Seaside bone (Hypogymnia heterophylla) COSEWIC assessment and status report: chapter 5

Habitat requirements

Hypogymnia heterophylla is restricted to the branches and terminal twigs of conifers, especially shore pine (Pinus contorta var. contorta) in exposed seaside habitats along the northwest Pacific coast. H. heterophylla occupies the driest subzones of the Coastal Western Hemlock Zone of the British Columbia Biogeoclimatic Ecosystem Classification System (Meidinger & Pojar 1991), in a region of rainshadow-induced Mediterranean climate. In Canada, this species appears to be restricted to younger forest stands and may therefore be indirectly dependent on early seral forest attributes. Its distribution, for the most part, may further be controlled by a requirement for salts associated with sea spray (Goward 1996). Other lichens are similar, for instance, Glavich (2003) speculates that the epiphytic species Bryoria pseudocapillaris and B. spiralifera may be dependent on oceanic salts because they are only found in locations in close proximity to the coastline in the Pacific Northwest.

The climate in the region of Vancouver Island in which the four locations containing Hypogymnia heterophylla were found can be characterized as oceanic. The Sheringham Point weather station, located approximately 8 km west of the Sooke area, reported a mean annual temperature of 10.4°C, mean December minimum temperature of 3.7ºC, extreme minimum temperature of –3.5ºC, mean maximum August temperature is 17.9ºC and extreme maximum temperature is 29.8 º C (1996–2004) (Environment Canada 2006).

Though actual data are lacking, the microsites colonized by Hypogymnia heterophylla are expected to have a distinct thermal profile due to their locations along the outer coast and are subject to a strong moderating influence from the adjacent ocean. In addition, Coxson et al. (1984) demonstrated that thallus temperatures in Hypogymnia (specifically H. physodes (L.) Nyl.) exposed to full sunlight are much higher than adjacent air temperatures, notwithstanding strong convective wind cooling. Because H. heterophylla occurs primarily in rather exposed, well-illuminated sites, elevated temperatures must constitute an important part of its operating environment (Goward 1996).

Precipitation occurs predominantly in the winter months between October and March with the mean annual precipitation of 96 cm at the Sheringham Point weather station (Environment Canada 2006). Drought conditions often occur in the summer months with the extreme monthly minimum rainfall of 1.6 mm in August 2002. In nearby Washington state, Hypogymnia heterophylla is limited to the Puget Sound area in the rain shadow of the Olympic Mountains; here, as in British Columbia, humidity and summer fog likely help to offset low rainfall in the summer to allow limited growth in morning and evening.Under such conditions, H. heterophylla grows mainly in the winter months when temperatures and solar radiation are lower and precipitation occurs. Also, early spring is conducive to spore germination on the growing twigs of shore pine where there is no competition from other lichens for space. Shore pine can tolerate the summer drought and extreme conditions of the coastal habitat where other coastal tree species require year round precipitation. Fire does not appear to be a critical factor in the coastal shore pine communities in Canada where H. heterophylla is found.

Wind data are not available from Sheringham Point weather station; at Gonzales Heights in Victoria, prevailing winds are from the southwest and west-southwest throughout the year and secondarily from the north and northeast in winter (Environment Canada 1975). Conditions are calm only 3% of the time, so the desiccating effects of wind on Hypogymnia heterophylla may be a physiological advantage in maintaining the wetting and drying cycle required by this and other lichens (Goward 1996; Kershaw 1985; Nash 1996).

Habitat trends

Hypogymnia heterophylla occurs on primarily rocky, windswept ledges, predominantly southwest to west facing in early to intermediate seral shore pine (Pinus contorta) forests. Shore pine trees in these locations are somewhat stunted and prone to branch destruction and damage from offshore winds and winter storms. The severe winter storms of 2006/2007 blew down several individual trees in East Sooke Regional Park (J. Miskelly, pers. comm. 2007). As part of storm clean-up, all downed vegetation will be left on site, so any Hypogymnia heterophylla on trees damaged in the storms, could be viable for a limited time.

Hypogymnia heterophylla occurs in Washington, Oregon and California as far south as the Los Angeles/Santa Barbara/Channel Islands area at favourable windswept sites in close proximity to the coast (Figure 2). This is one of few epiphytic macrolichens specifically tied to localities in close proximity to the outer coast. Its colonization of southern Vancouver Island from the islands of Puget Sound was probably effected via spores being transported northward either by wind or by migratory birds (Bailey & James 1979; M. Raymond, pers comm. 2006). Potential habitat for H. heterophylla would be restricted to the Gulf Islands in the Strait of Georgia and southern, coastal Vancouver Island.

Habitat protection/ownership

East Sooke Regional Park is part of the Capital Region District’s parks system. The ‘Park’ designation ensures the habitat will not be intentionally violated but inadvertent damage could occur due to the passage of hikers, cyclists and dogs along trails through the shore pine stands. Hypogymnia heterophylla on Bentinck Island is potentially in danger of being damaged or eradicated due to Department of National Defence (DND) use of the island for demolitions; the DND liason person with Environment Canada is aware of the locations of H. heterophylla (A. Robinson, pers comm. 2006).

At Sheringham Point the habitat is on a steep cliff and thus well protected from hikers, cyclists and dogs. The area is currently owned by Fisheries and Oceans Canada, which operates a navigation light at the point of land. There are no plans to alter the Hypogymnia heterophylla habitat on trees along the coast (N. Taylor, pers. comm. 2007).

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