Sinks or stores? World’s forests are key to fight against global warming
Posted: October 16, 2017Source: Treesource by
Source: U.S. GeologicalSurvey
In 2013, Slovenian archaeologist Ivan Sprajc hacked his way through a remote jungle on Mexico’s Yucatan peninsula. The modern-day Indiana Jones was on the trail of a huge discovery: a lost Mayan city obscured by centuries of nearly impenetrable forest.
It wasn’t the first ancient city unearthed from the jungle: Sprajc found two others in the area the following year, and archaeologists have been discovering Mayan, Incan and Aztecan cities deep in Central and South American jungles for a century or more.
More importantly, for archaeology and for the planet, it probably won’t be the last.
For archaeologists, these ancient cities, buried and hidden by centuries of jungle growth, offer clues to civilizations long past. For the rest of us, they offer a glimmer of hope in the face of a changing climate.
The very forces that obscured these sites for so long – sprawling canopies of rain-soaked trees and thick, verdant jungle – have spent centuries doing the work of forests around the world, turning carbon dioxide into woody biomass and oxygen.
Forests and Carbon: Sources and Sinks
According to the Food and Agricultural Organizations of the United Nations, forests play four major roles in climate change:
- They currently contribute about one-sixth of global carbon emissions when cleared, overused or degraded.
- They react sensitively to a changing climate.
- When managed sustainably, they produce wood fuels as a benign alternative to fossil fuels.
- Finally, they have the potential to absorb about one-tenth of global carbon emissions projected for the first half of this century into their biomass, soils and products and store them – in principle in perpetuity.
At the most basic level, forests are both sources of the greenhouse gas carbon dioxide and “sinks” for CO2.
Trees release carbon when they die, decay or burn – and are thus a source of atmospheric carbon.
When they grow, they take carbon dioxide from the atmosphere, combine it with sunlight, water and nutrients from the soil and turn it into biomass – and are thus a sink for atmospheric carbon.
Pretty much all plants are carbon sources when they die and sinks when they grow, but trees’ long life spans and large sizes mean they play a larger role in both sides of the equation than most other plants.
That’s why the United Nations notes that trees “contribute about one-sixth of global carbon emissions” while at the same time, they “have the potential to absorb one-tenth of global carbon emissions.”
When cleared, slashed and burned or otherwise eliminated, forests release carbon into the atmosphere. Additionally, they stop absorbing carbon because they are no longer photosynthesizing.
But when forests are managed sustainably, or even intentionally to store as much carbon as possible, they have the potential to sequester significant amounts of carbon from the atmosphere – fully one-tenth of all emissions, according to the UN.
The numbers aren’t precise, but generally at a global scale, forest ecosystems sequester nearly 4 billion tons of atmospheric carbon dioxide every year. That represents roughly 60 percent of annual carbon emissions from burning fossil fuels and cement manufacturing.
Unfortunately, deforestation of tropical forests is a net source of carbon, or about 3 billion tons of carbon per year, rendering the global carbon sink attributed to forests about 1 billion tons per year.
You don’t have to be a mathematician or a climate researcher to see that curbing deforestation is one of the most effective ways to prevent new greenhouse gasses from entering the atmosphere.
In fact, deforestation is the second largest anthropogenic source of atmospheric carbon dioxide, behind the burning of fossil fuels. This simple fact means that one of the most effective ways humans can leverage forests to combat global climate change is by keeping forests as forests.
But if the goal is to reduce the amount of carbon in the atmosphere now or to offset new emissions by sequestering carbon in real time, then planting trees is arguably more effective than simply leaving forests standing.
Yes, mature forests hold a lot of carbon in their trunks, branches, leaves and soil, but they don’t pull new carbon from the atmosphere as quickly as do young forests. Old-growth forests, while critical for a number of ecosystem functions including storing the carbon they’ve already pulled from the atmosphere, don’t actually sequester much carbon in the present.
Aggressively mitigating climate change requires young trees that sequester carbon at faster rates than their older brethren. To employ forests as effectively as possible in the battle against climate change, forest managers must reduce wholesale deforestation, especially tropical deforestation, which dumps huge amounts of carbon into the atmosphere, while also planting new trees that turn atmospheric carbon dioxide into wood.
