Large old trees have always had a special place in our culture. Trees like the President giant sequoia in California, the Jōmon Sugi in Japan or the Llangernyw Yew in Wales are revered, visited by thousands and the subject of endless stories, some of them true, many based on folklore. Yet through all the stories of these magnificent trees there has been a common thread, especially among professional biologists and foresters – that these are trees in decline, past their prime. Forests consisting mostly of very large old trees were said to be senescent and losing value.
A worldwide group of scientists has now shown us that we were entirely wrong about big old trees. While they do not grow much in height, and even become shorter, their growth never slows. They continue to accumulate biomass throughout their lives. One reason it took so long to recognize this strange fact about tree biology is that these trees are so large that it is difficult to measure the addition of biomass to the large surface area of these trees. Foresters had long known that large old trees stop growing in height and even become shorter; it was logical to assume that volume growth slowed as well.
The group of thirty eight scientists from all over the world assembled a huge data set, including careful measurement of 670,000 trees representing 403 species, both temperate and tropical. Their measurements compared the largest trees of each species with smaller ones and showed that more than 95% of the largest trees accumulated mass faster than the smaller trees of the same species.
This is not just an interesting result, it is also extremely important. As we begin to address solutions to climate change, forests are increasingly important for their ability to store large amounts of carbon. We now know that the largest trees are the most efficient at accumulating mass and therefore carbon. Wood is mostly carbon – a 10 ton tree stem contains about 4.1 to 5.5 tons of carbon. The longer that tree is allowed to grow, whether in a forest or your front yard, the more carbon it stores. Forests of very large trees also support huge amounts of dead wood, and therefore of carbon.
This finding should give renewed emphasis to the preservation of old growth forests in tropical and temperate climates, old trees in urban areas, and the giant trees of our woodland pastures. The production of wood products from our forests is important, and also provides an opportunity for the long term storage of carbon in buildings, furniture and other wood products. Given the importance of large old trees for growth and carbon storage, we should seek wood products from smaller trees and leave the giants alone.