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COSEWIC assessment and status report on the Boreal Felt Lichen in Canada

Biology

General

The life cycle of E. pedicellatum is still incompletely known and will remain so until the life cycle of its photobiont Scytonema has been studied in detail. The distribution and micro-ecology of this cyanobacterium within the forests containing E. pedicellatum are presently insufficiently known. Even the vectors for the distribution of the spores of E. pedicellatum have not been fully recognized; however, the junior author has investigated flying insects as potential vectors of Erioderma spores in a Masters thesis currently under completion. For most components of the life cycle, only hypothetical considerations can be given.


Reproduction

Only sexual reproduction is known for E. pedicellatum. Therefore the lack of vegetative propagules makes E. pedicellatum dependent upon a de novo synthesis of new thalli. This means that initiation of a new life cycle is a chance encounter between the more or less full complement of eight spores from a single ascus and a suitable strand of the free-living form of the cyanobacterium, Scytonema. The process of lichenization then begins. Such chance encounters would have a much greater probability of being successful if both the spores and the photobiont partners were to find refuge in a microbiologically suitable microenvironment, such as in the watersacs of Frullania (Scheidegger, 1996).


Dispersal

Most recently Yetman (unpublished data from Masters thesis) has shown that the spores of E. pedicellatum are ejected either individually or as groups of 8 per ascus. This has been confirmed using scanning electron micrograph imaging. Based on recent laboratory experiments, it is likely the spores are ejected under suitable weather conditions, usually after a dry weather period when the humidity of the air goes up and fog is rolling in or when the first drops of rain are falling.

Favoured by the frequent incidence of fog or rain, by the favourable characteristics of the lichen forest's understory and tree canopy, as well as by the availability of adjacent wetlands, it may be assumed that the primary spore dispersal mechanism of E. pedicellatum is operative during most if not all parts of the year. Exceptions to this rule may be imposed by the few weeks of hot weather during the summer and during snowy and frosty periods in the winter. Studies on Xanthoria parietina in the British Isles and Oregon have shown that viable spores of this species can be obtained throughout the year (Christmas 1980). On the other hand, there are lichens such as Rhizocarpon lecanorinum, that show a peak performance in the spore dispersal during a specific time of the year (Clayden 1997b). So far, according to the junior author (unpublished data for Masters thesis) it seems that at least E. pedicellatum does not disperse over the hot, dry months of summer.

The following vectors may assist in spore dispersal of E. pedicellatum to adjacent or more distant suitable habitats:

  1. Strong moisture-laden winds that will carry the released spores up to a few hundred metres into an adjacent younger woodland (Scheidegger, 1996).  The prevailing windstorms on the Avalon Peninsula blow either from East to West (during the winter months) or in the opposite direction from Southwest to Northeast (during the summer months). Analogous prevailing wind directions have also been recorded for Cape Breton Island, according to data from the Point Aconi Weather Station (see Maass and Richardson 1994). On the mainland of Nova Scotia, e.g., in Halifax County, the winds often come directly from the North (during the winter) or from the South (in the summer and during the hurricane season). The presence of variable and moderate winds in these semi-closed canopy forests would undoubtedly carry spores to adjacent younger balsam fir stands.

  2. Insects as possible dispersal agents. Through a series of studies it was shown that insects could be dispersal agents of Erioderma spores (unpublished data for Masters thesis by Yetman). Initially in laboratory experiments, fruit fly (Drosophila melanogaster) larvae were allowed to mature and roam for 48 hours in an experimental chamber containing a mature Erioderma thallus. Following this, the fruit flies were anaesthetized and viewed under a scanning electron microscope. Erioderma spores were positively identified, adhered to the leg bristles of several fruit flies. These findings were reconfirmed in the field in the summer of 2001 in mature forest stands in Lockyer’s Waters, Newfoundland. Erioderma spores were identified on the segmented antennae of Anapsis rubis, a small flying beetle, using SEM. It is probable that such flying insects disperse viable spores to far reaching forest stands; this may result in the creation of a thallus but only if at least two spores remain adhered in a clump while landing on a Frullania to germinate and make contact with a suitable Scytonema symbiont in the water sacs of this hepatic. Although spores of E. pedicellatum also adhere to the legs of mosquitoes, no proof has yet been obtained as to how far these insects can travel (Yetman, unpublished data). We suspect that mosquitoes do not get air-borne during windstorms.

  3. Birds. Woodpeckers, and possibly other birds, are an additional potential vector for the long distance dispersal of viable spores of E. pedicellatum from over-mature balsam firs. Spores could be picked up involuntarily on their front and/or tail feathers while the birds forage for bark-inhabiting insect larvae immediately following a brief dry weather period when most of the ripened spores are normally ejected.


