Climate change might reduce the amount of carbon that forests can store, in part because photosynthesis decreases at high temperatures.

Emily Meineke and her colleagues at North Carolina State University in Raleigh studied willow oaks (Quercus phellos) in the local area, where — as in other large cities — pavement and other hard surfaces absorb and slowly radiate the Sun's heat. This increases the temperature in some urban regions to a level comparable to that predicted for the next century as climate warming continues. The team measured trees' photosynthetic rate, as well as factors such as water stress and pest prevalence, in hotter and cooler areas of Raleigh.

Hotter areas saw less tree growth. The team calculates that the 'urban heat island' effect reduced carbon sequestration in these trees by 12%. The reduction in growth was driven mainly by the effect of water deficits on photosynthesis, not increased herbivore activity.

Proc. R. Soc. B 283, 20161574 (2016)