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Older eastern white pine trees and stands sequester carbon for
many decades and maximize cumulative carbon
Robert T. Leverett1, Susan A. Masino2, William R. Moomaw3,*
1American Forests National Champion Tree Program, Washington, DC, USA; Native Tree
Society (www.nativetreesociety.org); Friends of Mohawk Trail State Forest, Florence, MA, USA
2Trinity College, Hartford, CT; Harvard Forest (2018-2019), Petersham, MA, USA
3The Fletcher School and Global Development and Environment Institute, Tufts University,
Medford, MA, USA; Woodwell Climate Research Center, Falmouth MA USA
Correspondence:
William R. Moomaw, Ph.D.
William.moomaw@tufts.edu
1 Abstract 2 Pre-settlement New England was heavily forested, with some trees exceeding 2 m in diameter. New 3 England’s forests have regrown since farm abandonment and represent what is arguably the most 4 successful regional reforestation on record; the region has recently been identified as part of the 5 “Global Safety Net.” Remnants and groves of primary “old-growth” forest demonstrate that native tree 6 species can live for hundreds of years and continue to add to the biomass and structural and ecological 7 complexity of forests. Forests are an essential natural climate solution for accumulating and storing 8 atmospheric CO2, and some studies emphasize young, fast-growing trees and forests whereas others 9 highlight high carbon storage and accumulation rates in old trees and intact forests. To address this 10 question directly within New England we leveraged long-term, accurate field measurements along with 11 volume modeling of individual trees and intact stands of eastern white pines (Pinus strobus) and 12 compared our results to models developed by the U.S. Forest Service. Our major findings complement, 13 extend, and clarify previous findings and are three-fold: 1) intact eastern white pine forests continue to 14 sequester carbon and store high cumulative carbon above ground; 2) large trees dominate above-15 ground carbon storage and can sequester significant amounts of carbon for hundreds of years; 3) 16 productive pine stands can continue to sequester high amounts of carbon for well over 150 years. 17 Because the next decades are critical in addressing the climate crisis, and the vast majority of New 18 England forests are less than 100 years old, and can at least double their cumulative carbon, a major 19 implication of this work is that maintaining and accumulating maximal carbon in existing forests – 20 proforestation - is a powerful near-term regional climate solution. Furthermore, old and old-growth 21 forests are rare, complex and highly dynamic and biodiverse, and dedication of some forests to 22 proforestation will also protect natural selection, ecosystem integrity and full native biodiversity long-23 term. In sum, strategic policies that grow and protect existing forests in New England will optimize a 24 proven, low cost, natural climate solution for meeting climate and biodiversity goals now and in the 25 critical coming decades. 26 27 Keywords: carbon accumulation, proforestation, chronosequence, tree volume measurements, old-28 growth forest, ecological integrity, ecological resilience 29
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30 Running title: Carbon in eastern white pines and stands 31 32 Introduction
33
A global priority for the climate has long been reducing ongoing emissions of heat-trapping 34 greenhouse gases (GHGs) produced by burning carbon-based fuels. While essential, less attention has 35 been given to the importance of simultaneously increasing carbon dioxide (CO2) removal (CDR) by 36 natural systems. Clearing forests, draining and developing wetlands, and degrading soils account for 37 one-third of all the CO2 added to the atmosphere by humans since the beginning of the industrial 38 revolution. Together these ongoing actions continue to add approximately 1.5 PgC/year (1 Pg is 1015 39 grams or 1 billion metric tonnes). Burning wood and plant-derived liquid fuels adds even more CO2, 40 and current forest management practices keep forests relatively young and limit their potential to 41 accumulate carbon above and below ground and keep it out of the atmosphere. Two recent 42 Intergovernmental Panel on Climate Change (IPCC) reports outlined the urgent and unprecedented 43 imperative to halt CO2 emissions and remove additional CO2 from the atmosphere (IPCC, 2018, 2019). 44 These reports, including the recent 1.5oC IPCC special report, identify forests as playing a major role. 45 However, for CDR they focus primarily on afforestation (planting new forests) and reforestation 46 (regrowing forests) and do not take into account the climate mitigation and adaptation benefits of 47 growing existing natural forests termed “proforestation” (Moomaw et al., 2019; Cook-Patton et al., 48 2020). 49
Even achieving the goal of “zero net carbon” to limit global average temperatures to 1.5oC will still 50 increase temperatures above the current rise of 1.1oC. This will result in additional disruption of the 51 climate system and additional adverse consequences presently experienced; impacts are 52 interconnected. To avoid ever-more serious consequences of a changed climate, the goal must be to 53 become net carbon negative as soon as possible. Using the strategies of natural reforestation and 54 particularly proforestation are among the most effective and least costly means for reducing the 55 atmospheric stock of carbon as will be illustrated by the findings reported in this paper. The power of 56 natural solutions – particularly growth and regeneration of natural forests – varies regionally, and has 57 recently been identified as being even more robust than previously considered (Cook-Patton et al., 58 2020). 59
