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Land use strategies to mitigate climate change in
carbon dense temperate forests
Beverly E. Law
a,1
, Tara W. Hudiburg
b
, Logan T. Berner
c
, Jeffrey J. Kent
b
, Polly C. Buotte
a
, and Mark E. Harmon
a
a
Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR 97333;
b
Department of Forest, Rangeland, and Fire Sciences,
University of Idaho, Moscow, ID 83844; and
c
EcoSpatial Services L.L.C., Flagstaff, AZ 86004
Edited by William H. Schlesinger, Duke University, Durham, NC, and approved January 22, 2018 (received for review November 16, 2017)
Strategies to mitigate carbon dioxide emissions through forestry
activities have been proposed, but ecosystem process-based in-
tegration of climate change, enhanced CO
2
, disturbance from fire,
and management actions at regional scales are extremely limited.
Here, we examine the relative merits of afforestation, reforesta-
tion, management changes, and harvest residue bioenergy use in
the Pacific Northwest. This region represents some of the highest
carbon density forests in the world, which can store carbon in
trees for 800 y or more. Oregon’s net ecosystem carbon balance
(NECB) was equivalent to 72% of total emissions in 2011–2015. By
2100, simulations show increased net carbon uptake with little
change in wildfires. Reforestation, afforestation, lengthened har-
vest cycles on private lands, and restricting harvest on public lands
increase NECB 56% by 2100, with the latter two actions contribut-
ing the most. Resultant cobenefits included water availability and
biodiversity, primarily from increased forest area, age, and species
diversity. Converting 127,000 ha of irrigated grass crops to native
forests could decrease irrigation demand by 233 billion m
3
·y
−1
.
Utilizing harvest residues for bioenergy production instead of leav-
ing them in forests to decompose increased emissions in the short-
term (50 y), reducing mitigation effectiveness. Increasing forest carbon
on public lands reduced emissions compared with storage in wood
products because the residence time is more than twice that of wood
products. Hence, temperate forests with high carbon densities and
lower vulnerability to mortality have substantial potential for reduc-
ing forest sector emissions. Our analysis framework provides a tem-
plate for assessments in other temperate regions.
forests
|
carbon balance
|
greenhouse gas emissions
|
climate mitigation
Strategies to mitigate carbon dioxide emissions through for-
estry activities have been proposed, but regional assessments
to determine feasibility, timeliness, and effectiveness are limited and
rarely account for the interactive effects of future climate, atmo-
spheric CO
2
enrichment, nitrogen deposition, disturbance from
wildfires, and management actions on forest processes. We examine
the net effect of all of these factors and a suite of mitigation strat-
egies at fine resolution (4-km grid). Proven strategies immediately
available to mitigate carbon emissions from forest activities in-
clude the following: (i) reforestation (growing forests where they
recently existed), (ii) afforestation (growing forests where they did
not recently exist), (iii) increasing carbon density of existing for-
ests, and (iv) reducing emissions from deforestation and degra-
dation (1). Other proposed strategies include wood bioenergy
production (2–4), bioenergy combined with carbon capture and
storage (BECCS), and increasing wood product use in build-
ings. However, examples of commercial-scale BECCS are still
scarce, and sustainability of wood sources remains controversial
because of forgone ecosystem carbon storage and low environmental
cobenefits (5, 6). Carbon stored in buildings generally outlives
its usefulness or is replaced within decades (7) rather than the
centuries possible in forests, and the factors influencing prod-
uct substitution have yet to be fully explored (8). Our analysis
of mitigation strategies focuses on the first four strategies, as
well as bioenergy production, utilizing harvest residues only and
without carbon capture and storage.
The appropriateness and effectiveness of mitigation strate-
gies within regions vary depending on the current forest sink,
competition with land-use and watershed protection, and envi-
ronmental conditions affecting forest sustainability and resilience.
Few process-based regional studies have quantified strategies that
could actually be implemented, are low-risk, and do not depend
on developing technologies. Our previous studies focused on re-
gional modeling of the effects of forest thinning on net ecosystem
carbon balance (NECB) and net emissions, as well as improving
modeled drought sensitivity (9, 10), while this study focuses mainly
on strategies to enhance forest carbon.
