Within-crown foliar plasticity of western hemlock, Tsuga heterophylla, in relation to stand age

Anatomy and morphology of needles from six different positions within the crowns of western hemlock, Tsuga heterophylla(Raf.) Sarg., were studied to investigate the plastic response of leaf structure to crown position, in relation to stand age. Trees were sampled from three stands, all of which originated following catastrophic fires, representing a chronosequence (15, 55 and 145 years old) of stand development. Needles from the highest and outermost crown positions showed morphological and anatomical responses normally associated with 'sun leaves'. As a measure of expressed plasticity, we calculated a relative trait range (RTR) index to quantify differences in expressed traits between needles from the upper outer crown (UO needles) and those from the lower inner crown (LI needles). RTR was positive for most morphological and anatomical traits, indicating larger trait measurements on UO needles relative to LI needles (e.g. UO needles were thicker and had more vascular tissue than L I needles). However, the degree to which sun and shade leaf traits were expressed varied between stands. In the youngest stand, UO needles had higher maximal rates of photosynthesis than LI needles (positive RTRPs). but this trend was reversed in the oldest stand (negative RTRPs), Mean RTR across all morphological and anatomical traits was higher in the 145-year-old stand than in either the 55-year-old stand (P = 0.03) or the 15-year-old stand (P < 0.01). While this probably relates mainly to differences in light conditions between the three stands, it may also indicate a connection between ontogeny and phenotypic plasticity. (C) 2001 Annals of Botany Company.