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POTENTIAL IMPACTS OF CLIMATE CHANGE ON PACIFIC NORTHWEST
FORESTS
George A. King
ManTech Environmental Technology Incorporated
and
David Tingey
Environmental Protection Agency
US EPA Environmental Research Laboratory
200 SW 35th Street
Corvallis, OR 97333
This document is a preliminary draft. It has not been formally released by the U.S.
Environmental Protection Agency and should not be construed to represent Agency policy. It
is being circulated for comments on its technical merit and policy implications. Do not release.
Do not quote or cite.
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TABLE OF CONTENTS
FINDINGS i
INTRODUCTION 1
CURRENT STATUS OF NORTHWEST FORESTS 1
Common Forest Types 1
Current Land Use 2
Future Resource Condition Without Climate Change 2
SENSITIVITY OF FORESTS TO CURRENT AND FUTURE CLIMATE 3
Current Climate 3
Adaptation of Forests to Current Climate 4
Sensitivity to Future Climate Change 4
METHODOLOGY 5
Generation of Climate Scenarios 5
Vegetation Modeling - Application of the Holdridge Life-Zone Classification .... 6
Vegetation Modeling - Local Climate-Forest Zone Correlations 7
Vegetation Modeling - Forest Gap Models 7
Carbon Model Results 9
RESULTS 9
Holdridge Vegetation Scenarios 9
Simulation of Current Vegetation 9
Estimated Vegetation Change 9
Assumptions/Limitations 10
Local Climate-Forest Zone Correlations 10
Forest Gap Models - CLIMACS Results 11
Simulation of Current Forests 11
Estimated Changes in Forest Compositions 11
Assumptions/Limitations 11
Forest Gap Models - ZELIG Results 12
Simulations of Current Vegetation 12
Estimated Changes in Forest Composition 13
Assumptions/Limitations 14
Potential Changes in Carbon Storage 14
Expert Judgement - Effects of Climate Change on Forest Disturbances 15
Major Uncertainties 16
Transient Dynamics 16
Direct Effects of C02 17
CONCLUSIONS .t 19
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Climate Induced Changes in Forest Composition and Distribution 19
19
Implications for Forest Management 19
ACKNOWLEDGEMENTS 21
LITERATURE CITED 22
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FINDINGS
Trace gas induced climate change could
significantly alter the climate of the Pacific
Northwest. Mean annual temperatures could
increase 2° to 5° C. The seasonality of
precipitation will likely remain the same, but
with annual totals remaining unchanged or
increasing 20%.
The potential effects of these climate
changes on Northwest forests have been
estimated using a variety of modeling
approaches and climate scenarios. Overall,
26 to 90% of the area in the Northwest
would change from one general vegetation
type to another. Forest area in the
Northwest could decrease 5 to 25%.
Remaining forest land would differ in
species composition, and likely be less
productive than current forests. In Oregon,
Douglas-fir dominated forests would increase
in area, whereas the more productive western
hemlock forests would decrease. Forest
vegetation zones would increase in elevation
from 500 to 1000m. Alpine and subalpine
forests could disappear from all but the
highest elevations in the region.
Forest disturbances such as fire and
pest/pathogen outbreaks would likely
increase in frequency, speeding vegetation
change in response to climate change.
There are two key limitations to the
data presented here. First, the transient
(time-dependent) dynamics of change have
not been adequately addressed. How forest
dynamics respond to a rapidly changing but
variable climate is uncertain. Second, the
direct effects of enhance«jCO2 concentrations
on forest species growth has not been
considered in any of the modeling
simulations. Laboratory experiments suggest
the potential for increased drought tolerance
by individual plants under higher C02
concentrations. The landscape scale impacts
of higher C02 concentrations are uncertain.
Forest managers are thus presented
with a difficult problem. Specifically, how
should current forests be managed given 1)
our uncertainty as to the magnitude and
direction of future climate change and 2) the
potential for large changes in forest
composition and distribution if the climate
does change as currently simulated by state-
of-the-art climate models?
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INTRODUCTION
The forests of the northwestern United States are dominated by evergreen conifers that
are long-lived and grow to sizes unmatched in other parts of the world (Waring and Franklin
1979, Franklin 1988). Biomass storage is higher than any other vegetation type in the world
(Waring and Franklin 1979, Franklin 1988, Harmon et al. 1990). In sum, these forests are unique
among north temperate forest types, in which deciduous trees are usually the dominant life-form.
Besides their intrinsic value as unique ecological systems, Northwest forests serve as an important
source of timber and are a focus of the tourism industry. The potential for significant changes
in the regional climate caused by global increases in radiatively important trace gases (IPCC
1990) has raised concern over the future of Northwest forests. The purpose of this chapter is to
summarize our current understanding of the potential effects of climate change on this important
regional resource. We will focus on the forests in Washington and Oregon, with some results
and analysis applicable in the state of Idaho.
CURRENT STATUS OF NORTHWEST FORESTS
Common Forest Types
Forests occupy 72 million acres of land in Washington, Oregon, and Idaho, or about 46%
of the region (Figure 1, Table la,b). Forests occupy higher elevations east of the Cascades, and
dominate the land west of the Cascades. Douglas-fir (see Table 2 for scientific nomenclature)
and western hemlock are the most important trees in forests ranging from sea-level to 700-1000
m west of the Cascades (Franklin 1988). Douglas-fir typically dominates younger forest stands,
with hemlock and western red cedar increasing in importance after 400-600 year* Grand fir and
western white pine ef&ebe minor components of forests west of the Cascades. In central Oregon,
incense cedar, sugar pine, and ponderosa pine may occur. Hardwoods are rare in these forests,
occupying recently disturbed sites or riparian zones. Big leaf maple and red alder are the most
common hardwood species. On drier sites, Pacific madrone, golden chinkapin and Oregon white
oak are common (Franklin 1988). Understory composition varies widely in this forest zone,
controlled generally by moisture availability (Franklin 1988).
Along the Pacific coast, a narrow band of Sitka spruce - western hemlock forest prevails.
Sitka spruce is only found within a few tens of kilometers of the coast, forming pure stands along
the coast (or with lodgepole pine) because of its salt tolerance. Hemlock is usually more
abundant, however (Franklin 1988).
At higher elevations in the Cascades and Olympic Mountains, mixed conifer forests occur
(Franklin 1988). Common tree species include silver fir, western hemlock, noble fir, Douglas-fir,
western red cedar, and western white pine. At higher elevations and towards the interior,
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lodgepole pine, subalpine fir, Englemann spruce, western larch, mountain hemlock, and Port-
Orford-Cedar can be present. Western hemlock often forms pure stands at the highest elevations
in the Cascades and Olympic mountains that can support forests.
Complex geology and steep gradients in moisture and temperature make the Klamath
Mountains in southwestern Oregon and northern California the most diverse forested region in
the study area (Whittaker 1960, Franklin 1988). Mixed conifer and evergreen hardwood forests
are typical here. Douglas-fir is usually the dominant conifer, and the evergreen trees tanoak,
canyon live oak, Pacific madrone, and golden chinkapin can form a hardwood understory.
Carbon storage in old growth Douglas-fir - western hemlock forests averages over 600
Mg/ha, while second growth 60 year old forests average 250 - 275 Mg/ha (Harmon et al. 1990).
Productivity in eastern Oregon forests (east of the Cascade crest) is significantly lower.
In contrast to conifer forests elsewhere in North America, disturbances are infrequent in
Pacific Northwest forests. The natural fire rotation in these forests is about 400-500 years and
tends to be shorter in the southern part of the range (Franklin 1988). Wind storms can produce
widespread tree mortality, but are of lesser importance in the Cascades than along the coastal
mountains. Pest and pathogen outbreaks are not as frequent as in other western conifer forests
(Franklin 1988).
