D. Loustau et al., TRANSPIRATION OF A 64-YEAR-OLD MARITIME PINE STAND IN PORTUGAL .1. SEASONAL COURSE OF WATER FLUX THROUGH MARITIME PINE, Oecologia, 107(1), 1996, pp. 33-42
The transpiration, sap flow, stomatal conductance and water relations
of Pinus pinaster were deter mined during spring and summer in a 64-ye
ar-old stand in Ribatejo (Portugal). The transpiration of the pine can
opy was determined from sap flow or eddy covariance techniques. Canopy
conductance values (g(c)) were estimated from inversion methods using
eddy covariance or sap flow data, respectively, and from scaling-up m
ethods using stomatal conductance values measured in the field and lea
f area index (LAI) values. The transpiration was closely controlled by
the stomatal conductance of pines (Omega was 0.05-0.15). For wet soil
conditions, the various estimates of g(c) showed reasonable agreement
. g(c) peaked in the morning at 0.01 m x s(-1), exhibited a midday dep
ression and showed a secondary peak in late afternoon. This behaviour
could be predicted simply on the basis of the stomatal sensitivity to
air vapour pressure deficit. On a seasonal basis, monthly average valu
es of g(c) decreased from 4 x 10(3) m x s(-1) in spring to 1.7 x 10(-3
) m . s(-1) in late summer. Accordingly, the transpiration peaked at 3
mm x d(-1) on wet soil in May. It decreased progressive ly during the
summer drought to 0.8 mm x d(-1) at the end of August. The minimal va
lue of needle water potential was maintained at -1.9 MPa but predawn v
alues de creased from -0.6 MPa in May to -0.9 MPa in July. It may have
reached lower values in August. The amount of water stored in the tru
nk accounted for a 12% (10 kg x tree(-1) x day(-1)) of the daily trans
piration in spring. The storage capacity of the canopy was within the
same order of magnitude. The trunk storage increased to 25% (13 kg x t
ree(-1) x day(-1)) of the daily transpiration at the end of summer und
er drought conditions. The sap flow beneath the crown lagged according
ly behind transpiration with a time constant estimated between 26 min
in spring and 40 min at the end of summer.