by tire or soilremoval of und.lend soil providen large, multi~~cies may heCOIre high enoug~ It)regeneration of It)

C\Imopy. The tree ...led in a new gaI the canopy 0lilable resourCI

ne of two waY1Jenmg, or CTO'

p extend laterallY 111II&s. lateral closure;.~ne forests, gaps" '11;

b maJ!c ecause unde~t~rea beneath the gap'

can~)pytree speC!~~torest the composlbal.'.,

ta~ned solely by ~.

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I:anopy gaps. Evell'~ntain a small numbci~1rlmghthe infrequ~i'pical forests characiit has been hypothe_1tan be differentiated.t..ies across a rangeor~ .".,s suggests thatgapsI.;pecies richnessand'~

.(nems of the biota~i:animals. gaps may~cal habitat thatdoesi a..lopies. Bird species! "

ms been foundtobe.

I.,

' E,

:>ed areas of forest.~

'e important habitl[l~~

,ty-nesting birds and: .

ns that use decayed~.The production of,ft

:st shrub and herb,me species m.aybe~

the. understones 01i:nany forests" g~PSIIt provide a shlftmg£"lai"i". th' """"

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ntireforestecos~,CLI.\IATOLOGY; Fo/t":fi

GR/:;£NHOUS£ t:Ff1')

.

f Science & TeC~

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,

.

,

.ThomasA. S~.' .

T. A. Spies (orga- "'osvstems (sym~., '

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: concentrations of fI. I t

c change.C "ria~..P. Neilsonet aI....

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I' ~ eMf1,00 k- 0 f 5C;eM c.e ~ Tee4t481';)11 ("'1)Forestecology 169

'uvity of ecological landscapes and regions toclimatic change, Environmental Protection

y, EPA~3-89-073. 1989;D. .~. Perry et al:,iesmigrationand ecosystem stablhty ~unng ch-change: The belowground connectIOn, Con-Bioi., 4:266-274, 1990; S. T. A. Pickett and

. S.'White(eds.), The Ec~logy of Natural Distur-ce and Patch DynamICs, 1985: W. J. Platt and

R.StrOng(eds.), Special feature: Treefall gaps and't dynafi1ics,Ecology, 70(3):535-576, 1989;

$, H.Schneider,The greenhouse effect: Science andpolicy,Science. 243:771-781, 1989; W. E. Shands.,d J. S. Hoffman, The Greenhouse Effect, Cli-trIIlleChange, and U. S. Forests. 1987; C. Uhl etiI.. Vegetationdynamics in Amazonian treefall gaps,Ecology,69(3):751-763,1988.

!!!!!!!=

Forestecology

The forest canopy is the layer of the forest ecosystemthatis photosyntheticallyactive, and is an importantinterfacebetween the atmosphere and the biosphere.Thestructureof the canopy determines the diversity of

. habitatand food resources for associated plants andanimals. Canopy interactions with the atmospherecontributeto forest health, air quality, and climate.

Canopystructure. Canopy structure develops asforestsage, often culminating in dense. multilayeredcanopiesthat may exceed 225 ft (70 m) in height intemperate and tropical rainforests. Forest canopies inlessfavorable environments are more open, as a resultof wider tree spacing, and often reach only a fewmeters in height. Canopy structure is detenninedprimarilyby resource availability and disturbances.

Effectof resourees.Canopy structure reflects tree.adaptationsto acquire adequate water and light. Wateravailability and snow accumulation influence tree

crownheight, shape, and spacing. The conical shapeof boreal conifers is an adaptation for shedding snowand reducing winter breakage; the funnel shape ofcrownsin xeric forests is an adaptation for channelingYiaterto roots; the arching form of rainforest trees is anadaptation for shedding water.. Forests with closed canopies or foliage characteris-~ that limit light penetration have shallower cano-r than do forests wi~ open canopies. Similarly,~orestsat low latitudes with high sun angles (tropicalorests)maximize interception of light from above

~11hshallo~ly domed crowns (Fig. 1). Subcanopies::at heIghts where cones of light penetratinglow upper canopies intersect to form a zone of

