Gross Modifications in Certain Plant Species Tolerant of Calcium Sulfat Dunes

Abstract

The survival of vegetation in dune regions ordinarily depends upon a species plasticity which permits one or more of several possible adjustments to moving sand. A dune tolerant plant can usually endure temporary complete or permanent partial burial of the crown, elongate beyond accumulating drifts, greatly increase absorptive surface, withstand extensive exposure of the root system, or check the undermining action of wind erosion. Burial of vegetation, which usually results in defoliation and death, in certain plant species causes stem elongation which keeps young foliar portions of the crown sufficiently aibove the sand surface for survival. Such stems ordinarily produce adventitious roots at any level covered by moist sand. Certain species anchor the drifting substrate by a tangled growth of stems or roots. A number survive by the meristematic activity of creeping rhizomes. A few, chiefly the annuals, propagate successfully on and between the dunes by seed alone. The present study was initiated to discover gross modifications, especially in the main axes, of plant species growing in the nitrogen deficient gypsum dunes of the White Sands south and west of Alamogordo, New Mexico, and to compare this flora with that of certain quartz sand dunes. Locale.-This drifting gypsum deposit, which covers approximately 224 square miles, differs from the usual dune area in that the substrate is 96-97% calcium sulfate (Coville and MacDougal, 1903; Byers, 1936). Over a range approximately 28 miles long and 8 to 10 miles wide brilliantly white dunes 20 to 40 feet high alternate with narrow, wind-formed hollows. Windward slopes are long, rising gently at about a 300 angle, while leeward slopes are formed by sand slipping over the dune crests and settling at an angle of approximately 700. The darker, lighter weight impurities are blown free of the drifts where the winnowed gypsum, being heavier, remains. The water table in the flats lies 2 to 3 feet below the surface, and the pH of the substrate is 7.5 (Byers, 1936). Only one of the several soil chemists who have analyzed this gypsum sand reports any trace of nitrogen. Quantitative tests also indicate insufficient amounts of potassium and phosphorus for normal plant growth. Evolution of the dunes.-The White Sands lie in the Tularosa basin, which at this point contains gypsum carried from the Chupadora formation in the mountains on either side and deposited as valley fill, much of it in standing water, to a depth of 1000 feet (Potter, 1938). Gypsum forming the gradually growing dunes continues to 'be derived by wind excavation of an adjacent alkali flat, which represents the surface of the original de-