I’m a field research scientist. What does this mean? I enjoy being outside, in forests and wetlands, studying the environment up close and personal. One of my favorite places to work and explore over the course of my career has been Howland Research Forest in central Maine.
Dominated by red spruce, eastern hemlock, and red maple, this mature northern forest feels old. There is a 400 year old yellow birch that was already a mature tree during the American revolution. The ground is soft— spongy with a lot of “holes” where past trees have fallen and roots decomposed. My feet often plunge into these holes, which can sometimes be filled with water.
The Howland Forest Research station was established in 1986 by the University of Maine in partnership with a packaging and paper company, International Paper. My first trip to Howland Forest was in 1998 and at the time the research center was just a collection of trailers housing equipment. I had never seen so much mouse poop in a building.
Howland was one of the first sites ever dedicated to measuring the net exchange of carbon between a forest and the atmosphere. Its support comes from the Ameriflux Network, a grass roots, science driven network of research stations spread across North and South America that monitors the flow of carbon and water across ecosystems. In these early years, Howland forest also served as a training site for testing out NASA’s remote sensing capabilities. At one time, Howland Research Forest was the most photographed site on earth from space. Soon the well used trailers were replaced with multiple buildings to accommodate the ever expanding research. The mice were evicted.
Howland forest was selectively harvested over 100 years ago, evidenced by cut stumps, but the forest has remained intact, growing under natural conditions since then. Most trees range between 100-120 years old. In 2007, International Paper was scheduled to harvest these mature trees. Recognising the value of maintaining a continuous long-term record of observations, scientists from Woodwell Climate Research Center, The University of Maine (UMaine Orono), and the U.S. Forest Service (USFS) partnered with the Northeast Wilderness Trust (NEWT) to purchase the forest. The Howland Research forest, now owned by NEWT, was protected in a forever wild state. This science and conservation partnership saved an invaluable mature natural forest and research site. As scientists continued to collect data over the next decades, we would learn just how important this partnership was to our understanding of mature forests.
Long-term measurements of carbon exchange between the forest and the atmosphere are being taken from the top of a tower, as part of the Department of Energy (DOE) supported Ameriflux Network, and paired with measurements on the ground. It’s the measurements on the ground where I come in. Myself and collaborators at UMaine Orono, USFS and a host of other scientists and students over the decades have measured carbon exchange from soils, tracked changes in temperature and moisture, and taken tree inventories.
Mature forests contain large stores of carbon in their tree stems, foliage, roots, and within the soils, accumulated over decades of growth and decomposition. Allowing mature forests to continue to grow, untouched, is beneficial to maintaining carbon stores along with the natural biodiversity and water cycling, often collectively called “ecosystem services”.
Over the last 25 years, Howland Research Forest has seen the warmest, driest, and wettest years. Observations show an increasing trend in the net uptake of atmospheric carbon (as carbon dioxide) into this mature forest, meaning that Howland forest is continuing to take up and store more carbon each passing year.
If the forest had been harvested in 2007, observations spanning that shorter time frame would have indicated a decreasing trend in net net carbon uptake, meaning that Howland Forest was taking up less carbon each passing year.
Although Howland Forest continues to take up carbon, the overall number of live trees has been declining (17% decline since 2001 in live trees, particularly red spruce and northern white cedar) and the number of dead trees has nearly doubled since 2001. Theoretically, fewer live trees would indicate less carbon uptake, but that is not happening. The mature, large diameter trees continue to grow; although there may be fewer in number, they continue to take up significant amounts of carbon.
Tree species can differ in how they respond to environmental changes as well as how carbon is allocated within the tree and across a mature forest ecosystem. Teasing out these complex, multi-scaled, multispecies responses requires long term studies. However, given the challenges to acquiring and sustaining funding for long-term studies, it’s unusual to have this type of paired dataset like we have at the Howland Research forest. This would not have been possible without the forward-looking vision of scientists and NEWT, and the consistent support from the Ameriflux Network.
Thanks to its preserved, forever-wild status, a new generation of scientists has the opportunity to continue this work, building on our understanding of the mechanisms driving climate resilience in this mature northern forest.
The partnership between science and conservation is a victory for both. Results from the Howland Research Forest demonstrate the need to continue supporting long-term studies to fully understand how natural, mature forests respond to a changing climate. Conservation organizations and land trusts are preserving and restoring critical habitats across the U.S. and the globe. This is an opportunity to build alliances between science and conservation, to inform how natural ecosystems function and the impact of restoration efforts on the ecosystem services that we all benefit from, while preserving natural spaces for future generations.
When it comes to reversing climate change, trees are a big deal. Globally, forests absorb nearly 16 billion metric tonnes of carbon dioxide per year, and currently hold 861 gigatonnes of carbon in their branches, leaves, roots, and soils. This makes them a valuable global carbon sink, and makes preserving and maintaining healthy forests a vital strategy in combating climate change.
