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Developmental changes in Sitka spruce as indices of physiological age I. Changes in needle morphology.

There is a paucity of data regarding developmental changes in trees. Measurements of length, width, length/width ratio, perimeter/width and width/height ratios of tranverse sections, projected and total surface areas, dry weight, specific leaf area and the weight of epicuticular waxes per unit leaf area were made on needles sampled from the uppermost whorl of Sitka spruce trees aged 3, 5,10,15, 20 and 38 years. All characteristics, except needle length and the weight of epicuticular waxes, showed asymptotic changes with age that could adequately be described by the Gompertz growth function, and offer potential as indices of physiological age. Asymptotes were reached at different ages (width and the length/width ratio having the slowest rates of change and the perimeter/width ratio, specific leaf area, and the width/height ratio having the fastest) suggesting a predictable sequence of changes. The observed changes in needle morphology are interpreted as a transition from shade to sun leaves.