Abstract:
• Hydraulic vulnerability of Douglas-fir (Pseudotsuga menziesii) branchlets
decreases with height, allowing shoots at greater height to maintain hydraulic
conductance (Kshoot) at more negative leaf water potentials (Ψl).
• To determine the basis for this trend shoot hydraulic and tracheid anatomical
properties of foliage from the tops of Douglas-fir trees were analysed along a height
gradient from 5 to 55 m.
• Values of Ψl at which Kshoot was substantially reduced, declined with height by
0.012 Mpa m−1. Maximum Kshoot was reduced by 0.082 mmol m−2 MPa−1 s−1 for
every 1 m increase in height. Total tracheid lumen area per needle cross-section,
hydraulic mean diameter of leaf tracheid lumens, total number of tracheids per needle
cross-section and leaf tracheid length decreased with height by 18.4 μm2 m−1,
0.029 μm m−1, 0.42 m−1 and 5.3 μm m−1, respectively. Tracheid thickness-to-span
ratio (tw/b)2 increased with height by 1.04 × 10–3 m−1 and pit number per tracheid
decreased with height by 0.07 m−1.
• Leaf anatomical adjustments that enhanced the ability to cope with vertical
gradients of increasing xylem tension were attained at the expense of reduced water
transport capacity and efficiency, possibly contributing to height-related decline in
growth of Douglas fir.
