The Heat is On for Oak Adaptation

To most of us, the oak leaf has an almost unmistakable shape. Its characteristic lobed sides make it one of the first trees we learn to identify, but leaf shape is also helping researchers identify the impacts of changing climate on ecosystems and their key species.

Kyra Prats is a Master’s student at the Yale School of Forestry and Environmental Studies and spent the summer of 2015 in Spain taking samples for a study on the impacts of drought on cork oaks. Working with a team of three field assistants, she found a way to observe the effects of climate change over time: The changing form and shape of cork oak leaves.

“We wanted to know how climate change is affecting cork oaks in Spain, which serves as a good model for an already dry Mediterranean ecosystem that is getting drier and hotter. The best way to do this is to see whether the trees have responded to changes over the past few decades, a comparison over time. Because properties like leaf thickness can’t be maintained in a leaf sample for long, we found ourselves focusing on shape as an indicator of these changes,” said Prats.

“So, how does leaf shape change with average temperature and the amount of water available to a tree, and why?”

“Actually, leaf shape is really influential in a tree’s ability to regulate temperature. A large edge or perimeter relative to area means that leaves can regulate their temperature and cool off more efficiently. Other functional traits like the thickness of the leaf and the stomata [small pores in the leaf surface] characteristics can tell us about the ability of a tree to assimilate carbon and how well it retains water.”

To understand how trees may have changed in the changing climates of Northern and Southern Spain, Kyra Prats went to the extensive herbaria in Barcelona and Madrid  for leaf samples from as far back as the 1890s. Each herbarium sample was collected from a specific location that Prats and her team planned to revisit in 2015 to cut new leaves from the canopy and compare them to their dried and mounted predecessors.

“We tried to find the exact locations of collection for each of the herbarium samples we worked with so that our comparisons would be exact. This meant we had to spend days on the road, traveling to find the right cork oak forests for comparison. But because we covered such a large area we can actually analyze our data over a spatial gradient as well as time,” says Prats. This means they can compare trees in the south of Spain to those in the north and today’s trees to those that grew 30 years ago.

Now that they have been collected, Kyra Prats is analyzing her samples in the lab. Though the results are not yet in and the project is set to develop throughout the year, there are indications that trees are indeed changing to meet new, challenging conditions. According to the researchers, this can be key to understanding the larger functioning of the ecosystems that surround these cork oaks. Oaks form the backbone of many culturally and economically important ecosystems worldwide, as work profiled in a previous Field Notes article also highlights. The preservation of these species in these times of rapid climate change is a priority that cannot wait.