Current Land Use
About 75% of the forest land in the Northwest are managed for timber production (Table
1, USDA 1989). Over the region, about 63% of the timberland is publically owned, although
in Washington State timberland is equally divided between public and private ownership. Forest
industry owns about half of the private forest lands in the Northwest
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timber supplies in the future, while maintaining large tracts of heretofore unprotected old growth
forest.
Another management challenge is to assess the environmental effects of the conversion
of natural stands into managed plantations (Swanson et al. in press). A key issue is how to
disperse cutting patterns on a forested landscape. Franklin and Foreman (1987) analyze the
effects of various cutting strategies on landscape patterns. An important result was that the
widely used practice of dispersed 10-20 ha cuts through a virgin landscape will eventually
fragment the remaining forests into smaller and smaller contiguous tracts. This effectively
reduces the amount of interior forest available as habitat for old growth dependent species.
Te&hniques at modelng landscape patterns have been developed to help evaluate alternative
cuJfmg patterns (Hemstrom 1990, Cissel 1990, Swanson et al. in press).
SENSITIVITY OF FORESTS TO CURRENT AND FUTURE CLIMATE
Current Climate
"Climatically the [Pacific Northwest] region experiences wet mild winters and warm dry
summers. The dormant season, when shoot growth is inactive, is characterized by heavy
precipitation with daytime temperatures usually above freezing. Away from the coast, the
growing season is characterized by warm temperatures, clear days, and little precipitation. Water
storage in snowpack, soils and vegetation - as well as pulses of fog, clouds, or cool maritime air
which reduce evapotranspiration - obviously are more important during a summer drought"
(Waring and Franklin, 1979). The forests, in this region, grow under a wide range of temperature
and moisture regimes. Mean annual temperature range is about 5° C, while precipitation varies
about an order of magnitude (approximately 300 cm) (Franklin et al., in press).
In general, both summer and winter temperatures decrease from south to north (Fig. 2).
The north to south temperature gradient is steeper in the winter than the summer. Focusing again
on the region west of the Cascade creset, at Gold Beach, OR the mean January temperature is
8.3° C while at Forks, WA it is 3.8° C. In the summer, mean July temperatures are 18.0° C at
Andrews, and 15.7° C at Forks, WA. Similarly, there is a large decrease in the duration of the
growing season which ranges from 365 days at Gold Beach, OR to 281 days at Forks, WA.
Similarly, the growing season becomes shorter and the temperatures lower as the elevation
increases. Summer (mean monthly) temperatures only reach 13.2° and 12.4° C at Rainier/Paradise,
WA and Crater Lake, OR with 5 to 6 months displaying mean monthly temperatures below 0°
C.
t>
There is a distinct seasonality in precipitation with the winters tending to be wet and the
summers dry (Figure 3). Depending on the temperature and elevation, the precipitation falls as
either rain or snow; significant snow pack occurs at higher elevations. During the winter, monthly
precipitation ranges from 40 to 80 cm, while during the summer months precipitation can be less
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than 10 cm/month. There are stronger orographic and elevational influences on precipitation than
a south to north influence. Rain shadow effects are particularly evident east of the crest of the
Olympic Mountains and Cascade Mountains.
Adaptation of Forests to Current Climate
The strong seasonality of climate in the Northwest is a strong determinant of regional
vegetation patterns. Evergreen conifers are well-adapted to this climatic regime, in contrast to
hardwood species. These adaptations are summarized by Waring and Franklin (1979) and 1)
include the ability to photosynthesize under the cool winter temperatures, 2) needle-shaped leaves
which reduce leaf temperature and respiration during the warm dry summer, 3) a large volume
of sapwood that stores water which can be utilized during dry summer days, and 4) the ability
to reestablish their vascular water column if it's broken during evaporative stress (whereas many
hardwood species can not; Sucoff 1969, Burke et al. 1976, Sakai 1983).
Within the region soil moisture and temperature are important controls of ecotones
between forest types (Daubenmire 1943, Zobel et al. 1986, Franklin 1988). Lower elevational
zones are controlled primarily by soil moisture, whereas upper elevational zones are controlled
by temperature. Snowpack and wind can be important controls at uppermost elevations in the
Cascades and Olympic Mountains (Scott 1980). Also, Leverenz and Lev (1987) point out the
seed chilling requirement needed by Douglas-fir for successful regeneration in coastal regions.
Sensitivity to Future Climate Change
A logical extension of the hypothesis that climate controls the dominant vegetation
patterns in the region is simply that future climate change will affect regional vegetation patterns.
The key question is: how much can climate change before major vegetation redistribution takes
place? One way to answer this is to look at the response of vegetation to past climate variation.
Within the historical record, short-term droughts had little impact on vegetation (Graumlich
1987). Over the past few centuries, droughts similar to those of the 1920s and 1930s occurred
at least once every century since 1675, as determined by tree ring analysis (Graumlich 1987).
This variation obviously affected tree growth, but did not change regional vegetation patterns.
In the longer term, temperatures were about 2° C warmer during the early Holocene
(10,000 - 7,000 years before present) (Brubaker 1988) during which time the vegetation was
much different from today's. Douglas-fir was more important than today in western Washington,
and oak savanna extended north of its present limit in the Williamette Valley in Oregon to the
southern part of the Puget Trough (Barnosky et al. 1987). Thus it appears that climate changes
of greater than 2° C will likely have significant impact on regional vegetation patterns.
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A complicating factor in assessing forest sensitivity to climate change is the long life
spans of trees in the Northwest (500 - 1000+ years; Franklin 1988; Brubaker 1986, Franklin et
al. in press). Adverse climate conditions may eliminate seedling establishment and sexual
reproduction, while mature trees survive for several centuries. Trees which can reproduce
asexually may persist "out of equilibrium" with climate for thousands of years (e.g. Neilson and
Wullstein 1983). The key definition that needs to be made here is "adverse". It most certainly
has an upper and lower bound in this context, since mature trees can be killed by severe climate
conditions as well. Unfortunately, the autoecological databases for making such a determination
are too incomplete at this time for defining environmental tolerances of seedlings and mature
trees of many species in the Northwest However, modeling exercises using forest gap models
(discussed below) may help define the term for certain species.
METHODOLOGY
The sensitivity of Northwest forests to climate change has been analyzed using a variety
of modeling techniques as well as expert judgement based on an understanding of how climate
controls the current distribution of forest types in the region. As will be described, the modeling
analyses are complimentary, as no single vegetation model can provide the specific detail and
geographic context needed in an impacts assessment. The specific limitations and assumptions
of the specific modeling analyses will be described in the results sections. Also, expert opinions
on climate change impacts (e.g. Franklin et al. in press) are also discussed in the results section.
Generation of Climate Scenarios
Four low resolution general circulation models (GCMs), OSU (Schlesinger and Zhao
1989), GISS (Hansen et al. 1983), GFDL (Manabe and Wetherald 1987), and UKMO (Wilson
and Mitchell 1987) were used to project future climate conditions assuming a doubling of
"current" ambient CO2 concentrations. Because the grid sizes of each model (typically about 4°
latitude by 5° longitude) is large relative to the area of the Pacific Northwest, we can not expect
these models to exactly reproduce the current or projected climate but rather to provide insight
into changes over broad regions when the atmospheric CO2 is increased (Jenne, 1988). To
convert the low resolution GCM output to specific locations, the ratios of 2X: IX CO2 model runs
were used to^multiple^current mean monthly temperature and precipitation for each location.
Temperatures were first convened to °K before calculating the ratios. Current climate data for
selected locations in Washington and Oregon was provided by Charles B. Halpern, Department
of Forest Science, Oregon State University from long-tem NOAA summaries. The length of the
time-series used to create the mean values varies with location, and ranged between 30 and 60
years.
