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Universidad de Buenos Aires (2001)

Arquitectura hidráulica y relaciones hídricas de árboles de sabanas neotropicales : efectos de la disponibilidad de agua y nutrientes

Bucci, Sandra Janet

Titre : Arquitectura hidráulica y relaciones hídricas de árboles de sabanas neotropicales : efectos de la disponibilidad de agua y nutrientes

Auteur : Bucci, Sandra Janet

Université de soutenance : Universidad de Buenos Aires

Grade : Doctor en Ciencias Biológicas 2001

Résumé
The neotropical savannas are characterized by a continuous herbaceous layer with isolated trees and shrubs. The objective of this study was to assess the functional significance of morphological and physiological traits of woody species in response to temporal and spatial variation in resource availability. This study was carried out in one of the most important neotropical savannas, the Brazilian Cerrado. The variations in resource availability in Cerrado ecosystems are determined by factors such as seasonal precipitation and changes in the air saturation deficit (ASD), soil nutrient content and tree density which affect soil water availability. Tree density increases from campo sujo (960 individuals per ha) to cerradão (3343 individuals per ha), with other physiognomies containing intermediate tree densities. Soil water content differed by about 10% between wet and dry seasons in the upper 300 cm of soil. Air saturation deficits were substantially higher during the dry season. Leaf water potentials of several tree species were more negative during the dry season compared to the wet season. In some cases 2,6 MPa water potential differences were observed between both seasons. Also in both seasons the trees exhibited a strong stomatal control of transpiration. Average stomatal conductance, however, was 50 % lower during the dry season. Despite lower stomatal conductance during this period total daily sap flow per unit leaf area tended to be similar among plants but daily water loss per individuals was significantly lower. As a consequence of a lower total leaf surface area during the dry season, the ratio between leaf area and sapwood cross sectional area (LA/XA) was also lower, resulting in an increase in the water transport efficiency from soil to leaves. Nocturnal transpiration was relatively high due to high ASD, particularly during the dry season. Nocturnal transpiration accounted up to 24% of the total daily water use per tree. Soil water availability was lower in the cerradão below the first 100 cm soil depth compared to the campo sujo sites. The cerradão trees experience passive osmotic adjustment due to the decrease in the symplastic water content. However the leaf water potentials were not different between both sites. The water transport efficiency in each individual tree, and therefore their water economy, were influenced by the size of the internal water storages. Trees with higher transpiration rates did not experienced larger water deficits due to the relatively larger water storages and more efficient water transport system. Diurnal changes in specific hydraulic conductivity observed in two tree species suggest that embolism formation and repair occurred on a daily basis. N and P fertilizations resulted in substantial changes in the hydraulic architecture of trees due to variations in total leaf surface area and stem diameter. Trees in P fertilized sites improved their water relations due to an enhancement of the water transport efficiency (lower LA/XA) which allowed them to maintain higher leaf water potentials. Trees in N fertilized sites had opposite responses in terms of water transport efficiency. The morphological and physiological acclimation responses observed in Cerrado trees have an adaptive value allowing the trees to maintain high flow rates even during the dry season without experiencing water deficits, which allow them to expand new leaves during this unfavorable period for growth.

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Page publiée le 28 septembre 2017, mise à jour le 13 février 2020