Life Cycle

Scheidegger (1996) states the following concerning the life cycle of Erioderma pedicellatum. [The following life cycle scenario has not been conclusively demonstrated.]

  1. Only one generation of Erioderma occurs during one successional cycle of the so-called lichen forest.

  2. During the approximately 15 to 25 years of the over-mature to decaying phase of the forest, previously established thalli of E. pedicellatum are able to achieve a high growth rate due to favourable light conditions. The reproductive phase of the lichen is restricted to this period when microscopically small diaspores are dispersed [each spore measuring about 4-6 μm in length, Yetman, unpublished] through hypothetical vectors to a considerably younger tree, possibly in an adjacent stand corresponding to an earlier successional stage of the fir-dominated coniferous forest.

  3. The life cycle of Erioderma would therefore begin again in another stand of suitable ecology and successional stage, at a distance of up to a few hundred meters from the original population. The capture of a suitable cyanobacterium (Scytonema) would lead to the development of minute individuals by a process that could last for more than ten years, whilst the stand might be reaching its optimal phase of growth.

Scheidegger's life cycle model has validity in as far as it applies to those forests that have adjacent blocks of even-aged trees at different developmental stages. Such a model would then apply to stands selectively cut during the past two hundred years, such as the forests in Lockyer’s Waters, Newfoundland. The same might be accomplished by epidemic outbreaks of forest pests and forest fires. The “wave forests” on the West Coast of the Great Northern Peninsula of Newfoundland also show alternating strips of synchronized tree growth. The same applies to the wave forests on St. Paul’s Island and those in the most north-easterly region of Cape Breton, although the wind velocities in most of these highly exposed habitats are too forbidding for the establishment of a new generation of thalli.

Young regenerating forest stands within which new life cycles of E. pedicellatum are initiated have rarely been encountered. The only possible exception is site NF-21b in Jipujijkuei Kuespem Park where a fairly large juvenile population of E. pedicellatum has been discovered (Yetman, 1999). The earliest stages in the colonization of a synchronized young woodland site by E. pedicellatum have so far remained undetected.

Scheidegger's scenario of the amount of available light becoming increasingly greater during the life cycle of Erioderma may not be totally inclusive. The presence of adult thalli in half-open very hydric habitats appears to support the claim that the de novo re-synthesis of thalli of E. pedicellatum requires a fair amount of light. Older well-established thalli of E. pedicellatum are probably quite adaptable to changes in the light intensity as long as sufficiently high levels of humidity are supplied. Such changes in the light intensity might be caused by opening or closing of the canopy, by the death of individual trees adjacent to the phorophyte, or by the maturation of younger trees in the understory beneath the canopy of the forest.

Concerning the life cycle of Erioderma pedicellatum, it seems more relevant to conclude that as relatively young and small natural stands of balsam fir continue to grow, the amounts of light available to the lichen (or its free living photobiont) might either increase or decrease to the point below which E. pedicellatum and Scytonema containing Frullania can survive depending on the stability of the moisture regime. Such occasional habitats are encountered in high humidity areas of Newfoundland (sites NF-2 and NF-25) where E. pedicellatum thalli have remained exclusively confined to the branches.

Nevertheless, based on numerous field observations it is important to maintain either a mosaic of forest stand age classes adjacent to one another or multi-age stands.


Growth Rate

Growth rates of mature thalli, in the exponential phase of growth, are somewhat comparable on balsam fir and spruce. The highest annual growth rates had been observed on balsam fir, with growth index (g.i.) values of up to 13 and 14 mm/yr on branches and trunks respectively (thallus # 2-b on balsam fir, # 2 at Salmonier Nature Park (SALM); thallus # 2-t on balsam fir, # 13 at Fitzgerald Pond Park (FITZ)). The largest g.i. value on black spruce was recorded for thallus # 3-b on tree # 4 at SALM, i.e., 11 mm/yr. This was in spite of the fact that the thallus had become strongly necrotic above its holdfast area during the growth period of a little over 11 months between subsequent measurements.

The growth rate measurements on balsam fir had also included two immature thalli. One of them (thallus # 2-t on tree # 8) had been encountered at FITZ and the other one (thallus # 1-t on tree # 3) at Goobies, NF. The initial measurements, taken on two consecutive days in October of 1980, had been the same for both thalli, namely 6 x 4 mm/yr. The subsequent annual growth increments had been 3 x 4 mm/yr at FITZ and 3 x 2 mm/yr at Goobies, NF. In addition, a juvenile thallus had been studied on spruce at SALM (thallus # 2-b on tree # 3). During the 11-month period between 2 Oct.1980 and 7 Sept.1981 it had increased its growth in length and width by about 50% (12 mm= x 10 mm per yr). This is similar to growth rates measured for young thalli on balsam fir during a comparable time interval.