How will climate change affect the way trees allocate carbon to root and shoot ?

and any change in climate which alters the length of each phase is likely to alter the root shoot ratio, with effects on forest respiration and photosynthesis. Hence, we need accurate estimates of root and shoot growing periods to predict growth in a changing environment but most models of carbon and nitrogen allocation in trees do not take account of seasonal phases of growth (eg. Thornley, 1991). In this report we have reanalysed a detailed study of Sitka spruce (Picea sitchensis) by McWilliam (1972) which suggests strong links between shoot extension and xylem growth, and between root growth and the formation of next year's buds. The analysis provides a framework for building future models. McWilliam described the sequence of growth in 22-year-old Sitka spruce trees in Fetteresso Forest, Scotland. His data for 1970 are summarised in Fig. 1 which shows that: 1. budburst was synchronised throughout the tree in the second week in May; 2. the growing season ended throughout the tree in the second week in Oc-tober; 3. there were two phases of shoot growth. In the first, the previous year's buds expanded to form new shoots. Extension then gave way to a phase in which next year's bud was formed by laying down needle primordia. The switch was latest in the leading shoot, earliest in the lower branches. Such 'determinate growth' is common among conifers, but McWilliam went on to show that 4. xylem cell division was closely synchronised with shoot extension at all levels in the crown (Fig. 1). They started together at bud burst, throughout the tree, and ended together at any one height. 5. elongation of fine roots (and to some extent, phloem growth) was suspended during the phase of shoot extension.