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Accueil du site → Doctorat → États-Unis → 1980 → THE DISTRIBUTION OF DECIDUOUS AND EVERGREEN TREES IN MEXICO : THE ROLES OF COMPETITION AND NUTRIENT AVAILABILITY

University of Arizona (1980)

THE DISTRIBUTION OF DECIDUOUS AND EVERGREEN TREES IN MEXICO : THE ROLES OF COMPETITION AND NUTRIENT AVAILABILITY

Goldberg, Deborah Esther

Titre : THE DISTRIBUTION OF DECIDUOUS AND EVERGREEN TREES IN MEXICO : THE ROLES OF COMPETITION AND NUTRIENT AVAILABILITY

Auteur : Goldberg, Deborah Esther

Université de soutenance : University of Arizona

Grade : Doctor of Philosophy (PhD) 1980

Résumé partiel
A plant growth form-soil fertility pattern is described from the Sierra Madre Occidental, Sonora, Mexico. Evergreen oak woodland occurs on acid, nutrient-poor soil derived from hydrothermally altered rock, while subtropical deciduous forest occurs on circumneutral, nutrient rich soil derived from unaltered rock. The soil-based pattern is embedded in an elevational gradient from deciduous forest at low elevations to evergreen oak woodland at higher elevations, where it occurs on both soil types. Quantitative vegetation samples from along the elevational gra dient were classified into deciduous forest or oak woodland types based on a Wisconsin indirect ordination. The two groups are floristically distinct and the deciduous forest has significantly higher plant density and more species present. Two alternative models were proposed to explain the association of evergreen plants with nutrient-poor soils. The competition model is based on costs and benefits of leaf longevity : evergreen plants have a higher mineral use efficiency but pay for this efficiency by "expensive" leaves and slow growth. Hence, when nutrients are not limiting, they are outcompeted by rapidly growing deciduous plants. This model could account for the many examples of the association of evergreen plants in other climatic regimes as well. The abiotic model is based on the eco logical affinities of the particular plants present in the Sierra Madre and does not account for similar patterns elsewhere. Several reasons are proposed for the restriction of evergreen oaks to acid soils in this region : nutrient toxicity, insufficient moisture on nutrient rich soils, poor conditions for symbiotic mycorrhizae, and pathogens found only on nutrient-rich soils. The two models differ in their pre dictions about evergreen seedling growth and survival on nutrient-rich soil : the competition model predicts good growth and survival only in the absence of competition and the abiotic model predicts no growth and survival regardless of competitive regime. Both models assume that deci duous plants are not found on acid soils due to nutrient limitation. Field experiments were carried out in the Sierra Madre to test these predictions, using Lysiloma divaricata as representative of the deciduous species and Quercus albocincta as representative of the ever green species. In the oak woodland, nutrient availability was increased by raising the pH of the acid soil with calcium carbonate to test the assumption that deciduous plants are limited by nutrient availability and to test the nutrient toxicity hypothesis for evergreen plants. As expected, Lysiloma (deciduous) survived only in plots with calcium carbonate. Quercus (evergreen) showed no significant response to cal cium carbonate, thus eliminating the nutrient toxicity hypothesis. In the deciduous forest, competition was reduced by removing all combina tions of three structural components : herbaceous plants, shrubs and roots of canopy trees. Survival and growth of Lysiloma was highest in the complete removal. No plants survived in the control and only one seedling survived with removal of only one vegetation component.

Mots clés : Evergreens — Mexico — Sonora (State) ; Trees — Mexico — Sonora (State) — Growth. ; Plant competition — Mexico — Sonora (State) ; Plant-soil relationships.

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