Production of Branches and Foliage by Young trees of Pinus contorta and Picea sitchensis: Provenance Differences and Their Simulation

Cannell, M

The lengths and disposition of all the branches were measured on young trees, grown from seed collected at different locations in north-west America (provenances) which differed inherently in height and branching habit. There were ten trees of six provenances, plus one artificial hybrid, of 9-year-old Pinus contorta growing in Peeblesshire, and four trees of five provenances of 8-year-old Picea sitchensis in Argyll. The amount of needle tissue and stem-wood produced by the differing provenances were not closely related with differences in needle size or number per unit length of branch They were overwhelmingly determined by up to three-fold differences in total length of needle-bearing shoot per tree. This, in turn was controlled by (1) the amount of leader extension per year and (2) the numbers of lateral branches produced. (1) During the latter 4-5 years the southern coastal provenances of Pinus and Picea had grown consistently up to 35% and 100% taller respectively, than the small northern and/or inland provenances. Furthermore, all the branches had grown correspondingly longer. Among provenances of one species there were no appreciable differences either in `apical control', which defined the length of lateral relative to terminal shoots, or in `ageing', which defined the changing annual length growth of a terminal shoot. (2) The numbers of laterals borne by leaders, and first-, second- and third-order branches, were proportional to their lengths. Numbers ranged from 0.35 per cm in many coastal provenances of Pinus contorta to 0.25 in inland or heavily flowering provenances, while in Picea sitchensis numbers varied unpredictably among provenances from 0.85 to 0.60 per cm. However, these differences did not necessarily correspond with differences in total branch numbers, as these depended also on the lengths of the shoots, determined ultimately by leader growth. A model based on actual measurements of (1) shoot lengths and (2) branch number/parent shoot length relationships, showed how the numbers and total length of branches multiplied-up as a consequence of greater leader growth. It was thereby confirmed that some provenances branched more profusely than others simply because they grew more rapidly, and that phenotypes differing in height growth could mimick such differences as occurred, for example, between some north and south coastal Pinus contorta provenances. By contrast, inland provenances of P. contorta and the hybrid, were inherently sparsely branched, as were some types of Picea sitchensis.

Production of Branches and Foliage by Young trees of Pinus contorta and Picea sitchensis: Provenance Differences and Their Simulation

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