Tropical Forest Management and Conservation of ... · Tropical Forest Management and Conservation of Biodiversity: an Overview Introduction Parks and protected areas are essen-tial

Issues in International Conservation

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Conservation Biology, Pages 7–20Volume 15, No. 1, February 2001

This paper is a much abridged version of World Bank Environment Department Paper 75, “Biodiversity Con-servation in the Context of Tropical Forest Management,” by the same authors. It is the first paper in thebank’s Impact Studies Series, which addresses the broader questions of the positive and negative effects of hu-man activities on biodiversity. A more complete set of data supporting the arguments herein are in the appen-dices and text of the World Bank paper, which can be accessed from http://www.worldbank.org/biodiversity.

Tropical Forest Management and Conservation of Biodiversity: an Overview

Introduction

Parks and protected areas are essen-tial for biodiversity conservation butinadequate to assure the continuedexistence of the majority of naturallandscapes, ecosystems, communities,species, and genotypes in tropical for-ests. Even if a goal of 10–12% protec-tion were attained and reserved ar-eas were appropriately located andmanaged, up to 50% of tropical spe-cies would be expected to go ex-tinct during the next few decades(Soulé & Sanjayan 1998). If biodiver-sity outside protected areas is ne-glected, thousands of species arelikely to disappear. The first priorityshould be to increase the area of for-ests under strict protection while im-proving reserve management. Mech-anisms should be developed to haltroad building and commercial log-ging in forest wilderness areas aswell as in centers of diversity andendemism. But promoting more bio-diversity-sensitive management of for-ests outside protected areas is ofalmost equal priority, given the con-servation potential of these still vastareas.

In the search for land-use practicescompatible with biodiversity mainte-nance, many environmentalists havefocused on logging (for comprehen-sive reviews of the environmental ef-fects of logging in tropical forests,see Grieser Johns 1997; Haworth1999). This emphasis is not surpris-

ing given that of all forest uses, log-ging is often the most financiallylucrative and has the most severe en-vironmental effects. The indirect ef-fects of logging, particularly increasedhunting (e.g., Robinson et al. 1999)and the greater likelihood of defores-tation due to improved access (re-viewed by Kaimowitz & Angelsen1998), have also been highlighted re-cently.

What has emerged from the dust,mud, chainsaw noise, and dieselfumes is the idea that conservationof some components of biodiversitycould be facilitated by collaborationbetween loggers and environmental-ists. Some of the latter oppose theidea of promoting better forest man-agement as a means for achievingoverall conservation goals (or at leastoppose financial investment in suchefforts; Rice et al. 1997; Bawa &Seidler 1998; Bowles et al. 1998).This opposition notwithstanding, con-sideration of the inevitability of log-ging in much of the tropics, themany constraints on expansion ofnature reserves in tropical countries,the challenges of protecting and man-aging the parks already demarcatedon paper, sovereignty issues, devel-opment needs, and the implicit adop-tion of the “use it or lose it” assump-tion motivate other conservationiststo continue holding sustainable for-est management as a worthy con-servation goal in forests outside ofprotected areas (e.g., Dickinson et

al. 1996; Chazdon 1998; Poore et al.1999; Whitmore 1999).

General conceptual approaches tozoning forests for different uses havebeen presented recently by variousauthors, including Noble and Dirzo(1997 ) and Frumhoff and Losos(1998). Methods for integrating con-servation functions at different geo-graphical scales were reviewed re-cently by Poiani et al. (2000). Ourpaper focuses on the forests zonedfor timber production. Our goals areto (1) contribute to the developmentof a heuristic construct for consider-ing the range of effects of forestryactivities on tropical forest biodiver-sity at the levels of landscapes, eco-systems, communities, species, andgenes; (2) indicate the topics on whichfurther research will be particularlyuseful in evaluating the effects of dif-ferent forestry activities on the vari-ous components and attributes of bio-diversity; and (3) suggest ways tomitigate the deleterious effects offorestry activities on tropical forestbiodiversity.

To help elucidate some of the fac-tors on which the compatibility oftropical forest logging and biodiver-sity protection depend, we first at-tempt to disaggregate the terms

log-ging

and

biodiversity.

We discussthe wide range of logging intensities,logging methods, collateral damage,and silvicultural approaches appro-priate for tropical forests. A frame-work for considering the effects of

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Tropical Forest Management Putz et al.

Conservation BiologyVolume 15, No. 1, February 2001

logging and other management ac-tivities on the various forest compo-nents (landscapes, ecosystems, com-munities, species/populations, andgenes) and attributes (structure, com-position, and function) of biodiver-sity is presented. We use the compo-nents and attributes of biodiversityto review the effects of logging andother silvicultural activities on tropi-cal forests.

Disaggregating “Biodiversity”

Biodiversity

refers to the natural va-riety and variability among living or-ganisms, the ecological complexesin which they naturally occur, andthe ways in which they interact witheach other and with the physical en-vironment. This definition and theelucidation below are based on workby the Office of Technology Assess-ment (1987), Noss (1990), and Red-

ford and Richter (1999). Climate, ge-ology, and physiography all exertconsiderable influence on broad spa-tial patterns of biotic variety; localecosystems and their biological com-ponents are further modified by en-vironmental variation (e.g., local cli-matic and stream flow fluctuations)and ecological interactions. This nat-ural variety and variability is distin-guished from biotic patterns or con-ditions formed under the influenceof human-mediated species introduc-tions and substantially human-alteredenvironmental processes and selec-tion regimes (Noss & Cooperrider1994; Bailey 1996).

Biological diversity can be measured

in terms of different components (land-scape, ecosystem, community, popu-lation/species, and genetic), each ofwhich has structural, compositional,and functional attributes (reviewed inTable 1).

Structure

refers to the physi-cal organization or pattern of the ele-

ments.

Composition

refers to theidentity and variety of elements ineach of the biodiversity components.

Function

refers to ecological and evo-lutionary processes acting among theelements.

