In Kruger National Park, along the northeastern edge of South Africa, Madelon Case looks for trees the elephants knocked down. Maddy, a second year Ecology and Evolutionary Biology PhD candidate in the lab of Dr. Carla Staver, travelled to south Africa to collect tree samples. Her elephant-facilitated data will help her understand the complex relationships between tree growth and rain—the drivers of this ecosystem.
Maddy started by walking the park.
“I’ve been looking for the trees for the tree ring study. I haven’t been taking cores for this: it has to be cross sections because these trees have really dense wood. Using an increment borer as people often do for dendrochronology doesn’t work. Because I have to take these cross sections—and I’m in a national park: you’re not allowed to cut down living trees—I’ve been going around to trees that have been toppled by elephants. Which, it turns out, happens pretty often.”
The elephants bring these trees down to get at the high leaves: good food. The toughness of her subject, Acacia nigrescens or “knobthorn," means Maddy and a park ranger take cross sections with chainsaws. They look in groves that are set back from the road, she said. Park administrators don’t want cutting debris in a high tourist area.
“I picked the tree based on being widespread. I would be able to find it on multiple soil types in multiple sites with rainfall records—but it seems like it helps that it’s also a favorite tree of elephants.”
“You get to a place and it just seems like they were knocking over every tree.”
Maddy will use the elephant damage to stalk a more difficult prey: the answers to theoretical questions.
“Savannas are defined as places that have a continuous grass layer and scattered trees, so one of the big questions here is what allows them to co-exist. How might changes in rainfall variability affect the interactions between the trees and the grasses? This is a system that has very important fire feedbacks—how might that propagate through the systems to give you a change in vegetation?”
There isn’t a simple relationship, Maddy says, between trees and water. The relationships between all the players on the savanna, including water, trees, grass, and fire, are complex. What will happen in response to rain is not clear. Instead of just encouraging tree growth, rain might produce much more grass—and then all that grass would die and become fuel for fire. The fire limits tree growth. How these feedbacks and relationships work, Maddy suspects, is written in the wood.
“I’m looking at the long term records of growth that trees keep themselves, because there aren’t any long term records of trees in these systems.” In particular, Maddy is seeking signs of delayed or long term responses to changes in precipitation patterns. Like the drought in which Kruger is currently embedded.
A theoretical as well as plant ecologist, Maddy is fascinated by the complexity of systems responses. Multiple realities can exist for a given set of conditions. How does one become actual over another? In an area that has been protected and monitored as a national park for over a century, that possesses long-term climate data sets, she can track this complexity, following signs of related events.
Understanding ecosystem responses to rain has far-reaching implications. It addresses processes with social as well as ecological impact.
“A lot of human livelihoods worldwide depend on using savannas as rangelands, where people graze cattle and other livestock. Woody plant encroachment can be a big problem because it reduces how much grass is available to feed animals. Here in Kruger, people depend on the landscape in a different way—it’s a major engine of tourism. Woody encroachment here changes the quality of the animal habitat and the aesthetics of the landscape.”
“I hope that my research will help us understand more about the responses of plants and ecosystems to environmental variability. No organisms live in a perfectly static environment.”
And as global change potentially gives us a more erratic world, Maddy adds, understanding how slight changes affect how living things grow, interact with each other, and coexist will be essential to land management.
She is finishing harvesting her samples, claiming data from those felled knobthorns. Now, Maddy’s biggest concern is lugging her sixty cross-sections of dense wood across an ocean. Then the real tracking begins, back in the Yale lab.
Maddy Case is currently in Kruger National Park on a dissertation pilot grant from YIBS.
Article and Interview by Juliana Hanle