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Forest Structure: A Key to the Ecosystem

Thomas A. Spies, USDA, Forest Service, Pacific Northwest ResearchStation, Corvallis, Oregon, USA, 97331

Pages 34-39 in J.A. Trofymow and A. MacKinnon, editors. Proceedingsof a workshop on Structure, Process, and Diversity in SuccessionalForests of Coastal British Columbia, February 17-19, 1998, Victoria,British Columbia. Northwest Science, Vol. 72 (special issue No. 2).

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Thomas A. Spies, USDA, Forest Service, Pacific Northwest Research Station, Corvallis, Oregon, USA, 97331

Forest Structure: A Key to the Ecosystem

AbstractForest structure is both a product and driver of ecosystem processes and biological diversity. It has become apparent in recentyears that changes in forest structure as a result of management for timber production have undesirable consequences for othercomponents of forest ecosystems. The objective of this paper is to provide an overview of what we have learned about theecological roles of forest structure in the Pacific Northwest and how forest structure changes as a result of disturbance andsuccession. Forests are structurally diverse, but many structures derive from the same processes of disturbance and growth.Consequently, measurements on a few structural attributes can be used to estimate many other structural conditions. Particularlyimportant components of forest structure include live-tree sizes, vertical foliage distributions, horizontal variation in canopydensity, and coarse woody debris. Knowledge of the ecological roles of these structures has increased in recent years and we nowhave a general understanding of how these structures change during succession. Although the ecological values of forest struc-tures are now more widely appreciated, we still have many significant knowledge gaps including the ecological roles of below-ground structure, woody debris, and landscape pattern.

IntroductionForests are three-dimensional systems whose bio-physical structure plays major roles in ecosys-tem function and diversity. Forest structure canbe thought of as both a product of forest dynam-ics and biophysical processes and as a templatefor biodiversity and ecosystem function. Conse-quently, understanding forest structure can helpunlock an understanding of the history, function,and future of a forest ecosystem. The importanceof structure is especially apparent in coastal for-ests of northwestern North America where treescan reach great heights and diameters (Waringand Franklin 1979). In these forests, structuresplay many roles in the ecosystem, e.g. large leafareas intercept radiation and precipitation, gapsin dense canopies allow trees, shrubs, and herbsto regenerate, and large live and dead trees pro-vide specialized habitats for many species (Franklinet al. 1981, Spies and Franklin 1988, Franklinand Spies 1991). Forest structure is shaped bynatural forces such as wind, fire, and succession.Increasingly, the structure of forests at stand andlandscape scales is controlled by forest manage-ment (Spies and Cline 1988, McComb et al. 1993).Managers have typically manipulated and restrictedvariation in forest structure to maximize timberoutputs (Franklin et al. 1981 Hansen et al. 1991).However, as the range of forest structures becomeslimited, so does the diversity of wildlife habitatand other values. Knowledge of patterns of varia-

tion in forest structure over time and space canserve as the basis of forest management strate-gies that seek to sustain a broad array of forestgoods and services (Spies et al. 1991, McCombet al. 1993).

Our knowledge of forest structure, its dynam-ics, and its significance in ecosystems has advancedconsiderably since some of the first efforts tounderstand the ecological importance of foreststructure (Franklin et al. 1981, Harmon et al 1986,Spies et al. 1990a, Ruggiero et al. 1991, Spies1997). However, knowledge gaps remain, leav-ing considerable uncertainties about the ecologi-cal role of forest structure. In this paper I willbriefly review some of what we have learned aboutforest structure in the Pacific Northwest and identifyareas where our knowledge is especially deficient.In particular, I will examine what we know abouthow forest structure changes as a result of distur-bance and succession in coastal Douglas-fir/west-ern hemlock forests (Pseudotsuga nienziesii/Tsugaheterophylla). I will focus on four major compo-nents of forest structure: live-tree size distribu-tion; vertical foliage distribution; horizontal pat-tern; and coarse woody debris.

