Older Eastern White Pine Trees and Stands Accumulate Carbon for Many Decades and Maximize Cumulative Carbon
Robert T. Leverett1,2,3 , Susan A. Masino4,5 and William R. Moomaw6,7*
1 American Forests National Champion Tree Program, Washington, DC, United States, 2 Native Tree Society (www.nativetreesociety.org), Florence, MA, United States, 3 Friends of Mohawk Trail State Forest, Florence, MA,
United States, 4 Trinity College, Hartford, CT, United States, 5 Harvard Forest (2018–2019), Petersham, MA, United States,
6 The Fletcher School and Global Development and Environment Institute, Tufts University, Medford, MA, United States,
7 Woodwell Climate Research Center, Falmouth, MA, United States
Pre-settlement New England was heavily forested, with trees exceeding 2 m in diameter. The forests have regrown since farm abandonment, representing what is arguably the most successful regional reforestation on record and identified recently in the “Global Safety Net.” Temperate “old-growth” forest and remnant stands demonstrate that native tree species can live several hundred years and continue to add to forest biomass and structural and ecological complexity. Forests globally are an essential natural climate solution that accumulate carbon and reduce annual increases in atmospheric CO2 by approximately 30%. Some studies emphasize young, fast-growing trees and forests while others highlight carbon storage and accumulation in old trees and intact forests. We addressed this directly within New England with long-term, accurate field measurements and volume modeling of individual trees and two stands of eastern white pines (Pinaceae: Pinus strobus) and compared our results to models developed by the U.S. Forest Service.
Within this sample and species, our major findings complement and clarify previous findings and are threefold: (1) beyond 80 years, an intact eastern white pine forest can accumulate carbon above-ground in living trees at a high rate and double the carbon stored in this compartment in subsequent years; (2) large trees dominate above-ground carbon and can continue to accumulate carbon; (3) productive stands can continue to accumulate high amounts of carbon in live trees for well over 150 years.
Because the next decades are critical in addressing the climate emergency, and most New England forests are less than 100 years old, a major implication of this work is that maintaining and accumulating carbon in some existing forests—proforestation— is a powerful regional climate solution. Furthermore, older and old-growth trees and forests are rare, complex, highly dynamic and biodiverse: dedication of some forests to proforestation will produce large carbon-dense trees and also protect ecosystem integrity, special habitats, and native biodiversity long-term. In sum, strategic policies to grow and protect suitable existing forests in New England will optimize a proven, low cost, natural climate solution that also protects and restores biodiversity across the landscape.
Keywords: proforestation, intact forest, ecological resilience, carbon accumulation, chronoseqeuence, old- growth and second-growth forest, tree volume, ecological integrity
INTRODUCTION
A global priority for the climate has long been reducing ongoing emissions of heat-trapping greenhouse gases (GHGs) produced by burning carbon-based fuels. While this is essential, it is not sufficient for halting the rise in global temperatures. It is necessary to also simultaneously increase carbon dioxide (CO2) removal (CDR) and keep carbon stored within natural systems. Clearing and harvesting forests, draining and developing wetlands, and degrading soils account for one-third of all the CO2 added to the atmosphere by humans since the beginning of the industrial revolution (Simmons and Matthews, 2016). Together, these ongoing actions continue to add approximately 1.6 PgC/year (1 Pg equals 1 Gt or 1015 grams or 1 billion
metric tons; Friedlingstein et al., 2020). Burning wood for heat and electricity adds additional CO2, and current forest management practices limit the potential of this natural solution to accumulate carbon above and below ground and keep it out of the atmosphere (Sterman et al., 2018).
Two recent Intergovernmental Panel on Climate Change (IPCC) reports identify the urgent and unprecedented imperative to simultaneously and rapidly reduce Carbon Dioxide Emissions and achieve additional Carbon Dioxide Removal (CDR) from the atmosphere (Intergovernmental Panel on Climate Change, 2018, 2019). These reports identify forests as playing a major role in accumulating carbon out of the atmosphere. However, for CDR the focus is primarily on afforestation (planting new forests) and reforestation (regrowing forests) and ignores the more rapid climate mitigation and adaptation benefits of additional growth by existing forests, termed “proforestation” (Moomaw et al., 2019).
