Height-related trends in leaf xylem anatomy and shoot hydraulic characteristics in a tall conifer: safety versus efficiency in water transport

Hydraulic vulnerability of Douglas-fir (Pseudotsuga menziesii) branchlets decreases with height, allowing shoots at greater height to maintain hydraulic conductance (K-shoot) at more negative leaf water potentials (psi(1)). 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 psi(1), at which K-shoot was substantially reduced, declined with height by 0.012 Mpa m(-1). Maximum K-shoot 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 mu m(2) m(-1), 0.029 mu m m(-1), 0.42 m(-1) and 5.3 mu m m(-1), respectively. Tracheid thickness-to-span ratio (t(w)/b)(2) increased with height by 1.04 x 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.