Endemism in Rock Outcrop Plant Communities of Unglaciated Eastern United States: An Evaluation of the Roles of the Edaphic, Genetic and Light Factors

Abstract

Within the unglaciated eastern United States, where the natural vegetation is mostly deciduous forest, edaphically-controlled herbaceous plant communities occur on unshaded rock outcrops in shallow soil developed from serpentine, shale, granite, limestone, dolomite, sandstone, chert, rhyolite and other rock types. The most distinctive features of the flora of these communities are their endemic taxa. Many of the endemics are not restricted to a single geological substratum, and they grow equally well, or better, in non-outcrop as in outcrop soil. Thus, it seems unlikely that they are restricted to the outcrops because of a requirement for some chemical, physical or biological element found there but nowhere else. Several of the endemics exhibit intraspecific variability in one or more of the following characteristics: morphology, physiology, breeding system, flower colour and enzyme pattern. Thus, it is unlikely ' that lack of genetic variation per se is the cause of endemism. A requirement that the endemics have in common for growth and survival is a high photosynthetic photon flux density. The obligate nature of the high light requirement is supported by: (1)field observations that the endemics grow on well-lighted portions of the outcrops but not in adjacent forests, (2) studies which show that growth and photosynthetic rates are maximal in high light but are severely inhibited at low light levels, and (3) field and glasshouse experiments which show that the endemics are shade intolerant and that they compete poorly with taller plants. Thus, of the three hypotheses examined to account for the proximal causes of endemism in the rock outcrop endemics, the requirement for a high level of light is the most important.

The Edaphic Factor in the Origin of Plant Species

“…This sequence encompasses an evolutionary history from the initial tolerance of the habitat by certain preadapted variants to clear-cut species formation. These stages can be appropriately applied to other forms of geoedaphic challenges-mine soils with heavy metals (Antonovics et al, 1971;Bradshaw et al, 1990), guano (Gillham, 1956;Ornduff, 1965;Vasey, 1985), vernal pools (Holland andJain, 1977, 1981), granite outcrops (Wyatt and Fowler, 1977;Ornduff, 1986), salt marshes (Flowers et al, 1986), gypsum (Turner, 1973;Turner and Powell, 1979), dolomite (Mooney, 1966;Lloyd and Mitchell, 1973), and limestone (Baskin and Baskin, 1988;Quarterman et al, 1993)-also leading to the formation of edaphically endemic taxa.…”

Occupation of bare habitats, an evolutionary precursor to soil specialization in plants

“…Understanding why some species become restricted to particular soils has challenged biologists for almost a century because most soil endemics are able to grow in more benign substrates (9,(32)(33)(34)(35)(36). Costs associated with adaptations to harsh environments are hypothesized to result in reduced competitive ability in zonal (regionally common) soils (9,37), and have been the main paradigm to explain narrow soil endemism (9,35,36).…”

“…Costs associated with adaptations to harsh environments are hypothesized to result in reduced competitive ability in zonal (regionally common) soils (9,37), and have been the main paradigm to explain narrow soil endemism (9,35,36). Lately, tradeoffs in competitive ability associated with defense against herbivores and pathogens have also been implicated in the restricted distributions…”

Differences in Functional Trait Distribution between Inselberg and Adjacent Matrix Floras

“…Inselbergs are harsher and drier environments compared to the matrix in which they are embedded. Inselbergs shed up to 95% of precipitation, the soils are skeletal, humic, and highly acidic [10,12,[14][15][16][17][18][19][20][21][22]. Temperatures up to 18 ∘ C higher than the ambient air and in excess of 60 ∘ C within the soil have been recorded; greater exposure to wind and solar radiation means that the thin soils rapidly dry out [16,20,[22][23][24][25].…”