United States 

Department Atmospheric and Biospheric Interactions

of Agriculture 

Forest Service of Gases and Energy in the Pacific Region 

Pacific Southwest of the United States, Mexico, and Brazil 

Research Station 

General Technical Report 

PSW-GTR- 161 
Publisher: 

Albany, California 

Mailing address:

PO Box 245, Berkeley CA 

94701-0245 

510 559-6300 

http://www.psw.fs.fed.us 

April 1997 

Pacific Southwest Research Station 

Forest Service 

US Department of Agriculture

Abstract 

Bytnerowicz, Andrzej, technical coordinator. 1997. Atmospheric and biospheric 

interactions of gases and energy in the Pacific region of the United States, Mexico, 

and Brazil. Gen. Tech. Rep. PSW-GTR-161 Albany, CA: Pacific Southwest Research 

Station, Forest Service, U.S. Department of Agriculture; 42 p. 

Anthropogenic activities of the past century have caused a dramatic increase in global 

air pollution. This process has accelerated in the past few decades, and emissions of carbon 

dioxide, nitrogen oxides, or chlorofluorocarbons caused serious changes in the earth's 

climate, e.g., increased temperatures or elevated ultraviolet-B radiation. These changes, 

together with more severe droughts, forest fires, and air pollution (ozone, nitrogen or 

sulfur compounds), may have pronounced effects on terrestrial ecosystems. Changes in 

global and regional carbon and nitrogen cycles as well as changes in water resources and 

cycling have also taken place. The current and predicted atmospheric and biospheric 

interactions of gases and energy in the Pacific Region of the United States, Mexico, and 

Brazil are described in this document. The role and status of simulation modeling for 

weather predictions, production and transport of smoke from biomass burning, and air 

pollution uptake by forest canopies are discussed. To meet growing needs for 

environmentally sound forest management, priorities for research on air pollution, forest 

fire effects, nutrient cycling, water resources, and development of models are listed. 

Retrieval Terms: air pollution, climate change, forests, nutrient cycles, plant responses, 

simulation modeling 

Technical Coordinator 

Andrzej Bytnerowicz is Ecologist and Project Leader-Atmospheric Deposition Effects in 

Western Forest Ecosystems research unit at the Station's Forest Fire Laboratory, 4955 

Canyon Crest Drive, Riverside, CA 92507. 

Acknowledgments 

We thank Laurie Dunn for technical editing and Lola Thomas for word processing. 
Atmospheric and Biospheric Interactions of Gases 

and Energy in the Pacific Region of the United States, 

Mexico, and Brazil 

Andrzej Bytnerowicz, Technical Coordinator 

Contents: 

In Brief .............................................................................................................. ii 

Introduction......................................................................................................... I 

Andrzej Bytnerowicz, Sue Ferguson, and Mark Poth 

Part I : Synthesis of the Current State of Knowledge about Atmospheric and 

Biospheric Interactions ............................................................................ 3 

Chapter 1. Exchanges of Gases and Aerosols Between Atmosphere 

and Terrestrial Ecosystems ...................................................................... 5 

Nancy Grulke, Paul Miller, Roger Ottmar, Mark Poth, and Philip Riggan 

Key Greenhouse Gas and Aerosol Emissions ....................................................... 5 

Forest Fire Emissions of Radiatively Important Trace 

Gases and Aerosols ................................................................................... 8 

Emissions of Other Anthropogenic Air Pollutants ................................................ 10 

Chapter 2. Nutrient Cycles and Energy Flows ................................................. 13 

Andrzej Bytnerowicz, Mark Fenn, Sue Ferguson, and Nancy Grulke 

Global Carbon Cycle and Carbon Resources of the Western Forests ..................... 13 

Global Nitrogen Cycle and Nitrogen Deposition to Western Forests ....................... 15 

Changes in Streams, Snow, and Permafrost Caused by Changing Climate ............ 16 

Chapter 3. Plant Responses to Climate and Air Pollution Changes ................. 21 

Mark Fenn, Nancy Grulke, and Paul Miller 

Changing CO2 Levels ....................................................................................... 21 

Increased Nitrogen Deposition .......................................................................... 22 

Other Air Pollutants and UV-B Radiation ............................................................ 23 

Part II: Simulation Modeling: Role and Status ................................................. 27 

Andrzej Bytnerowicz, Sue Ferguson, Francis Fujioka, Roger Ottmar and 

Mark Poth 

Regional Scale Weather Models ........................................................................ 27 

Production and Transport of Smoke Pollutants from Biomass Burning Models ........ 30 

Models Simulating Uptake of Gaseous Pollutants by Forest Canopies .................. 30 

Part III: Research Needs for the Pacific Region .............................................. 33 

Andrzej Bytnerowicz, Paul Miller, and Mark Poth 

References....................................................................................................... 35 

Pacific Southwest 

Research Station 

USDA Forest Service 

General Technical 

Report PSW-GTR-161 

April 1997 
In Brief... 

Bytnerowicz, Andrzej, technical coordinator. 1997. Atmospheric and biospheric 

interactions of gases and energy in the Pacific region of the United States, Mexico, and 

Brazil: Gen. Tech. Rep. PSW-GTR-161. Albany, CA: Pacific Southwest Research Station, 

Forest Service, U.S. Department of Agriculture; 42 p. 

Retrieval Terms: air pollution, climate change, forests, nutrient cycles, plant responses, 

simulation modeling 

ncreased concentrations of air pollutants or "greenhouse" gases (carbon dioxide, 

methane, chlorofluorocarbons, nitrous oxide, and others) in the earth's atmosphere 

inc I 

rease air temperature as well as deplete the stratospheric ozone layer causing elevated 

ultraviolet-B radiation-which may be deleterious to sensitive plants, animals, and 

humans. Anthropogenic activities, such as human-caused forest fires, are major sources of 

greenhouse gases, as well as the use of fossil fuels that cause increased emissions of air 

pollutants such as nitrogen and sulfur oxides and ozone formation which may be toxic to 

various forms of life. 

Elevated emissions of carbon dioxide, methane, and nitrogen oxides affect global 

cycles and pools of two elements of major biological importance: carbon and nitrogen. 

However, other human activities, especially harvesting of old-growth forests in the 

Pacific Northwest of the United States or in the Brazilian Amazon, may affect global 

carbon resources even more. Whether boreal forests will continue to be a carbon sink (an 

absorber of increasing amounts of carbon dioxide) will depend on the extent of land use 

conversion from forests to other uses, as well as the degree of global warming and the soil 

temperature and moisture status of these forests. Elevated nitrogen deposition, among 

other effects, may lead to shifts in species composition in severely affected ecosystems. 

For instance, grasses may expand to other species' habitats in various ecosystems 

worldwide. Increased emissions of nitrogen may also cause some beneficial effects, but 

only in the short term. More available nitrogen in soil may initially improve vegetation 

growth and increase the capability of forests and other ecosystems to sequester increasing 

amounts of carbon dioxide. 

Climatic changes may also affect water availability in the Pacific region of the United 

States. Increased temperatures could cause a significant reduction in snow cover, melting of 

glaciers and permafrost, and changes in stream flow. As a consequence of these 

changes various effects can be predicted, such as increased surface warming (less solar 

energy reflected from the snow-covered earth surface) or reduction of water availability 

during the vegetation growing season. These effects may cause major changes in western 

ecosystems and agricultural activities in the west. 

Interactions between atmospheric and biospheric processes are very complex, and 

various responses of ecosystems to the occurring changes can be expected. Some of the 

expected changes due to elevated concentrations of carbon dioxide could be increased 

yields of some plant species, altered susceptibility of plants to drought, and shifts in species 

composition of natural ecosystems. Some of the effects of the increasing nitrogen emissions 

and deposition may include changes in plant growth rates and susceptibility to air 

pollutants and other environmental stresses. 

To predict the effects of air pollutants on forest ecosystems, simulation modeling plays 

a key role. Although the absolute results of these models (i.e., geographically specific 

"predictions" or "forecasts") should be viewed with great caution, they are one of the few 

tools available for assessing the most sensitive components of the environment. Some 

models that have provided the most useful results include regional scale weather models, 

models simulating production and transport of smoke pollutants from biomass burning, 

models simulating uptake of gaseous pollutants by forest canopies, and terrestrial 

ecosystem biogeochemical models. By using these models, forest managers can be provided 

with useful data to help them implement the best environmental policy decisions, such as 

regimes of prescribed burning, fertilization, or timber harvesting. Future research needs will 

include assessment of emissions of trace gases from forests, better understanding of a 

relationship between nitrogen and carbon global and regional cycles, evaluation of nitrogen 

deposition to forests, etc., which will also provide new information to help managers with 

revising environmental policy. 

ii USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Introduction 

Andrzej Bytnerowicz,

1

 Sue Ferguson,

2

 and Mark Poth1 

Because of human activities, such as the use of fossil fuels and prescribed 

forest fires that cause unprecedented rates of global climate change, the 

earth's average surface temperatures are expected to increase from 1.5 to 4.5°C 

before the end of the 21st century. Although the magnitude and rate of expected 

climate change will not be steady, this continued warming by increased 

concentrations of air pollutants or "greenhouse" gases (carbon dioxide, methane, 

chlorofluorocarbons, nitrous oxide, and others) will be enhanced because of cold 

periods caused by a number of internal and external climate forcings. These 

should be short-lived, however, compared to the greenhouse gas contribution to 

warming (Intergovernmental Panel on Climate Change 1990, 1992). 

