Flooded jellyskin (Leptogium rivulare) COSEWIC assessment and status report: chapter 6

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Biology

General

The genus Leptogium is one of the jelly lichens, and is characterized by its thin, skin-like thallus containing cyanobacteria (Nostoc). The cynaobacteria are not confined to a narrow "algal layer" as in most other foliose lichens, and this genus is therefore considered to be "non-stratified." Cyanobacterial lichens are most often shade and moisture-loving, and they often are found in forests or close to water. This is especially true of Leptogium. Leptogium rivulare has a special requirement for periodic flooding, making it much more limited in its habitat potential. Cyanobacterial lichens are also notoriously sensitive to air pollution, especially pollution due to sulphur dioxide (Ferry et al. 1973).

Leptogium rivulare has previously been known almost exclusively from old herbarium material and the briefest of accompanying notes on habitat and substrate. Although an investigation of its biology is beyond the scope of this report, certain observations made in the field by Robert Lee (pers. obs., 2002) may be pertinent.

Reproduction

This lichen is always abundantly fertile. Apothecia develop when the lichen is small and young, and spores are readily produced. Presumably spores are the major factor in the lichen's reproduction and dispersal. Like all lichens, though, Leptogium rivulare can also reproduce by fragmentation. Tiny lobules are formed in some populations and could easily break off the parent thallus and serve as propagules.

If the spores were water-borne, rather than air-borne, as would be more usual in a lichen, this could explain the apparently restricted distribution within a much broader range, and the apparent absence from much suitable habitat.

Leptogium rivulare is regarded as a small lichen, and could probably reach full size (6 cm across) within a decade. But heavy encrustations up to 0.5 m across have now been observed, and if these do not simply result from the confluence of many smaller thalli, the maximum size, and associated age, could be greater.

Leptogium rivulare grows below the high-water mark, mainly on flood-zone trees, in a zone from which most other lichens are excluded. It is the largest of the few lichens that do grow there, and the only one that could overgrow those around it. It may be that lack of competition with other lichens is important. But L. rivulare is sometimes found among thick growths of moss over its usual substrate. Some members of the genus (e.g., L. cyanescens (Rabinh.) Korber) are able to grow vigorously in such circumstances. Others (e.g., L. dactylinum Tuck.) seem to die out as moss grows over them and their substrate. From what has been observed by Robert Lee, L. rivulare seems most like L. dactylinum, with thalli disappearing under thick mats of rapidly growing moss.

Nothing is known about its growth rate or life span, except what has been observed by Robert Lee. One thallus 2 cm across has been found on a Dogwood stem only 4 years old (as determined by counting both rings and whorls of leaf scars). This suggests a potential growth rate of 2.5 mm per year, which is comparable to other arboreal foliose lichens (Brodo et al. 2001).

Most growth may well take place during or immediately after spring immersion, as the margins of several different thalli, measured against pins driven into the bark a month after the waters receded, extended outward only 0.2 to 0.6 mm between early August and November in 2002. Growth was equally limited during the whole of 2003, when floodwaters did not rise high enough to immerse the lichen.

Survival

The survival of this species depends on periodic or at least occasional inundation, coupled with subsequent prolonged exposure to the air. This necessitates its inhabiting somewhat unstable habitats. Such flooding, however, does not appear to be absolutely necessary for the year-to-year survival of individual lichens. In recent years, at some ponds, spring water levels have fallen 25 to 50 cm short of known maximums, and equally short of covering all the existing lichen. Most, but not all of the lichen, which has been stranded for at least six seasons, appears to be healthy.

In most of the ponds where Leptogium rivulare occurs, it occupies only a part of the substrate available, whether tree bark or rock. There would seem to be room for further growth of the population in these places. There are also innumerable places within the Canadian range with suitable habitat. The problem may well be one of dispersal.

Physiology

Little is know about the physiology of even the most common species of Leptogium, other than they seem to require fairly shaded, humid habitats (Brodo, pers. comm., 2002).

The most northerly region where Leptogium rivulare is known to have occurred in Canada (near Flin Flon, in Manitoba) has a subhumid high boreal ecoclimate, with mean annual precipitation of 400-500 mm, and mean temperatures summer and winter of 12.5º and -18.5º C, respectively (Ecological Stratification Working Group, 1995). The Ontario localities are more humid, with 800-1000 mm of precipitation, and warmer (16º and -7º C). L. rivulare is a cool-temperate to boreal species, and given its greater range in Europe (north to Finland, and south to the Seine in France), it may be able to withstand greater temperature extremes than the limited Canadian distribution would indicate.