Components and Patterns

The term 'forest structure' encompasses manythings and can be described in numerous ways.Essential attributes of forest structure include:structural type, size, shape, and spatial distribu-

34 Northwest Science, Vol. 72, Special Issue No. 2, 1998© 1998 by the Northwest Scientific Association. All rights reserved.

TABLE 1. Components of forest structure.

FoliageLeaf areaVertical distributionLeaf shape, densityCanopy gaps and horizontal pattern

Tree CrownsShapeLengthLife form (e.g. deciduous, coniferous)Diameter, area, densityPosition in standBranch characteristicsCavities, breakage, decay

Tree BarkTextureThickness

Tree BolesDiameterHeightCavities, breakage, decayGaps and spatial patternAge distribution

Wood TissuesVolumeBiomassType (e.g. sapwood, heartwood)

Standing Dead TreesDiameterHeightDecay stateVolume, massCavities

Fallen TreesDiameterHeightDecay stateVolume, mass

Shrub, Herb, and Moss LayersBiomass, volumeHeightLife formSpatial pattern

Forest Floor and Organic LayersDepthDecay state

Pit and Mound TopographyAreaIleight/depth

RootsSizeDensity, decay stateBiomassSpatial pattern

Soil StructureAggregationsOrganic matter distribution

Landscape StructureStand/patch type distributionPatch sizePatch shapeHabitat connectivityEdge density

tion (vertical or horizontal) of components. Foreststructure comprises numerous components (Table1), many of which are fundamental to the func-tioning and diversity of ecosystems. For example,forest canopies vary both vertically and horizon-tally and play major roles in intercepting radia-tion, controlling microclimate, and determininghabitat. In addition, the perspective of the observer,(i.e. above canopy, forest interior, or below ground)influences the appearance and kind of ecologicalfunctions observed. From above the canopy wecan measure the effects of canopy surface struc-ture on reflectance and gas fluxes, and we areable to study epiphytic habitat structure and canopydecadence. From below the canopy, we can studythe effect of structure on transmission of lightthrough canopies and gaps, and measure relative

crown position and shape. Within stands, mea-surements of tree sizes, biomass, and distributionsof shrub and herb layers are important to under-standing forest growth and habitat potential. For-est structure of the soil surface and below groundare not well studied (Spies et al. 1990b, Griffithset al. 1996) but are very important to forest growth,nutrient cycling, and habitat for vertebrates andinvertebrates. In general, our ability to see andmeasure patterns (e.g. spatial distribution of for-est trees and stands) and structure is ahead of ourunderstanding of the role of structure in ecosys-tem processes.

Quantifying relationships among different foreststructures simplifies the process of measuring,understanding, and managing forest structure. Forexample, scientists have used al lometric relation-

Forest Structure: A Key to the Ecosystem 35

ships within trees to predict crown characteristicsfrom stem diameters (Waring and Schlesinger1985). At the stand level, overstory canopy char-acteristics such as stem density and gap size havebeen linked to composition and dynamics of treeregeneration (Gray and Spies 1996, 1997). Struc-tural attributes also change with time. A compre-hensive study of stand structure over a 900-yearchronosequence of Douglas-fir/western hemlockstands in western Washington and Oregon (Spieset al. 1988; Spies and Franklin 1988, 1991) indi-cates that two general trends exist (Figure 1, Table2). One structural trajectory, followed by attributessuch as tree size, biomass, and forest floor depthfollows an 's-shaped' pattern during successionafter stand-replacement disturbances. This path-way is related to stand growth phenomena thatoccur following stand replacement disturbances.Tree growth and biomass increase slowly at first,then increase rapidly until growth rates and bio-mass accumulations slow as trees reach maximumsize and sites reach maximum capacity to sup-

100 200 300 400

500

600

700

Stand Age

Figure 1. Idealized changes in ecosystems attributes duringsuccession in Douglas-fir forests. (See Table 2 fordescription of curves, adapted from Spies andFranklin 1988.)