Our study region is Oregon in the Pacific Northwest, where
coastal and montane forests have high biomass and carbon se-
questration potential. They represent coastal forests from northern
California to southeast Alaska, where trees live 800 y or more and
biomass can exceed that of tropical forests (11) (Fig. S1). The
semiarid ecoregions consist of woodlands that experience frequent
fires (12). Land-use history is a major determinant of forest carbon
balance. Harvest was the dominant cause of tree mortality (2003–
2012) and accounted for fivefold as much mortality as that from fire
and beetles combined (13). Forest land ownership is predominantly
public (64%), and 76% of the biomass harvested is on private lands.
Significance
Regional quantification of feasibility and effectiveness of forest
strategies to mitigate climate change should integrate observa-
tions and mechanistic ecosystem process models with future cli-
mate, CO
2
, disturbances from fire, and management. Here, we
demonstrate this approach in a high biomass region, and found
that reforestation, afforestation, lengthened harvest cycles on
private lands, and restricting harvest on public lands increased net
ecosystem carbon balance by 56% by 2100, with the latter two
actions contributing the most. Forest sector emissions tracked
with our life cycle assessment model decreased by 17%, partially
meeting emissions reduction goals. Harvest residue bioenergy use
did not reduce short-term emissions. Cobenefits include increased
water availability and biodiversity of forest species. Our improved
analysis framework can be used in other temperate regions.
Author contributions: B.E.L. and T.W.H. designed research; B.E.L., T.W.H., and P.C.B. per-
formed research; M.E.H. contributed new reagents/analytic tools; B.E.L., L.T.B., J.J.K., and
P.C.B. analyzed data; B.E.L., T.W.H., L.T.B., and M.E.H. wrote the paper; and M.E.H. con-
tributed the substitution model.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This open access article is distributed under Creative Commons Attribution-NonCommercial-
NoDeriv atives L icense 4. 0 (CC BY-N C-ND).
Data deposition: The CLM4.5 model data are available at OregonState University (terraweb.
forestry.oregonstate.edu/FMEC). Data from the >200 intensiv e plots on forest carbon
are available at Oak Ridge National Laboratory (https://daac.ornl.gov/NACP/guides/
NACP_TERRA-PNW.html), and FIA data are available at the USDA Forest Service
(https://www.fia .fs.fed.us/to ols-data/).
1
To whom correspondence should be addressed. Email: bev.law@oregonstate.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1720064115/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1720064115 PNAS Latest Articles
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ENVIRONMENTAL
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SUSTAINABILITY
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Many US states, including Oregon (14), plan to reduce their
greenhouse gas (GHG) emissions in accordance with the Paris
Agreement. We evaluated strategies to address this question: How
much carbon can the region’s forests realistically remove from the
atmosphere in the future, and which forest carbon strategies can
reduce regional emissions by 2025, 2050, and 2100? We propose
an integrated approach that combines observations with models
and a life cycle assessment (LCA) to evaluate current and future
effects of mitigation actions on forest carbon and forest sector
emissions in temperate regions (Fig. 1). We estimated the recent
carbon budget of Oregon’s forests, and simulated the potential to
increase the forest sink and decrease forest sector emissions under
current and future climate conditions. We provide recommenda-
tions for regional assessments of mitigation strategies.
Results
Carbon stocks and fluxes are summarized for the observation
cycles of 2001–2005, 2006–2010, and 2011–2015 (Table 1 and
Tables S1 and S2). In 2011–2015, state-level forest carbon stocks
totaled 3,036 Tg C (3 billion metric tons), with the coastal and
montane ecoregions accounting for 57% of the live tree carbon
(Tables S1 and S2). Net ecosystem production [NEP; net primary
production (NPP) minus heterotrophic respiration (Rh)] aver-
aged 28 teragrams carbon per year (Tg C y
−1
) over all three
periods. Fire emissions were unusually high at 8.69 million metric
tons carbon dioxide equivalent (tCO
2
ey
−1
,i.e.,2.37TgCy
−1
)in
2001–2005 due to the historic Biscuit Fire, but decreased to
3.56 million tCO
2
ey
−1
(0.97 Tg C y
−1
)in2011–2015 (Table S4).
Note that 1 million tCO
2
eequals3.667TgC.
