As global temperatures increase, trees are growing faster, but forests are storing less carbon because their so-called carbon residence time — the length of time trees stores carbon — may be getting shorter, according to a new study led by the University of Cambridge and published on 15 May in Nature Communications (1).
Trees absorb carbon dioxide (CO2) from the atmosphere for photosynthesis. Along with water, CO2 is used to make energy for plants to grow. The CO2 removed from the atmosphere is replaced with oxygen and carbon is incorporated into the structure of the tree, where it remains – often for hundreds of years until the tree eventually dies. Slow-growing trees living at higher altitudes, such as pines and other conifers found in the Pyrenees and Altan Mountains, have the ability to store this carbon for centuries.
In theory, planting more trees should remove more carbon from the atmosphere. Furthermore, faster growth caused by warming temperatures is thought to be a boon for carbon storage. However, based on these new findings, the “live fast die young” hypothesis appears to be true for trees. The relationship between growth rate and lifespan is well-known in the animal kingdom – animals with faster heart rates tend to grow faster but they also do not seem to live as long.
In other words, accelerated tree growth may not translate into enhanced carbon storage. As global temperatures increase, trees will grow faster but tend to die younger, releasing the carbon they store back into the carbon cycle much sooner than expected. Ultimately, this means climate change mitigation strategies based on simply planting new trees to take up more carbon may be flawed. And current estimates of future carbon sequestration may be inaccurate.
To come to this conclusion, the international team of researchers from Germany, Spain, Switzerland, and Russia analysed ring width measurements from the past 2,000 years of more than 1,700 trees, both living and dead mountain pines and larch trees located in mountainous regions of Spain and Russia that have remained untouched for centuries.
Annual tree rings hold an abundance of information on past climates. Importantly, scientists can use this to dissect how the Earth’s ecosystems responded to temperature changes in the past. In this case, the researchers used this data to model the total lifespan and juvenile growth rates of trees both before and after the industrial revolution.
They found that cold temperature slow tree growth but the resulting trees are much stronger, and thus, live longer – up to 25 years longer. Whereas, trees growing faster during their first 25 years die much sooner than their slow-growing relatives and therefore, release carbon into the atmosphere much sooner.
This is the first time the concept of carbon residence time, first proposed in Prof Christian Körner from the University of Basel, has been demonstrated with data. The findings challenge the assumption that a climate warming-induced increase of tree growth translates into large-scale carbon sequestration.
According to the study, forests are storing less carbon. But it remains unclear what the future impact of this will be. “This may have far-reaching political, ecological, and economic consequences,” the authors write.
(1) Büntgen, U. et al. Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming. Nature Communications (2019). DOI: 10.1038/s41467-019-10174-4