But not every forest absorbs and stores carbon in the same way, and the threats facing each are complex. A nuanced understanding of how carbon moves through forest ecosystems helps us build better strategies to protect them. Here’s how the world’s different forests help keep the world cool, and how we can help keep them standing.
Tropical forest carbon
Tropical rainforests are models of forest productivity. Trees use carbon in the process of photosynthesis, integrating it into their trunks, branches, leaves, and roots. When part or all of a tree dies and falls to the ground, it is consumed by microorganisms and carbon is released in the process of decay. In the heat and humidity of the tropics, vegetation grows so rapidly that decaying organic matter is almost immediately re-incorporated into new growth. Nearly all the carbon stored in tropical forests exists within the plants growing aboveground.
Studies estimate that tropical forests alone are responsible for holding back more than 1 degree C of atmospheric warming. 75% of that is due simply to the amount of carbon they store. The other 25% comes from the cooling effects of shading, pumping water into the atmosphere and creating clouds, and disrupting airflow.
In many tropical forest regions, there is a tension between forests and agricultural expansion. In the Amazon rainforest, land grabbing for commodity uses like cattle ranching or soy farming has advanced deforestation. Increasing protected forest areas and strengthening the rights of Indigenous communities to manage their own territories has proven effective at reducing deforestation and its associated emissions in Brazil. “Undesignated lands” have the highest levels of land grabbing and deforestation.
Fire has also become a growing threat to the Amazon in recent years, used as a tool to clear land by people illegally deforesting. When rainforests have been fragmented and degraded, their edges become drier and more susceptible to out-of-control burning, which weakens the forest even further. Enforcing and strengthening existing anti-deforestation laws are crucial to reduce carbon losses.
In Africa’s Congo rainforest, clearing is usually for small subsistence farms which, in aggregate, have a large effect on forest loss and degradation. Mobilizing finance to scale up agricultural intensification efforts and rural enterprise within communities, while implementing protection measures, can help decrease the rate of forest destruction. Forests and other intact natural landscapes such as wetlands and peatlands could be the focus of climate finance mechanisms that encourage sustainable landscape management initiatives.
Temperate forest carbon
Much of the forest carbon in the temperate zone is stored in the trees as well— particularly in areas where high rainfall supports the growth of dense forests that are resilient against disturbances like drought or disease. The temperate rainforests of the Northwestern United States, Chile, Australia, and New Zealand contain some of the largest and oldest trees in the world.
Two thirds of the total carbon sink in temperate forests can be attributed to the annual increase in “live biomass”, or the yearly growth of living trees within the forest. This makes the protection of mature and old-growth temperate forests paramount, since older forests add more carbon per year than younger ones and have much larger carbon stocks. Timber harvesting represents one of the most significant risks to the carbon stocks in temperate forests, particularly in the United States where 76% of mature and old growth forests go unprotected from logging. Fire and insects are also significant threats to temperate forests particularly in areas of low rainfall or periodic drought.
Maintaining the temperate forest sink means reducing the area of logging, by both removing the incentive to manage public forests for economic uses and by providing private forest owners with incentives to protect their land. Low-impact harvesting practices and better recycling of wood products can also help bring down carbon losses from temperate forests. In areas threatened by increasingly severe wildfires, reducing fuel loads especially near settlements can help protect lives and property.
Boreal forest carbon
In boreal forests, the real wealth of carbon is below the ground. In colder climates, the processes of decay that result in emissions tend to lag behind the process of photosynthesis which locks away carbon in organic matter. Over millennia, that imbalance has slowly built up a massive carbon pool in boreal soils. Decay is even further slowed in areas of permafrost, where the ground stays frozen nearly year round. It estimated that 80 to 90% of all carbon in boreal forests is stored belowground. The aboveground forest helps to protect belowground carbon from warming, thaw, decay, and erosion.
Wildfire— although a natural element in boreal forests— represents one of the greatest threats to boreal forest carbon. With increased temperatures, rising more than twice as fast in boreal forests compared to lower latitudes, and more frequent and long-lasting droughts, boreal forests are now experiencing more frequent and intense wildfires. The hotter and more often a stand of boreal forest catches fire, the deeper into the soil carbon pool the fire will burn, sending centuries-old carbon up in smoke in an instant. Logging of high-carbon primary forests is also a big issue in the boreal.
The number one protection for boreal forest carbon is reducing fossil fuel emissions. Only reversing climate change will bring boreal fires back to the historical levels these forests evolved with. In the meantime, active fire management in boreal forests offers a cost effective strategy to reduce emissions— studies found it could cost less than 13 dollars per ton of carbon dioxide emissions avoided. Strategies for fire management included both putting out fires that threaten large emissions, and controlled and cultural burning outside of the fire season to reduce the flammability of the landscape.