the role of aborists in providing wildlife habitat and...

160 Dunster: Providing Wildlife Habitat in the Urban Forest

THE ROLE OF ABORISTS IN PROVIDING WILDLIFEHABITAT AND LANDSCAPE LINKAGESTHROUGHOUT THE URBAN FORESTby Julian A. Dunster

Abstract. In recent years, great advances have been madein arboriculture, but the main design professions, thegeneral public, and some arborists still lack understandingabout the ecological functions of trees for other organisms,and how these ecological functions and processes connectacross larger landscapes. Consequently, many opportuni-ties for the retention or creation of wildlife habitat in theurban forest are still being lost. Some of these opportunitiesare discussed, along with examples from British Columbia.A selection of references from related fields is included toprovide a basis for further reading and understanding aboutlandscape ecology, greenways, wildlife trees, and howarborists might integrate the principles from these fieldsinto their practice.

Keywords. Urban forest; wildlife habitat; landscapeecology; linkages; wildlife trees.

Arborists play an important role in managing ur-ban forests. Our prescriptions, management, andmaintenance activities determine the structure andfunction of individual trees to stands of trees andentire forests in the larger landscape on publicand private lands. Yet the contemporary focus ison the single tree, or on a group of trees in re-sponse to a single client. The situation is not helpedby the pronounced division of responsibilities seenin most local government structures: the parks de-partment looks after street trees and greenspaces,the planning department looks after new develop-ments and redevelopments, and all too often, theengineering department works in isolation fromeveryone else.

Complicating this fragmented governance is anall-too-common lack of awareness, especially atthe senior decision-making and political levels,about contemporary science and the need for in-terdisciplinary design approaches. How manymembers of the design professions such as engi-neering, architecture, planning, biology, and land-scape architecture actually know what trees andwildlife require? Arborists would probably respond,

"Not many." Conversely, how many arborists un-derstand the mindset of these other professions?Again, the likely answer is "Not many."

The profession of arboriculture has advancedconsiderably in recent years. We now have abetter sense of how good pruning should be un-dertaken; we know more about roots, stress re-sponses, good and bad retention, planting, andplant care practices. But all of this has focusedon what the arboriculture profession considersto be the best means of maintaining a healthytree. Less attention has been given to the roleof these trees within an urban or rural ecosys-tem, and not all arborists may be aware of thecontribution that trees make to the larger land-scape in terms of ecosystem function and eco-system processes. Consequently, we miss animportant opportunity to advise the design pro-fessions about what other ecological functionsa tree might provide.

In the field of landscape ecology, the notion ofconnectivity between larger patches or nodes ofhabitat is central to the function of a healthy sys-tem (Forman and Gordon 1986; Hodge 1995).Stream corridors and their associated riparianhabitats, parks with forest components, and singletrees or groups of trees on private and publiclands all contribute to a mosaic of landscape con-nectivity. For many species of wildlife, connec-tions between larger nodes of habitat may beessential to survival because fragmentation, andsubsequent isolation of populations and their as-sociated habitats, may lead to long-term loss ofgenetic flux, insufficient habitat area, and poten-tial extirpation from the area, perhaps even ex-tinction. A large body of literature is devoted tothe theme of landscape linkages, which in theurban and rural context have been termed"greenways" (Hudson 1991; Labaree 1992;Smith and Hellmund 1993; Ministry of ELP 1995).

Journal of Arboriculture 24(3): May 1998 161

Along similar lines, it is common to see land-scapes formerly denuded of vegetation being re-stored as forests. This aspect is particularlyevident in parts of Europe where lands formerlyunder agriculture or industrial uses are now aban-doned. The literature on forest restoration, or re-creation of forests, is plentiful and containsexcellent guidance for arborists wishing to betterunderstand the larger role of trees in a functionalecosystem (Forestry Commission 1990, 1991;Hodge 1995; Rodwell and Patterson 1995). Thetheme of ecosystem functions within urban for-ests is also well documented (Bradley 1995; Kollin1994, 1995) and provides further insight abouthow trees provide so much more than shade, aes-thetics, climate control, etc. The management ofurban forests, especially small remnant patches,and the manner in which these can be used tocreate important patches of wildlife habitat (Agee1995; Milligan Raedeke and Raedeke 1995) areintegral to understanding the larger role that ar-borists could assume. Similarly, at the industrialscale of forest management, much new evidenceis emerging about the manner in which wildlifehabitat can be better integrated with commercialharvesting (Bradford et al. 1996).

