Forest Dynamics

Trees are the dominant and distinguishing life form of forests; they create the structures that make forests unique. Not surprisingly then, the knowledge gained from studying trees is fundamental to ecological forest management. This knowledge must be understood and applied at larger spatial scales, however, because managing individual trees is not practical. This need forges a link with community and landscape ecology, which study the patterns and processes of forest growth and the interactive effects of competition, succession, and disturbance dynamics.

Reproduction and Establishment

Information about how different tree species reproduce is chief among the needs for managing forests in ways that sustain them and the people who rely upon them. Different tree species have different reproductive strategies. Some species sprout from existing root systems, others rely on birds and other animals to disperse their seeds, still others evolve seed dispersal strategies that require fire, wind, or some other disturbance. The different strategies confer different abilities to establish and grow in a particular set of conditions. Trees that reproduce vegetatively can have an advantage over trees that reproduce via seed on flooded or burned sites. In contrast, seeds that remain viable in soil over decades can ensure survival of early colonizers that get shaded out during long intervals without disturbance. Species life histories offer clues to the conditions under which different trees evolved and provide guidance for managing forests to favor some species over others. Soil conditions, including structure and fertility, are vital for seedling establishment and growth.

Productivity

The combination of site fertility, climate, and species composition influences forest productivity. Productivity is a measure of potential energy of a site and can be divided into either gross productivity, which includes all production, or net productivity which excludes production used in respiration. A group of trees growing together on a site of similar soil and climate and having similar structure is called a stand. Individual tree growth can be thought of as annual growth increment, whereas stand growth is the total sum of all trees in an area. Some

Case Study 2 Warm Springs Indian Reservation, USA

An integrated approach to resource planning and management on the Confederated Tribes of the Warm Springs Indian Reservation is mandated by current tribal law and inspired by the beliefs and practices of generations. This approach requires that all resources be considered when setting objectives and envisions the coexistence of a self-reliant society and a healthy environment in perpetuity. The management plan for the forested area of the reservation provides guidelines for the stewardship of all forest resources so that future generations have the same range of options that are available today.

The Warm Springs Indian Reservation is located in north-central Oregon, USA. Approximately 50% of the 259 000 ha reservation is forest land. It is home to the Warm Springs, Wasco, and Paiute tribes. Most of the 4000 members of the Confederated Tribes of Warm Springs live on the reservation, of which less than 2% is in non-Indian ownership.

The crest of the Cascade Range forms the western boundary of the reservation; the slopes are covered with mixed-conifer forests (including Tsuga mertensiana, Abies procera, Pseudotsuga menziesii, Pinus ponderosa, Abies grandis) that gradually descend into pine-dominated forests (Pinus ponderosa, Pinus contorta) and extend east into high desert communities of bunchgrasses and juniper (Juniperus occidentalis). Geologically, most of the forested area is tied to the formation of the Cascades, which were created by uplift during the Pliocene. The deep valleys and long, gentle to steep slopes and dissected ridges of the area reveal subsequent glacial melt and volcanic activity, which deposited andesites overlaid with pumice and ash. Deep soils typically occur at mid to lower slope positions, whereas upper slopes often have shallow, gravelly soil.

The mixed-conifer forests are managed to provide an array of commercial, subsistence, and ceremonial products, including timber, deer (Odocoileus spp.), and huckleberries (Vaccinium spp.). Conditional use areas on the reservation are designated to protect cultural resources. The tribal forest management plan has been certified by the Forest Stewardship Council (FSC) (see Case study 3 for details on the FSC).

Mixed-severity fire regimes with return intervals of 30-200 years interact with root disease and defoliating insects to create a patchy landscape comprised of multi-aged stands. One aim of ecological forest management is to use silvicultural and harvesting systems that match site conditions and forest dynamics. Accordingly, shelterwood systems, variable-intensity thinning, and aggregate retention clearcuts are favored on the reservation. Depending on slope, harvest systems include ground-based and cable operations.

Warm Springs Forest Products Industries, a tribal industry, operates an FSC-certified mill on the reservation, so most logs are hauled there over a network of local, state, and federal roads. The mill produces framing and industrial lumber, mostly from Douglas fir, white fir, and ponderosa pine; logs of other species are sometimes sold off the reservation to maximize income. An important objective of Warm Springs Forest Products Industries is to provide jobs to tribal members (see Figure 3), and its mission is to maximize the value of the forest resource for the Confederated Tribes of the Warm Springs.

