Download Impacts, Adaptations and Uncertainty in the face of Anthropogenic

The North American boreal forests: impacts, adaptations and uncertainty in the face of
Anthropogenic Global Warming.
The boreal forests of the world like other forests are sensitive to climate change. The
boreal forests are both affected by and contribute to climate change, as its presence over large
areas of the earth has a significant effect on the radiative balance of the planet (ACIA, 2005)1.
Today mostly due to deforestation and the burning of fossil fuels, the chemical composition of
the atmosphere is changed and the greenhouse effect is enhanced, leading to an energy
imbalance that results in the earth’s gradual warming (about 0.5 degrees globally since 1950
alone) (Hansen., 2004)2. Because today’s climate change is more influenced and a result of
human activities instead natural variations, this work will refer to this type of climate change as
Anthropogenic Global Warming (AGW).
Boreal forests are vulnerable to AGW. Trees may expand into the tundra but die back
along southern prairie ecotones, there may be a loss of evergreen trees and a shift towards
deciduous trees, some boreal forests may collapse in some areas, while becoming more
evergreen in the north, and increased disturbances from fires and insect out breaks will shift parts
of the boreal forest to a younger age class (Bonan., 2008)3. The boreal forest like the rest of the
planet is influenced and impacted by AGW. The purpose of this paper is to illustrate the impacts
that AGW is projected to have, and is currently effecting the boreal forests of North America,
while at the same time, analyzing how this forest is adapting to its changing environment.
The boreal forest is defined as the overall forested area within the boreal zone. The boreal
forest is sometimes referred to as the boreal zone itself, because forests dominate this landscape.
The boreal is mainly characterized by a small number of coniferous species including spruce,
pine, larch and fir, and a limited number of broad-leaved species such as birch and poplar
(ACIA, 2005)4. This forest is found across a broad circumpolar vegetation zone in the high
latitudes (Natural Resources Canada, 2012)5, although in this paper we focus only on the boreal
forests of North America.
Image 1
Image 1: Global temperature anomalies between 1880 and 2011. 2011 is the 9th
warmest year on record. Baseline used between 1951-1980. Source: National
Aeronautics and Space Administration, Goddard Institute for Space Studies.
Image 2
Range and growth changes
The range of the boreal forest of North America is
shifting in latitude and altitude. The vegetation changes in
northern North America are significant. In Alaska along
the Arctic to Sub-Arctic boundary the tree line has moved
about 10km northwards within the past 50 years. Also 2%
of Alaskan tundra has been displaced by forest within the
same time frame (IPCC, AR4, WG2, CH 15)6. The tree
line is advancing generally in both a northern direction as
well as higher in elevation. These Arctic and alpine tree
lines are an important indicator of change in the boreal
region, because they mark the very limit of the boreal
forest (LLOYD., 2005)7. These changes in range are
adaptations to (in part) warmer temperatures that allow
for the range shifts to occur.
The boreal forest is expanding its range in latitude
and altitude as a result of AGW, however this is only
where soils are adequate for this expansion to take place.
In these areas it is projected that species richness will
increase as relatively species rich forests displace tundra.
Some species in isolated favorable microenvironments
which are considerably further north of their main
distribution, are also likely to spread rapidly as a result
and adaptation of AGW. A “moderate” projection for
2100 for the replacement of tundra by boreal forest is
about 10%, however estimates of up to 50% have been
published (IPCC, AR4, WG2, CH 15)8.
Temperature changes cannot be considered in
isolation because they are not the sole control of species
distribution, or of the expansion of the boreal forest.
Other factors, including soil characteristics, nutrient
availability and disturbance regimes may prove to be
more important than temperature in controlling future
ecosystem dynamics (Climate Change
Image 2: Present and projected vegetation and minimum sea-ice
Impacts and Adaptation, A Canadian
extent for Arctic and neighbouring regions. Vegetation maps based on
9
floristic surveys (top) and projected vegetation for 2090-2100,
Perspective 2004) . For example cold soil
predicted by the LPJ Dynamic Vegetation Model driven by the
temperatures act as the limit to the extent of
HadCM2 climate model (bottom) modified from Kaplan et al. (2003)
in Callaghan et al. (2005). Also shown are observed minimum sea-ice
the northern Canadian boreal forest,
extent for September 2002, and projected sea-ice minimum extent,
together with potential new/improved sea routes (redrawn from
nstanes et al., 2005; Walsh et al., 2005). Source: AR4, IPCC WG2 CH
15
compared to the southern Canadian boreal where forest appears to be influenced more by
interspecies competition and moisture conditions, than by temperature tolerance (Brooks et al.,
1998)10.
