Multi-species management plan for Athabasca Sand Dunes 2013: chapter 1

3. Species Information

3.1 Species Description

Large-headed Woolly Yarrow (Achillea millefolium L. var. megacephala (Raup) Boivin) is a perennial herb in the Aster or Composite Family (Asteraceae) (Figure 1). Plants are densely woolly and bear large, compact clusters (4-10 cm in diameter) of flowers (6 mm or more in diameter) on erect stems that are 30-50 cm high. The finely divided leaves form a cluster at the base of the plant and also occur alternately along the flowering stems. The flower clusters are composed of numerous flower heads, each of which consists of many tiny flowers surrounded by a circle of white rays (Harms 1999a). Large-headed Woolly Yarrow is part of the Achillea millefolium complex, which is found over much of the northern hemisphere, and which has evolved a variety of ecological races adapted to specific environments (Purdy and Bayer 1996). It is similar in appearance to Common or Woolly Yarrow (Achillea millefolium var.lanulosa), but it is more densely woolly overall, and has larger flower heads with larger and more prominent rays (Harms 1999a).

Figure 1. Large-headed Woolly Yarrow (© Rob Wright).

Figure 1 is a photograph of Large-headed Woolly Yarrow.

Athabasca Thrift(Armeria maritima(P.Mill.) Willd. ssp.interior (Raup) Porsild) is a perennial herb in the Leadwort Family (Plumbaginaceae) (Figure 2). It has a branched woody base and one to several clusters of leaves at ground level (rosettes). Flowering stems may arise from each rosette, and the small pink flowers are found in a dense, almost round head. The plant grows in small clusters or as occasional individuals (Argus 1999a).

Figure 2. Athabasca Thrift (© Rob Wright).

Figure 2 is a photograph of Athabasca Thrift.

Mackenzie Hairgrass (Deschampsia mackenzieana Raup) is a perennial grass (Grass Family, Poaceae) (Figure 3). The plant consists of a dense bunch of stems growing from fibrous roots. The narrow leaves are mostly at the base of the plant. Ligules (leaf-like structure at the junction of the leaf sheath and blade) are large (3-5 mm long). The flower heads are open branching structures (panicles) which are 10-20 cm long and light straw-coloured. The spikelets (the individual parts of the flowering head) are 6-12 mm long, with 2 or 3 flowers (Harms 1998). Mackenzie Hairgrass developed from the widespread species Tufted Hairgrass (Deschampsia caespitosa) by a doubling of the number of chromosomes, leaving it reproductively isolated from its progenitor (Purdy and Bayer 1995b).

Figure 3. Mackenzie Hairgrass (© Rob Wright).

Figure 3 is a photograph of Mackenzie Hairgrass

Sand-dune Short-capsuled Willow(Salix brachycarpaNutt. var. psammophila Raup) is a shrub in the Willow Family (Salicaceae) (Figure 4). Plants are short and stiffly erect, 90 to 120 cm high, with grayish, erect or spreading branches. The roughly oval-shaped, flat leaves are either stalkless or with short stalks less than 2 mm long. The leaves are 1.5 to 3 cm long, have rounded or heart-shaped bases, and do not have serrated edges. Both their upper and lower surfaces are covered with dense, silky, greyish-white hairs. The flowering heads are catkins (i.e. dense cylindrical clusters of tiny flowers arranged along a stem), with male flowers on some plants and female flowers on others (Harms 1999b, Flora of North America Editorial Committee 1993+). Sand-dune Short-capsuled Willow is a variety with a narrow range, within the more widespread species Short-capsuled Willow (Salix brachycarpa) (Harms 1999b).

Figure 4. From left to right, Felt-leaf Willow, Sand-dune Short-capsuled Willow, Tyrrell’s Willow, and Turnor’s Willow (© Rob Wright).

Figure 4 is a photograph of Felt-leaf Willow, Sand-dune Short-capsuled Willow, Tyrrell’s Willow, and Turnor’s Willow.