More simply: Planting new forests is a key strategy to sequester carbon already in the atmosphere and stopping deforestation or other loss of forest cover is the best strategy for reducing forests’ contributions to atmospheric carbon.
But you can’t have a young forest unless there’s space for it. So managers turn to two options: afforestation – planting trees where no prior forest existed (at least in recent memory) or reforestation – planting trees after a forest dies (harvests, fire, insects).
Somewhat paradoxically, demand for wood products, whether lumber, pellets, paper or otherwise, can actually encourage both options.
Perpetually Renewable
Trees are a renewable resource, provided they are regenerated after harvest. And they can store carbon after they’re harvested, provided they’re not left to rot on the ground or burned.
Therefore, sustainably managed forests coupled with economic demand for wood products can produce a win-win scenario.
First, wood products like dimensional lumber store carbon in the form of houses, apartment buildings and other durable products. Additionally, wood products can further reduce carbon emissions by replacing carbon-intensive products like cement and steel.
Second, other wood products, like wood pellets and wood used in biomass energy facilities, directly replace fossil fuels. While burning these products does produce atmospheric carbon, biofuel proponents note that trees grown to replace those burned in a pellet stove or biomass plant will sequester carbon as they grow, making wood a “benign alternative to fossil fuels” in the words of the United Nations.
Some environmental groups like the Natural Resources Defense Council aren’t convinced that wood is truly a “benign” alternative to fossil fuels. They argue that even if the tree burned for energy is replaced with a seedling (and without sustainably managed forests, that’s not guaranteed), there’s a decades-long gap before that “replacement” is large enough to store an equivalent amount of carbon as was released when the first tree burned.
During that time period, the first tree could have continued to sequester carbon while also providing other ecosystem benefits like water and air filtration and wildlife habitat.
How the carbon accounting is done on the forest growth and whether the wood used is residual matter from harvests or manufacturing are the main differences in these two perspectives.
Sensitive to a Changing Climate
According to the U.S. Forest Service, forests in the United States – public, private, commercial and urban – sequestered about 13 percent of U.S. fossil fuel emissions in 2011.
Generally, this nation’s forests sequester between 10 percent and 20 percent of all carbon emissions on an annual basis, according to the agency.
That figure hasn’t fluctuated much in the last 50 years or so, but that doesn’t mean it won’t start to shift. While we tend to think of forests as enduring and ancient, the UN notes, “[forests] react sensitively to a changing climate.”
In the U.S., this sensitivity to climate is manifesting in two significant ways: increasing wildfires and shifting tree distribution patterns.
Whether you live in the West or the East, it’s hard to escape each wildfire season’s news reports. According to a Forest Service report from 2015: “Climate change has led to fire seasons that are now on average 78 days longer than in 1970. The U.S. burns twice as many acres as three decades ago and Forest Service scientists believe the acreage burned may double again by mid-century.”
This longer fire season and increased burned acreage is driven by multiple factors, including increasing temperatures, shifting precipitation patterns and longer, more intense droughts – all driven by and/or exacerbated by climate change.
When huge swaths of forests burn, as they do across the American West each summer, significant amounts of carbon are released into the atmosphere and the burned-over forests no longer actively store carbon until they begin to regrow, sometimes decades later.
Tree planting can help offset the atmospheric carbon released through fires, and helps improve water quality and wildlife habitat immediately following a fire. Unfortunately, in the arid West, it can take several decades for trees to grow large enough to store significant amounts of carbon.
Of course, wildfires aren’t driven entirely by climate. Historic management practices excluded the natural role of fire from most forested ecosystems. This exclusion left many forests overstocked with small-diameter trees that have little commercial value but high potential to burn.
With no private economic incentive to harvest these small trees and limited federal funding to thin these overstocked forests, public land agencies like the Forest Service, struggle to prevent the worst wildfires from impacting large swaths of forest.
But climate change is playing a major role in the current state of the national forests – lengthening the fire season, changing precipitation patterns and driving drought, all of which make forests more susceptible to fire. Once those forests burn, they may not fully recover, which means they won’t sequester as much carbon as they did when they were standing.
