Allometric equations quantify accelerated growth and carbon fixation in trees of northeastern north America

Abstract

A tree's basal area (BA) and wood volume scale exponentially with tree diameter in species-specific patterns. Recent observed increases in tree growth suggest these allometric relationships are shifting in response to climate change, rising CO₂ levels, and/or changes in forest management. We analyzed 9,214 cores from nine conifer and 11 broadleaf species grown in managed mixed-species stands in the upper Midwest to quantify how well diameter (diameter at breast height (DBH)) serves to predict BA growth and above-ground wood and carbon (C). These samples include many large trees. We fit mixed models to predict BA growth and above-ground biomass/C from diameter, tree height, and the BA of nearby trees while controlling for site effects. Models account for 55%–83% of the variance in log(recent growth), improving predictions over earlier models. Growth-diameter scaling exponents covary with certain leaf and stem (but not wood) functional traits, reflecting growth strategies. LogBA increment scales linearly with log(diameter) as trees grow bigger in 16/20 species and growth actually accelerates in Quercus rubra L. Three other species plateau in growth. Growth only decelerates in red pine, Pinus resinosa Ait. Growth in whole-tree, above-ground biomass, and C accelerate even more strongly with diameter (mean exponent: 2.08 vs. 1.30 for BA growth). Sustained BA growth and accelerating wood/C growth contradict the common assumption that tree growth declines in bigger trees. Yield tables and silvicultural guidelines should be updated to reflect these current relationships. Such revisions will favor delaying harvests in many managed stands to increase wood production and enhance ecosystem values including C fixation and storage. Further research may resolve the relative roles of thinning, climatic conditions, nitrogen inputs, and rising CO₂ levels on changing patterns of tree growth.