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Root Respiration-Vital Signs of a Forest

By Lon Tegels
College of Forestry, Agriculture and Natural Resources
The University of Arkansas at Monticello

Fast facts

  • When Do Tree Roots Go Dormant?
  • Soil Temperatures Connected to Root Respiration
  • Manager’s Decide When to Flood Green Tree Reservoirs

Doing forest research can be a lonely occupation. Ask the University of Arkansas at Monticello graduate student Jonathan Kressuk. The sun isn’t even up yet. It’s the first hard freeze of the season in Southern Arkansas. Kressuk is wading through a foot of water, hauling bags of ice into the forested wetlands in Cut-Off Creek Wildlife Management Area near Dermott, Arkansas.

His mission is to chill the stems of the willow oak seedlings growing naturally on the forest floor. He will measure tree roots’ respiration, akin to breathing in animals, at various air temperatures. 

“Most people think of the leaves of trees taking in carbon dioxide for photosynthesis and releasing oxygen, but roots use oxygen and sugars produced by leaves to get their energy-releasing carbon dioxide, just like humans and animals do when they breathe,” said Babst.

Kressuk is part of a five-year research project with Ben Babst, UAM associate professor and Arkansas Agricultural Experiment Station researcher with the Arkansas Forest Resources Center. Ultimately, it will provide forest landowners and forest managers with information about when it is safest to flood their bottomland hardwood forests within greentree reservoirs (GTRs) without damage to the trees or their root systems. Ducks love flooded forests. Certain species of oak trees provide small acorns that ducks thrive on, along with other ecosystem benefits. The key is to flood the forest, providing an attractive environment for ducks without killing off the oaks that comprise their food supply.

Kressuk is trying to determine the role air temperatures play in tree dormancy. “Today, I’m testing the effects of two different air temperatures on root activity on willow oak seedlings, trying to see if air temperature affects how roots are growing,” said Kressuk. 

The experiments are part of a larger project related to human-made flooding of southern Arkansas’s bottomland hardwoods. Kressuk’ s master’s degree involves measuring tree roots’ activity when the stems experience different air temperatures. 

Roots grow too slowly to measure growth in a single day. But root respiration tends to track growth, giving us a snapshot. This root respiration tells us how much overall root activity is happening now. Kressuk said he also spent a good deal of his research measuring root growth directly by pulling up the roots from the ground, drying them out, and measuring their weight.  

Historically, bottomland hardwood forests used to flood naturally and seasonally. But as European civilization migrated west, much of that natural flooding was eliminated when flood control measures were implemented to allow farming. Decades ago, landowners saw ducks flock to these forests when they were flooded and saw the value to both ducks and duck hunters in making this flooding more reliable. 

Evidence now clearly indicates the challenge for landowners and forest managers is to find balance. If land managers flood too soon or leave the water too late in the season, tree roots can’t breathe and eventually die. According to Babst, some bottomland forests have already suffered tree death because of years of constant flooding. 

“During the last few decades, people realized that trees were dying in these GTRs. These are forests flooded with the help of small dams to replicate what the hydrology used to be before Europeans settled and built levees along many of the rivers and streams in this region.”

The Arkansas Game and Fish Commission (AGFC) is funding the Arkansas Forest Resources Center research project at the University of Arkansas at Monticello, in part with grant funds from the Pittman-Robertson Wildlife Restoration Program (Grant #AR-W-F14AF00290) of the U.S. Fish and Wildlife Service. The research aims to determine the best conditions to flood GTRs and how long the water can sit without doing damage to trees. This information will help inform the AGFC’s ongoing efforts to ensure long-term, sustainable management of these limited habitats. 

On this day, Kressuk’ s effort involved packing ice around seedlings growing in forest wetlands. He measures the respiration rates (emission of carbon dioxide) of the roots from seedlings that were chilled by ice or exposed to ambient air temperature. 

According to Babst, we don’t know a lot about root respiration in trees. But what is known comes from research focused on roots during the growing season, not during winter dormancy. His project involves exposing the roots of willow oak seedlings to decreasing temperatures. His research team is trying to measure how and when root growth and respiration change during the transition from fall to the dormant winter season. Babst’s research tries to determine when root dormancy begins and ends and what kinds of conditions must exist for dormancy to occur. 

Those conditions may involve above-ground air temperatures and below-ground soil temperatures and the species of trees being flooded. Some tree varieties handle the water load better than others. “When root activity and oxygen demand are low, we think the roots will be more protected from the damage ordinarily caused by flooding, which reduces the amount of oxygen available to roots,” said Babst. “If this is correct, and if landowners can time flooding their forests more accurately, they may be able to provide habitat while doing minimal or no damage to the tree roots.” 

Babst said his research is made up of two different components, temperatures, and flooding. The next stage is to test whether roots are more protected during winter dormancy. His research team wants to know what happens to root activity during the fall when changes in the day/night cycle and air temperature are happening. 