~uate light intensity. By contrast, trees at high~ ltu~s and low sun angles (boreal forests) maximizeght Interception from the side with narrow crowns

~I.conifers). Light penetration under these con-"'uons .fife IS tn~ufficient for subcanopy development.

trice::. 01dIsturbance. All forests periodically experi-~h..sturbances that alter canopy structure. Cata-~ h~C.disturbances, such as wildfIre, landslides,~e: Tneanes, remove the canopy over relatively

areas every 50-500 years and initiate canopy

development. Small-scale disturbances that injure orkill individual trees or small groups of trees are morefrequent; such disturbances include wind and stormdamage, drought, breakage under the weight of snowor accumulatedepiphytes,factorsthat increasecom-petition for light and other resources, and infestationsof insects and pathogens.

All tree species are adapted to survive some adverseconditions that kill or suppress other tree species. Forexample, shade-adapted tree species tolerant of lowlight intensities wil\ be favored in gaps that are smallrelative to sun angle or canopy height; shade-intoleranttree species requiring full sunlight will be favored inlarge gaps. In the absence of further disturbances orother limiting factors, shade-intolerant species areeventually replaced, through the process of succes-sion, by shade-tolerant species that can germinate andgrow under the canopy. Therefore. disturbances tendto maximize forest diversity by maintaining a patch-work of varied canopy structures.

Canopycomposition.Half of all knownorganismsmay inhabit forest canopies. Older. more complexcanopies with moderate temperature and humidity areespecially diverse. Epiphytic plants intercept waterand nutrients,and provideadditionalfood resourcesand, in many cases. aquatic habitats. Small pools intree cavities and epiphytic bromeliads are protectedhabitats for many amphibiansand invertebrates.Avariety of vertebrate and invertebrateherbivores,predators, and decomposers also make their homes inforest canopies and influence forest processes. Insectsare especially well represented. with perhaps severalmillion species in tropical canopies.

Most canopy organisms are highly specialized andrarely. if ever. are seen living near the forest floor.Many are restricted to particular tree species or canopylayers (Fig. 2). In turn, the importance of a treespecies to canopy structure and function often reflectsthe presence of associated species that affect pollina-tion, seed dispersal, growth rate, or branching pattern.

...

'................

........

llC

canopy width, m

Fig. 1. Schematic representation of the canopy of a boreal forest (left) and a tropical forest(right) drawn to scale. The limiting angle for light penetration through the boreal canopy Is15" from the vertical, and for the tropical canopy 60". 1 m = 3.3 ft. (From J. Terborgh, Thevertical component of plant species diversity in temperate and tropical forests, Amer.Natural., 126:760-776,1985)

60

50 ...."

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170 Forest ecology

50

10

oFig. 2. Layering of environments in the rainforest. Each level of the forest has its own array of plants and animals. includingpollinating insects. The two bee species at the top left were found only at the upper margin 01 the canopy, and the lower twospecies only In the lorest below. 1 m = 3.3 It. (From D. R. Perry, The canopy of the tropical rain foresl, Sc/. Amef'~251(5}:138-147, 1984)

Nutritional quality and density of foliage and otherplant tissues. size and orientation of branches. depthof bark crevices. and size of sunlit openings varywidely. especially in older forests. The variety ofthese resources detennines the diversity of associatedplant and animal species. For example. large epi-phytes with high moisture requirements are restrictedto large branches low in the canopy; insects that feedon young, succulent foliage occur on the outer mar-gins of the canopy.

Canopy resources vary in availability. suitability,and visibility as food or habitat for associated organ-isms. Availability varies spatially over the patchworkof canopy structures and often varies seasonally intrees that shed foliag~ during unfavorable periods.Suitability as food is detennined by chemical compo-sition. Nutritional quality varies with tree species andwith the age and condition of the foliage. Healthytrees of all species produce a characteristic array ofdefensive chemicals (such as phenols, terpenes, alka-loids, and animal honnone analogs) to protect sensi-tive tissues from ultraviolet radiation, and to provideprotection against animal and pathogen species thathave not adapted to detoxify or avoid these chemicals.Some of these defensive compounds are small, highlyvolatile molecules. When carried on the airstream,these chemicals become powerful signals that adver-tise plant location to adapted animal species that trackhost aerosols.

