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Ecological Studies of the Vegetation of the Great Smoky Mountains of North Carolina and Tennessee. I. Soil Reaction and Plant Distribution

Stanley A. Cain
Botanical Gazette
Vol. 91, No. 1 (Mar., 1931), pp. 22-41
Stable URL: http://www.jstor.org/stable/2471074
Page Count: 20
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Ecological Studies of the Vegetation of the Great Smoky Mountains of North Carolina and Tennessee. I. Soil Reaction and Plant Distribution
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Abstract

1. Soils show striking variations in reaction: (a) For all soil samples tested the range was from pH 2.8 to 8.2, representing a range in "active acidity" from 16,000 to zero, and from zero to 16 in "active alkalinity." (b) The plant associations show ranges in active acidity of considerable extent, the most acid sample in an association being from about 3 to 300 times as acid as the least acid sample. In terms of pH this means a range from 4.3 to 4.8, as in the beech orchard, and from 4.6 to 6.8, as in the chestnut, based on 120 and 63 tests respectively. (c) Variations between three tests per sample averaged about 0.1 pH from the mean of the three tests, which has increased significance with higher acidities, of course, because of the logarithmic nature of the numbers. Variations ranged from zero divergence of three tests to as much as from pH 4.5 to 5.2. (d) Surface soils tend to show greater variations than subsoils. 2. In respect to the vegetation, certain conclusions as to soil reaction may be drawn: (a) Plant associations show wide ranges of tolerance of hydrogen-ion concentration. (b) No two contiguous associations can be separated on a basis of reaction alone because of the considerable extent of overlapping both in surface and subsoils, although one association may be definitely more acid than another as a general rule (on a basis of averages). (c) Despite this situation it is apparent that the higher acidities particularly exert a considerable influence on the floristic composition of the plant associations. This is probably partly direct (influence of acidity) and partly indirect, because of the influence of peat soils and concomitant factors on the elimination of species. 3. In respect to the general high acidity, certain phenomena are conspicuous: (a) Surface soils are generally more acid than subsoils. (b) Soils of high altitudes are generally more acid than soils of lower altitudes. In a broad way this situation is apparent, but peculiar features of certain associations may enter to upset the perfect altitudinal sequence, as the evergreen habit and peat formation. The altitudinal difference is clearer when we compare identical associations, as chestnut woods at 3000 feet, with an "active acidity" of 55 and at 4700 feet with an "active acidity" of 250, or the heath bald with subsoil acidities of 980 at 5000 feet and 1600 at 6600 feet altitude. 4. Concrete evidence of one type of vegetation being replaced by a more acid-tolerant type, as a result of the tendency of soils in a humid climate to become progressively more acid, with an accompanying altitudinal depression of the limits of zones, has not been obtained as yet but is in all probability a historical fact.

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