Richard A. Birdsey and John
L. Hom - Part 2
USDA Forest Service, NERS, Newtown Square, PA 19073
Monitoring and Predicting Regional Environmental
Information about historical climate is used in the analysis of
ecological data to develop models relating ecosystem processes and
responses to climate, and to develop analogs for future climate
scenarios at regional to local scales. NGCRP researchers have developed
techniques for modeling historical and current climate at specific
locations of interest to land managers, and can produce estimates
of precipitation and ionic deposition at any point in the NGCRP
region. We have the capability to project regional climate changes
by nesting a mesoscale climate model within a specific area covered
by much larger scale global climate models that are unable to resolve
important local features such as topography and large water bodies.
Responses of Northern Tree Species to Regional Stress
Our understanding of how and why individual trees respond to change
is inadequate because multiple interacting stresses may produce
responses that are considerably different from reactions to any
one factor. Since controlled experiments are limited in scope to
evaluation of a few factors and their interactions, we have linked
carefully selected experimental studies with models of physiological
mechanisms to predict the complicated tree responses more realistically.
Our ongoing objective has been to conduct comparative experiments
on immature and mature trees, with the eventual goal of conducting
experiments at the stand or ecosystem level.
We have featured research on the effects of chronic CO 2 and ozone
exposure, interacting with nutrient and moisture limitations. Key
responses include changes in carbon allocation, altered ecosystem
productivity, and effects on susceptibility to insect defoliation
and disease. Extrapolation of these results to the ecosystem level
and larger regional scales of interest to land managers and policy
makers is based on mechanistic models at different scales.
A sizable body of research indicates that deposition of nitrogen
and sulfur compounds via clouds, rain, snow, and wind is impacting
the health of some tree species in the northeastern United States.
Several NGCRP studies are advancing our understanding of the affects
of acid deposition on spruce physiology and cold tolerance. Additional
studies examine the effects of acid deposition on ecosystems through
changes in nutrient cycling.
Several long-term studies have been initiated or adopted by the
NGCRP to improve understanding of the physiological and genetic
mechanisms of tree resistance to stresses such as physical and biological
damage, disease, and water stress. An understanding of the mechanisms
by which trees adapt to rapid change is critical to planning for
climate change. Adaptation may be a significant alternate survival
strategy to migration for some species, and will surely have an
impact on future species composition, biodiversity, and the status
of sensitive ecosystems.
Responses of Ecosystem Processes to Regional Stress
To successfully manage forests in a changing environment we must
understand the complex dynamics of forest ecosystems as well as
the responses of individual trees. Unless we have long-term records
of environmental variables such as temperature, soil solution chemistry,
and vegetation responses such as tree growth and mortality, we have
no yardstick against which to measure our predictions about ecosystem
responses to environmental change or to test the results of our
models. The NGCRP has continued observations at seven intensive
research sites established during the National Acid Precipitation
Assessment Program (NAPAP). Measurements of climate variables, atmospheric
deposition, throughfall chemistry, and soil solution chemistry have
been made for up to six years. Continuing such measurements over
a long period of time can provide complete characterization of the
variability in deposition and mineral cycling rates, and their relation
to tree health.
Northern forest soils are susceptible to increases in the amount
of soluble aluminum as a result of acidic deposition. Elevated concentrations
of aluminum are toxic to roots and can inhibit the availability
of calcium, an important nutrient. Establishing a cause-effect link
between acid deposition, soil chemistry, and tree health was one
of the major challenges of the NAPAP program; however, there are
many factors affecting northern forest ecosystems and it became
extremely challenging to establish a direct causal link. Under NGCRP,
research has continued on these important soil-mediated effects
of acidic deposition. Several experiments have been established
to study how a changing climate may affect tree growth through alterations
in soil processes.
Forests in the North are dynamic and typically in a state of recovery
or succession following the last disturbance, whether timber harvesting,
land clearing and reversion to forest, or natural events such as
hurricanes. Understanding ecosystem responses to environmental change
requires a solid understanding of how disturbance impacts the processes
that govern successional change. NGCRP scientists are studying the
impacts of harvesting and natural disturbance on microclimate, species
succession, nitrogen saturation, cation depletion, and carbon dynamics.
Forest and Landscape Responses to Regional Stress
and Management Activities
A significant alteration in the mix of tree species, in the productivity
of forest lands, or in the health of existing forests could have
a substantial impact on aesthetic and commercial values of forests,
as well as wildlife populations and associated forest values. Understanding
how basic forest parameters are affected by environmental change
is the objective of a series of observational and modeling studies.
Where possible, long-term permanent plots have been relocated and
remeasured to detect changes associated with disturbance, acid deposition,
and climate. These data sets are used to both explain observed vegetation
characteristics, and to parameterize models so that predictions
about future changes over large areas can be made.
Some tree species and ecosystems are more sensitive to climate
and atmospheric chemistry than others, and are more rapidly affected
by change. The susceptibility of vegetation to change, and the rate
of change, will be influenced by the interaction of weather patterns
and such factors as soil chemistry, elevation, and land use. Studies
of vegetation history have shown that the occurrence of an individual
species or a whole biome is closely related to past climate changes.