A second and perhaps even more urgent priority is the strong protection of intact biodiverse natural 60 systems as outlined in the Global Assessment Report on Biodiversity and Ecosystem Services 61 (Intergovernmental Science-Policy on Biodiversity and Ecosystem Services, 2019). This joint 62 climate/biodiversity priority was reiterated in the peer-reviewed declaration of a Climate Emergency 63 signed by over 13,000 scientists in late 2019 which highlighted proforestation as a global climate 64 solution (Ripple et al., 2020), as did a recent post on behalf of the International Union for the 65 Conservation of Nature (Kormos et al., 2020). We can use forest-based solutions to rapidly and 66 substantially close the gap between CO2 emissions and removals by maximizing a range of nature-67 based solutions (Griscom et al., 2017), and the critical role of protecting intact ecosystems was 68 quantified in a report that documents “wilderness” as reducing species’ extinction by half (Di Marco et 69 al., 2019). Intact forests can simultaneously protect natural selection and biodiversity long-term, reduce 70 extinction, and provide pathways for migration while they continue to accumulate atmospheric CO2 71 and thereby moderate temperature increases (Friedlingstein et al., 2019). Taken together, it is practical 72 and possible to immediately protect ecosystems and prevent extinction while we increase CDR rates 73 and accumulate additional carbon in forests and forest soils. 74
To date, significant attention has been focused on tropical forests (Mitchard, 2018), yet temperate 75 forests are also biodiverse (Hilmers et al., 2018), benefit human health and well-being in highly 76
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populated areas (Karjalainen et al., 2010), and provide many essential ecosystem services (United 77 States Forest Service, 2020a). They also have a large additional potential for CDR (Cook-Patton et al., 78 2020) and New England Acadian Forests are part of the “Global Safety Net” and recently identified as 79 a Tier 1 climate stabilization area (Dinerstein et al., 2020). Current forest CDR in the United States is 80 estimated to remove an amount of atmospheric CO2 equal to 11.6% of added annual CO2 equivalent 81 emissions from the nation’s Greenhouse Gas emissions (United States Environmetal Protection 82 Agency, 2020), with the potential for much more (Keeton et al., 2011; Moomaw et al., 2019). 83 Consistent with the IPCC 1.5°C report that identified forests as key to increasing accumulation rates, 84 Houghton and Nassikas estimated that the “current gross carbon sink in forests recovering from 85 harvests and abandoned agriculture to be -4.4 PgC/year, globally” (Houghton and Nassikas, 2018). 86 This potential carbon sink from recovering forests is nearly as large as the gap between anthropogenic 87 emissions and removal rates, -4.9 Pg/year (Friedlingstein et al., 2019). 88
In the context of resource production and forest management, some carbon is stored in lasting wood 89 products, and responsible forestry provides a reliable wood supply. However, a natural forest does not 90 require management, and multiple analyses have found that a majority of carbon removed in a timber 91 harvest is lost to the atmosphere. For example, Hudiburg et al. demonstrated that just 19% of the 92 original carbon stock in Oregon forests in 1900 is in long lived wood products – approximately 16% is 93 in landfills, and the remaining 65% is in the atmosphere as carbon dioxide (Hudiburg et al., 2019); 94 Harmon found that the carbon storage in wood products is overestimated between 2 and 100-fold 95 (Harmon, 2019). Furthermore, Harris et al. has shown that biogenic emissions from harvesting are 640 96 MtC/year, exceeding the commercial and residential building sectors, and fossil fuel emissions from 97 harvesting add an additional 17% CO2 to the atmosphere (Harris et al., 2016). Strategic planning for 98 responsible resource production can both mitigate these emissions and ensure a protected network of 99 intact natural areas. 100 101 The US Climate Alliance highlights the importance of “net carbon accumulation” in forests across the 102 landscape (United States Climate Alliance, 2020). This is already occurring and demonstrates the 103 power of nature to help us restabilize the climate. A more impactful and explicit goal is to maximize 104 carbon accumulation by utilizing some forests for responsible resource production and protecting other 105 forests for maximal carbon accumulation for climate protection, long-term biodiversity, and human 106 health and well-being. At a global level, if deforestation were halted, and existing secondary forests 107 allowed to continue growing, a network of these intact forests would protect the highest number of 108 species from extinction (Di Marco et al., 2019; World Wildlife Federation, 2020) and it is estimated 109 that they could sequester ~120 PgC in the 84 years between 2016 and 2100 (Houghton and Nassikas, 110 2018). This is equivalent to about 12 years of current global fossil fuel carbon emissions, and these 111 global numbers are conservative as outlined in recent analyses (Cook-Patton et al., 2020) and they do 112 not factor in the enhanced regional CDR potential and high cumulative carbon that can be achieved 113 with proforestation – for example, of carbon-dense temperate forests such as in the Pacific Northwest 114 (Law et al., 2018) and New England (Nunery and Keeton, 2010; Keeton et al., 2011; Moomaw et al., 115 2019; Dinerstein et al., 2020). 116 117 Because these global and regional projections can be difficult to translate locally, particularly over 118 time, we focused on a detailed analysis of individual trees and stands in New England. Historically, 119 between 80% and 90% of the New England landscape was heavily forested, and early chroniclers 120 describe pre-settlement forests with many large, mature trees reaching 1 to 1.5 m in diameter. Fast-121 growing riparian species like sycamores and cottonwoods could reach or exceed 2 m. Today, New 122 England trees of this size are mostly found as isolated individuals in open areas, parks, and old estates. 123 Old-growth forests (primary forests) and remnants and are currently less than 0.2% of northern New 124 England’s landscape, and less than 0.03% in Southern New England, with ongoing attempts to 125 document their value and identify their locations (Davis, 1996; Kershner and Leverett, 2004; Ruddat, 126
