Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K (S)), leaf blades (K (lb)), petioles (K (P)) and branches (i.e. leafless stem; K (B)) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200-250 mu mol m(-2) s(-1), using different light sources. K (lb) depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A (L)). K (lb) increased from crown base to tree top, in parallel with vertical patterns of A (L). However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A (L) the higher K (lb). Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K (S), K (B), K (lb)) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K (S) increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.