The overall effect of climate change in the Pacific Northwest |s that forests will become
significantly warmer. The GCMs project a 2° to 5° C increase in annual mean temperatures
(Table 3) with projected temperatures decreasing from south to north. The UKMO predicts the
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greatest warming, with January temperatures increasing 5-6° C, and July temperatures increasing
9-11° C. The temperature seasonality is projected to persist under climate warming with all
months showing an increase (Figure 2). Growing degree days, a measure of the cumulative
increase in temperature over the growing season, is projected to increase significantly (Table 3).
The OSU GCM projects a 37 to 73% increase in growing degree/days while the GISS model
projects significantly more warming (83 to 133%).
Under the projected climate change, there are no seasonal shifts in precipitation; the
pattern of relatively dry summers and wet winters will persist (Figure 3). Although the
seasonality is maintained, the proportions of rain and snow may change from the current
conditions. Two of the GCMS (OSU and GFDL) predict unchanged annual precipitation, while
the other two (GISS and UKMO) predict about 20% greater annual precipitation. In terms of the
impact on vegetation, changes in soil moisture is of greatest interest. This depends in a large part
on potential evapotranspiration (PET), estimates of which are listed in Table 4. The projected
changes in PET are dependent on the method of calculation and must be viewed as preliminary
estimates at this time (Marks 1990). Overall, the UKMO scenarios present^ the largest change
from current conditions, particularly the large increases in summer temperature.
The climates projected from the GCMs represent a significant shift from present
conditions. When viewed in a south to north transect, the climatic change is equivalent to
shifting current climates from 200 to 500 km north from the present locations, i.e., moving the
climate of northern California into northern Oregon and the climate of northern Oregon into
northern Washington (Franklin et al., in press). Similarly, when these changes are viewed from
an elevational perspective, it is equivalent to moving current climatic conditions at the base of
mountain 500 to 1000 m upward (Franklin et al., in press).
Vegetation Modeling - Application of the Holdridge Life-Zone Classification
The Holdridge Life-Zone classification system has been used to simulate the effect of
climate change on global vegetation patterns (Emanuel et al. 1985a,b; Prentice and Fung 1990,
Smith et al. submitted). We will summarize the results of Smith and Leemans (1990) for the
Pacific Northwest. Although the model was applied at a relatively coarse resolution (0.5° x 0.5°)
for application to a regional case study, the Holdridge results are the only published data
available that provide estimates of regional changes in the distribution of forests in the Northwest.
The Holdridge Life-Zone classification system relates major vegetation formations with
mean annual biotemperature, precipitation, and the ratio of potential evapotranspiration (PET) to
mean annual temperature. Biotemperature is basically an index of the growing season. PET as
defined by Holdridge (1967) is a linear function of biotemperature, and thus is not an
independent variable in this model. Using these climate variables, Holdridge created a triangular
axis system relating climate and vegetation (Figure 4). Smith and Leemans (1990) applied the
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Holdridge classification system to a gridded, global database of mean monthly temperature and
mean annual precipitation (Leemans 1990) for current climate, and simulated future climate using
GCM output described above. The climate data base has a resolution of 0.5°, which consequently
is the resolution of the vegetation scenarios.
Vegetation Modeling - Local Climate-Forest Zone Correlations
Franklin et al. (in press) used correlations of climate and forest zonation in the central
Oregon Cascades (Franklin 1988) to estimate changes in the areal extent of forest communities
under two climate change scenarios. In particular, the current temperature for a particular
elevation was assumed to increase by either 2.5° and 5° C. The current relationship between
temperature and forest types were used to define the new elevational bands the forest types would
occupy under these two scenarios. Then the areal extent of the forest zones at their new
elevations was determined using a elevation model relating area with a given elevational band.
These results are more detailed than the Holdridge simulations, but are limited to the central
Oregon Cascades.
Vegetation Modeling - Forest Gap Models
Models that simulate forest dynamics on small plots (usually 1000 m2) have been
developed for a variety of forest communities worldwide (Shugart and West 1980). The models
were initially devised in order to simulate forest succession in a gap formed in a closed forest
produced by the death of a overstory tree. In recent years, these models have been used to
estimate changes in species composition in response to climate change (Solomon 1986, Botkin
et al. 1989, Dale and Franklin 1989, Urban and Shugart 1989, Bonan et al. 1990). The models
simulate tree growth for every individual on a plot, as well as seedling establishment and tree
mortality. Mathematical functions of key demographic processes (e.g. annual diameter growth)
are derived for the maximum potential behavior of each tree species included in the model.
Growth rates or other processes are then reduced according to environmental constraints such as
shading, soil moisture, and temperature (Urban and Shugart 1989). Thus, environmental
feedbacks are an important component of these models. As a forest gap closes, available light
decreases at the forest floor, shifting the probability of sapling establishment from shade
intolerant species to shade tolerant species. Since the models incorporate stochastic processes,
a large number of plot simulations are run and the average results presented as the model output.
Forest gap models were initially developed for eastern deciduous forests (Botkin et al.
1972, Shugart and West 1977), but several gap models have been developed for western forests
(Dale and Hemstrom 1984, Kercher and Axelrod 1984, Urban et al. 1990). Two models have
been used to simulate climate change effects in the Pacific Northwest, CLIMACS (Dale and
Hemstrom 1984, Dale and Franklin 1989) and ZELIG (Urban 1990, Urban et al. 1990).
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CLIMACS was modified from FORET for application to the Northwest, specifically for
western Washington and Oregon. The major modifications include introducing a moisture factor
in the growth equations, calculating regressions of height to diameter for use in the annual growth
equations, making mortality dependent on successional class of the species, and incorporating
natural disturbances and clear-cut logging (Dale and Hemstrom 1984, Dale et al. 1986). Twenty-
one tree species are included in the model. The model was run assuming a 5° C warming in
northwest Oregon, using data from Cascade Head on the Oregon coast. The climate change
scenario was applied to a typical stand structure of a forest 140 years old at Cascade Head.
ZELIG is an updated version of the FORET model as well (Urban 1990) and was used
to simulate climate change impacts at three sites in Washington and Oregon (Urban et al. 1990,
unpublished report). The major effort in implementing the model in the Northwest was
developing a set of species parameters (e.g. growing degree day limits) describing life history and
growth characteristics for each Northwest tree species. In contrast to eastern United States where
relief is substantially less, tree species in the mountains of the Northwest often have upper and
lower elevational range limits. Correlations of climatic variables and these range limits were thus
established at a number of latitudinal bands from northern California into British Columbia.
Because of sparse climate data, regional regression models were developed to predict mean
monthly temperature (Urban et al. 1990). Substantial latitudinal variation occurs in the climatic
limits of some of the major species in the region (to be discussed more later).
ZELIG was tested initially at the H.J. Andrews LTER site (44.2° N, 122.2° W) in the
central_OregQiL_Cascades because of large amount of forest stand data available for model
verification. The model was applied at three elevations, 500m, 1000m, and 1500m. The climate
parameters defining the range limits of species at the Andrews latitude (44° N) were used in the
model run. Model simulations started from bare ground (in contrast to the CLIMACS
simulations), ran for 500 years, and were replicated for 20 plots. Two climate change scenarios
were used, those generated from the OSU GCM (Schlesinger and Zhao 1988) and GISS (Hansen
et al. 1983). The sensitivity of forest dynamics at Mount Rainier, WA (46.8° N, 121.7° W,
1654m) and Gold Beach, OR (42.4° N, 124.4° W) to climate change was also tested, using the
species parameters set for 44° N. These results must be viewed as more tenuous than the results
for the Andrews, because of differences in species characteristics at these two different latitudes.
