Lichen / Bryophyte Nonvascular Boulderfields and Outcrops
Vegetation almost entirely dominated by lichens and bryophytes occupies exposed, minimally weathered boulderfields and associated massive outcrops on mountain ridges of western Virginia. Boulderfield habitats have resulted from periglacial phenomena and the collapse of resistant strata from weathering and erosion of weaker underlying rocks. The most numerous and extensive exposed boulderfields are composed of resistant sandstone or quartzite, with a few occurrences on metabasalt (greenstone) at higher elevations of the northern Blue Ridge. These habitats, where few vascular plants survive, are typically densely populated by overlooked or cryptic species of lichens and moss.
Massive, sheltered rock faces that are shaded and periodic wet often support large colonies of the umbilicate "rock tripe" Umbilicaria mammulata, one of the largest lichens in Virginia. Other umbilicate lichens, shade-tolerant foliose lichens such as Flavoparmelia baltimorensis, and many crustose lichens may also occur. Vascular associates include scattered individuals of evergreen wood-fern (Dryopteris intermedia) and Appalachian rock polypody (Polypodium appalachianum).
Rock tripes also dominate dry quartzite and sandstone boulderfields, with Lasallia papulosa and Lasallia pensylvanica characteristically abundant. Also common are golden moonglow lichen (Dimelaena oreina), Hypogymnia physodes, Physcia subtilis, Xanthoparmelia conspersa, Xanthoparmelia plittii, and Melanelia culbersonii, along with numberous crustose species. Flat surfaces and interstices that have thin deposits of organic matter often support broom-mosses (Dicranum spp.), Hedwig's rockmoss (Hedwigia ciliata) and other bryophytes. These, in turn, provide substrates for a variety of fruticose lichens, including Cladonia rangiferina, Cladonia uncialis, Cladonia crispata, Cladonia furcata, Cladonia macilenta, Cladonia ochrochlora, and Cladonia squamosa . Along the edges of the boulderfields, scattered individuals of Virginia creeper (Parthenocissus quinquefolia), blueberries (Vaccinium spp.), and other vascular plants may occur in transition zones with forests or woodlands.
Rare sandstone and quartzite bounderfields above 1,200 m (4,000 ft) elevation in the Ridge and Valley and Allegheny Mountains of Virginia and West Virginia support a different association of lichens characterized by the dominance of Umbilicaria muehlenbergii and the presence of boreal-alpine species such as Melanelia stygia and Arctoparmelia centrifuga. Boulderfields of basic metamorphic rocks are confined to higher elevations of the northern Virginia Blue Ridge, where the most resistant beds of metabasalt (greenstone) are present. Characteristic species here include Stereocaulon glaucescens, Lasallia papulosa, Chrysothrix chlorina, Usnea halei, Aspicilia cinerea, Diploschistes scruposus, Porpidia spp., Rhizoplaca subdiscrepens, and numerous other crusts. Several boreal-alpine or geographically isolated species are present in this community, including including Cladonia coccifera, Melanelia stygia, Microcalicium arenarium, Parmelia omphaloides, Porpidia tuberculosa, Rhizocarpon geographicum, and Umbilicaria caroliniana.
Progressive, long-term weathering of exposed boulderfields results in slow invasion by trees such as yellow birch (Betula alleghaniensis) and American mountain-ash (Sorbus americana) at higher elevations, and sweet birch (Betula lenta) at lower elevations. Open boulderfields are favored by timber rattlesnakes (Crotalus horridus horridus), which often locate their hibernacula in the rocky substrates. These small-patch community types are uncommon and are primarily threatened by air pollution and acid precipitation.Reference: Fleming et al. (2007), Fleming and Coulling (2001).
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|Large-block quartzite boulderfield with sparse lichen cover, at Blackrock, Rockingham County (Shenandoah National Park).Photo © Gary P. Fleming.|
|The umbilicate "rock tripe" Lasallia papulosa is the most conspicuous lichen on quartzite blocks at Blackrock, Rockingham County (Shenandoah National Park). Photo © Gary P. Fleming.|
|Northern lowbush blueberry (Vaccinium angustifolium) invades the edge of a red sandstone talus field near the summit of Jack Mountain, Highland County (Highland Wildlife Management Area). Photo: Gary P. Fleming / © DCR Natural Heritage.|
|Deeply piled metabasalt (greenstone) debris with diverse crustose lichens on a high-elevation, northern Blue Ridge boulderfield. Hawksill, Shenandoah National Park. Photo: Gary P. Fleming / © DCR Natural Heritage.|
High-elevation acidic boulderfield on sandstone talus at 1280 m (4200 ft) elevation below summit of Sounding Knob on Jack Mountain, Highland County (Highland Wildlife Management Area). The two dominant lichens, Umbilicaria muehlenbergii and Melanelia stygia, form dark patches on the exposed rock surfaces. Photo: Nancy Van Alstine.
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