Felt-leaf Willow (Salix silicicola Raup) is a shrub in the Willow Family (Salicaceae) (Figure 4). Plants are erect, 1 to 2 m (sometimes up to 3 m) high, with spreading branches and twigs that are densely covered with felted white hairs. The roughly oval-shaped, flat leaves are borne on 2 to 10 mm long stalks. Leaves are 3 to 6 cm long and 2 to 3.5 cm wide, have a rounded or broadly wedge-shaped base, prominent veins, and no serration along the margins. Both their upper and lower surfaces are covered with thickly felted white hairs. The flowering heads, which are covered with long white hairs, are catkins, with male flowers on some plants and female flowers on others (Harms 1999c, Flora of North America Editorial Committee 1993+). Felt-leaf Willow is thought to have evolved from the more widespread northern species Alaska Willow (Salix alaxensis) (Purdy and Bayer 1995). Felt-leaf Willow differs from Alaska Willow by having shorter and broader stipules, and broader, densely pubescent leaves (Harms 1999c).

Turnor’s Willow (Salix turnorii Raup) is a shrub in the Willow Family (Salicaceae) (Figure 4). Plants are erect and narrow, 1 to 2 m (sometimes up to 4 m) tall, with pale grayish-green bark that is usually smooth on branches that are at least a year old. Younger twigs have reddish or purplish bark that may be hairy at first but then becomes smooth. The elongated, roughly oval, flat leaves are noticeably longer (2 to 3.5 cm) than they are wide (0.5 to 1 cm), and have prominently serrated (toothed) edges. They have a rounded base but come to a sharp point at the tip. Both surfaces are pale green, but the undersurface is paler. They can be hairy at first but become almost smooth when mature. The leaf stalks are 2 to 5 mmlong. The flowering heads are catkins, with male flowers on some plants and female flowers on others (Harms 1999d, Flora of North America Editorial Committee 1993+).

Turnor’s Willow is thought to have evolved from Yellow Willow (Salix eriocephala var.famelica), a widespread species of the southern prairies (Harms 1999d). Turnor’s Willow differs from Yellow Willow by having redder young twigs, pale gray-green bark on second-year and older stems, shorter and thicker leaves not acuminate-tipped, shorter catkins, more reddish capsules, and longer stipes (Harms 1999d).

Floccose Tansy (Tanacetum huronense Nutt. var. floccosum Raup) is a perennial herb in the Composite Family (Asteraceae) (Figure 5). Plants are stout and grow 20 to 40 cm high from rhizomes (horizontal stems running through the soil). Plants have finely divided leaves at their bases and alternately along their stems. Leaves and stems are covered by densely matted, white, woolly hairs. The typical composite flower heads have yellow rays and are arranged in loose clusters at the end of the upwardly branching stems (Harms 1999e). Floccose Tansy is a variety with a restricted range, within the more widespread species Lake Huron Tansy (Tanacetum huronense) (Harms 1999e). It differs from its nearest relative (Tanacetum huronense var. bifarium) by the hairiness of the leaves and stems (Harms 1999e).

Figure 5.Floccose Tansy (© Rob Wright).

Figure 5 is a photograph of Floccose Tansy.

3.2 Populations and Distribution

All of these species are endemic to (found only in) the Athabasca Sand Dunes in Saskatchewan, with the exception of a specimen of Felt-leaf Willow collected in Nunavut in 1966 (Argus 2010, G. Argus, pers. comm.), a specimen of Mackenzie Hairgrass collected in the Northwest Territories in 1927[1] (B. Bennett, pers. comm.), and a few incidental observations nearby on the shores of Lake Athabasca (see below). The Athabasca Sand Dunes is a complex of active and stabilized dunes extending about 100 kmeast-west along the south shore of Lake Athabasca, a large lake in the northwest corner of Saskatchewan (Figures 6 and 7). The complex is made up of a series of large dune fields, including (from west to east) the William River, Thomson Bay, Cantara Lake, Archibald Lake, and MacFarlane River dune fields, plus other smaller areas (Figure 7). Athabasca Sand Dunes Provincial Park (ASDPP) which encompasses almost all of the ranges of the seven species, has an area of 1925 square kilometres (Figure 6). The park is in a remote area with no road access, and the few visitors arrive by float-plane or by boat from communities such as Uranium City and Fond du Lac, Saskatchewan. It is located in the Boreal Shield Ecozone and has a northern boreal climate. The town of Cree Lake, located about 200 km southeast (Figure 6), has mean January temperature of -22.7°C, mean July temperature of 15.7°C, and annual precipitation of 446 mm (1971-2000 normals; Environment Canada 2011). The vegetation on stabilized soils in the area consists mainly of open jack pine forest.

Figure 6. Map of Saskatchewan showing Athabasca Sand Dunes Provincial Park (red) on the south shore of Lake Athabasca (blue).

