Warming cities stunt trees

As our planet warms, we know that high levels of atmospheric carbon are associated with that warming. Trees play a vital role in combating the rise of carbon in the atmosphere as they pull it out of the air and convert it into biomass. Yet, in a sad paradox, the very warming that the trees may help to prevent is stunting the growth of the trees themselves.

To study the effect of warming in urban environments on trees, researchers chose 20 pairs of willow oak trees (Quercus phellos) in Raleigh, North Carolina. At each site, one of the pair of trees was treated with an oil that kills insect pests and the second was left untreated. Air temperature was also monitored at each site across the two years of the experiment.

Growth of the trees was assessed by measuring the tree’s trunk circumference and how much specific branches grew on each tree. Photosynthesis was also measured as it is the process by which trees capture carbon and is a measure of tree health.

The researchers found in fact that warming reduced carbon storage by 12 per cent.

For a start, the researchers found that the incidence of pests like scale insects and spider mites was greater at hotter locations. They also found, however, that hotter areas also negatively affected photosynthesis and growth regardless of whether pests were present. Trees without pests had much more branch growth but in all sites the hotter it was, the less the trunk growth, meaning less biomass and less carbon captured. They found in fact that warming reduced carbon storage by 12 per cent. In Raleigh, that means 27 metric tons every year not being stored.

The researchers believe these findings would be true for other tree species beyond oaks as well. As well as maintaining urban trees, we need to ensure the conditions are right for them as well.

Source: Proceedings of the Royal Society B: Biological Sciences