Because of the rapid urban growth in manyparts of British Columbia, the British Columbia Min-istry of Environment issued an informative seriesof publications dealing with development and theenvironment. We now have detailed guidelines fordevelopments next to streams and rivers and ar-eas of specialized habitat (Chillibeck 1993; Minis-try of ELP 1994), as well as guidelines forintegrating development with greenways (Minis-try of ELP 1995). Coupled with the emerging shifttowards the creation of greenways and landscapelinkages across entire municipalities or regionallandscapes is the increasing need for a better un-derstanding of trees. What makes them suitablefor retention(Dunster 1995, 1996)? Which prac-tices maximize the chances for long-term survival?What is the best approach for design andpostconstruction management? When a broaderview is taken, it is clear that arborists can assistother professionals in deciding how best to de-sign and manage for sustainable urban forests andthe creation of wildlife habitat.

Trees as a Source of Wildlife HabitatTrees are a large and highly visible componentof greenways. With the increasing focus on theselandscape linkages, typically situated on a mixof public and private lands, arborists can antici-pate a larger role in managing these systems.However, management of trees within greenwaysystems requires a focus beyond simple prun-ing and health care. In many cases, a dead ordying tree is just as important to the overallhealth of the ecosystem as the live andhealthy trees.

The traditional response to dead or dyingtrees has been to remove them either becausethey are hazards, or to "tidy up" the landscapeto avoid an anthropocentric perception of un-kempt appearances. But this tidying-up ap-proach can be ecologically damaging, andarborists need to better understand the role ofdead or dying trees as a source of habitat. Forexample, large pieces of a tree, known in forestmanagement as coarse woody debris, providea source of food for many insects and fungi. In-sects are a food source for the birds, which them-selves help maintain insect populations atendemic levels. Small mammals find refuge andbreeding areas in decayed logs, and it has beenshown that dispersal of beneficial forest mycor-rhizae is a direct function of small mammal popu-lations and the dispersal of spores in their fecalpellets (Maser 1988; Machmer and Steeger1993).

In industrial forest management, particularly inthe United States and more recently in Canada,there has been a strong drive to manage the for-ests for more than just timber. Wildlife trees, de-fined as "tree[s] that provide ... present or futurecritical habitat for the maintenance or enhance-ment of wildlife" (Dunster and Dunster 1996) havevalue for many different species and are used formany different purposes including nesting, perch-ing, feeding, or roosting. The species that use wild-life trees can be sorted into 5 categories(Maser 1988; Backhouse 1992; Wildlife TreeComm. 1992):

1. Primary cavity excavators: Examples in-clude woodpeckers, flickers and sapsuck-ers, chickadees, and nuthatches, all of


which use their hard beaks to dig out holesin soft, decaying wood. These speciesmake new cavities each year, leaving theold ones for other species to use.

2. Secondary cavity users: These speciescannot excavate cavities by themselves,but use abandoned holes to nest and raiseyoung, to store food, or simply as shelter.Examples are owls, tree swallows, blue-birds, and wood ducks, in addition to mam-mals such as martens, raccoons, flyingsquirrels, and deer mice.

3. Open nesters. These are the larger birdsthat require a place for heavy nests—typi-cally birds of prey such as eagles, ospreys,or great blue herons. They can use deador live trees but usually prefer trees with abroken top or flat crown that can supportthe nest and yet retain a clear field of viewin all (or most) directions.

