Old-growth redwood forests I: Separating effects of tree size and age on trunk growth

Background/Question/Methods

Among unsuppressed trees in the world’s tallest forest (i.e., along Bull Creek in Humboldt Redwoods State Park, California), the annual rate of aboveground growth increases with tree size through old age such that the tallest and largest trees produce the most wood annually.  Here we extend whole-crown measurements of Sequoia sempervirens to include suppressed and unsuppressed trees throughout the geographic range of the species in California.  We installed eleven 1-hectare plots in old-growth S. sempervirens forests from Jedediah Smith Redwoods State Park (extreme north) to Landels-Hill Big Creek Reserve (extreme south). Fifty-five trees 18.1 to 115.6 m tall were crown-mapped, which included direct measurements of all trunks and appendages as well as increment coring of trunks at multiple heights from base to treetop.  A total of 160 branches, stratified by basal diameter (0.6 to 33.5 cm) and height (4.0 to 108.7 m), were randomly selected for destructive sampling and dissected to quantify leaves, bark, cambium, sapwood, heartwood, and decaying wood.  These data were used to develop equations to predict values for all branches on the 55 trees.  Summing actual and predicted values yielded whole-tree estimates of branch components, and combining these estimates with measurements obtained from trunks and increment cores yielded accurate aboveground estimates for whole trees.  Dendrochronology permitted estimation of minimum ages (110 to 1770 years), quantification of annual wood growth (2 to 660 kg), and reconstruction of annual height growth for each tree’s main trunk. 

Results/Conclusions

Confirming results from our previous study of unsuppressed trees, multivariate analysis of seven whole-tree structural variables yielded highly significant principal components representing size (PC1) and energy balance (PC2, aboveground respiratory demand scaled by photosynthetic capacity). In stepwise regression, PC1 and PC2 explained over 61 % of the variation in annual mass growth of the main trunk (P < 0.0001), and minimum age explained significant additional variation (7 %, P = 0.0025).  The proportion of trunk volume growth converted to heartwood increased with tree age such that by 1000 years, over 90 percent of the annual volume increment is converted to heartwood.  Height growth declined linearly with tree height, though heights of the tallest trees are still increasing by 2 to 20 cm per year.  Examination of main trunk growth over time revealed that annual rates of wood production increased during the 20^th century in eight of the eleven plots and that this increase was independent of tree size.