Figure 6 is a map of Saskatchewan showing the location of the Athabasca Sand Dunes Provincial Park, which is found along the south shore of Lake Athabasca, which is located in the very northern part of Saskatchewan and which intersects the Alberta border.

Lamb et al. (2011) reported on surveys done in 2009-2010. A total of 224 transects (250 m long) were surveyed in the William River, Thomson Bay, Cantara Lake, Archibald Lake, and MacFarlane River dune fields. Changes in habitat type along the transects were recorded, and numbers of individuals of endemic plants were counted in a belt 10 mwide for willows and 4 m wide for herbaceous species. The relative abundances of the endemic species are shown in Table 2. These relative abundances were calculated from the total number of individual plants observed, with willow numbers adjusted to account for the wider search area. However, Lamb et al. (2011) cautioned that the number of willow individuals may be inflated due to the counting of stems rather than clumps.

Figure 7. Major dune fields referred to in the text: A – William River; B – Thomson Bay; C - Cantara Lake; D – Archibald Lake; E – MacFarlane River. The red line is the boundary of Athabasca Sand Dunes Provincial Park.

Figure 7 is an orthophotograph showing the location of the major dune fields found within the boundary of Athabasca Sand Dunes Provincial Park.
Table 2. Percent relative abundances of the seven species of special concern in the Athabasca Sand Dunes Provincial Park, based on 2009-2010 surveys (after Lamb et al. 2011).
Species Name % Relative Abundance
Mackenzie Hairgrass 30%
Sand-dune Short-capsuled Willow 24%
Felt-leaf Willow 20%
Turnor's Willow 13%
Floccose Tansy 12%
Large-headed Woolly Yarrow 1%
Athabasca Thrift 0.6%

Mackenzie Hairgrass is the most abundant of the herbaceous endemics (Table 2). It is widely distributed across all of the dune fields that were surveyed, and is also found on the beaches of Lake Athabasca (Lamb et al. 2011, Lamb and Guedo 2012, Figure 8). It occurs as scattered large tufts or in small groups of 3-5 or more plants (Harms 1998). There is no population trend information available but no indication that the population is declining (Harms 1982). Mackenzie Hairgrass has also been observed on active dunes just outside of the Park boundary south of the William River dune field (B. Godwin, personal observation), and on beaches at the Gunnar mine site on the north side of Lake Athabasca (Harms 1982).

Figure 8.Distribution of Mackenzie Hairgrass in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 8 is an orthophotograph showing the distribution of Mackenzie Hairgrass in the Athabasca Sand Dunes.

Sand-dune Short-capsuled Willow is the most abundant of the endemic special concern willows (Table 2). It is widely distributed in the Thomson Bay dune field, common around the margins of the William River, MacFarlane River, and Archibald Lake dune fields, and is also found on the beaches of Lake Athabasca (Lamb et al. 2011) (Figure 9). It has also been recorded on the tailings at the Gunnar mine site on the north side of Lake Athabasca (B. Godwin, pers. comm.). No population trend data are available, but the population is believed to be stable (Harms 1999b). It is relatively widespread on active dunes, where it is often associated with Felt-leaf Willow (Hermesh 1972).

Figure 9.Distribution of Sand-dune Short-capsuled Willow in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 9 is an orthophotograph showing the distribution of Sand-dune Short-capsuled Willow in the Athabasca Sand Dunes.

Felt-leaf Willow is relatively common, comparable to Sand-dune Short-capsuled Willow (Table 2). It is widely distributed in the Thomson Bay and MacFarlane River dune fields, common around the margins of the William River and Archibald Lake dune fields, and is also found on the beaches of Lake Athabasca (Lamb et al. 2011) (Figure 10). Although no population trend data are available, the population is believed to be stable (Harms 1999c). Felt-leaf Willow has been observed outside of the Park, westward along the shoreline of Lake Athabasca (B. Godwin, pers. comm.). Little is known about the reported occurrence of this species at Pelly Lake in Nunavut (Harms 1999c, Argus 2010).

Figure 10.Distribution of Felt-leaf Willow in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 10 is an orthophotograph showing the distribution of Felt-leaf Willow in the Athabasca Sand Dunes.

Turnor's Willow is less abundant than the other willows (Table 2). It is widely distributed in the Thomson Bay and MacFarlane River dune fields, common around the margins of the William River dune field, but present only on beaches at the margin of the Archibald Lake dune field (Lamb et al. 2011) (Figure 11). It occurs scattered over active dunes (Harms 1999d). Although no population trend data are available, the population is believed to be stable (Harms 1999d).