It is inevitable that species distributions will shift, either from
climate change or other natural and human-induced environmental
Changes in disturbance patterns are likely to be one of the more
striking features of a changing climate. Studies of individual disturbances
and relationships to global change will eventually lead to a more
comprehensive understanding of the long-term role of disturbance
in shaping forest communities. At the landscape scale, the NGCRP
has focused disturbance research on drought and fire regimes, and
the possible role of global change in patterns and intensities of
these disturbances. The climate changes predicted by Global Climate
Change Models (GCMs) would also lead to altered patterns of insect
infestation. To anticipate these changes and their implications
for forest health and productivity, the NGCRP has undertaken research
on the potential effects of climate change on insect populations.
Human-Forest Interactions and Regional Change
A growing number of NGCRP studies address natural resource policy
issues and management in a changing environment. Estimating carbon
sequestration or release for managed forest ecosystems is crucial
to understanding the role of terrestrial ecosystems under changing
climates. This estimation requires a reliable assessment of the
quantities of carbon stored in various ecosystem components such
as the plant biomass, forest floor, and soil, along with a recognition
of spatial patterns of ecosystem carbon variability. Regional-scale
estimates must consider patterns of carbon variability and interactions
with environmental factors and land use change to accurately estimate
the effects of change on forests at landscape and regional scales.
The NGCRP includes a series of studies designed to quantify how
carbon in forested landscapes changes over time. Attempts to quantify
the role of Northern forests in the global carbon cycle, and to
understand the effects of alternate management activities on carbon
storage, have been hampered by a lack of quantitative information.
These studies are intended to fill this knowledge gap.
An Integrated Model of the Effects of Global Change
on US Forests
Environmental change could be rapid relative to the ability of
a species to adapt or migrate. The capability to project successional
change is particularly important because rapid environmental change
could induce forest health problems during a transitional change
from one vegetation type to another. Significant problems with forest
health would in turn affect forest resource use and the people dependent
on the multiple resource values of forests, from the timber industry
to the subsistence user.
NGCRP scientists are participating in the development of a national
integrated model of global change effects on forests. The integrated
model combines submodels of the physical, biological, and social
systems. Climate models at global and regional scales, and hydrologic
models represent major physical systems. Several different models
of ecosystem change are being developed and evaluated at different
temporal and spatial scales. The human dimensions are partially
captured with econometric models of the forest sector, which can
project land use change, harvesting activity, and impacts of change
on the forest products industry.
Development and application of the integrated model are based on
models used to conduct national assessments required by the Resources
Planning Act (RPA). The integrated model provides analyses of the
effects of scenarios of global change on forests, and can be used
as a tool for evaluating alternate policy responses to projected
changes. Initial efforts are focused on improving projections of
potential forest vegetation distribution, forest ecosystem composition,
forest growth, and the national carbon budget. As capability to
project vegetation changes improves, the modeling system will be
extended to project changes in other forest system attributes such
as biodiversity and wildlife habitat.
An ongoing synthesis of basic forest statistics and development
of carbon accounting models has highlighted the past and prospective
role of US forests in the global carbon cycle and provided input
to policy decisions regarding the effects of alternate strategies
for offsetting greenhouse gas emissions through forestry actions.
The US carbon budget model (FORCARB) predicts carbon in major forest
components: trees, understory vegetation, the litter layer and coarse
woody debris, and soils. Although still under development, early
versions of FORCARB have provided input to national decisions regarding
the effects of alternate policies for offsetting greenhouse gas
emissions through forestry actions. Using statistics from a nationwide
inventory of trees and data from site-specific studies, the model
makes continental-scale projections of the carbon that would be
released and/or sequestered by various management activities that
could result from national policy decisions.
Analyses at the national scale may obscure important regional changes
that are likely to occur in specific ecoregions. There is a need
to develop integrated models at the regional scale that are optimized
for the specific domains under study -- large watersheds, river
basins, multi-state areas, economic regions, and the like. At the
same time, there is an opportunity to establish feedbacks between
national- and regional-scale models so that information developed
at the national scale provides the context for regional assessments,
and regional studies (that may contradict national studies) can
be used to verify national-scale results or investigate possible
regional effects in more detail. Model comparisons at different
spatial and temporal scales are important to understanding the capabilities
and limitations of models that are becoming widely used in assessments.
Boreal Forests and Global Change
Boreal forests play a major economic, social, and ecological role
in the global environment. Circling the northern latitudes, boreal
forests occur within the borders of Russia, Canada, the United States
(Alaska), Finland, Norway, and Sweden. With an area of 920 million
hectares they comprise 29 percent of the world's total forest cover.
Boreal forests may be the single largest terrestrial carbon sink,
with an estimated 40 billion tons of the world's stored carbon in
Siberia's forests alone.
Boreal forest range and health are closely tied to prevailing climate
conditions. Current projections indicate that global warming will
be detected earlier and most strongly at high latitudes. NGCRP scientists
are part of a cooperative international effort to study boreal forests
and the implications of global change for this critical ecosystem.
Summary: Part 1