4. Other mammals: This category includesmammals such as bears (which often builddens in the hollow base of larger trees) andother large mammals such as caribou,which use the lichens growing on branches.Smaller mammals, such as bats and mice,find opportunities for shelter and nestingunder loose bark and in other smallcavities.

5. Amphibians: Amphibians use dead anddying trees as a shelter and brooding habi-tat, especially once the wood is in an ad-vanced stage of decay and is soft andspongy enough to tunnel into, yet moistenough to protect the occupants.

In addition, decaying trees are often a sourceof nutrition at various levels of the food chain. Forexample, insects that attack dying trees often in-troduce fungal agents that help accelerate thedecay processes. These insects become food forforaging birds, and fungi make the wood suitablefor the cavity excavators. The use of wildlife treesin industrial forestry is well established (Thomas1970; Bull et al. 1986; Morrison et al. 1986;Bradford et al. 1996; Parks et al. 1997). Trees withuseful cavities for wildlife can be live, or can bepartly or wholly dead, as shown in Figure 1.

Figure 1. Cavities occur in many forms and havemany uses for different species (from Thomas1979).

In British Columbia, over 90 species are knownto depend to some extent on wildlife trees, andabout 18% of all bird species breed in the cavi-ties of wildlife trees, including several rare andendangered species (Wildlife Tree Comm. 1992;Machmer and Steeger 1993). The establishmentof the British Columbia Wildlife Tree Committee,jointly administered by the Ministry of Forests,the Ministry of Environment, Lands, and Parks,and the Workers Compensation Board, has givensome credibility to the creation and retention ofwildlife trees in industrial operations. This com-mittee has established a training function to per-mit the identification and retention of treesconsidered necessary for wildlife (Wildlife TreeComm. 1992), and the retention of such trees hasnow been extended into urban situations andgreenways.

One of the simplest means of defining a land-scape corridor (connection) on the ground is toretain the vegetation alongside creeks, streams,and rivers. In the British Columbia developmentguidelines, a 15 m (50 ft) setback from the top ofthe bank is now considered a minimum amount


of land to be left undeveloped. This setback zoneprotects the water body and retains a narrow areaof riparian habitat, including trees. However, sucha narrow band of remnant forest is quite oftenunsuitable for safe tree retention if it contains talltrees next to new residential developments(Dunster 1995). From a fisheries and environmentviewpoint, trees provide shade for the water;cooler water holds higher oxygen concentrations,which in turn helps ensure fish survival. At thesame time, litter from the trees provides nutrientinputs for the various insect fauna, and hence, asource of food for fish, and the tree roots stabi-lize the bank slopes. As a unit, the entire riparianzone provides a narrow band of habitat alongwhich certain species of animal can travel, rea-sonably free of outside disturbance, although ina 30-m (1OO-ft) band of forest, this is sometimeshard to achieve.

A common problem is that the retained trees,so greatly desired for environmental purposes,are often too tall, too isolated, and too hazard-ous to justify their retention. This often producesan immediate source of friction between thosewanting the trees for their environmental benefitsand those wanting them removed or treated toreduce the hazards. To reduce the levels of fric-tion, alternative approaches have been devel-oped. The width of the setback zone design canbe designed according to the development den-sity, terrain, and vegetation types. Areas withyoung forest cover that can adapt to changed mi-croclimatic and hydrological conditions resultingfrom adjacent developments may be more suit-able for retention than areas covered with ma-ture or semi-mature forest that may be unstableand often unable to adapt to the changed condi-tions fast enough, if at all. Such stands typicallysuffer decline once isolated and often fail to pro-duce the benefits originally envisaged over longerperiods of time.