Figure 3 Sawmill operations on the Warm Springs Indian Reservation, Oregon USA.

The mill can produce FSC-certified kiln-dried lumber and currently operates a 3 MW biomass power generation facility. It uses an on-site boiler to generate steam energy from mill residues. A proposed biomass energy plant might be co-located at the mill site.

By S. Hummel evidence exists that forest productivity can increase with species mixtures because of the different ways resources are exploited and space is shared. At different levels of competition, a tradeoff exists between individual tree growth and the growth of a stand, as illustrated by Figure 4.

Development

In some forest stands, development begins with a severe disturbance; hence, the developing plant community is generally even-aged. When less-severe disturbances affect forests, the result is often mixtures of age classes

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Figure 4 Tradeoff between tree growth and stand density as compared to stand growth and density. Both cannot be maximized at the same time in a single even-aged stand. Reproduced from Long JN, Dean TJ, and Roberts SD (2004) Linkages between silviculture and ecology: Examination of several important conceptual models. Forest Ecology and Management 200: 249-261, with permission.

Figure 5 Mixed-conifer forest in the Jack Creek watershed of the central Oregon Cascades USA.

that may be intermingled spatially or dispersed in clumps (see Figure 5).These uneven-aged or multi-aged stands are often more complex than even-aged stands because rates of growth affecting each age class differ. Similarities include the processes of trees getting larger, but declining in numbers, and continual competition for resources or growing space.

Stages of stand development can be classified by key stand structures and processes typical of each stage

Figure 6 Schematic representation of stand development from a regenerating even-aged stand (a), through a series of competitive stages (b and c), to mortality (d), and regeneration of a second cohort (e). Reproduced from Long JN, Dean TJ, and Roberts SD (2004) Linkages between silviculture and ecology: Examination of several important conceptual models. Forest Ecology and Management 200: 249-261, with permission.

Figure 6 Schematic representation of stand development from a regenerating even-aged stand (a), through a series of competitive stages (b and c), to mortality (d), and regeneration of a second cohort (e). Reproduced from Long JN, Dean TJ, and Roberts SD (2004) Linkages between silviculture and ecology: Examination of several important conceptual models. Forest Ecology and Management 200: 249-261, with permission.

(as illustrated in Figure 6), although patterns of stand development vary by species and site. Stand structure includes the horizontal and vertical distribution of forest elements, including the heights and diameters of live and dead trees and the arrangement of foliage, crown layers, shrubs, herbs, and down wood. Although such classification methods can help link ecosystem processes and stand development, they also have limitations because they group continuous variables like diameter and foliage distribution, light conditions, and inter- and intraspecific competition into categories. Patterns of development are discernable in forests of all latitudes, but the rate of change varies with available resources and along gradients of temperature and moisture.

Disturbances redistribute carbon in a system and influence resource availability. They can occur at the scale of an individual tree or be much larger. Understanding the disturbance regime under which a forest evolved can provide important information on the possible continuum of intensity, frequency, and severity of management treatments. Scientific debate exists on the relation between disturbance frequency and biodiversity. A contemporary proposition is that silvicultural practices patterned on natural disturbance dynamics may conserve biodiversity in managed landscapes.

Silvicultural Principles

Silviculture applies theories and models from population ecology and biometrics to predict forest development. Two predominate. Size-density theory implies that in plant populations - such as a stand of trees - growth reaches a point where for some trees to get bigger, others must die to make resources available. Figure 7 represents a conceptual self-thinning trajectory of an even-aged stand undergoing density-dependent mortality over time. Whereas size-density theory relates to the trees per unit area of an average size, growth-growing stock relations describe the expected rate of growth for any number of trees per unit area and stage of stand

Figure 7 Self-thinning trajectory of an even-aged stand in relation to an upper boundary or maximum relative density line. In theory, any combination of tree size and density is possible below the upper boundary, but not above it. Reproduced from Jack SB and Long JN (1996) Linkages between silviculture and ecology: An analysis of density mangement diagrams. Forest Ecology and Management 83: 1-3, with permission.

Figure 7 Self-thinning trajectory of an even-aged stand in relation to an upper boundary or maximum relative density line. In theory, any combination of tree size and density is possible below the upper boundary, but not above it. Reproduced from Jack SB and Long JN (1996) Linkages between silviculture and ecology: An analysis of density mangement diagrams. Forest Ecology and Management 83: 1-3, with permission.

development. Understanding how individual tree species respond to density is essential for ecological forest management.