Disaggregating “Logging”

When properly planned and con-ducted, logging is an integral compo-nent of forest management systemsdesigned to promote sustained timberyields (STY) or the more all-encom-passing goal of sustainable forest man-agement (SFM). Unfortunately, log-ging in tropical forests all too oftenrepresents a timber “mining” activitycarried out without regard for re-newability of this natural resource(Putz et al. 2000

a

). Due mostly tothe desire to obtain voluntary third-party certification of good manage-ment from the Forest Stewardship

Table 1. Components and attributes of tropical forest biodiversity that might be influenced by logging and other silvicultural activities.*

Components Structure Composition Function

Landscape size and spatial distribution of habitat patches (e.g., seral stage diversity and area); physiognomy; perimeter-area relations; patch juxtaposition and connectivity; fragmentation

identity, distribution, and proportion of habitat types and multihabitat landscape types; collective patterns of species distributions

habitat patch persistence and turnover rates; energy flow rates; disturbance processes (e.g., extent, frequency, and intensity of fires); human land-use trends; erosion rates; geomorphic and hydrologic processes

Ecosystem soil (substrate) characteristics; vegetation biomass, basal area, and vertical complexity; density and distribution of snags and fallen logs

biogeochemical stocks; life-form proportions

biogeochemical and hydrological cycling; energy flux; productivity; flows of species between patches; local climate effects

Community foliage density and layering; canopy openness and gap proportions; trophic and food web structures

relative abundance of species and guilds; richness and diversity indices; proportions of endemic, exotic, threatened, and endangered species; proportions of specialists vs. generalists

patch dynamics and other successional processes; colonization and extinction rates; pollination, herbivory, parasitism, seed dispersal, and predation rates; phenology

Species/population sex and age-size ratios; range and dispersion; intraspecific morphological variation

species abundance distributions, biomass, or density; frequency; importance or cover value

demographic processes (e.g., survivorship, fertility, recruitment, and dispersal); growth rates; phenology

Genetic effective population size; heterozygosity; polymorphisms; generation overlap; heritability

allelic diversity; presence of rare alleles; frequency of deleterious alleles

gene flow; inbreeding depression; rates of outbreeding, genetic drift, and mutation; selection intensity; dysgenic selection

*

Modified from Noss (1990) and Redford and Richter (1999).

Putz et al. Tropical Forest Management

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Council (FSC), the transition fromforest mining to forest managementhas finally started to occur in someareas in the tropics (Nittler & Nash1999). Even within certified foreststhere are questions about how tomost effectively and efficiently mini-mize the deleterious environmentaleffects of logging and other silvicul-tural activities.

Forest interventions of all types,from harvesting of fruits for homeconsumption to clearcutting for tim-ber, have effects on forests that needto be understood and often deservemitigation (Peters 1996). Logging of-ten is the most damaging and gener-ally the most financially lucrative ofsuch forest interventions (Pearce etal. 1999). The compatibility of log-ging with biodiversity conservationis complicated because logging iscarried out over a huge range of in-tensities with a variety of techniquesthat may be applied carefully or inways that result in a great deal ofavoidable damage. The following sec-tions focus on the issues of harvest-ing intensities, yarding methods (howtimber is extracted from the stumpto haul roads), and ways of reducinglogging damage.

Logging Intensities

Logging intensities span more thantwo orders of magnitude (

,

1 m

3

/hato

.

100 m

3

/ha; Fig. 1), complicatingthe challenge of generalizing the ef-fects of such activities. At a smallscale (1–10 ha), the typical aggre-gated distributions of tropical trees(Hubbell 1979) leads to locally se-vere logging effects unless harvest-ing controls are implemented. Thelocalized but severe direct effects ofroads, log landings, and skid trailshardly need to be emphasized (Guar-iguata & Dupuy 1997). At a slightlylarger scale (10–100 ha), stands varyin stocking of commercial speciesand in their accessibility due to ter-rain or edaphic factors. Where log-ging is carried out in areas desig-nated for each year of management

(annual coupes), logging effects tendto be aggregated at larger scales.Logging intensities also vary overtime, tending to increase with localtimber shortages, improved access,and greater willingness of markets toaccept lesser-known species (Plump-tre 1996).

The substantial number of studiesconducted on the effects of loggingin tropical forests all conclude thatsoil effects and damage to the resid-ual forest all increase with increas-ing logging intensity (Ewel & Conde1980; Sist et al. 1998). Proportionsof both soil and residual trees dam-aged by logging range from 5% to50%, depending on harvesting inten-sity, yarding method, and the carewith which the operations are car-ried out. Interpretation of data per-taining to the relationship betweenlogging intensity and residual standdamage is complicated by concomi-tant change in residual stand den-sity; at the extreme, there is no re-sidual stand in clearcuts. Furthercomplicating assumptions about log-ging damage is the fact that few

tropical forests are logged only once.

Log Yarding Methods

Much of the direct damage to tropicalforest caused by logging occurs whilelogs are being extracted from thestump to roads or riversides fromwhich they are then hauled or towedout of the forest (yarding). Yardingmethods utilized in tropical forestsvary in technological sophistication,the collateral damages with whichthey are associated, and yarding costs(Conway 1982). The range of effectson biodiversity at the landscape, eco-system, community, population/spe-cies, and genetic levels varies greatlyalong the continuum of technologi-cal sophistication of yarding methodsthat stretches from manual extractionto the use of helicopters (Fig. 2).

Yarding can be carried out in an en-vironmentally and silviculturally sen-sitive manner, or it can be extremelydestructive. The rankings of environ-mental effects in Fig. 2 are sugges-tions of the typical amounts of dam-

Figure 1. Logging intensities (m3/ha) for tropical forests. In most of these studies, as in most logging areas in the tropics, felling was done with chainsaws and yarding with bulldozers or articulated skudders with rub-ber tires.