“We had temperature controls on the soil. We used large 6-foot by 4-foot water baths, each with chillers and heaters, to maintain the water temperatures. We then put potted seedlings (housed in bags protected from flooding) into the water baths to maintain soil temperatures. 

“We had sets of plants that we eased into three different temperatures, 60°, 50°, and 40° Fahrenheit. Soil temperatures here in Arkansas are typically between 40° and 50° F during winter. Once those winter temperatures were reached, we flooded half of the seedlings at each temperature.” 

Babst said that most seedlings survived at all three temperatures, but trees in soil maintained at 60° F showed significant fine roots death. He said the number of dead roots decreased as soil temperatures dropped to 50° F, and there were virtually no dead roots when soil temperatures fell to 40° F. The conclusion was that the lower the temperature, the lower the root respiration, and more roots survived. 

Babst said a lot of forestry professionals already suspected that the timing of flooding is important. Still, it is crucial to have scientific data to support decision-making in the field, where erring on one side or the other could be costly. 

The key now, said Babst, is to find the “right time” to apply water to the forests. He said narrowing down the best window of time to start flooding greentree reservoirs will require more research, but forest land managers now have something to hang their decisions on instead of merely guessing. 

Babst believes soil temperature has a more substantial impact than what’s happening above ground. He said this might mean that GTR managers might have to take soil temperatures combined with other existing weather data to determine when to flood the forests. “Ideally, we would like to come up with a way for the forest manager to take readily available weather data and plug it into a formula to determine, ‘Is it time yet,'” he said.

According to Babst, his initial experiments were done in a controlled greenhouse. What made Kressuk’ s research specialty is that he took his experiments to the next level. He moved his root respiration experiments into the field. 

Babst said it’s inevitable that you will find different results in the field than in a lab or greenhouse setting, no matter how much you try to replicate the environment. “By manipulating the stem temperature and putting a bag of ice around it, despite the warming air temperatures and plenty of sunlight, does it slow down the respiration in roots by disrupting the phloem function?” Babst asked. 

“Phloem is like little pipelines through the stem that deliver sugars made by leaves to roots,” he said. “Disrupting that flow of ‘fuel’ to the roots could slow down root activity. If you look at a tree, it looks like it is not doing anything, but it is quite active. If you dig up the root system, it will continue to do what it’s doing, at least for a short period.” Jonathan would then take those roots, clean off the dirt, and put them in a chamber and measure the amount of CO2 the roots emit.”

“It wasn’t surprising that we see respiration decrease with lower temperatures,” said Babst, “We had a hypothesis, but now we have firm data. What AGFC and our research partners want from this research is actual hard data. So, when they are making decisions, they know they are making the best decisions that are supported by hard evidence,” said Babst. Babst partnered with researchers at the U.S. Forest Service Center for Bottomland Hardwoods Research, University of Missouri, and Missouri Department of Conservation to ensure what he learns applies broadly in the region. 

Babst said he now is in his third year of research and expects it to last at least two more years. He adds that there hasn’t been much study of root respiration in southern tree species, especially about the dormant season and flooded forests. 

Babst said his greenhouse research so far indicates that the ideal time to begin flooding bottomland hardwood forests is probably somewhere between late November to mid-December if you want to play it safe. If your only concern is protecting the trees, start draining the water by the end of February. More prolonged flooding killed many seedlings in his study. 

However, Babst said, “if the low temperature is driving the changes in roots, rather than declaring when the ‘right time’ is to flood, we should be identifying the right temperature conditions. The weather is a little different each year, and so the date when those conditions are reached could change somewhat from year to year.” 

He said there are many variables to consider in the field. “This year, we saw a big impact of freezing air temperatures. We need to keep looking to see if that effect of a hard frost is consistent across years. And we need to narrow that window of time when it will be safe to start flooding.”

About the College of Forestry, Agriculture and Natural Resources and the Arkansas Forest Resources Center

The College of Forestry, Agriculture and Natural Resources, and the Arkansas Forest Resources Center, a University of Arkansas System Center of Excellence, brings together interdisciplinary expertise through a partnership between the University of Arkansas at Monticello and the University of Arkansas System Division of Agriculture. The College and Center are headquartered at the University of Arkansas at Monticello campus, but their programs range statewide with the mission of developing and delivering teaching, research, and extension programs that enhance and ensure the sustainability and productivity of forest-based natural resources and agricultural systems. Academic programs are delivered by the College of Forestry, Agriculture, and Natural Resources through the University of Arkansas at Monticello. Through the University of Arkansas System Division of Agriculture, research is administered by the Arkansas Agricultural Experiment Station, and extension and outreach activities are coordinated by the Arkansas Cooperative Extension Service.

The University of Arkansas at Monticello and the University of Arkansas System Division of Agriculture offer all of their programs to all eligible persons without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and are Affirmative Action/Equal Opportunity Employers.

About the Division of Agriculture

The University of Arkansas System Division of Agriculture’s mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation’s historic land grant education system.

The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on five system campuses.

The University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.

Wesley is the owner of South Ark Weather, LLC which owns and operates searkweather.com You may contact him at wesley@searkweather.com

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