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Summers in Boulder are hot andÌý. But the large trees that occupy the city’s parks, yards and green spaces can provide a welcome patch of shade for pedestrians, critters and understory plants.

One INSTAAR researcher is hyper-focused on this phenomenon.ÌýPhD student Advyth Ramachandran is investigating the cooling power of various species of urban trees. Each week, his field crew downloads data from carefully-placed temperature sensors and gathers samples of leaves and wood. The goal is to create guidance for urban planners and foresters looking to cool down cities with natural solutions.

Today, Ramachandran will present his methodology and preliminary findings at theÌý. He is one ofÌý.

Earlier this week, INSTAAR sat down with Ramachandran to learn more about his project and how it came to be.

What is the origin story of your interest in urban trees?

Advyth Ramachandran clips a twig of an American linden tree in Boulder in August, 2025. Back at the lab, Ramachandran and his team will use the sample to assess wood density and leaf area for the tree. (Photo courtesy of Advyth Ramachandran)

I didn't have a very outdoorsy childhood, but my family would walk around a lot in urban parks, so I was always really interested in vegetation and biodiversity in urban areas. Then, when I was in college, three of my friends, who are computer engineers,Ìý. We were interested in soil moisture and how it impacts urban tree survival. Through that project, I got fascinated with urban forests, and I decided that it would be cool to study urban ecosystems more in graduate school.Ìý

When I started my PhD inÌý, I pitched the urban tree traitsÌý project to Katie and she gave me the green light. The idea is to study urban trees using some of the same methods that we use in natural ecosystems and also to think about urban vegetation dynamics through the frameworks we use in restoration ecology. One of my committee members,ÌýLaura Dee, connected me with theÌý, which led to a collaboration with them that is ongoing.

This is such a unique research question. How did it come about?

We know trees are good. We know they're good for a lot of reasons, but importantly they reduce urban temperatures. At this point, the evidence of that is overwhelming.Ìý

But, a lot of the evidence that we have for the role of trees in reducing temperatures actually comes from satellites, which are sensing land surface temperature — not actually sensors on the ground that are measuring the air temperature that people experience. The year before I came to CU,Ìýstudents in Laura Dee’s lab put out temperature sensors around Boulder and found that vegetation does actually cool down air temperatures in Boulder.Ìý

My research question ultimately arose from this finding, the existing literature and my interest in plant functional traits — basically characteristics that influence how plants interact with the environment. We converged on this idea of how tree traits might predict which species are the best at cooling.Ìý

Today in Boulder, we really don't know which tree species are the best to plant for cooling. When foresters decide which trees to plant they base that on a lot of things, such as disease resistance, drought tolerance, and fire risk. We’re trying to add an understanding of which species might work best for reducing temperatures to that selection process.

This is really an example of one of the main thrusts of my dissertation research, which broadly focuses on how plant functional traits can be used to design nature-based solutions.Ìý, and one of the things that we learned is that this area is really understudied.Ìý

So, I did a pilot study in 2024 and then this year was the real deal. We got useful data last year, but we didn't have enough to really see clear differences among species. This year, I was finally able to see some clear differences arise after refining the methodology and expanding the size of the study.

What insights have you gleaned so far?

CU undergraduate researchers Jen Dugdale (left) and Rhiannon Danborn select and preserve leaf and wood samples In Boulder in August 2025. (Photo courtesy of Advyth Ramachandran)

The first insights came from last year's pilot study. We found was that the tree canopy has a significant effect on temperature, as you might expect. It can actually cool the air by around four degrees Fahrenheit. We also found that soil moisture and grass cover are really important determinants of temperature. One major takeaway was that temperature varies on really fine spatial scales. You could have sensors that are a stone’s throw away from each other and see a difference in average summer afternoon temperatures of five degrees Fahrenheit.Ìý

Coming into 2025, we decided to start zeroing in on the impacts of individual tree species by controlling for all of these factors that we identified last year. We only put sensors under isolated trees located in urban parks that are irrigated and green. Once we incorporatedÌý all these factorsÌý into our statistical model, we started to see differences between species. In the end, we were able to identify a gradient of tree species based on their cooling effects.

Can you get specific about which trees are the best at cooling?

I'm not ready to provide planting recommendations just yet, because we still need to refine our analyses and go through peer review. One thing we did find, though, is that dense foliage seems to be a useful trait for cooling. Interestingly, there are some trees, like blue spruce for example, that grow dense foliage and seem to be relatively drought-tolerant. More research is needed, but this is a promising indication that we might be able to find tree species that are both tolerant of a dry climate and good at cooling cities.