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age associated with different yardingtechniques. Impact is affected byboth choice of yarding equipmentand the care with which yarding op-erations are carried out, as indicatedby the contrasts between conven-tional and reduced-impact logging(RIL) in Fig. 2. Unfortunately, al-though the silvicultural, environmen-tal, and economic benefits of theplanning of log-extraction routes havebeen recognized for many decades(review by Putz et al. 2000

a

), well-planned logging operations are stillthe exception in tropical forests.

Most destructive yarding in tropi-cal forests is carried out with bull-dozers (crawler tractors). Bulldozersare excellent devices for construct-ing roads but are unfortunately ver-satile enough to yard timber as well.The excessive damage to soils andresidual trees during conventionalbulldozer yarding is well known, es-pecially at high intensities on steepslopes logged during wet weatherby untrained crews, but bulldozersstill yard much of the timber in com-mercial logging areas in the tropics.

With the loss of forest in accessi-ble areas in much of the tropics,logging is increasingly being rele-

gated to flooded, steep, rocky, orotherwise adverse terrain. Becauseground-based yarding from suchsites can be prohibitively expensive,they often have been left as un-logged refugia within harvested ar-eas. Their status as refuges is jeopar-dized by helicopter yarding, becausehelicopters can yard timber fromeven the most adverse sites. Obtain-ing timber from these sites is becom-ing more cost-effective under someconditions. Although areas fromwhich timber is harvested by heli-copters are not dissected by skidtrails, haul roads are still needed,and with them come all the prob-lems associated with increased ac-cess. An equally important concernabout helicopter yarding is that ar-eas traditionally avoided by loggersare rendered accessible and arelikely to be harvested. Because heli-copters leave no obvious trails, itwill be challenging to monitor theirharvesting effects.

Logging and Other Silvicultural Treatments

Logging can be either a cause of agreat deal of avoidable damage or

one of a series of silvicultural treat-ments designed to promote the re-generation and growth of commer-cial timber species while protectingecosystem services and biodiversity.In logging areas where sustainedyield of timber is a priority, varioussilvicultural treatments can be usedin combination with the appropriatelogging regime to promote the re-generation or growth of commer-cial species. Carrying out silvicul-tural treatments in conjunction withlogging reduces their cost while re-inforcing the idea that logging itselfcan be silviculturally useful.

The most common method for con-trolling timber harvesting in tropicalforests is simply to set a minimumstem diameter for felling. Althoughtheoretically easy to implement andmonitor, minimum-diameter rules of-ten are inimical to achieving silvicul-tural goals. The problem with thissystem is obvious where minimum-diameter limits are set below the sizesat which trees start to reproduce(Appanah & Manof 1991; Plumptre1995). Minimum-diameter rules alsodo not prevent harvesting of clustersof trees and thereby create silvicul-turally unsatisfactory conditions, aswhen commercial species are favoredby small canopy gaps or vines prolif-erate in large ones.

Logging is often the most severe ofsilvicultural interventions, but thereare other prescriptions designed to in-crease the stocking of commercialtree species or to increase the growthof trees already present. Retainingseed trees in harvested stands is onepossible way to increase stocking, butfor species that require mineral soilseed beds or minimal competition forgermination, establishment, and sub-sequent growth, seed-tree retentionneeds to be combined with variousother treatments. Seed beds can bemodified and competition can be re-duced through controlled burning,mechanical scarification, or herbicidetreatment of plants competing withseedlings of the crop species. Wherenatural regeneration fails, or whereparticularly high stocking levels are

Figure 2. Generalized ranges of direct biodiversity effects of timber yard-ing methods used for tropical forest logging as a function of level of techno-logical sophistication required (assumes equal volumes of timber yarded). For mechanized ground-based yarding operations, conventional (non–RIL), and reduced-impact logging (RIL) effects are contrasted.


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desired, many foresters have triedplanting seedlings of commercial spe-cies in gaps or along lines clearedthrough the forest. For stands inwhich regeneration of the commercialspecies is already established, variousthinning and weed-control treatmentsare often prescribed to accelerate treegrowth and to promote “stand im-provement.” Thinning around poten-tial crop trees, often referred to bytropical foresters as “liberation thin-ning,” and vine cutting are two com-monly prescribed but less commonlyapplied silvicultural treatments. Inexperimental areas where liberationtreatments have been applied to en-hance volume increments of commer-cial species, treatments are often pre-scribed at intervals of 10 years or more(de Graaf et al. 1999). As in the case oflogging, all of these other silviculturaltreatments have effects on biodiver-sity, but they have been much lesswell studied.

Reducing the Effects of Logging and Other Silvicultural Treatments

Substantial attention has been givenrecently to reduced-impact logging(RIL). Most of the practices in thevarious RIL guidelines that have beenpromulgated of late have long beenrecognized as being environmentallysound and silviculturally appropriate(Bryant 1914). Full application ofRIL techniques represents a majorstep toward SFM, but RIL alone doesnot guarantee sustainability. Especiallywhere tree species being harvestedregenerate only in large clearings,the required silvicultural interven-tions to assure sustained yield of thesame species often may be substan-tial (e.g., mimicking the effects ofslash-and-burn agriculture or hurri-canes followed by fires; e.g., Snook1996; Fredericksen 1998; Dickinson& Whigham 1999; Pinard et al. 1999;Fredericksen & Mostacedo 2000). Thissilvicultural challenge notwithstand-ing, careful planning and implemen-tation of harvesting guidelines wouldrepresent a big step toward sustain-able forest management.

For all yarding methods, loggingdamage can be substantially reduced,and logging costs reduced as well,by proper design, construction, andmaintenance of road networks. Muchof the cost of harvesting timber anda large proportion of the hydrologicaldamage (e.g., stream sedimentation)due to logging is associated withroads (e.g., Bruijnzeel 1992). Guide-lines for road construction are readilyavailable and are well outlined in theFAO Model Code of Forest HarvestingPractices (Dykstra & Heinrich 1996).Unfortunately, forest engineering stan-dards in most tropical logging areasare extremely low, and there are toofew experienced forest engineers in-volved in most tropical logging oper-ations. Because soil damage due topoor skid trail design or improperuse, for example, reduces produc-tivity, increases surface erosion, andhas various other deleterious envi-ronmental effects, adhering to RILguidelines has advantages regard-less of whether the forest is allowedto regenerate or is replaced by anoil palm plantation or a maize field(e.g., Congdon & Herbohn 1993;Nussbaum et al. 1995; Pinard et al.1996).

