Urban Warming Slows Tree Growth, Impacting Urban Trees’ Role in Cutting Carbon

 

Increasingly, policy makers and community leaders across the world are looking to find ways that they can reduce the carbon emissions that contribute to global climate change. Alongside renewable energy, efficiency measures and cleaner transportation, tree planting is often held up as an effective way to draw down carbon from the atmosphere. In urban environments, it can also help provide shade, which reduced energy demand from air conditioners and counteracts the significant urban heat island effect.

 

There’s only one problem with this strategy: New research from North Carolina State University finds that urban warming reduces growth and photosynthesis in city trees. In other words, as the world gets warmer, city trees grow less.

Emily Meineke, lead author of a paper entitled “Urban warming reduces aboveground carbon storage,” which was published this month in the Royal Society journal Proceedings B, explains why this matters:

 

“Earlier studies have shown that urban warming increases pest abundance in street trees. We wanted to know how urban warming and pest abundance affect tree growth, since trees pull carbon out of the atmosphere and convert it into biomass. This is important, because we know that high levels of atmospheric carbon play a role in climate change.”

 

Meineke and her team studied 20 pairs of willow oak trees across Raleigh, North Carolina, treating one tree in each pair with pesticides and leaving the other tree untreated. The sites selected showed a range of average urban temperatures and researchers monitored air temperature at each site during the experiment.

 

While the presence of tree pests like scale insects and spider mites did impact branch growth, the studies showed that higher average air temperature was a much more significant influence on both trunk growth and photosynthesis. Specifically, Meineke and her team found that urban warming reduced carbon storage by all of Raleigh’s willow oaks by 12%, a figure which translates to 27 metric tons per year in Raleigh alone.  

 

“We think the findings are generalizable to other tree species and other cities, especially hotter cities like Atlanta, but additional work needs to be done to determine whether that’s the case,” says Meineke.

 

If this does indeed prove to be the case, then these findings should have significant implications for how we assess the potential of urban forestry and tree planting strategies to mitigate climate change. Ecosystem service assessments that do not consider urban conditions run the risk of overestimating urban tree carbon storage, say the report’s authors.

 

That’s not to say, of course, that cities should stop planting trees. From New York City’s Million Trees NYC initiative to Louisville, Kentucky’s massive investment in urban trees, many cities are rediscovering the value of urban forestry. In fact, if anything, there may be a case to be made to increase tree planting based on these findings. After all, if each tree we plant is growing slower and absorbing less carbon, then we may be well advised to start planting more. And if a warming world means slower growing trees, then we’d better plant more trees to stop the world from warming.