Even achieving the goal of “zero net carbon” will only “probably” limit global average temperatures to 1.5◦C (Intergovernmental Panel on Climate Change, 2018) above the pre-industrial global temperature and a significant increase above the current level (∼1.2◦C). This additional temperature increase will result in greater disruption to the climate system and will accelerate ecological decline. To avoid ever more serious consequences of a changed climate, the goal must be to become net carbon negative as soon as possible. Growing suitable existing forests is an effective and low cost means for reducing the atmospheric stock of carbon as others have noted (Fargione et al., 2018; Hudiburg et al., 2019; Moomaw et al., 2019; Mildrexler et al., 2020) and will be demonstrated by the findings reported in this paper. Natural regeneration of forests has recently been found to accumulate more carbon in the first 30 years than managed reforestation (Cook-Patton et al., 2020).
A second and perhaps even more urgent priority is the strong protection of intact biodiverse natural systems (Watson et al., 2018), as verified in the Global Assessment Report on Biodiversity and Ecosystem Services (Intergovernmental Science- Policy on Biodiversity and Ecosystem Services, 2019) and the recent “Global Deal for Nature”(Dinerstein et al., 2019). A global review with a dual focus on carbon and biodiversity identified regions that are part of a “Global Safety Net” (Dinerstein et al., 2020), and the safety net must be now be translated to local levels. This joint climate/biodiversity priority was also highlighted in the peer-reviewed declaration of a Climate Emergency signed by over 13,000 scientists in late 2019 and which highlighted proforestation as a global climate solution (Ripple et al., 2020).
There is scientific consensus that we can substantially close the gap between CO2 emissions and removals by maximizing a range of nature-based solutions (Griscom et al., 2017; Fargione et al., 2018). Regarding biodiversity, the beneficial role of protected areas in supporting species abundance and diversity was confirmed in a global meta-analysis (Coetzee et al., 2014), and the benefit of protecting intact ecosystems was quantified by comparing the probability of extinction in the six major global regions. On average, “wilderness” reduces the rate of species’ extinction by half due to higher rates of species loss in unprotected areas (Di Marco et al., 2019); the quantified benefit of wilderness in preventing extinction is even higher in regions, including the Eastern United States. Biodiverse intact forests can simultaneously provide long-term protection to natural processes and biodiversity, reduce extinction, and provide pathways for migration while accumulating atmospheric carbon moderating local and global temperature increases (Friedlingstein et al., 2020). Taken together, it is practical and possible to act immediately to protect ecosystems and prevent extinction while we maintain increased CDR rates and store and accumulate additional carbon in forests and forest soils.
Forest conservation studies tend to focus on high-biodiversity tropical forests (Mitchard, 2018), yet temperate forests are also biodiverse (Hilmers et al., 2018), benefit human health and well- being in highly populated areas (Karjalainen et al., 2010), and provide many essential ecosystem services (United States Forest Service, 2021). They also have a large additional potential for CDR that has been underestimated by 32% (Cook-Patton et al., 2020). New England Acadian Forests are the only region in the lower 48 United States identified as part of the “Global Safety Net” as a Tier 1 climate stabilization area (Dinerstein et al., 2020). Current forest CDR in the United States reduces annual net nation-wide greenhouse gas emissions by 11.6% (United States Environmental Protection, and Agency, 2018), with the potential for much more (Keeton et al., 2011; Moomaw et al., 2019). Houghton and Nassikas (2018) estimate the current gross carbon sink in forests recovering from harvests and in abandoned agriculture to be −4.4 PgC/year (negative means removal) globally, consistent with the IPCC 1.5◦C report that
identified forests as key to increasing accumulation rates. This potential carbon sink from recovering forests is nearly as large as the gap between anthropogenic emissions and removal rates, 5.1 PgC/year (Friedlingstein et al., 2020).
