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Abstract

The boreal forest is one of Earth’s most climatologically sensitive regions, and changes in the cover and structure of its vegetation pose a positive carbon-climate feedback on atmospheric greenhouse warming. The region has also experienced more than three times climatological warming of any forested biome in recent decades. While ecological models predict a northward shift of boreal tree cover in response to climate change, comprehensive data have not been available to test the hypothesis. Here we report a test of the magnitude, direction, and significance of changes in the boreal canopy based on the longest and highest-resolution record of calibrated satellite maps to date. The boreal canopy increased in density and shifted northward from 1984 to 2020, with the largest and most significant gains in its northern latitudes. Net forest gains occurred despite stable rates of disturbance across all but the region’s southernmost latitudes, implicating widespread release of climatological limitation on growth over changing distribution of fire, harvest, insect, and other disturbances. These new forests will sequester carbon as they mature, increasing its residence time in woody biomass, and will play a key role in how the terrestrial biosphere attenuates atmospheric CO2 increases.
1
Northward migration of the boreal forest
1
confirmed by satellite record
2
3
4
The boreal biome is Earth’s most expansive, ecologically intact, and climatologically sensitive
5
forest. The boreal forest comprises a third of the global forest area and accounts for 20.8% of the
6
total forest carbon (C) sink 1. The boreal region contains 38 ± 3.1 Pg C of above-ground biomass
7
2 and is underlain by 1672 Pg C, summing to total biomass rivaling the tropics and half of global
8
soil C—of which 88% is locked in permafrost 3,4. Boreal vegetation structure also controls the
9
reflective and thermal balance of solar radiation of the high northern latitudes via canopy albedo,
10
posing a positive feedback mechanism for greenhouse atmospheric warming 5–8.
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The boreal region has experienced the fastest climatological warming of any forest biome, with
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annual surface temperatures increasing > 1.4° C over the past century 9. Boreal forest dynamics
13
are highly correlated to climate 1012, and increases in vegetation productivity have been observed
14
across the northern high latitudes 13. Meanwhile, regional increases in the frequency and severity
15
of windthrow, fire, insect, and disease events have been reported as well 3.
16
While theory predicts a northward shift of the boreal forest, the net effect of the many opposing
17
factors on the region’s tree canopy remains an untested hypothesis. Coupled climate-vegetation
18
models predict a net-northward migration of boreal vegetation due to warming 14,15, supporting
19
the dominance of growth processes, and multiple studies 1618 have reported vegetation
20
“greening” based on general indices of plant productivity. However, the slow productivity of
21
boreal tree cover requires long-term analyses, which have been either confined to regional scopes
22
or poorly calibrated data 1921. As a result, the net effect of growth and mortality on the
23
2
distribution of the boreal tree canopy, and the resulting effect on carbon budgets, remain
24
unconfirmed.
25
Here we report the results of a global test of the magnitude and direction of boreal-forest change
26
from 1984 to 2020, as observed through historical satellite records of tree cover. We calibrated
27
machine learning algorithms 22,23 to 224,026 Landsat images covering the boreal forest and
28
adjacent tundra on the Amazon Web Services (AWS) cloud-computing architecture to estimate
29
tree-canopy cover over space and time. The resulting 30-meter, annual-resolution dataset—the
30
most extensive and highest-resolution record of boreal tree cover to date—was then subjected to
31
time-series trend analysis to estimate and map the historical direction, rate, and significance of
32
change across the region.
33
Distribution of boreal tree-canopy cover
34
The tree canopy is densest in the southern portions of the biome and thins with increasing
35
latitude (Fig. 1) 23. Sparse conifer forest, woodland, herbaceous, and non-vegetated cover
36
increase in frequency into and across the taiga-tundra ecotone, and tree cover is nearly absent
37
above 71°N. Including unforested tundra, wetlands, and inland water bodies, the most common
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range of tree cover is below 5%.
39
The boreal forest increased in density from 1985 to 2020 (Fig. 2). Trees covered 7.153 million
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km2 (41.44 %) of the region in 1985 and 7.997 million km2 (46.32 %) in 2020, increasing
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linearly 0.023 million km2/yr (0.12%/yr) over the 36-year period (percent cover = 0.116 x year –
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187.6, R2 = 0.79, p < 0.001). Given the 10-30% range of tree-canopy cover defining forests
43
within the United Nations Framework Convention on Climate Change 22, the region held
44
3
between 8.95 to 12.41 million km2 of forest in 2000 and increased to between 9.41 and 13.26
45
million km2 of forest in 2020.