port vegetative growth and biomass. The othertrajectory, tollows a 'u-shaped pattern and in-cludes attributes such as amount of coarse woodydebris, biomass, and diversity of the shrub andherb layers. This pathway is followed by compo-nents that attain high levels in early stages of suc-cession, either because of carryover from the pre-vious mature stand (e.g. woody debris) or becausethe open conditions are favorable to growth (e.g.shade-intolerant plants). These components thendecline with decay or canopy closure and thenincrease again later in succession as stands pro-duce more dead wood and gaps open in the cano-pies. Although individual stands develop in a widevariety of ways, general tendencies allow one topredict the characteristics of one type of foreststructure from knowledge of another (e.g. foli-age height distributions from tree dbh variation)(Spies and Franklin 1991) and to predict futurestates of a population stands from knowledge oftheir current forest structure (e.g. knowledge ofcurrent size/age distributions and species of livetrees can be used to estimate future characteris-tics of dead trees).

Four Important Components of ForestStructure

Particularly important components of forest struc-ture include: (1) tree size/age distribution, (2)vertical foliage distributions, (3) horizontal canopydistribution, and (4) dead wood. The traditionaland most common measures of forest structureare the size and age distributions ofthe trees (Smith1986). Size distribution of living trees is closelylinked to many other structural features (e.g. fo-liage distribution, crown attributes) or the poten-tial to produce other features (e.g. dead wood ofdifferent sizes). Size distribution and densities perunit area are used to calculate growth and yield

High

U)

a)Ta

•=t-

Low

0

TABLE 2. Idealized patterns of change for forest structural characteristics during natural succession in Douglas-fir forests(adapted from Spies and Franklin 1988).

Characteristics following a `u-shaped' curve Characteristics following an `s-shaped' curve

Amount of coarse woody debris

Number of large snags

Dead trees as a percentage of ecosystem biomass

Spatial heterogeneity of herbs, shrubs, and tree regeneration

Plant species diversity

Vertebrate species diversity

Susceptibility to fire

Average size of dominant trees

Diversity of tree sizes

Incidence of broken tops and other signs of decadence

Forest floor depth

Surface area of boles and branches

Vertical foliage diversity

Live biomass

36 Spies

as well as make decisions about harvesting orthinning forest stands. Size distributions are alsorelated to important habitat elements such ascanopy layering and nest site availability. Treesize is indicative of age distributions but the rela-tionship between age and size is frequently notvery strong. However, even-aged stands tend tohave relatively narrow bell-shaped size distribu-tions, whereas in uneven-aged stands, diameterdistributions approximate negative exponentialfunctions or exhibit a series of peaks that repre-sent establishment events (Smith 1986).

Size distributions of trees in unmanaged co-niferous forests are strongly related to disturbancehistory and time since the last stand-replacementdisturbance. Typical patterns of size distributioncan be identified, although many stands will de-viate from idealized patterns. In centuries-old, late-successional forests, frequency distributions oftrees typically approximate a negative exponen-tial distribution. Intermediate disturbances suchas partial fires can remove understory and over-story trees, altering horizontal and spatial patternof canopy foliage. In some cases, different dis-turbance histories can produce similar size dis-tributions of trees. For example, where partial firesleave large remnant Douglas-firs, late-successionaldiameter distributions (negative exponential) canarise in less than 100 years instead of four or fivecenturies (Spies, unpublished data). Processes orspecies that are sensitive to conditions in multi-layered canopy forests may not distinguish be-tween the two cases.

If similar forest structures can arise from dif-ferent stand histories then there may be optionsto create desired stand structures through silvi-cultural practices. It is becoming increasing clearthat traditional forest management practices willnot produce the structures found in old-growthstands or will not produce them at the same rateas in natural stands. For example, Tappeiner etal. (1997) found that growth rates of individualyoung trees in dense forest plantations are muchslower than those of old-growth trees in unmanagedforests when those trees were young. If the objec-tive of management is produce old-growth struc-tures from forest plantations, future forests willprobably not have the same structural characteris-tics as current natural old-growth forests, unless standdensities in plantations are reduced now.

