Interannual variability in ozone damage to tropical forests

Fire emissions include the ozone precursor NOx, which is often the limiting precursor in remote locations such as the tropical forests. In fact, interannual variability in tropical fire activity, which depends on meteorology and human activity, is highly correlated with variability in surface ozone concentration over the tropics. Additionally, drought years in Asia and South America show consistently higher fire activity and surface ozone concentrations compared to years without droughts. Since surface ozone is known to decrease plant productivity, higher ozone concentrations during drought events may reduce strength of the land carbon sink. On the other hand, drought events may protect plants from ozone damage by causing a decrease in stomatal conductance. Thus, the net impact will be the balance of these opposing effects, and will likely vary by region. As climate change may increase the frequency of drought events in the tropics, an understanding of present-day relationships between drought events, fire activity and surface ozone concentrations will help inform of current and future risks of plant-ozone damage in the tropics.

Using climate model predictions of surface ozone concentration from 1996 – 2015, we show that annual mean ozone concentrations over the Amazon are up to 10 ppb higher during drought years compared to years without drought due to variability in fire activity. We then use a land surface model to show that net primary productivity loss due to plant-ozone damage in the Amazon is largest during drought years at the basin scale. The majority of the productivity loss occurs around the arc of deforestation in the Southern Hemisphere, whereas a reduction in stomatal conductance protects the Northern Hemisphere Amazon from ozone damage during drought years. Given that the interannual variability in carbon lost from plant-ozone damage is predicted to be of similar magnitude to that from direct fire emission (~ 200 Tg C), we highlight a need to consider plant sensitivity to ozone, especially for agriculture and secondary forests in the arc of deforestation.