46
The boreal forest also shifted northward from 1985 to 2020. The mean latitude of boreal tree-
47
canopy cover increased half a degree, from 57.37 °N to 57.66 °N over the period (mean latitude
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= 0.0075 x year + 42.6, R2=0.79, p < 0.001). Median latitude increased at a faster rate than the
49
mean (median latitude = 0.0124 x year + 32.5, R² = 0.88, p < 0.001), implying widespread
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growth across the entire biome rather than changes at either its outlying northern or southern
51
margins.
52
53
4
54
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Fig. 1 | Current (2020) distribution of tree-canopy cover across boreal and arctic tundra ecoregions. The tree-canopy
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cover was estimated at 30-meter pixel resolution by machine learning algorithms applied to all available Landsat satellite
57
images from the year 2020; data gaps due to clouds were filled with estimates from earlier years. Ecoregions were defined
58
by 24. The bottom panel shows northward migration of the distribution of boreal tree-canopy cover from 1984 to 2020.
59
Tree-canopy cover (%)
90°W90°E
0°
180°
60°W
120°W
150°W150°E
120°E
60°E
30°E
1.5
2.0
2.5
3.0
3.5
Area (106km2)
Tree -canopy cover (%)
010 20 30 40 50 60 70 100
0
50
100
25
75
025 50 75 100
y = 0.116x -187.6
R² = 0.79
y = 0.185x -325.3
R² = 0.83
35
37
39
41
43
45
47
49
51
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
tree-canopy cover (%)
Year
mean median Linear (mean) Linear (median)
5
The pace and pattern of boreal forest change
60
These global totals comprise the balance of strong geographic variation (Fig. 2). Net canopy
61
gains occurred at every latitude above 53°N from 1984 to 2020, with the strongest increases
62
occurring between 64 to 68°N. While not seeking a distinct line per se, net gains in the region’s
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highest latitudes support the hypothesis of a positive shift in the northern limit of tree cover, or
64
“northern tree line”. In contrast, net canopy losses were smaller in magnitude and confined to the
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lower boreal latitudes (45-51°N), where human activity is most intense (Fig. 3).
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In North America, significant net gains were concentrated in the northernmost portion of the
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domain, where increases in shrub and grass cover have also been reported 25. Regions of
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significant net losses corresponded to areas of widespread forest disturbance, including fire and
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bark beetle (Dendroctonus spp.) outbreaks in British Columbia 26, spruce budworm
70
(Choristonura sp.) outbreaks in Quebec 27, and fires across the central Canadian provinces and
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interior Alaska 28.
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In Eurasia, hotspots of forest loss included the eastern Russian-Chinese border and agricultural
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regions of the southern boreal margin east of the Ural Mountains, as well as areas of forest
74
felling near the Russia-Finland border in the 1990s 29 and increased fire frequency and intense
75
selective logging 30. Net losses were notably rare in Europe 31. Corroborating reports of
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increasing coverage of tall shrubs and larch (Larix spp.) in the Siberian ecotone 32, regions of
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significant net gain included areas of post-Soviet agricultural abandonment and reforestation, as
78
well as larch forests underlain by permafrost in the Yakutsk region of eastern Siberia. In these
79
forests, permafrost thawing has been hypothesized to result in increased forest productivity 33,
80
and vegetation recovery from wildfires in the 1990s is ongoing 34.
81
6
82
TCC c hang e rat e
y = 0.0075x + 42.6
R² = 0.79
y = 0.0124x + 32.5
R² = 0.88
57
57.1
57.2
57.3
57.4
57.5
57.6
57.7
57.8
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
Latitude (°N)
Year
mean median Linear (mean) Linear (median)
7
The pan-boreal increase in tree cover occurred against a backdrop of relatively stable rates of
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disturbance over the period (Fig. 3). The region-wide rate of disturbance accelerated from 53,546
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km2/yr in 2000 to 60,275 km2/yr in 2020— equating to an increase of 1.8%/yr (1,100 km2/yr)
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(y=1,099.57 x - 2,157,378.02, R2=0.27, p-value = 0.016) or 0.2 to 0.4% of the maximum forested
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area of the region over the period. In contrast to net gains, the latitudinal distribution of
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disturbances fluctuated from year to year while remaining stationary over the period as a whole
88
(y=0.04 x - 25.16, R2=0.14, p-value: 0.023). !
