Crown architecture of short-rotation, intensively cultured Populus II. Branch morphology and distribution of leaves within the crown of Populus 'Tristis' as related to biomass production

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Abstract
The branch morphology and the distribution of leaves and specific leaf weight were determined within the crown of 5- and 6-yr old Populus ''Tristis No. 1'' (P. tristis Fisch. .times. P. balsamifera L.) trees grown in northern Wisconsin [USA] under short rotation, intensive culture at 1.2- and 0.6-m square spacings, respectively. The relationship of leaf area to aboveground biomass productivity was also evaluated for the same trees. The 1st-order branches within the trees exhibited acrotony and were predominantly long shoots. No branching higher than 3rd-order was observed. Leaf size and specific leaf weight were greatest on the current terminal shoot and decreased from the upper portion of the crown to the base. When the 6-yr old trees were divided into 8 1-m vertical strata, over 80% of the total number of leaves and total leaf area was in 3 middle strata (4-7 m), and most of the 1-m strata contained leaf area attached to 2 or more height growth increments. Leaves attached to branches on a single height growth increment of the 6-yr old trees occurred in up to 4 1-m vertical strata. Long shoots comprised 53% of total leaf area in the 5-yr-old trees and 66% in the 6-yr old trees, short shoots the remainder. Ninety-five percent of the long shoots in the 6-yr old trees were in the 3 uppermost vertical strata (5-8 m), and 95% of the shoot shoots were in the lowermost leaf-containing vertical strata (3-6 m). Long-shoot leaves had higher specific leaf weights than short-shoot leaves attached to branches on the same height growth increment. Leaf-area indices (LAI) were 7.6 and 8.8 m2 .cntdot. m-2 for the 5- and 6-yr old stands, respectively. Leaf area per tree was linearly related to the aboveground biomass of the tree. The linear regression line for the relationship leaf area and D2H (diameter2 .times. height) for the 6-yr old trees in the study was statistically different from that of the 5-yr old-trees. This relationship may serve as a useful quantitative index of crown closure in poplar stands. Some crown morphological criteria useful for selection and breeding of improved poplar trees for short-rotation intensive culture are presented.