Unearthing Secrets of the Forest 

Forests are a defining feature for large areas of the Pacific

northwestern United States from northern California to Alaska.

Coniferous temperate rainforests in the western Cascade and 

coastal mountain ranges are appreciated for their aesthetic 

value and abundant natural resources. Few people recognize 

the riches beneath the forest floor; yet, soil is a key ecosystem 

component that makes each type of forest unique. Soils harbor 

immense biological diversity and control the release of water and 

nutrients that support life above ground. 

Understanding how carbon and nutrients cycle in forests, 

known as forest biogeochemistry, is crucial for evaluating forest 

productivity, composition, diversity, and change. At the U.S. 

Geological Survey (USGS) Forest and Rangeland Ecosystem 

Science Center, research in the Terrestrial Ecosystems 

Laboratory focuses on nutrient cycling in five themes: climate 

change, nutrition and sustainability, fire effects, restoration, and 

forest-stream linkages. This research is essential to understand 

the entire forest ecosystem and to use the best science available 

to make informed policy and management decisions.

Carbon and Nutrient Cycling

carbon

nitrogen

nutrient 

losses

nutrient inputs

decomposition

respiration

CO CO 2 2

uptake

calcium

weathered

bedrock

organic

matter

microorganisms

Carbon, nitrogen, calcium, and other key nutrients are transferred 

from plant to soil to air and back to plant again through processes

such as uptake, respiration, and decomposition. This nutrient 

cycling is a ‘biogeochemical’ process because nutrients follow 

pathways that are mediated by biological, geological, and chemical 

players. These players include plants, soil microorganisms, 

weathered rock, and gases in the air and soil. This diagram represents

only some of the possible pathways involved in nutrient cycling.

Western Cascades

Coast Range

Klamath Siskiyou 

Olympic Peninsula

Lowlands/interior valleys

Seattle

Portland

Corvallis

CA

OR

WA

Pacific Ocean

Forested regions where USGS biogeochemistry 

research is conducted.

Black Box Beneath the Big Trees

Old-growth forests of the Pacific 

Northwest store more carbon per unit 

area than any other biome, anywhere 

on Earth. Much of this carbon is tied up 

in the huge trees. The regional temperate 

climate creates ideal soil conditions and 

properties that hold secrets to why these 

trees grow so big. Soil acts as a ‘black 

box’ in forest ecosystems because the 

vital processes are hidden underground. 

Soil fertility, or the ability of soil to 

provide plants with sufficient nutrients 

and water, is not visually obvious, but 

it can be measured using scientific 

methods and analytical equipment. USGS 

scientists are examining the black box 

to provide important information for 

balancing human needs and sustaining 

forests. The following examples highlight

completed and ongoing research. 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3078

September 2009 Printed on recycled paper Printed on recycled paper
Climate Change

Weathering the Storm

Precipitation and its effects on soil moisture shape 

forests across the Pacific Northwest, from the dense 

rainforests of the coast to the semi-arid woodlands of inland 

areas. Climate change likely will bring 

warming temperatures across the 

region, intensifying summer drought and 

moisture stress, which can limit tree 

growth. Climate predictions also call for 

wetter winters overall, with increased 

rain and declining snowpack. The USGS 

currently is studying how forests cope 

with the direct effects of changing rainfall 

and the indirect effects resulting from 

higher temperatures. Overall, the work 

seeks to understand how climate change 

will alter the amount of carbon stored in 

forests, and how this connects local and regional forests to 

rising levels of atmospheric carbon dioxide globally. 

Storms from the Pacific Ocean slam into the 

massive Olympic Mountains and drop a yearly 

average of 15 feet of precipitation on its wettest 

western forests. The resulting rain shadow on 

the eastern side of the mountains receives as little 

as 3 feet of precipitation each year. In this natural 

laboratory, USGS scientists are comparing sites 

across the rainfall gradient and transplanting 

soil cores between sites to mimic how changing 

climate may alter forest soils. By varying soil exposure to 

different levels of precipitation, scientists can predict which 

areas are likely to experience changes in nutrient availability 

that shape carbon storage capacity. Future climate warming 

that decreases soil moisture during the growing season is 

expected to decrease nutrient availability, plant growth, and 

carbon storage in many forests of the region, particularly in 

areas that already experience significant summer drought.

