What do old trees tell us about future water?
Looking through a microscope Dendrochronologist with the U.S. Forest Service Justin DeRose circled what could be seen as a "false ring" on a piece of douglas fir at the dendrochronology workspace at Utah State University on Friday, March 13, 2015.
A microscope illuminates a douglas fir sample at the dendrochronology workspace at Utah State University on Friday, March 13, 2015. Graph paper is used as a measuring tape to captures patterns in tree rings to help identify the years of each ring. This information can be used to identify years of excess rain and drought.
Dendrochronologist with the U.S. Forest Service Justin DeRose shows off some of the douglas fir and Utah juniper samples they have brought back to study at the dendrochronology workspace at Utah State University on Friday, March 13, 2015. A measuring tape along the edge of the table captures patterns in tree rings to help identify the years of each ring. This information can be used to identify years of excess rain and drought.
Trees respond to water like all living things, and they make useful records of dry and wet seasons.
When it’s wet, the trees flourish. When it’s dry, they’re stressed. All those responses show up in a tree’s rings. Using those ring records, dendrochronologists have been able to take a look back in time and get a sense of water and drought in the West.
Last month, scientists at Utah State University, Brigham Young University and The U.S. Forest Service announced they’d traced the Bear River’s stream flow back 1,200 years. That’s long before Mormon pioneers started building the first towns and cities in the area, and longer than any other tree-ring record in northern Utah to date.
“One of the key messages is there is no ‘normal,'” said Roger Kjelgren, a professor and plant scientist at USU. “(Northern Utah) really is like a grandfather clock. It oscillates back and forth, moving between orbiting around a dry period and then shifting and oscillating back to a wet period.”
The trick is lining up all that wet-dry variation with climate models. Climate models do a good job at predicting temperatures, but they’re not as good at predicting future precipitation. That’s where the trees can help.
“I wouldn’t say it’s a match made in heaven, but it’s kind of a jigsaw-puzzle fit, taking the past tree record from the tree rings and combining it with the climate models to get an idea of these cycles,” Kjelgren said.
During a wet season, trees drink in water and grow proportionally. As the season dries, they harden and cell walls get darker. That’s how tree rings form. During dry years, the rings will be tiny, sometimes requiring a microscope to see.
The Bear River tree-ring study went back 1,200 years with the help of Utah juniper trees, a particularly finicky and telling species when it comes to water.
“It maintained its sensitivity going back in time,” said Justin DeRose, a U.S. Forest Service scientist and the lead author of the study. “So we decided to go with Utah juniper against the advice of some folks.”
When dendrochronologists study trees, they don’t just look at live ones. They scavenge for dead wood in the area, then line up the tree-ring records of dry and wet periods. The process, called cross-dating, helps scientists take the chronology back further in time.
“It also helps us work out any problems if there are particularly bad years, and some trees deiced not to grow a ring on a part of the stem which is called an absent ring, by having a couple of cores per tree or cross sections, can find out where those occur across multiple trees,” DeRose said.
Juniper are “sensitive” because their variability in tree-ring width is high compared to other trees.
“For Utah juniper, everything’s’ this year. It lives for today, it’s very Zen-like,” Kjelgren said. “It lives in the now, and if there’s no water it doesn’t grow. If there is water, particularly in the springtime, it puts everything it can into foliage and trunk growth.”
In drier climates, like the Southwest, juniper trees often have false rings because the region typically has two wet seasons — spring runoff and summer monsoons. The region is also prone to long drought, which means junipers won’t grow any rings at all.
That’s not the case in northern Utah.
“Up here, we have one distribution of precipitation — we get precipitation in the winter, we get snowmelt, then in the summer it’s dry,” De Rose said. “It’s one of the reasons we love living here.
Utah juniper thrives in dry, rocky areas, where older specimens are often better preserved. It’s also one of the most prolific trees species in the state.
The ancient Utah juniper DeRose analyzed in his study found significant cycles of drought in northern Utah. One, during the medieval period, lasted seven decades. Most periods of wet and dry in the region lined up with climate models for the region.
“Some of these groups of rings are big, and you can see them with your naked eye, and there are some individual years that are small in there,” DeRose said. “This isn’t surprising coming from northern Utah because what the climatologists have found in the instrumental record is we tend to have this 10 to 15 year cycle of wet and dry conditions.”
Teaming with climate scientists, dendrochronology researchers can help make climate models stronger, especially in terms of precipitation. And in northern Utah, where so much of our urban water supplies comes from snowmelt, they’re also a useful tool for water managers.
“We really want to try and work with them to understand, there is no normal here,” Kjelgren said. “It’s better to work with the public, the water consuming public, to understand that moving back and forth in a pendulum that may be amplified by climate change.”
It’s a foregone conclusion that the climate is changing across the planet, and those changes will have a big impact on Utah’s water supply. Good climate models are one of policymakers’ and water manager’s best defenses.
“We don’t want to create a scary movie-like topic of ‘climate change,'” Kjelgren said. “It is what it is, and we need to come to terms with that. How do we manage that water, and the potential amplifications that will come from climate change? So let’s not use the word ‘scary,’ let’s just say we’re going to be adaptable.”