Cess and others (1995) found disagreement between the measured and 

predicted absorption of solar radiation by clouds, indicating a very high degree 

of uncertainty in predicting the rate of the expected warming. Clouds have 

been shown to reduce net solar radiation at the sea surface more efficiently than 

previously believed-much higher amounts of radiation are trapped by water 

contained in clouds than current models would predict (Ramanathan and 

others 1995). On the basis of revised simulation models, future climate changes 

may be less dramatic than previously thought. However, research has shown 

that the future climate should be warmer, with less precipitation and slower 

surface winds (Kerr 1995). 

Because of this inability to predict the exact rate of change, precisely how 

regional climates will change is unknown. However, studies have shown that 

due to a consistent poleward migration of atmospheric moisture, and the normal 

latitudinal distribution of incoming solar radiation, most regional changes in 

climate are expected to occur first over land areas in the northern latitudes 

(Ferguson 1995, Hare 1988, McBeath 1984, Weller 1993). This means that most 

climate change in the Pacific states of the United States is expected to occur first 

in interior Alaska, with less change elsewhere. Small changes in climate, however, 

can cause dramatic biological responses in areas with steep elevation gradients, 

like the mountainous western United States (Barbour and others 1991). 

Interactions between climate/ atmospheric chemistry and ecosystems in 

the United States Pacific region determine if current natural landscapes will 

remain intact (U.S. Congress 1993). Unlike farm land or other areas in which 

management can radically and quickly adapt to changing conditions, natural 

landscapes cannot be managed (Intergovernmental Panel on Climate Change 

1990). For instance, climate and atmospheric deposition are important 

variables that affect ecosystem processes such as primary production or 

decomposition. These processes directly affect climate and atmospheric 

systems by modifying fluxes of water and various greenhouse gases. Such 

effects may result in immediate changes in climatic properties. This difficulty 

is because of the complexity of interactions between atmospheric and 

biospheric processes as a result of various time and spatial scales of their 

individual components (fig. 1, Intro) (Ojima and others 1991). 

Various responses of ecosystems to these changes and interactions may be 

expected. In many cases the strategy of the key ecosystem components is 

endurance (plant surviving a drought) or avoidance (wildlife leaves for better 

areas). Because of the very complex interactions between all facets of ecosystems 

over very long time scales, the ability to manage physically and biologically 

complex natural landscapes is limited for typical successional time scales (50 to 

300 years for ecosystems that include woody plants). 

1

Ecologist and soil microbiologist, 

respectively, Pacific Southwest 

Research Station, USDA Forest 

Service, 4955 Canyon Crest Dr., 

Riverside, CA 92507. 2

Atmospheric scientist, Pacific 

Northwest Research Station, 

USDA Forest Service, 4043 

Roosevelt Way NE, Seattle, WA 

98105. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 1 
Figure 1-Spatial and temporal 

representation of (a) major climatic 

processes and (b) biospheric 

processes (Ojima and others 1991). 

Reprinted with permission from D.S. 

Ojima and others: Critical issues for 

understanding global change effects 

on terrestrial ecosystems. Ecological 

Applications 1(3), p. 319. Copyright 

1991, Ecological Applications. 

Given the expected changes, the future of natural areas and forests may be 

at stake. Hence the scope of our work is both geographically and biologically 

wide ranging. Our region, which reaches from the tundra to the tropics, 

includes unique biological resources. Many species such as giant sequoia and 

bristle cone pine hold special ecological value. Other large areas hold both 

ecological and economic importance, such as the commercial forests of the 

Pacific Northwest. 

This report describes the state-of-the-science studies of the Pacific Global 

Change Research Program of the Pacific Southwest Research Station on the 

effectiveness of ecosystem management practices and simulation models to 

monitor greenhouse gases, nutrient cycles, carbon cycles, and energy flows in 

the atmospheres and biospheres of the Pacific region of the United States, 

Mexico, and Brazil. 

2 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Part I 

Synthesis of the Current State of Knowledge 

about Atmospheric and Biospheric 

Interactions 
Chapter I 

Exchanges of Gases and Aerosols Between 

Atmosphere and Terrestrial Ecosystems 

Nancy Grulke,

1

 Paul Miller,1 Roger Ottmar,2 Mark Poth,

1

 and Philip Riggan1 

Emissions of certain air pollutants (often called "greenhouse gases and aerosols") 

have pronounced effects on global temperatures, the amount of ultraviolet (UVB) radiation reaching the earth, and the cycles of carbon and nitrogen. Forests 

and other terrestrial ecosystems may serve as sources and sinks for these 

pollutants. Rates of exchange of gases and aerosols between atmosphere and 

terrestrial ecosystems depend on many biotic and abiotic factors. Climate 

warming, forest fires, nutritional status of soil, or application of various 

management practices may be listed as some of the most important factors in this 

regard. Primary air pollutants, such as hydrocarbons and nitrogen oxides, can 

serve as precursors for formation of secondary pollutants, such as ozone, 

peroxyacetyl nitrate, nitric acid or nitrate aerosols. These secondary pollutants 

may have direct toxic effects on plants, animals, and humans, or may significantly 

affect forest health and productivity. 

Key Greenhouse Gas and Aerosol Emissions 

Although consequences of increasing greenhouse gases on the terrestrial 

ecosystems are still not very well-defined, there is no doubt that their 

concentrations in the atmosphere are increasing. In the atmosphere, the major 

carbon constituents that contribute to greenhouse warming are, among others, 

carbon dioxide (CO2), methane (CH4), and chlorofluorocarbons (CFCs) (fig. 1, 

Chapter 1). These gases have an important role in global carbon cycle and can be 

of natural and anthropogenic origin. 

From pre-industrial times to 1990, the atmospheric CO2 concentration has 

increased 26 percent (Watson and others 1990), and currently this gas contributes 

Figure 1 -The relative contribution 

of the major greenhouse gases to 

global warming during the 1990's 

(Ashmore 1990). 

1

Plant physiologist, plant pathologist, soil microbiologist, and soil 

scientist, respectively, Pacific 

Southwest Research Station, 

USDA Forest Service, 4955 Canyon Crest Dr., Riverside, CA 

92507. 2

Atmospheric scientist, Pacific 

Northwest Research Research 

Station, USDA Forest Service, 

4043 Roosevelt Way NE, Seattle, 

WA 98105. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 5 
Figure 2-Concentrations of atmospheric CO2 measured from the 

Mauna Loa observatory, Hawaii. 

Reprinted with permission from L.G. 

Simpson and D. B. Botkin: Vegetation, 

the Global Carbon Cycle, and Global 

Measures. In: Dunnette, David A.; 

O'Brien, Robert J., eds. The Science 

of Global Change: The Impact of 

Human Activity on the Environment. 

ACS Symposium Series 483. 

Washington, DC: American Chemical 

Society. Copyright 1992, American 

Chemical Society. 

to about 50 percent of the warming. The effect of individual gases on radiation 

balance may by calculated from a knowledge of the atmospheric concentrations 

of the gas and its rate of increase, the relationship between concentrations and 

radiation absorption of that gas, and the extent to which its absorption bands 

overlap those of other gases. However, it should be emphasized that the 

presented values of contribution of individual gases to global warming are only 

estimates (Ashmore 1990). 

Concentrations of atmospheric CO2 measured at the Mauna Loa 

observatory in Hawaii from the late 1950's to 1990 reflect these changes (fig. 2, 

Chapter 1). Similar trends of increasing CO2 concentrations are predicted for 

the near future. However, the carbon-to-nitrogen ratio (C:N) is expected to 

vary throughout the next century depending on the mix of energy source 

chosen. More nitrogen in the emissions can produce a cofertilization effect in 

the terrestrial biomes, which would lead to sequestration of additional carbon. 

The uncertainty in the C:N ratio will create uncertainty in the measurements 

of atmospheric concentrations of CO2 as high as 20 ppm (Keller and Goldstein 

1994). The major natural sources of CO2 are decomposition, respiration, 

wildfires initiated by lightning, volcanic emissions, as well as inorganic 

sources such as exchanges between substrates rich in carbonates and water 

(Solomon and others 1985). Anthropogenic sources of CO2 include fossil fuel 

burning, changes in land use (high C density ecosystems converted to urban 

or agricultural uses with lower C density, low or no C storage, and net C loss 

to the atmosphere) (Kurtz and Apps 1992), and slash pile and field burning 

(very rapid carbon release back into the atmosphere, but short-term increase 

in nutrient status) (Ashmore 1990). The major sinks for CO2 are photosynthetic 

uptake by plants and CO2 absorption within oceans (about 17 percent and 15 

percent, respectively, of the atmospheric pool). Atmospheric residence time 

is about 50 to 230 years (Bolin 1986), and the rate of increase is 0.4 percent or 

1.5 ppm per year (Keeling 1983). One-sixth of the increase of atmospheric CO2 

concentrations is attributable to deforestation (Anderson 1992). The more 

recent estimates show 7.3 to 9.2 Gt C yr

-1 as a sum of global emissions. Out of 

that figure, 5.8 to 6.2 Gt C yr

-1 is attributed to fossil fuel burning and 1.5 to 3.0 

Gt C yr

-1 to tropical deforestation (Gifford 1994). 

Methane is the most abundant, reactive trace greenhouse gas (Ehhalt 

1985, Khalil and Rasmussen 1990). The major biogenic sources include 

wetlands and tundra ecosystems, termites, and by-products of digestion. 

Anthropogenic sources include various agricultural practices such as 

formation and maintenance of rice paddies, domestic ruminants, biomass 

burning, natural gas exploitation, and landfills (Ashmore 1990, Ehhalt 1985). 

The major sink for methane is atmospheric reaction with hydroxy (OH) 

6 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
radicals. The atmospheric residence time of methane is 10 years, and the 

current rate of increase is 0.9 percent or 0.015 ppm per year (Blake and 

Rowland 1988, Ehhalt and others 1983). 