The effects of other silviculturaltreatments on biodiversity (e.g., thin-ning and vinecutting) depend on theintensity with which they are ap-plied and on the proper designationof areas deemed inappropriate forstand “improvement.” For example,vine cutting can enhance tree growthbut undoubtedly has negative effectson a wide variety of animals (Putz etal. 2000

b

). Both biodiversity effectsand labor costs of vine cutting de-pend on whether only selected fu-ture crop trees are liberated or vinecutting is carried out as a blanketprescription.

Effects of Forest Management on Biodiversity

Human activities have an enormouseffect on a global scale, as three ex-

amples illustrate: 40% of the Earth’sterrestrial primary productivity is ap-propriated by humans (Vitousek etal. 1997); 25–35% of the primaryproductivity of continental-shelf ma-rine ecosystems is consumed by hu-mans (Roberts 1997); and 26% of to-tal evapotranspiration and 54% of allrunoff in rivers, lakes, and other ac-cessible sources of water are appro-priated by humans (Postel et al.1996). Despite these statistics oncurrent human effects, many peo-ple still maintain it is possible toboth use and preserve biodiversitywith no costs to either side (e.g.,Huston 1993). This claim is made re-gardless of human overexploitationof resources that began in prehistory(Goudie 1990) and is manifested mostrecently in the negative effects oftropical logging (Frumhoff 1995; Bawa& Seidler 1998) and exploitation ofnontimber forest products (Homma1992; Coomes 1995). This thinking isdangerous because it allows its ad-herents to believe that there areeasy, cost-free solutions to the prob-lems intrinsic to exploitation of theplanet.

Summarizing the effects of for-estry activities on biodiversity intropical forests is an effort fraughtwith problems, in large part becauseof the immense diversity foundtherein. Even at the species level,the diversity is difficult to imagine,and each of the literally millions ofspecies in tropical forests respondsto different logging effects in dis-tinct ways. Although some general re-sponse patterns are obvious and oth-ers have emerged from field research,the idiosyncrasies and apparent in-consistencies of species responsesneed to be recognized. For exam-ple, chimpanzee (

Pan troglodytes

)populations have been reported toincrease (Howard 1991; Hashimoto1995), decrease (White 1992), andrespond not at all to logging (Plump-tre & Reynolds 1994). In contrast,studies of terrestrial and bark-glean-ing insectivorous birds consistently re-port negative effects of logging (Putzet al. 2000

b

).

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Landscape Effects

Logging affects the landscape com-ponent of biodiversity by changingland forms and ecosystem types acrosslarge geographic areas. Logging shiftsthe regional mosaic of land uses. Al-though the landscape component ofbiodiversity is the least sensitive tologging, changes in the size, spatialdistribution, and connectivity of hab-itat patches across the landscape oc-cur, especially as the intensity ofmanagement interventions increases.These changes in the habitat mosaicalter species distribution patterns,forest turnover rates, and hydrologicprocesses. The most severe effectsdescribed in this section, however,result from indirect consequences oflogging, such as increased access toremote areas (leading to hunting andland conversion), fragmentation, andaltered fire regimes (Holdsworth &Uhl 1997; Cochrane & Schulze 1999;Cochrane et al. 1999).

STRUCTURE

The size and spatial distribution oftropical forest patches and the juxta-position and connectivity of differ-ent forest patches across the land-scape are most affected by theindirect effects of logging. One ofthese effects—increased access forhumans—often is deleterious, be-cause logging is almost invariably ac-companied by increased huntingpressure and is often followed by de-forestation as lands are cleared foragriculture. Another indirect effectof logging—fragmentation of previ-ously contiguous or otherwise con-nected forest patches—may havecomplex effects. For example, widelogging roads may represent un-crossable barriers for some forest-interior species, but roadsides withsecondary vegetation attract manylarge ungulates, where they are moreeasily hunted (Robinson & Bennett2000).

The degree of fragmentation de-pends on whether logging is dis-persed over large areas or is concen-

trated in small areas (Fig. 3). Wherepreservation of forest-interior biodi-versity is the priority, concentratinglogging in small areas is generallythe pattern recommended by con-servation biologists (Noble & Dirzo1997). Fortunately, due to associ-ated cost savings, concentration oflogging activities is one of the stepstoward improved forest managementthat loggers may find acceptable.

Long-term species maintenance isalso influenced by whether loggedstands are interspersed within spe-cies-rich forest or in a low-diversitylandscape dominated, for example,by pulpwood plantations. Even small,unlogged patches within harvest ar-eas can serve as source populationsfor some species after logging.

COMPOSITION

Logging activities may directly andindirectly affect the identity, distri-bution, and proportion of habitattypes in tropical forests. Forestrymay directly affect the compositionof the landscape component of bio-diversity by intentional creation ofnew types of habitats (e.g., forestsconverted into plantations). Further-more, if silvicultural objectives areuniform across the landscape, inter-stand diversity is sacrificed by wide-spread application of the same stand“improvement” treatments. Perhaps

most important, improved accessprovided by logging roads indirectlyfosters post-logging habitat changesby human forest colonizers, weeds,and wildfires.

Setting aside reserves within log-ging areas may mitigate some of thedeleterious effects of logging andother silvicultural treatments. Thespecific location of reserves substan-tially influences their value in biodi-versity conservation. Optimally, thefull range of landscape features andhabitats should be represented withinprotected areas.

