Into the Woods at Dartmouth

August 2, 2022

An Earth and Environmental Sciences grad broadens her soil studies with a prestigious postdoc.

Gisselle Mejia
Gisselle Mejía begins her postdoc in forest ecology at Dartmouth College this month. (Photo credit: Steven Deheeger)

On any given day, billions of microbes process carbon and nitrogen that supply essential nutrients to soil ecosystems in New York City’s urban forests, said Gisselle Mejía, who worked as a forester with the city Department of Parks & Recreation before joining the Graduate Center Ph.D. Program in Earth and Environmental Sciences.

Mejía, a native New Yorker, defended her doctoral dissertation this summer. She recently arrived in New Hampshire to begin a postdoctoral fellowship at Dartmouth College, where she’ll continue her study of soil microbes, only this time in a rural, New England forest.

Mejía talked to the Graduate Center about her Ph.D. experience and her work in forest ecology.

The Graduate Center: You study how plants and soil interact in urban forests. Can you tell us a little bit about your research?   

Mejía: So, I look at microbial communities in soils. Basically, microbial communities in soils are responsible for processing carbon and nitrogen, which are essential nutrients for the development of forest ecosystems.

I measure how microbial communities process carbon and nitrogen. From there, we're able to see whether carbon and nitrogen are being retained in the soil or they're being released into the atmosphere, which is not what we want. We want more retention of carbon and nitrogen in the soil so that it gets recycled and retained by plants so that they can grow big and healthy, and be sustainable over time.

Learn More About the Ph.D. Program in Earth and Environmental Sciences

GC: How does city soil differ from rural soil in terms of nutrients and support to plant life?

Mejía: One of the things that we discovered, in New York City for example, looking at the soils in different parks, is that large, preserved areas in the city tended to have relatively natural soils.

For example, Pelham Bay Park in the Bronx and Marine Park in Brooklyn. The city has been managing those parks as natural areas. So, there had been a lot of focus on preserving these areas from being impacted by human activity. There's been a lot of focus on making sure that these are more intact forest areas, which harbor diverse habitats for wildlife. Another one is a very popular park, Inwood Hill Park, in north Manhattan.

Because they've been relatively intact for so many years, these parks have been managed in order to be preserved as natural areas by the city, so they can be compared to rural forests.

GC: Does air pollution affect soil quality in New York City? Does it lead to poor soil quality?

Mejía: It definitely does. These forests, regardless of whether they’re more natural, they're all impacted by the atmosphere in the city. For example, nitrogen pollution is something that occurs through atmospheric deposition. That can be wet, by rain, or dry through particulate matter.

So, an abundance of nitrogen in soil can be polluting in itself, which is not good for the plants. It can also end up not being processed in soil and end up in waterways, which then pollutes coastal ecosystems in New York City.

Gisselle Mejia
Gisselle Mejía takes a soil core sample in Brooklyn’s Marine Park as part of the MillionTreesNYC initiative in 2019. (Photo courtesy of Gisselle Mejía)

GC: What's one surprising thing about urban forests that people don't know?

Mejía: One thing that doesn't get emphasized enough is that these are important ecosystems for wildlife. These parks are real homes to diverse species. They’re also really important for people's psychological well-being.

It's an equity issue as well. I say that because, for people who can't afford to travel, these parks are the closest thing they have to nature. So, being able to have access to nature is really important for people in the city, especially in low-income communities.

GC: How do urban forests help to mitigate the effects of climate change?

Mejía: Basically, forests are essential for sequestering carbon, either in their biomass, meaning in the trees themselves, or in the soil.

Most of the carbon that's taken out by trees from the atmosphere is actually sequestered in soils. That's the main mechanism by which climate change is mitigated — the trees take up carbon from the atmosphere and use it for themselves and distribute it in the soils, which are then used by all these microbial communities and all the other organisms that help decompose organic materials in soils. It’s part of the whole nutrient-cycling process that helps retain carbon and nitrogen in soils long term.

GC: What made you decide to join the Ph.D. program in Earth and Environmental Sciences?

Mejía: There were different reasons. One of the things that woke me up to pursue research in environmental sciences was Hurricane Sandy and realizing we need our ecosystems to be resilient, to take the impact that we're facing from all these weather extremes.

Then having a master's in forestry and having worked at the Parks Department doing urban forestry work, I wanted to expand on that experience. I considered going back to graduate school and doing a Ph.D., but I wanted to be in the city. I heard about the Graduate Center because I’d taken a GIS class at Hunter College, in the Geography and Environmental Science Department. I enjoyed the class and then I was added to the department listserv. Through the listserv I heard about the Graduate Center.

I reached out to some professors in the department to learn more about it. They invited me to a colloquium and I was really impressed with how great the faculty and the students got along. They were very welcoming. Everyone was really laid back and it just seemed like a diverse and surprisingly friendly environment, which is not what I was expecting from a Ph.D. program.

GC: How did the Graduate Center help to prepare you for your career in forest ecology?

Mejía: One of the great things about the Earth and Environmental Sciences program is they emphasize interdisciplinary work.

One of the key things that I came away with from the program is being able to focus not just on biogeochemistry, but to also look through the social lens by working with colleagues and other Ph.D. students who are in the social sciences — for example, in the geography program — and the exposure through all those different programs to socio-economics, or race issues, or even literature. … There’s a well-rounded community at the Graduate Center.

GC: What will your research focus on at Dartmouth?  

Mejía: I’ll be moving away from urban ecology and focusing more on rural forests. Dartmouth College has a forest area that they own to do research. I don't have a specific project that I've developed yet. But the area I'll be working on will look at fungi communities and how they connect to carbon and nitrogen cycling in forest soils.

GC: What advice would you have given to yourself at the beginning of your Ph.D. program?

Mejía: Sometimes there's this expectation when you're doing a Ph.D. that you should be able to figure everything out on your own, that you’re already an expert. But that was a self-imposed expectation. I was afraid to ask for help.

The people at the Graduate Center were always willing to talk to me, give me guidance, and point me in the right direction. Those resources are there. …  Sometimes we can silo ourselves as graduate students. We tend to think that we just need to figure things out by ourselves and that asking for help is some kind of weakness. It’s not.

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