One of the largest globally significant ecosystem sources of methane are 

rice paddies (70-170 x 109

 kg CH4/yr). The Sacramento River Delta in 

California is one of the largest centers of rice production in the United States. 

However, the boreal and arctic biomes are other wet ecosystems that 

contribute globally significant sources of methane (40-110 x 109

 kg CH 4/yr). 

Cattle alone contribute 49-70 x 109

 kg CH4/yr. Biomass burning contributes 

20-110 x 109

 kg CH4/yr, and methane emissions from wildfires in the subpolar 

regions, Pacific Northwest of the United States, and the tropical savannas are 

also likely to contribute at a globally significant level (Ehhalt 1985). Emissions 

of methane may increase because of increases in isoprene emissions. 

According to the doubling of the CO2 scenario, the isoprene emissions would 

be about 25 percent higher than the current emissions mainly because of the 

expansion of tropical humid forests that have the highest annual emission 

rates (Turner and others 1991). 

Chlorofluorocarbons (CFCs) are the most rapidly increasing greenhouse 

gas compounds (Prinn 1988). They are important because they react with and 

destroy stratospheric ozone. Some of the species of CFCs are stable long enough 

to act as global atmospheric tracers. Most of the CFCs are anthropogenic in 

origin and include aerosols, refrigerants, and foam packaging (the exception is 

methyl chloride). After release into the atmosphere, there is a 1- to 3-year delay 

for net transfer to the stratosphere where CFCs are chemically broken down 

(Rowland and Molina 1975). The residence time varies for different species, but 

is in the range of 6 to 170 years (Prinn 1988). The current rate of increase is 4 

percent, or 0.015 ppb per year (Ashmore 1990). 

The best estimates of all the sources and sinks of nitrous oxide (N20), a long-lived 

greenhouse gas, leaves about 30 percent of the observed increase unaccounted for 

(Keller and others 1986, Khalil and Rasmussen 1992, Matson and Vitousek 1990). The 

global budget for nitric oxide (NO), a more reactive gas important in tropospheric 

photochemistry and ozone formation, is similarly out of balance (Davidson 1991). 

Forest soil may be the origin of some of the N trace gases; according to Anderson and 

others (1993), forest soil microbes responsible for the production of nitrous oxide 

produce at a much higher rate than their agricultural soil counterparts (fig. 3, Chapter 1) 

We know that ecosystem soils can respond to disturbance (fire, N additions) by 

increasing nitric oxide and nitrous oxide emissions (Anderson and Poth 1989, Castro 

and others 1993). This suggests that emissions from forest soils may be significant and 

that proper management practices should be applied to minimize these emissions. 

Comparison of Trace Gas Production: Figure 3-Nitrous oxide production 

Forest vs. Agricultural Soil Bacteria by microorganisms in forest soils and 

agricultural soils (Anderson and 

others 1993). 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 7 
Forests and wildlands can serve as sou

rces and sinks for N

2O, CO2

, and CH4 

depending on how these lands are managed. The Pacific Global Change Research 

Program (PGCRP) at the Pacific Southwest Research Station, USDA Forest 

Service, has been gathering baseline data on current rates of exchange in 

undisturbed forests and related ecosystems; this information is a reference point 

for future comparisons. In addition, for ecosystems that have demonstrated 

regional as well as potentially significant global contributions of trace gases, 

researchers in the PGCRP have begun to evaluate the influence of management 

on trace gas exchange. Typical forest management practices such as timber 

harvesting (Anderson and others 1993) and forest fertilization (Matson and 

others 1992) directly influence carbon storage and cycling, as well as the 

emissions of greenhouse gases from forest soils, as a result of stimulating nitrogen 

availability or physically changing soil water availability or temperature regimes. 

Thus, the choice of land management practices determines the amount of 

emissions. In some cases, offsite, long-term storage in forest products can offset 

the C loss from forest ecosystems. This effect is greatest when the forest products 

are long-lasting and forest regeneration rates are fast (Kurtz and Apps 1992). 

Forest Fire Emissions of Radiatively Important 

Trace Gases and Aerosols 

The emitted greenhouse gases affect the earth's radiation balance. Wildland and 

agricultural fires may now be contributing to climate change depending on their 

global extent and the strength of their emissions. Best available estimates are 

considerably uncertain but do show that biomass combustion is likely a primary 

anthropogenic source of atmospheric aerosols, methane, carbon monoxide, carbon 

dioxide, and some nitrogen oxides (Crutzen and Andreae 1990, Radke and others 

1991). Yet fire is a natural process in many of the world's terrestrial biomes, and 

thus only a portion of contemporary emissions can contribute to climate change. 

Fires of human origin could be important in the net flux of radiation-absorbing 

trace gases if, over the long term, they reduce the amount of carbon accumulated in 

standing biomass or soils or if they emit a higher proportion of CH

4, 

CO, or N2O 

than would alternative processes of organic matter decomposition. Anderson and 

Poth (1989) found that burning increases production of CO

2

, NO, and N

2O from 

ponderosa pine, chaparral soils, and tropical savanna. Trace gas flux is a function 

of soil mineralization and other processes that respond to fire intensity and 

frequency (Dunn and others 1979, Fenn and others 1993, Poth and others 1995). 

Various aspects of the effects of fires have to be considered when the budget of 

emissions of radiatively important gases is calculated. However, forest fires may 

reduce net production of these gases if total emissions resulting from fires are 

lower than emissions from natural decomposition processes in forests. 

Emissions of particles higher than those found in natural fire regimes 

could well dominate and compensate for the radiative effects. A high 

proportion of smoke particles are active as cloud condensation nuclei (CCN) 

(Hobbs and Radke 1969, Radke 1989, Radke and others 1991, Rogers and 

others 1991), and their entrainment in clouds can produce more numerous 

droplets of smaller diameter, thereby increasing cloud albedo and possibly 

liquid water content and cloudiness. Combined with their direct radiative 

effects, aerosols from biomass combustion could be responsible for a global 

cooling as high as 2 watts m-2, comparable in mag

nitude and opposite in sign 

to the radiative effects expected from increases of greenhouse gases during 

the 20th century (Penner and others 1991, 1992). 

Current estimates of global fire extent, and thereby emissions, are inexact 

and come from uneven reporting of fire occurrence and from calculations based 

on growth of the human population and land use (Seiler and Crutzen 1980). They 

8 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
do show that tropical savanna and deforestation of primary tropical forests are 

likely the greatest global sources of fire emissions (Crutzen and others 1985, 

Seiler and Conrad 1987). 

Emission rates of NOx, HCN (hydrogen cyanide), and CH3CN (acetonitrile) 

from biomass burning are sufficient to contribute significantly to the global 

budgets of carbon and nitrogen. Possibly, half of the biomass nitrogen can be 

converted to molecular nitrogen, N2, leading to an estimated annual loss of 12-

28 x 1012 g of biomass N (pyrodenitrification) equal to 9 to 20 percent of the 

estimated global rate of terrestrial nitrogen fixation (Lobert and others 1990). 

Cicerone (1994) has also shown that NO, NO2, and CO can produce ozone in 

troposphere downwind from burning sites, which is also found in polluted urban 

air after a series of photochemical reactions. We have found that tropical fires are 

a strong source of methyl bromide, which is important in the destruction of 

stratospheric ozone, and have made the first quantitative, synoptic measurements 

of radiant energy release from wildland fires (Riggan and others 1993). 

The PGCRP is supporting an assessment of the role of tropical fires in global 

change through a cooperative program with the government of Brazil (figs. 4, 5, 

Chapter 1). The scientists of the Pacific Southwest Research Station, USDA Forest 

Service, and the National Aeronautics and Space Administration (NASA) Ames 

Research Center in California are working with the government of Brazil and 

other United States agencies to assess the extent and global environmental 

impacts of widespread burning in central Brazil. 

Figure 4-By using a specialized fire 

remote sensing system, fire spread 

can be mapped and emissions of 

greenhouse gases and smoke particles 

important in climate change can be 

estimated. Recent fire scars are in 

red and several active fire fronts are 

shown in white in a portion of 

Pantanal, Brazil, one of the world's 

largest wetlands. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 9 
Figure 5-The USDA Forest 

Service is working with Brazilian 

scientists and fire fighters to improve 

and demonstrate techniques in fire 

management and to restore a more 

natural role for fire in Brazilian 

national conservation areas 

(photograph by Philip Riggan). 

Emissions of Other Anthropogenic Air Pollutants 

Climate warming is expected to have an influence on the natural and anthropogenic 

emissions of primary air pollutants including nitrogen oxides (NOx), non-methane 

volatile organic compounds (NMVOC), and sulfur oxides (SOx) (Melillo and others 

1993). NOx and SOx emissions may result from the fuel combustion associated with 

increased energy needs for air conditioning and refrigeration if current energyproducing technologies are continued. NMVOC's will increase because of more 

rapid volatilization of petroleum fuel and solvents, as well as from increased 

emissions of isoprene and terpenes from vegetation (Knox and Scheuring 1991). 

The formation of secondary pollutants, including ozone and the peroxyacyl 

nitrates, is temperature dependent (Smith and Tirpak 1990). Currently, a 

correlation can be observed between air temperature and ozone concentration 

particularly in urban or near-urban atmospheres (Kuntasal and Chang 1987). Most 

forested areas where sensitive species now show symptoms of ozone injury are 

influenced by near-urban atmospheres. The enlargement of urban centers expected 

with population growth will probably exacerbate the problem. In the western 

United States, examples of rapid growth and accelerated air pollution problems 

can be found in Seattle, Washington, and the California Central Valley (Bakersfield, 

Fresno, and Sacramento). The forests in the adjacent mountain ranges (Cascades, 

western Sierra Nevada) are predicted to experience negative effects of that growth. 

