Kala Point

Forest Ecology in Washington

by Donald P. Hanley & David M. Baumgartner
Washington State University & Dept. of Natural Resources
Extracts of Pub. EB1943 – 2002

1. Introduction

A forest ecosystem is not just a collection of trees. Forests contain living, or biotic components, and nonliving, or abiotic components. Besides trees, the living portion of the forest includes herbs, shrubs, other plants, animals, and microorganisms like bacteria and fungi. Nonliving parts of the forest include snags, logs (also known as large organic debris or LOD), the underlying rocks from which soil is formed, and the soil itself, which provides water, nutrients, and support for the plants. The atmosphere and climate have an effect as well. Fires, frost, drought, windstorms, and other disturbances regularly influence forest growth and development.

Together, the living and nonliving pieces that make up the forests often are referred to as a forest ecosystem. Forest ecologists study how all the parts of a forest are related. Just as members of a household influence each other, changes that happen in one part of a forest ecosystem influence other parts. Timber harvesting and natural disturbances also change the forest. It is important to learn to recognize the changes that occur and how they influence the total forest ecosystem.

As a field of study, forest ecology is very broad and theoretical, but also exciting and useful. From explaining how one population of soil bacteria relates to another, to describing why ponderosa pine will grow well on some sites and not on others, forest ecology deals with virtually everything relating to the forest as a whole. Foresters and landowners know different organisms in an ecosystem depend on each other and that changes to one organism can have an impact on others. This understanding has made people think about how they manage forested lands and realize that management practices must be based on sound principles of forest ecology.

Forest ecology is one of the most important courses forestry schools offer. It forms the basis of silviculture—the art and science of tending forests to achieve management goals. Forest ecology is complex. This publication presents key concepts as an aid to good forest management.

2. Structure, Function, and Change

Forest ecosystems have three major properties: structure, function, and change. Structure includes the underlying rock and soil, the living and dead tree and plant material, and the atmosphere. Function refers to the movement and exchange of matter and energy. This occurs between the physical environment and the living community and between living community components within and between ecosystems. For instance, the food chain or web represents the flow of energy from the producer level (green plants) through various consumer levels (plant-eating animals, carnivores, insects) and decomposer levels (bacteria, fungi). Many functions are described in terms of cycles—the energy cycle, the carbon cycle, the water cycle, and the nutrient cycle. Ecosystem structure and function also interact and change over time. This is why we say forests are dynamic.

3. Scale

Scale is the frame of reference we use to define an ecosystem. The term “ecosystem” does not imply any particular size. Ecosystems range from quite tiny to very large. The population of lichens on the bark of a Douglas-fir forms an ecosystem. All the plants and animals in a particular watershed also form an ecosystem. Both are ecosystems having a different scale. If we start by thinking about one scale, that of the individual tree, smaller scales would include a leaf on that tree, or even a single cell within that leaf. We can think about and observe larger scales: a recognizable unit of the forest, called a stand, or even an entire drainage or landscape made up of many interacting ecosystems.

Changes in structure and function may influence additional ecosystems. Effects become apparent at different scales. An event that occurs at one scale may not produce noticeable results at a greater scale, or may cause effects that magnify at increasingly larger scales. Interactions among lichen populations on the bark of an individual Douglas-fir may not be apparent at the scale of the forest stand. If insects defoliate a few needles, the effect will not be visible at the scale of the entire tree, which will survive and continue to grow. If insects eat many needles, they can seriously weaken or even kill the tree. While the death of an individual tree is usually not serious at the stand and landscape levels, the death of trees in an entire stand or landscape is serious because of the scale. Similarly, the effect on forest floor temperature and light will be different if many trees die or if only a few trees die. If a fire disturbs a small area, the effects on soil erosion and water quality will likely be less than if an entire drainage burns.

In the past, forest ecology concentrated on studying interactions at the scale of individual stands. Now, landscape-scale interactions also are considered important. Think of a forest landscape as a mosaic or collage of many forest stands. Landscapes can be fairly homogenous—all stands can be similar in age and species composition. Heterogeneous, or mixed landscapes are made up of stands that differ from each other in age, composition, or both. Diverse landscapes usually result from past disturbances (fire, weather, insects, diseases) and topographic features.

4. Landscapes

A landscape-scale ecosystem is made up of groups of smaller interacting ecosystems. Like all ecosystems, landscapes have the attributes of structure, function, and change. Landscape structure refers to the sizes, number, kinds, and configurations of ecosystems that make up the landscape. Landscape function refers to the flows of energy, materials, nutrients, and species among ecosystems. Landscape change refers to alterations in the structure and function of the mosaic of ecosystems over time.

Historically, landscape patterns were created and maintained by natural disturbance. In managed landscapes, political boundaries and management decisions affect these patterns and often the character of disturbances. Ecosystems and landscapes change over time as a result of vegetation development and disturbances.

In western Washington, large, infrequent fires created and maintained relatively homogeneous landscapes. Urban development and timber harvesting have fragmented this landscape, altering these ecosystems. Eastern Washington landscapes were historically patchy because frequent, low to moderately severe fires created a mosaic of different-aged forests across the landscape. During the past century, fire suppression has allowed dense fire-prone stands to establish and grow across the landscapes. This, coupled with poor selective logging or “high grade” logging practices earlier in this century, has created much more uniform landscapes than existed historically. These landscapes are more susceptible to outbreaks of insects, diseases, and catastrophic fires.