In the context of resource production and forest management, some forest carbon is stored in lasting wood products, and responsible forestry can provide a reliable wood supply from a semi-natural forest. However, multiple analyses have found that more carbon associated with timber harvests is lost to the atmosphere than is stored in the harvested wood products (Nunery and Keeton, 2010; Harris et al., 2016). For example, just 19% of the original carbon stock in Oregon forests in 1900 is in long lived wood products; approximately 16% is in landfills, and the remaining 65% is in the atmosphere as carbon dioxide (Hudiburg et al., 2019). Updated models indicate that the product substitution benefits of wood products are overestimated between 2 and 100-fold (Harmon, 2019) and any near-term carbon benefit relies on product subsitution (Hudiburg et al., 2019; Leturcq, 2020). Biogenic emissions from harvesting in the United States are estimated to be 640 MtC/year or 85% of total forestry emissions, exceeding the commercial and residential building sectors, and fossil fuel emissions from harvesting add an additional 17% CO2 to the atmosphere above biogenic emissions (Harris et al., 2016).
Strategic planning for responsible resource production can both mitigate these emissions and ensure a protected network of intact natural areas. For example, the US Climate Alliance underestimates the importance of “net carbon accumulation” in forests (United States Climate Alliance, 2021). Forests do accumulate net carbon now, but carbon above and below ground is far below historic levels and far below its potential (Law et al., 2018; Hudiburg et al., 2019). A critical and explicit goal is to increase and optimize carbon accumulation by utilizing some forests for responsible resource production as needed and protecting other forests for climate protection, long-term full biodiversity, science, and human health and well-being.
At a global level, if deforestation were halted, and existing secondary forests allowed to continue growing, a network of these intact forests would protect the highest number of species from extinction (Di Marco et al., 2019; World Wildlife Federation,
2020) and it is estimated that they could accumulate ∼120 PgC
in the 84 years between 2016 and 2100 (Houghton and Nassikas,
2018). This is equivalent to about 12 years of current global fossil fuel carbon emissions. These global numbers are conservative as outlined in recent analyses (Cook-Patton et al., 2020) and they do not factor in the enhanced regional CDR potential and high cumulative carbon that can be achieved with proforestation in such carbon-dense temperate forests of the Pacific Northwest (Law et al., 2018) and New England (Nunery and Keeton, 2010; Keeton et al., 2011; Moomaw et al., 2019; Dinerstein et al., 2020).
Because these global and regional projections can be difficult to translate locally, particularly over time, we focused on a detailed analysis of individual trees and stands in New England. Historically, between 80 and 90% of the New England landscape was heavily forested, and early chroniclers describe
pre-settlement forests with many large, mature trees reaching 1–1.5 m in diameter (Whitney, 1996). Fast-growing riparian species like sycamores and cottonwoods could reach or exceed 2 m. Today, New England trees of this size are mostly found as isolated individuals in open areas, parks, and old estates. Old- growth forests (primary forests) and remnants are currently less than 0.2% of northern New England’s landscape, and less than 0.03% in Southern New England. Ongoing attempts to document their value and identify their locations is underway (Davis, 1996; Kershner and Leverett, 2004; Ruddat, 2020). Secondary forests in New England consist mostly of smaller, relatively young trees (on average less than 100 years old). The U.S. Forest Service estimates that fewer than 7% of the nation’s forests exceed 100 years in age. Our goal in this study was to measure carbon directly in individual trees and in an “average” vs. an older stand of eastern white pine (Pinaceae: Pinus strobus) in New England. Most forest carbon studies focus on large geographical areas, and utilize “net” carbon data gathered from LIDAR (Light Detection And Ranging) and satellite technology, as well as statistical modeling based on the US Forest Service methods. Upon examining these options we note that carbon estimates from different tools and models can lead to disparate results at the level of individual trees—and these errors can be extrapolated to stands (Leverett et al., 2020). Therefore, we capitalized on the extensive tree-measuring protocols and experience of the Native Tree Society (NTS) to conduct highly accurate direct field measurements and measure volume precisely in younger vs. older trees growing in stands (Native Tree Society, 2021). We used direct measurements to evaluate volume-biomass models from multiple sources and developed a hybrid—termed FIA-COLE— to capitalize on the strengths of each model. We calculated the live above-ground carbon (in metric tons) in individual eastern white pines and individuals of other species in pine stands using conservative assumptions and direct measurements in pines up
to 190 years old.