Our LCA showed that in 2001–2005, Oregon’s net wood
product emissions were 32.61 million tCO
2
e(Table S3), and 3.7-
fold wildfire emissions in the period that included the record fire
year (15) (Fig. 2). In 2011–2015, net wood product emissions were
34.45 million tCO
2
e and almost 10-fold fire emissions, mostly due
to lower fire emissions. The net wood product emissions are
higher than fire emissions despite carbon benefits of storage in
wood products and substitution for more fossil fuel-intensive
products. Hence, combining fire and net wood product emis-
sions, the forest sector emissions averaged 40 million tCO
2
ey
−1
and accounted for about 39% of total emissions across all sectors
(Fig. 2 and Table S4). NECB was calculated from NEP minus
losses from fire emissions and harvest (Fig. 1). State NECB was
equivalent to 60% and 70% of total emissions for 2001–2005 and
2011–2015, respectively (Fig. 2, Table 1, and Table S4). Fire
emissions were only between 4% and 8% of total emissions from
all sources (2011–2015 and 2001–2004, respectively). Oregon’sfor-
ests play a larger role in meeting its GHG targets than US forests
have in meeting the nation’s targets (16, 17).
Historical disturbance regimes were simulated using stand age
and disturbance history from remote sensing products. Comparisons
of Community Land Model (CLM4.5) output with Forest Inventory
and Analysis (FIA) aboveground tree biomass (>6,000 plots) were
within 1 SD of the ecoregion means (Fig. S2). CLM4.5 estimates of
cumulative burn area and emissions from 1990 to 2014 were 14%
and 25% less than observed, respectively. The discrepancy was
mostly due to the model missing an anomalously large fire in 2002
(Fig. S3A). When excluded, modeled versus observed fire emis-
sions were in good agreement (r
2
=0.62; Fig. S3B). A sensitivity
test of a 14% underestimate of burn area did not affect our final
results because predicted emissions would increase almost equally
for business as usual (BAU) management and our scenarios,
resulting in no proportional change in NECB. However, the ratio
of harvest to fire emissions would be lower.
Projections show that under future climate, atmospheric carbon
dioxide, and BAU management, an increase in net carbon uptake due
to CO
2
fertilization and climate in the mesic ecoregions far outweighs
losses from fire and drought in the semiarid ecoregions. There was not
an increasing trend in fire. Carbon stocks increased by 2% and 7%
and NEP increased by 12% and 40% by 2050 and 2100, respectively.
We evaluated emission reduction strategies in the forest sector:
protecting existing forest carbon, lengthening harvest cycles, re-
forestation, afforestation, and bioenergy production with product
substitution. The largest potential increase in forest carbon is in the
mesic Coast Range and West Cascade ecoregions. These forests are
buffered by the ocean, have high soil water-holding capacity, low
risk of wildfire [fire intervals average 260–400 y (18)], long carbon
residence time, and potential for high carbon density. They can
attain biomass up to 520 Mg C ha
−1
(12).AlthoughOregonhas
several protected areas, they account for only 9–15% of the total
forest area, so we expect it may be feasible to add carbon-protected
lands with cobenefits of water protection and biodiversity.
Reforestation of recently forested areas include those areas im-
pacted by fire and beetles. Our simulations to 2100 assume regrowth
of the same species and incorporate future fire responses to climate
and cyclical beetle outbreaks [70–80 y (13)]. Reforestation has the
potential to increase stocks by 315 Tg C by 2100, reducing forest sector
net emissions by 5% by 2100 relative to BAU management (Fig. 3).
The East and West Cascades ecoregions had the highest reforestation
potential, accounting for 90% of the increase (Table S5).
Afforestation of old fields within forest boundaries and non-
food/nonforage grass crops, hereafter referred to as “grass crops,”
had to meet minimum conditions for tree growth, and crop grid
cells had to be partially forested (SI Methods and Table S6). These
crops are not grazed or used for animal feed. Competing land uses
may decrease the actual amount of area that can be afforested.
We calculated the amount of irrigated grass crops (127,000 ha)
that could be converted to forest, assuming success of carbon
offset programs (19). By 2100, afforestation increased stocks by
–
FireNPP
–
R
h
–
Harvest
NECB =
Fig. 1. Approach to assessing effects of mitigation strategies on forest
carbon and forest sector emissions. NECB is productivity (NPP) minus Rh and
losses from fire and harvest (red arrows). Harvest emissions include those
associated with wood products and bioenergy.
