Seaside centipede lichen (Heterodermia sitchensis) COSEWIC assessment and status report: chapter 5

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Habitat

Habitat requirements

Compared with most other tree-dwelling lichens, H. sitchensis has remarkably stringent habitat requirements. Its occurrence appears to be determined by the superposition of three essential ecological factors: 1) proximity to the ocean and seaward orientation of the site; 2) old Sitkaspruce trees with defoliated twigs in the lower canopy; 3) slow growing branches; and 4) nitrogen and calcium enrichment. These and other field characteristics are briefly discussed below.

1) Seaside habitats

All finds of H. sitchensis to date have been restricted to the outer coast, within the spray zone of the Pacific Ocean. Interestingly, the same is true of virtually all western Heterodermia species, i.e., including H. japonica, H. leucomela, H. namaquana, and H. obscurata. Given the intense level of sampling for epiphytic macrolichens in coastal British Columbia, the strictly seaside occurrence of these lichens is certainly real, not a mere artifact of undercollecting; see Figure 3.

2) Sitka spruce

Heterodermia sitchensisis an epiphytic (tree-dwelling) lichen known only from Sitka spruce (Picea sitchensis). Because epiphytic macrolichens are seldom confined to a single tree species, a careful search of other potential host trees was made, including red alder (Alnus rubra), amabilis fir (Abies amabilis), western red-cedar (Thuja plicata), western hemlock (Tsuga heterophylla), and western yew (Taxus brevifolia). These efforts yielded no additional host trees for this lichen.

The occurrence of Sitkaspruce is controlled in part by a requirement for periodic disturbance. In the Broken Group Islands of Pacific Rim National Park, for instance, it occurs abundantly on exposed, outer islands. In more sheltered, inner islands, by contrast, it is virtually absent. This observation doubtless has profound implications for the occurrence of H. sitchensis.

3) Twigs

Heterodermia sitchensisis mostly restricted to twigs less than 8 mm in diameter, though a few twigs 12 mm in diameter were found to support this species. As a rule, only the defoliated portions of twigs are colonized, the adjacent foliated portions being perhaps too young or too ecologically unstable to support it. Twigs larger than about 10 to 12 mm in diameter characteristically bear mosses and hepatics, these presumably tending to outcompete H. sitchensis.

In common with other lichens of small twigs, H. sitchensis appears to be a short-lived species, completing its life cycle within about 10 to 15 years. It dies when its position on a given branch has been usurped by other more aggressive lichens and bryophytes. This observation is probably key to its status as a rare species; for though H. sitchensis has evolved a requirement for frequent recolonization, it appears to be conspicuously inefficient at dispersal and/or establishment (see below).

4) Branch elongation

Heterodermia sitchensisis mostly restricted to slow-growing twigs, with growth rates less than about 10 cm per year. This observation is consistent with its characteristic association with old trees, and could also explain its occasional presence on young trees growing on rocky outcrops. In both cases, branch growth would be expected to be slow.

Heterodermia sitchensisis probably most "abundant" following periods of stress to its host trees. Moderate insect defoliation, for example, appears to favour colonization on even rapidly growing branches. Population dynamics in this species are thus probably cyclic, with episodes of relative abundance at landscape scale alternating with periods of decline. Observations in 2002 suggest that H. sitchensis was then entering a period of "abundance" following a defoliation event in about 2000. Most favourable for colonization are twigs that survive defoliation, and hence continue to put on new growth in subsequent years. By contrast, branch death eventually leads to loss of bark and, with it, H. sitchensis.

5) Canopy position

Heterodermia sitchensisappears to be restricted to the lower forest canopy, typically growing within about 4 m of the forest floor. Goward (1994) interpreted this as reflecting sensitivity to desiccation, though recent observations of healthy thalli growing 13 m above the ground suggest this may not always be the case (Goward & Wright 2003). An alternative explanation, not necessarily precluding the first, is that H. sitchensis is absent from the middle and upper canopy owing to an unavailability of nitrogen and other nutrients needed in order to establish. According to this hypothesis, nitrogen is likely to be present in concentration only in the lower canopy, perhaps owing to leaching from higher in the tree.