5. Energy Flows and Photosynthesis

Forest ecosystems are driven by solar energy through the process of photosynthesis. Trees and other green plants convert light energy from the sun into chemical energy. The leaves or needles capture solar energy (light) and convert carbon, hydrogen, and oxygen to simple sugars. These are then converted into more complex compounds such as cellulose, the main component of wood fiber. A mature forest produces many tons of sugars and other compounds per acre each year. Of these compounds, the leaves use approximately 70%; about 5% is converted to wood, branches, and fine roots, and 25% maintains tree functions. This maintenance is referred to as respiration. As forests age, more energy captured each year goes into respiration instead of into the production of wood, branches, roots, or leaves.

Although a complete understanding of photosynthesis is beyond the scope of this publication, the faster and more efficiently a tree carries on photosynthesis, the faster it will grow and the less vulnerable it will be to insects and disease. This is primarily a function of the size of the live crown, actually the total leaf area (to capture light), and available soil nutrients and water. When a tree or parts of a tree die, it begins to decompose. The microbes and other organisms that decompose the twigs, leaves, and wood use some of the energy stored in the tree parts. Much of it remains in organic material that accumulates on the forest floor. Following a disturbance that removes most of the vegetation from an area, the increased light and temperature at the forest floor increase the rate of decomposition—that is, the rate at which the energy stored in the organic matter is released. Recovery of ecosystem structure and function following a disturbance is powered by solar energy stored in the forest floor.

Some of this stored energy is lost through grazing and browsing by herbivores, such as squirrels, rabbits, grouse, deer, and elk. As these animals eat, they use the energy they consume to build tissue, such as muscle, fat, and bone. In turn, these herbivores become food for forest carnivores and the energy is transformed again, used in the development of body mass for these species. When an animal dies, the energy once again is transformed into food for decomposers and soil microbes. This transformation of energy from plant to animal tissues also contributes to the dynamic nature of the forest ecosystem. Through photosynthesis, forests accumulate biomass and stored energy, which cannot build up forever. Ultimately, this energy will be released through microorganisms, fire, or human activity.

6. Cycles

Ecosystem function—the exchange of energy and material within and between ecosystems—can be characterized as the interaction of several cycles, such as water and nutrient cycles. Cycle rates vary. Some cycles operate at regional or global scales and over long periods of time, so that material movement at smaller scales appears to go only in one direction. Soil movement is a good example. Soil moves down slope. It can move rapidly and in great quantities, as in a landslide, or gradually, as gravity moves particles slowly from ridge top to valley bottom. All cycles are powered by solar energy.

     a. Water Cycle

The water cycle also is referred to as the hydrologic cycle. Rain or snow falling through the atmosphere first strikes the forest canopy. Some is intercepted by needles or leaves and may evaporate back into the atmosphere or be delayed before reaching the ground. Some water directly reaches the ground. In areas where vegetation is sparse and the soil is compacted, the soil cannot absorb much of the water, and the water flows over the soil surface, picking up soil particles. Eventually, this overland flow may reach a stream channel and transfer to another ecosystem, along with the soil particles and nutrients dissolved in it. If the forest floor is intact, most precipitation reaching the ground will move into the soil. The roots of trees and other vegetation usually take up water in the soil before the water reaches a stream channel. The trees will store some of this water, while some will return to the atmosphere through transpiration. Basically, transpiration is the process by which water is pulled first from the soil by tree roots, then pulled up the tree through xylem tissue and into the leaves or needles. The leaves contain small openings called stomata that continue the pull of moisture, allowing it to escape back into the atmosphere.

     b. Nutrient Cycle

A properly functioning forest ecosystem is able to acquire, store, and recycle nutrients.

Energy flows power the interconnected water and nutrient cycles. Through transpiration, the water cycle drives much of the circulation of nutrients within the ecosystem. Many dissolved nutrients are held within the soil water. These nutrients are lifted into the canopy by transpirational pull and eventually returned to the soil through needle fall and decomposition. Transpiration also controls photosynthesis. When not enough moisture is available in the soil for trees to transpire effectively, plants close their stomata to reduce transpiration. Photosynthesis also is reduced. This is why trees do not grow as fast when moisture is limiting.

Figure: Nutrient Cycle.


A forest ecosystem, burned so severely that little organic matter remains in the soil, could lose much of its nutrient capital through erosion and streamflow. If no present vegetation is available to take up nutrients, and if few soil organisms are present to store the nutrients, they frequently are washed into streams and flow out of the area. In some cases, replacing these lost nutrients can take hundreds of years.

The woody parts of trees tie up only small amounts of nutrients. Traditional harvesting methods remove only minor amounts of a site’s nutrient capital. Most nutrients occur in the needles, leaves, or fine roots of trees. Whenever possible, leave these tree parts on the ground after logging so the nutrients will be available to be cycled.

