What is Carbon Sequestration, and Why Do We Need It?
Earth's atmosphere contains gases, primarily oxygen and nitrogen. But there are small amounts of other gases, including carbon dioxide (CO2), which is designated as a “greenhouse” gas. Why “greenhouse”? Because it acts like the glass of a greenhouse, in that it allows solar radiation to penetrate through the atmosphere to heat Earth, but does not allow heat to radiate back into space. This causes a rise in surface temperatures on the planet, resulting in climate change.
The gradual rise in temperatures causes a cascade of ecological and environmental problems, such as polar ice cap and glacial melting (and therefore rising sea levels), desiccation of forests, changing forest species composition and distribution, etc. With the rise of industrialization and burning of fossil fuels, carbon dioxide concentrations have increased substantially in the atmosphere.
We must find ways to reduce the level of CO2 in the atmosphere in order to reverse negative climate change effects. That's what “carbon sequestration” is all about. The carbon in carbon dioxide must be recaptured, or sequestered, from the atmosphere and stored long-term on Earth. How on earth (pardon the pun) can this be done?
Trees Sequester Carbon
Carbon is a major constituent of plants on land and in the oceans, because their process of photosynthesis uses carbon dioxide, water, and solar radiation to grow their tissues. In particular, woody plants such as trees contain relatively large amounts of carbon.
So, if we grow more trees, and let trees grow larger, they will sequester greater amounts of carbon in their wood, drawing CO2 out of the atmosphere. In theory, it's a very simple solution, and requires virtually no effort or great cost... just let more forests grow, and grow to old age! In reality though, we regularly harvest trees to satisfy our demand for wood and paper products. As long as wood products remain intact, the carbon they contain is still stored, although there is a significant loss of carbon in the harvesting process. But as they decompose (or are burned), their carbon re-enters the atmosphere. To reduce atmospheric CO2, we have to sequester much more carbon (principally, in the form of trees) than we allow to enter the atmosphere. There is no method more effective, less expensive, and quicker than letting trees already growing get progressively larger. Planting lots more trees is also helpful, but not nearly as effective in the short-term. It is becoming clear that large trees play the major role in sequestering carbon; here, we will look at carbon storage in large trees by focusing on big eastern white pines (Pinus strobus). But first, it is worthwhile to briefly pay a visit to New England past.
Primeval Eastern White Pines
Our New England woodlands are not associated with exceptionally large trees like those in California and the Pacific Northwest. But it wasn’t always this way. In the 1600s and 1700s, chroniclers described a landscape that featured giant eastern white pines, some claimed to be well over 200 feet in height, and up to seven or more feet in diameter. Romantic accounts exist of pines in Maine and New Hampshire reaching astounding sizes and achieving great ages, and of course, the species was famous as a resource for British ship masts.
The great whites became the replacement for the exhausted European Riga Fir (actually Scots Pine, Pinus sylvestris L.) used by the King’s Navy to hold up the sails of its warships. Trees of a certain size and shape were reserved exclusively for the Royal Navy. They were often marked by three slashes with an ax, called the broad arrow mark. In fact, the white pine was the foremost symbol of the region’s original virgin wilderness, but the time of those legendary pines came and went.
The intense lumbering of the region, especially in the 1700s and 1800s, depleted the rich old growth forests. By the early to mid-1900s, New England’s recovering woodlands consisted of younger trees, and it is safe to conclude that, based on what they saw, people’s perception of what the white pine (or any species for that matter) could achieve in size was greatly scaled down.
Today’s forest historians largely relegate chronicler accounts of giant pines to the pages of history. This is evidenced by descriptions of the species in popular 20th century field guides, which often listed the white pine as a tree capable of surpassing 100 feet in height, but commonly reaching only 75 to 100. More descriptive authors like Donald Culross Peattie reminded us of the historic heights, but most of these authors made it clear that such giants no longer grow. In fact, the stature of the species had been so diminished in the public eye that one hiking guide to trails in New Hampshire described a white pine, stated to be 125 feet tall, as exceptional. The guide was written in the mid-1980s, even though many pines then were already above that height, with specimens reaching to 150 feet and more in iconic places like Cook Forest and Hearts Content in western Pennsylvania, Hartwick Pines in Michigan, Pack Forest in the Adirondacks, and the Cathedral Pines in Cornwall, CT.