Table 1. Forest carbon budget components used to compute
NECB
Flux, Tg C·y
−1
2001–2005 2006–2010 2011–2015 2001–2015
NPP 73.64 7.59 73.57 7.58 73.57 7.58 73.60
Rh 45.67 5.11 45.38 5.07 45.19 5.05 45.41
NEP 27.97 9.15 28.19 9.12 28.39 9.11 28.18
Harvest removals 8.58 0.60 7.77 0.54 8.61 0.6 8.32
Fire emissions 2.37 0.27 1.79 0.2 0.97 0.11 1.71
NECB 17.02 9.17 18.63 9.14 18.81 9.13 18.15
Average annual values for each period, including uncertainty (95%
confidence interval) in Tg C y
−1
(multiply by 3.667 to get million tCO
2
e).
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94 Tg C and cumulative NECB by 14 Tg C, and afforestation
reduced forest sector GHG emissions by 1.3–1.4% in 2025, 2050,
and 2100 (Fig. 3).
We quantified cobenefits of afforestation of irrigated grass crops
on water availability based on data from hydrology and agricultural
simulations of future grass crop area and related irrigation demand
(20). Afforestation of 127,000 ha of grass cropland with Douglas
fir could decrease irrigation demand by 222 and 233 billion m
3
·y
−1
by 2050 and 2100, respectively. An independent estimate from
measured precipitation and evapotranspiration (ET) at our ma-
ture Douglas fir and grass crop flux sites in the Willamette Valley
shows the ET/precipitation fraction averaged 33% and 52%, re-
spectively, and water balance (precipitation minus ET) averaged
910 mm·y
−1
and 516 mm·y
−1
. Under current climate conditions,
the observations suggest an increase in annual water avail-
ability of 260 billion m
3
·y
−1
if 127,000 ha of the irrigated grass
crops were converted to forest.
Harvest cycles in the mesic and montane forests have declined
from over 120 y to 45 y despite the fact that these trees can live
500–1,000 y and net primary productivity peaks at 80–125 y (21).
If harvest cycles were lengthened to 80 y on private lands and
harvested area was reduced 50% on public lands, state-level stocks
would increase by 17% to a total of ∼3,600 Tg C and NECB would
increase 2–3TgCy
−1
by 2100. The lengthened harvest cycles re-
duced harvest by 2 Tg C y
−1
, which contributed to higher NECB.
Leakage (more harvest elsewhere) is difficult to quantify and could
counter these carbon gains. However, because harvest on federal
lands was reduced significantly since 1992 (NW Forest Plan),
leakage has probably already occurred.
The four strategies together increased NECB by 64%, 82%,
and 56% by 2025, 2050, and 2100, respectively. This reduced
forest sector net emissions by 11%, 10%, and 17% over the same
periods (Fig. 3). By 2050, potential increases in NECB were largest
in the Coast Range (Table S5), East Cascades, and Klamath
Mountains, accounting for 19%, 25%, and 42% of the total
increase, whereas by 2100, they were most evident in the West
Cascades, East Cascades, and Klamath Mountains.
We examined the potential for using existing harvest residue
for electricity generation, where burning the harvest residue for
energy emits carbon immediately (3) versus the BAU practice of
leaving residues in forests to slowly decompose. Assuming half of
forest residues from harvest practices could be used to replace
natural gas or coal in distributed facilities across the state, they
would provide an average supply of 0.75–1TgCy
−1
to the year
2100 in the reduced harvest and BAU scenarios, respectively.
Compared with BAU harvest practices, where residues are left to
decompose, proposed bioenergy production would increase cu-
mulative net emissions by up to 45 Tg C by 2100. Even at 50% use,
residue collection and transport are not likely to be economically
viable, given the distances (>200 km) to Oregon’s facilities.
