Transpiração em Árvores Isoladas de um Montado de Azinho. Evolução Sazonal e Condicionantes Hidráulicas^[1]

 

Teresa Soares David*, Isabel Ferreira**, João Santos Pereira**, Shabtai Cohen*** e Jorge Soares David**

*Investigadora Auxiliar

Estação Florestal Nacional. Departamento de Ecofisiologia e Melhoramento Florestal. Quinta do Marquês, Av. da República, 2780-159 OEIRAS

**Professor Catedrático

Instituto Superior de Agronomia. Tapada da Ajuda, 1349-017 LISBOA

***Investigador

Institute of Soil, Water and Env. Sciences, P.O. Box 6, Bet Dagan 50250, Israel

 

Sumário. A transpiração e suas condicionantes foram analisadas, durante cerca de dois anos, numa árvore isolada de Quercus rotundifolia Lam. integrada num montado de baixa densidade da região de Évora. Pretendeu-se contribuir para um melhor conhecimento das estratégias de adaptação da componente arbórea dos montados a condições edafo-climáticas potencialmente adversas, aspecto fundamental no equacionamento da sua sustentabilidade. A evolução sazonal da transpiração evidenciou valores máximos no Verão (3 mm dia^-1), quando o potencial evaporativo da atmosfera é particularmente elevado, apesar de o solo estar, então, seco. Este facto só é explicável pelo acesso directo do sistema radicular a um aquífero existente a 13 m de profundidade. O controlo estomático parece evitar que a transpiração exceda a capacidade máxima de bombagem de água pelas raízes, a partir do solo/subsolo, impondo um limiar máximo de transpiração e impedindo que o potencial foliar desça abaixo de -3,2 MPa, provável limiar crítico de cavitação do xilema.

Palavras-chave: Quercus rotundifolia Lam.; fluxo de seiva; evapotranspiração; lençol freático

 

Abstract. Sapflow and related variables were monitored for two years in an isolated Quercus rotundifolia Lam. tree, in a low density oak woodland (montado) near Évora. The study aimed at identifying tree strategies in adverse soil and climatic conditions in order to understand the conditions for survival and sustainability. Seasonal variations in transpiration, mainly related to solar radiation and vapour pressure deficit, showed a peak in summer (3 mm day^-1) even in dry soil conditions. Water availability to the roots did not change much over time, due to the likely access of the root system to an aquifer located at 13 m depth. Stomatal control prevents transpiration from exceeding the maximum water uptake capacity by the roots. An upper limit is thus imposed on transpiration, preventing leaf water potential to decrease below -3,2 MPa, which may probably be the threshold for cavitation.

Key words: Quercus rotundifolia Lam.; sapflow; evapotranspiration; aquifer

 

Résumé. On a etudié, pendant deux années, l'évolution du flux de sève et de variables dépendantes d'un arbre isolé de Quercus rotundifolia Lam., situé dans un peuplement de basse densité de chêne (montado) près d'Évora. L'étude a envisagé l'identification des stratégies adoptées par les arbres vis-à-vis de la sécheresse climatique et du sol, fondamental pour la compréhension de sa survie. L'évolution saisonnière de la transpiration, correlée surtout avec le rayonnement solaire et le déficit de saturation de l'air, montre un maximum en été (3 mm jour^-1), même en sol sec. Les racines ont certainement une forte capacité d'extraction d'eau d'une nappe phréatique située à 13 m de profondeur. La régulation stomatique évite que la transpiration surpasse la capacité maxime de pompage d'eau des racines, en imposant des contraintes aux taux maximaux de transpiration pour maintenir le potentiel hydrique foliaire au dessus de -3,2 MPa, probable seuil de cavitation xylémique.

Mots clés: Quercus rotundifolia Lam.; flux de sève; évapotranspiration; nappe phréatique

 

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Submetido para publicação em Setembro de 2002

Aceite para publicação em Outubro de 2002

 

^[1]O presente trabalho resume alguns dos aspectos essenciais da 2ª parte da Dissertação intitulada "Intercepção da precipitação e transpiração em árvores isoladas de Quercus rotundifolia Lam.", apresentada por Teresa Soares David, no ISA (UTL), para obtenção do grau de Doutor em Engenharia Florestal em Abril de 2001.

*E-mail: teresa.david@efn.com.pt