MATERIALS AND METHODS
This paper centers primarily on (1) individual eastern white pines (Pinaceae: Pinus strobus), (2) a representative older pine stand in Western Massachusetts, named the Trees of Peace (TOP: located in Mohawk Trail State Forest, Charlemont, MA), and (3) a nearby younger pine stand (∼230 m center to center from the TOP). Both stands regenerated naturally from pasture and they share abiotic conditions such as a similar elevation, soil type (Hinkley loamy), temperature and precipitation. The younger stand is slightly downslope, and neither shows evidence of major recent disturbance. In 1989 the TOP lost 6 trees in a storm. Currently the TOP has 76 pines covering 0.6–0.7 ha. While not discussed in detail herein, we have also collected and analyzed data from NTS measurements in 38 other sites with white pines in the Eastern United States. Since 1990, NTS has taken thousands of on-site direct measurements of individual trees in dozens of stands of eastern white pines (see examples Supplement 1). Measurements are published on the society’s website (Native Tree Society, 2021) and comprehensive measurement protocols were adopted from those developed by NTS (Leverett et al., 2020) and incorporated into the American Forests Tree Measuring Guidelines Handbook (Leverett and Bertolette, 2014). A brief description of the measurement methods and models is provided in “Height and Diameter Direct Measurement Methodology,” Supplement 2 and in Leverett et al. (2020). Here, in all cases, the best mathematical processes were applied, e.g., the sine instead of the tangent height method and the best statistical models.
In the pine stands, a point-centered plot was established with a radius of 35.89 m, covering 0.403 hectares (subsequently referred to as 0.4 ha), with the goal of evaluating a standard acre (radius: 117.75 ft), and thus relevant to forestry conventions in the U.S. Within the TOP, 44 mature white pine stems were tallied along
with 20 hardwoods and eastern hemlocks greater than 10 cm in diameter at breast height (DBH, 41 511 or 1.37 m from the ground).
The measured acre had 50 pines in July 1989 when six trees were lost in a wind event. The pines are ∼160 years old; the hardwoods and hemlocks are estimated to be between 80 and 100 years old.
Height and Diameter Direct Measurement Methodology
We quantified the volume of the trunk and limbs of each tree from heights and diameters measured with laser-based hypsometers, monoculars with range-finding reticles, traditional diameter tapes, and calipers (described in detail in Leverett et al., 2020). Each instrument was calibrated and independently tested for accuracy over a wide range of distances and conditions (see Supplement 2 for an example). Absolute accuracies of the two main infrared lasers were verified as ±2.5 cm for distance, surpassing the manufacturer’s stated accuracy of ±4.0 cm. The tilt sensors were accurate to ±0.1◦, meeting the manufacturer’s stated accuracy. The combination of these distance and angle error ranges, along with the most accurate trigonometric methods noted above (sine vs. tangent method), gave us height accuracies to within 10–15 cm on the most distant targets being measured and approximately half that on the closest targets. We distinguished the rated and/or tested accuracy of a particular sensor of an instrument (such as an infrared laser or tilt sensor) from the results of a measurement that utilized multiple sensors. Tree heights were measured directly for each pine with a visible top, using the sine method (Supplement 2) whenever possible rather than the traditional tangent method. Our preference for the sine method is supported by NTS, the US Forest Service (Bragg et al., 2011) and American Forests (Leverett and Bertolette, 2014). The more traditional tangent method often over/under-estimates heights by treating the sprig being measured (interpreted as the top), as if it were located vertically over the end of the baseline. The heights of 38 white pines in the TOP with visible tops were measured directly using
the sine method.