ZELIG was also used to begin studying the transient response of Northwest forests to
climate change. Transient climate changes were applied to the model as linear increases in
growing degree days (GDD) starting at a specific year and ending at specific years. The
increase was started at year 0, 200, and 500 in separate simulations, and either a 600 GDD or
1300 GDD warming was applied. A 600 GDD corresponds to the OSU 2xC02 equilibrium
scenario, and the 1300 GDD increase corresponds to the GISS 2xC02 equilibrium scenario.
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Carbon Model Results
Potential changes in carbon storage in the central Oregon Cascades was analyzed using
the DFC model (Harmon et al. 1990). This model tracks carbon stored onsite (e.g. changes in
ecosystem storage) and offsite (e.g. the fate of harvested wood). A simple 5° C warming
scenario was applied at three elevations in the Oregon Cascades (Urban et al. 1990).
RESULTS
Holdridge Vegetation Scenarios
Simulation of Current Vegetation
Before presenting the climate change results it is important to consider how well the
vegetation model simulates today's vegetation patterns. The Holdridge classification presents a
much coarser taxonomic resolution of vegetation in the Northwest (Table 5, Rgures 5-8).
Generally, the Holdridge model does a good job of separating forest from non-forested land. The
main forest types in the Holdridge model under current conditions are temperate forest and boreal
forest, whereas the SAP classification has eight major forest types delineated based on the
dominant tree (e.g. Douglas-fir forests). All the forests west of the Cascade crest are considered
temperate forests, while subalpine forests are predicted for parts of the Cascade crest, and
portions of the Northern Rocky Mountains in Idaho. The Holdridge model does not depict the
heterogeneity of forest vegetation in central Idaho. The Holdridge estimate of forested land in
the region reasonably approximates the Forest Service estimates of forested land (Holdridge
318,000 km2 vs. 290,000 km2 USDA 1989).
Estimated Vegetation Change
Major shifts in vegetation patterns occur in the Northwest under four climate change
scenarios, according to the Holdridge system (Smith et al. submitted). Overall, 26% (OSU) to
90% (UKMO) of the tri-state region will change from one Holdridge vegetation class to another
(Table 5, Figures 5-8). Most vegetation change in predicted under the UKMO climate scenario,
with least change predicted under the OSU scenario. Total forested area decreases under all
scenarios from 5 to 25%. Boreal (subalpine) forests decrease by at least 50%, while temperate
forests generally contract (except for the OSU scenario in which these forests increase by 20%).
Temperate forests remain or expand at higher elevations in the region. In contrast, warm
temperate forest expand many fold.
Interpreting what these changes in broad vegetation classes means in terms of more
specific forest types identified in the introduction is somewhat problematic. If one compares the
current distribution of forest types and the Holdridge classes, temperate forests in the Northwest
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correspond to the entire suite of conifer forests in the region. Warm temperate forests are only
found in the southeastern United States, corresponding to oak dominated forests in the middle
of the eastern United States (Holdridge 1967, Kuchler 1964). Thus it appears that the Douglas-fir
- hemlock forests decrease in regional extent, being replaced by more drought tolerant oaks.
Assumptions/Limitations
The limitations of this analysis are numerous and should be kept in mind when analyzing
the output. First, the climate change analysis assumes that the Holdridge system adequately
defines current vegetation patterns in the region, and that current climate-vegetation correlations
are unchanged in the future. Although the Holdridge system only correctly classifies 40% of the
globe's vegetation, the model does a reasonable job of separating forest and non-forest vegetation
in the Northwest. It is uncertain whether current climate-vegetation correlations will remain
unchanged in the future. New broad-scale vegetation models are being developed (Neilson et al.
in press, Prentice) which are based on a mechanistic understanding of how climate controls
vegetation patterns. These models should be more robust under different climates. All
vegetation models used in climate change analysis should be extensively tested, either in different
parts of the world, or at different times in the past (e.g. last 15,000 years).
The model also does not simulate the dynamics of vegetation change. The climate change
results must be viewed as a snapshot of future vegetation patterns after a double CO2 climate
change has occurred and the vegetation has come back into equilibrium with the regional climate.
In other words, forest dynamics (tree establishment and migration) are not simulated by the
model.
The model only simulates potential natural vegetation; land use is not considered in the
model. The influence of soils on vegetation is not factored into the analysis, nor are the direct
effects of higher CO2 concentrations on plant growth. Finally, the life zone classifications are
very broad and applied on a coarse resolution. Interpreting the vegetation change scenarios in
terms of actual species distribution is problematic.
Local Climate-Forest Zone Correlations
Mean annual temperatures of major plant zones in the region differ by about 1.5° to 2.0°
C at Mount Rainier, WA (Franklin 1988) and 2.5° C in southwestern Oregon (Atzet and Wheeler
1984, Franklin et al. in press). Thus, a 2° degree wanning would completely shift a forest type
one zone upward; a 4° degree warming would shift them two zones upward (Franklin et al.
1990).
The potential impact of climate change on the elevation and area! extent of forest zones
in the central Oregon Cascades was estimated using current correlations between forest zones and
mean annual temperature. Two simple climate warming scenarios (+2.5°C and +5°C) were used
10
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to drive the correlational model (Franklin et al. in press). Douglas-fir dominated forests would
increase from 8% of current forested land to 39 or 27% (Table 6, Figure 9). The moister western
hemlock forests would decrease in areal extent. Subalpine and alpine forests would decrease in
areal extent. Decreases in area of upper elevational forests is simply a function of less area
occupying a given elevational band as elevation up a mountain range increases. Total forested
area would decrease east and west of the Cascade crest (Table 6).
This correlational approach has the same limitations as those discussed for the Holdridge
approach, except the taxonomic resolution is much finer. Also, the approach assumes that current
forest zones will not change in composition, all the species in them will move an equivalent
distance upslope. However, paleoecological data clearly indicate that this assumption is incorrect.
Species moved independently in the past in response to warming after deglaciation (Davis 1981,
Webb ). Each species has its own unique set of climate limits. Thus vegetation change in the
future will certainly be more complex than suggested here.
Forest Gap Models - CLIMACS Results
Simulation of Current Forests
Before future simulations of forest composition as simulated by CLIMACS are discussed
it is important to evaluate how well the model simulates current forest dynamics. CLIMACS has
been verified against data from HJ. Andrews (Hemstrom and Adams 1982) and validated for the
western Olympic peninsula. For xeric and mesic sites at Andrews the model correctly predicts
Douglas-fir as the dominant tree and modeled and measured leaf area and basal area compare
favorably. For the Olympic Peninsula site, the model correctly projects Douglas-fir as dominant
with western hemlock and silver fir in the understory (after 500 years). The size distribution is
similar for both modeled and measured stands (Dale et al. 1986).
Estimated Changes in Forest Compositions
CLIMACS was run under a 5° C warming scenario for Cascade Head on the Oregon coast
(Dale and Franklin 1989). Soil moisture status was not changed in the scenario. The warming
scenario was imposed on a stand 140 years old. Under current climate conditions, the
composition of the stand would change from one dominated by Douglas-fir to one dominated by
western hemlock (Figure 10). Silver fir would be a co-dominant. Under the warming scenario,
western hemlock also succeeds Douglas-fir as a the dominant tree. However, grand fir replaces
silver fir as a codominant (Figure 10).
Assumptions/Limitations
The limitations of gap models will be discussed fully, after the ZELIG results are
summarized. One limitation unique to the CLIMACS analysis jije that soil moisture was not
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altered so the drought effects of warming were not analyzed. These could be quite pronounced
in the Northwest under the GFDL and UKMO scenarios.
Forest Gap Models - ZELIG Results
Simulations of Current Vegetation
Much of the work in adapting ZELIG to run in the Northwest was spent developing a list
of species parameters (e.g. growing degree limits) for inclusion in the model (Urban et al. 1990).