Silvicultural Guidelines

Research has extended the principle of size-density relations to forest management by developing guidelines that avoid the subjectivity and site-dependent nature of absolute measures of density (trees per unit area, for example) and instead use mean size parameters such as quadratic mean diameter. Density management diagrams are based on theories of self-thinning populations and forest production and thus link forest ecology and management. They have been constructed for dozens of temperate species and a handful of tropical and boreal ones. Such diagrams illustrate volume and diameters associated with different levels of trees per area and help managers recognize how a stand is stocked relative to a maximum density. Thinning schedules can then be designed to achieve desired objectives, such as timber production or habitat structure. The diagrams are species specific and derived primarily from research in even-age, single-species stands.

Similar tools are generally not as developed for more-complex stand structures. Better knowledge is needed about the dynamics of mixed-species or uneven-aged stands and the response of such stands to specific treatments before such tools can be developed. For example, data on the effects of residual trees on the growth and yield of regeneration are limited, as is information about maximum density in mixed-species stands. Recent research in this area is linking more fundamental concepts related to stand productivity with management. For example, stand productivity is a function of photosynthesis which, in turn, is closely related to the amount of leaves in a stand. The amount of leaves is often represented by a leaf area index which is equal to the average number of layers of foliage projected to the ground. This information is useful because it is related to light reflectance, water usage, interception of precipitation, and productivity. Silviculturists are developing ways to use this as a stocking tool that links to other uses of this information. A multi-aged stand might have leaf area index divided by age class as a basis for stocking control. Similarly, a mixed-species stand could have its leaf area divided by species to determine proportions of different species.

Implementing Ecological Forest Management

A variety of methods to harvest and regenerate trees have been developed for forests of different types. Some have been studied experimentally and benefit from knowledge gained over decades, whereas others are still being developed and adapted. The methods rely on information about ecological and economic conditions of a site, including soil characteristics and life-history requirements of plants and animals, plus human cultural practices which favor the establishment and growth of some species over others. In implementing ecological forest management, the focus is on sustaining and developing the forms and functions of specific forest types. In some places, conditions favor stands comprised of few species and age classes, whereas in others the interaction of resource availability, species life histories, and disturbance patterns give rise to complex structures and composition.

Silvicultural Systems

In simple terms, silviculture manages the growing space of individual trees through treatments that regenerate and space trees. Trees can be spaced farther apart to create open stands that favor more understory plant development and larger trees with large branches, or they can be spaced closer together to create dense stands with little understory and smaller trees with smaller branches, but more overall tree volume production. Spacing can be managed when seedlings first establish or over time with intermediate thinning treatments.

A silvicultural system is a series of treatments applied to a forest stand to create target conditions. A given stand can be directed on many pathways, depending on management objectives. This is accomplished by altering the timing and intensity of silvicultural treatments to control light conditions and species composition. Traditional silvicultural systems represent a gradient of tree removal; the heaviest removals typically promote development of structurally and compositionally uniform conditions while the lightest removals may have little effect on diversity. Treatments between these extremes often promote the most complexity.

Even-aged silvicultural systems produce a single cohort of regeneration, which means the newly establishing trees are close in age. Clearcutting is an even-aged system that removes almost all trees, creating a fully exposed site. It is sometimes referred to as a monocyclic system, particularly in tropical forests. Seed tree systems also result in a single age class of regeneration. In this system, almost all trees are removed at harvest, but a few are retained so their seeds can produce the next generation. In contrast, a shelterwood system regenerates a stand with two age classes. It is distinguished from a seed tree system by using shade intentionally to give the seedlings of desired species a competitive advantage over other vegetation during their germination and early growth. If the shelter trees are removed after regeneration, the stand is even-aged.

Uneven-aged silvicultural systems regenerate a stand with three or more age classes. The objective is a forest with trees of different ages or sizes intermingled, typically accomplished with some form of selection system. In these systems, mature and immature trees are felled to create or maintain uneven-aged stands. Single-tree selection fells individual trees and generally tends to increase the proportion of shade-tolerant species in mixed-species stands. Group selection creates small openings and, therefore, maintains a higher proportion of shade-intolerant species in mixed species stands than does single-tree selection. Partial cutting is a general term denoting something other than a clearcut and can include selection systems. In tropical regions, selection systems are also referred to as polycyclic systems.