Discussion
Earth system models have the potential to bring terrestrial ob-
servations related to climate, vulnerability, impacts, adaptation,
Change in carbon stock from BAU
(Tg C)
0
100
200
300
400
500
600
Afforest
Reforest
Reduced
harvest
Combined
A
94
315
118
527
Period
2021−2030
2041−2050
2091−2100
Change in NECB from BAU
(Tg C yr 1)
0
1
2
3
4
Afforest
Reforest
Reduced
harvest
Combined
B
14
53
152
220
Fig. 3. Future change in carbon stocks and NECB with mitigation strategies
relative to BAU management. The decadal average change in forest carbon
stocks (A) and NECB relative to BAU (B) are shown. Italicized numbers over
bars indicate mean forest carbon stocks in 2091–2100 (A) and cumulative
change in NECB for 2015–2100 (B). Error bars are ±10%.
Annual carbon flux (million tCO2e yr 1)
−100
−50
0
50
100
NECB
Fire
Harvest
Forest Sector
Energy Sector
Total Emissions
Carbon sink
Carbon source
Period
2001−2005
2006−2010
2011−2015
Fig. 2. Oregon’s forest carbon sink and emissions from forest and energy
sectors. Harvest emissions are computed by LCA. Fire and harvest emissions
sum to forest sector emissions. Energy sector emissions are from the Oregon
Global Warming Commission (14), minus forest-related emissions. Error bars
are 95% confidence intervals (Monte Carlo analysis).
Law et al. PNAS Latest Articles
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ENVIRONMENTAL
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and mitigation into a common framework, melding biophysical
with social components (22). We developed a framework to
examine a suite of mitigation actions to increase forest carbon
sequestration and reduce forest sector emissions under current
and future environmental conditions.
Harvest-related emissions had a large impact on recent forest
NECB, reducing it by an average of 34% from 2001 to 2015. By
comparison, fire emissions were relatively small and reduced NECB
by 12% in the Biscuit Fire year, but only reduced NECB 5–9%
from 2006 to 2015. Thus, altered forest management has the po-
tential to enhance the forest carbon balance and reduce emissions.
Future NEP increased because enhancement from atmospheric
carbon dioxide outweighed the losses from fire. Lengthened har-
vest cycles on private lands to 80 y and restricting harvest to 50%
of current rates on public lands increased NECB the most by 2100,
accounting for 90% of total emissions reduction (Fig. 3 and Tables
S5 and S6). Reduced harvest led to NECB increasing earlier than
the other strategies (by 2050), suggesting this could be a priority
for implementation.
Our afforestation estimates may be too conservative by limit-
ing them to nonforest areas within current forest boundaries and
127,000 ha of irrigated grass cropland. There was a net loss of
367,000 ha of forest area in Oregon and Washington combined
from 2001 to 2006 (23), and less than 1% of native habitat remains
in the Willamette Valley due to urbanization and agriculture (24).
Perhaps more of this area could be afforested.
The spatial variation in the potential for each mitigation option
to improve carbon stocks and fluxes shows that the reforestation
potential is highest in the Cascade Mountains, where fire and
insects occur (Fig. 4). The potential to reduce harvest on public
land is highest in the Cascade Mountains, and that to lengthen
harvest cycles on private lands is highest in the Coast Range.
Although western Oregon is mesic with little expected change
in precipitation, the afforestation cobenefits of increased water
availability will be important. Urban demand for water is pro-
jected to increase, but agricultural irrigation will continue to
consume much more water than urban use (25). Converting
127,000 ha of irrigated grass crops to native forests appears to
beawin–win strategy, returning some of the area to forest land,
providing habitat and connectivity for forest species, and easing
irrigation demand. Because the afforested grass crop represents
only 11% of the available grass cropland (1.18 million ha), it is
not likely to result in leakage or indirect land use change. The
two forest strategies combined are likely to be important con-
tributors to water security.
Cobenefits with biodiversity were not assessed in our study.
However, a recent study showed that in the mesic forests, cobe-
nefits with biodiversity of forest species are largest on lands with
harvest cycles longer than 80 y, and thus would be most pro-
nounced on private lands (26). We selected 80 y for the harvest
cycle mitigation strategy because productivity peaks at 80–125 y
in this region, which coincides with the point at which cobenefits
with wildlife habitat are substantial.
Habitat loss and climate change are the two greatest threats to
biodiversity. Afforestation of areas that are currently grass crops
would likely improve the habitat of forest species (27), as about
90% of the forests in these areas were replaced by agriculture.
