Microhabitat Relationships of Six Major Shrubs in Navajo National Monument, Arizona

Abstract

Six shrub species were studied to determine their microhabitat relationships as well as their effect on the immediate environment. Analysis of site characteristics and mineral composition of soils in open areas adjacent to shrubs and beneath shrubs allowed for comparison of the different habitats following shrub establishment. Soil pH differs beneath the various shrubs and all six species tended to create more alkaline soils beneath their canopy. All species showed increased soil salinity beneath their canopy. However, the concentration of total soluble salts in the soil surface beneath the shrubs varied with the species and was highest beneath fourwing saltbush. Significant increases in the concentration of magnesium and potassium ions beneath shrubs were observed. Nitrogen and phosphorus were also found in greater concentration beneath the shrub canopy. Soil depth differed beneath the shrub species, with sagebrush and fourwing saltbush growing on the deeper more highly developed types. There was a positive relationship between the presence of shrubs and the depth of the soil profile. It has long been known that shrubs influence the soil characteristics beneath their canopy (Fireman and Hayward 1952). Studies done in the last decade show that shrubs influence both the horizontal and vertical patterning of soil chemicals (Charley and West 1975). Sharma and Tongway (1973) studied the effects of two species of saltbush (Atriplex nummularia and Atriplex vesicaria) on soil salinity and related properties. They recognized an accumulation zone beneath the shrubs, a zone of compensation where leaching of minerals replaced those absorbed, and ‘a depletion zone from which mineral absorption exceeded the rate of replacement. Salt accumulation under shrubs, therefore, represents a redistribution of salts from peripheral regions of root activity (Charley and West 1975). Plants employ several “strategies” for the removal of mineral wastes. Leaching of minerals from leaves is accomplished when the solution of the leaf surface connects with that of the intercellular spaces of the mesophyll and ions are lost by diffusion. Salts are also excreted with the aid of salt glands, as is the case with many Atriplex species. Salts are also concentrated in the extracellular spaces of leaves during the process of transpiration, thus accumulating minerals in leaves. Leaf fall subsequently serves as a mechanism for the elimination of mineral wastes (Epstein 1972). Charley and West (1975) used the distributional patterns of carbon and nitrogen under and between shrubs as a measure of