To successfully implement an effective wild-life habitat plan, it is essential to have an inter-disciplinary approach, in which experts in all fieldscan freely discuss ideas and requirements at theinitial design stages—not at the last minute. Inmany cases, the planners can relax certain de-velopment requirements to permit a slight den-

sity gain in one location in order to offset devel-opment losses due to environmental protection.In other cases, it may be necessary to carefullyremove the unstable forest overstory (such asedge trees with no windfirmness, or suppressedtrees with little or no rooting system) using spe-cialized logging techniques involving a suspendedlift of the felled trees to minimize ground distur-bances. The relatively undisturbed ground is thenreplanted. In some instances, the material beingremoved is placed in a stable position on theground and left to decay in place, thus ensuringno net loss of biomass from the site. Such tech-niques have been used successfully by the au-thor; the coarse woody debris is important habitatand declines in place over time, while replantedtrees develop very rapidly to create a new forestarea that is better adapted to the new site condi-tions (Dunster 1995).

The creation of a good wildlife tree requiresunderstanding its purpose. As a tree declines anddies, it provides habitat for a range of species.According to a a well-defined classificationscheme for wildlife trees (snags) and logs on theground, each stage of decomposition has its ownparticular uses for various species. The schemeshown in Figure 2 is typical.

A Class 1 wildlife tree is live and healthy. It mayposses some small cavities that are a result of de-cay, not primary excavation. The crown foliage andbranches are used for nesting, perching, or as ter-ritorial markers. If large lateral limbs are present,large birds of prey may nest or perch on them.

A Class 2 tree is at the initial stages of evidentdecline but still living. Dieback in the crown, usu-ally as a result of internal decay (root rots or otherfungal agents) has softened the center wood,leaving a hard outer shell. The tops or branchesmay be breaking off, and insect damage may beevident. Primary cavity excavators move in andnew cavities appear annually. Foraging birds takeadvantage of the abundant insect life, while bro-ken tops and limbs become nests and perchingsites for raptors or other species such as owls.

A Class 3 tree is newly dead. The heartwoodis usually hard, needles and twigs are still present,and the branches and bark are still intact. Theroots are generally stable, but if root rot has killed


Stage 1Live

Stage 2Declining

Stage 3Dead

Stage 4Loose bark

Stage 5Clean

Stage 6Broken

Stage 7Decomposed

Stage 8Downmaterial

Stage 9Stump

Log decompositionclass 1





Figure 2. The classification of wildlife trees and fallen logs. The illustrations shows a conifer; similarstages would be seen in deciduous trees (from Thomas 1979).

the tree, this stability may be short lived. Class 3trees are used by primary cavity excavators aswell as secondary cavity users. The deadbranches are used for hunting and hawkingperches, roosts, and large nests. As insect popu-lations increase, the tree will get more use fromforagers.

A Class 4 tree has no needles or twigs, and atleast 50% of the branches have been shed. Theheartwood is still reasonably hard, but the barkis loose and the top may have fallen off. Cavityexcavators are still active, but weaker speciesare now able to dig out smaller cavities. The aban-doned cavities are actively used by secondarycavity users, and bats and other insect feedersare well established.

A Class 5 tree has decayed enough that it hasspongy heartwood. Most branches are lost, thebark has fallen off, and internal decay is well ad-vanced. On a larger tree, the roots may still bestable, but on smaller trees they may be ap-proaching instability due to decay. Cavity usersare still active, along with the insect feeders. As

the wood gets softer, nesting and shelter oppor-tunities for smaller mammals and amphibians in-crease.

A Class 6 tree has extensive internal decayand is starting to break up. The sapwood andheartwood starts to fall off from the upper trunkarea, the smaller roots have decayed, and thelarger roots are marginal. Cavity users no longerfind it attractive habitat, but insect feeders, smallmammals, and amphibians may increase.

A Class 7 to 8 tree is in the final stages ofdecay. A stub of the tree remains, showing ex-tensive decay. A hard outer shell may remain,but the softer central wood is almost completelyabsent, and lateral roots are almost gone. At thisstage, the tree is mainly used by amphibians,small mammals, and insect-feeding birds.

Finally, a Class 9 tree is a stump or small stub.It provides good habitat for salamanders, smallmammals, and foraging birds, and may be usedas a territorial marker or a drumming log forgrouse. It is also a nutrient source within theecosystem.