Nutrition and Sustainability

Administering the Multivitamin

Forest ecosystems need a ‘multivitamin’ of nutrients to 

balance and sustain growth. Nutrient deficiencies can have 

detrimental effects on trees, making them susceptible to 

disease, root rot, and attack by 

insects. Historically, nitrogen has 

been considered the most limiting 

nutrient in Pacific Northwest 

forests, and as a result, nitrogenbased fertilizers are widely used 

to stimulate tree growth in young 

plantations. Interestingly, many 

forests in Oregon and Washington 

are not nitrogen-limited, mainly due 

to the presence of nitrogen-fixing 

red alder trees. USGS research 

has shown that naturally high levels of soil nitrogen result in 

low levels of soil calcium and associated calcium deficiency 

in Douglas-firs. These nutritional imbalances may predispose 

Douglas-firs to a fungal disease called Swiss 

needle cast, which can reduce a tree’s growth up 

to 50 percent.

These concerns led USGS researchers to 

consider sources and cycles of calcium in these 

forests. In some cases, most of the calcium 

used for tree growth comes from rainfall and 

atmospheric dust rather than the breakdown of 

minerals in rocks. Over time, trees accumulate 

this calcium in their wood and needles to support growth. At 

harvest, this calcium source is removed permanently from 

the ecosystem. Building on this information, scientists and 

forest managers have joined forces to investigate how different 

fertilizers can be used to improve and sustain tree growth on 

nutrient poor soils. Tree growth may be stimulated and disease 

averted through the addition of one or more of these 

critical nutrients to a site.

Most plants take up nitrogen from the soil, but some 

form unique relationships with bacteria that can 

convert nitrogen gas from the atmosphere into forms 

of nitrogen the plants can use. Common nitrogenfixing plants native to the Pacific Northwest include 

red alder trees, Ceanothus shrubs like deerbrush 

and snowbrush, and herbs such as Kincaid’s lupine. 

Nitrogen-fixing plants can ultimately enrich ecosystem 

fertility and benefit other plant and animal species.

Nitrogen-Fixing Plants
Fire Effects

Inheriting a Burning Legacy

Wildfire can produce drastic changes to soil carbon and

nutrients that alter the structure of the regenerating forests for

years. USGS scientists and university partners are currently 

studying the immediate and long-term effects of fire on soil

nutrients. In the Klamath–Siskiyou region in northwest 

California and southwest Oregon, shrubs such as deerbrush 

immediately colonize a burned area. Deerbrush can fix 

atmospheric nitrogen gas into mineral forms available to 

plants, restore soil organic matter, and renew soil fertility after 

wildfire. Current research focuses on how deerbrush aids the 

growth of longer-lived conifers by restoring soil fertility in 

areas that experienced catastrophic wildfire 10–20 years ago. 

By studying these processes at sites along a climate gradient, 

the scientists also hope to understand how the recovery of 

forests and soil fertility after wildfire may be shaped by future 

climate change.

The long-term effects of fire on soil carbon and nutrients 

can last for centuries. A USGS study in the western Cascade 

Range in Oregon compared soil nutrients in forested areas 

that burned 550 years ago with other areas that burned 150 

years ago. Scientists detected more carbon and nitrogen 

in the forest floor material in the older forests. Additionally, 

nitrogen was more available in the older forests than in the 

younger forests. Study findings suggest that more frequent 

fires may prevent accumulation of carbon and nitrogen and 

may ultimately decrease the role of forests in long-term carbon 

storage. In this way, policy decisions on managing wildfires 

can have multi-century effects that go well beyond the shortterm effects usually considered by forest managers. 