Climatic correlates with tree species range limits were established for a number of latitudinal
bands in the Northwest, ranging from a species southern geographic limit to its northern limit.
A complicating result of this analysis in terms of modeling forests throughout the Northwest is
the fact that growing degree day (GDD) limits for a species vary with latitude. For some species,
the maximum at higher latitudes is greater than the minimum at lower latitudes. There are
several explanations for this, ranging from genetic variation within a species, to the possibility
that some other factor (soil moisture, snow pack, wind) besides GDD limits species distributions
in some parts of their range.
This complicates the forest gap modeling, because a decision needs to be made as to what
speciesparameters should be input into the model when the model is run at a particular site.
EitheTIhe local parameter suite, &f the regional limits are the two choices. In the ZELIG
analysis, parameters for the latitude of the HJ. Andrews site were used in the model runs. For
climate change analysis, this assumes that seed sources will be restricted to the area surrounding
the site (within about 100 km).
Despite the problems the genetic variability of a species presents to modeling forest
response to climate change, the variability will provide a species the ability to better respond to
climate change. The probability of a genotype being present in a species adaptive to a future
climate is increased with greater genetic variability within a species.
The results from the ZELIG simulations for current and future conditions at Gold Beach,
OR, HJ. Andrews, OR, and Mount Rainier, WA are summarized in Figure 11. The model does
a poor job of simulating current vegetation patterns at HJ. Andrews. At 500 m, Douglas-fir and
western hemlock are the dominant trees in the current forests. In the model simulation, chinkapin
and ponderosa pine are dominant. Douglas-fir and western hemlock are only minor components
of the modeled stand. In essence the model predicts vegetation adapted to much drier sites than
occur at the Andrews. The model performs better at 1000m, where Douglas-fir and western
hemlock dominate natural stands and modeled stands. Douglas-fir is still underestimated by the
model, however.
A priori one would expect the model not to perform as well at the two other sites because
of the apparent latitudinal differences in species adaptations to climate. At Gold Beach (where
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annual precipitation is greater than at the Andrews, and temperatures are 1.3° C warmer), current
forests are dominated by western hemlock and Sitka spruce (Franklin and Dyrness 1988). At the
end of the 500 year run, the model predicts chinkapin and tan oak as dominants. Hemlock is a
very minor component (sitka spruce was inadvertently left out of the species set, will be rerun
but won't change outcome). Again the model is biased towards predicting a drier type of
vegetation (even more so than at Andrews) than what actually occurs at the site.
At 1600m elevation at Mount Rainier, mountain hemlock and silver fir are dominant with
subalpine fir present (Franklin and Dyrness 1988, Franklin et al. 1988). Silver fir, mountain
hemlock, western white pine, subalpine fir are simulated as dominant species by the model. In
terms of the dominant trees, the model does a good job (better than at the other two sites) of
simulating the forest stand composition. In sum, ZELIG appears to be biased towards simulating
drier types of vegetation at warm sites, but does a better job of forest simulations at higher sites.
It does not simulate the dynamics of Douglas-fir well (Hansen et al. unpublished).
Estimated Changes in Forest Composition
ZELIG predicts major changes in species composition under the OSU and GISS climate
scenarios (Figure 11). Under the OSU scenario, the dominant trees simulated under current
conditions are replaced by tanoak and red alder at Andrews. At Gold Beach, dominants under
current conditions are replaced by red alder. At Rainier, vegetation change is less, with the major
change being a replacement of mountain hemlock with western hemlock. The driving factor
behind these changes is the increase in GDD at each site. Even under the moderate wanning of
the OSU scenario, only a few species have maximum GDD limits less than that of the predicted
climate. At Gold Beach, only red alder and tanoak will grow.
The increase in temperature under the GISS scenario is so great that GDD limits at
Andrews and Gold Beach exceed the maximum values for any species included in the Northwest
modeling effort. Thus, the model simulates no tree growth at these sites. At Rainier, silver fir
becomes a minor component with western hemlock and Douglas-fir becoming more important.
In essence the model predicts an upward movement of species distribution in the Washington
Cascades.
The general response of mid-elevation forests at Andrews to the transient climate scenario
varies with the age at which the warming applied (Urban et al. 1990). If the warming is applied
at a stand age of 200 or 500 years, a short episode of high tree mortality occurs and the total
biomass on the plot decreases. The total stand biomass doesn't return to prewarming levels
because of the growing degree constraints on tree growth as discussed above. If the warming
began from the beginning of the simulations, succession to more heat tolerant species proceeds
without significant forest dieback.
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These results are distinctly different from the CLIMACS results, in which total biomass
did not drop after a climate change was imposed on a young stand.
Assumptions/Limitations
A number of assumptions are made when the two forest gap models are applied in the
Northwest. In developing correlations between climate and species distribution, it is assumed that
their current distribution reflects their environmental tolerance, and that the species grow best at
the center of its range (Urban et al. 1990). Temperature (through growing degree day totals) is
assumed to be the primary control of species range limits. Genetic variability with a species
parameter set is assumed to be non-existent. Seeds of all the species included in the simulation
are assumed to be^ instantaneously available if the climate is favorable for their germination.
Other assumptions are discussed by Urban et al. (1991). The form of the species response
functions to climate is assumed to be correct and similar for all species.
The limitations of Northwest forest stand models are substantial at this point in their
development. The ZELIG model has problems simulating low elevation (warm) forests in the
Northwest, and consequently the 2xC02 scenarios can only be interpreted in terms of relative
change in dominance. Specific changes in species dominance are likely inaccurate, except
perhaps for the Rainier site. Thus problems probably exist both in the species parameterization
and form of the model algorithms.
More importantly, the models do not adequately account for the importance of soil
moisture in controlling species distributions at lower elevations (Daubenmire 1968, Zobel et al.
1976, Waring and Franklin 1979, Neilson et al. 1989). Moreover, the simulation of soil moisture
is biased towards simulating wetter conditions than what actually occurs at a site, because
vegetation cover does not affect available soil moisture in the model. Also the contribution of
snowmelt to soil moisture during the spring and early summer in the growing season is
underestimated. Finally, other factors besides temperature and soil moisture may limit species
distributions in particular regions, such as chilling requirements along the coast (Leverenz and
Lev 1987) and snowpack at higher elevations (Scott 1980).
The model simulations do not consider the direct effects of enhanced CO2 concentrations
on water use efficiency or changes in disturbance frequencies (Urban et al. 1991). Model output
also depends in part on the species assemblages input as potential occupants of a stand. Many
of the species specific parameters are expressed relative to the other species in the assemblage.
These could change as assemblages change, they remain constant in these model runs.
Potential Changes in Carbon Storage
A climate warming of 5° C could decrease detrital carbon stores by 30% across a diverse
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array of sites (Urban et al. 1990). At low, middle, and high elevations in the Oregon Cascades
this could result in fluxes of 40, 55, and 75 Mg C per hectare to the atmosphere. Production
would have to increase by about 10% to offset these losses. The increase in production is
unlikely except at middle elevations. Low elevations could be constrained by low soil moisture
and replacement of Douglas-fir by slower growing species. Shallower soils at higher elevation
may limit production increases there. Thus, assuming no change in production, and weighting
the detrital carbon loss by the area! extent of each elevation zone as done by Franklin et al. (in
press) suggests that there could be a net loss of 60 Mg carbon per hectare from the Oregon
Cascades.
It should be emphasized that this analysis does not factor in changes in species
composition, nor the potential for catastrophic forest dieback (Neilson personnal communication)
to inject a pulse of carbon into the atmosphere. Predicted changes to less-productive forests
adapted to warmer and drier conditions could substantially reduce the amount of carbon stored
in the Northwest.
