A global assessment of tree carbon accumulation rates
Forests are key components of the global carbon cycle, potentially contributing substantial feedbacks to ongoing climatic changes. It is therefore remarkable that no consensus has existed about how tree growth – and thus the rate at which individual trees accumulate carbon from the atmosphere – changes with tree size. One widely-held belief is that trees have a lifetime growth pattern similar to humans: rapid growth early in life followed by declining growth later in life. However, a small group of studies have instead found continuously increasing growth rates in trees, with the largest trees growing the fastest. So, we have been confronted with two mutually exclusive generalizations about the fundamental nature of tree growth, but have lacked a global assessment to distinguish between the two.
To address this information gap, USGS scientists assembled an international team of researchers to analyze growth measurements of 673,046 trees belonging to 403 tree species, representing tropical, subtropical, and temperate regions across six continents.
Results of the analyses were unequivocal. For the vast majority of species (97%), mass growth rates continued to increase up through the largest trees in the data set. At the extreme, a giant tree can add the mass equivalent of an entire medium-sized tree to the forest in a single year.
These results add to the growing body of evidence suggesting that although trees may age (i.e., suffer cumulative exogenous injuries through time), they do not senesce (suffer an inevitable, endogenous physiological decline). Most significantly, the results highlight the disproportionately important role of large trees in determining rates of carbon exchange between forests and the atmosphere – information that will improve our ability to forecast the role of forests in the global carbon cycle and to devise appropriate adaptation and mitigation strategies for managing forests in the face of rapid climatic changes.
The paper, published in Nature, is available at: https://pubs.er.usgs.gov/publication/70124417