The methods used to manage structural and compositional diversity depend on site-specific conditions as well as on management objectives. For example, thinning is one silvicultural technique applied in various ways and for various objectives. It is a treatment to reduce tree density and alter spacing. Thinning can also be used to modify the stand structure to achieve greater vertical structural diversity, modify species composition, or encourage the development of new cohorts of trees or other plants. Precommercial thinning implies that trees cut are not merchantable, whereas in commercial thinning the trees are valuable enough to recover some of the costs of their harvest. When trees are cut from the lower crown classes to favor those in the upper classes, the term used is thinning from below. In contrast, thinning from above involves cutting dominant and co-dominant trees to favor the best trees of these classes. By targeting specific diameter classes or strata, thinning can be used to alter wildfire behavior, to recruit specific understory plants, to provide thermal and hiding cover for animals,

Figure 8 Variable retention harvesting in Pacific Northwest of North America. Some trees are left within the cutting unit in clumps.

and to grow trees with specific characteristics for wildlife habitat. For example, spatial patterns of residual trees can be altered to leave random or clumpy patterns depending on management objectives (see Figure 8). Although contemporary practices like variable density thinning do not have the benefit of long-term data supporting their intensity or design, they do rely on established theories of plant density and tree growth. Further, extensive silvical information on the shade tolerance and growth form of various species can be used to predict responses of mixed-species stands to various types and intensities of thinning.

Harvest Systems

What occurs during and after intermediate treatments and final harvest determines whether and how a forest will be renewed, and its future composition. Chief among the effects are the condition of the soil and the condition of any advanced regeneration and its composition. Because harvest systems alter forest canopy and structure, it is important to control the removal of products in ways that site microclimate and soil are sufficient to regenerate desired tree species. This involves simultaneous consideration of the forest itself and available labor and machinery. Designing a harvest system involves weighing biotic and abiotic factors of forest inventory and topography. Minimizing damage to residual trees, other plants, animals, and soil is possible by the methods chosen and the timing of forest operations. The same property that makes wood so useful, lightness relative to strength, also makes it bulky in relation to its weight; so harvesting and transportation can be expensive. Harvest systems, which include falling, skidding, and hauling, are an important part of harvest planning. These systems include ground-based, cable, and helicopter; they differ in the amount of associated soil disturbance and in their cost. System selection considers factors like slope, value and size of trees removed, equipment availability, and site objectives. Helicopter logging disturbs soil and site

Figure 9 Helicopters can be used to harvest timber from areas where road construction and skidding of logs over the terrain would potentially cause severe erosion. The helicopter shown here is a heavy-lift machine capable of lifting an external load of about 11.31 under optimal conditions.

vegetation the least (see Figure 9), but it is the most expensive system. In contrast, ground-based systems are less costly, but affect a larger area. The systems all rely on equipment to cut trees and remove logs to an area where they can be loaded on trucks or other transportation. At its simplest, this could include using a handsaw to fell trees and delimb them, then attaching ropes or cables to drag, or skid, them from a site with a tractor or animal.

Increased mechanization can occur at each step. Specialized machines have been invented for use on a range of terrain and in a variety of forest conditions. In tropical forests, where ground-based systems predominate, reduced-impact logging (RIL) emphasizes minimizing damage to the residual stand with preharvest planning and postharvest assessments. It includes designing roads to provide access to a harvest unit, while minimizing soil disturbance and protecting streams, using directional felling of trees, winching logs to planned skid trails, and using yarding systems that protect soils and residual vegetation. A case study from the Republic of Congo (see Case study 3) illustrates the value of harvest planning. In some tropical forests, cutting vines in advance of falling operations is necessary to avoid pulling down neighboring trees during harvest, which can endanger workers and create large gaps. By limiting the season of harvest operations and the type of equipment used, it is possible to moderate immediate disturbances to nesting and breeding animals and soil structure. On sites with heavy rainfall and soils with high-erosion potential, operations can aim to maintain some canopy cover. Similarly, on sites where shrubs, herbs, or non-native plants are a concern, reducing their competitive advantage by keeping them in shade can be a useful practice. Postharvest effects such as erosion can be anticipated by installing water bars and culverts in roads.

Regeneration

Postharvest regeneration of trees is essential. Preharvest planning can promote favorable conditions for seedling establishment or continued growth of advance regeneration following harvest operations. On some sites or for some objectives, seedlings might be planted rather than naturally regenerated. This could be done to offset expected mortality on harsh sites, to increase the proportion of a particular species, or to regulate spacing. Advances in seedling production and nursery operations are occurring around the world.

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