The climatic feature most associated with prolonged pollution episodes is the 

formation and persistence of the high pressure systems, including the Pacific High 

affecting the west coast of the United States in summer and the Subtropical High 

Pressure Belt influencing central Mexico in winter. These systems will be more 

frequent and persistent, particularly in coastal areas because temperature contrast will 

be higher between the near-surface stable marine layer and warm overlying 

continental air. Recent comparisons of the annual cycle of ozone formation near Los 

Angeles and Mexico City have shown that warmer winters near Mexico City result in 

higher winter-month ozone levels. Conversely, the hotter, rain-free summer months 

near Los Angeles result in ozone concentrations higher than those seen near Mexico 

City in the wet summer season (fig. 6, Chapter 1) (Miller and others 1994). 

10 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Figure 6-Precipitation and ozone 

concentrations in two mountain 

locations: Desierto de los Leones near 

Mexico City, and Crestline, San 

Bernardino National Forest near Los 

Angeles (Miller and others 1994). 

"Precipitation" graph reprinted with 

permission from Atmospheric 

Environment, volume 28, Paul R. Miller, 

Maria de Lourdes de Bauer, Abel 

Quevado Nolasco, and Tomas 

Hernandez Tejeda, "Comparison of 

ozone exposure characteristics in 

forested regions near Mexico City and 

Los Angeles," pages 141-148, 1994, 

with kind permission from Elsevier 

Science Ltd, The Boulevard, Langford 

Lane, Kidlington 0X5 IGB, UK. 

The change to warmer prevailing air temperatures and lower rainfall could 

lengthen the season during which severe episodes of photochemical smog may be 

expected. This may be partly caused by increasing emissions of isoprene from the 

forests (Turner and others 1991). A similar scenario has been presented for western 

Europe: large-scale, ground-level ozone concentrations could reach 0.10 ppm with a 

possibility of peak values exceeding 0.15 ppm (Feister and Warmbt 1987). 

Warmer temperatures will also increase the oxidation rate of NO2 and SO2 to 

nitrate and sulfate forms; thus, dry deposition of aerosols to forest canopies may 

increase, which could significantly affect nutritional status of the forest plants. Lowvolume rain events in air masses from heavily polluted airsheds will continue to 

result in hydrogen ion deposition causing immediate changes in the acidity of some 

alpine lakes and gradual changes in poorly buffered alpine soils (Brown and Lund 

1991, Melack and Stoddard 1991). Furthermore, the increased amount of sulfate haze 

will further impair visibility in areas where scenic vistas are the major attraction. 

Sulfate haze from the troposphere may contribute to a layer of haze now being 

monitored in the upper atmosphere. This layer is believed to be transparent to 

incoming solar radiation but tends to inhibit night-time reradiation of long-wave 

radiation from the troposphere. This phenomenon may reinforce the greenhouse 

effect (Hofmann 1990). 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 11 
Chapter 2 

Nutrient Cycles and Energy Flows 

Andrzej Bytnerowicz,1 Mark Fenn,

1

 Sue Ferguson,2 and Nancy Grulke1 

Carbon and nitrogen are the key elements for any form of life on the earth. These two 

elements are also essential constituents of various air pollutants affecting global 

climate. Better knowledge of carbon and nitrogen global cycles, including 

understanding of their mechanisms and quantification of their flows, is needed for 

proper planning of management practices in terrestrial ecosystems. For the same 

reason a sound knowledge of the climate-caused changes in distribution and rates of 

water resources, including streams, snow, and permafrost, is also critically needed. 

Global Carbon Cycle and Carbon Resources 

of Western Forests 

Carbon is an essential element for all living organisms. The fixations of carbon 

dioxide (CO2) by plants through geologic time have resulted in the presence of 

oxygen in the earth's atmosphere that sets the oxidation potential for the entire 

planet. Through oxidation and reduction processes the cycles of other elements are 

closely tied to the global cycles of carbon (Schlesinger 1991). The largest fluxes of 

the current carbon cycle are those linking the atmospheric CO2 to terrestrial 

vegetation and the oceans (fig. 1, Chapter 2). The carbon contained in forest 

ecosystems globally has been estimated to be equivalent to about one-half of 

Figure 1-Current carbon cycle. 

All pools are expressed in units of 

1015 g C and all annual fluxes in units of 

1015 g C/yr (Schlesinger 1991). 

Reprinted with permission from 

William H. Schlesinger: Biogeochemistry-An Analysis of Global 

Change. San Diego, CA: Academic 

Press, Inc. Copyright 1991, 

Academic Press. 

1

Ecologist, plant pathologist, and 

plant physiologist, respectively, 

Pacific Southwest Research Station, USDA Forest Service, 4955 

Canyon Crest Dr., Riverside, 

CA 92507. 2

Atmospheric scientist, Pacific 

Northwest Research Station, 

USDA Forest Service, 4043 

Roosevelt Way NE, Seattle, WA. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 13 
(Simpson and Botkin 19

92), two-thirds of (Jarvis 1989, Schlesinger 1991, Watson 

and others 1990), or even two times (Woodwell 1991) the carbon contained in the 

atmosphere. The predicted climate warming and the accompanying increase in 

mean soil temperature could accelerate terrestrial emissions of CO

2

. Soil respiration 

and N mineralization have been shown to increase with soil temperature when 

adequate water is available (Alexander 1977). A 4 °C increase in air temperature 

increases the CO2 efflux fro

m upland tundra soil by 26 percent (Oberbauer and 

others 1986). Increased atmospheric CO2 concentra

tions and higher air 

temperatures are expected to be accompanied by changes of summer soil wetness 

(Manabe and Wetherald 1986). Billings and others (1983) found that lowering the 

level of the water table alone changed the net carbon balance of a coastal wetland 

soil from a net sink (+119 g CO

2 m-2) to a net source (-476 g CO2 m-2) 

during a 

growing season. Carbon budget response of forests to future global change 

scenarios is speculative because responses of mature trees to CO

2 enrichm

ent and 

the effects of temperature on terrestrial sources and sinks of CO2 hav

e not been 

determined (Luxmore and others 1993). 

Arctic tundra has changed from a sink to a source of CO

2

. Approximately 

one-third of terrestrial organic carbon is stored in northern peatlands (boreal 

forests and Arctic tundra) (Post and others 1982). The large C stores are believed 

to have accumulated during the early Holocene (10,000 B.C.) when the climate 

was warmer than at present. Currently, net ecosystem CO2 balance is negative in 

coastal wetlands, and increasingly positive with soil drainage (Oechel and others 

1993). Increasingly drier soils are expected with global warming in these northern 

ecosystems (Manabe and Wetherald 1986), which could further exacerbate global 

warming (Billings and others 1983). Carbon is also being exported to aquatic 

ecosystems (Hobbie 1980): as the underlying permafrost melts, surface subsides, 

and water runoff increases, the areas of lower topography may accumulate water 

and thus become sources of methane. Modeling of soil chemical and physical 

properties will help to predict the extent that this biome will become a net 

emitter of greenhouse gases (Reynolds and Leadley 1992). 

The estimate of the net flux between the biosphere and atmosphere is 

particularly uncertain and has been currently a matter of considerable 

controversy as to whether the biosphere is a source of CO2 or a sink for CO2 

(Jarvis 1989). This controversy may not be resolved until the sizes of the 

vegetation carbon pools and associated carbon fluxes are better assessed. A 

proper actual and potential role of vegetation in the global carbon cycle requires 

accurate information about carbon storage and change over time (Simpson and 

Botkin 1992). Recent ecosystem-scale nutrient studies indicate that 1.0 to 2.3 Gt C 

yr-1 of carbon storage ma

y be stimulated by anthropogenically caused increases 

in nitrogen deposition in the 20th century (Hudson and others 1994). Calculations 

suggest that northern continents are a major sink for carbon and that nitrogen-

stimulated carbon uptake may more or less balance global carbon losses to the 

atmosphere from deforestation and agriculture (Schindler and Bayley 1993). 

The most important factor affecting carbon resources of the Pacific states of the 

United States is harvesting of old-growth forests. Mass balance calculations indicate 

that the conversion of old-growth forests to second-growth plantations west of the 

Cascade Mountains in the past 100 years has added 1.5 to 1.8 x 1012 kg C to the 

atmosphere (Harmon and others 1990). Boreal forest ecosystems contain 13 percent 

of the terrestrial biomass (Whittaker and Likens 1975) and also 13 percent of the 

earth's terrestrial organic soil carbon (Post and others 1982). Whether boreal 

ecosystems will continue to be a carbon sink will depend on the degree of global 

warming and soil moisture changes (Oechel and Billings 1992), as well as the extent 

of harvesting and management practices in boreal ecosystems. 

Another source of carbon resources are the tropical savannas. The conversion of 

these lands to agricultural fields and the frequency and extent of anthropogenic 

fires in this (Riggan and others 1993) and other ecosystems is a significant source of 

14 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
carbon release to the atmosphere. Olson (1981) summarized carbon release as a 

function of mean frequency of average fires per year for about 25 different 

ecosystems. If these estimates are correct, nonfossil carbon release by fires is at least 

slightly below the 5 x 1012 kg C yr

-1 currently released by fossil fuel consumption. 

Chaparral, temperate marshes, boreal forest and mixed conifer forests all had 

comparable high carbon release (103 kg available carbon per square meter) with 

relatively low burn frequency (every 10-100 years). The ecosystem with one of the 

highest carbon release rates was the understory of giant sequoia, which also had 

one of the most frequent burn rates. However, this comparison did not differentiate 

wildfire from anthropogenically caused fires (Olson 1981). 