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2020). Secondary forests in New England consist mostly of smaller, relatively young trees (less than 127 150 years, and on average less than 100 years old). Without proactive protection, and in the face of 128 programs that almost exclusively incentivize active management (typically for young forests and/or 129 timber production), we risk a future where the vast majority of the landscape will be managed and 130 performing well below its carbon accumulation and biodiversity potential. 131 132 Our goal herein was to measure carbon directly in individual trees and in an “average” versus an older 133 stand of eastern white pine (Pinus strobus) in New England. Most forest carbon studies focus on large 134 geographical areas, and utilize “net” carbon data gathered from LIDAR (Light Detection And 135 Ranging) and satellite technology, as well as statistical modeling based on the Forest Inventory and 136 Analysis (United States Forest Service, 2020b) and Carbon On Line Estimator (National Council for 137 Air Stream Improvement, 2020), products of the US Forest Service. We explore these options and note 138 that carbon estimates from different tools and models can lead to disparate results at the level of 139 individual trees – errors that can therefore be extrapolated to stands (Leverett et al., 2020). Therefore, 140 we capitalized on the extensive tree-measuring protocols and experience of the Native Tree Society 141 (NTS) to conduct highly accurate direct field measurements and measure volume precisely in younger 142 vs. older trees growing in stands (Native Tree Society, 2020). We used direct measurements to 143 evaluate volume-biomass models from multiple sources and developed a hybrid – termed FIA-COLE – 144 to capitalize on the strengths of each model. 145 146 For all aspects of this analysis we calculated the live above-ground carbon (in tonnes) in eastern white 147 pines and individuals of other species in a pine stand using conservative assumptions and direct 148 measurements wherever possible and well as direct measurements of individual dominant pines up to 149 190 years in age. Our basic analyses likely apply to other northeastern conifers such as red pines 150 (Pinus resimosa), eastern hemlock (Tsuga canadensis), and red spruce (Picea rubens). 151 152 2. Materials and methods 153 154 This paper centers on the study of individual eastern white pines of a representative older stand in 155 Western Massachusetts, collectively named the Trees of Peace (TOP) located in Mohawk Trail State 156 Forest, Charlemont, MA. The TOP has 76 pines covering 0.6 to 0.7 ha. We also collected and analyzed 157 data from NTS measurements in 38 other sites in the East (Supplement 1). Since 1990, NTS has taken 158 thousands of on-site direct measurements of individual trees in multiple stands of eastern white pines 159 (Pinus strobus) (See Supplement 1 for list of sites). Measurements are published on the society’s 160 website (NativeTreeSociety), comprehensive measurement protocols (Leverett et al., 2020) were 161 adopted from those developed by NTS (Leverett et al., 2020) and incorporated into the American 162 Forests Tree Measuring Guidelines Handbook (Leverett and Bertolette, 2014). A brief description of 163 the measurement methods and models is provided in section 2.1, Supplement 2 and (Leverett et al., 164 2020). 165 166 In the pine stands, a point-centered plot was established with a radius of 35.89 m, covering 0.403 167 hectares (subsequently referred to as 0.4 ha), with the goal of evaluating a standard acre (radius: 168 117.75 ft), and thus relevant to forestry conventions in the U.S. Within the TOP, 44 mature white pine 169 stems were tallied along with 20 hardwoods and eastern hemlocks down to a diameter of 10 cm at 170 breast height. The measured acre had 50 pines in July 1989, and since then six trees were lost in a wind 171 event. The pines are ~160 years old, and the hardwoods and hemlocks are estimated to be between 80 172 and 100 years old. 173 174 175 2.1 Height and diameter direct measurement methodology 176 177
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We quantified the volume of the trunk and limbs of each tree from heights and diameters measured by 178 state-of-the-art laser-based hypsometers, monoculars with range-finding reticles, traditional diameter 179 tapes, and calipers (Leverett et al., 2020). Each high-performance instrument was calibrated and 180 independently tested for accuracy over a wide range of distances and conditions. Absolute accuracies 181 of the two main infrared lasers were verified as +/- 2.5 cm for distance, surpassing the manufacturer’s 182 stated accuracy of +/- 4.0 cm. The tilt sensors were accurate to +/- 0.1°, meeting the manufacturer’s 183 stated accuracy. The combination of these distance and angle error ranges, along with the best 184 measurement methodology, gave us height accuracies to within 10 to 15 cm on the most distant targets 185 being measured and approximately half that on the closest targets. We distinguished the rated and/or 186 tested accuracy of a particular sensor of an instrument (such as an infrared laser or tilt sensor) from the 187 results of a measurement that utilized multiple sensors. 