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PREDICTING FOREST CHANGES
ASSOCIATED WITH CLIMATE
WARMING: POTENTIAL USES
OF GIS TECHNOLOGY
David L. Verbyla
With CO2-induced climate warming, similar vegetation
zone migrations in latitude and elevation are expected
(Emmanuel and others 1985; Payette and Filion 1985).
Where should permanent plots be established to monitor these shifts in vegetation zones? If permanent plots
are randomly established, and are therefore mostly on
"normal" sites (near the center of a species' distribution),
the effect of climate warming might not be detectable for
decades or centuries.
However, permanent plots established at the hot or cold
extremes ofa vegetation zone would be more sensitive to
immediate climate warming. For example, at its northern
distribution limit, white spruce (Picea glauca) has expanded significantly in response to recent climate warming (Payette and Filion 1985). To monitor the early effects of climate warming, a method is needed to efficiently
delineate the "hot" and "cold" extremes of vegetation
zones.
ABSTRACT
Research is needed to understand potential local forest
responses to climate warming. Geographic Information
Systems (GIS) technology can be used to establish permanent plots at hot or cold extremes of vegetation zonesareas where immediate effects of climate warming may
be detectable. Empirical studies using GIS technology
and solar radiation indices can be used to test hypotheses
of expected forest changes associated with recent climate
warming. However, because ambient carbon dioxide is
likely to increase and may ameliorate the effects of climate
warming on forests, long-term predictions from climate
warming studies should be interpreted with caution.
INTRODUCTION
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Most research that predicts forest changes associated
with climate warming has been regional or global studies
using computer simulation models (for example, Davis
and Botkin 1985; Dale and Franklin 1989; Pastor and
Post 1988). However, much of our forest management
occurs at the forest or ranger district level. How will
climate warming affect soil and vegetation processes
at this scale? For example, will Armillaria root disease
within a Douglas-fir habitat type become a more serious
problem if the climate warms and drought stress in
Douglas-fir increases? Will mountain pine beetle in lodgepole pine decline because of a less favorable (hotter)
microclimate, or will lodgepole pine trees become more
stressed and therefore more susceptible to beetle attacks?
Will nitrification rates significantly increase in sprucefir habitat types if the climate warms? The objective of
this paper is to outline an empirical approach aimed at
addressing such research questions.
PREDICTING CHANGES
ASSOCIATED WITH CLIMATE
WARMING
How will forest insect and disease problems change
with climate warming? How will soil properties and processes change? These types of questions can be addressed
by examining current conditions within stands sampled
from a range of "cool" to "hot" sites.
For example, Amman and others (1988) hypothesized
that microclimate is a principle factor controlling mountain pine beetle (Dendroctonus ponderosae) infestations
in lodgepole pine (Pinus contorta). Will mountain pine
beetle infestations in lodgepole pine become less severe
if the climate becomes hotter? Geographic Information
Systems (GIS) can be used to efficiently select sample
stands from "cool," "normal," and "hot" sites within the
lodgepole pine series. Trends in mountain pine beetle
density could then be examined across the continuum
of "cool" to "hot" lodgepole stands and predictions could
be made about future beetle infestations in response to
climate warming.
How can GIS be used to rank stands as "cool," "normal,"
or "hot" sites? Digital Elevation Models (DEM) can be
purchased from the U.S. Geological Survey for certain
areas of the Rocky Mountains. IfDEM data are not available, they can be derived by digitizing contours from topographic maps. They also can be generated photogrammetrically with an analytical stereoplotter. Given a DEM, the
POTENTIAL CHANGES IN
VEGETATION DISTRIBUTION
In the past, forest communities have shifted in response
to climate warming during the Pleistocene and Holocene
periods (Baker 1983; Van Devender and Spaulding 1979).
Paper presented at the Symposium on Management and Productivity
ofWestero-Montane Forest Soils, Boise, ID, April 10-12, 1990.
David L. Verbyla is Visiting Assistant Professor, Department of Forest
Resources, University ofIdaho, Moscow, ID 83843.
193
The effect of increased CO2 on ecosystem changes is
poorly understood and yet is important in predicting
forest and soil changes that may occur due to climate
warming. For example, tree growth may actually increase at some sites due to the fertilization effect of increased carbon dioxide. The growth rates of subalpine
trees in Nevada and California have been reported that
exceed growth rates expected due to climatic trends but
is consistent with increased carbon dioxide concentrations
(LaMarche and others 1984).
Increased carbon dioxide may also ameliorate the effects of water stress in some plant species. For example,
Tolley and Strain (1984) found that sweetgum (Liquidambar styraciflua) seedlings exposed to water stress and
grown at elevated CO2 conditions had final dry weights
significantly greater than seedlings grown under wellwatered and normal CO2 conditions. Hurt and Wright
(1976) found similar results with knobcone pine (Pinus
attenuata) and Coulter pine (Pinus coulteri).
Despite these problems, research is desperately needed
for rational forest management in the event of climate
warming. Until now, much of the climate warming research has been on a global and regional level. We need
to begin research on the local forest level. GIS technology
and radiation indices are tools that can be used at this
local level address the question: what trends can we expect in our forests if the climate becomes significantly
hotter in the 1990's?
potential solar radiation of any site can be computed
as a function oflatitude, slope gradient, slope azimuth
(aspect) and Julian day (Flint and Childs 1987; Garnier
and Ohmura 1968; Harrington 1984; Kaufmann and
Weatherred 1982; Lee and Baumgartner 1966; Swift
1976). Ea~h stand can then be assigned a "radiation
index" value by integrating potential solar radiation
over the water-limited season. Stands can be selected
while controlling for other important factors, for example,
parent material, age class, and species composition.
Therefore, changes in radiation indices among stands
are assumed to be the dominant factor that influences
the microclimate of each stand. This is similar to comparing stands from north-facing ("cool") and south-facing
("hot") slopes where each stand is similar in terms of parent material, age class, and species composition.
Potential solar radiation has been used successfully in
many diverse areas including prediction of the distribution of frozen soils (Zuzel and others 1986), prediction of
rock glacier development (Hassinger and Mayewski 1983),
prediction of watershed runoff yield (Lee 1964), and vegetation ordination studies (Dargie 1984; Parker 1989).
The GIS approach has several advantages. Stands can
be efficiently selected to control for other confounding
factors. For example, it is easy with a GIS to select all
stands from a Douglas-fir series, on limestone parent
material, with a certain basal area and age class. Second,
this approach of using a radiation index to rank stands is
an empirical approach that can be used to test hypothesis
generated by theoretical computer simulation models.
Third, this empirical approach can be used to address
research questions that would take decades to answer
with controlled experimentation.
ACKNOWLEDGMENTS
I thank Brian Clark, Tom Lee, and Pete Wolter for
reviewing the manuscript and offering constructive
suggestions.
POTENTIAL PROBLEMS
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Many technical problems need to be resolved. Potential
solar radiation varies daily. What is the appropriate
period to integrate radiation over-the entire growing
season, the period of maximum drought, or the entire
year? East- and west-facing slopes receive the same
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