Production of branches and foliage by young trees of Pinus contorta and Picea sitchensis: provenance differences and their simulation.
Measurements were made of the length and disposition of all branches on young trees of P. contorta (a) planted in Peeblesshire, Scotland, in 1966, and P. sitchensis (b) planted in Argyll, in 1968, from seed of various provenances from NW America with inherent differences in height and branching habit. Data for (a) were collected from 10 trees of each of 6 provenances plus one artificial hybrid during winter 1971-72, when the trees were 9 years old, and from 4 trees of each of 5 provenances of 8-year-old trees of (b) in 1972-73. The amounts of needle tissue and stemwood produced by the different provenances were not closely related to differences in needle size or number per unit length of branch, but were strongly 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. Results showed that S. coastal provenances of (a) and (b) grew consistently up to 35% and 100% taller respectively than N. and/or inland provenances; branch lengths showed corresponding variations. Among provenances of one species there were no appreciable differences either in 'apical control', which defined the length of lateral branches relative to terminal shoots, or in 'ageing', which defined the changing annual growth increment of a terminal shoot. 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 (a) to 0.25 per cm in inland or heavily flowering provenances, while in (b) numbers varied unpredictably among provenances from 0.85 to 0.60 per cm. However, these differences did not necessarily correspond to differences in total branch numbers, as these depended also on the lengths of the shoots, determined ultimately by leader growth. A model based on measurements of shoot lengths and branch-number/parent-shoot-length relations, showed how the numbers and total length of branches multiplied 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 mimic such provenance differences as occurred, e.g. between some N. and S. coastal provenances of (a). By contrast, inland provenances of (a) and the hybrid were inherently sparsely branched, as were some provenances of (b).