ESTIMATING TREE CANOPY WATER-USE VIA XYLEM SAPFLOW IN AN OLD NORWAY SPRUCE FOREST AND A COMPARISON WITH SIMULATION-BASED CANOPY TRANSPIRATION ESTIMATES
B. Kostner et al., ESTIMATING TREE CANOPY WATER-USE VIA XYLEM SAPFLOW IN AN OLD NORWAY SPRUCE FOREST AND A COMPARISON WITH SIMULATION-BASED CANOPY TRANSPIRATION ESTIMATES, Annales des Sciences Forestieres, 55(1-2), 1998, pp. 125-139
Tree xylem sapflow rates of 140-year-old Norway spruce (Picea abies) w
ere scaled to the stand level and compared to canopy transpiration pre
dicted by the stand gas exchange model STANDFLUX. Variation in sapflux
densities between individual sensors was high (coefficient of varianc
e = 0.4) and included both variation within and between trees, but it
was not different between two applied sapflow methodologies (radial fl
owmeter according to Granier, variable heating tissue heat balance met
hod according to Cermak and Kucera). During the morning, a time-lag of
typically 2 h elapsed between sapflow (E-f) and predicted canopy tran
spiration rate (E-p). During this time total water use was as high as
0.3 mm, which was less than the estimated capacity of easily available
water in the tree canopy (0.45 mm, on average 14 L per tree). Canopy
conductance derived from stand sapflow rates (g(f)) and from STANDFLUX
(g(p)) was in the same range (g(tmax): 10 mm s(-1)), but a stronger d
ecline with increasing vapor pressure deficit of the air (D) was obser
ved for g(f) as compared to g(p) with current model parameterization.
Tree water uptake measured by xylem sapflow was higher during spring a
nd somewhat lower during summer compared with E-p. Seasonal sums of tr
anspiration from April to October amounted to 108 and 103 mm season(-1
) for E-f and E-p, respectively. Estimated tree water uptake during ni
ght increased with D up to 0.5 mm per dark period (on average 16 L per
tree) which was 10-140 % of total daily flux. Because this flow rate
did not increase with further increases in D during night, it is concl
uded that it reflects the refilling of easily exchangeable water in th
e trees rather than a rate of night transpiration. ((C) Inra/Elsevier,
Paris.).