Cryptic paw (Nephroma occultum) COSEWIC assessment and status report: chapter 5

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Habitat

Habitat requirements

Nephroma occultumis confined to moist forested regions at elevations below 1200 m (most populations occur between about 400 and 800 m).  In Canada, all populations occur in the Coastal Western Hemlock Zone and the Interior Cedar-Hemlock Zone of the British Columbia Biogeoclimatic Ecosystem Classification system (Meidinger and Pojar 1991). No other zones are colonized, nor do extreme maritime (hypermaritime) portions of the Coastal Western Hemlock Zone appear to support this species. Climatic variables common throughout the range of N. occultum are: 1) rather continuous high humidity; and 2) moderate summer temperatures. Its absence from the hypermaritime is not easily explained, but could possibly involve either a sensitivity to salt spray or an inability to withstand strong competition from epiphytic bryophytes.

In Canada, Nephroma occultum usually occurs in old growth forests characterized by high humidity, stable environmental conditions and nutrient-rich soils. These forests provide stable conditions for slow dispersing lichens such as N. occultum and also provide protection from summer drought, one of the key distribution constraints of this species. Within such forests, trees of all age classes are colonized. In Oregon, however, N. occultum has been occasionally found in younger forests (McCune and Geiser 1997). Young stands in humid macro-climates like the Oregon Cascades (which feature frequent summer fog) support N. occultum where soredia are able to disperse from neighbouring older forests. A humid macro-climate may also explain why N. occultum inhabits the upper forest canopy of coastal ecosystems, whereas in inland localities this species is essentially restricted to the lower forest canopy where humidity is maintained throughout the summer by shading and moisture retention from older trees and associated vegetation (Goward 1995b).

In the Canadian portion of its range, Nephroma occultum tends to grow on living branches, usually near the branch tips among live needles. It is less common or absent over large branches or on the trunks of trees. In Oregon, however, it is often on larger branches near the trunk (Wetmore 1980, Rosso et al. 2000), which may reflect the humid macro-climate and competition from bryophytes.

Nephroma occultumis an acidophytic species which colonizes a broad range of trees. The major phorophytes, as reported by Goward (1995a), are listed here in decreasing order of importance: Abies lasiocarpa, Tsuga heterophylla, Picea sitchensis, Abies amabilis, Picea glauca, Tsuga mertensiana and Betula papyrifera. In Oregon it is also known from Pseudotsuga menziesii and Acer macrophyllum (Wetmore 1980, Rosso et al. 2000), and at one B.C. locality, from a shady mossy outcrop (Goward 1995a).

Habitat trends

The preferred habitat of Nephroma occultum, humid old growth cedar-hemlock forests, is threatened by both natural and human caused disturbances. Both sources of disturbance are causing a progressive decline in the remaining N. occultum habitat.

The primary natural source of Nephroma occultum habitat loss is defoliation of old growth forests by hemlock looper (Lambdina fiscellaria lugubrosa). This is especially true of inland cedar-hemlock forests where canopy defoliation reduces understory humidity, causing mortality amongmoisture sensitive lichens such as N. occultum (Goward 1995a). Hemlock looper infestations do not always result in complete canopy tree mortality; some infected stands are able to recover within years and may continue to support N. occultum. Rates of defoliation attributed to hemlock looper in cedar-hemlock stands (in British Columbia) were approximately 44,277 ha in 2002, 42,542 ha in 2003, and 5,750 ha in 2004 (Westfall 2002, 2003, 2004); the total area of the Interior Cedar-Hemlock zone is 5,035,476 ha (Marvin Eng pers. comm.). The rates varies in response to a four-year life history cycle. In comparison, fire consumed 5,179 ha (2002), 33,331 ha (2003), and 210,008 ha (2004) across the entire province. Ecologists speculate that outbreaks of hemlock looper are facilitated by warmer than average mean annual temperatures. If true, then temperature changes predicted by climate models will cause an even greater prevalence of hemlock looper infestation in the years to come (Sutherland et al. 2004). This poses a considerable threat to the habitat of inland populations of N. occultum.