The restriction to periodically flooded places implies a physiological capacity to survive immersion that is evidently lacking in all but a select few other lichens observed in the same or similar habitats.  And while it can withstand immersion at depths of up to 2 metres, for periods as long as three months, this lichen also seems to require eventual exposure to the air.

Although, as a species, Leptogium rivulare has an absolute requirement for substrate that is flooded periodically, this seems to be related to some very early stage in the lichen’s life cycle, rather than physiology. While this could have to do with some alteration of the substrate (such as leaching of bark) necessary for spore germination, it more probably relates to spore dispersal.

Dispersal

The general dispersal ability of Leptogium rivulare is probably low, as indicated by its very restricted distribution, occurring at very widely scattered points across an extensive geographic range, despite the widespread availability of habitat. Conversely, very local dispersal, within the range of a single wetland, appears to be excellent. Dispersal at intermediate range, between nearby ponds, is spotty.

This pattern of distribution could be explained if water were the medium for dispersal of the spores. Long-range dispersal would be extremely unlikely; intermediate-range dispersal by four-footed animals acting as carriers would be possible, but limited, and dispersal within the reach of spore-bearing waters could be highly effective.

Several observations lend support to this possibility. Firstly, as noted above, the lichen’s requirement for flooding seems to have to do with spore dispersal or germination, rather than long-term survival.

Secondly, RobertLee (pers. obs., 2002) has found spores that appear to be those of Leptogium rivulare in the floodwaters around trees that bear the lichen, while spores from almost all other lichens are absent. These spores adhere to substrate surfaces (such as plastic “microscope slides,” which were set out experimentally) as the water levels fall.

Thirdly, on some trees and boulders in one of the ponds with unique characteristics, Leptogium rivulare sometimes forms a thick growth like a collar or bathtub ring just below the high water mark. This pond differs from the others in that it floods to the same level every year (additional water spills out into adjacent swamps). This would repeatedly bring water-borne spores that are floating on the surface to that point. All of the other ponds with L. rivulare occupy basins that lack such well defined sills. Meltwater and spring rains fill them to different depths every year. In these places, the lichen occurs irregularly down the tree trunks.

Although the waterborne mechanism of dispersal is not proven, and other interpretations could be advanced, the possiblity does have implications.

Along waterways, downstream dispersal by water ought to be very effective. But waterways are subject to damming and siltation, and the habitat may well have been seriously disrupted or lost in much of eastern North America and Western Europe.

The widely scattered points of occurrence, usually separated by many hundreds of kilometers, could also derive from water-borne spores, as the result of long-distance migration of ducks, as carriers. Two species, the Wood Duck (Aix sponsa) and the Hooded Merganser (Lophodytes cucullatus) have regularly been observed courting on some of these ponds in full flood, before their final dispersal for breeding. But it is obvious from the lichen’s rarity that, if it occurs, dispersal by such means is not very effective.

It has been suggested by Jørgensen & James (1983) that these scattered occurrences could also represent relict populations from a more widespread distribution. This is suggested by the amphi-Atlantic distribution, which may date from a time when Europe and North America had a more continuous flora.

Without intervention, repopulation from sources outside Canada seem most unlikely, for Leptogium rivulare is at least as rare everywhere else. If the spores are waterborne, however, it may be possible to introduce the species to new sites. That this might turn out to be rather easy is suggested by the circumstances surrounding the anomalous site in Pakenham Township, discussed below under Population Sizes and Trends.

Nutrition and interspecific interactions

Nothing is known, other than the fundamental symbiotic relationship between the fungus that gives form to the lichen, and its photobiont, the cyanobacterium Nostoc. Through photosynthesis, the latter generates the carbohydrates that are used by both partners.

Adaptability

Leptogium rivulare is extremely habitat-specific throughout its global range, and within that habitat effectively requires a substrate provided by only those few species of deciduous trees that can withstand regular flooding. On such trees, it grows almost exclusively on the bark, and not on exposed wood; hence the trees must be alive, or else serve as a substate only until the bark falls off. It does grow on shrubs and saplings, as well as mature trees, so has the potential to establish itself in recently disturbed and also newly developed habitats. However, almost all occurences on shrubs, rocks, and unusual tree species have been in the midst of heavy populations of the lichen on surrounding trees, where extraordinarily abundant reproduction may flood all substrates in the immediate area with spores.