Foliage layering or vertical foliage distribu-tion is another component of forest structure that

plays important roles in wildlife habitat, absorp-tion of solar radiation, and in the microclimate ofthe forest. Forests can have distinctive horizontallayers of vegetation, but typically foliage is dis-tributed more continuously from the forest floorto the upper canopy with peaks in the profile.During succession, forest foliage distributions tendto increase in height and evenness. Evidence ismounting that other forest species such as birdsand epiphytic lichens also respond to this verti-cal gradient of structure and microclimate. Forexample, vertical foliage diversity may facilitatethermoregulation by northern spotted owls (Strixoccidentalis caurina) (Gutierrez 1996). Epiphyiticlichen species are distributed at different heightswithin multilayered Douglas-fir forests (McCuneet al. 1997). Until recently, it was nearly impos-sible to measure foliage distributions directly andmost studies had to use surrogates such as treeheight and canopy depth to evaluate foliage dis-tributions in stands. New remote sensing tools suchas laser altimetry now give us the ability to di-rectly measure this attribute of forest structureand examine its variability across landscapes(Weishampel et al. 1996. Lefsky et al. in review).

Forests are horizontally structured into a mo-saic of different canopy densities and gaps. Be-cause light is limited within most forest systems,variation in foliage density plays important rolesin regeneration and understory development. Gapscontribute to spatial diversity, facilitate tree re-generation, and enable herb and shrub species togrow and reproduce within late-successional forests(Eck 1984; Stewart 1984; Taylor 1990; Spies etal. 1990a; Lertzman 1992; Gray and Spies 1996,1997; Van Pelt and North 1996). The role anddynamics of gaps change with succession. Canopygaps become larger and close more slowly as standsbecome older (Spies et al. 1990a). In contrast togaps, patches of remnant canopies help creatediversity in early successional forests (Goslin1997). Remnant trees can retain epiphytic lichenspecies and become a source of propagules topopulate the canopies of younger conifer stands(Sillett and Neitlich 1996).

Coarse woody debris plays many roles in for-est ecosystems, including wildlife and fish habi-tat, water storage, nutrient cycling, and soildevelopment (Maser and Trappe 1984, Harmonet al. 1986). Its role in terrestrial and aquatic habitatis generally understood. Many species of terrestrialvertebrates, invertebrates, plants, and fungi usedecaying wood as shelter, as substrate, and as an

Forest Structure: A Key to the Ecosystem 37

energy source (Maser and Trappe 1984). In streams,dead wood helps Li eate habitat complexity lotsalmonids (Sedell et al. 1988). The role of deadwood in site productivity is less clear than its roleas habitat. It can contribute nitrogen to soil eco-systems via fixation; however, this contributionmay be small relative to other sources (Harmonand Chen 1991). On dry sites, decayed wood maycontribute structures to the soil, increasing waterholding capacities (Harmon et al. 1986).

The abundance and distribution of dead woodin a forest is strongly controlled by disturbance his-tory. Although old forests typically accumulate rela-tively large amounts of dead wood, the highestamounts are found in very young forests that origi-nate following disturbances that kill overstory trees(Spies et al. 1988). Consequently, the greatest dif-ference in the structure of managed vs. natural for-ests is probably in young stands which typicallyhave low amounts of dead wood under traditionaltimber management systems (Spies and Cline 1988).Coarse woody debris in streams comes from twomajor sources: (1) streamside disturbances andmortality; and (2) landslides and debris flows inheadwall areas, which entrain wood and deliver itto larger stream channels. Management of streamchannel structure and watersheds will need to takeboth sources of wood into account.

Conclusion

Studies over the last 20 years have shown us thatforest structure is more than just variation in tree

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Forest Structure: A Key to the Ecosystem 39