Global Nitrogen Cycle and Nitrogen Deposition 

to Western Forests 

The atmosphere is the largest pool (3.8 x 1021 g N) in the global nitrogen cycle 

and relatively small amounts of N are found in terrestrial biomass (3.5 X 1015 g) 

and soil organic matter (95 x 1015 g) (fig. 2, Chapter 2). The mean C:N ratios for 

terrestrial biomass and soil organic matter are about 160 and 15, respectively. - The pool of inorganic N in the form of NH 4

+

 and NO3 on land is very small. In 

general, the transformations of N in the soil and uptake of N by organisms are so 

rapid that little N remains in inorganic form, despite a large annual flux through 

this pool (Schlesinger 1991). 

Concentrations of N air pollutants in a dry form (mainly as NO2, HNO3, NH3, - - particulate NO3 and NH 4

+

) and NO3 and NH 4

+ in wet precipitation in the Southern 

California Air Basin have been elevated (Blanchard and Tonnessen 1993, Brewer 

and others 1983, Grosjean 1983, Russell and others 1985) for at least 40 years. 

Although, in general, little is known of the concentrations of N pollutants in the 

mountains surrounding the Southern California Air Basin, recent studies suggest 

Figure 2-The global nitrogen 

cycle. Pools and annual fluxes are 

expressed in 1012 g N (Schlesinger 

1991). Reprinted with permission 

from William H. Schelesinger: 

biogeo-chemistry-An Analysis of 

Global Change. San Diego, CA: 

Academic Press, Inc. Copyright 1992, 

Academic Press. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 15 
that some of the mixed coniferous forest locations in the San Bernardino Mountains 

(Fenn and Bytnerowicz 1993) as well as the chaparral stands of the San Gabriel 

Mountains (Bytnerowicz and others 1987a, 1987b, Grosjean and Bytnerowicz 1993) 

may receive as much as 30 kg N ha-1 yr-1. These values are among the highest 

reported in North America and may lead to changes in internal cycling and export 

from the watershed. - Elevated levels of NO3 and NH 4

+ deposition also occur in western Sierra 

Nevada forests in California and are about six- and four-fold higher than in the 

eastern Sierra Nevada locations (fig. 3, Chapter 2) (Bytnerowicz and others 1991, 

1992). Although the absolute values of dry deposition of N in the western Sierra 

Nevada were lower than in the Southern California Air Basin, the concentrations 

of reduced N compounds (NH3, particulate NH 4

+

) were higher in the Sierra 

Nevada sites (Bytnerowicz and Riechers 1995). 

In recent years much has been learned about the chemistry of wet 

precipitation in California (Blanchard and Tonnessen 1993). Estimates of wet and 

dry N deposition for California based on the California Acid Deposition 

Monitoring Program network have also been attempted. Although estimates of 

Figure 3-Atmospheric dry deposition 

+ of anions and cations, including NH4 

- and NO3 , is much higher in the western 

(Emerald Lake) than in the eastern 

(Eastern Brook Lake) subalpine Sierra 

Nevada sites. Elevated levels of 

deposition in the western Sierras are 

caused by the long-range transport of 

the polluted air masses from the 

California Central Valley (Bytnerowicz 

and others 1991, 1992). 

wet deposition in the Southern California Air Basin bear uncertainties of less 

than 20 percent, the estimates for the northern and southeastern portions of 

California are much less accurate (as high as 100 percent uncertainties). Estimates 

of dry deposition based on the California Acid Deposition Monitoring Program 

database are limited in many respects (mainly because of the limitation to only 10 

sites in the network) and are subject to uncertainties as high as 50 percent 

(Blanchard and Michaels 1994). For other western states, only sparse results on 

wet and especially dry deposition are available. 

Changes in Streams, Snow, and Permafrost 

Caused by Changing Climate 

Most useable water in Pacific coastal regions of North America comes from the 

mountain snowpack. This important resource is strongly influenced by climate 

change. For example, a typical moist adiabatic lapse rate for mid-latitude winter 

16 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Figure 4-The snow line of the mountain near the Columbia River Basin (just above the central valley in early spring) could 

shift up near the ridge crest because of a minor degree of winter warming, causing a severe decrease in snow cover 

(photograph by Sue Ferguson). 

is about 4 °C/km (Barry and Chorley 1987). This means that a 2 °C increase in 

winter-averaged temperature could raise the seasonal snow line by about 500 

meters. According to current estimates of snow cover in the Columbia River 

Basin in the Cascade Mountains, a 500-meter rise in the snow line would cause a 

44 percent reduction of snow cover (fig. 4, Chapter 2) (Fleagle 1991). This could 

cause a significant reduction in water resources and play a dramatic role in 

shifting ecological gradients in steep terrain, especially in the Sierra Nevada and 

Cascade mountains where the snow pack already shows a strong response to 

climate change (Cayan 1996). 

Loss of snow cover not only reduces water supply, but produces a positive 

feedback to atmospheric warming. Seventy-five to 90 percent of incoming solar 

radiation is reflected back into the atmosphere by snow and ice. On the other 

hand, only 25 to 50 percent is reflected by soil and vegetation (Berry 1981). The 

rest is absorbed and re-radiated at longer wave-lengths, then trapped by the 

near-surface atmosphere. This feedback mechanism allows for small initial 

changes in snow cover to significantly increase land cover absorption of heat, 

which adds to the original heating. The most dramatic change in snow albedo 

feedback and subsequently greatest change in surface temperature is expected in 

central and northern Alaska where large areas of snow cover and sea ice could 

disappear completely (Ferguson 1995). 

Large areas of the northern latitudes are covered by permafrost. In Alaska, 

north of the Brooks Range, some of the permafrost was formed thousands of 

years ago (Piexoto and Oort 1992). This is a region of continuous permafrost that 

is several tens to several hundreds meters deep. The top meter is an active layer 

that melts during the summer. Current winter temperatures in northern Alaska 

average between -20 °C and -30 °C, and continuous permafrost can persist at 

average annual temperatures as high as -7 °C (Ives 1974, Untersteiner 1984). 

Therefore, even the most substantial increase in global temperatures should 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 17 
cause little d

egradation in continuous permafrost (Osterkamp 1984). The active 

layer, however, may deepen (Maxwell 1992) and could refreeze more slowly 

during autumn and thaw more quickly during spring (Ferguson 1995). 

Unlike continuous permafrost, regions of discontinuous permafrost, like 

interior Alaska, should be strongly affected by expected global warming 

(Osterkamp 1984). Melting permafrost in this region should cause dramatic 

increases in marsh and lake areas (Ferguson 1995). 

The amount of water held in glacier ice in the Pacific region of North America 

equals or surpasses all the water in the lakes and streams. Nearly all glaciers in 

the United States exist in the Pacific region. Alaska is covered by 75,000 km2

 of 

glacier ice; 75 percent of glaciers in the contiguous United States are in Washington 

State; Oregon and California also have glaciers, mainly on the volcanic peaks 

(Ferguson 1991). Water flowing from Alaskan glaciers contributes more than 

186,000 billion liters of water into the stream-flow system (Meier 1985). 

Glacier-fed streams exhibit peak flows during mid-summer. This contrasts to 

snow-fed streams that usually peak during spring. The delayed discharge 

provides a more continuous and reliable water source in many coastal rivers 

(Fountain and Tangborn 1985). 

Advancing glaciers can choke rivers and streams with silt or glacier flour. 

This makes a rather inhospitable habitat for many plants and animals. Retreating 

glaciers, on the other hand, contribute little silt and have higher discharge rates 

(Ferguson 1991). Most mountain glaciers in the Pacific region have been 

retreating during the past 100 years and are expected to continue retreating 

during the next 100 years (fig. 5, Chapter 2) (National Research Council 1984). 

Many of the small glaciers, especially in California, could completely disappear. 

Larger glaciers that accelerate their retreat should produce greater stream 

discharges with less silt. Glacial retreat also may increase hazards in mountainous 

areas. Retreating ice may expose unstable slopes, increasing mass wasting from 

rock- and landslides and debris flows. Temporary dams formed by ice or 

moraines may fail, producing highly destructive debris-laden floods and flows 

with potential to travel long distances. 

Changing climate can significantly affect both the seasonality (i.e., timing), 

total volume, and variability of runoff in streams. These effects can be quite 

pronounced and immediate where both rainfall and seasonal snowpacks 

contribute to runoff. Under these circumstances, relatively small changes in 

temperature or precipitation amount or type can dramatically influence the 

amount of water delivered to streams at different times of the year. For example, 

in the western Cascades, streams receive runoff from a rain-dominated zone 

below 400 m elevation, a "transient snow zone" between 400 and 1,400 m where 

snow falls but snowpacks tend to melt rather quickly during subsequent rainfall, 

and a seasonal snow zone above 1,400 m. With as little as a 2 °C warming, the 

lower boundary of the rain-on-snow zone may become as high as 1,000 m, 

effectively eliminating rain-on-snow events from some watersheds and changing 

the location and timing of melting of snowpacks in other basins (Swanson and 

others 1992). These changes will have consequence to the overall distribution of 

runoff within the year, such as reducing late-season low flows in basins where 

winter snowpacks have been depleted. Such changes may increase summer 

water temperatures, further stressing aquatic ecosystems (Hicks and others 1991). 

The central and northern Sierra Nevada, and south-central and south-east 

Alaskan mountains, where similar snowpack fluctuations exist, could experience 

similar effects. 

In southern California, where a large percentage of streams currently are 

ephemeral, many streams could completely disappear if warmer temperatures 

and/or decreased precipitation were to occur. In addition, the currently perennial 

streams in this area may become ephemeral under a significant climate change 

(Peter Wohlgemuth, personal communication). 