FUNCTION

Logging may markedly alter severallandscape-level ecological processessubsumed under the functional at-tribute of the landscape componentof biodiversity. For example, loggingroads and activities associated withtheir construction can greatly influ-ence the permanence of the forestfragments they create by alteringlandscape-level disturbance regimes.In large part, disturbance is alteredbecause roads and skid trails provideready access to the forest for bothcolonists and fire. High-intensity andwidespread logging, especially if notcarefully controlled, also influenceshydrological processes at all levels,perhaps including the regional cli-mate. Where logging roads are wideand logging intensities are high,

Figure 3. Some effects of logging on biodiversity as a function of distribu-tion of logging activities (percentage of area logged) and logging intensity (m3/ha of timber harvested).


13


landscape-level movements of ani-mals can be disrupted (Goosem1997), as can gene flow in plantswhen pollinators are restricted toisolated fragments by inhospitablesurroundings. It should be noted,however, that the effects of roadsdepend on where and how they areconstructed and change with time asthe road and its margins mature (Lugo& Gucinski 2000).

Ecosystem Effects

The deleterious ecosystem-level ef-fects of logging on tropical forestsare widespread, substantial, enduring,and well studied, especially com-pared with landscape and geneticcomponents. The ecosystem compo-nent of biodiversity is somewhatmore sensitive to logging than thelandscape component in part be-cause management activities are usu-ally implemented at this scale. In con-trast to the landscape component,most ecosystem-level effects are a di-rect consequence of logging activi-ties. Logging purposely removes bio-mass from ecosystems, but it alsoalters their vertical complexity andsoil characteristics. Depending onthe silvicultural objectives, changesin structural heterogeneity may beintended. Whether intended or not,the structural effects of logging alterthe relative proportions of life forms,biogeochemical stocks, nutrient andhydrologic cycling, productivity, andenergy flows.

STRUCTURE

Logging may affect the structural at-tribute of the ecosystem componentof biodiversity by changing the bio-physical properties of soils, spatialheterogeneity of forest stands, andbiomass. The extent and types of eco-system damage to these structural at-tributes depend on logging intensity,the yarding system, and the care withwhich the operations are conducted.Soil compaction, for example, is amajor problem during ground-basedyarding operations, especially where

skid trails are unplanned and yardingcontinues during wet weather. Com-paction negatively affects hydrology,which in turn alters the characteris-tics of watercourses. Where compac-tion is severe, soil permeability andbulk density often require many de-cades to recover. Exposure of min-eral soil after litter layers and rootmats are bladed off by bulldozers isalso a concern. Disruption of mineralsoil occurs during bridge building,road construction and maintenance,and skidding operations—forest-man-agement activities that affect ecosys-tem structure and thereby biodiver-sity.

Another effect of logging on eco-system-level structure is reduction inbiomass and alteration of necromass.Losses of biomass due to forest-man-agement activities include the amountsin the removed timber, damage totrees in the residual stand that resultin mortality, and silvicultural treat-ments that result in tree death. Bio-mass losses range from 5–10 Mg/haat the lowest logging intensities tosubstantially greater amounts whereheavy logging is followed by poisongirdling of noncommercial trees inthe residual stand. Necromass, includ-ing coarse woody debris, increasesimmediately after logging but maythen decrease to levels below pre-logging conditions because of in-creased temperatures near the groundand associated increases in decom-position rates. Wildfires promotedby construction of logging roads andcanopy opening and controlled burnscarried out for silvicultural purposescan have obvious effects on biomassand necromass stocks in tropical for-ests.

Maintenance of healthy communi-ties, species populations, and genepools is predicated upon protectionof hydrological functions, nutrientcycles, and other ecosystem proper-ties. Fortunately, methods for miti-gating the ecosystem-level effects oflogging on tropical forests are wellknown. Switching from ground-basedto skyline yarding techniques, forexample, greatly reduces damage to

residual stands, soils, and streamsbut allows harvest on steep slopes.The opposite trend in technologicalchange also can have environmentalbenefits: log yarding with draft ani-mals or by manual means generallyresults in substantially less loggingdamage than yarding with bulldoz-ers (Cordero 1995). To reap thesepotential benefits, the expertise ofexperienced forest engineers shouldbe called upon more often wherelogging does have to occur.

COMPOSITION

Logging affects ecosystem-level com-positional attributes of biodiversityby changing biogeochemical stocks.For example, soil compaction re-duces water-holding capacity, whichin turn leads to increased surfacerunoff. Limited storage capacity innatural streams is further reduced bysedimentation, which means flowregimes can be greatly modified bylogging, especially during the firstyears after logging is completed. Var-ious RIL techniques, such as installa-tion of cross drains on skid trails,can greatly diminish these effects.

FUNCTION

Logging affects the functional at-tributes of ecosystem-level biodiver-sity by adversely affecting hydrologi-cal and biogeochemical fluxes aswell as productivity. Reduced plantproductivity results in part from im-peded root growth, a further conse-quence of logging-induced soil com-paction. Because most of the availablenutrients are usually found near thetop of the soil profile, blading of thesoil surface also diminishes nutrientavailability in local areas and other-wise interferes with nutrient cycling.In more extensive portions of log-ging areas where RIL guidelines arenot followed, nutrient cycling and hy-drological functions are greatly mod-ified by reduced canopy interceptionof rain and mist, decreased uptake ofwater and nutrients by the diminished

14



biomass, and increased occurrenceof surface erosion and landslides as-sociated with improperly located andpoorly constructed roads and skidtrails.

Changes in carbon storage andflux associated directly and indi-rectly with logging and other silvi-cultural activities influence whetherforests are net sources or sinks of“greenhouse” gases. For example,substantial logging-induced transfersof living trees to coarse woody de-bris can have substantial effects onunderstory structure and dynamics,leading to more carbon release. Thedeleterious effects of logging on for-est carbon balance can be greatlydiminished by application of RILtechniques (Pinard & Putz 1996).Substantial biodiversity benefits arealso likely to result from RIL, butthey have not been well studied.Other silvicultural treatments, suchas fire management, weed control,thinning, and enrichment planting,have various effects on both green-house-gas emissions and biodiversitywhich should be investigated.

