Water flux in a hybrid poplar stand.

We studied water flux in a four-year-old stand of hybrid Populus during midsummer 1992. Study trees ranged in height from 11.0 to 15.1 m and in diameter from 8.3 to 15.1 cm. The large-leafed Populus hybrid was relatively poorly coupled to the atmosphere. The average value of the stomatal decoupling coefficient, Omega, was 0.66, indicating that, on average, a 10% change in stomatal conductance would result in only a 3 to 4% change in transpiration. During the middle of the summer, the smallest study tree used between 20 and 26 kg of water per day, whereas the largest tree used between 39 and 51 kg day(-1). The maximum observed rate of stand water loss was 4.8 mm day(-1) in this Populus clone. Maximum rates of sap velocity within the xylem were as high as 12.5 m h(-1); measured rates for exposed sunlit branches approached 90% of this maximum. Within-canopy patterns of stomatal conductance generally reflected patterns of incident radiation. Stomatal conductance of foliage grown in shade, even when exposed to non-limiting light and water source conditions, did not increase appreciably. Patterns of stomatal conductance under limiting and non-limiting conditions suggested that both stomatal conductance and leaf specific hydraulic conductivity (LSHC) were linked with the ability to exploit the light resource.