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The rainfall intercepted by an isolated olive tree was measured in a traditional olive-grove/ pasture system with a sparse canopy cover. Results from a two-year period of observations are presented. The data are then used to test models of the interception process in this type of agricultural system. Modelling was performed at the single tree level using the sparse-forest version of the Gash analytical model combined with two other methodologies: the wet bulb approach, to estimate the evaporation rate from the wet canopies of individual olive trees, and a newly developed procedure to estimate the canopy structure parameters. Good model performance was achieved at the storm level with model simulations within 1.5% of the observed value, clearly within the expected error of interception loss measurements.
Contém referências bibliográficas
Dissertação de Doutoramento em Engenharia Florestal, apresentada ao Instituto Superior de Agronomia da Universidade Técnica de Lisboa
Mediterranean evergreen oak woodlands of southern Portugal (montados) are savannah-type ecosystems with a widely sparse tree cover, over extensive grassland. Therefore, ecosystem water fluxes derive from two quite differentiated sources: the trees and the pasture. Partitioning of fluxes according to these dif- ferent sources is necessary to quantify overall ecosystem water losses as well as to improve knowledge on its functional behaviour. In southern Iberia, these woodlands are subjected to recurrent droughts. Therefore, reaction/resilience to water stress becomes an essential feature of vegetation on these ecosys- tems. Long-term tree transpiration was recorded for 6 years from a sample of holm oak (Quercus ilex ssp. rotundifolia) trees, using the Granier sap flow method. Ecosystem transpiration was measured by the eddy covariance technique for an 11-month period (February to December 2005), partly coincident with a drought year. Pasture transpiration was estimated as the difference between ecosystem (eddy covari- ance) and tree (sap flow) transpiration. Pasture transpiration stopped during the summer, when the sur- face soil dried up. In the other seasons, pasture transpiration showed a strong dependence on rainfall occurrence and on top soil water. Conversely, trees were able to maintain transpiration throughout the summer due to the deep root access to groundwater. Q. ilex trees showed a high resilience to both sea- sonal and annual drought. Tree transpiration represented more than half of ecosystem transpiration, in spite of the low tree density (30 trees ha1) and crown cover fraction (21%). Tree evapotranspiration was dominated by transpiration (76%), and interception loss represented only 24% of overall tree evaporation.
