Cryptic paw (Nephroma occultum) COSEWIC assessment and status report: chapter 6

Biology

General

Nephroma occultum is a moisture-sensitive cyanolichen endemic to western North America. N. occultum is limited by an inefficient dispersal mechanism, and increasingly by the availability of suitable habitat. Within Canada, its habitats are humid old growth forests in the Coastal Western Hemlock Zone and the Interior Cedar-Hemlock Zone. These forests are do not experience summer drought, which appears to be another important limitation for distribution for this species. N. occultum is a poor competitor and is susceptible to displacement by epiphytic bryophytes in very humid localities.

Reproduction

Nephroma occultum reproduces via asexual soredia composed of clusters of cyanobacterial cells surrounded by fungal hyphae. The soredia of N. occultum are coarsely granular, and occasionally intermixed with isidia. These vegetative structures are abundant on most specimens, including those in early development, and are present in virtually all thalli larger than 1.0 cm in width (Goward 1995a). Soredia arise on lobe margins, and eventually on ridges of the upper surface of older individuals.

Sexual structures (apothecia) are not known for Nephroma occultum, and it is assumed that genetic recombination is infrequent and variation is low. This suggests that N. occultum is poorly suited to adapting to rapid changes in the environment.

At most sites, Nephroma occultum was found on or near the extreme branch tips of understory conifers. The branch tips were estimated to be less than 5 years old indicating that N. occultum is presently successfully regenerating at all known localities. Growth rates for this species have not been measured; however, an estimate based upon the size of specimens and their proximity to the growing tips of branches suggests a growth rate range of between 1-10 mm/yr along the longest axis, with an average of approximately 5-6 mm/yr.

Dispersal

On observing the inconsistent distribution of the lichen within a single tree, and among trees within a single stand, Rosso et al. (2000) hypothesized that Nephroma occultum is dispersal limited. N. occultum relies upon the dispersal of asexual soredia for reproduction. In this species, the soredia are coarsely granular (70-330 µ m broad), and may be too large to disperse efficiently. Poor dispersal capability is recognized as a characteristic of old growth dependent lichens (Sillett et al. 2000). Vectors that might assist in distributing the large soredia of N. occultum include water, wind and animals.

Rain splash probably helps distribute lichen soredia within a forest stand; however, this could only account for short distance dispersal and would not allow lichens to disperse across spatial barriers (such as inappropriate habitat) measuring more than a few metres.

Wind is almost certainly an important vector for dispersing soredia (Torno et al. 2001, Muñoz et al. 2004). Goward (1994) suggests that wind may be important in coastal localities where Nephroma occultum occurs in the upper canopy, but is probably not a factor in inland sites where this lichen grows in the sheltered lower-canopy. While the lower-canopy is protected by boundary layer effects, penetrating gusts from occasional storms could potentially distribute lichen soredia considerable distances, particularly from the edges of stands. The coarse, granular soredia of N. occultum, however, are probably less easily transported by wind than the soredia of many other lichens.

Animals vectors, and birds in particular, are a possible source for the medium and long distance dispersal of Nephroma occultum (Goward 1995a). One study (Bailey and James 1979) demonstrated that lichen propagules adhered to the feet of birds, potentially resulting in the establishment of new lichen colonies elsewhere. Dispersal by birds would help explain the irregular distribution of N. occultum within and among suitable habitats.

While poor soredia dispersal appears to be an important factor in determining the limited distribution of this species, the diminishing availability of suitable habitat is an increasingly important constraint.

Growing conditions

The specific growing conditions preferred by Nephroma occultum have been described in detail by Goward (1995a) and are briefly summarized here.

In Canada, Nephroma occultum grows where the mean annual temperature is between 4-10 °C, with an annual mean temperature range of about 15-26 °C, depending on latitude. These conditions occur in the Coastal Western Hemlock Zone and the Interior Cedar-Hemlock Zone of British Columbia (Meidinger and Pojar 1991). 

One of the key attributes of the Canadian localities of Nephroma occultum is the absence of summer drought. Both the Coastal Western Hemlock Zone and the Interior Cedar-Hemlock Zone receive at least 75 mm/month during the summer months. In contrast, N. occultum is seldom seen in regions that have little or no precipitation during summer. Summer fog allows N. occultum to occur in southern localities (Oregon for example) which receive less rainfall.

Nephroma occultum is an nutriphytic lichen which grows on a range of nutrient-enriched conifers, but is rarely found growing on deciduous trees (with the exception of Betula papyrifera and Acer macrophyllum). This lichen most frequently occurs in forests with nutrient-rich soils, typically with toe-slope topography where soils are enhanced by moisture and nutrient additions from surrounding elevated landforms.

Adaptability and competitive interactions

Nephroma occultum is intolerant of summer drought and produces soredia that disperse inefficiently. These biological attributes are common among several old growth dependent lichens (see Goward 1994, 1995b; Goward and Pojar 1998; and Sillett et al. 2000) and signify adaptation to the stable environmental conditions associated with very old forests. In particular, inland populations of N. occultum are intolerant of disturbances that dramatically alter humid micro-climates. The dynamic environmental conditions of young managed stands do not provide protection from summer drought and are unsuitable habitat where the macro-climate is dry.Where the macro-climate is suitable, for instance in coastal regions or in Oregon, protecting representative individuals in remnant old growth patches as sources for short distance soredia dispersal may be an effective conservation strategy for maintaining N. occultum in younger managed stands.

Nephroma occultum is a poor competitor. Goward (1995a) suggests that it is displaced by mosses and liverworts, especially in coastal ecosystems where bryophytes are the dominant epiphytes. N. occultum may persist in coastal ecosystems by colonizing recent growth on branch tips or by occupying branches higher in the canopy, habitats that are less favourable to bryophytes.

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