Feeding pressure by organisms adapted to plantdefenses favors plants that produce new compounds.The biochemical diversity found in canopy plants andother organisms is gaining increased attention. Ani-

mals produce their own array of biochemicals, used todetoxify host chemicals. attract mates, and deterpredators. Although the potential wealth of canopycompounds has barely been tapped, chemical screen-ing, especially of tropical species, is yielding promis-ing new pharmaceutical chemicals and natural pesti-cides. Natural products may regain attention aspetroleum-derived products become more expensive.

Canopyfunction. Forestcanopies are recognized fortheir importance to photosynthesis, evapotranspira-tion, and reduced soil disturbance and erosion. Re-search on canopy function has been limited by thedifficulty of canopy access. However, development ofcanopy access and remote sensing techniques in the19805has stimulated canopy research. Elaborate rope-and-pulley systems suspended among neighboringtrees have increased canopy access from the ground,permitting the first detailed observation of canopycommunities. Sky cranes (tall structures with !Cvolv-able booms) are being used to access larger canopyareas, to map canopy structure, and to install moni-toring equipment. Remote sensing via satellite andlaser imagery now permits studies of canopy produc-tivity, evapotranspiration, thennal flux, and foliar andatmospheric chemistry over extensive areas. As aresult of these new techniques, canopy processescontributing to forest health, air quality, and globalclimate are becoming better understood. Figure 3summarizes these canopy processes.

Foresthealth.Thediversityof canopyspeciesfunc-tions to maintain foresthealth and minimize disruptionof ecological processes. Ground cover by the canopYreduces precipitation impact and erosion, and main-

Centrisfusciventris

>.a.0c:'"uEpicharisalbofasciata

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50

;, Includinge lower two

,cals, used to. and deterh of canopynical screen-

:.ling promis- i

latural pesti- ~attention aste expensive. ~cognized.fort

.

~.

lpotransplI3-f

erosion. Re- ~nited by the ~lelopment of jliques in the ;lborate rope- "neIghboring ~

the ground.1 of canopywith revolv-

Irger canopY-installmoni-!satelliteandi10Py produc- ~nd foliar andareas. As a

'y processes.and global1. Figure 3

ipecies func-ze disruption

y the canopYI, and maiD-

.'

::'? '1\5the modemte environment essential for survival, .,. many key ~pecies. The vas~ surface area of tre.e

, ofr ge and epiphytes serves to Intercept water, nutn-(013and other particulate materials from the atmo-

~re. Some epiphytic lichens, algae, and bacteriare and convert atmospheric nitrogen, a critical

~ften limiting nutrient, into usable organic nitro-throughthe process of nitrogen fixation. Decom-

~itiOn of dead plant and animal tissues within thecanopymakes nutrients available for canopy pro-cesses.Nutrients eventually reaching the forest floorjncre3SI:soil fertility.

Evenherbivorous insects, which are often viewed

as pests, contribute to long-tenn forest health bypruningplant tissues that are photosynthetically inef-ficientor poorly defended, and releasing nutrientsfromthese tissues. Nutrient availability and forest

productivitycan be enhanced for decades followingjnsectpopulation outbreaks.

Diverseand healthy canopies tend to prevent out-breaksof particular species, Because many canopyorganismsare restricted to a particular tree species. adiversecanopy limits species ability to increase pop-ulationsize and exhaust resources. Necessary re-sourcesare widely scattered. chemically defended.andhiddenby mixing of host and nonhost aerosols intheairstream. In contrast. forest canopies composedof one or a few tree species provide abundant re-sourcesfor adapted organism<;. Adverse conditionsstressmost trees simultaneously in such forests. in.creasingthe likelihood of pest outbreaks and forestdecline.

Canopy processes are essential to the long-termhealthandstability of forest ecosystems. Disruptionoftheseprocesses through crop species selection. re-ducedbiodiversity, .::mopy removal. and other man-agementpmctices can reduce soil fertility and forestproductivity, exacerbating adverse conditions andleadingto forest decline.

