Serpentine Geoecology of Eastern North America: A Review

Serpentine outcrops are model habitats for geoecological studies. While much attention has been paid to serpentine outcrops worldwide, the literature on eastern North American serpentine and associated biota is scant. This review examines the available literature, published and unpublished, on geoecological studies conducted on serpentine in eastern North America, from Newfoundland through Québec and New England south to Alabama. Most serpentine outcrops in the region have been mapped, but there have been few intensive mineralogical and pedological investigations. The limited soil analyses available suggest elevated levels of heavy metals such as Ni, near-neutral pH values, and Ca∶Mg ratios < 1, characteristic of serpentine soils worldwide. Botanical studies to date have largely focused on floristic surveys and the influence of fire exclusion and grazing on indigenous vegetation. To date, 751 taxa of vascular plants belonging to 92 families have been reported from serpentine outcrops in the region. Two taxa, Agalinis acuta and Schwalbea americana, are federally endangered in the United States while many others are listed as rare, endangered, or imperiled in one or more states or provinces. Globally, six species, Adiantum viridimontanum, Minuartia marcescens, Pycnanthemum torrei, S. americana, Scirpus longii, and Symphyotrichum depauperatum are listed as imperiled (G2) while one species, Agalinis acuta, is listed as critically imperiled (G1). Cerastium velutinum var. villosissimum is the only recognized serpentine endemic plant for eastern North America while Adiantum viridimontanum, Aspidotis densa, M. marcescens, and S. depauperatum are largely restricted to the substrate. Based on current distributions, we propose that A. viridimontanum and M. marcescens be considered endemic to serpentine substrates in eastern North America. Studies on cryptogams list 165 species of lichens and 146 species of bryophytes for the region. None of the species found appear to be restricted to the substrate. Compared to other regions of the world, ecophysiological and evolutionary investigations are scant. Biosystematic investigations are restricted to the taxa Adiantum aleuticum, C. velutinum var. villosissimum, and S. depauperatum. Plant-soil relations, especially the capacity to hyperaccumulate metals such as Ni and the ecological consequences of metal accumulation, are also under explored. One report from eastern Canada lists Arenaria humifusa, M. marcescens, Packera paupercula, and Solidago hispida as hyperaccumulating Ni although the findings have yet to be confirmed by subsequent investigations. Overall, serpentine geoecology in eastern North America remains largely unexplored.