve forage for game species (elk and deer) (Turner and Bell 1971).

Frequent low-intensity burns killed young red alder and Douglas-fir and checked the growth of trembling aspen and most shrub species – notably Symphoricarpos albus and Rosa nutkana. The resulting increase in light levels and decrease in competition favoured the growth of herbaceous plants such as yellow montane violet. Even the composition of the herb layer altered, since many highly competitive plants decrease under a regime of frequent burning.

First Nations fire management practices also played a significant role in the development (and therefore fertility) of soils. The organic component of the upper mineral horizon was not greatly reduced by low-intensity fires because it accumulated through the in situ decomposition of root material. In contrast, the surface organic materials did burn rather than accumulate, releasing nutrients. Since the main inputs of organic matter came from herbs rather than coniferous trees, the upper mineral horizon also had a relatively neutral reaction in sharp contrast to the acidic nature of soils under Douglas-fir forests (Broersma 1973). As well, the frequent fires provided a continuous supply of ‘safe sites’ where the small seeds of yellow montane violet may have been able to germinate and grow without the stifling influences of surface organic horizons.

At some sites, including Duncan 3 which has 30-40% of the Canadian population, the native shrub Symphoricarpos albus appears to have expanded into much of the habitat formerly available to yellow montane violet. Symphoricarpos albus is often abundant on sites where fire and grazing have been removed.

Trampling

Populations of yellow montane violet are often concentrated along paths, where the expansion of woody plant populations is held in check by frequent human foot traffic. In such circumstances, and on sites where woody plants are not a threat but footpaths still pass through populations of yellow montane violet, trampling damage is usually evident and often damages a significant proportion of the population.

Stochastic events

Some populations of yellow montane violet are threatened simply by their small size and area of occupancy, which predisposes them to stochastic events. Six populations are in very poor condition, with 50 or fewer mature plants, in most cases occupying less than 50 . Saanich 4, Saanich 7 and Duncan 4 have fewer than 10 mature plants and appear to be especially vulnerable.

Herbivory

Herbivory poses a threat of uncertain impact on extant British Columbia populations of yellow montane violet.    Two large populations (Duncan 2 and Duncan 3), which account for about 40% of the British Columbia population, occur in areas that were grazed by livestock until recently.  The single largest population in British Columbia (Saltspring 2) received such heavy grazing pressure from native ungulates that the plants tend to be quite small and their flowers are often removed.  The fact that this population remains high despite intense grazing pressure suggests that grazing by large mammals may not pose a serious threat to large populations. In some cases, yellow montane violet appears to benefit from the removal of taller competing vegetation, which likely compensates for direct tissue damage.

In or near gardens, yellow montane violet may be heavily damaged by non-native slugs (pers. obs.). Gardens provide high concentrations of seedlings and vegetables, which provide a high-quality food source. As well, gardens tend to have ample harbour sites such as boards and grass clippings. As a result, populations of yellow montane violet in the vicinity of gardens may suffer significant damage. This threat should not be overstated, however, since signs of significant slug damage were rarely encountered even in surveys of populations in urban environments (pers. obs.).

MacDougall (pers. comm. 2006) found that a significant number of seeds may be damaged by small, boring insects. It is not clear whether this is a common phenomenon or if it extends beyond Duncan 2.

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