Airquality.The filtering capacity of the canopy isinstrumentalin removing particles, including pollut-ants,fromthe air. Deposition of atmospheric particlesoncanopysurfaces augments other sources of essen.lialnutrients. Filtering of fog and min removes addi-tionalmaterials from the atmosphere; conifer forestcanopiesmay intercept and filter as much as 30% ofbulkprecipitation, and broadleaved canopies 20%.

Unfortunately, this filtering capacity also aggm-vat~ canopy vulnembility to atmospheric pollutants.~clddeposition and toxic chemicals affect a substan-tialPOrtionof the canopy during air passage through!he.forest. Toxic aerosols and acid precipitation de-POSitedon foliage or absorl:>edthrough stomata de-Stroyplant tissues and disrupt photosynthesis.Air!"Jllutantshave been implicated in forest declines inIl\dut .. ~ nal areas of Europe and eastern North America.

I Cllmale.Forest canopies have a substantialeffectonoca) and global climate. Shading provided by the~opy reduces soil tempemture and convective radi-

~tion,stabilizing diurnal and regional temperatures.triv~potransPiratio_nfurther cools the forest and con-

Utesto cloud fonnation and regional rainfall and

---------

Forest ecology 171

o

, I '

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~ " I 'carbon r, ' j interception,dioxide ---i> PhotoSyntheSIS\ cleaner air, I... partlc.le ~., / '

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reduced lightand heat

reduced droplet impact.increased soil fertility

FIg. 3. Canopy processes influencing interactions between the biosphere and theatmosphere. 'The net result 01 the process 01 pholosynthesls and respiration is reducedatmospheric carbon dioxide.

humidity. Deforestation can raise regional tempem-lUres and reduce precipitation.

Incorporation of atmospheric carbon dioxide, agreenhouse gas, into forest biomass through photo-synthesis in the canopy regulates the absorption ofinfrared radiation by the atmosphere. Forest biomassaccounts for at least 60% of global organic carbonstorage. Young trees are more efficient photosynthet-ically than are older trees. leading some resourcemanagers to contend that conversion of older forests toyounger forests will reduce atmospheric carbon diox-ide. However, young forests harvested every 50-100years cannot accumulate as much carbon dioxide as isreleased by harvest, burning, or conversion of olderforests. Hence, harvest likely will increase the carbondioxide content of the atmosphere. Increased atmo-spheric carbon dioxide, in the absence of coolingfactors. is projected to increase global temperatures.

For background infonnation SEE EVAPOTRANSPIRA-

TION; FOREST AND FORESTRY; FOREST ECOLOGY in the

McGraw-Hili Encyclopedia of Science & Technol-ogy.

Timothy D. Schowalter

Bibliography.R. E. Dickinson (ed.), The Geophys-iology of Amazonia, 1987; A. W. Mitchell, The

--- - - - -

172 Forest management and organization

Enchanted Canopy: A Journey of Di.~coverytothe Last Unexplored Frontier, the Roof of theWorld's Rainforests, 1986;E. A. Norris,AncielllForests of the Pacific Northwest, 1990;T. D.Schowalter,W. W. Hargrove,and D. A. Crossley,Jr., Herbivoryin forestedecosystems,Annu. Rev.Entomol., 31:177-196, 1986.

ForestmanagementandorganizationConflicts between environmental and commodity in-terests over management of forest lands have in-creased dramatically during the last several decades.Increased scientific knowledge of the detrimentalecological impacts of traditional forestpractices, suchas use of even-aged forest monocultures based uponperiodic clear-cutting. has contributed to the conflict.Issues being debated include the potential impacts ofthese traditional practices on nongame wildlife spe-cies, watershed values (including levels of sedimentproduction and tlooding). tish production. and long-term site productivity. Another issue has been theincreased societal concern for protectionof biologicaldiversity, as retlected in the National Forest Manage-ment and Endangered Species Acts.