To put the potential carbon loss in perspective to recent land use changes, conversion of
old growth forests to younger plantations in western Oregon and Washington has resulted in
declines in carbon storage of 305 - 370 Mg per hectare (Harmon et al. 1990).
Expert Judgement - Effects of Climate Change on Forest Disturbances
Although as yet not quantitatively modeled in the Northwest, the effects of climate change
on forest composition and structure could be felt initially and most extensively through altered
disturbance regimes (Overpeck et al. 1990, Franklin et al. in press). Disturbances destroy the
resilience of the existing forests and coupled with climate change provide conditions for forest
reestablishment that may be more severe than previously existed. As noted by Brubaker
(1986)"[disturbances] should also mitigate the lagging effects of long tree lifespans by
accelerating rates of population decline when climate change makes conditions unfavorable for
seedling establishment."
Fire frequencies are likely to increase in the region given increased temperatures,
unchanged precipitation and higher potential evapotranspiration. In southern Oregon increased
fires could increase the dominance of some hardwood species that sprout after fires, (e.g. tanoak;
Franklin et al. in press). The primary effect of fire would probably be one of speeding the
conversion of a forest to one favored under warmer conditions.
New or more severe insect problems are probable given projected climate change. The
altered climate may provide a more favorable environment for the insect or the trees may become
more susceptible to the insect pest as a consequence of climate-change ino-easesln tree stress.
For example, the balsam woolly aphid (Adelges piceae) is an introduced p^st.can be a serious
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problem in the Pacific Northwest in Pacific silver fir and low-elevation occurrences of subalpine
fir (Mitchell, 1966; Franklin and Mitchell, 1967, Franklin, 1991). Currently, the aphid has been
restricted to low and middle elevations by temperature limitations. During the summer the second
generation of the aphicUmust reach the first instar stage to survive the winter. The higher
subalpine zones of the (CQ^al and Cascade mountains rarely experience sufficient heat for the
second generation to develop sufficiently to overwinter* fyence, to^few aphids attain the critical
stage to produce dense populations (Mitchell, 1966; Franklin et al., 1991).
However, given the large increase in growing degree/days (Table 3) and mean temperature
increases of 2° to 5° C, it would be possible for the aphid to successfully reproduce and spread
at the higher elevations where subalpine fir is a major component (Franklin et al., 1991). Mature
subalpine fir are susceptible to the aphid; consequently high levels of subalpine fir mortality are
likely (Franklin et al., 1991).
Increased frequency and severity of pest outbreaks are likely in forest stands subjected
to increased physiological stresses associated with climate change (Mattson and Haack, 1987).
Even under conditions of stable climate, the majority of pest outbreaks are associated with
increased host-plant stress (Franklin et al., 1991).
Disturbances can be viewed as events that hasten the adjustment of forest vegetation to
the current climate conditions. But this could be disruptive as climate change may create more
severe conditions for forest reestablishment which can cause significant changes in the forest
composition and function (Franklin et al., 1991). Transitions in vegetation types will be a
problem as forest loss may occur faster than forest reestablishment, especially at the lower and
upper tree lines. There will likely be a shift in area from forest to non-forest vegetation (Franklin
et al., 1991). "In general, natural forest ecosystems, with their greater compositional and
functional redundancy, are expected to show greater resistance to change and recover more
rapidly following disturbance than are intensively managed forests." (Franklin et al., 1991).
Major Uncertainties
Transient Dynamics
The model results presented here assume an instantaneous climate change, and simulate
the equilibrium response of vegetation to that climate change. That is, the models simulate
vegetation patterns in the region after species have responded to the climate change. One reason
for this assumption is the lack of transient climate change scenarios (two GISS transient scenarios
are available however, Hansen et al. (1983)). Simulating transient climate change using
atmospheric models is a difficult task, necessitating better ocean-atmosphere feedbacks.
Incorporating a more realistic ocean circulation model in a climate model greatly increases the
computer power required to operate the model, and taxes the fastest computers now available.
Other technical and conceptual issues need to be resolved as well before transient scenarios can
be produced.
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Ecosystem models also are limited in their ability to simulate transient dynamics. Broad-
scale vegetation models that simulate the equilibrium response of vegetation to climate change
are still being refined; development of regional scale models to depict transient behavior of
vegetation is still a ways off. Local scale models, such as forest gap models, are available for
driving by a transient climate scenario. Several scenarios of the transient response of forests to
climate change have produced in other parts of the U.S. (Urban et al. 1989, Botkin et al. 1989?).
However, forest gap models in the Northwest still need refinement in their simulation of current
forests before their simulations of future climate impacts can be considered to be more accurate.
Interestingly, the simple transient scenario used in the ZELIG approach suggests that
biological inertia (e.g. the long lifespan of the forest trees in the Northwest) may not significantly
delay changes in forest composition. The magnitude of the climate warming is so great that
forest dieback occurs even in mature trees. This result is driven by the temperature limitations
imposed on the species in the modeling runs which may not be relevant in lower elevational
forests. Still, changes in soil moisture could be substantial enough that mature trees rapidly
dieback to be replaced by more drought adapted species.
One difference between the eastern and western U.S. in terms of transient responses of
vegetation is the differences in dispersal distances required for a species to track its favored
climate. The generally flat terrain in the eastern U.S. means a 1° C temperature change will
move a climate zone a considerable distance latitudinally. In the west, the mountainous terrain
means that a 1° C change will move up only a few hundred meters of elevation and perhaps only
a few km in actual distance. Thus, western tree species will have shorter distances to migrate
to track their favored environment (as long as the temperature change does not raise the preferred
elevational band above the highest elevation in the local region). Thus mountain vegetation will
more likely stay in equilibrium with the changing climate than eastern forests.
Genetic variation in certain western tree species is relatively high (e.g. Douglas-fir).
Thus, within a small region, the probability of a genotype being present that is adapted to a
potential future climate is relatively high. However, current forest management techniques
limiting the range of genotypes being replanted on cutover sites may decrease the ability of local
^forests to successfully adapt to future climate change.
>
Direct Effects of CO2
The other major uncertainty in predicting forest response to climate change is the potential
effect of higher CO2 concentrations on forested ecosystems. Many assessments of the impacts
of climate change on forests have focused on the potentially negative direct effects of increased
temperature and drought on the trees, but have not considered the potential impacts of elevated
CO2 on forests.
There is a substantial body of literature that photosynthesis is increased and stomatal
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conductance decreased by supra ambient levels of CO2 (Agren, et al., 1991; Hollinger, 1987;
Surano et al., 1986). However, the magnitude of the changes, especially for mature trees is
uncertain. Also it is largejyonknown if the changes in photosynthesis and conductance will
persist as the tree acclimati^^o the elevated CO2 or if the increases can be supported by current
nutrient levels in the forest soils.
The extent to which possible nutrient limitations in forest soils will limit the direct effects
of CO2 on productivity is also unknown. In a review of a series of experiments of the response
of seedlings and young trees to CO2, Jarvis (1989) concluded that tree growth was stimulated by
added C02 even under severe nutrient limitations. However, it is not known if these stimulations
will persist in mature trees in nutrient-poor soils.
Studies with loblolly pine (Pinus taeda L.) seedlings found that drought stress did not
preclude a growth response to elevated CO2^and that the effects of drought stress were largely
mitigated by elevated CO2 (Tschaplinski and Norby, 1991). The reported decreases in stomatal
conductance and increases in photosynthesis yieldtf an increase in water-use efficiency by trees
(Conroy et al., 1988).
A key issue in the CO2 debate is how(wifl>egetation over a landscaperespond to higher
concentrations. The scientific problem is one of developing the techniques to extrapolate
experimental work to a forest stand. One approach is to use simulation models. Solomon
/and West Q investigated the impact of greater water-use efficiency of forest population dynamics
with a simulation model. They concluded that forest response to elevated CO2 was dampened by
the processes of tree regeneration and death. The increase in water-use efficiency was not
sufficient to compensate for the probable increase in drought severity expected from a warmer
and possible drier climate.
