Flooded jellyskin (Leptogium rivulare) COSEWIC assessment and status report: chapter 5

Habitat

Habitat and substrate requirements

Like collections elsewhere, almost all the occurrences in Canada have been in virtually identical habitats: the periodically inundated bases of trees, usually around the margins of basins that fill with meltwater each spring to form seasonal ponds. The water in these places is typically still and clear, beginning as ice-cold meltwater in March, and if still present in June, warming up considerably in the sunshine.

The trees in these places are those few species that, like Black Ash (Fraxinus nigra), can thrive in, or at least withstand, substantial springtime flooding to a depth of as much as 2 metres, for 3 to 12 weeks each year. About 98% of the lichen observed by Robert Lee in 2002 occurs on the bark of flooded tree bases. None is known to occur on permanently flooded trees.

To the old records of ash (Fraxinus sp., especially F. nigra), observations at the new localities add as the most common bark substrates: Red Maple (Acer rubrum), Silver Maple (A. saccharinum), Red Ash (Fraxinus pennsylvanica), and American Elm (Ulmus americana). Bur Oak (Quercus macrocarpa) and Balsam Poplar (Abies balsamea) are less frequently available. Still less commonly, Red Osier Dogwood (Cornus stolonifera), wild grape (Vitis riparia), and willow (Salix sp.) provide significant amounts of substrate. In one or two instances, the lichen also occurs on White Cedar (Thuja occidentalis) and Glossy Bluckthorn (Rhamnus frangula). Other reported bark substrates are shrubs (e.g., Buttonbush [Cephalanthus sp.] and alder [Alnus sp.]). About 1% of the lichen is growing on shrubs and fallen branches.

Almost all of the currently known sites are, in spring, watery openings in forests, in which various forms of herbaceous vegetation (ferns, sedges, Poison Ivy) may or may not appear after the water subsides. To some extent the flood zone habitat is therefore a forest edge. However, the lichen occurs both at the more sunlit margin, and in the more deeply shaded portions of the zone, and on shady southern shores as well as the more exposed northern side of the ponds.

The best growths of Leptogium rivulare occur on rough bark, and the most extensive are usually on old, rough bark. Bare, weathered wood does not normally support the lichen, even when the two substrates are contiguous (weathered lignum contributes about 0.1% to the total substrate coverage).

Rock has rarely been reported as a substrate, and remains extremely rare even where boulders and bedrock occur among trees that bear the lichen (Robert Lee, pers. obs. 2002). However, once established, growth there can be good, and the coverage on just 2 large boulders accounts for almost 1% of the total known. Leptogium rivulare grows directly on the mineral surface, even when intermixed with moss. One minute instance of growth on soil has been observed by Robert Lee.

Seasonally submerged tree bark is an extremely unusual habitat for lichens, probably because most corticolous lichens cannot withstand immersion. Just as the lichen growth on those parts of rocks above the high water mark provides an obvious waterline around lakeshores, so the growth of all the usual arboreal lichens extends down the tree trunks only to the high water mark in seasonal ponds. By contrast, Leptogium rivulare grows only below that mark. Usually there is a gap of several to many centimetres between them. On just three out of many hundreds of trees examined has there been any overlap, and then by only a few millimetres.

Although aquatic and semiaquatic lichens on rocks along streams and lakeshores are well known and are not uncommon (e.g., Dermatocarpon luridum (With.) J.R. Laundon, Ionaspis lacustris (With.) Lutzoni, Rhizocarpon lavatum Hazsl., and Staurothele fissa (Taylor) Zwackh), the few species that have been observed to share the Flooded Jellyskin’s usual habitat are not so well recognized as being semi-aquatic.

One species, Lecania cyrtella (Ach.) Th. Fr., which is fairly common in the Ottawa region (Brodo 1988), has often been found by Robert Lee (pers. obs, 2002) to be abundantly present with Leptogium rivulare. It favours the moist lower parts of tree species that grow in seasonally flooded places, but is not restricted to them. Another, rather rare and minute species of Leptogium, L. tenuissimum, has been noted several times, but it also is clearly able to grow in habitats that are never flooded.

Several other species of minute to almost microscopic lichens, as yet unidentified as to species, or sometimes even genus, have also been noted, sometimes rarely, sometimes abundantly, but thus far only within the flood zone. Two are in the order Lichenales, and two others are in the family Collemataceae, genus Leptogium.

Forested swamp habitats such as those used by Leptogium rivulare are of frequent, albeit much fragmented, occurrence throughout the deciduous and mixed-woods regions of Ontario. The collection at Wawa would represent an area close to the northern limit of that vegetation zone. As several of the tree species that the lichen grows on occur well into the boreal forest, however, on that basis the range could conceivably extend north of Lake Abitibi, west to Lake Winnipeg, and east to the Maritimes. The recent collection from near Flin Flon, northwest of Lake Winnipeg, bears out this possibility.

