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Accueil du site → Doctorat → États-Unis → 1978 → ECOLOGY OF THE OAK COMMUNITIES ON THE EASTERN FOOTHILLS OF THE SIERRA MADRE OCCIDENTAL IN CHIHUAHUA

New Mexico State University (1978)

ECOLOGY OF THE OAK COMMUNITIES ON THE EASTERN FOOTHILLS OF THE SIERRA MADRE OCCIDENTAL IN CHIHUAHUA

PEREZ-GARCIA, ALBERTO

Titre : ECOLOGY OF THE OAK COMMUNITIES ON THE EASTERN FOOTHILLS OF THE SIERRA MADRE OCCIDENTAL IN CHIHUAHUA

Auteur : PEREZ-GARCIA, ALBERTO

Université de soutenance : New Mexico State University

Grade : Doctor of Philosophy (PhD) 1978

Résumé
During the summers of 1976 and 1977 thirty-six sites in the oak communities were sampled each year along that eastern foothills of the Sierra Madre Occidental in Chihuahua. These sites were located on areas with minimum disturbance. The vegetation was sampled using quadrat techniques. Parameters estimated for the herbaceous stratum included biomass, frequency and basal cover, and for tree and shrub strata the parameters estimated were canopy cover, density and height. Species checklists and collections of each species encountered were made for each site. Soil samples were collected from small pits dug at the center of each study site. Each sample was analyzed for pH, electrical conductivity, texture, field capacity, organic matter content and soil nutrients, including nitrogen, phosphorus, calcium and magnesium. Results from the quadrat survey showed that in the herbaceous layer seven species were important with regard to cover, frequency and constancy. These were Andropogon gerardi, Lycurus phleoides, Muhlenbergia emersleyi, M. montana, M. rigida, Elyonurus barbiculmis, and Aristida schiendiana, which accounted for 52.1% of the total cover of grass species in the herbaceous layer. Of the 17 tree species found in the tree layer, only five were important with regard to canopy cover, density and constancy. These were Quercus arizonica, Q. emoryi, A. grisea, Pinus cembroides and Juniperus deppeana. Of the 16 shrubby species in the shrub stratum, only five species were important. These were Mimosa biuncifera, M. dysocarpa, Rhus trilobata, Quercus depressipes and Acrospaphylos pungens. In 1976, total herbage biomass averaged 478 kg/ha compared to 367 kg/ha during 1977, indicating environmental variation among sites and effect of climatic conditions between years. In the two years, grasses contributed 96.8% of the total biomass, while forbs contributed only 3.2%. Herbaceous basal cover and biomass were significantly lower at higher elevations than at lower. Tree canopy cover and density were influenced by altitude and exposure ; they decreased from high elevations to low and were higher on north exposures than on southern exposures, probably in response to moisture conditions related to elevation and exposure. Significant differences in shrub canopy and density were found among years and latitude levels, but these results were attributed to environmental variations among sites and localization of southern sites. Factors that contributed most in predicting herbaceous basal cover were altitude, percent slope and percent nitrogen in upper horizon. No equation was developed to predict herbaceous biomass, indicating that none of the measured factors can be used to predict biomass. The site characteristics found most important in evaluation of tree canopy cover included altitude, percent sand content of the upper horizon, and extractable magnesium. Tree density can be predicted using exposure, elevation, depth of upper horizon and percent nitrogen. Environmental factors which contributed most in predicting shrub canopy and density were percent surface stoniness and soil pH. Simple linear correlations between site and vegetational characteristics illustrated relationships and did not contribute greatly to a complete understanding of site-plant complex because of the large number of interactions. The prediction of vegetational characteristics of oak communities as a measure of site potential exhibited wide variability and must be used with caution. Vegetational characteristics were evaluated for the conditions existing in 1976-1977. The predictions must be considered representative of conditions at that time and in relation to the study sites.

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