18 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Figure 5-Most glaciers in the Pacific 

mountains of the Northwestern 

United States have been retreating 

over the last 100 years and are 

expected to continue retreating during 

the next 50 to 100 years. Small glaciers 

like these valley and cirque glaciers 

could disappear completely with 

continued global warming (photograph 

by Sue Ferguson). 

Permafrost also affects streamflow. Permafrost-dominated catchments 

exhibit higher peak flows, lower base flows, and steeper recessions than adjacent 

permafrost-free basins (Hilgert and Slaughter 1987, Slaughter and others 1983). 

Therefore, decreasing permafrost areas could significantly alter streamflow 

regimes, especially in central Alaska where discontinuous permafrost areas 

could be disrupted significantly by changing climate. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 19 
Chapter 3 

Plant Responses to Climate and 

Air Pollution Changes 

Mark Fenn,1 Nancy Grulke,1 and Paul Miller1 

Changing climate may have significant effects on terrestrial plants. Increasing 

levels of CO2, elevated air and soil temperatures, diminished availability of 

water, elevated deposition of nitrogen, as well as direct phytotoxic effects of 

various air pollutants and UV-B radiation are the major factors in the presently 

observed and predicted changes in terrestrial ecosystems. 

Changing CO2 Levels 

Kimball (1983) compiled data from 430 studies of different crops grown at 

ambient and twice ambient CO2 levels and found that yields at elevated CO2 

concentrations were increased by an average of 30 percent. However, in natural 

ecosystems, response to elevated CO2 atmospheres is much more variable than in 

the agricultural systems. An intact salt marsh community dominated by a sedge 

yielded a prolonged, net production increase of 88 percent in a doubled ambient 

CO2 atmosphere (Drake and Leadley 1991). In contrast, exposure of intact tundra 

initially resulted in a net increase of 6 g C m-2 d-1, which was lost by the third year 

of the exposures (Oechel and others 1994). Evidence of acclimation to elevated 

CO2 was apparent within 3 weeks of the elevated CO2 exposure in a separate 

experiment with intact tundra (Grulke and others 1990). Physiological downregulation of photosynthetic capacity (acclimation) has been noted in a number 

of long-term CO2 exposure studies (Cure and Acock 1986, Delucia and others 

1985, Grulke and others 1993, Pearcy and Bjorkman 1983, Tissue and Oechel 

1987). The capacity of some plants to acclimate to elevated CO2 concentrations is 

an important factor to include in assessments of global carbon budgets because it 

may eliminate a source of buffering to the atmospheric increase. Elevated CO2 is 

also known to reduce stomatal aperture of some species, but tends to provide 

beneficial effect to others by increasing water use efficiency in the warmer, less 

humid global climate that is expected (Bazazz and Fajer 1992). For instance, in at 

least one study, there was an apparent "acclimation" of stomatal conductance to 

an elevated CO2 environment (Grulke and others 1993). If this study is correct and 

generalized, the findings will have significant repercussions for atmospheric water 

balance predictions. Ongoing research in the tropical rain forest is the only known 

study of elevated CO2 exposure of an intact ecosystem (Charles Korner, personal 

communication). 

Simulation models show that elevated CO2 and the subsequent changes in 

plant transpiration and gas exchange will increase sensitivity of forest 

communities to climate change (Woodward 1990). Temperature extremes or 

extended droughts could induce widespread declines of some forest trees known 

as "lead species": these are the first to fail and drop out of the community 

because of expected perturbations caused by fire, insects, and diseases (Auclair 

and others 1992). One of the important mechanisms starting the decline of tree 

species is xylem cavitation injury resulting from fluctuations in water availability 

1

Plant pathologist, plant physiologist, and plant pathologist, 

respectively, Pacific Southwest 

Research Station, USDA Forest 

Service, 4955 Canyon Crest 

Dr., Riverside, CA 92507. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 21 
and temperature during the winter. Winter injury and weakening of trees by 

extended droughts makes them more susceptible to killing from insect pests like 

bark beetles. Furthermore, warmer temperatures are expected to increase the 

number of generations and spread of insect pests, especially herbivorous species, 

in a single year (Cammell and Knight 1992). 

Elevated CO2 concentrations may change leaf area of plants-some authors 

predict increase of leaf area index (LAI) (Taylor and others 1994), while others 

foresee an opposite trend (DeLucia and others 1994). Assuming the first scenario, 

for those pollutants that are conserved and for which continental landscapes are 

a sink (e.g. Hg°, N pollutants), increases in LAI would proportionally increase 

deposition at the forest stand and catchment level (this condition is especially 

true for the pollutants in which deposition is independent of stomatal 

conductance, e.g. gaseous HNO

3) (Taylor and o

thers 1994). 

Elevated levels of CO2 can also have a positive feedback on soil C and N 

dynamics, producing greater N availability (Gifford 1994, Zak and others 

1993). For instance, a buildup of non-structural carbohydrates in foliage (and 

litter), followed by increased C exudation from the roots can result in higher 

availability of easily metabolized C in the soil (stimulating faster rates of 

mineralization of "old" organic C) and hence increased soil N mineralization 

(Oechel and Strain 1985). 

Increased Nitrogen Deposition 

Northern temperate forests have been traditionally considered N-limited. 

However, recent studies in the eastern United States and Europe confirm the 

growing concern that chronic N deposition can lead to the contrasting condition of 

excess available N, with potentially undesirable effects on forest ecosystems 

(Aber and others 1989, Driscoll and Van Dreason 1993, Emmett and others 1993, 

Hultberg and others 1994, Kahl and others 1993, Stoddard 1994). Nitrogen 

saturation of a forest has been variously defined, but basically refers to an 

ecosystem that cannot retain and process additional inputs of N, or when 

vegetation no longer shows a positive growth response to further N additions. Nitrogen-saturated forests are characterized by persistent loss of excess N either - as NO3 leached beyond the rooting zone or into streamwater, and/or as trace N 

gas emissions from soil denitrification and nitrification (Aber and others 1989, 

Stoddard 1994). Nitrogen saturation is of particular concern to ecosystem 

management because of the potential contamination of streamwater or 

- groundwater with NO3 , and the possibility of inc

reased emissions of trace N 

gases such as nitrous oxide (N

2

O) and nitric oxide (NO). High concentrations of 

-

NO3 in soil are believed to greatly increase denitrification rates and emissions of 

N greenhouse gases (Aber 1992, Matson and others 1992, Melillo and others 1989). 

Nitrogen saturation in forest ecosystems of the western United States can 

occur as a result of natural sources of excess N. Red alder (Alnus rubra) stands in 

Washington State are N saturated as a result of biological N fixation (Johnson 

and others 1991). Dahlgren (1994) also reported the occurrence of a naturally 

occurring N saturation in the Klamath Mountains in northern California as a 

result of weathering of ammonium from mica schist bedrock. 

However, the main source of the increasing levels of available nitrogen in 

these forests is from anthropogenic emissions. For instance, some of the chaparral 

ecosystems in southern California appear to have developed N saturation, - illustrated by the high concentrations and fluxes of NO3 in streamwater from 

grassland and chaparral watersheds exposed to high N deposition in the San 

Gabriel Mountains (Riggan and others 1985, 1994). Burning of chaparral stands - exposed to chronic N deposition has mobilized N and resulted in NO3 

22 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
concentration

s in streamwater as high as 69 mg L-1 (Riggan and others 1994). 

- High NO3 efflux from chaparral watersheds does not appear to b

e typical, since 

burned and unburned chaparral watersheds in Arizona (Longstreth and Patten 

1975), and in low-pollution areas of the Southern California Air Basin, did not 

- have high NO3 concentrations in streamwater (Rigg

an and others 1985). 

Similarly, a number of factors suggest that some mixed conifer stands in 

the western, highly polluted area of the San Bernardino Mountains are N 

saturated. Nitrate concentrations in soil solution were several-fold higher 

throughout the growing season in high deposition sites than in low deposition 

sites. High trace N gas emissions and high soil and foliar N concentrations, low 

C:N ratios in soil, high N:P (phosphorus) ratios in foliage, and a decreased 

foliar biomass in response to N fertilization have been observed at Camp 

Paivika, a high-pollution site in the San Bernardino Mountains (Fenn and 

others 1996). Furthermore, during the past 18 years C:N ratios and soil pH 

values have decreased considerably at Camp Paivika, but changed little or not 

at all at Camp Osceola, a low-pollution site in the San Bernardino Mountains 

(Fenn and others 1996). Although N saturation effects seem to be occurring in 

the highest N deposition sites, more widespread forested areas may benefit 

from a N fertilizer effect which is not visibly evident (Aber and others 1989). 

Critical loads of N for forest ecosystems are difficult to determine because they 

are highly site specific, varying on the basis of current and past forest and soil 

conditions (Cole and others 1992, Emmett and others 1993, Feger 1992). 

However, ecosystems in the Southern California Air Basin exposed to as much 

as about 20-30 kg N ha-1 yr-1 exhibit symptoms of N saturation based on loss of 

excess N from the ecosystem, suggesting that the critical load for these 

ecosystems may be <20 kg N ha -1 yr -1. 

Other Air Pollutants and UVB Radiation 

Air pollution potential effects in a changed global climate have been predicted to 

be higher if the increases in temperature materialize (Taylor and others 1994). 