188 189 Tree heights were measured directly for each pine with a visible top, using the sine method 190 (Supplement 2) whenever possible rather than the traditional tangent method. Our preference for the 191 sine method is supported by NTS, the US Forest Service (Bragg et al., 2011) and American Forests 192 (Leverett and Bertolette, 2014). The more traditional tangent method often over/under-estimates 193 heights by treating the sprig being measured (interpreted as the top), as if it were located vertically 194 over the end of the baseline. The heights of 38 white pines in the TOP with visible tops were measured 195 directly using the sine method. 196 197 2.2 Use of a form factor and FIA-COLE in determining pine volume 198 199 To compute directly the trunk volume from base to absolute top of a tree, diameters at base and breast 200 height were measured with conventional calibrated tapes according to the procedures established and 201 published by NTS. Diameters aloft were measured with the combination of laser range-finders and 202 high performance monoculars with range-finding reticles. A miniature surveying device, the 203 LTI Trupoint 300, was also used. Its Class II, phase-based laser is rated at an accuracy of +/- 1.0 mm to 204 clear targets. In the TOP, we computed the volume of each pine’s trunk and limbs using diameter at 205 breast height, full tree height, trunk form, and limb factors. (See Supplement 3 for a discussion on the 206 development of the form factor and its importance in measuring volume, with comparisons to other 207 methods of measurement). 208 209 Detailed measurements of 39 sample trees established an average form factor (see NTS measurements 210 in Supplement 3, Table S3.2). The volume of each sample tree was determined by dividing the trunk 211 into adjacent sections, with the length of each section guided by observed changes in trunk taper and/or 212 visibility. Each section was modeled as the frustum of a regular geometric solid (neiloid, cone, and 213 paraboloid; see Supplement 3 and Leverett et al., 2020, for formulas). Section volumes were added to 214 obtain trunk volume, the form factor was determined needed to equal the trunk volume, given the total 215 height and breast-high diameter of each pine. This produced an average factor that would fit the pines 216 growing in a stand. We applied the average form factor to all pines included in the TOP as one 217 determination of trunk volume. 218 219 For comparison to our direct volume measurements, we applied a hybrid volume-biomass model to 220 compute trunk volumes for pines in the TOP. This hybrid allowed us to make use of the extensive 221 analysis of the US Forest Service Forest Inventory and Analysis (FIA) program and database (which 222 determines volume and biomass through the use of allometric equations) as well as the Carbon On-223 Line Estimator (COLE). This hybrid was termed FIA-COLE. See Supplement 4 for a full explanation 224 of the variables and equations for defining trunk volume. We finalized volumes for the pines in the 225 TOP by averaging our direct measurements with those of FIA-COLE. 226 227
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For the total volume of the above-ground portion of a pine, we derived a factor for limbs, branches, 228 and twigs as a proportion of the trunk volume using the FIA-COLE model (Supplement 5). That model 229 includes all the branching in what is defined as the “top” in a biomass calculation and the limb factor 230 for large trees is typically an additional 15-16%. We ran the model for each of the individuals in the 231 TOP and calculated the volume. This was converted to biomass (density) and then to carbon mass 232 using the carbon mass fractional factor. 233 234 2.3 Analysis of individual pine trees and a representative stand 235 236 In addition to the TOP, and older exemplary pines, we quantified above-ground carbon in younger 237 trees and a representative stand. To determine an “average” pine at 50 years we defined two 238 populations: (1) trees at 50 years that are still alive today, and (2) trees that were alive at 50 years, but 239 are missing today. This allowed us to compute an average trunk size for the missing trees and the 240 associated carbon. We also measured white pines from young to older ages to estimate growth rates 241 and volumes. 242 243 We extensively studied an ~80-year-old stand of pines adjacent to the TOP (Supplement 6) growing on 244 a terrace located just downslope from the TOP in an area fairly well protected from wind and with 245 adequate soil depth. This age is more representative of the average stand of eastern white pine in New 246 England. We also considered the range of pines of known ages from stands within the vicinity and 247 elsewhere. Where we could, we examined ring growth and height patterns for individual pines during 248 their early years on a variety of sites in different geographical locations. In some cases, we examined 249 stumps and measured the average ring width. In other cases, we measured trees and counted limb 250 whorls to get age estimates. 251 252 We measured the tallest pine in great detail and over a long time-span (referred to as Pine #58, its 253 research tag number). Pine #58 has been measured carefully and regularly over a period of 28 years. In 254 1992 the tree was 47.24 m tall and 2.93 m in circumference. Since then, it has been climbed 4 times, 255 tape-drop-measured, and volume-determined. Pine #58 continues to grow and enabled us to quantify 256 the changes in carbon accumulation in a dominant tree over decades. See Supplement 7 for a detailed 257 measurement history of Pine #58. Additional trees were measured at sites listed in Supplement 1. 