About 45 mammal species are at risk because of range contraction
(28). Forests are more efficient at dissipating heat than grass and
crop lands, and forest cover gains lead to net surface cooling in all
regions south of about 45° latitude in North American and Europe
(29). The cooler conditions can buffer climate-sensitive bird pop-
ulations from approaching their thermal limits and provide more
food and nest sites (30). Thus, the mitigation strategies of affor-
estation, protecting forests on public lands and lengthening harvest
cycles to 80–125 y, would likely benefit forest-dependent species.
Oregon has a legislated mandate to reduce emissions, and is
considering an offsets program that limits use of offsets to 8% of
the total emissions reduction to ensure that regulated entities
substantially reduce their own emissions, similar to California’s
program (19). An offset becomes a net emissions reduction by
increasing the forest carbon sink (NECB). If only 8% of the GHG
reduction is allowed for forest offsets, the limits for forest offsets
would be 2.1 and 8.4 million metric tCO
2
e of total emissions by
2025 and 2050, respectively (Table S6). The combination of affor-
estation, reforestation, and reduced harvest would provide 13 million
metric tCO
2
e emissions reductions, and any one of the strategies
or a portion of each could be applied. Thus, additionality beyond
what would happen without the program is possible.
State-level reporting of GHG emissions includes the agriculture
sector, but does not appear to include forest sector emissions, ex-
cept for industrial fuel (i.e., utility fuel in Table S3) and, potentially,
fire emissions. Harvest-related emissions should be quantified,
as they are much larger than fire emissions in the western United
States. Full accounting of forest sector emissions is necessary to
meet climate mitigation goals.
Increased long-term storage in buildings and via product sub-
stitution has been suggested as a potential climate mitigation op-
tion. Pacific temperate forests can store carbon for many hundreds
of years, which is much longer than is expected for buildings that
are generally assumed to outlive their usefulness or be replaced
within several decades (7). By 2035, about 75% of buildings in
the United States will be replaced or renovated, based on new
construction, demolition, and renovation trends (31, 32). Re-
cent analysis suggests substitution benefits of using wood versus
more fossil fuel-intensive materials have been overestimated by at
A
B
Change in forest carbon from BAU
Fig. 4. Spatial patterns of forest carbon stocks and NECB by 2091–2100. The
decadal average changes in forest carbon stocks (A) and NECB (B) due to
afforestation, reforestation, protected areas, and lengthened harvest cycles
relative to continued BAU forest management (red is increase in NECB)
are shown.
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least an order of magnitude (33). Our LCA accounts for losses in
product substitution stores (PSSs) associated with building life
span, and thus are considerably lower than when no losses are
assumed (4, 34). While product substitution reduces the overall
forest sector emissions, it cannot offset the losses incurred by
frequent harvest and losses associated with product trans-
portation, manufacturing, use, disposal, and decay. Methods
for calculating substitution benefits should be improved in
other regional assessments.
Wood bioenergy production is interpreted as being carbon-
neutral by assuming that trees regrow to replace those that burned.
However, this does not account for reduced forest carbon stocks
that took decades to centuries to sequester, degraded productive
capacity, emissions from transportation and the production pro-
cess, and biogenic/direct emissions at the facility (35). Increased
harvest through proposed thinning practices in the region has
been shown to elevate emissions for decades to centuries regardless
of product end use (36). It is therefore unlikely that increased wood
bioenergy production in this region would decrease overall forest
sector emissions.
Conclusions
GHG reduction must happen quickly to avoid surpassing a 2 °C
increase in temperature since preindustrial times. Alterations in
forest management can contribute to increasing the land sink and
decreasing emissions by keeping carbon in high biomass forests,
extending harvest cycles, reforestation, and afforestation. For-
ests are carbon-ready and do not require new technologies or
infrastructure for immediate mitigation of climate change. Grow-
ing forests for bioenergy production competes with forest carbon
sequestration and does not reduce emissions in the next decades
(10). BECCS requires new technology, and few locations have
sufficient geological storage for CO
2
at power facilities with
high-productivity forests nearby. Accurate accounting of forest
carbon in trees and soils, NECB, and historic harvest rates,
combined with transparent quantification of emissions from the
wood product process, can ensure realistic reductions in forest
sector emissions.