Further diminishing the growth potential of the species in our eyes today, forest managers keep woodlands artificially young, largely for short-term economic reasons. Yet despite this reigning management paradigm, the eastern white pine is re-emerging to reclaim some of its former glory as our tallest eastern species.
The Return of Large White Pines in the Landscape
In some Massachusetts conservation areas, parks, state forests, and private lands, white pines are starting to once again reach impressive stature (often on recovering old fields). As of 2019, Mohawk Trail State Forest is home to at least 146 white pines reaching to over 150 feet in height as confirmed by the Native Tree Society; 25 of these surpass 160 feet, and two of those exceed 170.
One of the most impressive pines is a 180- to 200-year-old tree growing in western Massachusetts. It is presently 172.4 feet tall and has a circumference (at breast height) of 12.2 feet. It is an example of a class of emerging modern-day super pines that offers us an opportunity to better understand the long-term carbon storage capacity of very big trees and their rates of sequestration from youth to maturity and beyond. Our huge pine, which we named “Saheda”, is neither a young tree nor what we would classify as old growth. Based on the longevity of the species, it may have another 100 years or more of life.
How does a pine in Saheda’s size and age class perform in terms of its rate of carbon sequestration from youth until the present? Many may think that the growth performance of trees like Saheda is well understood, but white pines of its age and stature are rare today, since the species is typically harvested (at least in the Northeast) at 60 years or less. The prevailing belief in forest management has been that trees much over 100 years in age have plateaued in their annual growth, and are stagnant or senescent. This belief has led to a gap in our understanding of the growth performance of large dominant trees in the Northeast.
The calculated trunk volume of Saheda is 864 cubic feet (ft3). Its limbs add approximately 15.4% of the trunk’s volume, giving a trunk and limb total of 997 ft3 (the 15.4% is from a US Forest Service biomass model that we use). In today’s management paradigm, pines one third to one half this size are considered large. In fact, pines with diameters over 18 inches typically fall into the large category. By contrast, Saheda’s diameter is 46.6 inches, making it a super-pine.
Beyond the simple dimensions of diameter and height, there is volume. Determining the trunk, limb, and root volume of a tree is important because, from it, we can calculate the amount of carbon sequestered in the tree (and from that, the equivalent amount of CO2 removed from the atmosphere).
Those who favor using forests as the climate solution are divided on strategy. Some advocate concentrating on young forests, believing that they grow fastest and can sequester the most carbon in the near future. Many in this camp also believe that forests hit growth plateaus at 70 or 80 years. This faction is prone to making statements like: younger forests have a higher rate of carbon sequestration than older ones do. They seldom specify the dividing point between young and old.
A second group believes that older forests should be left to grow. Some believe this primarily because of the great carbon stores the old forests presently hold. Releasing large amounts of carbon by harvesting would work against the sequestration objective. Some of the second group believe that annual growth in the older forests exceeds that of their younger counterparts. With due respect to both groups, the truth lies somewhere in the middle, but favors the arguments of the older-forest champions over those of young-forest advocates, and substantially so (if "young" forests are those in the age range of 0 to 50 years, a not untypical age for stand rotation).
What follows is a discussion of the role large dominant eastern white pines can play in productively sequestering carbon for beyond a century and a half.
The Efficiency of Large Trees in Sequestering and Storing Carbon
A stand of white pines on a good growing site will gain carbon most rapidly between 40 and 80 years. However, growth from 80 to 120 years will outpace growth from 0 to 40 years. Growth from 120 to 140 years will outpace growth from 0 to 20. Beyond 160 years, annual sequestration in the living pines drops more quickly, partly due to continued self-thinning of the stand. However, other species progressively fill the gaps left by dying larger pines. Additionally, after logging, the soil releases carbon from the root systems for 10 to 30 years, partially offsetting gains from new growth. Consequently, for between 140 and 160 years, annual sequestration likely outpaces that of the first 40 years. So, how does this information impact the arguments given for young versus old forests?
|Young White Pine Stand|
One reason the older forest group’s position has gained ground among the scientific community is the high performance of the dominant trees. They continue to add carbon at accelerated rates for decades longer than commonly realized. Ordinary stand rotations of 50 years or less do not allow managers to assess the performance of really big trees.
