A new study suggests that trees accelerate their growth as they get older and bigger.
The findings contradict assumption that old trees are less productive and could have important implications for the way that forests are managed to absorb carbon from the atmosphere.
Old trees better CO2 absorbers
38 scientists from 16 countries studied measurements of 673,046 trees of more than 400 species growing on six continents. They found that large, old trees actively fix large amounts of carbon compared to smaller trees.
A single big tree can add the same amount of carbon to the forest in a year as is contained in an entire mid-sized tree, they found.
It also means that big, old trees are better at absorbing carbon from the atmosphere than has been commonly assumed.
Most living things reach a certain age and then stop growing, but trees accelerate their growth as they get older and bigger.
In absolute terms, trees 1 metre in trunk diameter typically add from 10-200 kg dry mass each year averaging 103 kg per year. This is nearly three times the rate for trees of the same species at 50 cm in diameter, and is the mass equivalent to adding an entirely new tree of 10-20 cm in diameter to the forest each year.
The findings back up a 2010 study which showed that some of the largest trees in the world, like eucalyptus and sequoia, put on extraordinary growth as they get older.
More important than thought
The study also shows old trees play a disproportionately important role in forest growth. Trees of 1 metre in diameter in old-growth western US forests comprised just 6% of trees, yet contributed 33% of the annual forest mass growth.
But the researchers said that the rapid carbon absorption rate of individual trees did not necessarily translate into a net increase in carbon storage for an entire forest. Old trees can die and lose carbon back into the atmosphere as they decompose.
However, findings do suggest that while they are alive, large old trees play a disproportionately important role in a forest’s carbon dynamics. It tells us that large old trees are very important, not just as carbon reservoirs. Old trees are even more important than previously thought.
Understanding of the role of big trees in a forest is developing rapidly even as they come under increasing threat from the fragmentation of forests, severe drought and new pests and diseases. Research in 2012 showed that big trees may comprise less than 2% of the trees in any forest but they can contain 25% of the total biomass and are vital for the health of whole forests because they seed large areas.