Use of a Form Factor and FIA-COLE in Determining Pine Volume
To compute trunk volume directly from the base to the absolute top of a tree, diameters at base and breast height were measured with conventional calibrated tapes according to the procedures established and published by NTS. Diameters aloft were measured with the combination of laser range-finders and high performance monoculars with range-finding reticles. A miniature surveying device, the LTI Trupoint 300, was also used. Its Class II, phase-based laser is rated at an accuracy of
±1.0 mm to clear targets and its tilt sensor is accurate to ±0.1
degrees. In the TOP, we computed the volume of each pine’s trunk
and limbs using diameter at breast height, full tree height, trunk form, and limb factors. (See Supplement 3 for a discussion on the development of the form factor and its importance in measuring volume, with comparisons to other methods of measurement).
Detailed measurements of 39 sample trees established an average form factor (see NTS measurements in Supplement 3, Table S3.2). The volume of each sample tree was determined by dividing the trunk into adjacent sections, with the length of each section guided by observed changes in trunk taper and/or visibility. Each section was modeled as the frustum of a regular geometric solid (neiloid, cone, paraboloid; see Supplement 3 and Leverett et al., 2020, for formulas). The form factor for each pine was computed by adding its section volumes to obtain total trunk volume and then dividing the result by the product of the pine’s height and breast-high cross-sectional area. This produced an average factor that would fit the pines growing in a stand. We applied the average form factor to all pines included in the TOP as one determination of trunk volume.
For comparison to our direct volume measurements, we applied a hybrid volume-biomass model to compute trunk volumes for pines in the TOP. This hybrid allowed us to make use of the extensive analysis of the US Forest Service Forest Inventory and Analysis (FIA) program and database (which determines volume and biomass through the use of allometric equations; United States Forest Service, 2020) as well as the Carbon On-Line Estimator (COLE; National Council for Air Stream Improvement, 2020). This hybrid was termed FIA-COLE. See Supplement 4 for a full explanation of the variables and equations for defining trunk volume. We finalized volumes for the pines in the TOP by averaging our direct measurements with those of FIA-COLE.
For the total volume of the above-ground portion of a pine, we derived a factor for limbs, branches, and twigs as a proportion of the trunk volume using the FIA-COLE model (Supplement 5). That model includes all the branching in what is defined as the “top” in a biomass calculation and the limb factor for large trees is typically an additional 15–16%. We ran the model for each of the individuals in the TOP and calculated the volume. This was converted to biomass (density) and then to carbon mass using a conservative carbon mass fractional factor of 48%.
Analysis of Individual Pine Trees and a Representative Stand
In addition to the TOP, and older exemplary pines, we quantified above-ground carbon in younger trees and a representative stand. To determine an “average” pine at 50 years we defined two populations: (1) trees at 50 years that are still alive today, and (2) trees that were alive at 50 years but are missing today. This allowed us to compute an average trunk size for the missing trees and the associated carbon. We also measured white pines from young to older ages to estimate growth rates and volumes. The number of pines alive at 50 years but not alive today was determined from stand density data coming from both field counts and FIA (United States Forest Service, 2020).
We extensively studied an ∼80-year-old stand of pines
adjacent to the TOP (Supplement 6) growing on a terrace located
just downslope from the TOP in an area fairly well protected from wind and with similar abiotic conditions and adequate soil depth. This age is more representative of the average stand of eastern white pine in New England (60–80 years; United States Forest Service, 2019). We also considered the range of pines of known ages from stands within the vicinity and elsewhere. Where we could, we examined ring growth and height patterns for individual pines during their early years on a variety of sites in different geographical locations. In some cases, we examined stumps and measured the average ring width. In other cases, we measured trees and counted limb whorls to get age estimates.