258 As noted, above-ground volumes were converted to mass using standard wood density tables (United 259 States Department of Agriculture, 2009). The air-dried density for white pine is 385.3 kg/m3 (0.3853 260 tonnes/m3). We calculated the amount of carbon in each pine using a conservative figure of 48% of 261 total air-dried weight (50% is used more commonly; the percentage of carbon content in different 262 species ranges from ~48% to 52+%). Therefore we calculated a cubic meter of white pine trunk or 263 limbs as holding 0.18494 tonnes of carbon. Note that the carbon in a cubic meter of wood varies 264 depending on the species and is usually higher in hardwoods (United States Department of Agriculture, 265 2009). 266 267 268 3. Results 269 270 Using conservative assumptions where needed, and direct measurements wherever possible, we found 271 that individual eastern white pines accumulate significant above-ground volume/carbon at least up to 272 190 years, that this volume/carbon accumulation can accelerate overtime, and that a stand of pines can 273 double its above-ground carbon between ~80 and 160 years. 274 275 3.1 Analysis of dominant individuals and averages for stand-grown pines 276 277
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As Pine #58 is the tallest and the largest tree (volume) in the TOP, its performance over time was 278 analyzed in great detail. It started growing as part of a more tightly packed stand, but presently has 279 ample space. Its circumference at breast height is 3.30 m, its height is 53.64 m, and its crown spread is 280 approximately 15.5 m. Over a period of 26 years, beginning in 1992, Pine #58 has grown in 281 circumference at an average rate of 1.39 cm per year and grown in height 23.71 cm per year. For a 282 chronosequence, we assumed that Pine #58 grew a lot when it was young – an average of up to 61 cm 283 per year in its first 50 years. Its trunk and limb volume was 23.02 m3 at the end of the 2018 growing 284 season (Supplement 7). 285 286 Figure 1 shows the increase in height, circumference and volume of Pine #58 within each 50-year 287 interval up to 150 years. Its estimated age is ~160 years, and we used a chronosequence to determine 288 previous epochs. For dominant pines in stands on good sites, ring widths for the first 50 years average 289 ~0.6 cm and thus a 1.88 m circumference at 50 years. (We measured one exceptional pine at 2.13 m in 290 circumference.) Heights of stands at age 50 depend largely on the site index (the average height of a 291 stand at 50 years), and indices for white pine on good sites usually range from 25.0 to 33.5 m. We used 292 an index near the upper range (30.5 m) and well above the average for Massachusetts to assume rapid 293 early growth. Based on these principles, the change in circumference and growth in height were 294 greatest in the first 50 years, and decreased in the next two 50-year periods, confirming young pines 295 “grow more rapidly” in terms of annual height and radial increases. However, volume growth, and thus 296 carbon accumulation, increased with age. This is primarily because volume increases linearly with 297 height but increases as the square of the diameter (see Figure 1 and Supplement 8). 298 299 As noted, we assumed Pine #58 had optimal rapid growth in the first 50 years. Even so, our analysis 300 supports the conclusion that the pine accumulated the majority of its current carbon after age 50 and at 301 a slightly increased rate. Pine #58 now stores 4.24 tC above ground and continues to grow. For 302 comparison, the carbon sequestered in the highest volume 50-year-old pine that we encountered (2.13 303 m circumference, 34.75 m height, and 0.4353 form factor) is 1.01 tC. Therefore, even in the best-case 304 scenario Pine #58 would have acquired less than a quarter of its current carbon by age 50. 305 306 The carbon advantage gained by the older trees accelerates with their increasing age and size, a finding 307 that has been affirmed globally (Stephenson et al., 2014). Figure 2 documents the average volume in 308 individual pines in the stands at ~80 and ~160 years as well as several additional large pines. MSF Pine 309 #1, the largest pine in Monroe State Forest, western Massachusetts, has a trunk volume of 35.9 m3 at 310 approximately 190 years (6.15 tC; Figure 2). Assuming its early years accumulated 1.01 tC at 50 years, 311 which is the fastest growing 50-year old pine we measured in all sampled locations, the large pine 312 added 5.14 tC between 50 and 190 years, or 1.84 tC per 50-year cycle after year 50. This is at least 313 1.82 times the rate of growth for the first 50 years. This compares to a 1.6 ratio for Pine #58. In both 314 cases more than 75% of the carbon they sequestered occurred after their first 50 years even when 315 assuming the most optimal growth observed during the first 50 years. 316 317 3.2 Stand measurements at ~80 and ~160 years 318 319 Detailed measurements were taken in comparable pine stands at ~80 and ~160 years (TOP). As noted, 320 the average tree in each stand is shown on Figure 2, and the distribution of tree sizes in the TOP is 321 shown in Figure 3A. The largest pine in the TOP holds 4.24 tC and the smallest holds 0.53, an eight-322 fold difference. A comparison of the stand density and above ground carbon at ~80 vs. ~160 yr are 323 shown in Figure 3B. 324 325 Complete data for 76 individual pines in the TOP (the 0.4 ha primary plot plus additional trees in the 326 stand) is provided in Supplement 9. For comparison, data from 0.4 ha was collected from an ~80-year 327 old stand growing on a terrace just downslope from the TOP in an area fairly well protected from wind 328