A December 2007 paper from the journal BioScience detailed how climate change can affect distribution of tree species in North America. By using a “climate-envelope” approach, the authors were able to predict where 130 common North American tree species would be able to exist in the year 2100.
Under two different scenarios the average predicted range decreased by 12 percent to 58 percent and moved 330-770 km north.
Does this mean that by 2100, all the trees that we now see will have shifted northward somewhere between 330 and 770 kilometers? Not necessarily.
Climate prediction is a tough game and uncertainty swirls around all predictions, no matter how rigorous or sound the modeling. But it does mean that public and private land managers need to be managing species composition differently than in the past.
In fact, a recent study from the journal Science Advances argues that trees in the eastern U.S. are moving not just north but also west, depending largely on the type of tree.
Using data from the Forest Service’s Forest Inventory and Analysis program, researchers mapped the distribution of 83 common tree species in the eastern U.S. What they found surprised them. Over the last 30 years, deciduous trees like oaks, maples and hollies have moved farther west, while coniferous trees have moved north.
The researchers attribute this not to changing temperatures, but to changing patterns of precipitation. The northeastern U.S. has seen a slight increase in precipitation over the last 30 years, while the Southeast has seen a significant drop.
Southern plains states like Oklahoma and Kansas have seen more rain. Rain-loving deciduous trees have followed this increase in precipitation westward, while less rain-dependent coniferous trees have shifted north in response to an overall increase in temperature. So how do these types of studies impact the people actually managing North America’s forests?
Creating Adaptability, Increasing Mitigation
According to a 2007 paper in the journal Ecological Applications by lead author and U.S. Forest Service researcher, Constance Millar, land managers should adopt several methods to manage forests in the face of an uncertain future.
Specifically, Millar notes that managers “will be challenged to integrate adaptation strategies (actions that help ecosystems accommodate changes adaptively) and mitigation strategies (actions that enable ecosystems to reduce anthropogenic influences on global climate) into overall plans.”
According to Millar, “Adaptive strategies include resistance options … resilience options … and response options.” Each of these helps forests survive the impacts that a changing climate produces.
Mitigation strategies include methods that maximize a forest’s ability to sequester carbon from the atmosphere, which can reduce the effects of climate change into the future. In other words, managers need to manage forests to both increase their adaptability to climate change while also using forests to mitigate the impacts of climate change.
But how exactly do they do that?
Millar points out that most forest managers have used “concepts of historical range of variability, natural range of variability and ecological sustainability to set goals and inform management decisions.”
In short, managers have looked to the past to learn how forests should be managed in the present. Millar argues that this is no longer tenable and “we cannot rely on past forest conditions to provide us with adequate targets for current and future management.”
Fortunately, forests in the U.S. cover about 766 million acres or about 33 percent of our country’s landmass, including Alaska. That’s a lot of trees and a lot of opportunity to learn, adapt and improve how we manage forests to survive climate change and help us solve it.
Forests from Cities, Forests from Farms
I grew up far from the jungles of Mexico, but the sprawling forests of Maine that provided the setting for my boyhood explorations do have something in common with the thick jungles where Ivan Sprajc made his archaeological discoveries.
Growing up, I thought those Maine forests were ancient and primeval, like the jungles that had obscured the Mayan cities for nearly 1,000 years. But as my rambles expanded past my backyard, I started to notice a hard-to-explain feature in the dense underbrush of serviceberry, ferns and mountain ash: rock walls, piled by hand and covered with lichen, moss and the varnish of time.
There, thousands of miles from the Yucatan Peninsula, a transformation had also taken place. Just like the jungles that had been cleared for Mayan cities a thousand years ago, much of Maine’s virgin forests had been cleared for farms.
Now, just a hundred or so years later, those farms and the rock walls that defined their boundaries are covered by thick forests of pine, oak, maple and hemlock. This regrowth happened all over New England, proof that forests can play a significant role in the battle against climate change.
Constance Millar is right, forest managers do need to look to the future to ensure that our forests will survive climate change. The rest of us may take some solace in the past, when ancient cities and pastoral farms once stood where carbon-sequestering trees now grow.
Greg M. Peters writes from Missoula, Montana, where he received a master’s of science degree in environmental studies at the University of Montana. He is a communications professional for a conservation nonprofit.