Community Effects

Logging, especially if followed by sil-vicultural treatments such as libera-tion of future crop trees from com-petition, can substantially changethe physiognomy, composition, andtrophic structure of forest stands. Toa large extent, these modificationsrepresent the goal of forest “refine-ment” treatments applied to increasevolume increments and relative den-sities of commercial timber species.This “stand domestication” by naturereduces species richness; rare, threat-ened, and endangered species maybecome locally extinct, especially ifthey have no perceived commercialvalue. These changes in compositionand structure affect numerous com-munity-level ecological processes, in-cluding colonization, predation andmortality rates, pollination, seed dis-persal, and timing and abundance offlower and fruit production.

STRUCTURE

The most obvious logging-inducedeffect on the structural attributes ofcommunity-level biodiversity is thechange in proportions of succes-sional stages in forest stands. De-pending on harvesting intensities,planning of roads and skid trails, andtraining and supervision of workers,logging can result in large changesin the proportion of forest in ma-ture, recovering, and early succes-sional stages. In some severely dis-turbed areas, succession might be“arrested” by post-logging prolifera-tion of vines, bamboo, and othernonarboreal growth forms. Silvicul-tural treatments such as thinningand vine cutting can increase therate of succession and increase theproportion of stand growth concen-trated in commercial species, butnot without affecting biodiversity inmore than the intended ways.

COMPOSITION

In the community component ofbiodiversity, logging affects composi-tion by changing (often purposefully)the relative abundance of species andguilds inhabiting forest stands. The rel-ative abundance of tree species withlight-demanding versus shade-tolerantregeneration, wind- versus animal-dis-persed seeds, vertebrate- versus inver-tebrate-pollinated flowers, and thickversus thin bark, for example, are allsubject to change in logged and other-wise silviculturally treated forests.Likewise, representation of differentguilds of animals (e.g., understory in-sectivores and arboreal folivores) is in-fluenced by forestry activities. De-pending on a great number of factorsrelated to the intensities, spatial scales,and modes of forest intervention, aswell as characteristics of the focaltaxa, effects of forestry activities canbe negative, positive, or neutral. Forexample, in eight studies that consid-ered the effects of logging on frugivo-rous birds, two reported positive ef-fects at the guild level, three reportednegative effects, and three reportedno change at all (Putz et al. 2000

b

).

FUNCTION

The functional aspects of commu-nity-level biodiversity include numer-ous key ecological processes, suchas pollination, herbivory, seed dis-persal, and predation, all of whichare affected by logging, especially un-der the most intensive managementinterventions. Many of the effects onthese processes are a direct conse-quence of altered resource abundance(e.g., fruit for frugivores or youngleaves for folivores), which in turn re-sult from the logging-induced changesin community structure and compo-sition. In addition to being influencedby resource-base changes, these eco-logical processes are also affected bychanges in forest microclimates thatare a result of exploitation, silvicul-tural treatment, and hunting.

Species Effects

The species component of biodiver-sity has received the most attentionfrom researchers concerned aboutthe effects of logging and other silvi-cultural treatments in tropical for-ests. The most obvious species-leveleffect of logging is on the abun-dance and the age and size distribu-tion of harvested and damaged trees.Depending on the intensity of log-ging and the care with which it iscarried out, the reproduction, growth,and survival of many species can beadversely affected. In reviewing thisliterature, it is important to note thatthe taxa studied were not selected atrandom. Instead, in many cases thespecies chosen were expected to besensitive to and thus good indicatorsof the effects of logging.

STRUCTURE

The most immediate and direct ef-fects of logging on the structural at-tribute of the species component ofbiodiversity are suffered by the har-vested tree species. Their popula-tions are often left greatly depleted,especially in the larger size classes of


15


reproductive individuals, when man-agement is based solely on minimum-diameter felling rules. Because of thespatial clustering characteristic ofmany commercial timber trees, therichest patches of forest are gener-ally the most severely disturbed un-less logging guidelines specify mini-mum spacing between harvestedtrees.

Changes in forest structure are suf-fered most by specialist species ofthe forest interior. After logging,many formerly shaded microenvi-ronments in the forest interior be-come drier, brighter, warmer, andmore easily exploited by some pred-ators. For example, severe canopyopening adversely affects litter inver-tebrates and their predators. For spe-cies that are generalists in their dietsand wide-ranging in their habitatuse, such as many frugivorous can-opy birds, the direct effects of log-ging vary from somewhat negative,to neutral, to positive. For the under-story species that are adversely af-fected by logging, the effects maypersist for decades (Wong 1985; butsee Lee et al. 1998).

The effects of logging and stand-refinement treatments on speciesare of particular concern in smallforest management units. Privatelandowners with

,

100 ha to man-age, for example, may be unwillingto set aside 10% of their forest forspecies preservation. If their forestsare surrounded by similarly managedor deforested areas, then blanket ap-plication of stand-refinement treat-ments or heavy logging can take asubstantial toll on commercial andnoncommercial species alike.

COMPOSITION

Logging affects the composition ofspecies-level biodiversity by chang-ing the abundance and distributionof species. Unless logging is accom-panied by other silvicultural treat-ments designed to foster their repro-duction and growth, the abundanceand population structure of the har-vested tree species are greatly modi-

fied by logging. Logging effects ontree populations continue for manyyears after logging is completed be-cause damaged trees suffer highmortality rates, proliferation of weeds(e.g., vines) interferes with tree re-production and survival, and popula-tion size reduction and fragmenta-tion can decrease pollination levelsand change the pattern and intensityof seed dispersal and predation. Thespecies composition of animals alsochanges in response to the direct ef-fects of logging such as canopyopening and associated indirect ef-fects such as increased fire fre-quency and intensity, hunting, andforest conversion. Changes in spe-cies composition in response to for-estry operations are by no meansconsistent across or even withintaxa. Our review of the literature onprimates, for example, revealed fewcases of consistent responses of spe-cies to logging. This variation can beattributed to differences in loggingintensity and to differences in theduration of post-logging populationmonitoring. Silvicultural treatmentsother than logging, especially vinecutting and crown liberation of fu-ture crop trees, might have moreconsistent deleterious effects on can-opy animals, but such effects havebeen little studied.