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and with adequate soil depth (Supplement 6). This age is more representative of the average stand of 329 eastern white pine in New England. Average values for both ages are summarized in Table 1. As 330 shown in Figure 2, we found an average of 0.66 tC per tree compared to 1.95 tC per tree in the TOP, a 331 near tripling of carbon in the average individual pine in the older stand. We found a lower stand 332 density in terms of number of stems, and a higher level of carbon in the TOP. Pines predominated both 333 plots, and non-pine species added ~10% to the total above ground carbon (Figure 3B). 334 335 We note these calculations only include above ground tree-based carbon – they do not include more 336 labile sources of additional carbon in the needles, leaves and understory plants, or the accumulation of 337 woody debris in older stands. Our measurements also do not include the large store of underground 338 carbon (the root system is typically estimated as an additional 15-20% of the above-ground tree 339 volume, and total soil organic carbon can be an additional 50% or more. Therefore, the total carbon is 340 significantly higher; we do not address those elements here. The above-ground tree-based carbon 341 measured directly in the primary acre in the 80 year old stand is 46.86 tC and the 160-year-old stand is 342 94.4 tC, translating to 117.15 and 236.0 tC per hectare, respectively. Approximately 10% of the tree-343 based carbon in the older stand is non-pine; non-pine carbon in the younger stand is negligible (Table 344 1).345
346 4. Discussion 347 348 The representative stands in this analysis approximate the average pine forest age (~80 years old) and a 349 comparable stand nearly twice that age. To determine the biomass and above ground carbon in living 350 trees as a function of tree size and age, we have used a combination of direct measurements and a 351 hybrid FIA-COLE volume and biomass model to quantify individual trees and stands of eastern white 352 pine, a common tree species in New England, We found that dominant individual trees accelerate their 353 accumulation of carbon well past 150 years, and more than 75% of the carbon in dominant pines up to 354 190 years is gained after the first 50 years. Despite a lower stand density (fewer stems), total above-355 ground carbon is greatest in older stands and continues to increase past 150 years. The carbon per 356 hectare quantified in these stands matches previous averages for the region and previous regional 357 estimates that New England forests can accumulate at least twice as much carbon. The total carbon 358 stored is even greater when below-ground carbon in roots, coarse woody debris, standing dead trees 359 and smaller plants and soils are included (Nunery and Keeton, 2010; Tomasso and Leighton, 2014). 360 361 Forest managers stress the high accumulation rates of younger forests as important in absorbing 362 atmospheric CO2. This is an important consideration for production forests to help optimize between 363 growing a wood resource and accumulating carbon. Younger individual trees do not sequester more 364 carbon than mature trees, and we did not find evidence for a significant benefit for a young stand 365 compared to an older stand but we did not estimate accumulation rates below 80 years. Clearing an 366 older forest to create a young forest creates a large carbon debt. Creating or maintaining this habitat 367 has other benefits (wood production, habitat for hunting, or specific successional species), but it 368 dramatically reduces forest carbon and eliminates the ability for that forest to host the full biodiversity 369 of some of our rarest species of plants, animals, insects, fungi, lichens, reptiles and amphibians found 370 in older and continuously forested areas (McMullin and Wiersma, 2019; Moose et al., 2019) as well as 371 climate-sensitive birds that may benefit from old-growth forests (Betts et al., 2017). These older 372 unmanaged forests also have fewer invasive species (Riitters et al., 2018). 373 374 The pine stands studied here grew from former sheep pastures, therefore not likely starting from a 375 severely disturbed condition and thus minimizing initial carbon elution. This raises the question about 376 the importance of site history in influencing growth, especially in the early years, since a disturbed 377 condition can continue to lose carbon for more than a decade. We recognize that at some point above-378 ground carbon in living trees will no longer be increasing since the trees eventually die. However total 379
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forest carbon continues to increase even in some primary (“old-growth”) forests (Mackey et al., 2015). 380 After tree death or forest disturbance there is a transfer of live carbon to dead wood and woody debris, 381 the litter layer, and into the soil. Here in the older pine stand there is also the increased prevalence and 382 growth of trees of other species (including more carbon-dense hardwoods) so that the species diversity 383 and total carbon load continues to rise. A challenge for future research is to understand tree and stand 384 carbon accumulation and dynamics in detail well beyond 200 years. 385 386 Public forests in New England are typically older than private forests (but still predominantly less than 387 100 years old), and provide the greatest possibility for intact forests across the landscape. Native tree 388 species can live for several hundred years (and in the case of eastern hemlock (Tsuga canadensis) and 389 black gum (Nyssa sylvatica), up to and exceeding 500 years). Despite the noted lack of old and old-390 growth forests, and the increasing level of natural disturbances from insects and storms creating forest 391 diversity and forest openings, a major focus on public land is clearcutting to “create young forest” or 392 “create resilience.” These programs assert that young forests prevent a suite of species from declining, 393 that they sequester carbon more rapidly, and that they are more resilient than their older counterparts 394 (Anwar, 2001). This approach has experimental merit, but at this time it needs more direct and long-395 term measurements and sufficient baselines and controls. It overlooks the dynamic evolution of these 396 habitats over time, creating niches for these species. It also overlooks the critical role of cumulative 397 stored carbon compared to sequestration, and the superior resilience of older forests to the stresses of 398 climate change (Thom et al., 2019). The details of age and location (tropical, temperate, boreal, etc.) 399 are also important in terms of what is reported “young” – in some cases considered up to 140 years 400 (Pugh et al., 2019). 