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Do hungry bugs stress nitrogen-fixers? Make like an insect to find out

The essential question I am exploring this summer is: Does herbivory present a major cost to all nitrogen-fixing plants in the temperate zone?

When I first arrived at Cary Institute, I was familiar with symbiotic relationships in nature – where organisms co-evolve together. But I hadn’t given a lot of thought to how these relationships were shaped by climate change. When it comes to discussing climate change impacts, most think: sea level rise, intensified weather events, emerging diseases. Rarely do we hear about disruptions to more subtle biological relationships. 

This summer, I am exploring how climate change and animal grazing shape the symbiotic relationship between ‘nitrogen-fixing’ plants and the bacteria that colonize their roots. Nitrogen-fixers are able to produce nitrogen, an essential nutrient that enriches soils and feeds plants, with the help of their bacteria neighbors. 

Since arriving at Cary, I have immersed myself in the literature on nitrogen-fixing plants and climate implications. I can hardly count the number of papers that I read in my first few weeks. Topics ranged from Mediterranean nitrogen-fixing plants to tropical species such as Inga edulis, aka the ice cream bean, which supposedly tastes like its namesake. I’ve read about varying techniques for ‘manual herbivory’ – a process wherein scientists mimic insects and ‘eat’ a plant by cutting its leaves. 

black locust saplings. Credit: Ava Adler.
Tiny black locust saplings I uprooted on the Cary property. These feisty legumes spread through root suckers, so all of the roots are interconnected. This makes it nearly impossible to uproot the plants unless you sever each root end. The plants are unfortunately not doing well in the greenhouse as a result of this traumatic event; they will probably not be included in the experiment. Credit: Ava Adler.

As these papers raced through my mind, I was in a nitrogen-fixing daze. Whenever my friends, family members, fellow REU students, and scientists at Cary asked what I was up to, I was elated to share all of the knowledge that I was absorbing. I was practically on the verge of bursting with plant species names and annual herbivory rates. I felt ready to tackle my project and contribute to the small, but now growing, body of research focused on nitrogen-fixing plants.

The essential question I am exploring this summer is: Does herbivory present a major cost to all nitrogen-fixing plants in the temperate zone? There is a substantial body of research focused on tropical nitrogen-fixers; however, few studies have looked at their temperate counterparts – those that occur between the subtropics and the polar circles. 

To help fill this gap, I set out to study the effects of herbivory on temperate nitrogen-fixing plants. First, I assembled a collection of woody nitrogen-fixers that grow in temperate regions, including black locust, speckled alder, and mountain mahogany. Then I began a series of herbivory treatments to understand how the plants respond when ‘eaten’ by insects. In this experiment, the ‘insects’ are me. By cutting parts of the leaves and spraying the plants with a substance called Jasmonic acid, which induces plant defense responses, I can observe the plants’ reactions to insect-induced stress. 

Speckled alder. Credit: Ava Adler.
Speckled alder (Alnus incana) that I purchased from an online nursery and potted. These saplings are living in the greenhouse for five weeks, receiving herbivory and nutrient treatments on a weekly basis; they're also watered daily. These pampered plants are truly living the life. Credit: Ava Adler.

I have already come across various challenges while undertaking this experiment. I’ve learned to anticipate problems and I’ve developed an appreciation for the importance of being nimble. For example, when an online shipment of black locust trees arrived too late for meaningful research to be conducted, I rerouted my initial plan. Instead, I uprooted black locust saplings from Cary’s property to use in my experiment. This isn’t a terrible loss to the system because while they are native, they are also a pest. Plus, I was grateful to get my hands in the dirt.

I am still trying to accept the fact that this project may not turn out as I had hoped. I may not be able to collect data that contributes meaningfully to the field of N-fixer plants. Regardless, I have learned so much about the challenges of undertaking a research project from emergence to commencement. Bit by bit, I have come to realize just how strong my passion for plants is. And I appreciate the opportunity to spend the summer digging up plants and caring for them – in the hopes of contributing to a better understanding of these organisms that are an integral part of our ecosystem.

scoby cc: Ava Adler
A ‘scoby’ (symbiotic colony of bacteria and yeast) that my mentor gifted me to make kombucha. I feel it shows the wholesome, caring, and conscious community that Cary Institute attracts and fosters. Credit: Ava Adler.

Ava. Adler, a student at Oberlin College, is part of Cary Institute's 2019 Research Experiences for Undergraduates (REU) program.

This summer, Ava is working with Cary scientist Sarah Batterman and Will Barker, from the University of Leeds, to study nitrogen-fixing plants in the temperate zone.