Small fragments of untouched for-est that remain within even heavilylogged forests serve as important ref-ugia for plants and animals. Wildlifedensities in these unlogged frag-ments can be very high during andshortly after harvesting, but then di-minish as animals recolonize the sur-rounding matrix. Many “unplanned”reserves are on steep or otherwiseadverse sites, which certainly influ-ences their function as refugia.

FUNCTION

Demographic processes (e.g., survi-vorship, fertility, and recruitment)and growth rates are two key func-tional attributes of the species com-ponent of biodiversity that are af-fected by logging. Populations of

many organisms are susceptible tolarge fluctuations after logging dueboth to the direct effects on forestconditions such as microclimate andfragmentation and to the indirect ef-fects of increased hunting, fire, andforest conversion. The proliferationof disturbance-adapted taxa in loggedforests, some species of which arenot native or were not previouslycommon in the area, can have largebut as yet little-studied effects on theresident flora and fauna.

Genetic Effects

The genetic component of biodiver-sity is likely to be the most sensitiveof all components to logging be-cause of reductions in effective pop-ulation size and interruptions ingene flow. At present, however, lit-tle is known about the genetic struc-ture of any tropical organisms, evencommercially valuable timber trees(Ledig 1992). Furthermore, the tech-niques required for assessing the ge-netic structure of populations are so-phisticated and expensive. Except ina few cases, concerns about dys-genic selection, genetic drift, andother genetic problems are based oncontroversial theory that is develop-ing rapidly as evidence accumulates.

STRUCTURE

Logging affects the structural at-tribute of the genetic component ofbiodiversity by reducing effectivepopulation sizes and heterozygosity.Effective population sizes of bothcommercial and noncommercial spe-cies are reduced by harvesting, othersilvicultural treatments, forest frag-mentation, weed proliferation, andwildfires. There are also good rea-sons to be concerned about the ef-fects of logging and stand-improve-ment treatments on dioecious speciesand in small forest-management unitsin which population sizes of all spe-cies are correspondingly small. Al-lelic frequencies of commercial spe-cies change after removal of a largeproportion of healthy reproductive

16



adults. For species with high densi-ties of advanced regeneration, the ge-netic structures of their populationsare unlikely to change dramaticallyafter selective harvesting unless col-lateral damage is severe. Timber stand-improvement treatments also may af-fect the genetic structure of speciestargeted for removal (e.g., woodyvines) and their associates, but theseeffects apparently have not been stud-ied. Given the high proportion ofvines and other plants that resproutafter cutting (coppice), large effectson genetic structure are unlikely.

COMPOSITION

The fact that most species are rare intropical forests implies that allelic di-versity will decrease with increas-ingly intensive management inter-ventions. Unregulated harvesting ofall merchantable individuals of acommercial species, for example,has immediate effects on allelic fre-quencies that continue to changedue to decreased effective popula-tion sizes. Deleterious recessive genesmay become more apparent as a re-sult of dysgenic selection, and het-erozygosity may decline due to thebottleneck effect in the small, iso-lated populations that result fromharvesting, forest fragmentation, andother direct and indirect effects offorestry activities (Styles & Khosla1976; Murawski et al. 1994

a

, 1994

b

;but see Newton et al. 1996).

FUNCTION

Logging may affect the functional at-tribute of the genetic component ofbiodiversity by interrupting gene flow,which in turn influences outbreed-ing rates. Decreased effective popu-lation sizes, coupled with losses ofpollinators and seed-dispersal agents,can result in reduced gene flow andinbreeding depression in populationsof both commercial and noncom-mercial species. Especially vulnera-ble are populations represented by

scattered mature individuals and fewjuveniles (e.g., many “long-lived pio-neers” such as the mahoganies).Given the high proportion of tropi-cal tree species that are dioecious orobligate outcrossers, only severe re-ductions in effective population sizeare likely to have much effect ongene flow (Ghazoul et al. 1998).

Overview of Biodiversity Conservation in Relation to Logging and Other Silvicultural Treatments

Figure 4 graphically displays the ef-fects of logging on tropical forestsbased on the various componentsand attributes of biodiversity. Alongthe vertical axis of our framework,the five components of biodiversityare arrayed in the order of increasingsusceptibility to logging effects: land-scape, ecosystem, community, pop-ulation/species, and genetic. The hor-

izontal axis arrays a variety ofapproaches to silviculture in orderof increasing intensity.

We assessed the effect of these sil-vicultural approaches on each biodi-versity component based on threecategories. First, each biodiversitycomponent was scored as “mostlyconserved” for cases in which theirattributes were expected to usuallystay within their natural range ofvariation. Second, biodiversity com-ponents were scored as “affected”for cases in which their attributeswere expected to frequently fall out-side their natural range of variation.Finally, biodiversity components werescored as “mostly lost” for cases inwhich their attributes were expectedto almost always fall outside theirnatural range of variation.

The scoring process used for Fig.4 was admittedly subjective; thescores were based on a reading ofthe literature and the authors’ expe-rience. We fully recognize that par-ticular situations might warrant dif-

Figure 4. Expected effects of a range of forest uses on the components of biodiversity. Abbreviations: NTFP, nontimber forest products; RIL, reduced-impact logging; reserves, protected areas with forests managed mainly for timber; refinement, silvicultural treatments such as liberation of future crop trees from competition, which can substantially change the physiog-nomy, composition, and trophic structure of forest stands that are applied to increase volume increments and relative densities of commercial timber species; CL, conventional logging; enrichment planting, increasing the stocking of commercial species by planting seedlings (or seeds) in logging gaps or along cleared lines.


17


ferent scores, and we emphasizethat our purpose is to illustrate whatis believed to be a useful analyticalprocess rather than to obtain per-fectly accurate scores. This frame-work also serves to suggest impor-tant research topics for conservationbiologists from a number of disci-plines.

