Seasonal Evolution of the Evapotranspiration Regime and Carbon Assimilation Over a Eucalyptus globulus Plantation

 

João Mateus*¹, Gabriel Pita**, Abel Rodrigues*** and Hélène Oliveira***

 

*Environment Engineer

**Assistant Professor

Departamento de Engenharia Mecânica. Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 LISBOA

***Senior Researcher

***Chemical Engineer

Departamento de Silvicultura e Produtos Florestais. Estação Florestal Nacional, Av. da República, Quinta do Marquês, 2780-149 OEIRAS

 

Abstract. Seasonal patterns of carbon assimilation and evapotranspiration of 2004 in Eucalyptus globulus plantation of the CarboEurope-IP Portuguese site of Herdade da Espirra are discussed. The atmospheric fluxes were obtained by the eddy covariance method. A separation of atmospheric carbon flux, or net ecosystem exchange (NEE), in gross primary production (GPP) and ecosystem respiration (R_eco), was made and analysed the variation of atmospheric fluxes with some micrometeorological variables. The plantation acted as a strong carbon sink, with a NEE of 7.9tonC.ha^-1.yr^-1. The diurnal NEE was mainly a function of global solar radiation, with which it is perfectly in phase all year around, except for the period between July and September. In these months, stomatal closure, strongly dependent on the water vapour pressure deficit in the atmosphere (WVD) high values and low water availability, was the main factor controlling carbon assimilation, a tendency already noticed in 2002 and 2003. The evapotranspiration is clearly controlled by water vapour deficit with decoupling factors ( Ω ) varying from 0.1 to 0.4, typical data for forest canopies. In the period from July to September evapotranspiration dependency on WVD increases as a consequence of the stomatal closure.

Key words: seasonal patterns; evapotranspiration; carbon; radiation; vapour pressure

 

Evolução Sazonal dos Regimes de Evapotranspiração e da Assimilação do Carbono numa Plantação de Eucalyptis globulus

Sumário. São discutidos os padrões sazonais de assimilação de carbono e do regime de evapotranspiração numa plantação de Eucaliptus globulus no site de Herdade da Espirra do Programa Europeu CarboEuroflux. Os fluxos atmosféricos foram obtidos pelo método de covariância turbulenta. Foi estabelecida a decomposição do fluxo de carbono atmosférico ou balanço líquido de carbono nas componentes de respiração do ecossistema e produção primária bruta e analisada a variação dos fluxos com alguns parâmetros meteorológicos. A plantação funcionou como sumidouro substancial de carbono, à taxa de 7.9tonC.ha^-1.ano^-1. O balanço líquido diurno foi especialmente dependente da radiação solar global, em regime de fase concordante ao longo de todo o ano, exceptuando o período entre Julho e Setembro. Nestes meses o encerramento dos estomas, fortemente dependente do elevado défice de pressão de vapor da atmosfera e baixo teor de água disponível, foi o principal factor de controlo da assimilação de carbono, tendência já evidenciada em 2002 e 2003. A evapotranspiração é claramente controlada pelo défice de pressão de vapor atmosférico com coeficientes de desacoplamento variando entre 0.1 e 0.4, valores típicos para cobertos florestais. No período compreendido entre Julho e Setembro a dependência da evapotranspiração relativamente ao défice de pressão de vapor aumenta em consequência do fecho dos estomas.

Palavras-chave: padrões sazonais; evapotranspiração; carbono; radiação; pressão de vapor

 

Évolution Saisonnière des Régimes d'Évapotranspiration et d'Assimilation de Carbone dans une Plantation d'Eucalyptus globulus

Résumé. Ci-après nous discuterons des modèles saisonniers d'assimilation de carbone et du régime d'évapotranspiration d'une plantation d'Eucalyptus globulus sur le site de la Herdade da Espirra et conformément au Programme Européen CarboEuroflux. Les flux atmosphériques ont été obtenus par la méthode de covariance turbulente. La décomposition du flux de carbone atmosphérique ou échanges de carbone liquides a été établie dans ses composantes, en respiration de l'écosystème et en production primaire brute. La variation des flux a été analysée suivant certains paramètres météorologiques. La plantation a fonctionnée comme un fort « puit » de carbone, présentant un taux de 7.9ton C.ha^-1.année^-1. Les échanges de carbone avec l'écosystème ont surtout été dépendants de la radiation solaire globale, en régime de phase concordante au long de toute l'année, sauf pour les mois de Juillet à Septembre, inclus. Pendant ces mois, la fermeture des stomates, fortement dépendante de l'élevé déficit en pression de vapeur atmosphérique ainsi que du bas taux d'eau disponible, a été le facteur principal du contrôle de l'assimilation du carbone, tendance déjà vérifiée en 2002 et en 2003. L'évapotranspiration est clairement contrôlée par le déficit de pression de vapeur atmosphérique présentant comme valeurs des coefficients de désaccouplement entre 0.1 et 0.4, valeurs typiques pour des couverts forestiers. Pendant la période comprise entre Juillet et Septembre la dépendance de l'évapotranspiration envers le déficit de pression de vapeur augmente due à la fermeture des stomates.

Mot clés: modèles saisonniers; évapotranspiration; carbone; radiation; pression de vapeur

 

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Entregue para publicação em Janeiro de 2005

Aceite para publicação em Junho de 2005

 

¹ 1º Author E-mail: jamateus@ist.utl.pt