Floccose Tansy is relatively common, but less so than Mackenzie Hairgrass or the willows (Table 2). It is widely distributed across all of the dune fields, as well as on the beaches of Lake Athabasca (Lamb et al. 2011) (Figure 12). It occurs as individuals, or more often as small clonal clusters var.ing from several to more than a dozen plants (Harms 1999e). Although no population trend data are available, the population is believed to be stable (Harms 1999e).

Figure 11.Distribution of Turnor’s Willow in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 11 is an orthophotograph showing the distribution of Turnor’s Willow in the Athabasca Sand Dunes.

Figure 12.Distribution of Floccose Tansy in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 12 is an orthophotograph showing the distribution of Floccose Tansy in the Athabasca Sand Dunes.

Large-headed Woolly Yarrow, like Athabasca Thrift, is less common than the other endemic species (Table 2). According to Harms (1999a), Yarrow is more common locally but more restricted in distribution, while Thrift is sparser locally but has a wider distribution. Large-headed Woolly Yarrow is patchily distributed in the Cantara Lake, Thomson Bay, and William River dune fields, but absent from the Archibald Lake and MacFarlane River dune fields (Lamb et al. 2011) (Figure 13). It also occurs on beaches of Lake Athabasca (Rob Wright, personal observation), but was not recorded on beach transects (Lamb et al. 2011). Although no population trend data are available, the population is believed to be stable (Harms 1999a).

Figure 13.Distribution of Large-headed Woolly Yarrow in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 13 is an orthophotograph showing the distribution of Large-headed Woolly Yarrow in the Athabasca Sand Dunes.

Athabasca Thrift is the least common of the Athabasca endemics (Lamb and Guedo 2012, Table 2). It is concentrated in the northeastern sector of the William River dune field, patchily distributed in the Thomson Bay and MacFarlane River dune fields, and absent from the Archibald Lake dune field (Lamb et al. 2011) (Figure 14). While no population trend data are available, there is no evidence to suggest that it has declined in recent years (Argus 1999a). Argus (1999a) considered the rarity of this plant to be related to the rarity of its gravel pavement habitat. Lamb et al. (2011) suggested that it may be particularly vulnerable because of its preferred habitat, growth form, and low population size.

Figure 14.Distribution of Athabasca Thrift in the Athabasca Sand Dunes (after Lamb et al. 2011). Red dots represent transects where present, and yellow dots represent transects where absent.

Figure 14 is an orthophotograph showing the distribution of Athabasca Thrift in the Athabasca Sand Dunes.

3.3. Needs of the Species

3.3.1 Habitat and biological needs

The main habitats used by most of the Athabasca endemics are active dunes (with a variety of aspects and slope positions) and moist inter-dune areas referred to as dune slacks. Another distinctive habitat is gravel pavement. This is a highly localized and fragile habitat created by wind action on a sandy gravel till. As the sand was winnowed out of the material by wind erosion, the surface became covered by a single layer of stones, a lag concentrate, which reduced subsequent erosion (Argus 1999a). Less commonly used habitat types include sandy beach ridges along Lake Athabasca and stabilized dunes covered by forest or lichen-crowberry heath occurring in proximity to the active dunes (Lamb et al. 2011, Lamb and Guedo 2012).

It is important to note the dependence of these species on naturally disturbed habitats. Dune stabilization would in fact eliminate the habitat for these species. This is a major concern in the dunes of the southern Prairie Provinces, where reduced disturbance has led to increased vegetation cover, reducing the habitat for active-sand specialists. However, there is no evidence of a trend towards stabilization in the Athabasca Sand Dunes. The human impact is much lower than in the southern Prairies, so whatever factors produced the active dunes are probably still in play. Disturbance by forest fires is probably one of these factors. Bihun (1998) observed charred wood in some open sand areas, suggesting that fire contributes to expansion of active dunes. The Athabasca region is still subject to a largely natural fire regime, because forest fires are not suppressed in this zone of Saskatchewan unless they threaten settlements (Parisien et al. 2004). Another factor that has been thought to affect dune stabilization is hydrological change (see Section 4.2, Altered Hydrology). More research is needed on the historic stabilization/destabilization trends in the Athabasca Sand Dunes.