7. Ecosystem Productivity

The measurement of productivity provides us with an indication of the rate of photosynthesis and biomass accumulation in a community. Although all biological activity in plants ultimately depends on received solar radiation, solar radiation alone does not determine gross primary productivity. All plants require sunlight, carbon dioxide, water, and soil nutrients for photosynthesis. Photosynthesis also depends on temperature, moisture, and nutrient availability. Temperature (heat) controls the rate of plant metabolism, which in turn determines the amount of photosynthesis that can take place. Water is a principal requirement for photosynthesis and the main chemical component of most plant cells. In dry regions, there is a linear increase in net primary productivity with increased water availability. In the more humid forest climates of the world, plant productivity begins to level off at higher levels of precipitation. The productivity of plants, especially at the local scale, also can be controlled by the availability of nutrients. About 20 to 30 elements generally are considered essential for plant metabolism and growth. These essential nutrients are sometimes grouped into two categories: macronutrients and micronutrients. Plants use the macronutrients for the construction of structural molecules and for building a variety of organic molecules used in metabolic processes. Only two of the macronutrients used by plants normally occur in limiting concentration for plant uptake: nitrogen and phosphorus. When limiting, these two nutrients control the amount of plant productivity that can occur. Plants require micronutrients in extremely small quantities for the creation of less common organic molecules or as ions to catalyze specific metabolic reactions. In general, micronutrients are common in abundance and do not limit plant production.

8. Site Quality

Foresters refer to the sum of all the natural factors that influence forest vigor, health, and growth as the site. Additionally, foresters frequently talk about site index, or refer to relative productivity of sites. What they really are talking about is the combination of environmental influences such as climate, soil texture, soil nutrients, slope, aspect, and elevation. These factors affect where tree species grow, how rapidly they grow, how healthy they are, and their form at maturity.

Slope, aspect, and elevation combine with local climate conditions to determine the microclimate of a site. For example, gently sloped, northeast-facing slopes tend to be cool and moist, while steep, southwest slopes, which face the sun in the afternoon, tend to be warm and dry. High elevation sites are colder and more exposed to wind than sites deep within a valley. However, frost pockets may exist in low-lying areas where air drainage is restricted. The orientation of a valley determines its exposure to prevailing winds.

Just by knowing the aspect, elevation, and slope of a site, one can make some predictions about its potential productivity and which species will grow well there. For example, in western Washington on northeast slopes below 1000 feet elevation, Douglas-fir—western hemlock forests dominate. Northern aspects often result in Douglas-fir—western hemlock—western white pine forests. Slope position is also important. On wet or poorly drained riparian areas, western redcedar, red alder, Oregon ash, or black cottonwood may dominate on the west side of the state.

One of the most important things to know about a site is the nature and condition of the soil. Forest soils are made up of four main ingredients: mineral particles, organic matter, water, and air. The minerals it contains largely determine soil fertility. Soils derived from basalt or volcanic ash tend to be more fertile than those derived from granite or sandstone, because basalt has a greater concentration of nutrient-bearing minerals than either granite or sandstone.

The physical properties of a soil—whether it is fine or coarse—depend on the type of minerals present and on the size of the soil particles. The three basic soil particle sizes are sand, silt, and clay. Sand is the coarsest size; clay is the finest. Sand is very porous and normally does not contain many nutrients. Clay is very nutrient-rich but drains poorly and is easily compacted. Soil texture refers to the proportions of sand, silt, and clay particles in a particular soil. Soils that contain a large proportion of clay and silt have a fine texture. Finer soils are usually more productive than coarse soils, but fine soils do not drain as quickly, are very susceptible to damage from compaction, and are more easily eroded than coarse soils. A soil made up of roughly equal amounts of sand, silt, and clay is referred to as a loam. Loams tend to be fertile and hold water, without becoming overly wet.

Organic matter—rotting debris such as needles, leaves, and twigs—strongly influences the physical and chemical properties of a soil. Soils having large amounts of organic matter have better structure and greater fertility. Organic matter also helps forest soils hold water. However, too much organic matter, such as in a poorly drained bog, can tie up nutrients, making them unavailable to living vegetation, including trees. Thick layers of organic matter on the forest floor can hinder seed germination for some tree species, such as ponderosa pine, Douglas-fir, and western larch.

More than half the volume in the upper layers of an undisturbed forest soil can be made up of air and water. The proportion of air and water is both affected and determined by the physical properties of the soil. Since roots need air to respire and water to supply the rest of the tree, the air and water components of a soil are very important. More than half the “feeder” roots in a forest occur in the top 6 inches of soil. Soil compaction reduces the amount of space available for air and water, and lowers site productivity. This is why it is so important for rubber-tired skidders to stay on established trails, especially on sites having wet or fine-textured soils that tend to compact easily.

Soils contain living matter too. Fungi, bacteria, insects, and a host of tiny creatures live on organic matter produced by trees and shrubs. Even though we cannot see most of them without a microscope, these organisms are absolutely essential to the growth and development of forests. Although some occasionally cause disease in trees, most soil organisms feed on fallen leaves and woody debris. Their main role is to recycle nutrients like phosphorous, potassium, and calcium tied up in dead vegetation and animals. During the process of decomposition, nutrients locked up in dead vegetation and animal carcasses are ingested by soil microorganisms and returned to the soil upon the death of these decomposers. Without them, the forest floor would be littered with debris accumulated over thousands of years, and trees would be starved for the nutrients locked up in this material.