It is often stated that young, vigorously growing forests are the most effective at sequestering carbon. That seems intuitive, doesn't it? One can rather easily witness the rapid growth taking place in a sapling from year to year. Young trees seem like they’re on steroids. Surely, they can outperform an old, hulking, grandfather of a tree. Or can they? An 8-foot tall pine sapling might put on another foot of height in the next year; as a percentage, that's an impressive 12.5% growth in height. But in terms of actual volume, that doesn't amount to very much wood. The growth looks impressively fast, but there's still not a lot of wood in a 9-foot tall pine sapling. A larger tree can add much more volume of wood in a year (and therefore sequester more carbon), but we don't tend to notice it because most of the growth is occurring aloft, and is spread over a large trunk diameter. A 1/8-inch increase in the radius of a 3-foot diameter tree represents far more wood than a 1/8-inch increase in the radius of a 1-inch diameter sapling.
So, just how effective is a huge pine like Saheda (mentioned above) in sequestering carbon compared to younger/smaller pines? We have performed considerable, accurate measurements of white pines, and careful calculations of their trunk volumes. Here, we will offer generalized results for the class of largest pines.
Imagine the space on the ground under Saheda's crown. If we were to replace Saheda with 20-year-old pines, approximately 27 would fit in the same space; but it would require about 402 of those young pines to equal Saheda's volume and carbon content! This would require ground space equal to 73% of an acre. It is apparent that large trees are efficient utilizers of ground space, since most of their bulk is aloft. In addition, young trees can grow beneath their crowns. That is a win-win situation.
|Grand White Pines|
A tree does not add a fixed volume of wood to its trunk and limbs each year. As it grows larger, its greater foliage area carries on more photosynthesis, thereby creating a greater volume of new wood. So, for a period of many years, as it grows larger, it increases its volume faster, and consequently outperforms young trees in sequestering carbon. Eventually, an older tree's growth will slow down. But its total carbon content is still there, stored in its trunk, limbs, and roots. What's more, when that huge tree comes crashing down in the wind and is lying on the forest floor, its large carcass will take much longer to decompose than a small log would, so its carbon remains stored longer, not released to the atmosphere.
So, how might a Saheda-sized pine gain trunk volume across a span of 180 years? A new stand development model we are employing gives the following cubic-foot gains at 20-year periods for this largest size class pine. Between 20 and 40 years, the largest class pines gain 56.5 ft3 of trunk volume on the model. Between 140 and 160 years, the amount is 97.9 ft3. Trunk growth stays above the first 35 to 40 years up to an age of 180. It is abundantly clear that most of the fast volume growth for this size class pine occurs after 40 years. If that were not the case, the tree would not have achieved such a huge size.
What's the Lesson Here?
There is great concern about climate change these days. Increased atmospheric CO2 has been identified as one of the main culprits, so we must take steps to reduce it. Harvesting trees on short rotations (e.g. 50 years) is counterproductive for climate change resolution. Managing for large tree size is an excellent strategy, as is retaining as much carbon on the forest floor as possible. Removing all downed coarse woody material from the forest floor during harvest operations, or chipping it up on site, releases stored carbon more rapidly. It also invites drying of the forest floor and introduction of non-native invasive plants, and compromises wildlife habitat.
|Carbon-rich Coarse Woody Debris|
And, certainly, burning trees as a biomass fuel is counterproductive, by not only removing still-growing carbon-storing plants, but putting their carbon directly back into the air. Some argue that those removed trees will be replaced by vigorous young trees that will quickly store carbon... yes, they will; but it has now been shown that those vigorous young trees can't come close to matching the carbon content of the large trees they're replacing, at least for many, many decades. And in the meantime, the carbon of the burned trees is making the problem worse.
Since we don’t have much time to make headway in getting CO2 emissions under control, the most straightforward and easiest solution, especially in our public forests, is allowing the trees to grow to their maximum sizes. Nothing else will be as effective, less costly, more ecologically beneficial, or easier. One approach to doing this is explained in the concept of “Proforestation”, which basically advocates letting as many of our forests as practicable grow without interference.
More information can be found at https://bit.ly/2L434Ln