In another simulation study of forest stand pfocpssies^ Shugart (1984) found that the
primary effect of elevated CO2 was to increase the rate^atgapsin the forest closed. Consequently,
forests recovered faster from disturbance^uid forests with more disturbances were more
responsive to additional CO2. "Although...slm/ulation studies of possible responses of forests to
enhanced levels of CO2 are based on simple assumptions and formulations, the indications that
population dynamics may override the consequences of CO2 fertilization are interesting. The
direct effects of CO2 must therefore be considered within the larger scale consequences of
population dynamics and responses of different species to CO2-induced climate change" (Agren
et al., 1991).
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CONCLUSIONS
Climate Induced Changes in Forest Composition and Distribution
Despite the uncertainties and concerns on model precision addressed above in the
discussion of climate change impacts on Northwest forests, some overall conclusions can be made
concerning climate change impacts on Northwest forests. The foremost of these is that the
distribution and composition of forests in Washington and Oregon could change substantially,
given the GCM scenarios of warmer temperatures and generally unchanged precipitation and
ignoring the potential mitigating effects of increased CO2 concentrations. The Holdridge,
Elevation-zone, and forest gap models (except for the CLIMACS results) all forecast shifts to
forests better adapted to warmer and drier conditions. Temperate forests in the Holdridge
scenarios are generally restricted to upper elevations and total forest acreage decreases 5 to 25%.
In central Oregon, total forested area is projected to decrease by almost half under a 5° C
warming. Forest zones could move up one complete elevation band under the same degree of
warming. Oak woodlands and Douglas-fir dominated forests are likely to increase in areal extent,
while the more productive hemlock forests will undergo significant contraction. Subalpine and
alpine vegetation is likely to disappear.
The forest modeling work, although potentially more precise, is still in need of additional
refinement. ZELIG results do show an upward movement of forest type^a)id a general decrease
in forest biomass. The model does not simulate low elevation or dry siWs'well. Upper elevation
simulations perform better, perhaps because temperature actually does becomes the limiting
constraint on forest growth as assumed by the model.
Expert judgement suggests that disturbance frequencies (fires and pest/pathogen outbreaks)
could increase under warmer conditions. Increased disturbance rates would speed the conversion
of forest types and decrease the biological inertia represented by long-lived trees. Even without
disturbance, the ZELIG results suggests that mature trees would dieback under the 2xCO2 climate
conditions simulated by GCMs.
Implications for Forest Management
Climatically induced changes of the magnitude predicted here raise a number of forest
management issues.
1). Given current rotation lengths and the possibility of substantial climate change over
the period of that rotation, what species should be planted now?
2). How much must current uncertainties in climate predictions and vegetation response
be reduced before irreverisble management practices predicated on climate change are
implemented?
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3). Are their "no regrets" management practices for adapting to climate change that can
be implemented without a substantial cost if climate change does not occur as expected?
4). How should current strategies at protecting endangered species (e.g. the spotted owl)
be altered considering the prospect of large changes in future climate?
5). How can forest management practices be altered to promote the sequestering of
carbon and slow atmospheric concentrations of CO2?
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ACKNOWLEDGEMENTS
Dean Urban from the University of Virginia and colleagues at the Pacific Northwest Lab of the
U.S. Forest Service supplied the ZELIG model and the initial 2xCO2 climate simulations through
Interagency Agreement DW12934129. Charles Halpern of the Department of Forest Sciences
at Oregon State University provided climate data for specific localities in western Oregon and
Washington. Terry Droessler at the Envrironmental Research Laboratory, Corvallis, performed
the 2xC02 simulations for Gold Beach and Rainier. The authors thank Don Phillips and Ron
Neilson for reading an earlier version of the manuscript.
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27
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Table la. Total forest land in the Pacific Northwest (USDA 1989). Data is for 1987.
State
Idaho
Oregon
Washington
Total
Total Total Timberland Reserved Other
Land Forest Timberland Forest
Area Area Land
(thousands of acres)
52692 21818 14533
61546 28057 22084
42483 21856 16848
156721
71731 53465
(290285 km2)
3051
1777
2765
7593
4234
4196
2244
10674
Table Ib. Timberland by ownership in the Pacific Northwest (USDA 1989). Data is for 1987.
State All Total Total Total Total Total
Ownerships Public Federal State Private Forest
Industry
(thousands of acres)
Idaho 14534
(percent)
OR 22085
(percent)
WA 16849
(percent)
Total 53468
(percent)
11435 10310
79 90
13706 12462
62 91
8652 5026
51 58
33934 27979
63 82
1036 3099
6 21
827 8379
6 38
2025 8197
23 49
3903 19734
12 37
1198
39
5114
61
4588
56
11000
56
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Table 2. Common tree species in the Pacific Northwest
Abies amabilis
Abies procera
Abies lasiocarpa
Abies grandis
Acer macrophyllum
Alnus rubra
Arbutus menziesii
Calocedrus decurrens
Castanopsis chrvsophylla
Chamaecvparis nootkansis
Larix occidentalis
Lithocarpus densiflorus
Picea sitchensis
Picea englemannia
Pinus monticola
Pinus lambertina
Pinus ponderosa
Pseudotsuga menziessii
Quercus chrvsolepis
Quercus garryanna
Thuja plicata
Tsuga heterophylla
Tsuga mertensiana
Silver fir
Noble fir
Subalpine fir
Grand fir
Big leaf maple
Red alder
Pacific madrone
Incense cedar
Golden chinkapin
Port-Orford-Cedar
Western larch
Tanoak
Sitka spruce
Englemann spruce
Western white pine
Sugar pine
Ponderosa pine
DouglaS-fir
Canyon live oak
Oregon white oak
Western red cedar
Western hemlock
Mountain hemlock
-------
Table 3. The comparison of the impact of climate change on mean annual temperature and growing degree/days.
The temperature is from historical records or calculated from the OSU and GISS GCMs while growing
degree/days is calculated from the Weather routine in the ZELIG simulation model.
Location
Forks, WA
Rainier, WA
Longview, WA
Astoria, OR
HJ Andrews, OR
Crater Lake, OR
Gold Beach, OR
Ashland, OR
Current
Temp. C
9.7
3.1
10.8
10.4
10.3
32
11.6
11.1
OSU
Temp. C
12.0
5.4
13.1
12.7
12.6
5.5
13.9
13.4
GISS
Temp. C
14.7
8.0
15.9
15.4
15.3
8.1
16.7
16.2
Current
Degree/Day
1584
511
2007
1773
1928
625
2225
2191
OSU
Degree/Day
2367
883
2794
2595
2645
984
NA
2933
GISS
Degree/Day
3340
1360
3774
3601
3663
1458
4066
3882
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Table 4. The comparison of the impacts of climate change on annual
precipitation and potential evaporation (PET). The precipitation is from
historical records or calculated from the OSU and GISS GCMs while PET is
calculated from the Weather routine in the ZELIG simulation model (Urban
1990, Urban et al. 1990) using the Thornthwaite relationship. The ratios for
precipitation and PET are GCM/Current condition.
Location
Forks, WA
Rainier, WA
Longview, WA
Astoria, OR
HJ Andrews, OR
Crater Lake, OR
Gold Beach, OR
Ashland, OR
Current
Prec. cm
302.3
289.3
1173
176.8
135.9
469.8
210.0
48.0
OSU
Ratio
1.45
1.45
1.45
1.46
1.46
1.46
1.47
1.46
GISS
Ratio
1.27
1.27
1.27
1.27
127
1.27
1.26
1.26
Current
PET cm
62.8
42.5
66.0
63.9
64.0
41.4
65.4
67.0
OSU
Ratio
1.10
1.16
1.10
1.09
1.10
1.18
1.09
1.12
GISS
Ratio
1.21
1.34
1.24
1.21
1.24
1.34
1.22
1.27
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Table 5. Areal extent of Holdridge life form classes before and after a double CO2 climate change. Area (thousands of km2) and
percent change (in parantheses) from current conditions are given for each life form catgory.