In the United States. debate over disposal of theremaining unreserved old-growth forests and preser-vation of the northern sponed owl in the PaciticNorthwest epitomizes the contlict between environ-mental and commodity interests. Conservationists fa-vor preservation of most of the remaining old-growthforests for their environmental and recreational val-ues. One major environmental concem is maintenanceof old-growth forest habitat for species such as thenorthern spotted owl. which was recently listed by theU.S. Fish and Wildlife Service under the EndangeredSpecies Act as a threatened species. The forest prod-ucts industry, on the other hand, favors logging of theremaining unreserved old-growth forests. arguing thatsufficient acreage of old-growth forests is alreadypreserved in National Parks, National Wilderness, andother reserved areas.

Traditionally, such conflicts have been resolved byaliocating lands to primary or single uses, such asintensive timber production or preservation. Newapproaches to forest management that better integratemaintenance of ecological values with commodityproduction, sometimes called New Forestry, haveemerged as an alternative to allocation. Althoughmany of the specific practices advocated under NewForestry have their historical roots in traditional for-estry, there are many differences. New Forestry in-corporates an ecosystem view of the forest and pro-vides tools that allow a more balanced weighting ofecological values with commodity values.

Scientific basis for New Forestry.New Forestrydraws heavily on modem ecological concepts to de-sign forest practices that retain ecological valueswhileproviding for production of wood products. Theecosystem paradigm. which recognizes the forest as abiological system with many essential and highly

~~'IE

integrated parts, is ~ne critical c~ncept. For example,.natuml fo~sts typicallyhave.hlg~ le~elsO! Spati~;heterogeneity and structural dlversuy (mcludmg trees~;varied in size, species, vigor, and soundness). Th' frichness in structural characteristics is one of the maj~ ·reasons that natural forests provide habitat for a Wide~variety of organisms and exert great intluence onfecological processes, such as those involved in regu.lation of nutrient and hydrologic cycling. For exam..pie, the immensesurface areas of the multilaYered~

canopies of the old-growth forests provide condensing ,surfaces for moisture and other atmospheric materials

A second concept, biological legacies, relates to~ ~recovery of natural forests following catastrophic dis- .

turbances. such as wildfire and windstorm. There afttypically large legacies of living organisms in formsranging from mature and seedling trees to spores andseeds found in the forest floor and fossorial animals.Extensive legacies of dead organic materials. includ-ing large structures. such as logs on the forest floorand standing dead trees (snags), also carry over fromthe ecosystem before the disturbance to the recoveringecosystem after the disturbance. For example. wildfiretypically kills trees but does not consume much of thewood. Because of these legacies. natural young for.ests are typically diverse ecosystems with high levelsof structural and compositional diversity rather thansimply communities of young trees. Sn: FORESTANDFORESTRY.

ConsiderJ.tionof large spatial scales. or landscapes,and longer temporal scales is a third concept under-pinning New Forestry. This concept addresses thecritical importance of issues such a."patch sizes andarrangements, edge phenomena, corridors to wildlife,and the size of watersheds. Increasing evidence ofecological dysfunction at the landscape level. such ascumulative negative impact" on water quality fromexcessive cutting or fragmentation offorest cover. hasspurred the interests of scientists and managers inlandscape ecology.

Applicationof conceptsat standlevel. A basic prin-ciple of New Forestry at the level of the forest stand ismaintenance of structural and compositional diversitywithin stands managed primarily for wood produc-tion. This contrasts with traditional forest practicesthat have emphasized simplification of forests, forexample, creation of even-aged monocultures.

Many techniques that will promote greater speciesrichness and structural diversity can be applied toyoung managed stands. An aggressive effort to estab-lish mixtures of tree species, rather than monocul-tures, is one approach. For example, hardwood treeScan be included within a stand dominated by conifers;or soil-building coniferous species, such as membersof the Cupressaceae, can be added to plantations.Early successional plant and animal species can alsobe retained for longer periods by using wide spacingsto delay tree canopy closure.

Maintaining a continuous supply of coarse woodYdebris (large standing dead trees and downed logs) isone technique increasingly used to provide structUraldiversity within managed forest stands and associated