Climate change has significant potential to negatively influence Nephroma occultum habitat throughout its range. In British Columbia, mean annual temperatures are expected to increase 1-4°C within the next decade (Fraser 2002). This may lead to larger and more frequent outbreaks of hemlock looper, but will also certainly lead to greater forest fire frequencies. Most N. occultum populations occur in old growth forests where moisture conditions are reflected in fire return intervals of hundreds of years or longer. While historically a minor threat, a changing fire regime may become significant determinant of N. occultum distribution.

Global warming will influence patterns of precipitation throughout the range of Nephroma occultum. Models generally predict a greater amount of total precipitation (up to 40% greater), but also an increase in the duration of summer drought (Fraser 2002, Sutherland et al. 2004). The absence of summer drought is thought to be a key determinant in the distribution of N. occultum (see Growing conditions). Changes to existing climatic conditions are certain; it is unclear how N. occultum will respond.

Forest harvesting is the largest current source of disturbance to Nephroma occultum habitat throughout its range from Alaska to Oregon. In Canada, the rate and location of forest harvesting is dependent upon international wood markets, the prevalence of insect outbreaks, and the proximity of mills in operation. For example, the rate of harvesting in the Kispiox Valley (where there are several known populations of N. occultum) has decreased considerably in the 2000s owing to the softwood dispute with the United States of America and the subsequent closure of two mills in the region. However, even in the Kispiox Valley there has been a significant loss of N. occultum habitat (Williston 2001), largely because cedar-hemlock forests are among the most commercially viable forest types in the Kispiox region, even under poor market conditions (Ken Smith pers. comm.).

Current management plans for this region do not adequately address the habitat requirements of this lichen. For example, in the Kispiox Landscape Unit (which is 56,659.1 ha in size and supports several different forest types), 9% (2,923.5 ha) of the humid cedar-hemlock forest (ICH-mc2 subzone) is planned to be managed as old growth forest (Roberts and Turney 2004). Of this, 1071.4 ha is situated in non-commercial forests (with low value as lumber, for example swampy forests). The remaining old growth (1,852.1 ha) will be secured through the recruitment of stands that are presently younger (due to historical disturbances such as forest harvesting or fire). An additional 6,138.8 ha of old growth forest remains in the timber-harvesting landbase and is unprotected (total historic old growth cover is not stated). This approach to forest management could lead to the regional extirpation of Nephroma occultum. Similar methods of planning are used throughout the Canadian portion of the range.

In contrast, where forest harvesting occurs on federal land in Washington and Oregon, N. occultum is among the seven lichens included in pre-disturbance surveys (USDA and USDI 2003). This has resulted in the development of protected areas and forest harvesting that emphasizes partially cutting (rather than clear-cutting). It has also contributed to a better understanding of the distribution and habitat availability of N. occultum in the southern portion of its range.

Forest harvesting, hemlock looper and fire (and cumulative effects among all three) continue to diminish the prevalence of old growth forests, the primary habitat of Nephroma occultum. This trend is expected to intensify as the demand for natural resources grows and mean annual temperatures increase in accordance with global warming.

Habitat protection/ownership

In Canada, most known populations of Nephroma occultum occur on provincially owned Crown land and, as such, are not protected by either provincial or federal legislation. Five populations are within parks or protected areas. One locality (41; see pages 9 to 12) is situated within a designated sensitive area (Williston 2002) and is protected as part of the Forest Development Plan for the McCully Creek drainage (a partially logged side-drainage of the much larger and heavily logged Kispiox Valley). Three other localities (11, 17 and 18) are within Wells Gray Provincial Park and a fifth population is protected within an ecological reserve (3).

Two additional populations (36 and 37) are adjacent to Glacier National Park and are situated in very old, old growth forests with humid, stable environmental conditions and rare species habitat, which mark them as high priority for national park expansion.

The remaining populations are in areas that are managed for forestry revenue, and barring the creation of additional parks and protected areas, many will be harvested. For example, in 1996 T. Goward identified 14 lichen-rich old growth forests in the McCully Creek watershed (several of which harboured Nephroma occultum); four years later, eight (57%) had either been logged or were designated for harvesting (Williston 2001). In effect, the protection of most Nephroma occultum habitat is determined by economics (the high costs associated with road building) and geography (the rugged topography of British Columbia), not by conservation measures.