Life Form Category
Cold Desert
Hot Desert
Steppe
Chapparral
Boreal (Subapline)
Forest
Temperate Forest
Warm Temperate
Forest
Tropical Semi-Arid
Tropical Seasonal
Forest
Tropical Dry Forest
Total Forest Area
Current Area
77
0
215
4
130
186
2
0
0
0
318
OSU
69
(-10)
4
225
(5)
37
(757)
63
(-51)
223
(20)
15
(571)
0
0
0
301
-5
GISS
9
(-88)
6
185
(-14)
144
(3192)
26
(-80)
171
(-8)
95
(4310)
2
0
0
292
-8
GFDL
56
(-27)
23
170
(-21)
148
(3301)
15
(-89)
142
(-24)
78
(3521)
2
0
2
239
-25
UKMO
2
(-97)
15
111
(-49)
255
(5760)
0
(-100)
93
(-50)
110
(4986)
30
18
4
254
-20
-------
Table 6. Percent of area in various vegetation zones in the central Oregon Cascade Range (latitude 44°30' north) under current climate
and with increases of 2.5 and 5 degrees C (Franklin et al. in press).
Climate
Zone
Western Slopes
Nonforested
Quercus woodland
Pseudotsuga
Tsuga heterophylla
Abies amabilis
Tsuga mertensiana
Alpine
Total Forested
Cold Snow Zone1
Eastern Slopes
Artemisia steppe
Juniperus occidentalis
Pinus ponderosa
Abies grandis
Tsuga mertensiana
Alpine
Total Forested
Cold Snow Zone2
1 includes halt ot Abies amabilis zone and all of the
2 includes Tsuga mertensiana and Alpine zones.
Current
0
0
8
56
24
9
3
97
24
0
29
22
19
17
13
58
30
Tsuga mertensiana and
+ 2.5° C
0
8
39
38
13
2
0
92
9
51
14
12
14
6
3
32
9
Alpine zones.
+ 5.0° C
0
39
27
24
2
0
0
53
1
77
11
7
4
1
0
12
1
-------
Figure Captions
Fig. 1. Forest types in the Pacific Northwest (USDA 1970).
Fig. 2. Current and predicted mean monthly temperatures for three sites in the Northwest
Fig. 3. Current and predicted mean monthly precipitation for three sites in the Northwest
Fig. 4. Holdridge life-zone classification system (Holdridge 1967).
Fig. 5. Vegetation redistribution in the Pacific Northwest under the OSU climate scenario using the Holdridge life-zone classification
systems (Smith et al. submitted). In the map on the lower right, red depicts the pixels that change from one vegetation type to another.
Fig. 6. Vegetation redistribution in the Pacific Northwest under the GISS climate scenario using the Holdridge life-zone classification
systems (Smith et al. submitted). In the map on the lower right, red depicts the pixels that change from one vegetation type to another.
Fig. 7. Vegetation redistribution in the Pacific Northwest under the GFDL climate scenario using the Holdridge life-zone classification
systems (Smith et al. submitted). In the map on the lower right, red depicts the pixels that change from one vegetation type to another.
Fig. 8. Vegetation redistribution in the Pacific Northwest under the UKMO climate scenario using the Holdridge life-zone
classification systems (Smith et al. submitted). In the map on the lower right, red depicts the pixels that change from one vegetation
type to another.
Figure 9. Potential shifts in forest zones under two different climate warming scenarios (Franklin et al. in press).
Figure 10. A. Simulations of current forests at Cascade Head using CLIMACS. B. Potential changes in forest vegetation at Cascade
Head as simulated by the CLIMACS model (Dale and Franklin 1989). In both simulations, the model was initiated at year 140 using
the stand structure of plots at Cascade Head was used to initiate the model at year 140.
Figure 11. Potential changes in forest tree species abundance at Rainier, WA, HJ. Andrews, OR, and Gold Beach, OR as simulated
by ZELIG (Urban 1990. Urban et aL 1990, 1991). ABAM = Abies amabilis, ALRU = Alnus rubra, LTDE = Lithocarpus densiflora,
PSME = Pseudotsuga menziesii, ABGR = Abies grandis, CACH = Castanopsis chrysophylla, THPL = Thuja plicata, ABLA = Abies
lasiocarpa, CHNO = Chamaecyparis nootkatensis, PIMO = Pinus monticola, TSHE = Tsuga heterophylla, ABPR = Abies procera,
LBDE = Libocedrus decurrens, PIPO = Pinus ponderosa, TSME = Tsuga mertensiana.
-------
Douglas-fir
Bortock-Sitka spr
f:| Ponferosa pine
Hhile pine
Lodgepole pine
Lard)
Fir-spruce
Hanhoods
PiDyon-jmiper
Non-forest
FIGURE 1
-------
Rainier/Paradise, WA
Lat. 46.8, Lon. 121.7, Elev. 1654m
Mean Monthly Temperature - °C
30
25
20
15
10
5
0
-5
1 234887891011 12
Months
H.J. Andrews Forest, OR
Lat. 44.2. Lon. 122.2, Elev. 500m
Mean Monthly Temperature - °C
30
25
20
15
10
5
0
-5
Gold Beach, OR
Lat. 42.4, Lon. 124.4, Elev. 15m
Mean Monthly Temperature - °C
1 234587991011 12
Months
Figure 2
-------
Rainier/Paradise, WA
Lat. 46.78. Lon. 12173. Elev. 1654m
Mean Monthly Precipitation - cm
8789
Months
80
SO
40-
30
20
10
H.J. Andrews Forest, OR
Lat. 44.2, Lon. 122.2. Elev. 500m
Mean Monthly Precipitation - cm
12348878910t112
Months
Gold Beach, OR
Lat. 42.4. Lon. 124.4. Elev. 15m
Mean Monthly Precipitation - cm
Figure 3
-------
Latitudinal
beli:
0 1.5 r--
J •/ .„,• \ / -r- \ T- Vrf *O O
«-
_ -\- -. W I -. >• f r \. <^f -
line
FIGTOE 4
-------
FIGURE 5
-------
FIGURE 6
-------
FIGURE 7
-------
.i 1 -.!--• Ul -.(I' 1
.)! <\r\ !'->f^
FIGURE 0
-------
POTENTIAL VEGETATION CHANGE
IN THE OREGON CASCADES
CURRENT
+2.5UC
7-
.H
LJ
UJ
1-
OREGON
WHITE OAK
MOUNTAIN
HEMLOCK
DOUGLAS
FIR
ALPINE
WESTERN
HEMLOCK
+5.0 C
PACIFIC
SILVER FIR
MANTECH / ERL-C =
FIGURE 9
-------
WESTERN HEMLOCK
AND OTHERS
140 240 340 440 540
STAND AGE (y«ar»)
640
100
WESTERN HEMLOCK
AND OTHERS
140 240 340 440 540
STAND AGE (y«ari)
FIGURE 10 A & B
640
-------
H.J. Andrews, Gold Beach & Mt. Rainier
Species Relative Importance (%), Year 500
SPECIES
ALRU
[777] PSME
BOG
GBeach
OTHER
THPL
BOG
Rainier
rrrr^n
I////J PIMO
r jr A TSHE
MODEL
LOCATICN
ABPR
LBDE
PIPO
TSME
FIGURE 11
------- |