Large, old trees aren't just stately elders due respect for their venerable size and age. Research published in Nature reports that large, old trees "actively fix large amounts of carbon compared to smaller trees." The research says a "single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree," and "rapid growth in giant trees is the global norm."
Photo: Stephen Kelly

Forests are major contributors to the global carbon cycle, e.g., "locking up carbon" and preventing it from escaping into the atmosphere, but according to research published on line Jan. 15, 2014 by the journal Nature ("Rate of tree carbon accumulation increases continuously with tree size," Stephenson, NL, Das, AJ, Condit, R, et al.), there is no consensus about the nature of productivity at the individual tree level.

The researchers thus did a global analysis of 403 tropical and temperate tree species (repeated measurements of 673,046 trees) to determine "whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age."

This broad empirical assessment showed that "for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree."

The researchers added: "The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density."

In old growth plots in the western U.S., the authors said the largest trees comprise six percent of the forest, but contribute a third of the annual growth in forest mass.

The researchers asserted the results "resolve conflicting assumptions about the nature of tree growth, inform efforts to understand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence."

For more, visit www.nature.com/nature/journal/vaop/ncurrent/full/nature12914.html