A group of soil organisms known as mycorrhizae-forming fungi actually colonize the roots of some trees and greatly improve their ability to take up water and nutrients. The tree and the fungus depend on each other; the tree provides food to the fungus while the fungus transfers water and nutrients to the tree and provides protection against harmful soil organisms. Mycorrhizal fungi are highly susceptible to changes in environmental conditions, especially those caused by soil compaction. Forestry operations must take into consideration these unseen, but very important components of forest ecosystems.

9. Forest Health

Forest health has been defined as the condition of a forest when it is

• resilient to change, it can recover from a disturbance.
• biologically diverse over a large area (landscape diversity), and
• able to provide a sustained habitat for vegetation, fish, wildlife, and humans.

A healthy forest is made up of trees and other organisms all dependent on each other. The presence of single or a small group of unhealthy trees does not necessarily indicate an unhealthy forest. For example, bark beetles can actually promote forest structural diversity. A professional forester can help you determine the severity of a forest health concern.

Just as humans need a certain combination of food, water, and exercise to maintain physical health, forests and the trees require certain inputs to maintain their health and growth. If one or more of these inputs is missing or insufficient, trees experience stress. Forest managers can influence these inputs through silvicultural practices.

One of the major health concerns in Washington forests is stress caused by having too many trees per acre, or overstocking. Over-stocking causes tree stress because it forces trees to compete with surrounding trees for limited light, water, and nutrients. Many silvicultural practices are effective because they reduce the number of trees per acre and, thus, the competition for these essential elements. Other stress factors may include air pollution, soil compaction, and climate changes.

The first requirement for healthy tree growth is light. Through photosynthesis, plants manufacture their own food by using the sun’s energy to convert carbon dioxide and water to a usable food source. Heavy shade, found underneath the closed canopy of a forest, provides insufficient energy for the smaller, less dominant trees to grow. Shade-intolerant species have great difficulty growing under these circumstances. These include pines and larches. Other species, such as Douglas-fir, Engelmann spruce, and most hardwoods, are considered moderately shade tolerant. They can grow in partial shade. The tolerant species—grand fir, hemlock, and western redcedar—can grow under conditions of heavy shade, although not very fast. A thinning operation can release slow-growing (suppressed) trees by providing them more light and space in which to grow.

The second requirement for healthy tree growth is water. Tree species vary considerably in their water needs and drought tolerance. Shade-intolerant species commonly grow in hot, sunny areas and are more drought resistant. Shade-tolerant species grow naturally in the cool, moist forest. When drought occurs, which happens frequently in all western states, these shade-tolerant species are more stressed than the shade-intolerant species. Thinning reduces the total number of trees competing for water and can relieve drought stress. However, overthinning (the removal of too many trees) may increase the amount of sunlight reaching the ground and dry out the area more rapidly. This is especially true on steep terrain.

The third requirement is a good nutrient supply. Trees take up minerals through their roots and incorporate them into developing cells. One of the basic determinants of potential tree growth is the level of nutrients available in the soil. Nutrient-poor soils will never produce large trees, and even rich soils cannot produce large trees if they are overstocked. A forest manager may thin a stand to reduce competition for nutrients. Although it is not always cost effective, using a fertilizer on forest soils can provide needed tree nutrients.

10. Vegetation Patterns

Northwest ecosystems contain many different vegetation patterns, ranging from brush fields to old growth forests, and including every successional stage in between. Collectively, the types, amounts, and distribution of vegetation patterns define water quantity and quality, timber resources, wildlife habitat, and many other important ecosystem characteristics. Vegetation patterns also impact forest processes such as streamflow, erosion, and succession.

Northwest forest landscapes are created and maintained through a balance of disturbance and recovery processes. Disturbance alters a portion of the current forest stand. A new forest grows, declines, and is again replaced. Ultimately, all living biomass is recycled. Disturbance and restoration processes create a sustainable cycle that conserves both biological capacity and options for future forests. Disturbance requirements within ecosystems vary greatly in intensity and frequency. The most common forms of disturbance that have influenced Northwest forest ecosystems in the past are fire, wind, ice, insects, and disease. We have altered historical forest ecosystems by creating unnatural disturbances, such as excessive logging, urbanization, and overgrazing, while suppressing fire. Consequently, these forests have different fire, insect, disease, and hydrologic disturbance cycles and processes. Fires in some areas are larger and more severe than in the past. Insect attacks last longer and spread wider. Sites favorable for tree disease are expanding. Vegetation cover in riparian areas has been diminished and stream structure has grown less complex, reducing fisheries habitat.

Human-caused disturbances can be used in management to proactively mimic natural disturbances (anthropogenic or human disturbances such as fires set by Native Americans have been a part of many landscapes for centuries) and to help with restoration activities.

11. Vegetation Development

Vegetation development refers to the processes of change that occur in forest stands and landscapes over time. In the absence of past disturbances, changes in the species composition of a forest are slow but continuous. The process of continual change is referred to as succession. Forest management is based on the fact that the direction of forest succession is both predictable and controllable. “Direction” refers to the gradual order of species replacement from intolerant to tolerant—or the scaling back of succession as a result of natural disturbances or silvicultural manipulation. If forest succession progresses to a more-or-less stable vegetative state over time, we define this vegetative association as a Habitat Type. Thus, Habitat Types reflect the climax vegetation in a location. Habitat Types often are defined by the dominant overstory tree species and the understory for forest floor complex.

