I usually study population dynamics in the upper Sonoran Desert near Tucson, but since I am still entering data from last summer’s fieldwork and analyzing it, I took my laptop down to Brazil for nearly a month. While I’m here, I also get to climb massive trees and learn about the questions and methods used by Scott Saleska’s lab to better understand the future of the Amazon under climate change.
Climate models predict wildly different things about how the Amazon Rainforest will respond to a new climate, but all agree that whatever happens will have a big effect on the rest of the world’s climate. There is a lot of carbon stored in these trees, and if they release that to the atmosphere through dying or burning, it will accelerate global warming. On the other hand, if they grow bigger and faster, they might actually buffer the climate by taking up more carbon dioxide.
One problem is that no one really knows how these trees respond to changes in the climate. Our best understanding, captured in mathematical models, predict that drought would cause the forest to die back, looking barer and browner. Yet during recent droughts, the forest appeared to green up! Was it because the forest is light limited and fewer clouds meant more light for the plants to grow? Would that response continue if the drought continue?
It turns out that in a very diverse forest like the Amazon, no one really knows how the leaves behave. Unlike in, say, New Hampshire, where trees all put out buds around the same time and then lose their leaves around the same time, this forest appears evergreen. Since leaves don’t live forever, we might assume trees replace their leaves. But how often? All at once, or on a sort of rolling basis? Regularly, or in response to some environmental signal? Tree species might behave very differently – after all, some grow fast in response to forest gaps, while others grow slowly in the shade of larger trees until they reach the canopy. So how similar are their leaf lifespans and replacement strategies? These answers are pretty difficult to answer, especially because walking up to a tree whose lowest branches are over 100 feet in the air makes it very difficult to look at their flowers – or even their leaves.
Fortunately, with improved arbor techniques, climbers can now safely access much more of the canopy to measure and tag individual leaves. Imaging and remote sensing might be calibrated to in the future provide an even faster way to measure leaf age and species identification. I’m enjoying learning both techniques for measuring leaf traits and leaf demography as well as for climbing into the canopies. And I’m especially enjoying thinking about a completely different set of scientific questions: questions about movement of water and carbon through a whole ecosystem, instead of competition, predation, and population dynamics.