|De Soyza, Amrita|
|Van Zee, J|
Submitted to: American Midland Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/1996
Publication Date: N/A
Citation: Interpretive Summary: Shrubs such as creosotebush have invaded and now dominate areas that once were mainly grasslands. In order to understand and perhaps predict the future course of these invasions we must understand what makes these shrubs so successfull in desert ecosystems. The shape of creosotebushes is very variable and we suggest this is related to each shrub's relative need for: 1) water, when it is trying to establish itself or 2) nutrients, when it i trying to grow and become larger. In locations where water is scarce, more of these shrubs tend to have steeply angled stems, giving the shrubs a cone shape. This allows the shrubs to direct water toward their stem bases and from there to the roots. In locations where water is more abundant creosotebushes tended to have less steeply angled stems, giving them a hemispherical shape. We found more plant litter under the hemispherical creosotebushes. Mineral nutrients in the soil also were greatest under hemispherical shrubs and least in the bare ground spaces between shrubs an under cone shaped creosotebushes. However, the expected greater quantities and greater diversity of short-lived annual plants under hemispherical shrubs (with more nutrients for plant growth) did not occur. Overall, because creosotebush shape affects nutrient accumulation, shrub shape is an important factor determining the distribution of resources in the warm deserts of the southwestern USA.
Technical Abstract: Differences in creosotebush (Larrea tridentata) crown morphology may reflect changes in the relative demand for water vs. nutrient resources, coinciding with shrub growth and development. Creosotebushes with inverted cone shaped crowns were more abundant in water-limited environments whereas hemispherical shaped creosotebushes were more abundant in less water limited environments. Cone shaped creosotebushes accumulated substantially less litter under their canopies than did creosotebushes with hemispherical shaped crowns. Soil nutrient concentrations under conical shrubs were similar to those in intershrub spaces and both of these were significantly less than soil nutrient concentrations under hemispherical shrubs. In ecosystems where overland flow of water exerted a greater influence on the movement of organic litter than did wind, shrub shape had little effect on long-term litter accumulation. No persistent differences in the biomass or rdiversity of ephemeral taxa exploiting undershrub areas were found, probably because the positive effects of greater nutrient resources under hemispherical shrubs were offset by the limitations imposed by the larger, more dense canopies of hemispherical shrubs. Overall, creosotebush morphology affected litter accumulation patterns and soil nutrient patterns, and must be considered when assessing the heterogeneity of desert ecosystems in the southwestern USA.