Air pollution effects will be mixed with or modified to some extent by other cooccurring variables or stresses. The occurrence of stress agents in both time and 

space will exhibit considerable variability (Miller 1993). This variability will be 

manifested by stresses acting either simultaneously or sequentially and by a 

patchiness of stress effects on the landscape. Heterogeneity is further enhanced 

because of the vast latitudinal range over which stresses may interact including 

north of the Arctic Circle in Alaska (70° N) to parts of Brazil below the equator at 20° S. Temperature changes by year 2050 over this vast region may increase by an 

average 2.5°C over the Northern Hemisphere and from 6 to 10°C between 50° 

and 60° N (Manabe and Wetherald 1975, Mitchel 1983). If this scenario is valid, 

this temperature range introduces more uncertainty into the interaction among 

stresses. 

The emphasis on single pollutants and acute effects has been replaced by 

recognition that most terrestrial ecosystems are not affected by one pollutant, 

that exposure and deposition are often chronic, and that effects in natural stands 

are largely the result of air pollution interactions with the suite of other 

environmental stresses (Taylor and others 1994). Plant species will be affected by 

a number of stresses either simultaneously or sequentially. Some of the more 

important stresses include effects of the higher CO

2 concentr

ations, increased 

UV-B radiation, extended droughts, temperature extremes, and increased 

damage from insects and diseases. For instance, for the past 40 years or so 

elevated concentrations of ozone (Miller and Ryan 1977) and increased deposition 

of nitrogen (Fenn and Bytnerowicz 1993) have interacted with changing climatic 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 23 
conditions in the San Bernardino Mountains in southern California, resulting in a 

combination of environmental stresses that are only partially understood. 

UV-B radiation flux through the troposphere is increasing particularly at 

high elevations (Blumenthaler and Ambach 1990) because of the gradual decay 

of the stratospheric ozone shield by chlorofluorocarbons. This decay has caused 

a great potential for various biological effects of the increasing UV-B radiation 

and its interactions with the increasing levels of CO2 and ozone (Krupa and 

Kickert 1993). However, the cuticles of conifers and other alpine species are not 

transparent to UV-B radiation (Day 1993, Sullivan and Teramura 1989), so effects 

would probably be seen first in species occupying lower elevations. In addition, 

because the tropospheric ozone acts as a shield for UV-B, ozone damage should 

not be expected to occur simultaneously. Therefore, no synergistic effects of 

ozone and increasing UV-B radiation levels on coniferous trees in the mountains 

of the western United States are expected. 

Natural hydrocarbon emissions from plants can contribute to ozone 

formation, causing concentrations that exceed ozone standards (Chameides and 

others 1988). The rates of emission of isoprene (Tingey and others 1979) and 

monoterpenes (Tingey and others 1980) from vegetation increase as temperature 

increases. These are the most important natural hydrocarbons participating in 

ozone formation. Therefore, predicted increases in average temperatures could 

lead to higher emission rates of natural hydrocarbons and increase in ozone 

formation. 

An important chemical outcome of the predicted climate changes will be an 

increase in trace gas levels through the lower atmosphere and the hydrosphere 

(Hansen and Lacis 1990, Mohnen and others 1993). Tropospheric O3 has been 

building up in many regions of the Northern Hemisphere (Mohnen and others 

1993), especially over Europe (Ashmore and others 1985). As a result of these 

changes, damage from gaseous pollutants could increase substantially, and the 

results may be non-additive if SO2, O3, and NOx increase simultaneously. In 

particular, the vulnerability of vegetation to climate extremes may be enhanced 

by high pollutant concentrations (Oppenheimer 1989). Some of these pollutants 

contribute directly and indirectly with other stresses on ecosystems to cause 

acceleration of turnover rates of sulfur (S), N, and C. These changes may lead to 

the net transfer of nutrients from land to coastal ocean accompanied by forest 

decline and coastal eutrophication (Oppenheimer 1989). 

The successful adaptation of individual species to new combinations of 

stresses will be the key process by which species may be perpetuated and move 

into new ranges where the general climate is most favorable to their needs. 

24 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Part II 

Simulation Modeling: Role and Status
Simulation Modeling: Role and Status 

Andrzej Bytnerowicz,1 Sue Ferguson,2 Francis Fujioka,

1

 Roger Ottmar, 

and Mark Poth1 

2 

Numerical model simulations can help to explain and summarize current 

information on atmosphere and biosphere interactions. However, absolute 

results (geographically specific "predictions" or "forecasts") are not possible 

because models have unknown errors both for spatial and temporal applications 

(Oreskes and others 1994), as well as limitations caused by variability and 

accuracy of input data. Thus, results of simulation models should be considered 

to be scenarios of possibilities rather than estimates of future conditions. Indeed, 

the general circulation models used for simulations of global climate cannot 

produce the common seasonal cycles of weather. Nevertheless, simulation 

models are valuable tools because they are designed on the basis of near-past 

and present conditions not future conditions, that are as yet unknown. 

To assess the impact of global change on forest and natural resources the PGCRP 

developed models on two scales. The first scale is geographic, and the goal is to 

produce results and understanding that are relevant to particular sites and 

regions. The second scale is thematic and addresses the issues of importance to 

land managers and policy makers. For instance, in the Pacific region of the 

United States, the thematic range includes local weather, fire and smoke 

dispersal, air pollution uptake by forests, and terrestrial ecosystem simulations, 

among others. 

Regional Scale Weather Models 

Climate in the Pacific region depends highly on ocean currents. The temperature, 

strength, and seasonality of ocean currents significantly affect storm patterns 

(Wallace and Gutzler 1981). The California current and Alaskan gyre are 

predominantly surface currents strongly affected by river discharge. Small 

changes in regional air temperatures could cause large changes in snow melt, 

resulting in discharge patterns (Chapter 2, this volume). This process can alter 

coastal current patterns (Ebbesmeyer and Tangborn 1992) and create a significant 

feedback mechanism for regional climate patterns, but it has not yet been fully 

explored. 

The Southern Oscillation (SO) is a phenomenon related to the displacement 

of warm water back and forth across the equatorial Pacific. Positive SO events (El 

Niño) have been correlated to warmer-than-normal temperatures and decreased 

snowfall in the Pacific northwest of the United States (Ferber and others 1992, 

Ferguson and others 1994) and wetter-than-normal conditions in the Pacific 

southwest (Schonher and Nicholson 1989). Negative SO events (La Niña) have 

been correlated to colder-than-normal temperatures and more snowfall in the 

Pacific northwest (Ferber and others 1992, Ferguson and others 1994). 

Periods of strong SO fluctuations have occurred from 1880 to 1920 and since 

about 1950 with a quieter period in between (Intergovernmental Panel on Climate 

Change 1990). The SO creates a considerable amount of climatic noise that can 

dominate temperature records. Coupled ocean-atmosphere general circulation 

models are just beginning to simulate SO patterns, but a definite relationship 

between climate effects and increasing levels of CO2 has not been observed. 

The complex terrain of the Pacific region of the United States profoundly 

influences their climate. The complexity of the spatial distribution of climate 

variables also depends on the time scale in which climate is studied. For example, 

1

Ecologist, research meteorologist, 

and soil microbiologist, respecttively, Pacific Southwest Research Station, USDA Forest Service, 4955 Canyon Crest Dr., Riverside, CA 92507. 2

Atmospheric scientist and research forester, respectively, 

Pacific Northwest Research Station, USDA Forest Service, 4043 

Roosevelt Way NE, Seattle, WA 

98105. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 27 
the spatial variability of temperature at a given hour will be different from the 

spatial variability of temperature for a given month. Greater variability implies 

greater uncertainty. Hence, any predictions based on climate scenarios include a 

margin of uncertainty proportionate to the inherent climate variability. 

Models that incorporate varying spatial and temporal scales have been used 

to generate climate scenarios. The most descriptive are three-dimensional, 

mesoscale atmospheric models, that have been used for high-resolution 

reconstruction of regional weather and local climate conditions. Nesting a fine 

grid within a coarse grid is an economical method of obtaining spatially 

descriptive simulations. 

• Regional Atmospheric Modeling System (RAMS) - Maui Island, Hawaii 

A study of Maui fire climate used the RAMS (Pielke and others 1992) in 

a hierarchical grid structure (Ueyoshi and others 1996) with a coarse 

grid spacing of 250 km, and embedded grids with spacings of 50 km, 

10 km, and 2 km (fig. 1, Part II). 

Figure 1-An example of a nested 

grid scheme used in weather modeling 

to link large- and small-scale features. 

In this case, weather simulations for 

Maui, Hawaii, used grid spacing of 2 

km (dots), 10 km (X's), 50 km and 

250 km (not shown). 

• Regional Spectral Model (RSM) - Los Angeles Basin, California 

The RSM (Juang and Kanamitsu 1994) provided a simulation of repeated 

wind events (called Santa Anas) in southern California. The study used a 

nested grid structure with spacings ranging from 250 km to 25 km (Chen 

and others, 1996). 

•MM5 - Cascade Mountains, Washington 

The MM5 model, originally developed at Pennsylvania State 

University, was used in simulations with a coarse grid spacing of 250 

km and hierarchical nesting to fine grid spacings of 27 km, 9 km, and 

3 km (Steenburgh, in press). 

Even with a nested grid structure, the computing costs of mesoscale models 

can be prohibitively expensive. More economical simulations are possible by 

28 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
reducing the model dimensions. A two-dimensional mesoscale model (WINFLO) 

has been used to simulate climate over the Columbia River Basin region at 10 km 

resolution (Hayes and Ferguson 1995). 

Statistical atmospheric models generally require less computing than 

mesoscale models. A variety of statistical methods (e.g., MTCLIM [Running and 

others 1987] and PRISM [Daly and others 1994]) are being used to interpolate 

coarse-resolution (500-1000 km) GCM data to 10 km resolution over the 

contiguous United States. A set of regression equations was developed to 

estimate monthly mean surface temperature, dew point, wind speed and 

precipitation frequency at first-order National Weather Service stations in the 

contiguous United States, given the 700 mb height anomaly field over the 

Northern Hemisphere (Klein and Whistler 1991). No computing is necessary for 

qualitative descriptions of expected climate change (e.g., Ferguson 1995, for 

Alaska and northern Canada). 