The rate at which succession proceeds can be increased or decreased by altering the species composition and density of species in the forest. Disturbances influence rate and direction of succession, while forestry practices mimic natural disturbances. Thinning small trees in the understory mimics ground fires or natural mortality in a stand. Harvesting the dominant and codominant trees in a stand can have results similar to those following a windstorm that removes the overstory.

A key concept to understanding why succession occurs is tolerance. Though tolerance is really the degree to which a species can successfully compete for site resources (light, moisture, and nutrients), it is most often used with respect to light and, thus, is referred to as shade tolerance (Table 1). Pioneer or early successional species such as red alder, western larch, and ponderosa pine are often extremely intolerant of shade. They are not able to grow or reproduce in shaded conditions. Mid-tolerant or intermediate species can grow in partial shade, and late-successional species are able to grow and reproduce in heavy shade. As a result, unless interrupted by disturbances that remove all or part of the canopy, forest succession usually proceeds toward more shade-tolerant species.

Table: Shade tolerance of commercially important conifers in Washington.


For example, in western Washington, Douglas-fir, western hemlock, and western redcedar occur together in many forest stands. Of the three, Douglas-fir is the least tolerant of shade and will almost always be found only in the overstory. Western hemlock and western redcedar, both very tolerant of shade, can exist in either the overstory or in the understory. As overstory trees die, cedars and hemlocks in the understory will grow into the overstory. If many centuries pass without fires or logging, the stand will increasingly be composed of western redcedar and western hemlock. In eastern Washington, western larch is a very shade-intolerant species that grows rapidly and may quickly dominate some forest stands following fires. More shade-tolerant species such as grand fir or subalpine fir are able to grow beneath the larch; however, because of its intolerance for shade, larch only appears in the overstory.

Two additional concepts provide a framework for understanding vegetation development patterns. These concepts are growing space and disturbance. Growing space refers to the availability of all the requirements a plant needs to grow. The most important requirements—or growth factors—are light (for photosynthesis), water, and nutrients. Each tree in a forest uses these growth factors until one or more becomes unavailable. When that occurs, the growing space is essentially filled. No new plants can establish, and the ones already there must compete with each other to gain more growing space. Plants that compete the best get more growing space and continue to grow. The losers often die. This competition ultimately causes forest succession.

The amount of growing space varies in time and in space. Light is most generally abundant for trees in the upper canopy but may be extremely limited at the forest floor. Sometimes light reduction at the forest floor can be as much as 97% of that in open conditions! Some species are able to tolerate growing conditions (such as low light levels) that are not adequate for other species. Differences among species are not great, but can give some species a competitive advantage on a particular site. For instance, ponderosa pine needs more light to grow than does Douglas-fir. In the understory, Douglas-fir seedlings have a competitive advantage over ponderosa pine. Growing space also can change over time. In much of the Pacific Northwest, moisture is limited during the dry summer months but seldom during the fall, winter, and spring. As a young stand matures, some nutrients may become tied up within the vegetation, limiting further growth, especially on coarse-textured soils.

Each tree species has a unique collection of silvical characteristics, or how the individual tree interacts with the environment. These characteristics are summarized in the table, below.

Table: Western Washington species characterization – A summary for young trees.


     a. Role of Disturbance

Disturbances play an important role in determining forest structure and species composition. Natural disturbances are quite common in all regions of the world. Every area has characteristic types of disturbances that occur at relatively predictable frequencies. Coastal Washington is subject to periodic windstorms or “blows” that knock down overstory trees, allowing seedlings in the understory to grow. On the western slopes of the Cascades, large, infrequent fires have provided conditions favorable for Douglas-fir to germinate and grow. Insects and diseases killed some trees, creating snags and logs that provided important habitat for wildlife. The type of wildlife using these structures depended on the disturbance that created them. For instance, stem decay fungi often produced soft snags useful for cavity nesting birds and animals. Douglas-fir mistletoe creates large witches-brooms used as nesting and hiding sites for some birds. But the brooms also can allow wildfires to move from the ground into the crowns of infested trees.

To create sustainable forest ecosystems, conserving disturbance processes is as important as conserving individual species. Timber harvests and prescribed fires can mimic many effects of wildfire and other disturbances. However, we need to balance such natural disturbances with needs for wildlife, aquatic resources, and sustainable commodity production. Ultimately, these goals may depend on sustaining the broader ecosystem through managed disturbance.

Disturbances change the availability of growing space. Disturbances that remove the overstory—for example windstorms or timber harvesting—change the amount of light that reaches the forest floor. Disturbances that remove organic matter and soil—some fires, landslides, and site preparation techniques—reduce the amount of nutrients and, thus, the total amount of growing space on a particular site. Disturbances that remove most of the vegetation on a site increase the amount of light, moisture, and nutrients available for new plants to utilize.

