UA 'Chemical Detectives' Use Tree Rings to Better Understand Monsoon
Matt Dannenberg, Paul Szejner and Erik Anderson core a ponderosa pine tree in Northern Arizona's Kaibab National Forest. (Photo by Emily Litvack/Research, Discovery and Innovation)
(From left) Erik Anderson, Jia Hu, Paul Szejner and Matt Dannenberg at Grand Staircase-Escalante National Monument during their fieldwork travels this summer. (Photo courtesy of Paul Szejner)
The team visited a site called Pine Valley in Utah's Dixie National Forest. (Photo courtesy of Paul Szejner)
A dusty Dodge Caravan rolls up a dirt road. The scientists park at a campsite and unload a cooler, a plastic tub full of wood slivers called "cores," a few tools for plucking cores from pine trees, a box of glass vials, a roll of blue tape, and a permanent marker. Here, in the Kaibab National Forest along the South Rim of Grand Canyon, they'll take cores from 25 mature, healthy trees before doing the four-hour-long trek back to the lab in Tucson.
The team of University of Arizona scientists includes Jia Hu, assistant professor of riparian ecology, Erik Anderson, a research specialist, and Matthew Dannenberg and Paul Szejner, postdoctoral research associates.
It's mid-June, and the team members are at the last of seven sites in the Southwest that they've visited over the course of eight days. They're ready for a real shower and a long nap but enthused about all they accomplished – that is, bringing pieces of nearly 200 ponderosa pine trees back to the UA Laboratory of Tree-Ring Research for study.
The goal: using tree rings to better understand the North American monsoon. How far north has the monsoon traveled over time? Has it changed much? How does it drive and stall the growth of pine forests in the region?
It's chemical detective work, because the answers are all recorded – in the form of oxygen and carbon – in the rings of trees.
Hu, Anderson, Dannenberg and Szejner begin walking through the forest, sizing up the ponderosa pine trees. They need ones that look big and healthy enough to make good samples. For fun, they guess the diameter of a tree before measuring it (Hu gets one right on the money). They race to see who can twist a core out of a tree fastest (Szejner, most of the time). When they've got samples from six trees, Anderson jokes, "We're a core-ter of the way done."
When they've finished, they wander back to the campsite and settle into chairs for a lunch of carrots, hummus and pita bread. They reflect on the time they've spent collecting data over the last week.
"With this project, we're looking at the annual variation of the monsoon in this region, how far north it goes, and how long it takes for a tree to recover after an extreme event," Szejner says.
Climate variation, including extreme events like drought, is a matter of life and death for ponderosa pine trees, which are widespread across the Southwest. People often assume a decrease in rain over the years is causing the demise of more and more pine trees. In fact, it's not that the Southwest is getting less rain. It's just becoming more arid.
"If we look at the last 100 years, there's been a lot of variability, but we're not getting less precipitation," Hu says. "And total annual precipitation alone isn't always enough to predict tree mortality. Aridity is a big, important culprit."
"And even if the mean precipitation hasn't changed much over the last century, how it's distributed is changing," Dannenberg adds.
And that matters because, put plainly, climate extremes like bone-dry summers are becoming more common.
"Now we see extremes every three years or so," Szejner says.
Increased aridity of the region is bad news for trees. Dry air makes it harder for them to transport nutrients from roots to needles, harder for them to grow, and makes their leaves dry out. Tree health depends largely on moisture in the air, so, unsurprisingly, the humid climate during the monsoon helps them thrive.
"We owe the Sonoran Desert to the monsoon. It's a very distinct source of moisture during the summer. Without it, the Sonoran wouldn't exist," Szejner says.
"The monsoon is a really unique feature of the climate system in the Southwest and a really important water source here," Hu adds.
Because ponderosa pine trees are always chemically recording information about their environment, the team can track the effect aridity has had on the trees for as long as they've been alive – sometimes 100 or 200 years.
"Aridity is the main variable we're after," Szejner says.
The trees put on annual rings that are related to precipitation. The wider the ring, the more rain that fell that year. When they take the cores back to the lab for analysis, the team first will measure the amount of oxygen and carbon isotopes in tree rings across seasons by carefully slicing each ring into three pieces where the wood looks distinctly different. For example, wood in the late summer looks dark and dense while monsoon-time wood appears pale. Where Szejner sees those shifts in appearance is where he'll slice each ring. It's meticulous, like a surgery, but useful.
"By capturing seasonality, we'll highlight the sensitivities of these trees even at the subannual scale," Hu says.
It's something that the scientific community hasn't done much but could provide very important information in a region like the American Southwest.
The "sensitivities" will appear as varying levels of oxygen and carbon isotopes. The fibers of these tree trunks record the chemical composition of their source water, such as rainfall, exactly as it came to the tree. By the time it gets to the pine needles, that water appears different. That change depends on aridity of the atmosphere. When it's especially dry, the needles will have a lower oxygen-16/oxygen-18 ratio compared with when it's wetter, because oxygen-16 is lighter and easier for the tree to transpire.
Because the data on ponderosa pine needle isotopes is already plentiful, the team will be able to compare exactly how different the source water in tree rings is from the water in pine needles. That difference shows what kind of effect aridity had on the tree that season.
"We might also be able to say that trees in the Southwest with a constant monsoon, like those in Tucson, will be better off than those that don't (have the monsoon), like in Flagstaff," Szejner says.
Not only will their findings offer a fresh perspective on the future of forests in the Southwest, but it has potential to be extremely valuable to forest managers, who could use the information to make decisions that preserve the forests.
"We can identify populations of trees that are vulnerable before they die because we'll see exactly how sensitive they are to aridity based on the tree ring data," Hu says.
A forest manager who receives an early alert for tree mortality can say "let's 'clean' this forest to reduce the amount of water it needs and we can get more water in the soil for the trees that need it," Szejner says.
The controlled burns Szejner is referring to come with the added benefit of reducing risk of wildfire.
The team's work, funded for three years by the National Science Foundation, will take its members right back to this Kaibab campsite in early September, when they plan to resample the same seven sites, after the monsoon has come and gone.
A version of this story originally appeared on the Research, Discovery and Innovation website.