Bridging the Generation Gap 

Less than two decades ago, most Federal forest land was 

managed to produce high yields of timber and associated 

wood products. A new regional forest plan implemented 

in 1994 dramatically shifted the old management goal for 

millions of acres of young forests. Many dense, young forests 

were incorporated into a network of conservation reserves 

to accelerate the development of old-growth structure. One 

approach Federal forest managers are taking to restore oldgrowth conditions is to create various-sized gaps in dense, 

young forests. Managers harvest wood to create gaps that 

mimic naturally occurring disturbances, such as wind damage, 

landslides, and insect infestations. Gaps are a significant 

attribute of old-growth forests that reduce competition for 

sunlight and other resources, resulting in forests that vary more 

in age, species, and structure.

Gap creation in forests alters soil nutrient availability in ways 

that can influence plant growth and recovery within gaps. Using 

a combination of laboratory studies and field manipulations 

at sites in western 

Oregon, USGS scientists 

determined that the 

availability of soil 

nitrogen was greater 

within created gaps 

than in the adjacent 

forest. Higher nitrogen 

in gaps results from 

reduced tree uptake of 

nitrogen when trees are removed, and from the rapid recycling 

of nitrogen by shrubs and herbs that initially colonize the 

gaps. Gaps, acting as high-nitrogen ‘oases,’ create a mosaic of 

nutrient supplies that can contribute to the development of more 

diverse conditions as young dense forests gain old-growth 

characteristics. 

Old-Growth Restoration
Forest-Stream Linkages 

Nourishing Food Webs

A riparian zone describes the forest-stream boundary area 

where land meets water. In the Pacific Northwest, coniferous 

and deciduous trees occur naturally along streamsides. When 

restoring riparian forests, managers emphasize retaining and 

growing large conifers, which are more effective than deciduous 

trees at providing shade and large wood for fish habitat. These 

functions, while critical in riparian forests, do not consider 

nutritional contributions from riparian plants that support streamand land-based food webs. Deciduous red alder and coniferous 

Douglas-fir trees are common species that managers can 

manipulate in riparian zones. The chemistry of leaf litter differs 

greatly between Douglas-fir and red alder trees, so managing for 

different tree species can have consequences to consumers, such 

as fish and amphibians, farther up the food web. 

As part of a multi-disciplinary study examining many aspects 

of food webs, USGS scientists assessed how the leaf litter 

produced by different tree species drives the base of food webs 

in riparian ecosystems. This work has shown that red alderdominated riparian zones contribute more leaf litter and nitrogen 

than conifer-dominated riparian zones, especially in autumn. Once 

on the ground, red alder leaves decay and release nutrients more 

rapidly than Douglas-fir needles, more effectively feeding the 

organisms that depend on these resources. Additional work by 

USGS scientists has determined that riparian red alders enrich 

soil nitrogen levels and fuel the subsequent growth of conifers for 

decades. Each of these studies highlights the importance of red 

alders in riparian areas and helps to educate managers about 

balancing shade, wood, and nutrients when restoring 

aquatic ecosystems. 

Forest Biogeochemistry Research 

 Addresses 

• Carbon uptake and storage

• Climate change 

• Tree nutrition and productivity

• Soil fertility

• Young forest management

• Old-growth requirements

• Watersheds and water quality

• Restoration techniques 

• Fertilization practices

• Fire effects and recovery

• Riparian food webs

• Native plant recovery

• Air pollution effects

• Resource protection

For additional information contact:

Steven Perakis

USGS Forest and Rangeland Ecosystem Science Center

3200 SW Jefferson Way, Corvallis, OR 97331

sperakis@usgs.gov / Ph: 541-758-8786 / Fx: 541-758-8806

Authors: Sarah Beldin and Steven Perakis

Photograph credits: Douglas squirrel, Amanita muscaria, yellow-rumped 

warbler and coast lily—Matt T. Lee; red alder forest, leaves and 

cones—Edward Jensen; old-growth Douglas fir—Bill Gibbs; 

all other images—public domain.

Graphic Design by Bill Gibbs

This factsheet and others are available at 

http://fresc.usgs.gov/.