Decomposed fallen trees have well-definedcharacteristics, and distinct species use the logs.Figure 2 shows the correlation between wildlifetree class and log decomposition class.

In a natural forested ecosystem, there will usu-ally be an array of wildlife trees and fallen logs inmany different stages of decay and use (Parkset al. 1997). In the managed ecosystem, manyof these wildlife attributes may have been lost.With careful understanding of tree decay patternsand of the manner in which wildlife use trees, ar-borists can help foster environmental benefits andcontribute to greater sustainability.

Creating Wildlife TreesAs a part of a provincial strategy to retain suit-able wildlife habitat, foresters, biologists, andgovernment officials in British Columbia haveadopted the requirement for wildlife trees. Wild-life trees can be found in several forms. The mostobvious is the live tree in use by wildlife such asraptors nesting in the crown (eagles, ospreys, andherons). If these trees are in active use by cer-tain species, they may be protected under wild-life laws; the arborist should know and recognizethese trees and associated statutes. Whether atree is in active use is not always easy to ascer-tain, but local sources of knowledge, such asnature clubs, local environmental organizations,and government authorities can help. In manycases, these trees will be designated as wildlifetrees once they have been documented.

A second form of wildlife tree creation is tak-ing an existing hazard tree, cutting it off at someheight above the ground, and then leaving it todecay. In other cases, trees have been removedand then stuck back in the ground like giant posts(Bradford et al. 1996).

In British Columbia, it is now common to re-tain actively used wildlife trees, as well as to cre-ate new wildlife trees of varying heights. Someare little more than small logs pushed into theground and will never serve as good raptor orwoodpecker trees. Many large trees, deemed tootall and too unstable to safely retain, have beencut to a height at which they will not be a hazardto adjacent developments when such trees even-tually fall down. These large trees, some of which

are 15 to 20 m (50 to 66 ft) high, provide perch-ing and nesting sites at the top and are alreadybeing explored by woodpeckers for nesting andforage. The cut top is notched and roughenedwith a saw to provide a sufficiently rough basefor nest building. Under natural forces, the lossof entire tree tops is common throughout the Pa-cific Northwest and up into British Columbia.Douglas-fir and western hemlock are often seenas tall stumps with their entire crowns having beensnapped off in heavy winds. Red alders andbigleaf maple, known for their propensity to de-cay from the top down, are common sites for cav-ity excavators.

Based on observations in the lower mainlandof British Columbia, it seems to take about 3 to 4years before coniferous trees are first colonizedby woodpeckers. Thereafter, colonization by otherspecies depends on rates of decay, but it seemslikely that many of these wildlife trees will remainstanding for 15 to 20 years before falling over. Thehardwood species decay faster, and crown diebackand loss of the main scaffold takes place withinabout 10 years. In several sites, the mosthazardous trees have been converted into wildlifetrees of varying heights, and the area is replantedto recreate a young forest. Careful extraction ofthe tree tops, replanting, and retention of theunderstory where feasible, makes it possible tore-create habitat with good structural and biologicaldiversity—one that can support a wide array ofwildlife species for many years to come.

Other Issues with Wildlife TreesIn all of this work, safety must be considered. Thecurrent guidelines for wildlife tree retention inBritish Columbia stipulate the creation of no-workzones around the tree, based on its condition,the degree of lean, and the slopes. This is gener-ally fairly easy to comply with, although in areaswhere a recreation trail passes close by, the man-ager of the area has to be fully aware of the snagsand undertake routine checks to ensure that theypose no hazard to pedestrians. In some cases, itmay require premature felling of the snags ratherthan letting them progress through to their natu-ral fall-down point. When a large tree cannot beretained, or when larger branches must be re-


moved, it is often feasible—and ecologically valu-able—to leave the larger chunks (the coarsewoody debris) on the ground to decay. In suchcases, these larger logs should be stable, whichusually requires all branches to be cut away sothat the log sits right on the ground. It is also im-portant to avoid using logs as bridges acrosscreeks. Such a structure provides a temptingchallenge to adventurous youngsters, but maybring with it a liability issue that should have beenavoided. Similarly, logs on slopes are best leftpointing downhill, rather than sitting across theslope. In the latter case, the log may roll andcause a hazard and associated liability.