As states and regions take a larger role in implementing climate
mitigation steps, robust forest sector assessments are urgently
needed. Our integrated approach of combining observations,
an LCA, and high-resolution process modeling (4-km grid vs.
typical 200-km grid) of a suite of potential mitigation actions
and their effects on forest carbon sequestration and emissions
under changing climate and CO
2
provides an analysis frame-
work that can be applied in other temperate regions.
Materials and Methods
Current Stocks and Fluxes. We quantified recent forest carbon stocks and
fluxes using a combination of observations from FIA; Landsat products on
forest type, land cover, and fire risk; 200 intensive plots in Oregon (37); and a
wood decomposition database. Tree biomass was calculated from species-
specific allometric equations and ecoregion-specific wood density. We esti-
mated ecosystem carbon stocks, NEP (photosynthesis minus respiration), and
NECB (NEP minus losses due to fire or harvest) using a mass-balance approach
(36, 38) (Table 1 and SI Materials and Methods). Fire emissions were computed
from the Monitoring Trends in Burn Severity database, biomass data, and
region-specific combustion factors (15, 39) (SI Materials and Methods).
Future Projections and Model Description. Carbon stocks and NEP were
quantified to the years 2025, 2050, and 2100 using CLM4.5 with physiological
parameters for 10 major forest species, initial forest biomass (36), and future
climate and atmospheric carbon dioxide as input (Institut Pierre Simon
Laplace climate system model downscaled to 4 km ×4 km, representative
concentration pathway 8.5). CLM4.5 uses 3-h climate data, ecophysiological
characteristics, site physical characteristics, and site history to estimate the
daily fluxes of carbon, nitrogen, and water between the atmosphere, plant
state variables, and litter and soil state variables. Model components are
biogeophysics, hydrological cycle, and biogeochemistry. This model version
does not include a dynamic vegetation model to simulate resilience and
establishment following disturbance. However, the effect of regeneration
lags on forest carbon is not particularly strong for the long disturbance in-
tervals in this study (40). Our plant functional type (PFT) parameterization
for 10 major forest species rather than one significantly improves carbon
modeling in the region (41).
Forest Management and Land Use Change Scenarios. Harvest cycles, re-
forestation, and afforestation were simulated to the year 2100. Carbon stocks
and NEP were predicted for the current harvest cycle of 45 y compared with
simulations extending it to 80 y. Reforestation potential was simulated over
areas that recently suffered mortality from harvest, fire, and 12 species of
beetles (13). We assumed the same vegetation regrew to the maximum
potential, which is expected with the combination of natural regeneration
and planting that commonly occurs after these events. Future BAU harvest
files were constructed using current harvest rates, where county-specific aver-
age harvest and the actual amounts per ownership were used to guide grid cell
selection. This resulted in the majority of harvest occurring on private land
(70%) and in the mesic ecoregions. Beetle outbreaks were implemented using
a modified mortality rate of the lodgepole pine PFT with 0.1% y
−1
biomass
mortalityby2100.
For afforestation potential, we identified areas that are within forest
boundariesthat are not currentlyforest and areas that are currently grasscrops.
We assumed no competition with conversion of irrigated grass crops to urban
growth, given Oregon’slanduselawsfordevelopingwithinurbangrowth
boundaries. A separate study suggested that, on average, about 17% of all
irrigated agricultural crops in the Willamette Valley could be converted to
urban area under future climate; however, because 20% of total cropland is
grass seed, it suggests little competition with urban growth (25).
Landsat observations (12,500 scenes) were processed to map changes in
land cover from 1984 to 2012. Land cover types were separated with an
unsupervised K-means clustering approach. Land cover classes were assigned
to an existing forest type map (42). The CropScape Cropland Data Layer (CDL
2015, https://nassgeodata.gmu.edu/CropScape/) was used to distinguish nonforage
grass crops from other grasses. For afforestation, we selected grass cropland
with a minimum soil water-holding capacity of 150 mm and minimum pre-
cipitation of 500 mm that can support trees (43).
Afforestation Cobenefits. Modeled irrigation demand of grass seed crops
under future climate conditions was previously conducted with hydrology
and agricultural models, where ET is a function of climate, crop type, crop
growth state, and soil-holding capacity (20) (Table S7). The simulations
produced total land area, ET, and irrigation demand for each cover type.
