Understanding How Tropical Forests Recover After Human Disturbance

Joanna M. Foster '08 for the Princeton Environmental Institute ・ High Meadows Environmental Institute

When Connor Stonesifer ’16 first arrived in Panama in the summer of 2015, his Spanish consisted of saying “I want” and having to point at things. When he asked the woman who ran the hostel where he was staying to turn up the air conditioning, he received a cup of pudding instead.

“It was not exactly an auspicious start to my first foray into independent field research,” said Stonesifer, an ecology and evolutionary biology major at Princeton University, who had traveled to Panama to study nutrient acquisition strategies among different tree species in recovering tropical rainforests as part of his senior thesis.

Yet, having been told by his faculty adviser Lars Hedin, chair of the ecology and evolutionary biology department, that the senior thesis project is as much about creatively working through problems as any particular research question, Stonesifer ate his pudding and decided that his foreign language skills were an obstacle and not a barrier and focused his attention on what had brought him to Panama in the first place – trees. Specifically, why trees that spend enormous amounts of energy fixing nitrogen, actually grow faster than non-fixing trees that get nitrogen for free.

For six weeks in the summer of 2015, Stonesifer’s research laboratory was a forest: the Smithsonian Tropical Research Institute’s (STRI) Agua Salud Project site near El Giral, Panama. The area is a rainforest recovery study site consisting of cleared and degraded land in various stages of recovery in the Panama Canal watershed.

Stonesifer’s research was funded by the Becky Colvin ’95 Memorial Award. The award is presented each spring to a rising senior by the Princeton Environmental Institute (PEI) and the Department of Ecology and Evolutionary Biology in support of field research projects critical to the senior thesis.

“Broadly speaking, this area is important to study because these tropical forests are one of the largest terrestrial carbon sinks on the planet,” said Stonesifer. “So understanding how these forests recover after human disturbance and what keeps them healthy has global climate consequences.”

A forest’s ability to regrow and recover depends, to a large extent, on the availability of the key soil nutrients nitrogen and phosphorus. Both nutrients are crucial to tree growth, but nitrogen is the limiting growth nutrient in young forests, while the abundance or scarcity of phosphorus sets the pace of growth in older stands.

Stonesifer said some researchers, observing the astounding growth rates of nitrogen fixing trees, have hypothesized that these species grow faster than non-fixers, because they invest excess nitrogen into capturing more phosphorus. An alternate theory, he added, states that the process of nitrogen fixing itself requires phosphorus and that phosphorus left over from this process could potentially be redirected to support growth.

In either case, Stonesifer reasoned that there should be evidence of increased activity to acquire phosphorus in nitrogen fixers when compared to non-fixers and that that difference should become less pronounced in older forests as nitrogen fixation slows down.

“Trees get phosphorus in two ways, phosphatase enzymes and mycorrhizal associations,” explained Stonesifer. “I like to think of phosphatase enzymes as little scavenging molecules that are sent out to get phosphorus for the plant. Mycorrhizal associations are more like trading posts where symbiotic fungi associated with a plant’s roots offer phosphorus in return for carbon.”

Swathed in Gore-Tex and leaving a trail of bug spray in his wake, Stonesifer collected root samples from three species of nitrogen fixing trees and three species of non-nitrogen fixing trees in both young (12 to 24-year-old) and older (50+ year-old) forest plots. Stonesifer had help finding the species he was looking for from a local STRI botanist, who showed up every morning in a pressed button-down shirt and carefully polished Oxford shoes.

“He always dressed like it was casual Friday at the office, while I was layered in protective gear ready for a day in the jungle,” said Stonesifer. “But somehow all the ants found me and gnawed through my gloves, while he stuck his hands into bushes seemingly at random and pulled out handfuls of butterflies.”

When he wasn’t in the field fending off ants and trying to untangle which roots belonged to what trees, Stonesifer spent his time in Panama analyzing his root samples for phosphatase activity in a lab at STRI. He then packed up the rest of his root samples and shipped them to the University of Georgia where he spent his intercession break that year glued to a microscope in a lab at the Odum School of Ecology. But one week was not enough to count every single filament and vesicle, so Stonesifer spent another month back at Princeton in his adviser’s lab tallying up his data.

Interestingly, Stonesifer found that fixers and non-fixers do not differ significantly in the extent or type of phosphorus acquisition strategy each employs. The only factor that predicts phosphorus acquisition is the age of the soil.

“My research is the first to look at both phosphatase activity and mycorrhiza associations in this context,” said Stonesifer. “And the findings contradict two of the current contending theories explaining the growth of nitrogen fixers in young forests.”

Stonesifer also found that in older forests, trees weren’t actually increasing the quantity of mycorrhizal associations: they were increasing the quality, by increasing the number of sites for nutrient exchange known as arbuscules. This strategy, Stonesifer believes, may allow trees to devote more carbon to growth, while increasing phosphorus uptake.

Stonesifer’s faculty adviser, professor of ecology and evolutionary biology, Lars Hedin, is eager to see Stonesifer’s results published.

“Connor’s research addresses one of the major unknowns about how tropical forests respond to climate change and increased carbon dioxide in the atmosphere,” said Hedin. “It’s important because if these trees have ways to get phosphorus, they will continue to serve as a carbon sink, but if they don’t, the carbon sink could shut off.”

“What sets Connor apart is not only his academic and intellectual abilities, but also his leadership skills and ability to work with others and navigate challenging situations,” added Hedin.

For his part, Stonesifer is grateful to Hedin for giving him the opportunity to discover his abilities.

“In academics, and especially lab work, there can be a lot of hand holding,” said Stonesifer. “Either you’re following step-by-step directions in your manual or your mentor is right there next to you talking you through everything. But in Panama, I had to take complete control of my project. I had to teach myself how to do certain things and when I couldn’t, I had to find the people who could help me. It was incredibly empowering. I realized that I had skills that I didn’t even know I had. I learned so much about how to be confident and go after something and make it happen.”

Stonesifer’s next big adventure is medical school, but first he intends to spend next year at home in Tampa, Florida working at a medical neuroscience lab studying traumatic brain injuries. He says he plans to lighten the intensity of the lab work by writing for comedy websites and reviewing comic books. 

In addition to Connor Stonesifer, Kathryn Grabowski ‘16, civil and environmental engineering major, was also awarded Colvin funds last spring. The Becky Colvin Memorial Award supports summer field research projects in support of the senior thesis. The award was established in 1995 by Dr. and Mrs. Robert Colvin in memory of their daughter, Becky Colvin ’95. Becky was an ecology and evolutionary biology major who was very interested in field research. Students are selected to receive the Colvin Prize by competitive application in the spring of their junior year.

Additional information about the Becky Colvin Memorial Award and prior recipients of the prize is available on PEI’s website.