Following a disturbance, surviving plants and new ones expand into the now-available new growing space. The plants that grow the fastest and capture the most growing space can dominate the stand for decades; such is the role of red alder on the west side of the Cascades. Trees in the open grow more quickly than those in the understory. In managed stands, thinning and harvesting can control the amount of light available to trees. Removing part of the stand allows more available light for the remaining trees. In a young stand, it may be only a matter of a few years before the crowns of residual trees grow into the spaces left by those removed.

     b. Dynamic Processes and Stand Development

Forests are dynamic. Changing vegetation patterns caused by disturbance or succession alter forest benefits and values. Ecosystem management anticipates and plans for change rather than simply responding to undesirable events.

Understanding succession is an important criterion when actively managing forests to create desired future conditions. Nature will continue to provide disturbances. Insight into potential vegetation patterns across adjoining land should help landowners, managers, and other ecosystem management cooperators plan how to better interact for their own needs.

Not all disturbances are stand replacing. Vegetation responses can be described by categorizing disturbances according to low, moderate, or high severity. Forest ecologists define high severity disturbances as those killing 70% or more of the trees in a stand. Moderate severity disturbances kill between 20% and 70% of the stand, and low severity disturbances kill less than 20%.

Succession depends on both the severity and type of disturbance. After a high severity fire, pioneer species such as grasses, forbs, brush, and tree species such as western larch and lodgepole pine, are usually favored. After a high severity windstorm, shade-tolerant advanced regeneration, such as western hemlock or western redcedar, is more likely to be favored.

Low severity disturbances kill individual trees or small groups of trees. Ground fires, small pockets of root rot, and small-scale insect attacks are examples of low severity disturbances.

12. Forest Development Phases

Following stand-replacing or catastrophic disturbances, forest development can be defined by four phases:

(a) Stand initiation or “open,”
(b) Stem exclusion or “dense,”
(c) Understory reinitiation or “understory,” and
(d) Old-growth or “complex.” Each phase is characterized by different structures, providing different wildlife habitats and forest products

Figure: Stand Development Phases as Defined by C.D. Oliver are Useful to Describe Stand Development.


While these four phases are useful to understand stand development over time, there are no absolute boundaries between them. Residual trees and shrubs are not often completely killed, resulting in “islands” of an older successional stage. “Legacy areas” act as “refugia” for later successional dependent plants and animals. As the new forest matures, these plants and animals are able to colonize that new forest area. These legacies often provide significant biological diversity to the forest site over time and provide wildlife habitats that differ from the adjacent areas by providing more vertical structural diversity. This can be very important to wildlife, especially birds. It was once thought that all forests developed from seral stages to stable old-growth stages in a predictable fashion. We now know that site conditions change over time, influencing the rate and direction of forest development.

     a. Stand Initiation or “Open” Phase

Death or removal of most trees from a site, whether by wind, fire, logging, or some other disturbance, greatly changes the environment at the forest floor. More sunlight reaches the ground, so temperature increases. Grasses, forbs, and shrubs are the dominant vegetation. Decomposition increases the availability of many nutrients that were bound up in living vegetation. Available water also may increase since trees are no longer taking it up from the soil and transpiring it through the leaves.

The soil and forest floor are very important. Energy and nutrients stored in the forest floor are released as the ecosystem adjusts from the disturbance. New plants grow, capturing and cycling the nutrients. The stand initiation phase ends when trees, shrubs, and other plants capture all the available growing space on a site. The many species of plants that grow on the site during the stand initiation phase include not only trees but shrubs, grasses, and other herbaceous plants. The diversity of plant life encourages a great diversity of animal species. The many flowering and fruiting plants near the ground provide habitats for a wide range of animals—from butterflies to bears. Shrubs and herbaceous plants provide browse for deer, elk, and rabbits. These animals provide food for cougars and bears. Seed-eating and insect-eating birds forage in these open areas. Some of the plant and animal species found in this phase are generalists; that is, species adapted to a wide range of conditions. Others, such as butterflies, are specialists and cannot survive in other conditions.

The coastal Douglas-fir forests owe their existence, in part, to severe fires that infrequently burned west of the Cascades. The large size of some of these fires would have made it difficult for the seed of trees and shrubs in adjacent stands to rapidly colonize the interiors of the burned areas. The stand initiation phase probably lasted for many decades before trees filled all the available growing space. An exception to this generalization occurs when red alder seed is available. Red alder has the ability to disperse seed great distances and colonize disturbed sites rapidly. Stand development following the eruption of Mount St. Helens is a good example of red alder’s colonization ability.

     b. Stem Exclusion or “Dense” Phase

When the trees on a site have captured most of the available growing space, their crowns touch. This point, called crown closure, marks the end of rapid and successful establishment of new shade-intolerant trees in the stand. Trees must compete with other trees for limited sunlight, water, and nutrients to survive, so they must grow larger than their neighbors. Larger trees or those having a competitive advantage are able to grow into the space occupied by less competitive individuals and reduce their growth rate or even kill them. The process by which dominant trees maintain their dominance is by out-competing their neighbors for site resources. This process results in crown differentiation. The trees that compete most successfully develop large crowns above the general level of the canopy. These trees are the dominants. Trees a little smaller, but still having large crowns, are the codominants. Intermediate trees have crowns quite crowded on all sides, and suppressed trees have crowns that fall below the general level of the canopy.