This section began with a caveat on accepting model outputs as 

predictions because of many and sometimes unknown factors that govern 

model performance. Whenever possible, model simulations should be verified 

by observations. The process of verification begins with comparison of 

observed and predicted fields (e.g., precipitation, as observed, then simulated 

by the Community Climate Model (CCM 1) (fig. 2, Part II). It then proceeds to 

rigorous statistical analysis. The field of spatial statistics is itself rapidly 

evolving, but newer, potentially better methods for spatial analysis have 

already been applied in air pollution studies (Cressie 1990). 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 

Figure 2 -Global precipitation as 

analyzed by the Global Precipitation 

Climatology Project (top) and 

simulated (bottom) by the National 

Center for Atmospheric Research 

Community Climate Model CCMI 

(Roads and others 1996). Scale at 

right is in millimeters per day. 

29 
Production and Transport of Smoke Pollutants

from Biomass Burning Models

Extensive research during the past several years has added considerably to the 

understanding of the production and transport of smoke pollutants from biomass 

burning. New fuel consumption models such as CONSUME (Ottmar and others 

1993) and FOFEM (Keane and others 1994) have been developed to enable managers 

to estimate the amount of biomass consumed during fires. Emission characterization 

research (Ward and Hardy 1991) has focused on the development of emission factors 

and smoldering decay factors for biomass burning in temperate forests of the United 

States, tropical forests in South America, and grasslands of Africa. Several smoke 

dispersion models such as PUFF (Hardy and Ferguson 1993), CALPUFF (Scire 1990), 

V-Smoke (Harms and others 1994), and SASEM (Riebau and others 1988) have been 

developed and are being evaluated and modified for immediate burn decisions to 

reduce impacts on downwind communities. Emission inventory models (Peterson 

1988, Peterson and Ward 1993) allow managers and regulatory agencies to estimate 

the total smoke produced. 

Models Simulating Uptake of Gaseous

Pollutants by Forest Canopies

Direct measurements of dry deposition of gaseous pollutants by plant canopies 

are difficult to make on a routine basis. Consequently, the use of models has been 

advocated as a means of deriving estimates of dry deposition fluxes (Hicks and 

others 1987). An example of such use is the "big leaf" multiple resistance model. 

The model is most applicable over relatively flat, horizontally homogenous 

terrain. The model is limited because of the omission of the within-canopy 

turbulent exchange process, its inherent inability to address questions about the 

flux of bi-directional pollutants such as NH

3, C

H4 and NO, and its lack of detail 

concerning transfer with soil, mesophyll, and cuticle (Baldocchi and others 1987, 

Hicks and others 1987). Various classes of models have been used in the past, 

including the "big leaf" multiple resistance model, and much more complex 

models such as the hybrid "big leaf"/ multi-layer model (Baldocchi and others 

1987), K-theory model (Baldocchi 1988), and higher-order-closure model (Meyers 

1987; Meyers and Paw U 1986). Their predictive capabilities were compared with 

direct eddy correlation measurements of SO

2 and 

O3 fluxes to a deciduous 

forest for both well-watered and water-stressed conditions. By increasing the 

degree of detail of the exchange-governing processes from the more simple 

single layer model to the more detailed multi-layer models, the predictions of 

the deposition rates improved by 40-50 percent as determined from the root 

mean square error (RMSE). Also, by including the effect of water stress in the 

stomatal resistance formulations, the RMSE of the predictions was decreased 

by 50 percent for the models examined (Meyers and Baldocchi 1988). 

However, use of the current models of dry deposition to forest canopies 

for conditions of the western United States is questionable. This difficulty is 

primarily because of the much higher complexity of the mountainous terrain, 

and the patchy character of vegetation cover (quite different from uniform 

canopies of forests in the eastern United States or agriculture crop canopies for 

which many deposition models have been developed). In addition, the available 

models are not designed for calculating deposition of particulate matter, which 

in dry conditions of the western United States accounts for substantial input of 

airborne nutrients to forests. And, the predictive ability of the multiple resistance 

"big leaf" models is limited in conditions when high concentrations of NH

3 are 

present, a situation quite typical for the western Sierra Nevada forests and other 

locations in California (Bytnerowicz and Riechers 1995). 

30 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
Part III 

Research Needs for the Pacific Region 
Research Needs for the Pacific Region 

Andrzej Bytnerowicz,1 Paul Miller,1 and Mark Poth1 

Trace gas exchanges with terrestrial ecosystems and their potential effects on 

ecosystems are not well understood. For instance, more information on emissions 

of terpenes and isoprenes from forests and their potential for ozone formation, 

including mechanisms of these processes and their rates, are needed to develop 

reliable global atmospheric chemistry models. For the same reason, smoke 

impacts on atmospheric radiation, ecosystems, and human health require further 

investigations. Quantification of trace gas emissions in sites with chronic N - deposition and high soil NO3 concentrations and the effects of different 

management practices (harvesting, prescribed fires, etc.) on these emissions is 

also required for a better understanding of N cycling and functioning of the 

terrestrial ecosystems. 

Carbon and nitrogen emissions both increase on a global and regional scale, 

and their biogeochemical cycles are closely linked and mutually dependent. 

Therefore, the relationships between these two elements should be studied in 

various geographical and ecological scenarios to better understand mechanisms 

of their interrelations and to be able to develop more reliable regional and global 

scale models. 

Increasing deposition of nitrogen seems to be one of the most obvious global 

changes affecting the Pacific region. Several research areas need further 

investigations. For instance, more field data should be collected to adequately 

characterize N deposition in the western United States. Although such 

monitoring efforts are very costly, they are essential in understanding regional 

distribution of dry and wet deposited nitrogen. In this regard, also understanding 

mechanisms of uptake and incorporation of various N pollutants by the terrestrial 

ecosystems is urgently needed to accurately estimate nitrogen cycling in various 

landscape forms (including forest stands). Available information in the literature 

on the subject is mostly from the eastern United States and Europe where 

climatic conditions are quite different from generally arid conditions of the 

western States. Accurate characterizing of nitrogen retention in terrestrial 

ecosystems is required for suggesting such management practices that could be 

consciously used in order to increase the capacity of sites to retain and use N 

from atmospheric deposition, N mineralization, or N fixation. Better 

understanding of the effects of chronic N deposition on sensitivity of important 

forest species, such as ponderosa pine, to ozone exposures is also needed. 

A better understanding of the changes occurring in streams, snow, and 

permafrost is required. This includes a need to characterize changes in mountain 

snow cover and resulting impact on water resources and streamflow patterns, 

e.g., seasonal snow patterns in complex terrain, rain-on-snow flood potential, 

snow accumulation, and snow melt. In this regard a better characterization of 

elevation position of the rain/snow interface and a significance of its impact on 

slope stability and certain elevation-specific vegetation species is also needed. It 

is also important to find ways to parameterize mountain snow cover patterns in 

relation to large-scale climate patterns to estimate mountain snow from general 

circulation models. In addition, methods should be developed for spatial 

interpolation of sparse ground observations and/or techniques to couple 

vegetation cover with snow cover for remote sensing applications (current remote 

monitoring techniques of snow cover from satellite imagery have failed in 

forested and mountainous environments and there is no quantitative information 

on snow cover distribution in most wildland areas of the Pacific region). 

1

Ecologist, plant pathologist, and 

soil microbiologist, respecttively, Pacific Southest Research Station, USDA Forest 

Service, 4955 Canyon Crest Dr., 

Riverside, CA 92507. 

USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 33 
More reliable predictions of climate change both on a global scale and for the 

Pacific coast are urgently needed. Because building climate scenarios in complex 

terrain is compounded by the lack of observation data, the installation of a longterm, high-density array of year-round remote meteorological stations is needed. 

To account for cold-air pooling and drainage patterns in complex terrain, we 

must increase efforts to spatially interpolate relative humidity and temperature. 

To build long-term and/or large-domain climate scenarios with 10 km or less 

spatial resolution, techniques for interpolating wind and precipitation should be 

developed. 

Studies across latitudinal gradients should be undertaken to predict possible 

changes related to warming of the climate. Present air pollution/climate 

scenarios from Mexico City may indicate possible future scenarios for other areas 

(e.g., the Los Angeles Basin). Changes occurring in Alaska, where temperature 

increases are hypothesized to be the greatest, could serve as an early warning 

system for the possible changes in lower latitudes. 

Because very little is known on levels and biological effects of UV-B radiation, 

which has the potential to affect some sensitive components of western terrestrial 

ecosystems (including the effects on forest ecosystems), it seems necessary to 

establish the UV-B monitoring network in the western United States. 

Because of great uncertainties in the present climate change models, 

especially in regard to the expected temperature increases, future biological 

effects research should focus on those changes that contain a relatively high 

degree of certainty (such as increasing concentrations of CO2, elevated N 

deposition, increasing UV-B radiation). Such studies should consider effects of 

other modifying factors, e.g., changing temperatures or soil moisture deficit. 

Model simulations and regional estimates research should focus on application 

of micrometeorological methods to estimate net primary production in the 

tropical savanna in Brazil; development of regional estimates of biomass 

consumption by fire; development of models for nitrogen deposition estimates 

which would take into account the patchy and mountainous landscape of the 

western forests; and establishment of a linkage between the Global Circulation 

Models and landscape level ecosystem models (e.g., ZELIG, TreeGro) that would 

be critical for understanding landscape level changes. 

34 USDA Forest Service Gen. Tech. Report PSW-GTR-161.1997. 
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