Climate Conversation: Jennifer Bhatnagar

Jennifer Bhatnagar studies the impact of human activities on the Earth's microbiome at Boston University. Her current field of study is how urbanization affects tree growth and health. Urban trees grow faster than rural ones, possibly due to elevated CO2 and adaptations to collect nutrients from the atmosphere. However, they die younger, with 50% dying in the first five years, likely due to a dysbiotic microbiome with higher pathogen concentrations. Jennifer and her team are using isotope tracers to measure nutrient uptake in urban and rural trees to discover more about the mechanisms behind the discrepancies in growth rates and healthspan of urban and rural trees.

Cleo Carney: So, firstly, to start off, can you tell me and our audience a bit about your research and how you got involved with it?

Jennifer Bhatnagar: Sure. So I'm an ecologist. I'm an ecosystem ecologist and a microbial ecologist, which means I study microorganisms in the environment. We call that the environmental microbiome. So that's the collection of all the smallest organisms that you cannot see with the naked eye: bacteria and fungi, archaea, viruses and things like that. [These bacteria] live in the environment, and I study how they work as a big community to drive some of the big environmental processes we see on Earth, like plant growth, movement of energy and elements through land systems into the atmosphere and the production of greenhouse gasses. My research really focuses on human activity and how human activities are changing the earth’s microbiome, and what the microbiome is doing. So I study things like elevated carbon dioxide, atmosphere, rising air temperatures, pollution, wildfire, [and] urbanization. All these types of changes caused by humans are impacting microbes and what they do in the environment. I got involved with [this line of work] because my mother was a botanist by training, and so I spent a lot of time outside as a kid. And I was always very interested in science. And so I ended up going to college for science, and I decided that I wanted to do environmental science as my career.

CC: Very cool. Thank you for giving me that background. I can only imagine that your field has a lot of new and exciting opportunities right now, for better or for worse, but also, there's a lot to study. I mean, the microbiome of the planet must be so vast — there are enough bacteria in our own gut microbiomes.

And so your research that you've recently been involved with found that urban trees grow four times faster than rural trees. And now, correct me, if I'm wrong, you and your coworkers are trying to look at why that is? Do you have any hypotheses of potentially why it is? Or is that something you cannot talk about? 

JB: Oh yeah, no, we do have hypotheses. It's been a mystery to us for some time. This was a discovery that was made by my collaborator, Lucy Hutyra. She discovered that city trees grow about four to five times faster than the same type of tree in a rural forest, and there are a lot of things that are different in a city. Some of those things would promote tree growth, like elevated carbon dioxide in the atmosphere can fuel photosynthesis by the tree. So you would imagine a tree would take advantage of that in a city. Another factor is that city trees are often planted at a distance from one another, so they’re not competing for light. But the city also has really tough conditions for a tree. It's much hotter, and the soil is much drier, at least in this part of the world. There are a lot of pollutants. The root systems are constricted by sidewalks, for example, when we plant trees in a street, and there aren't a lot of nutrients actually available below ground for them. So we have a hypothesis, this is a bit of a wild hypothesis. It's kind of ludicrous, but one thing we are thinking is that perhaps because there is so much pollution in the city from car exhaust and industry and things like that, that perhaps trees and cities are operating by a different rule of life, where they are shifting their strategies for collecting nutrients from below ground to above ground. Maybe they are collecting nitrogen and phosphorus, and some of these are elements they need to live directly from the atmosphere, as opposed to from their roots, which they typically would do in a forest. And so we are designing experiments to test that idea.

CC: Very fascinating. And what sort of experiments are you guys going to do to test that?