89
Previous studies have sought evidence of a northward shift of the boreal biome at the northern
90
limit of tree cover. While reports of advances in the northern tree line based on globally
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calibrated datasets have been contested for coarse categorization of the forest and poor
92
calibration 35,36, our observations based on regionally calibrated estimates corroborate the
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advancement hypothesis, as well as independent reports of disturbance and recovery 16,3740 and
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in situ measurements of changing woody structure near the northern limits of tree growth 32,41.
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Whereas we sought neither evidence of a discrete northern edge to the boreal forest or changes
96
therein, our results do show a biome-wide northward shift in the entire distribution of tree
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cover—including the gradient spanning its northern extreme.
98
8
Fig. 3 | Total area and median latitude of boreal stand-clearing disturbances from 1985 to 2020.
99
90°W90°E
0°
180°
60°W
30°W
120°W
150°W150°E
120°E
60°E
30°E
Year of forest
disturbance
53
55
57
59
61
63
0
20000
40000
60000
80000
100000
1985
1986
1987
1988
1989
1990
1991
1992
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1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
Latitude (°N)
Area (km2)
Year
Complete Incomplete Latitude
9
Distribution of boreal forest age
100
We also retrieved the spatial and frequency distribution of current forest stand age across the
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boreal biome. While most of the boreal forest area (8.19 million km2, 47.5% of the region) is
102
older than what can be measured from the 37-year satellite record (Fig. 4), some observations of
103
the region’s youngest forests can be made empirically. In 2020, 0.5 million km2 (or 5.29% of
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standing forests) were those that had been identified as forest in 1984, disturbed at some time
105
during the period, and recovered again to forest by 2020. The total of these recovering forests
106
and expanding “new” forests within the observable period have led to a weak mode of young
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stands between 9 and 21 years of age, as well as a current lapse in the youngest age classes.
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These young forests are especially prevalent in areas of intensive forestries, such as the industrial
109
plantations of Scandinavia, as well as areas recovering from wildfire. The latter is corroborated
110
by reports of increasing frequency and area of burns in Siberia since the end of the 20th century
111
42, the demographic effect of which is reflected in an increasing proportion of recovering forest <
112
20 years old.
113
10
Fig. 4 | The distribution of stand age (top) across the boreal ecoregion, and frequency distribution of boreal stand age
in 2020 (bottom). Forest age-class distribution is defined as years since the establishment of pixels that were forested in
2020.
Age in 2020
90°W90°E
0°
180°
60°W
30°W
120°W
150°W150°E
120°E
60°E
30°E
-
10,000
20,000
30,000
40,000
50,000
60,000
≤2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 ≥36
Area (km2)
Age
New Recovery
8.19 million km2
11
The carbon impact of young forests
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The impact of the young forests on the boreal carbon budget is significant, and it could explain
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the region’s increasing carbon sink 43. Forests with known stand ages (≤ 36 years since
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disturbance) hold between 1.1 – 5.9 Pg C, based on recent models 44. Ages of forests where no
117
disturbance was recorded during the observation period are unknown, yet plausible carbon-stock
118
values within the oldest forested age class may be bracketed between a lower, younger limit
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(19.1 – 58.4 Pg C) and an upper, older limit (300 years stand age, 42.4 – 89.2 Pg C). Based on
120
these estimates, all forested area of ages ≤ 36 years comprises 1.35% to 14.20% of the total
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carbon stock in aboveground biomass in the boreal forest, which increases with the fractional
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area of young (≤ 36-year) forest to total forest area (15.4%). Allowing all these young forests to
123
mature without disturbance would result in a potential additional carbon sink of 2.3 – 3.8 Pg C.
124
The amount of carbon in forests new to the satellite record is 0.8 – 3.5 Pg C, greater than carbon
125
in forests recovering from observed disturbances (0.3 – 2.4 Pg C). Over the next 36 years, these
126
“new” forests represent a potential additional carbon sink of 1.3 – 2.0 Pg C (0.036 - 0.18 Pg
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C/yr), compared to 1.0 – 1.8 Pg C (0.028 - 0.05 Pg C/yr) in recovering forest. The differences in
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present-day stocks and the carbon sink potential between new and regrowing forests can be
129
partly explained by the greater area of new forests compared to regrowing forests (7.6% and
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6.7% of total forest area, respectively), but also by the greater age of new forests compared to
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those forests recovering from disturbances within the record.