The responses summarized in Fig.4 actually represent a multitude ofeffects from a diversity of silvicul-tural approaches applied with vary-ing degrees of concern for biodiver-sity over a large range of scales.Figure 4 addresses neither the issueof profitability of different forestuses nor issues related to land-usecapability, biodiversity value, or thecapabilities and desires of localstakeholders. At least two other di-mensions of this multidimensionaltopic are captured in Fig. 5, whichindicates that every forest activitycan be carried out over a range of in-tensities. Correspondingly, each for-est-use activity generates timber vol-umes and financial profits that alsovary widely (Fig. 6). Recognizingthat forest-use practices vary overtime with, for example, market fluc-tuations and political change, andthat biodiversity effects are a func-tion of a multitude of interacting fac-tors operating at different temporaland spatial scales, we hope that Figs.4–6 present an accurate “snapshot”of the relationship between biodi-versity conservation and forest man-agement.

The effects summarized in Fig. 4and the ranks and ranges of effectsand profits in Fig. 5 are based on acombination of literature reviewsand the authors’ subjective estima-tions. For example, under the rangeof stocking levels, terrain, and acces-sibility in which logging is carriedout, reduced-impact and conven-tional logging overlap in both effectsand profitability. Nevertheless, par-ticularly where logging is conductedon steep slopes or under otherwiseadverse conditions, application ofmost RIL guidelines results in imme-diate profits lower than those of un-

constrained conventional logging.This observation helps explain whyareas where regulations are nonex-istent or unenforced are likely to belogged by conventional methods.Correspondingly, some of the ef-fects of high-intensity RIL overlapthose of low-intensity conventionallogging, but the former generally hasfewer adverse environmental effectsthan the latter.

Conclusions

Recognizing that all significant in-terventions in natural forests haveeffects on biodiversity, all silvicul-tural decisions necessarily representcompromises. Management for somegoods or services necessarily involvesmanagement against some others.What are “weeds” to timber-standmanagers are food sources, rare spe-cies, carbon stores, or intercrownpathways for other human and non-human stakeholders. The biodiver-sity compromises involved in decid-ing whether to cut vines, retain seedtrees, or enhance seedling establish-ment by carrying out controlled burnsshould be informed by research. Un-fortunately, little is known about howtropical forests can be best managed

to enhance biodiversity. Researchershave instead focused on enumerat-ing the deleterious environmental ef-fects of uncontrolled logging by un-trained and unsupervised crews. Toinform decisions about tropical for-est management and to assure thatbiodiversity is protected to the maxi-mum extent, more research is neededon how to maintain diversity in for-ests selected for logging. The largeand rapidly growing body of litera-ture on ecosystem management inboth northern and southern temper-ate forests (Kohm & Franklin 1997;Lindenmayer 1999) should provide in-spiration and starting points for trop-ical researchers intent on solving for-est-management problems associatedwith biodiversity.

The primary conclusions to derivefrom these analyses are that (1) dif-ferent intensities and spatial patternsof timber harvesting, along withother silvicultural treatments, resultin different effects on the differentcomponents of biodiversity; (2) somecomponents and attributes of biodi-versity are more sensitive than oth-ers to forest-management activities;and (3) only extremely limited use willprotect all components (i.e., large pro-tected areas are essential for biodi-versity conservation).

Figure 5. Generalized biodiversity effects plotted against expected short-term financial returns to forest owners or concessionaires (NTFPs, nontim-ber forest products).

18



The capacity to mitigate the dele-terious environmental effects of log-ging and other silvicultural treat-ments should not be construed asconstituting support for sustainableforest management as a conserva-tion strategy. Such an endorsementis unwarranted given widespread il-legal logging in the tropics, wide-spread frontier logging and loggingof areas of high priority for biodiver-sity protection, the persistence ofpoor logging practices despite sub-stantial efforts in research and train-ing, and the general slow rate atwhich most loggers are transformingthemselves from timber exploitersinto forest managers. Nevertheless,even the most harshly treated forestsmaintain more biodiversity than treefarms for pulpwood, oil palm planta-tions, maize fields, or cattle pastures.Furthermore, logging is often theleast environmentally damaging ofland uses that are also financially via-ble (Pearce et al. 1999). Given theseconclusions, effective mechanismsfor financing forest protection andenvironmentally sound forest man-agement are needed.

By focusing on the deleterious en-

vironmental effects of tropical forestmanagement activities, we often losesight of the fact that, from the per-spective of biodiversity maintenance,natural forest management (i.e., main-taining forests as forests) is prefera-ble to virtually all land-use practicesother than complete protection. Asforest-management practices improveunder market pressure or pressurefrom landowners, the deleterious en-vironmental effects of logging andother silvicultural activities are likelyto be substantially reduced. Foreststhat are carefully managed for tim-ber will not replace protected areasas storehouses of biodiversity, butthey can be an integral componentof a conservation strategy that en-compasses a larger portion of thelandscape than is likely to be setaside for strict protection. In otherwords, forests managed primarily fortimber will supplement and effec-tively extend the conservation estateif they are managed properly. Finally,it should be recognized that land-scape management is consistent withthe ecosystem approach emphasizedby the Convention on Biological Di-versity.

Acknowledgments

Substantial background material forthe full version of this paper wasprovided by D. Nepstad, M. Guari-guata, R. Noss, and C. Peters. Wealso acknowledge the contributionsof E. Bennett, J. Ewel, P. Frumhoff,E. Losos, K. MacKinnon, A. Moad, G.Platais, and A. Plumptre.

Francis E. Putz

‡

Geoffrey M. Blate

Department of Botany, P.O. 118526, University of Florida, Gainesville, FL 32611–8526, U.S.A.,

‡

email [email protected]

Kent H. RedfordRobert FimbelJohn Robinson

Wildlife Conservation Society, 2300 Southern Boulevard, Bronx, NY 10460–1099, U.S.A.

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