401 402 Our findings are consistent with Stephenson et al (2014) who found that absolute growth increases 403 with tree size for most of 403 tropical and temperate tree species, and a study of 48 forest plots found 404 that in older forests, regardless of geographical location, half of all above-ground biomass (and hence 405 carbon), is stored in the largest 1% of trees as measured by diameter at breast height (Lutz et al., 2018). 406 Keeton et al found an increase in carbon density per hectare as the age of the stand increased in the 407 Northeast U.S. (Keeton et al., 2011) and a recent study in China found that forests with older trees and 408 greater species richness had twice the levels of carbon storage than did less diverse forests with 409 younger trees (Liu et al., 2018). Earlier work demonstrated that intact old growth forests in the Pacific 410 Northwest contained more than twice the amount of sequestered carbon as did those that were 411 harvested on a fixed rotation basis (Harmon et al., 1990). Erb et al. concluded that forests are capable 412 of sequestering twice as much atmospheric carbon as they currently do (Erb et al., 2018). The potential 413 for natural reforestation as a climate solution has just been increased dramatically (Cook-Patton et al., 414 2020). 415 416 Proforestation - growing existing natural forests - and recognizing the role of older forests and large 417 trees in carbon accumulation and biodiversity protection, are critical components of a global strategy. 418 Rapidly moving large stocks of atmospheric carbon as CO2 into forests and reducing emissions is 419 essential for limiting the increase in global temperatures, and protecting intact and connected habitat is 420 essential in preventing extinction. An important implication of this finding is that the estimated 421 additional CDR achieved by future growth of secondary forests reported by Houghton and Nassikas is 422 likely an underestimate because it does not account for ongoing accumulation rates as trees age 423 (Houghton and Nassikas, 2018) – at least in regions with relatively young forests like those of the 424 Northeast United States. The global study of natural forest carbon accumulation by Cook-Patton et al. 425 provides quantitative evidence of the power of natural reforestation (Cook-Patton et al., 2020). 426 Considering these reports and the current findings increases the potential regional contribution for 427 increased carbon accumulation rates in the coming decades by Northeastern temperate forests. 428 429
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While the IPCC clearly identified forests as essential for sequestering additional carbon for climate 430 stability it focused on production forests that are currently recovering from being harvested or on 431 unforested areas where forests could be planted (afforestation). A report by Bastin et al. proposes 432 massive afforestation on 0.9 billion ha but acknowledges that it will take time before large amounts of 433 carbon would be sequestered (Bastin et al., 2019). Global tree planting efforts are under way but there 434 is little data on how to plant an ecosystem, and some of these tree planting efforts suffer from 75-80% 435 mortality of the young trees. In contrast, growing existing forests that already contain large carbon 436 stores and can rapidly sequester increasing amounts of atmospheric carbon dioxide and accumulate 437 diversity over time - a much more effective near-term and proven strategy (Moomaw et al., 2019). 438 While valuable, neither afforestation nor reforestation will remove as much atmospheric carbon as 439 proforestation in the next 50 years – the timeline when it is needed most to avoid irreversible 440 consequences of a changed climate. Protecting primary forests and secondary forests where possible is 441 also a far better option than the unproven technology of bioenergy with carbon capture and storage 442 (BECCS), also suggested by the IPCC report (Anderson and Peters, 2016; IPCC, 2018). Finally, letting 443 existing secondary forests grow provides equity, natural heritage and cumulative health benefits for 444 people in terms of respite and passive recreation - and does not compete directly with agriculture and 445 other demands for land use. 446 447 The highly accurate direct measurements at the tree and stand level in this paper are consistent with 448 parameterized and other studies at larger scale in verifying that larger trees (Lutz et al, 2018, 449 Stephenson et al, 2014) and stands of larger trees accumulate the most carbon over time compared to 450 smaller trees (Mildrexler et al. 2020). They support the proforestation strategy of growing existing 451 forests to achieve their natural capacity to accumulate carbon and achieve their biodiversity potential 452 (Moomaw et al., 2019) to redress the balance of carbon lost to the atmosphere from global forests 453 (Hudiburg et al., 2020). The important implication of these findings is that the trees and the forests that 454 we need most for carbon storage and CDR to help limit near-term climate change are the ones that are 455 already established. 456 457 Plantations and forests managed for forest products account for 71% of all forest area globally (IPCC, 458 2019), more than sufficient for resource production. Strategic planning can enable some to be 459 prioritized and repurposed for climate protection and research - and the remaining 29% should be 460 protected wherever possible. High levels of carbon accumulation and biodiversity protection can be 461 achieved in parallel with inherent resiliency to a changing climate – including by protecting species 462 networks, genetic diversity and epigenetic changes. These findings also specifically ground-truth the 463 capacity for New England pine forests to more than double their carbon in the coming decades. 464 Protection of public forests from unneeded intervention is urgent, and compensation programs should 465 be established for stewarding private forests based on numerous ecosystem services. 466 467 468 Acknowledgments 469 470