JB: Yes. So we have a whole suite of different ways that we are approaching this. We are focusing on a particular genus of tree: oak trees because we can find them in the city. They are planted as a shade tree, but we can find them in the rural forests here in New England. So we can look at the same type of tree across an urbanization gradient, from the very, very urban center of Boston, all the way out to Western Massachusetts. That’s quite [a] long [distance]. So we are studying the trees along that gradient, trying to quantify their sources of nutrients and how much pollution is getting deposited onto the trees along that gradient. We are also planning to do experiments this year where we will actually add a source of atmospheric nutrients to the tree, and we will add an isotope tracer to that nutrient solution. And what the isotope tracer does is, for example, for nitrogen, we will add nitrogen to the tree leaf, which is a heavy isotope of nitrogen. It has an extra neutron in the nitrogen atom, and it makes it heavier. The vast majority of nitrogen in our world is 14, and we are adding a 15 N heavy isotope of nitrogen. And because we add this — a very small amount of this — we can actually trace it through microorganisms on the leaf, through the tree leaf and into the tree stem to see how the tree takes up that form of nitrogen. And we can compare it with uptake from the roots to get a measure of the rate of nutrient uptake in these different parts of the tree. We are also going to do similar types of experiments in growth chambers, where it's much easier to manipulate trees. So we do saplings for that, much younger trees. We can remove micro organisms [in growth chambers]. We can add them, we can add pollutants. We can take them away, and we can see how these different components of an urban system interact to influence the tree's growth.

CC: That sounds very comprehensive; thank you for your simple and easy to understand explanation, because I am just trying to get my head around the idea that trees could be operating on a different rule of life. But it makes sense if they were to have this sort of shifted rule of life. 

I am intrigued, has the research you and your peers have done found that they live less long due to these environmental stressors and pollution?

JB: So, city trees die two times younger than the same tree in a rural forest. So they do die faster, too. The title of Lucy's 2019 paper that reported this is Live Fast Die Young. 

CC: Great name. 

JB: There are multiple reasons why city trees die faster than rural trees. We prune them, so that they don't fall into our houses and get tangled in our power lines, and that contributes to tree death. But even young trees will die in some parts of the city of Boston. The trees die at a rate of 50%, so 50% of the trees [in Boston] will die in the first five years. So we are actively trying to figure out why that is. We have recently discovered that trees in the city suffer from dysbiosis in their microbiome, which is similar to dysbiosis in the human microbiome. You [can get dysbiosis] if you are sick, that can cause a shift in your microbiome from having a lot of beneficial microorganisms [to harmful organisms]. For example, in your gut, [the organisms] that can help you digest fiber and things like that [can shift] to non beneficial microorganisms that contribute to obesity and other health issues. My PhD student Katie Atherton, just submitted this for publication: that trees in cities [have] a very high concentration of pathogens and they have lost many of their mutualists. [Mutualists] are fungi and bacteria that help give plants nutrients and help them tolerate stressors. So we are very concerned about the state of these trees [in cities]. They do grow quite quickly, but they are not necessarily healthy, and so we are likening this almost to…a diabetes patient, where you have, for example, an individual that is consuming a lot of, like, easily available resources. In the case of the city and a tree, it would be above ground pollutants, and that can cause really rapid biomass gain, but also increased disease state.

CC: That is incredibly interesting, that our human ways seem to be influencing the way plants live. I feel a bit bad.

JB: Yeah. Well, this is a hypothesis at this point. Urban places, like cities, were created by humans, for humans, and so the other organisms that live there have sort of been neglected. We have sort of left them behind in terms of our study of them. Many environmental scientists have not studied cities for a long time. They did not think that cities were really ecosystems worthy of study, and so we are really behind in understanding how non-human organisms survive and tolerate city conditions, and it's an important area of research, I think, because cities are growing very, very rapidly. [Cities are] the fastest growing biome, actually. 

CC: Wow. I look forward to seeing where your rules of life in the urban biome project goes, and we will definitely be checking back in once you have some findings we can see how the hypothesis plays out. Also, to discuss solutions that we can implement to try and help these trees to grow fast, but maybe not die so young. 

Thank you so much for chatting with me. 

JB: Cleo, so nice to talk with you.