During the stem exclusion phase, the stand begins to accumulate both living and dead matter. The trees in the stand have captured the available growing space, so nutrients are not readily washed out of the ecosystem. Smaller, weaker trees continue to die during this phase, while surviving trees grow taller and increase in diameter. In western Washington, the overstory casts a deep shade on the forest floor, and little or no vegetation may grow there. In parts of eastern Washington, moisture is more limiting than light. Stands may appear open, but beneath the surface, roots utilize all available growing space. During this phase, intense competition between trees often reduces the amount of shrubs and other plants in the ecosystem. Plant and animal diversity is frequently quite low. However, snowshoe hares and lynx frequently use dense, pole-sized stands of lodgepole pine and subalpine fir for hiding and to avoid intense heat and cold, while eating plants and prey in nearby stand initiation areas. Many managed forests are harvested sometime during the stem exclusion phase and, consequently, the last two phases of vegetation development are rarely achieved in traditional management of forests.

If trees are able to establish in a stand reproduced over a period of many decades, they will vary in size and ability to compete for growing space. If little difference exists in tree size and age, there may not be clear winners and losers in the competition for growing space. As a result, the stand may stagnate, with all trees developing small crowns. The size of the tree crowns will greatly influence the amount of energy produced in photosynthesis. Trees first allocate their energy to growing taller. If taller trees grow larger crowns, they capture more sunlight energy and grow large in diameter. In a stagnated stand, many trees continue to grow uniformly taller, so their crowns become small. Consequently, they grow slowly in diameter. Eventually, the trees become so tall and thin that they fall over. This occasionally happens in natural stands of lodgepole pine and also has occurred in managed plantations—especially during wind or snow storms. When trees are tall and thin, they also are vulnerable to insect attacks, and so become infested, die, and create fire hazards.

     c. Understory Reinitiation Phase

This phase marks the beginning of the end for the original trees that established following a major disturbance. Weather related disturbances such as wind and ice storms often start the reinitiation phase by killing or breaking up less dominant overstory trees, allowing some light to reach the forest floor. Shade-tolerant plants, including shrubs and trees, are able to establish, survive, and grow in the partial sunlight reaching the understory. Given little sunlight, these trees do not grow very tall. When a tree in the overstory dies and falls, it often leaves a large opening having more sunlight. A new age class of trees often establishes in the opening, and these trees grow more rapidly upward. During this phase, plant and animal diversity again increases with the addition of new age classes and structures, such as multiple canopy layers, snags, logs, and trees with forked or broken tops. Birds and animals that use large snags or broken-topped trees for nesting often thrive in the understory reinitiation and old growth phases of stand development. Several species of woodpeckers, including the pileated; Vaux’s swifts, northern goshawks, spotted owls, marbled murrelets, flying squirrels, pine martens, lynx, fisher, and wolverines choose these stages. Many beneficial insects such as carpenter ants need snags and down logs also. Deer and elk find winter shelter in these multi-canopied forests. Many of the birds and animals are habitat specialists that prefer older forests where overstory trees are beginning to die and multiple canopy layers are starting to form. Unlike generalist species that can adapt to a wide range of conditions, these specialists require specific forest structures or compositions. For instance, Vaux’s swifts use snags hollow at the top since they enter these snags from above. Marbled murrelets build their nests in living old trees whose gnarled broken tops provide suitable nesting platforms.

Many forests, such as Douglas-fir and western larch, on the west and east sides of the Cascades, respectively, are referred to as old-growth. However, they are actually in the understory reinitiation phase, since the Douglas-fir trees established following fire still make up a large proportion of the overstory. Stands in the stem exclusion phase can be manipulated silviculturally to bring about the understory reinitiation phase sooner than in unmanaged stands.

     d. Old-Growth or “Complex” Phase

In many forests, some sort of disturbance generally resets the biological clock before the old-growth phase of forest development is achieved. In a true old-growth forest, the multi-aged stand is composed of trees that established beneath the original overstory and eventually replaced it. Many plants and animals found in these forests are specialists. An interesting question is “how old does a forest have to be before it functions as old-growth”? Scientists are interested in finding if plants and animals believed to require old forest habitat can successfully live and reproduce in younger stands if they have some of the structures and compositions usually occurring only in older forests. These old growth complexes are also commonly known as habitat types.

     e. Natural and Human-Caused Disturbances Alter Development Phases

The stand development theory described above is under further study by ecologists and silviculturists, who now think, given periodic disturbances (fire, wind, floods, insect infestations, clearcut or partial cut logging), stand structures are much more dynamic as described in this figure, depicting how both natural and human-caused disturbances create a mosaic of diversity across the landscape as described below.



