Recent research has shown that plants help themselves grow by releasing volatile organic compounds. These chemicals form a mist of aerosols above the vegetation that blocks some of the direct light but enhances diffuse light. This boosts the solar radiation reaching the forest understory and increases growth.
Alexandru Rap from the University of Leeds, UK, and colleagues assessed the impact of plant volatiles on primary productivity by using atmospheric and vegetation models along with measurements of aerosols and plant productivity. Their findings, published in Nature Geoscience, show that globally plant volatiles boost vegetation productivity by around 1.23 Pg of carbon per year — equivalent to around 10% of the world’s fossil fuel carbon emissions.
“Amazingly we found that by emitting volatile gases, forests are altering the Earth’s atmosphere in a way which benefits the forests themselves,” says Rap. “While emitting volatile gases costs a great deal of energy, we found that the forests get back more than twice as much benefit through the effect the increased diffuse light has on their photosynthesis.”
Medium-height trees survive drought best
Plant productivity is influenced by a whole host of factors: the plant’s own chemical mist, air pollution and whether the weather is cloudy or sunny. A dose of windblown desert dust may boost productivity in the forest understory, but a cloud of traffic haze is more likely to suppress growth. When surface ozone enters a plant’s leaves, it dissolves in the water inside the plant and reacts with other chemicals, slowing down photosynthesis. Anthropogenic sources such as traffic fumes, power generation, industry and agriculture are major sources of ozone, as are forest fires.
To better understand the impact of forest fires on plant productivity, Xu Yue from the Chinese Academy of Sciences and Nadine Unger from the University of Exeter, UK, combined state-of-the-art models with global air quality and plant productivity measurements.
Their findings, published in Nature Communications, reveal that when all the sources of surface ozone are totted up, the reduction in plant productivity amounts to around 3.5%. Focusing on forest fires alone the researchers showed that between 2002 and 2011, wildfires pushed up surface ozone levels by an average of more than 5% over land, and increased aerosols by around 10%. The worst year was 2003 when fires raged through California, burning over 1 million acres of land.
Globally Yue and Unger estimate that ozone from forest fires knocks back plant productivity by an average of 0.91 Pg of assimilated carbon per year. The additional aerosols from the fires increase productivity by just 0.05 Pg of carbon per year — nowhere near enough to balance the negative impacts of surface ozone. In a nutshell, the diffuse-light-boosting effects of the aerosols are masked by dense clouds of ozone-laden smoke over forests. In total, ozone from forest fires accounts for around one fifth of the annual ozone-induced reduction in plant productivity.
Carbon budget calculations don’t currently include this indirect effect of forest fires but the numbers are significant.
“Our study shows that the impact of wildfire air pollution on productivity is substantially [around six times] larger than estimates of drought-induced losses over the same period,” says Yue.
This choking effect from wildfires is particularly apparent in the more pristine regions of the world, where background levels of air pollution are low. For example, the large forest fires in Indonesia in 2006 are estimated to have reduced local forest productivity by 3.6% the following year. Across tropical Africa, fire pollution is believed to cut productivity by as much as 2% every year. “Our study identifies Central West Africa as a particularly vulnerable hotspot to fire air pollution damage,” says Unger.
Looking ahead, climate change is likely to amplify the problem, with a warming future anticipated to increase fire activity. A study in Atmospheric Chemistry and Physics indicated that wildfires in North America are likely to increase significantly as climate warms, for example.
Understanding the balance of factors affecting plant productivity, mapping how air pollution moves, and anticipating change in the future are all questions that need to be answered.
