Three groups of lichens depend on riparian areas: non-epiphytic, in-stream species; streamside epiphytes; and species associated with forested wetlands, especially ash swamps. Selected species and habitats are illustrated below.
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These species are found on rock in streams. Although submerged much of the year, they have some degree of tolerance of being exposed during the dry season. This is especially true of the Dermatocarpon and Leptogium listed below. Examples:
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These species thrive on trees and shrubs in streamside and lakeshore forests and woodlands. See table of species more frequent in riparian areas.
References: Hutchinson (2001), Peterson & McCune (2001a), Rosso et al. (2000b), Ruchty (2000), McCune et al. (2001). Examples:
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Epiphytic species in forested wetlands, especially ash swamps (Fraxinus latifolia). See also Last Best Lichen Places. Examples:
See table of species more frequent in riparian areas. References: McCune (1998), Ruchty (2000). |
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In the Oregon Coast Range, Ruchty (2000) found that the lichens in riparian areas shift along a gradient from the headwaters to the lower reaches. Headwater streams tend to be at higher elevations, have narrower floodplains or closely constraining slopes and be more dominated by conifers. The constraining slopes make the riparian understory shady and prevents formation of a broad floodplain. Older stands in the headwaters tend to have more old-growth associated species, especially Alectoria sarmentosa, than do older stands along lower reaches. The abundance of Alectoria results at least partially from conifer dominance in the upper reaches.
"Species composition and abundance of epiphytic lichens and bryophytes differed among the seven riparian stand types. The factors hypothesized to underlie these differences included bark pH and the distribution of stand types along a valley continuum: as elevation and precipitation decreased, bottomland width expanded, stream reaches became less constrained, and riparian stands became more hardwood dominated (primarily due to increase in basal area of Oregon ash)... Along this continuum, overall lichen species richness increased, including green-algal foliose lichen and cyanolichen richness. The abundance of Lobaria oregana, an old-growth associated lichen... decreased along this continuum" (Ruchty 2000).
To some extent, old-growth dependency has been confused with habitat characteristics. Many of the species originally listed as old-growth associates in the Northwest Forest Plan turned out to be more strongly associated with hardwoods and shrubs than old-growth forests per se (Neitlich & McCune 1997). Similarly, many lichens considered old-growth associates in Fennoscandia are actually more strongly associated with forested wetlands and riparian areas than with old growth (Ohlson et al. 1997, Kuusinen 1996b, Rolstad et al. 2001).
The confusion is understandable, because some habitat characteristics may favor certain species and, at the same time, discourage disturbance by logging or fire. For example, ash swamps in the Pacific Northwest have slow growing trees with low timber values, occur in areas too wet for most crops, are less prone to fire, and are protected somewhat by state and federal wetland laws. These factors combine to make forested wetlands focal points of stability in an increasingly altered landscape. Furthermore, the more humid environment, and frequently broken, irregular canopy may foster lichen growth and diversity.
Similarly, rock outcrop areas may have brush and stunted trees that discourage logging. The relative stability of these gaps in the forest canopy and the diversity of woody plants occurring in outcrop areas appear to foster lichen diversity.
Disturbance rates depend on site characteristics; presence of old-growth associates depend on both disturbance rates and site characteristics. Future research will continue to clarify how disturbances and site characteristics interact to influence lichen communities.