13. Forests and Trees in Washington - West Side Forest Types

     a. Coastal Douglas-fir

Coastal Douglas-fir forests are among the most productive forests in the world. These stands are composed of at least 80% Douglas-fir and lesser amounts of other species, including western hemlock, grand fir, Pacific silver fir, noble fir, western redcedar, Sitka spruce, red alder, bigleaf maple, and Pacific madrone. Forests of this type are widespread west of the Cascade Range at elevations from sea level to approximately 1,500 feet. Douglas-fir forests regenerate naturally following fire with seed provided by scattered surviving trees. Planting following harvest also regenerates Douglas-fir forests. Mature stands may remain healthy for decades. When death of some of these trees creates gaps in the canopy, shade-tolerant species such as western hemlock, western redcedar, and grand fir become established. Unless another disturbance renews the cycle, these shade-tolerant species will eventually take over the site. Most coastal Douglas-fir forests are essentially even-aged; not until many centuries pass and shade-tolerant species become established do these forests become more diverse in age class structure.

     b. Douglas-fir/Western Hemlock

These forests are similar to coastal Douglas-fir forests except that together, Douglas-fir and western hemlock make up 80% of the species. These are mixed species stands. Douglas-fir is usually the most common species but on less fertile or very moist sites, hemlock may dominate. The most common associated species is western redcedar. Other associated species include grand fir and western white pine. At low elevations, Sitka spruce is often present. Noble fir may be present at higher elevations in the Cascades. This forest type thrives in mild, humid climates. Following fire, these mixed stands may convert to nearly pure stands of red alder. Hemlock often seeds in, recreating the mixed species type. Sometimes hemlock forms a substantial portion of the main canopy. When stressed by high temperatures or low soil moisture, hemlock remains in the understory as other species grow over the hemlock to form the overstory.

     c. Red Alder

These forests occur west of the Cascades, usually as pure stands. Stands of red alder typically grow below 1500-foot elevation, in riparian areas, in moist coves, or in early stages of succession following soil disturbance. Elsewhere in western Washington, red alder grows mixed with other short-lived hardwoods such as bigleaf maple, black cottonwood, and Pacific willow; or with conifers, including Douglas-fir, Sitka spruce, western hemlock, western redcedar, and grand fir. Forests dominated by red alder are always even-aged. Because red alder is very shade intolerant, only dominant or codominant trees survive. Starting at an early age, red alder produces abundant annual seed. This gives it a competitive advantage over most conifers. Since red alder has no serious insect or disease problems, it will grow readily on many sites infected by conifer root-rots. Red alder improves soil fertility through nitrogen fixing and produces large quantities of litter that decompose rapidly, adding nutrients and organic matter to forest soils. Shrubs are generally an important component of red alder forests. Common shrubs associated with red alder include Pacific red elder, blueberry elder, salmonberry, thimbleberry, and devils club.

     d. Sitka Spruce

Sitka spruce occurs primarily along the Pacific Coast. It is a shade-intolerant species unlike other spruce species found in the west. It often forms pure stands within 3 to 4 miles of salt water. The white pine weevil (Pissodes strobi), a native insect, often limits Sitka spruce range. On better sites Sitka spruce often grows with western redcedar and western hemlock. Red alder occurs where light reaches the forest floor.


Pubication EB1943, Copyright 2002 Washington State University
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Epiphytic Lichens and Bryophytes

Lichens and bryophytes are common epiphytes in forests of western Oregon and Washington. Epiphytes are nonparasitic organisms that grow on plants. They derive most of their moisture and nutrients from the atmosphere.

Lichens are symbiotic associations between a fungus and a photosynthetic partner (green algae, cyanobacteria, or both). Lichens generally are grouped into three forms on the basis of their overall habit and morphology. These forms are:

• foliose: leaflike, flat and only partially attached to the substrate.
• crustose: crustlike, tightly attached to the substrate along the lichen’s lower surface.
• fruticose: shrublike, standing out from the surface of the substrate.

The foliose and fruticose lichens together are known as macrolichens, which are often divided into three functional groups, based on their role in the ecosystem:

• forage lichens: generally fruticose lichens, also known as “alectorioids” (including Alectoria spp. and Bryoria spp.), used for forage by a variety of mammals.
• matrix lichens: the remainder of the macrolichens, which typically are dominant lichen species in young forests. These macrolichens sometimes are known as “green algal foliose lichens” to distinguish them from nitrogen-fixing and forage lichens.
• cyanolichens: lichens containing cyanobacteria as the primary photosynthetic partner, which enables them to fix atmospheric nitrogen. These lichens also are known as “nitrogen-fixing macrolichens.”

Bryophytes include mosses, liverworts, and hornworts:
• mats: spreading along the surface.
• tufts: standing out in a spherical to hemispherical arrangement from a main point of attachment.

Does Mulch Improve Plant Survival and Growth in Restoration Sites?
University of Washington, College of Forest Resources
Center for Urban Horticulture
Fact Sheet #38, September 2002

Successfully restoring wildland conditions in urban areas is often a difficult process. Determining management practices to improve long-term plant survival and growth and developing effective methods for long-term weed control are both critical. This is especially true for restoration projects, where weeding, watering, and other after care is often minimal.

A research project at the University of Washington’s Center for Urban Horticulture is investigating whether mulching an unmanaged restoration site prior to plant installation is more beneficial to plant growth and survivorship than applying herbicide to the site. The research hypothesis is that plants grown in the mulched plots will show higher rates of survival and growth than the plants grown in areas sprayed by an herbicide.

The research results suggest that all restoration sites, but especially those receiving no supplemental water, should be mulched. A thick layer of organic mulch will help retain the soil moisture crucial for plant survival while also reducing growth of weeds that compete for needed resources. The mulch will also help to reduce soil erosion, provide organic matter to the plants and soil organisms, and moderate soil temperatures. These benefits, combined with other good management practices, can improve the success and survival of restoration sites while reducing the need for costly after care.