Stephen C. Sillett, Humboldt State University and Robert Van Pelt, University of Washington.
The rate at which tree height increases slows as trees grows taller. For example, 60-m-tall Eucalyptus regnans and 70-m-tall Sequoia sempervirens both grow 43 cm yr-1 in height in 2006, but 90- and 110-m-tall trees of the same species at the same sites grew only 27 and 8 cm yr-1, respectively. Tree growth, however, is far more than an increase in height, because growth occurs across the entire surface of a tree’s cambium. Accurate quantification of whole-tree dimensions has now been achieved for many individuals of the 3 tallest tree species (S. sempervirens, E. regnans, and Pseudotsuga menziesii). For example, all branches, limbs, and trunks were non-destructively measured for size, structure, and location on 70 P. menziesii trees 50 to 650 years old in Washington. Principal components analysis of 24 tree-level variables revealed two orthogonal dimensions of structure that accounted for 71.3 and 12.4 % of the total variation. The first dimension represented a gradient of overall tree size and structural complexity that was positively correlated with stand age (R2 = 0.99). The second dimension represented a gradient of tree growth potential that was positively correlated with measured growth of main trunks at breast height during the 5-yr period after trees were mapped (R2 = 0.39). The strongest determinant of growth potential was the ratio of a tree’s total cambium surface area to projected leaf area. A tree’s growth potential may thus be unrelated to either size or structural complexity and depend instead on factors related to whole-tree carbon balance.