We acknowledge the critical technical assistance of Jared D. Lockwood in measuring pines in the 471 TOP, the younger pine stand, and elsewhere as needed. We thank Monica Jakuc Leverett for 472 invaluable assistance in editing and revising the original draft, and David Ruskin for tireless efforts 473 throughout the process. We thank Ray Asselin for photographing pines for further analysis, and for 474 ring and whorl counts to establish ages. Supported by Trinity College and a Charles Bullard 475 Fellowship in Forest Research (SAM), a Faculty Research Grant from the NASA Connecticut Space 476 Grant Consortium (RTL, SAM) and the Rockefeller Brothers Fund (WRM). 477
478 479
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480 Author contributions statement 481 482 RTL chose site locations and individual trees, established measurement methods and protocols, did the 483 on-site tree measuring, and performed the subsequent analysis. SAM analyzed and organized the 484 content and supplements and participated in drafting and finalizing the text. WRM framed the analysis 485 in the context of other studies and the larger context of climate change, assisted with data analysis and 486 presentation, and drafting and editing the text. 487 488 Conflict of interest statement 489 490 This work was not carried out in the presence of any personal, professional or financial relationships 491 that could potentially be construed as a conflict of interest. 492 493
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Table 1. Summary of key measurements within a 160-year pine stand (TOP) and a comparable ~80 494 year old stand (2018 – 2019 values) 495 496 Individual Values
~160 year old 0.4 hectare
Circumference at breast height (avg)
2.36 m
Diameter at breast height (avg)
0.75 m
Height (avg)
45.10 m
Tree Volume (trunk + limbs; avg)
10.47 m
3
Above-ground carbon per tree (avg)
~80 year old 0.4 hectare
Circumference at breast height (avg)
Diameter at breast height (avg)
Height (avg)
Tree volume (trunk + limbs; avg)
Above-ground carbon per tree (avg)
1.95 tC
1.56 m
0.50 m
38.4 m
3.58 m3
0.66 tC
Stand Values
Full Stand at ~160 years
Number of pines
76
Above-ground pine-based carbon
Above-ground non-pine carbon
Total above-ground tree carbon
Research Acre ~160 years (0.4 hectare)
Number of pines
Above-ground pine-based carbon
Above-ground non-pine carbon
Total above-ground tree carbon
Research Acre ~80 years (0.4 hectare)
Number of pines
Total above-ground pine-based carbon
(negligible non-pine carbon)
146.84 tC
14.90 tC
161.74 tC
44
85.8 tC
8.6 tC
94.4 tC
71
46.86 tC
497
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498 Figure Legends 499 500 501 502 Figure 1. Changes in circumference, height and volume of a stand-grown individual eastern white pine 503 (Pine #58) in three 50-y intervals. Upper panels - A: Change in circumference during 0-50, 50-100 and 504 100-150 y. B: Change in height between 0-50, 50-100 and 100-150 y. C: Change in above-ground tree 505 volume (trunk plus limbs) between 0-50, 50-100 and 100-150 y. Lower panels - D: Cumulative 506 circumference at 50, 100 and 150 y compared to cumulative above-ground volume. E. Cumulative 507 height at 50, 100 and 150 y compared to cumulative above-ground volume. On each panel initial 508 slopes were matched to reflect the rapid change in circumference and height during the first 50-y 509 interval. Note that volume is a proxy for above-ground carbon. Values for circumference, height and 510 volume of Pine #58 were determined by a combination of direct measurement and chronosequence and 511 described in the text and in Supplement 7. 512 513 Figure 2. Tonnes of above-ground carbon (tC) in an “average” eastern white pine in a measured 514 research acre (green locants) and in five individual trees (A,B,C,D,E) measured directly on site at three 515 separate locations in Massachusetts. Average tC and standard deviation is based on pines in a stand at 516 ~80 years (0.66 ± 0.38 tC) and ~160 years (1.95 ± 0.73 tC) as described in the text. Direct 517 measurement of tC is shown for individual trees in western Massachusetts at these ages and locations: 518 A, B - ~190 years (MSF #1 and #2, Monroe State Forest); C - ~160 years (Pine #58, Mohawk Trail 519 State Forest; more details of Pine #58 shown in Figure 1); D - ~150 years (Totem, Northampton, MA); 520 E – ~120 years (BB #2, Broad Brook, Florence, MA). 521 522 Figure 3. Carbon distribution, stand density and cumulative carbon in predominantly eastern white pine 523 stands at ~80 and 160 years. These two stands were regrown from land previously used as pasture (i.e. 524 not recovering from a harvest at time zero). A. Distribution of above-ground carbon (tC) among 76 525 eastern white pines of different sizes in the full TOP stand at ~160 years old. The majority contained 1-526 3 tC. B. Stand density and above-ground carbon measured directly on site in a research acre of eastern 527 white pine at ~80 and 160 years. Stand density (# of stems) declined while above-ground carbon 528 increased. The older stand includes some non-species that added to the number of stem and total 529 carbon (open locants). 530 531
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