Old, large trees may be even more important ecologically than long-believed, according to a new study in Nature. Looking at over half a million individual trees from over 400 species (both tropical and temperate), scientists have determined that most trees actually grow faster in their dotage than in their youth.
“In human terms, it is as if our growth just keeps accelerating after adolescence, instead of slowing down,” explains lead author Nate Stephenson with U.S. Geological Survey Western Ecological Research Center. “By that measure, humans could weigh half a ton by middle age, and well over a ton at retirement.”
Scientists have long-believed that as trees age their growth slows down, but the new study found that in fact 97 percent of trees (out of 403 species) see their growth speed up after maturity.
“Rapid growth in giant trees is the global norm, and can exceed 600 kg per year in the largest individuals,” write the researchers.
 Large western white pine (Pinus monticola) in California’s Sierra Nevada mountain range. Photo by: Rob Hayden. |
This comes with big ecosystem consequences: the older the tree the more carbon it sequesters every year.
“Large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees,” the scientists write, “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.”
Scientists have long recognized that old trees play important ecological roles, including niche wildlife habitat, fruit and flowers, and carbon storehouses, but the new study finds that these old trees are hugely-productive at sequestering carbon as well. However, these findings don’t necessarily translate to tree stands, since as tree stands age, individuals often perish, leaving fewer—albeit—larger survivors.
Old trees are vanishing worldwide. Not only are these often the first trees cut down by loggers, but they are also particularly vulnerable to drought and wildfires, both of which are thought to be occurring more frequently and intensely due to global warming. A study last year in Science warned that the world was at risk of losing its arboreal giants.
“Just as large-bodied animals such as elephants, tigers, and cetaceans have declined drastically in many parts of the world, a growing body of evidence suggests that large old trees could be equally imperiled,” the scientists wrote.
A large, old Scots pine (Pinus sylvestris) in the Sierra de Baza of southern Spain. Photo by: Asier Herrero.
Citations:
- Stephenson et al. (2013) Rate of tree carbon accumulation increases
continuously with tree size. Nature. doi:10.1038/nature12914 - David B. Lindenmayer, William F. Laurance, Jerry F. Franklin. Global Decline in Large Old Trees. Science 7 December 2012: Vol. 338 no. 6112 pp. 1305-1306 DOI: 10.1126/science.1231070.
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