The tree species that thrive in flood-zones are able to establish themselves early in ecological succession. Of those that can grow in the flood zone, and that are being used by the lichen as substrate, Black Ash, Bur Oak and American Elm are also able to persist into old-growth conditions. Leptogium rivulare has been found on trees approaching 150 years old, and does not seem bound to any particular stage of succession.

Historically, some of the ponds where Leptogium rivulare now occurs have been subject to a major forest fire (>130 years ago), agricultural use of some kind (ca. 100 years ago), and bulldozing for sandpit operations (ca. 50 years ago). In each of these cases, it could have survived either on unaffected substrate (rock or old trees) or in nearby, undisturbed habitat where it is now absent.

Leptogium rivulare may be able to re-establish rapidly, growing on the first woody plants to appear in the flood zone, within a very few years. This is suggested by its occasional appearance on small bushes, such as Red Osier Dogwood. However, as of 2003, Robert Lee has observed this lichen on shrubs only in places that already have heavy growths of L. rivulare on nearby trees. It may not spread to bushes until they are flooded with spores from large, existing populations.

Excessive sediment loads in rivers, which leave tree trunks in the floodplain coated in silt when the water subsides, would surely be harmful to Leptogium rivulare. Such conditions occur wherever agriculture has become a significant part of a watershed. This degree of siltation has been observed by Robert Lee (during the 1990s and in 2002) in otherwise suitable habitat on islands like Upper Duck Island in the Ottawa River.

Although sediment is not a problem in seasonal ponds, sometimes Leptogium rivulare becomes so coated with dried-up algae (Scytonema sp.) after the water recedes that its true colour is not evident. Whether this harms the lichen is not yet known.

At the two main locations currently known, the underlying bedrock is limestone and the pH of the floodwater should be well buffered. The historical locations at Lake Temagami and Wawa, however, would have been on soils based on acidic bedrock. In this connection, where the lichen has been observed on rock, that rock has also been granite -- not the sign of a lichen susceptible to acidity. When bark is a substrate, it may also be either neutral (elms) or acidic (maples). Among conifers, the only one noted as a substrate, Eastern White Cedar, has bark most like that of deciduous trees (I.M. Brodo, personal communication 2002).

As one or two trees at a given location have been seen to be sufficient to support a vigorous growth of Leptogium rivulare, and since it is found in wet places as little as 5 or 10 metres across, there is probably no minimum size of habitat patch, as long as flooding is adequate.

Habitat trends

As trees are the usual substrate, and flooding dictates the type of forest in the flood zone, rather than the stage of ecological succession, the general opening up of the landscape by European colonization probably did not create any additional habitat for Leptogium rivulare. Indeed, clearing for agriculture would have removed the trees that are the lichen’s necessary substrate around ponds, and, through muddy runoff, would have rendered riverine habitat unusable. In less settled regions, only dams for lumbering and for hydroelectric power are likely to have removed habitat, by drowning it. These are mainly, but not entirely, historical factors. Dams are still being built for small-scale power generation, and for habitat modification.

At present, the threats to habitat known to be occupied by Leptogium rivulare are incremental degradation associated with increasingly heavy recreational use at one site, and development outside a major urban centre at the other.

If there is a trend towards a warmer, drier climate, however, then lower springtime flood levels will shrink the habitat and significantly reduce both its horizontal and vertical extent.

Protection/ownership

Unless good populations of Leptogium rivulare can be rediscovered in old locations, or found at more than just the two principal localities currently known, its range in Canada must considered to be very restricted. For now, then, the details of land ownership and protection in these places are especially important.

The wetland complex occupied by Leptogium rivulare in the locality west of Bells Corners, in Ottawa, is on land owned by the National Capital Commission (NCC). This land has been held for almost 50 years as part of a conservation-oriented greenbelt, and may reasonably be expected to remain in that category. The NCC develops management plans for sensitive features of the environment. But the land in question has no definite protection in law or regulation, and other NCC lands have been sold, traded away, used for highways, and opened to certain forms of development deemed acceptable.

In Pakenham Township, the lichen is present on private land. Although the waterway is subject to both natural and artificial impoundments, these have taken place upstream without appearing to affect the population, minute as it is, or the stream-flow conditions that support it.

The other significant locality, in Darling Township, comprises seven small ponds scattered over about a kilometre of forested land. Ownership is divided among at least seven owners, including the Province of Ontario, Lanark County, and five or more private owners. Two of the ponds are themselves subject to multiple ownership.

The Crown lands are subject to general forestry management practices, including logging. The County land is in the form of an unopened road allowance that cuts across the middle of one of the ponds. In both of these cases, protection could be achieved through formal planning processes -- but for only about 6% of the local population.

At present, three of the private landowners are aware of the rarity of Leptogium rivulare on their properties, and, with reservations, are informally agreeable to protecting the habitat. (They fear that restrictions might be imposed on them.) Two others are aware, but have not stated their intentions.

It is possible that still more owners are already represented, because some properties are in the process of being subdivided. The location is adjacent to a major recreational lake, and is within commuting distance of Ottawa.

Given the divided and changing ownerships, future protection of the Darling Township locality is in doubt.

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