Down to Earth: Climate consequences of Hemlock’s decline

  • Danielle Ignace Jim Gipe Photographer—Jim Gipe Photographer © 2016...

  • Licor soil CO2 flux meter at the study site. Courtesy photo

  • A soil chamber measures CO2 respired from the soil. Courtesy photo

  • Danielle Ignace, researcher, carrying the soil CO2 flux meter to the study site. Courtesy photo

  • A soil chamber measures CO2 respired from the soil. Courtesy photo

  • This photo shows elongate hemlock scales (brown) and some evidence of hemlock wooly adelgid insects (white), an invasive pest. Courtesy photo

  • Licor soil CO2 flux meter at the study site. Courtesy photo

  • Hemlock wooly adelgid insects (white), an invasive pest, found on the hemlock twig. Courtesy photo

Published: 12/12/2018 9:42:50 AM

Editor’s note: This month’s Down to Earth column was written by guest columnist Zoe Merrell. Regular columnist Naila Moreira will return next month.

At a field station in rural Whately, Massachusetts, a sparse stand of birch trees provides a window into the future of American forests.

The stand has been called an “accidental experiment.” Thirty years ago, the director of the field station, Karl Davies, decided to log a portion of the original hemlock forest. What he couldn’t have known was that hemlock trees would soon become threatened by invasive pests. The logged plot is now invaluable to scientists trying to understand the implications of the decline of such an essential tree species. Recent findings suggest that the loss of hemlocks may have dramatic effects on climate.

Danielle Ignace and Jesse Bellemare, professors of plant science at Smith College, are studying the birch-dominated plots and neighboring hemlock-dominated plots at the college’s Ada & Archibald Macleish Field Station. The differences tell a startling story about future forest ecology on a national scale.

“As we transition from mature hemlock to black birch, all the carbon that was held up in the soil decomposes quickly, and that carbon is emitted into the atmosphere, which contributes to global warming,” said Ignace.

In hemlock groves, decomposition of leaf litter is relatively slow, enabling them to capture and store an abundance of carbon. When carbon isn’t stored in reservoirs like forests, it exists in the atmosphere as CO2, the main greenhouse gas contributing to climate change. Forests rich in species like eastern hemlock are therefore essential in mitigating our own CO2 emissions.

Eastern hemlock also drastically alters its environment, creating conditions and habitats for a unique set of other species. At least eight types of birds and 10 mammal species are strongly associated with hemlock-dominated forests. The trees form dense, evergreen canopies that create cool shade and deep, acidic, nutrient-rich soil, making them an essential component of Northeastern forests.

There are nearly one million hectares of hemlock-dominated forest in the U.S., according to a paper published by McWilliams and Schmidt in 2000 — almost enough to cover the state of Connecticut.

Sadly, most populations of eastern hemlocks are struggling because of a surge in invasive pests. The hemlock wooly adelgid was first introduced to the eastern U.S. in the 1950s from Asia. The invasion gradually spread northward, leaving a trail of sickly trees in its wake, and has now reached the southern parts of Vermont and New Hampshire.

The insect is unassuming — a fluffy, aphid-like white tuft that latches onto the underside of hemlock needles and sucks sugars from their veins. In large numbers, adelgids severely damage hemlocks’ ability to grow and maintain healthy populations. Together with elongate hemlock scale, another non-native insect that feeds on hemlock needles, these infestations can prove deadly.

“What we typically see is that eastern hemlock forests usually turn into black birch forests,” said Ignace. “My research goal is to understand the implications of that transition for ecosystem function.”

Black birch creates a very different sort of forest when it replaces hemlock. The lanky, fast-growing deciduous trees let more light reach the ground, and their nitrogen-rich leaves decompose faster, resulting in shallow, nutrient-poor soils.

Since 2013, Smith students and faculty have been working to characterize the leaf litter, soil respiration, soil nutrient levels, and soil organic layers in the hemlock and birch forests. Ignace, Bellemare and Smith alumna Aliza Fassler published their latest paper, “Decline of a foundation tree species due to invasive insects will trigger net release of soil organic carbon,” in Ecosphere this past August.

Ignace said that it wasn’t until the summer of 2015, when they decided to include a mature birch stand in their study, that she knew the study was going to be a big deal. When she finally saw the results, she felt both shocked and validated. “I was surprised that the results were so clear and so strong, and shocked that … we found something that no one else had found. I knew it would make a big splash.”

The researchers found that hemlock decline will cause decomposition on the forest floor to speed up, affecting ecosystem function. The loss of hemlock trees and subsequent decomposition will release an estimated 4.5 tons of carbon per hectare, “a massive amount of carbon,” Ignace said.

Although the study took place in only six 10-by-15 meter plots, each about the size of a living room, some quick calculations show that if all of the country’s hemlock forests were converted to deciduous forests, it could trigger the release of over 4.19 million tons of carbon. This quantity is equivalent to the CO2 emissions of all the cars in the state of Michigan being driven for an entire year — enough to potentially intensify climate change.

Sydne Record, a professor of biology at Bryn Mawr College, noted, “It might take decades before we see this soil organic carbon released.” Unfortunately, it’s difficult for most people to pay attention to a change occurring slowly over 30 to 80 years. “We may not be thinking in terms of the age of a tree,” she said. “It’s sometimes difficult for us to wrap our heads around as humans.”

She said Ignace, Bellemare and Fassler’s study “is able to show something to people that they wouldn’t be able to discover if they didn’t have this experimental design. It’s almost like having a time machine.”

Ignace said that she has received unsolicited messages of gratitude from teachers and professors who love the paper and are using it in their biology classes. She is surprised by how positive the reaction has been: “I don’t think I’ve quite grasped how much of an impact it made yet.”

“When you lose a tree, it’s more than just something that you see above the ground, it’s also those below-ground consequences,” says David Orwig, Senior Ecologist and Forest Ecologist at the Harvard Forest. “I think this paper starts to highlight those changes that occur.”

While this study paints a grim picture of what will happen if we lose the eastern hemlocks, there is still a bit of hope for the species. Small predator beetles imported from Asia might be the secret to controlling the hemlock wooly adelgid. The Forest Service’s Forest Health Protection program has begun rearing and releasing these predators across the country as part of the national HWA Initiative.

If successful, this initiative will be more economical and less destructive than other methods of control such as chemical treatment and selvedge logging. Ignace also pointed out that the hemlock trees further north are dying more slowly than the ones further south, which she hopes may indicate some kind of natural acquired resistance or systemic defense within certain trees.

The site at the MacLeish Field Station will continue to be used as a research plot by Smith College students and faculty. “I view this as just the start,” said Ignace, who feels very supported by the college and hopes that more students will get involved in research, outreach, and classroom learning at the field station.

The crisp, evergreen smell and cool shade of hemlocks have defined the landscapes of the northeast throughout history. Generations to come may not be able to experience the unique twilit grandeur of our eastern hemlock forests. If a solution to the hemlock wooly adelgid infestation isn’t found, we risk losing more than just carbon storage — we will lose a cornerstone of majesty and heritage in our country’s hardwood forests.

Zoe Merrell is currently a senior at Smith College, studying environmental science and biology, and is originally from Pennsylvania, where the state tree is the eastern hemlock.


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