Margaret Evans displays tree cores from the recovered collection.
Institute of the Environment 2017

Using the Past to Predict the Future

Tuesday, June 28, 2016

by Paulina Jenney, Institute of the Environment

Thousands of tree cores, little rods of wood containing year-by-year information about a tree’s growth, lay forgotten in a federal storage room, gathering dust, until the collection—a research treasure trove—found its way to University of Arizona dendrochronologist Margaret Evans.

To Evans, an assistant research professor in the Laboratory of Tree-Ring Research, the cores represented a wealth of knowledge that had yet to be tapped­—an entirely new set of data that could help predict how climate change will affect our future forests.

Photo of mounted tree core

Each ring on a tree core represents one rainy season. Thicker rings indicate years with more cumulative rainfall. Credit: David Archuleta

“Historically, in the field of dendrochronology, trees were sampled in order to reconstruct the past,” she said. “In order to do this, samples were taken from trees that were particularly sensitive to climate.”

This resulted in a biased data set, one in which the most extreme examples of trees, the biggest and oldest individuals from the highest and lowest elevations possible, made up the majority of tree core inventories.

“The purpose of my research is to fill in everything in between, your sort of average Joe-shmoe forest,” she said. “This helps us to better understand how climate affects all sizes of trees.”

With this information in hand, Evans and her team can begin predicting how changes in temperature and precipitation will affect many different types of forests, including those in popular recreation spots in the Santa Catalina and Santa Rita mountain ranges near Tucson.

In 2015, Evans was awarded a Faculty Exploratory Research Grant from the Institute of the Environment, which enabled her to hire a team of undergraduate students to help organize, catalogue, and analyze the boxes of tree cores sent to them by the Forest Service. The cores represent almost 1,000 different forest plots from all over Arizona and the interior western states.

Her work using the cores is particularly relevant here in the desert Southwest, where temperatures are increasing faster than in other parts of the U.S. Additional environmental stress, such as extreme heat and drought, has the potential to reduce the amount of plant life in the region.

Understanding how trees have responded to climate change in the past can help forest managers predict how trees will respond to present-day climate change and take appropriate action.

“Forest managers can’t change the climate,” Evans said. “But since we have samples from many, many plots with different stand conditions, we can start to tease out statistical information about how they can respond to climate change.”

For example, she said, if the evidence suggests that thinning out forests will give trees the best chance against drought stress, managers can begin implementing thinning strategies, which in the long-run could provide for a greener and cooler Arizona.