The growth rate of trees – and thus their accumulation of carbon – increases continuously with tree size. Even though the leaves of smaller, younger trees are more efficient (more productive per unit area of leaf surface), larger trees have more total leaf surface area and thereby grow at a faster rate than their smaller counterparts. “For example, in our western USA old-growth forest plots, trees > 100cm in diameter comprised 6% of trees, yet contributed 33% of the annual forest mass growth” [Stephenson 2014: 92].
Since trees use atmospheric carbon to grow, the more they grow, the more carbon they sequester. “Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees” [Stephenson 2014: 90].
Stephenson, N.L., et al., 2014, Rate of tree carbon accumulation increases continuously with tree size, Nature 507, https://www.nature.com/articles/nature12914/.