T.S. David · M.I. Ferreira · J.S. David · J.S. Pereira Transpiration from a mature Eucalyptus globulus plantation in Portugal during a spring-summer period of progressively higher water deficit Received: 26 January 1996 / Accepted: 20 October 1996 Abstract The rates of transpiration from a mature Eu- calyptus globulus Labill. stand in Portugal were evalu- ated during a drying period of the spring-summer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental period daily transpiration varied between 3.64 and 0.50 mm day )1 . For high- transpiration days, a good agreement was observed be- tween Penman-Monteith estimates and sap flow mea- surements, both on a daily and on an hourly basis. However, for low-transpiration days, the Penman- Monteith model overestimated transpiration in com- parison with the sap flow method. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods through a progressive stomatal closure. As soil moisture deficit increased, the daily maximum sto- matal conductance decreased from 0.46 to 0.14 cm s )1 . The results also show that, on a seasonal basis, stomatal conductance and daily transpiration were mainly related to predawn leaf water potential and, thus, to soil mois- ture content. Key words Transpiration · Eucalyptus globulus Sap flow · Water deficit Introduction Eucalyptus globulus has been extensively planted in Portugal for pulpwood production. Despite its economic importance, large-scale planting of this exotic species has been criticised because of reported nutrient and water resources depletion. Among other issues, the en- vironmental impact of this fast-growing species is not yet clear due to difficulties in the evaluation of the water consumption through evapotranspiration. One of the primary objectives of this study was to estimate the water use from a mature E. globulus stand under long-term soil water deficit. Transpiration, which is the largest component of evapotranspiration under such conditions, is not however easy to assess in natural conditions due to limitations of the currently used models, difficulties in measuring accurately all the vari- ables and parameters required by the more complex models, and technical problems and usually high costs of direct measurements in tall and aerodynamically rough vegetation. In order to overcome some of these prob- lems, two independent approaches were used in this study to evaluate E. globulus transpiration: the Granier sap flow measuring technique (Granier 1985, 1987a,b) and the Penman-Monteith model (Monteith 1965). Possible discrepancies between the results obtained from these two different procedures would be of great value as a tool for the diagnosis of limitations both on the measuring technique and on the model used. Addition- ally, the validation of a predictive model against mea- sured data would allow a reliable extrapolation of the results, both in time and in space, in future work. Another important objective of this study was to improve the understanding of the evolution in time of transpiration from E. globulus canopies during drying periods, by assessing the role of the main controlling factors. Previous work on E. globulus stands has shown that transpiration at the leaf level under plant water stress was mainly controlled by stomatal closure in re- sponse to vapour pressure deficit and correlated with predawn leaf water potential (Pereira et al. 1986, 1987; Oso ´rio and Pereira 1994). It was also found in those studies that the part of the photoperiod during which stomata were open decreased as soil water deficits in- creased. In this study the seasonal changes in stomatal Oecologia (1997) 110:153 – 159  Springer-Verlag 1997 M.I. Ferreira · J.S. David (&) 1 · J.S. Pereira Instituto Superior de Agronomia, Tapada da Ajuda, 1399 Lisboa Codex, Portugal fax: 00 351 1 3645000; e-mail: Prostep@isa.utl.pt Present address: 1 Estac ¸a ˜o Florestal Nacional, Rua do Borja 2, 1350 Lisboa Codex, Portugal