6) Branch chemistry

Goward & Fredeen (2006, in prep.) provide indirect evidence that H. sitchensis has a strong physiological requirement for bark with an elevated pH. At first inspection, this would seem to be out of keeping with its occurrence on Sitkaspruce which, in common with other conifers, tends to have acidic bark (Barkman 1958). Still, it is well known that conifer bark subject to nutrient enrichment of one kind or another can register a rather high pH (e.g., Goward & Arsenault 2000). A few instances of nutrient enrichment are briefly discussed below:

7) Bird perches

Droppings excreted by perching birds, especially Bald Eagles, create vertical columns of enrichment that extend downward to the forest floor. These nutrient columns appear to favour the establishment of H. sitchensis, even in trees otherwise with rather acidic bark. Once the supporting bird perch is abandoned (owing, for example, to breakage or crowding by younger branches), the nutrient column gradually dissolves. Such a phenomenon might explain, for example, the gradual loss of H. sitchensis from the holotype locality between 1983 and 2002 (see above).

8) Sea lion wintering grounds

One of the most productive sites for H. sitchensis is located on the north shoreof Wouwer Island, well known as a winter haulout for California and Steller's sea lions, which gather here in the hundreds. The sea lions occupy rock shelves, beaches, and even penetrate a short distance into the forest. To judge from the results of recent elemental analyses of Sitka spruce twigs collected from this locality (Goward & Fredeen 2006, in prep.), the resulting aerosols impregnate nearby trees, considerably elevating bark pH. Not surprisingly, such localities provide prime habitat for H. sitchensis.

9) Base-rich bedrock

In common with WouwerIsland, Florencia Island has significant populations of H. sitchensis. This island is also notable for its largely calciferous bedrock. In our opinion, these observations are likely to be causally related. Calciferous bedrock has been hypothesized to benefit base-loving arboreal lichens through the uptake of nutrients via tree roots (Goward & Arsenault 2000). The translocated nutrients are later released into the forest canopy. Evidence for nutrient enrichment in at least some stands of Sitka spruce can be deduced from the presence, as on Florencia Island, of unusually rich assemblages of cyanolichens (i.e., species in which the photosynthesizing partner is a cyanobacterium).

10) Seabird colonies

In the case of FlorenciaIsland, a supplementary source of enrichment could derive from intensive use by nesting seabirds. Glaucous-winged Gulls, Pelagic Cormorants, Black Oystercatchers, and Pigeon Guillemots all frequent this island throughout the year, but are especially abundant during the breeding season. In addition to creating localized nutrient columns of the kind already described, the tendency of Glaucous-winged Gulls to defecate while ridge-soaring above the forest edge could contribute an additional, more generalized source of enrichment. Possibly, H. sitchensis benefits from such "fecal bombing."

11) Old village sites

Another possible source of enrichment are the shell middens that mark the presence of old Aboriginal villages. According to this hypothesis, the former habit of native peoples of concentrating seashells in their village sites could now provide a localized and highly concentrated source of nutrients. Once translocated into the forest canopy, these nutrients doubtless elevate bark pH and hence favour the establishment of nitrophilous lichens like H. sitchensis. Disturbance associated with these village sites more than a century ago now promotes a shoreline fringe of pioneering Sitkaspruce and, by extension, future habitat for H. sitchensis.

12) Small islands

Small islands provide a measure of defence against predation, thereby encouraging regular and prolonged use by nesting seabirds, wintering sea lions, and other animals. The occurrence of a rich fauna doubtless promotes high levels of nutrient enrichment, which in turn could favour colonization by H. sitchensis. This observation, at any rate, could explain the relative frequency of H. sitchensis on small islands. Also apparently favourable to this species are capes, headlands, spits, peninsulas – all convenient resting places for migrating birds.

Habitat trends

In Canada, H. sitchensis is restricted to seaside oldgrowth forests along the west coast of Vancouver Island. Here it has probably been protected from resource extraction by the ruggedness of the topography, as well as by this region's relative inacessibility. Because, however, provincial regulations on forestry practice do not explicitly prohibit logging operations along the outer coast, it is highly likely that H. sitchensis has suffered at least some habitat loss, especially given the exceptional commercial value of its host tree, Sitka spruce. At a few southern localities on the Ucluth Peninsula, housing developments have already locally extirpated one population.

Habitat protection/ownership

Five of the ten extant Canadian localities for H. sitchensis are situated in Pacific Rim National Park Reserve, where they are strictly protected from logging and other forms of resource extraction. Four of the remaining localities are located on crown land on small, undeveloped islands. The last locality, also on crown land, is located near a tidal marsh on the Ucluth Peninsula 1 km from Ucluelet. See Table 1 for further details.