Increased CO2 concentrations are currently influencing forest growth rates. Not only is
the boreal’s traditional range shifting, its rate of growth is increased currently by a small amount.
Increased forest growth rates are a direct adaptation to increased CO2 emissions. It is likely that
a further increase of temperature of a few degrees will accelerate productivity of the boreal
forest. Satellite-based measures of the greenness of the boreal forest zone, illustrate a
lengthening of the growing season over the past two decades (Millennium Ecosystem
Assessment, 2005)11.
The degree to which changes in climate and in this case AGW, have already affected and
continue to affect productivity of forests and their ability to supply services, varies across space
and time (Millennium Ecosystem Assessment, 2005)12. Temporal patterns of temperature and
precipitation are the two most fundamental factors determining the distribution and productivity
of vegetation. Because AGW alters these two characteristics of an ecosystem, climate change is
currently and will continue to cause forest adaptation, by geographically shifting the ranges of
individual species and general vegetation zones across the boreal region (Millennium Ecosystem
Assessment, 2005)13.
On a continental scale however, increased net productivity of North American
ecosystems is projected, due to the expansion of the boreal forest into the tundra, and longer
growing seasons. Forest growth is currently slowly accelerating in regions where tree growth has
historically been limited by low temperatures and short growing seasons (IPCC, Climate Change
and Water, 2008)14.
As climate changes, individual tree species will adapt by migrating, the same way they
have done so in the past. However, concerns are present that the rapid rate of AGW will test this
generation of trees, and its dispersal abilities. The successful migration of tree species can also
be slowed and affected by additional factors such as habitat fragmentation, competition from
exotic species, changes in timing and rate of seed production (which may limit migration rates)
as well as unsustainable forestry practices (Climate Change Impacts and Adaptation, A Canadian
Perspective 2004)15.
Ecosystem Disturbances:
An ecosystem disturbance is defined as a change in the state of condition that disrupts the
way in which the system has been functioning (for example: water regulation, photosynthesis
etc), causing it to reinstate succession development. These disturbances can vary by cause, rate,
extent, intensity, timing, frequency and duration (ACIA 2005)16. Disturbances such as wildfire
and insect outbreaks are currently increasing as a result of AGW, and are likely to intensify in a
warmer future with drier soils and longer growing seasons (IPCC, AR4, WG2, CH 14)17.
Fire Disturbances
Every year Canada’s boreal forest is affected by disturbances such as fires, insects and
disease. In 2010 roughly 3.15 million hectares of forest area was burned in Canada, much of it
the boreal forest (Natural Resources Canada, 2011)18. Fire disturbances are influenced by
weather conditions. Increases in warm and dry weather conditions should result in increased
frequency and severity of fires throughout Canada’s boreal forest (The Canada Country Study,
1998)19. The area burned by forest fires in the Canadian boreal has increased over the past four
decades, at the same time as summer season temperatures have warmed. AGW has been found to
have a detectable influence on the area burned by forest fire in Canada over the recent decades.
Research suggests that AGW has significantly contributed to this recent trend, toward increasing
severity of forest fires in the boreal region. Variations in temperature were found to explain
much of the variability in the area burned (Gillet et al., 2004)20.
Fire season severity in the Canadian boreal is generally projected to increase in the future
due to AGW. Influencing factors for this projection include a longer fire season, drier conditions
and more lightning storms. However there is still uncertainty in this subject as research is not
always consistent, and warm weather and dry conditions do not necessarily lead to a bad forest
fire season. This was exemplified across Canada in 2001, where despite extreme heat and
dryness, wildfire frequency as well as the total area burned was the lowest on record
(Environment Canada, 2009)21. Vegetation type influences changes in fire frequency and
intensity. For example, conifers are more likely to experience intense fires than deciduous or
mixed wood stands. Species migrations in response to AGW will also affect future fire behaviors
by changing fuel types. Other factors that influence fire seasons particularly in the boreal include
wind, lightning frequency, antecedent moisture conditions and fire management mechanisms
(Climate Change Impacts and Adaptation, A Canadian Perspective 2004)22.
Wildfire-burned area in the North American boreal region has increased from 6 500
km /year in the 1960s, to 29 700 km2/year in the 1990s. This warming climate resulting from
AGW encourages wildfires through longer summer periods that dries fuels, promoting easier
ignition and faster fire spread. Earlier spring snowmelt also leads to longer growing seasons and
drought, where this is especially relevant in higher elevations, where increases in wildfire have
been greatest. In the Canadian boreal region warmer May to August temperatures of 0.8 degrees
Celsius since 1970, are strongly correlated with the total area burned (IPCC, AR4, WG2, CH
14)23.
