Effect of microtopography on composition and structure of vegetation in an elm-hackberry forest

Date

1987-08

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Publisher

Texas Tech University

Abstract

This study correlated topo-edaphic factors and vegetation pattern in a bottomland hardwood forest near Houston, Texas. Elevation-induced, seasonal flooding occurred in swales within the flat alluvial deposits of the Brazos River, creating an aeration gradient which influenced vegetation composition and structure. Environmental variables measured included relative elevation, water level (checked monthly), soil water content, and general textural classes of horizons in the profile. Swales were inundated from fall to spring, and always had higher soil water content than ridges or flats. The soils of swales were made up of heavy clays, but more elevated areas often had coarser horizons. Soil nutrient status was also examined: swales had higher levels of phosphorous, potassium, and salinity than flats and ridges. This topo-edaphic complex created an aeration gradient strongly influencing vegetation.

Reciprocal averaging ordination was the major tool in vegetation pattern analysis. Woody vegetation had species selectively removed with decreasing relative elevation, and only five tree species could tolerate flooding stresses associated with this site: Celtis laevigata, Fraxinus pensvlvanira. Acer neaundo, Ulmy§ crassifolia, and R. rubra. These species were common in unflooded areas as well.

Herbaceous vegetation was split into two types: a species-poor type inhabited swales, and a diverse type, which shared few species with swales, was present in flats and ridges. Swales were dominated by Panicum aymnocarpon with few other species present, those germinating mostly after waters receeded. Flats were dominated by sedges (Carex spp.), and as relative elevation increased, a gradual exchange of species occurred, until ridges were dominated by Oplismenus hirtellus.

Vegetation structure was moderately affected by flooding. Tree density was lower in swales, and tree size was generally smaller. Additionally, tree size distribution was affected by the aeration gradient: swales had fewer seedlings and fewer large, mature trees than did unflooded flats and ridges.

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