Genetic and environmental control of crown development in Picea sitchensis and its relation to stem wood production

Differences in stem wood production were found among four clones of Picea sitchensis (Bong.) Carr. at both an agricultural and a forest site. Clonal rankings were not consistent between the sites. Four variables were identified that may influence stem wood production; foliage amount, total above-ground production per unit foliage, proportional allocation of production to new foliage and to branch wood thickening. Comparison of clonal performance between sites showed that stem wood production could be influenced by differences in each of these variables. The four variables were themselves determined by components of crown structure, these were (i) production of new branches from the main stem, (ii) the numbers and lengths of branchlets supported on branches from the main stem, (iii) foliage production per unit branchlet length and foliage longevity, and (iv) branch wood thickening per unit branch length. These components varied both among clones and between sites. Branch production from the mainstem varied among clones, and for some clones it varied substantially between sites. In some clones branch production was positively correlated with mainstem height increment. There were differences among clones in the way that branchlet production varied between the agricultural and forest sites. In the absence of needle fall, foliage weight/branch length varied threefold among clones, but for each clone, varied little between sites. Branch wood weight/branch length was generally greater when foliage weight/branch length was large but total branch wood increment/tree was strongly influenced by total branch length. The dynamics of crown development and its relation to stem wood production are discussed with reference to (i) the importance of needle longevity, (ii) the importance of tallness and narrowness of crowns and (iii) the importance of branch thickening, a process that competes with stem growth in the utilization of photosynthate. It is concluded that although each of these factors may be important none singly accounts for the observed differences in stem wood production among genotypes and the way these change in response to the environment.