2
Image 3
Image 3: Anomaly in 5-year mean area burned
annually in wildfires in Canada since 1930, plus
observed mean summer air temperature
anomaly, weighted for fire areas, relative to
1920 to 1999 (data from Gillett et al., 2004).
Source: IPCC AR4, WG2, CH 14.
Insect and Disease Disturbances
AGW tends to accelerate insect development rates, facilitates range expansions of pests
and increases over-winter survival rates. Because of their short life cycles, mobility, reproductive
potential, and physiological sensitivity to temperature, even modest climate change will have
rapid impacts on the distribution and abundance of many forest insects and pathogens (Ayres and
Lombardero., 2000)24. These qualities allow for insects and pathogens to quickly exploit new
conditions and take advantage of new opportunities.
The range and development increases brought to insects from AGW temperature
increases currently affect, and will continue to increasingly affect the Canadian boreal forests.
Periodic insect epidemics kill trees over large regions, providing dead fuels for large wildfires.
These epidemics are related to aspects of insect life cycles that are climate sensitive. Many
boreal insects have a two year life cycle; however warmer winter temperatures allow a larger
fraction of overwintering larvae to survive. For example recently in the Alaskan boreal, spruce
budworm has completed its life cycle in one year, rather than the previous two years. In addition
to this the mountain pine beetle has expanded its range in British Columbia into areas that were
previously too cold (IPCC, AR4, WG2, CH 14)25.
From 1998 to 2010 the mountain pine beetle has killed more than 700 million cubic
meters of pine in British Columbia. Aided by warming temperatures, the range of this beetle is
expanding eastward from mature pine forests on the eastern slopes of the Rockies, across into
Alberta. Future eastward expansion of the beetle depends on its ability to survive the winter, the
frequency of summer drought, suitability of boreal forest pines to host the beetle, and the
effectiveness of intensive efforts to control the beetle populations (Natural Resources Canada,
2011)26. AGW has greatly aided the beetle expand its range, and it is likely this will continue as
temperatures across the boreal region increase.
Image 4
Image 4:. Annual anomalies of land-surface air
temperature in the Arctic (60º to 90º N) for the period
1900 to 2003 using the GHCN dataset (updated from
Peterson and Vose, 1997). Anomalies are calculated
relative to the 1961–1990 average. The smoothed curve
was created using a 21-point binomial filter, which
approximates a 10-year running mean. Note: When
compared to a GLOBAL temperature anomaly, you can
clearly see that the warming in the Arctic is much more
significant. Source (Arctic Climate Impact Assessment
2005)
It is possible that AGW will have further indirect effects on boreal forest disturbance by pests.
For example, longer drought conditions and ecosystem instability caused by species migrations
may increase the sensitivity of trees to insect defoliation. This is significant because in many
regions, defoliation by pests represents the most important controlling factor of tree growth
(Climate Change Impacts and Adaptation, A Canadian Perspective 2004)27. Although increases
in anthropogenic carbon emissions may increase forest productivity, these increases may further
reduce tree defenses against insects and diseases. AGW may also affect insect outbreaks by
altering the abundance of insect enemies, mutualists and competitors. For example changes in
the range of predators and prey of these insects could alter ecosystem dynamics by reducing the
biological controls on certain pest populations (Climate Change Impacts and Adaptation, A
Canadian Perspective 2004)28.
Uncertainties:
There are many uncertainties regarding how the boreal forests of North America,
influence and will be influenced by AGW. Extreme weather impacts on the boreal forest are an
example of uncertainty. The 1998 Ice storm that hit eastern Canada clearly demonstrated that
extreme climate events in the boreal can damage trees comparably to that of the most destructive
windstorms and hurricanes recorded anywhere. Wind related events may also have consequences
for other forest disturbances such as fires and insect outbreaks. A warmer climate may be more
conductive of extreme wind events, however there is much uncertainty on this issue. Wind
phenomena are localized events and are poorly understood (Climate Change Impacts and
Adaptation, A Canadian Perspective 2004)29. Together with the fact that reliable modeling of
future wind event frequency is not available it is impossible to project a universal trend regarding
the increases and severity of wind related storms in the boreal.
Uncertainties also remain about the influence of the boreal forest on each of the key
processes that determine the global carbon balance. For instance, the uptake of atmospheric CO2
by tree and other plant growth in the boreal may increase or decrease with increasing
temperatures, depending on the species, the geographic region where the growth occurs, the
range of temperature increase, and other climate factors such as precipitation that are likely to
change in a changing climate (ACIA, 2005)30.