The 2009-2010 field survey classified habitat types along transects (Table 3). The densities of endemic plant species by habitat type are shown in Table 3, while the percentages of habitat units (i.e. contiguous stretches of a given habitat type along a transect) with at least one individual of a given species are shown in Table 4.

Table 3. Mean density (individuals ha-1, ± standard deviation) of the species of special concern in each habitat type, based on 2009-2010 surveys (after Lamb et al. 2011). Note that the densities for Saline Inter-dune Slack may be unreliable because they are based on a small sample area.
  High- slope Dune1 Low- slope Dune2 Wet
Inter-dune Slack3
Saline Inter-dune Slack4 Gravel Pavement5 Lichen-Crowberry Heath6 Woodland7
Large-headed Woolly Yarrow 3 28 390     59 26
Large-headed Woolly Yarrow  (±32)  (±294)  (±1350)      (±397) (±101)
Athabasca Thrift 5 11 9 9 50 2 11
Athabasca Thrift  (±58)  (±122)  (±39)  (±27)  (±274)  (±20)  (±80)
Mackenzie Hairgrass 310 1126 1018 741 446 27 3
Mackenzie Hairgrass  (±839) (±4320)  (±4093)  (±1480)  (±2136)  (±182)  (±27)
Sand-dune Short-capsuled Willow 499 978 3083 3843 123 989 514
Sand-dune Short-capsuled Willow (±2067) (±5677)  (±8239)  (±9202)  (±1363)  (±4100)  (±4292)
Felt-leaf Willow 648 820 8189 75 58 177 11
Felt-leaf Willow (±2497) (±5718) (±41193)  (±145)  (±389)  (±906)  (±101)
Turnor's Willow 433 586 2333 2903 51 75 26
Turnor's Willow (±1985) (±6159)  (±7808) (±5718)  (±378)  (±427)  (±154)
Floccose Tansy 1199 360 3082 604 35 217 154
Floccose Tansy (±6938) (±1130)  (±8550) (±1675)  (±181)  (±678)  (±474)

1 Dry high slope gradient dune. The dominant substrate is open sand with slopes generally greater than 15-20°.
2 Dry low slope gradient dune. The dominant substrate is open sand with slopes generally less than 15-20°. Relatively level areas between dunes without evidence of a high water table were included in this category.
3 A level or nearly level habitat with a high groundwater table and moist soils. Open water is occasionally present. It may have sandy substrate or more or less extensive herbaceous or bryophyte ground cover.
4 Same description as for Wet Inter-dune Slack, but with evidence of salt deposits on the soil surface.
5Dominant surface cover is rocks or pebbles lying on a sandy substrate.
6 Dry areas with well developed layers of lichens, bryophytes, and low-growing ericaceous shrubs over the soil surface, but without extensive tall shrub or tree cover.
7Extensive woody vegetation (generally jack pine forest or birch scrub). Substrates between trees generally similar to Lichen-Crowberry Heath.

Table 4. Percentage of habitats units (contiguous stretches of a given habitat type along a transect) with at least one individual of a given species (after Lamb et al. 2011). Note that the percentages for Saline Inter-dune Slack may be unreliable because they are based on only 8 habitat units of this type.
  High- slope Dune Low- slope Dune Wet
Inter-dune Slack
Saline Inter-dune Slack Gravel Pavement Lichen-Crowberry Heath Woodland
Large-headed Woolly Yarrow 1 3 18 0 0 8 10
Athabasca Thrift 1 2 7 13 8 1 2
Mackenzie Hairgrass 28 46 22 38 35 7 1
Sand-dune Short-capsuled Willow 11 15 49 50 6 18 12
Felt-leaf Willow 18 19 33 38 14 18 2
Turnor's Willow 17 13 18 38 10 7 6
Floccose Tansy 43 31 64 38 9 26 20

Athabasca Thrift appears most dependent on gravel pavement (Tables 3 and 4). It appears poorly adapted to moving sand habitats because of its cushion-like growth form (Lamb et al. 2011). According to Argus (1999a), seedlings become established both on gravel pavements and on moist to wet dune slacks. Plants growing in dune slacks are young and vigorous, but are not able to grow up through sand and are eventually buried by moving dunes. Old plants are only found on the relatively stable gravel pavements. Lamb et al. (2011) observed that many of the plants recorded on dune slopes were adjacent to gravel pavements.