Current grass seed crop irrigation in the Willamette Valley is 413 billion m
3
·y
−1
for 238,679 ha and is projected to be 412 and 405 billion m
3
in 2050 and 2100
(20) (Table S7). We used annual output from the simulations to estimate irrigation
demand per unit area of grass seed crops (1.73, 1.75, and 1.84 million m
3
·ha
−1
in
2015, 2050, and 2100, respectively), and appliedittothemappedirrigatedcrop
area that met conditions necessary to support forests (Table S7).
LCA. Decomposition of wood through the product cycle was computed using
an LCA (8, 10). Carbon emissions to the atmosphere from harvest were cal-
culated annually over the time frame of the analysis (2001–2015). The net
carbon emissions equal NECB plus total harvest minus wood lost during
manufacturing and wood decomposed over time from product use. Wood
industry fossil fuel emissions were computed for harvest, transportation, and
manufacturing processes. Carbon credit was calculated for wood product
storage, substitution, and internal mill recycling of wood losses for bioenergy.
Products were divided into sawtimber, pulpwood, and wood and paper
products using published coefficients (44). Long-term and short-term prod-
ucts were assumed to decay at 2% and 10% per year, respectively (45). For
product substitution, we focused on manufacturing for long-term structures
(building life span >30 y). Because it is not clear when product substitution
started in the Pacific Northwest, we evaluated it starting in 1970 since use of
concrete and steel for housing was uncommon before 1965. The displacement
value for product substitution was assumed to be 2.1 Mg fossil C/Mg C wood
use in long-term structures (46), and although it likely fluctuates over time, we
assumed it was constant. We accounted for losses in product substitution as-
sociated with building replacement (33) using a loss rate of 2% per year (33),
but ignored leakage related to fossil C use by other sectors, which may result
in more substitution benefit than will actually occur.
The general assumption for modern buildings, including cross-laminate
timber, is they will outlive their usefulness and be replaced in about 30 y (7).
By 2035, ∼75% of buildings in the United States will be replaced or renovated,
based on new construction, demolition, and renovation trends, resulting in
threefold as many buildings as there are now [2005 baseline (31, 32)]. The loss of
Law et al. PNAS Latest Articles
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ENVIRONMENTAL
SCIENCES
SUSTAINABILITY
SCIENCE
the PSS is therefore PSS multiplied by the proportion of buildings lost per year
(2% per year).
To compare the NECB equivalence to emissions, we calculated forest sector
and energysector emissions separately. Energysector emissions [“in-boundary”
state-quantified emissions by the Oregon Global Warming Commission (14)]
include those from transportation, residential and commercial buildings, industry,
and agriculture. The forest sector emissions are cradle-to-grave annual carbon
emissions from harvest and product emissions, transportation, and utility fuels
(Table S3). Forest sector utility fuels were subtracted from energy sector emissions
to avoid double counting.
Uncertainty Estimates. For the observation-based analysis, Monte Carlo sim-
ulations were used to conduct an uncertainty analysis with the mean and SDs
for NPP and Rh calculated using several approaches (36) (SI Materials and
Methods). Uncertainty in NECB was calculated as the combined uncertainty of
NEP, fire emissions (10%), harvest emissions (7%), and land cover estimates
(10%) using the propagation of error approach. Uncertainty in CLM4.5 model
simulations and LCA were quantified by combining the uncertainty in the
observations used to evaluate the model, the uncertainty in input datasets
(e.g., remote sensing), and the uncertainty in the LCA coefficients (41).
Model input data for physiological parameters and model evaluation data
on stocks and fluxes are available online (37).
ACKNOWLEDGMENTS. We thank Dr. Thomas Hilker (deceased) for producing
the grass data layer, and Carley Lowe for Fig. 1. This research was supported by
the US Department of Energy (Grant DE-SC0012194) and Agriculture and Food
Research Initiative of the US Department of Agriculture National Institute of
Food and Agriculture (Grants 2013-67003-20652, 2014-67003-22065, and 2014-
35100-22066) for our North American Carbon Program studies, “Carbon cycle
dynamics within Oregon’s urban-suburban-forested-agricultural landscapes,”
and “Forest die-off, climate change, and human intervention in western North
America.”
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