Projections of AGW have uncertainties especially on a regional scale. In North America,
there is greater uncertainty about future precipitation than future temperature. This considerably
expands the uncertainty of a broad range of impacts on boreal ecosystems (IPCC, AR4, WG2,
CH 14)31. Climate change adds major uncertainty to basic assumptions about future forest
condition, growth, and uses that are critical in making decisions in the present. Major climate
shifts would alter these factors in ways which are not entirely understood, however it is very
likely these will be very disruptive (ACIA, 2005)32.
There are uncertainties in the detection and projection of terrestrial changes for the boreal
region, leading to implications for resource use and climatic feedbacks. There is also uncertainty
in current and future regional carbon balances over northern landscapes, as well as their abilities
to drive global climate change. Part of the reason for such uncertainty in the boreal is due to the
remoteness and harsh environments of the North American Boreal region. Data collection is
constrained, due to observational networks being sparse and only recently established (IPCC,
AR4, WG2, CH 15)33.
Conclusions:
As a result of warming temperatures from AGW, the boreal forest is slowly adapting by
shifting its range in two directions. These directions are northern in latitude and higher in
altitude. The boreal tree line has been found to follow these directions (IPCC, AR4, WG2, CH
15)34. This is significant because these Arctic and alpine tree lines are an important indicator of
change in the boreal region, because they mark the very limit of the boreal forest (LLOYD.,
2005)35. Continentally in the North America boreal region, increased net productivity of
ecosystems is projected, due to the expansion of the boreal forest into the tundra, and longer
growing seasons. Forest growth is currently slowly accelerating in regions where tree growth has
historically been limited by low temperatures and short growing seasons (Climate Change and
Water, 2008)36.
Although recent climate trends from AGW have increased vegetation growth in the
boreal forests of North America, continuing increases in disturbances are likely to limit carbon
storage, facilitate invasive species and disrupt ecosystem services. Warmer summer
temperatures are expected to extend the annual window of high fire ignition risk by 10-30%, and
could result in increased area burned by 74-118% in Canada by 2100. Over the 21st century,
pressure for species to shift northwards and to higher elevations will fundamentally rearrange
North American boreal ecosystems. Different capacities for range shifts as well as constraints
from development, habitat fragmentation, invasive species, and broken ecological connections
will alter ecosystem structure, function and services (IPCC, AR4, WG2, CH 15)37.
Due to complex interconnecting ecosystem processes, as well as the general remoteness
and harshness of the boreal forests, data collection is constrained which leads to observational
networks being sparse and relatively new. As a result of this there are still many uncertainties in
understanding how AGW will impact the boreal system, how the boreal system will adapt, as
well as how the boreal system will in turn affect further climate change(IPCC, AR4, WG2, CH
15)38.
This paper illustrated some of the impacts and adaptations the boreal forests of North
America are going through, as a result of AGW. It is important to study this because the boreal
forest contains trees growing at the highest latitudes on earth, where its northern margin merges
with the circum polar tundra. These high latitudinal impacts of AGW can act as indicators of
change in other regions. This is because the impacts from AGW are enhanced in the high
latitudes due to polar amplification (Moritz et al., 2002)39. The boreal forest is both affected by
and contributes to climate change. The radiative balance of the earth is affected by the rough
textured dark surface of land, covered with boreal forest canopy which intercepts and absorbs
high amounts of solar radiation converting it into heat. In high latitude regions where snow
covers the ground for extended periods of the year, the albedo effect of tundra vs. boreal forest
cover is magnified. Further expansion of the forest into present day tundra regions resulting
from a warming climate would thus amplify the warming further (ACIA, 2005). Understanding
these AGW processes, impacts and adaptations of the boreal forest will help policy makers make
more informed decisions on how to mitigate and prepare for further AGW impacts on the rest of
Canada.
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Image References:
Image 1) GISS Surface Temperature Analysis (GISTEMP), Annual summation Report (2011)
http://www.giss.nasa.gov/research/news/20120119/
Image 2) Anisimov, O.A., D.G. Vaughan, T.V. Callaghan, C. Furgal, H. Marchant, T.D. Prowse,
H. Vilhjálmsson and J.E. Walsh, 2007: Polar regions (Arctic and Antarctic). Climate Change
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Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F.
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Cambridge, 653-685. Retrieved from the worldwide web at http://www.ipcc.ch/pdf/assessmentreport/ar4/wg2/ar4-wg2-chapter15.pdf
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The North American boreal forests:
Impacts, Adaptations and Uncertainty in the face of Anthropogenic
Global Warming.
Work presented to Melanie McCavour
Class: GEOG 474: Sustainable Forest Management
Finalized by Alexander do Rio, on November 11th 2012