Finally, there is always a question of how muchclean-up should be undertaken. This will dependon the area, the species, and the goals of themanagement plan. In an urban situation, it is gen-erally wise to clean up smaller branches and twigsto reduce the potential for fire hazards. This ma-terial can be chipped on site and the mulchblown back in a thin layer. Avoid large mulch piles:they might spontaneously ignite due to the heatof decomposition or will decay anaerobically andproduce undesirable fermentation products.Evenly distributed thin layers decay rapidly andhelp stabilize the forest floor.

The other consideration is pest problems. Forexample, in British Columbia the Douglas-fir barkbeetle is known to breed in the branch and twiggymaterial; cleaning and chipping this material re-duces the chances of creating a bark beetleepidemic.

Rather than completely removing the wholetree, arborists can work with other professionalsto determine a safe height for a wildlife tree. Per-haps only the top needs removing, or some ofthe higher branches, or maybe the whole tree hasto come down but the larger logs can be left onsite to decay slowly. Whatever the approachadopted, arborists have an opportunity to expandtheir own skills and play a useful role in helpingcreate and maintain healthier urban forest eco-systems. The key to success lies in taking an in-terdisciplinary approach and understanding therole of trees in the ecosystem and how these func-tions can be incorporated into standard arbori-culture practices. With this knowledge, arborists

can actively promote these alternative practicesand, over time, refine them. All of these small ef-forts will help to make our urban forests moresustainable.

Literature CitedAgee, J.K. 1995. Management of greenbelts and forest

remnants in urban forest landscapes, pp 128-138.In Bradley, G.A. (Ed.). Urban Forest Landscapes:Integrating Multidisciplinary Perspectives.University of Washington Press. Seattle, WA.

Backhouse, F. 1992. Wildlife Tree Management inBritish Columbia. Ministry of Environment, Lands,and Parks, Victoria BC.

Bradford, P., T. Manning, T, and B. I'Anson. (Eds.).1996. Wildlife Tree/Stand-level BiodiversityWorkshop Proceedings. Ministry of Forests and BCEnvironment, Victoria, BC.

Bradley, G.A. (Ed.). 1995. Urban Forest Landscapes:Integrating Multidisciplinary Perspectives.University of Washington Press, Seattle, WA.

Bull, E.L., C.G. Parks, and T.R. Torgersen. 1997. Treesand Logs Important to Wildlife in the InteriorColumbia River Basin. Gen. tech. rept. PNW-GTR-391. USDA Forest Service, Oregon.

Bull, E.L., J.W. Thomas, and K. Horn. 1986. Snagmanagement on the national forests in the PacificNorthwest—1984. West. J. Appl. For. 1(2):40-43.

Chillibeck, B. (Ed.). 1993. Land DevelopmentGuidelines for the Protection of Aquatic Habitat.Ministry of Environment, Lands, and Parks,Victoria, BC.

Dunster, J.A. 1995. Effective tree retention in newdevelopments: An undisturbed landbase is the keyto success, pp. 125-131. In Neely, D., and G.Watson (Eds.). Trees and Building Sites.International Society of Arboriculture, Savoy, IL.

Dunster, J.A. 1996. Hazard tree assessments:Developing a species profile for western hemlock.J.Arboric. 22(1):51-57.

Dunster, J.A., and K. Dunster. 1996. Dictionary ofNatural Resource Management. UBC Press,Vancouver, BC.

Forestry Commission. 1990. Forest NatureConservation Guidelines. HMSO, London.

Forestry Commission. 1991. Community WoodlandDesign Guidelines. HMSO, London.