We measured the tallest pine in the TOP over a long time-span (referred to as Pine #58, its research tag number). Pine #58 has been measured carefully and regularly over a period of 28 years. In 1992 the tree was 47.24 m tall and 2.93 m in circumference. Since then, it has been climbed 4 times, tape-drop-measured, and volume-determined. Pine #58 continues to grow and has enabled us to quantify the changes in carbon accumulation in a dominant tree over decades. See Supplement 7 for a detailed measurement history of Pine #58.
Live tree above-ground volumes were converted to mass using standard wood density tables (United States Department of Agriculture, 2009). The air-dried density for white pine is 385.3 kg/m3 (0.3853 metric tons/m3). As noted above, we calculated the amount of carbon in each pine conservatively as 48% of total air-dried weight, whereby a cubic meter of white pine trunk or limbs holds 0.18494 metric tons of carbon (at least 50% is used more commonly; the percentage of carbon
content in different species ranges from ∼47% to 52+% and there is evidence that pine is at the upper range (Nicodemus and Williams, 2004). Note that the carbon in a cubic meter of wood varies depending on the species and is usually greater in hardwoods (United States Department of Agriculture, 2009).
RESULTS
Our measurements indicate that individual eastern white pines can accumulate significant above-ground volume/carbon up to at least 190 years, that this volume/carbon accumulation in an individual tree can accelerate beyond 100 years, and that a stand of pines can double its above-ground live carbon between
∼80 and 160 years.
Analysis of Dominant Individuals and Averages for Stand-Grown Pines
As Pine #58 is the tallest and the largest tree (volume) in the Trees of Peace (TOP), its performance over time was analyzed in great detail. It started growing as part of a more tightly packed stand, but presently has ample space. Its circumference at breast
height is 3.30 m, its height is 53.71 m, and its crown spread is approximately 15.5 m. Over a period of 26 years, beginning in 1992, Pine #58 has grown in circumference at an average rate of 1.39 cm per year and grown in height 23.71 cm per year. For a chronosequence, we assumed that Pine #58 grew a lot when it was young—an average of up to 61 cm per year in its first 50 years. Its trunk and limb volume was 23.33 m3 at the end of the 2018 growing season (Supplement 7).
Figure 1 shows the increase in height, circumference and volume of Pine #58 within each 50-year interval up to 150 years and includes a photo of the tree. Its estimated age is ∼160 years, and we used a chronosequence to determine previous epochs. For
dominant pines in stands on good sites, ring widths for the first 50 years average ∼0.6 cm and thus a 1.88 m circumference at 50 years. (Note that we measured one exceptional pine at 2.13 m in circumference.) Heights of stands at age 50 depend largely on site characteristics and expressed as site index (the average height of a stand at 50 years). The average index for white pine in Massachusetts is approximately 20 m (William Van Doren, Massachusetts Department of Conservation and Recreation, pers. comm.). For Pine #58 we calculated a much higher index to assume rapid early growth in the first 50 years. Based on these principles, the change in circumference and growth in height were greatest in the first 50 years, and decreased in the next two 50-year periods, confirming young pines “grow more rapidly” in terms of annual height and radial increases. However, volume growth, and thus carbon accumulation, continued to increase in the epochs studied here. This is primarily because volume increases linearly with height but increases as the square of the diameter (see Figure 1 and Supplement 8).
As noted, we assumed Pine #58 had optimal rapid growth in the first 50 years. Even so, our analysis supports the conclusion that the pine accumulated the majority of its current carbon after age 50 and at an increased rate during subsequent epochs. Pine #58 now stores 4.33 tC above ground and continues to grow. For comparison, the carbon stored in the trunk of the highest volume 50-year-old pine that we encountered (2.13 m circumference, 34.75 m height, and 0.4346 form factor) is 1.16 tC. Therefore, even in the best-case scenario Pine #58 would have acquired only a quarter of its current carbon by age 50. Note that the same crown area occupied by multiple younger trees cannot achieve the carbon in this larger tree (Leverett, unpublished observations).