Urban forestry... in the desert?

Cities, like Phoenix, burn a lot of fossil fuels and release a lot of CO₂ into the atmosphere. Think of all of the cars on the road, houses using electricity, and businesses and industries that use fossil fuels, too! The CO₂ is a greenhouse gas that influences the climate of the entire planet. How can cities reduce how much CO₂ is released into the atmosphere? One way is to burn less fossil fuels. We try this through efforts like increasing cars' fuel efficiency and switching to solar power. Another way is to try to re-absorb the CO₂ that was put into the atmosphere. So, even though the fossil fuels are being burned and CO₂ is going into the atmosphere, we can try to pull some of it back into the city so that it doesn't stay in the atmosphere. This lets cities "mitigate" the amount of CO₂ they create.

There are a lot of scientists who are trying to engineer fancy new technology to absorb CO₂ from the atmosphere. But there is also a simple way that just uses nature: trees! Trees pull a lot of CO₂ from the atmosphere when they photosynthesize, and they hold onto that carbon for a long time. Trees live for decades or more, which means the CO₂ stays in their biomass for decades. ("Biomass" is the scientific term for the entire mass of the plant, and about half of that biomass is made up of carbon!) Every year they shed parts of their biomass, like leaves and twigs, onto the ground, which gets decomposed and turned into "soil organic matter". That soil organic matter can become very stable in the soil, meaning it only continues decomposing VERY slowly. The soil organic matter builds up over time in the soil as trees shed their leaves, bark, and twigs, locking it into the soil instead of the atmosphere. So, not only do trees hold a lot of carbon, they also build up carbon in the soil where it can be stored for hundreds of years.

The carbon cycle in an urban forest ecosystem: The trees take up CO₂ through photosynthesis, and that carbon is stored in the tree for decades or more. When the tree sheds its dead parts (called "litterfall"), it decomposes in the soil, creating a form of soil organic matter called "humus". The humus can slowly decompose, which will release some of the carbon back to the atmosphere through respiration, but it is VERY SLOW. More litterfall every year creates humus faster than it is decomposed, which means there's a buildup of humus over the years, storing carbon in the soil for centuries!

Growing trees is easy in a lot of cities and has other benefits, like providing shade, cooling the surrounding area thanks to transpiration, making a home for birds and other animals, and generally looking pretty! "Urban forestry" is becoming popular in some cities. But... we live in a desert! Can Phoenix use urban forestry as a way to mitigate our CO₂ emissions? Many trees that have been planted in yards and along streets aren't native to the desert, so they require a lot of extra water to survive. That creates its own problems! Is there a way that Phoenix can grow native trees without having to give them a lot of irrigation? Can urban forestry be a sustainable way to capture CO₂ in a desert city?

That's what Arizona State University is trying to find out. We have planted an "urban forest" on campus, using only native trees that are good at surviving in a desert climate (without needing to be watered every day). The forest was planted in an unused area of the West Campus where some mesquite trees had "volunteered" to grow. But, most of the area was bare. One thousand mesquite and palo verde saplings were planted into those bare areas!

This is the area of campus, just before the forest of saplings was planted.

1,000 mesquite and palo verde saplings were planted into the bare areas to grow into a forest.

Right now, the trees are little saplings. As they grow, they will capture CO₂ during photosynthesis and store it in the tree biomass, and as they shed their leaves and wood, they will start to build up the carbon stored in the soil, too. However, they grow slowly, especially since we're only giving them water early in their life until they establish. After that, they are on their own to find water! 

So, how much carbon can a slow-growing desert forest absorb? We will be measuring that every year. We are measuring the growth of the trees, to estimate how much carbon is being stored in the tree biomass. We are also measuring how much carbon is being stored in the soil. The trees and soil are the two main places we expect to see the carbon stored.

One of the saplings whose biomass we measure every year. Hopefully we will see its biomass increase year-after-year, meaning it has removed CO₂ from the atmosphere and stored that carbon in the plant.

We are also making other measurements, to better understand the carbon cycling in the forest. We are measuring processes like photosynthesis and respiration rates (where CO₂ is moved between the forest and the atmosphere), the amount of "litterfall" every year, and the biomass of microbes living in the soil who do the respiration.

Kevin measures soil respiration using an infrared gas analyzer.

David measuring microbial biomass living in the soil using a process called "chloroform fumigation extraction".

It will take a long time before we have results from our experiment, because the trees grow slowly. If it is successful, it will grow into a beautiful mesquite bosque that not only stores carbon, but also provides shade, a habitat for birds, and a food source for pollinators. It will take decades to reach maturity! Until then, we will keep making our measurements every year, as we (hopefully) watch the trees grow.