Forman, R.T., and M. Gordon. 1986. LandscapeEcology. Wiley, New York, NY.


Hodge, S.J. 1995. Creating and Managing WoodlandsAround Towns. Forestry Commission Handbook 11.HMSO, London.

Hudson, W.E. (Ed.). 1991. Landscape Linkages andBiodiversity. Island Press, Washington, DC.

Kollin, C. (Ed.). 1994. Growing Greener Communities.Proceedings of the 6th National Urban ForestConference. American Forests, Washington, DC.

Kollin, C. (Ed.). 1995. Inside Urban Ecosystems.Proceedings of the 7th National Urban ForestConference. American Forests, Washington, DC.

Labaree, J.M. 1992. How Greenways Work: AHandbook on Ecology. National Park Service andAtlantic Centre for the Environment, Ipswich, MA.

Machmer, M.M., and C. Steeger. 1993. The EcologicalRole of Wildlife Tree Users in Forest Ecosystems.First Draft. Ministry of Forests, Prince George, BC.

Maser, C. 1988. The Redesigned Forest. R. & E. Mills,San Pedro, CA.

Miliigan Raedeke, D.A., and K.J. Raedeke. 1995.Wildlife Habitat Design in Urban Forest Landscapes,pp 139-149. In Bradley, G.A. (Ed.). Urban ForestLandscapes: Integrating MultidisciplinaryPerspectives. University of Washington Press,Seattle, WA.

Ministry of Environment, Lands, and Parks. 1994.Stream Stewardship: A Guide for Planners andDevelopers. MOELP, Victoria, BC.

Ministry of Environment, Lands, and Parks. 1995.Community Greenways: Linking Communities toCountry, and People to Nature. MOELP,Victoria, BC.

Morrison, M.L., M.F. Dedon, M.G. Raphael, and M.P.Yoder-Williams. 1986. Snag requirements of cavity-nestings birds: Are the USDA Forest Serviceguidelines being met? West. J. Appl. For. 1(2):38-40.

Parks, C.G., E.L. Bull, and T.R. Torgersen. 1997. FieldGuide for the Identifiaction of Snags and Logs inthe Interior Columbia River Basin. Gen. tech. rept.PNW-GTR-390. USDA Forest Service, Oregon.

Rodwell, J., and G. Patterson. 1995. Creating NewNative Woodlands. Forestry Commission Bulletin112. HMSO, London.

Smith, D.S., and P.C. Hellmund. (Eds.). 1993. TheEcology of Greenways: Design and Function ofLinear Conservation Areas. University of MinnesotaPress, Minneapolis, MN

Thomas, J.W. (Ed.). 1979. Wildlife Habitats inManaged Forest: The Blue Mountains of Oregonand Washington. Agriculture Handbook No. 553.USDA Forest Service, Washington, DC.

Wildlife Tree Committee of British Columbia. 1992.Wildlife/Danger Tree Assessor's Course WorkBook. Ministry of Forests; Ministry of Environment;Workers Compensation Board of British Columbia,Victoria, BC.

Dunster & Associates EnvironmentalConsultants Ltd.

P.O. Box 109Bowen Island, BCCanada VON 1G0

Resumen. Los profesionistas de diseno, algunosarboristas y el publico en general, carecen de unentendimiento acerca de las funciones ecologicas de losarboles para otros organismos, y como estas funciones yprocesos ecologicos se conectan a traves de grandesextensiones en el paisaje. Consecuentemente, se pierdenmuchas excelentes oportunidades para la creacion y/oretencion de habitats de vida silvestre en el bosque urbano.Algunas de estas oportunidades son discutidas, junto conejemplos de la Columbia Britanica. Se incluye una seleccionde literatura de aspectos relacionados, para proveer lasbases de lecturas adicionales y el entendimiento de laecologia del paisaje, corredores verdes, arboles silvestres yla manera como los arboristas pueden integrar estosprincipios dentro de sus practicas.

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