132
The amount of carbon sequestered by the new forests could be large enough to offset the effect
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of warming on boreal ecosystem respiration, estimates of which vary from 5 Pg C to 28 Pg C
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from 1985 to 2020 (SI). Climate warming and CO2 fertilization are expected to increase
135
productivity in these regions 45; and interestingly, the observed spatial pattern of net canopy
136
12
growth confirms model predictions of enhanced seasonal CO2 exchange at latitudes > 40°N 13.
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However, several factors could yet reduce the offset of forest expansion on a temperature-
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mediated increase in respiratory fluxes, including: the temperature effect itself can be
139
temperature dependent 46, sink capacity eventually decreases with age 47, thawing of carbon
140
locked in permafrost will accelerate respiration 48, and changes in fire regimes and wood harvest
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could overshadow stocking from forest development 3,42. It also remains unclear to what extent
142
the expansion of trees, with longer lived carbon pools than herbaceous vegetation, can be
143
structurally sustained by boreal soils 49, or on how disturbed area might increase from human
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activity. Each of these dynamics is already taking place across the boreal domain, and strategies
145
to quantify the potential tradeoffs between autotrophic and heterotrophic dynamics are key to
146
understanding the role of forest management in mitigating the causes and consequences of
147
climate change in the boreal domain.
148
Conclusions
149
A pan-boreal test of the magnitude, direction, and significance of boreal forest change has
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confirmed the climatological and ecological hypothesis of northward migration of the boreal
151
biome. Machine learning was used to retrieve the longest, highest resolution, and most complete
152
calibrated record of boreal-forest change from the historical satellite record to date. Time-series
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analysis of each of the 1.9 x108 30-m pixels over 37 years revealed increasing canopy density
154
and northward migration of the boreal forest from 1984 to 2020 despite relatively even rates of
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forest disturbance over the period. Recent models of the relation of forest age to biomass stocks
156
and change suggest the changing distribution of age will significantly affect the region’s
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contribution to the global carbon budget in coming decades.
158
13
While globally significant, the trends belie tremendous variation over space and time, as well as
159
in the processes underlying the observable changes. A deeper understanding of the full
160
complexity of the causes and consequences of canopy changes across the boreal forest will
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require analysis against coincident measurements of canopy structure and the environmental
162
determinants of growth and mortality. Further, translating the resulting information into action to
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forestall and adapt to climate change will require effective communication across scientific,
164
government, and commercial domains.
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Online content
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The current distribution of boreal tree canopy cover and its changes over time can be explored
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publicly at https://www.terraPulse.com/terraView/ccs.
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The impact of climate change on forests is expected to vary globally and regionally. Canada’s Acadian Forest Region lies in the transition between the North American boreal and temperate forest biomes and may be particularly sensitive to changes in climate because many of its component species are currently at their southern or northern climatic range limits. Although some species may be lost, others may exhibit major productivity boosts—affecting the goods and services we derive from them. In this study, we use a well-established forest ecosystem simulation model, PICUS, to provide the first exploration of the impact of climate change on the composition and growth of the Acadian Forest Region for the period 2011 to 2100 under two radiative forcing scenarios, RCP 2.6 and RCP 8.5. In the short term (2011–2040), little to no changes in forest composition or growth were projected under either forcing scenario compared with current forest conditions (simulated for 1981–2010 baseline climate); however, by mid-century, PICUS projected increasing departures from the baseline simulations in both composition and growth, with the greatest changes occurring under RCP 8.5 during the late 21st century (2071–2100). Our study indicates that under rapid 21st century warming, Canada’s Acadian Forest Region will begin to lose its boreal character (i.e., “deborealize”) as key tree species fail to regenerate and survive. Furthermore, increased growth and establishment by warm-adapted, temperate tree species may be unable to keep pace with the rapid loss of boreal species. This potential “lag effect” may lead to a temporary decrease in forest growth and wood supply during the late 21st century.
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We assess progress toward the protection of 50% of the terrestrial biosphere to address the species-extinction crisis and conserve a global ecological heritage for future generations. Using a map of Earth's 846 terrestrial ecoregions, we show that 98 ecoregions (12%) exceed Half Protected; 313 ecoregions (37%) fall short of Half Protected but have sufficient unaltered habitat remaining to reach the target; and 207 ecoregions (24%) are in peril, where an average of only 4% of natural habitat remains. We propose a Global Deal for Nature—a companion to the Paris Climate Deal—to promote increased habitat protection and restoration, national-and ecoregion-scale conservation strategies, and the empowerment of indigenous peoples to protect their sovereign lands. The goal of such an accord would be to protect half the terrestrial realm by 2050 to halt the extinction crisis while sustaining human livelihoods.
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