The other six species of special concern have generally been described as occurring on active dune slopes and dune slacks, as well as sandy beach ridges along Lake Athabasca (Harms 1998, 1999a, 1999b, 1999c, 1999d, 1999e). The 2009-2010 surveys (Tables 3 and 4) provided a more quantitative basis for describing habitat preferences. Strikingly, all of these species except Mackenzie Hairgrass had higher density and frequency on wet inter-dune slacks than on low-slope or high-slope dune habitats (Tables 3 and 4). Although the wet inter-dune slack habitat comprises a relatively small proportion of the total dunal area, it is thought to be important for seedling establishment for many of these species (Argus 1998, Lamb et al. 2011).

Felt-leaf Willow, Turnor’s Willow, and Sand-dune Short-capsuled Willow showed similar patterns, with highest density on wet inter-dune slack, and lower density on low-slope dune followed by high-slope dune. The pattern for Felt-leaf Willow and Turnor’s Willow is consistent with establishment of young plants on wet inter-dune slacks, with mature individuals persisting on dune slopes as large clumps following burial by moving sand (Lamb et al. 2011). Willows continue to grow vertically as their lower parts are buried (Abouguendia et al. 1981). However, Lamb et al. (2011) observed that mature Sand-dune Short-Capsuled Willows were more concentrated than the other willows in wet inter-dune slacks, and related this to the shorter stature of this species. Sand-dune Short-capsuled Willow was also more abundant in lichen-crowberry heath and woodland habitat types than the other willows (Table 3). All three willows also occur along the beaches of Lake Athabasca (Lamb et al. (2011).

Floccose Tansy is most dependent on wet inter-dune slacks, followed by high-slopes dunes (Tables 4 and 5). Tansy was also found on the upper margins of the beaches of Lake Athabasca (Lamb et al. 2011). It is similar to the willows in that it appears to germinate in wet inter-dune slacks, with mature individuals persisting as large clumps on dune slopes following burial by moving sand (Lamb et al. 2011).

By contrast, Mackenzie Hairgrass occurred more often (Table 5) and at slightly higher density (Tables 4) on low-slope dunes than on wet inter-dune slacks. Lamb et al. (2011) observed that Mackenzie Hairgrass frequently forms large stands of seedlings on areas of open sand, suggesting that it is more successful at establishment on dune slopes than the other species. The active dune habitat of Mackenzie Hairgrass contrasts sharply with the marshy habitat used by the close relative Tufted Hairgrass (D. cespitosa) (Harms 1998). However, Mackenzie Hairgrass was found on the beaches of Lake Athabasca (Lamb et al. 2011).

At the other extreme, Large-headed Woolly Yarrow appeared to be most dependent on wet inter-dune slacks and was almost absent from dune slopes (Tables 3 and 4), suggesting low tolerance for burial by moving sand (Lamb et al. 2011). Wet inter-dune slacks are clearly the most important habitat type for this species (Lamb et al. 2011). There was more Yarrow than expected on the small areas of stabilized habitats (lichen-crowberry heath and woodland) included in the 2009 survey (Tables 3 and 4).

With respect to other biological requirements, Large-headed Woolly Yarrow, Floccose Tansy, Athabasca Thrift and the willows rely on insects for pollination, whereas Mackenzie Hairgrass is wind-pollinated. Yarrow, tansy and the willows also spread vegetatively from rhizomes or from branches buried by moving sand. By contrast, Mackenzie Hairgrass and Athabasca Thrift are bunch-like plants, lacking the ability to spread vegetatively, and therefore depend on seed for seedling establishment to expand and disperse. Mackenzie Hairgrass seeds apparently require a winter cold-treatment prior to germination in spring. A long-day photoperiod seems needed for flowering induction. Numerous seeds and seedlings have been observed in nature, and seed viability appears high (Harms 1998).

3.3.2 Limiting factors

The obvious limiting factor for these species is the small spatial extent of their habitat. This implies that their populations are relatively small, which in turn implies that they are more vulnerable to extinction as a result of habitat change or stochastic events, compared to species with larger or more widespread populations. Moreover, at least two species (Felt-leaf Willow and Mackenzie Hairgrass) have reduced genetic diversity compared with their wider-ranging relatives (Purdy et al. 1994, Purdy and Bayer 1995a, 1995b), which may mean they have reduced capacity to adapt to habitat change (e.g. climate change).

More specific limiting factors are not yet understood. It has been suggested that some species may be limited by the small area and relative proportion of wet inter-dune slack habitat, which is important for seedling establishment (Argus 1998, Lamb et al. 2011).

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