STREET TREE DIVERSITY MAKING BETTERCHOICES FOR THE URBAN LANDSCAPE
NINA L. BASSUKUrban Horticulture InstituteCornell UniversityIthaca, NY 14853
Arguably the loss of the Americanelm (Ulmus americana) from our citystreets gave rise to the current highlevel of interest and activity in thefield of urban forestry and urbangreening. These magnificent treessuccumbed to Dutch elm disease in theUnited States over a period offorty-odd years beginning in the1930s. Because this one species wasso prevalently planted (it is estimatedthat 45% of all street trees in Chicagoin 1971 were elms), its demise left agaping hole which city foresters havebeen filling ever since (Dreistadt, S.H.,et al. 1990).- -
The devastation caused by Dutchelm disease also called attention to thedangers of planting monocultures, orextensive plantings relying on only avery few species. These prevalentplantings become increasinglyvulnerable by encouraging the build-upof pests and diseases.
Unfortunately, the lessons of theAmerican elm are just recently beingheeded. But during and directly afterthe loss of the elm, a replacement wassought to fill in the gaps left by deadelms. Instead of looking for a
diversity of tree species, manymunicipalities repeated the mistake ofthe past by overplanting a few species.In the 1960s, Gleditsia triacanthos,honeylocust, was thought to be a toughurban contender being fairly tolerant ofdrought, high pH soils, and salt and iseasy to transplant. Only recently arewe seeing a build-up of insect pestson Gleditsia (honeylocust plant bug andspider mite to name just two) whichcan be associated with the vastlyincreased food supply (Bassuk et al.- -1988). Sugar maples (Acer saccharum)which were also over-planted in theNortheastern U.S. are nowexperiencing a decline. Cities with20% -50% sugar maple are having toreplace vast numbers each year asthese trees die. Still, the emphasisamong some parks departments andcity foresters has ken to find theperfect urban tree which canwithstand the multitude ofenvironmental stresses encountered bystreet trees. Overplanting of Norwaymaples, green ash, little leaf linden,London plane and others seem to beanother manifestation of the sameproblem. Not only is this short-sighted, but it does not take intoconsideration the fact that the urban
environment is really a series ofheterogenous microclimates. One needonly to look at the non-uniformgrowth of identical cultivars of streettrees to see that differences inenvironmental variables such asdrainage, soil fertility, pH, salt and theamount of rooting space can createwidely differing site conditions within avery short space. Proper siteassessment should precede plantselection if street tree plantings are tobe successful. The match-up of sitelimitations with tree adaptability iscommonly called the right plant in theright place. By carrying out siteassessments, good plant selection willmake more of an impact and diversityshould be encouraged.
Having made the case for diversity,it is interesting to note that our urbancenters are actually repositories for awide range of diverse plant materials.Most cities seem to have upwards of100 or more species on the street, withsome milder climates having thegreatest number of diverse species(Table 1). However, for many cities,a very few species still make up thegreatest percentage of the population,so that the danger of monoculturalplantings remains real. It is interestingto compare the number of woodyspecies found in native habitats such asan example from Cattaraugus County,in western New York (Table 2). Veryfew species colonize these habitats.Adding all of them together would onlytotal 54 (Eaton et al. 1987.).- -However, a significant differencebetween natural areas and urban streettree plantings is the formers ability toregenerate. If a disease or insect
should decimate the species within sucha natural area, regeneration wouldassure that barren spaces would notexist for any appreciable length oftime. In the urban environment, whichis heavily managed and interfered with,this regeneration would necessitate theactive removal and replacement of thetrees.
HOW CAN WE QUICKLYGATHER INFORMATION ABOUTSPECIES HEALTH ANDDIVERSITY WITHIN THE URBANENVIRONMENT?
Typically, the way in which wehave gathered information about streettrees has been by the use of street treeinventories - a laborious cataloguingof all trees on all streets. We havedeveloped a new technique using arandomized sample that will providestatistically reliable data on suchquestions as species makeup, totalnumber of trees, number of unplantedspaces, diameter size class, andtree health and maintenance.
This is based on technologyby such polling organizations asNielsen and Gallup. The basicpremise of the technique is that
in alarge population, a random selection ofapproximately 2000 individuals shouldprovide meaningful data. Theimportant point is that each of the2000 sample individuals should have anequal chance of being chosen. Whatmakes this technique exciting is thatthe sample size remains the sameregardless of the size of the city or
Table 1 Survey of Cities with Street Tree Inventories
City Number of Species Total number ofMaking up 65% - 70% Species/Cultivarsof Total Street Tree Found in the StreetPopulation Tree Population
*Worcester, MA 1*Mobile, AL :;Poughkeepsie, NY ; 65Orlando Hills, IL*Ravenna, OH z -5;*Franklin, IN 4 64Ithaca, NY 103*Forest Park, OH 5*Providence, RI
Rochester, NY - -*Rockford, IL
Syracuse, NY - -**Eureka, CA**Lancaster, CA z 1z*Novi, MI**Santa Barbara, CA : 1g*Arlington Co, VA**Manteca, CA ;
*Falls Church, VA*Lakeland, FL ::
**Santa Ana, CA 1;**Pasadena, CA :; 253**Redondo Beach, CA
**Sunnyvale, CA 202**West Hollywood, CA 120**Monrovia, CA :: 127**Riverside, CA
**San Buenaventura, CA 169**Palo Alto, CA
*Vancouver, BC**Encinita, CA 26 z!
*Information provided by ACRT, Inc., Kent, OH 44240
**Information provided by Gold Coast Environmental Services, Inc., Irvine, CA92714
Table 2. Woody Species in NaturalHabitats/ Cattaraugus County, NewYork
Beech/Birch/Maple/Hemlock Forest 5 species
Bottomlands 10 species
Oak Forests 13 species
Conglomerate Boulders 13 species
street tree population. We haverecently conducted surveys of Ithaca,Syracuse, Rochester and Brooklyn,New York containing 5,600, 33,000,48,000 and 111,000 trees respectivelyusing this technique. Accuracy wasexcellent compared back to fullinventories conducted previously.
Described simply, a city isdivided into zones so that the2000~tree sample can be assured ofa good distribution throughout thecity. A pre -sample is conducted todetermine the distribution of treenumbers on each block and theestimated percentage of street treeswithin each zone. The final sampleis then made so that we can comeclose to our desired 2000~treesample. Actual data are taken by ateam driving around in a car so thatthe survey is typically completed inone to two days.
We can see by our results that avery few species make up the vastmajority of trees in these citiesalthough the overall breadth ofspecies is often quite impressive.(Tables 3-5)
HOW CAN BETTER PLANTSELECTION HELP OVERCOMESITE LIMITATIONS ?
There are many environmentalvariables which contribute to theearly mortality of urban trees;however, the problems of soilcompaction, poor drainage andaeration, high soil pH, road salt andlimited rooting space arc common tonumerous sites and can have severeconsequences on tree growth. Inpoorly drained sites, plant selectioncan be very effective in overcomingthese problems. Such trees asQuercus bicolor, Nyssa sylvatica,and Taxodium distichum are tolerantof standing water. There are moretrees that can tolerate high pH soilsthan those that require acid soils;however, some of our most populartrees such as Acer rubrum and- -Quercus palustris are intolerant ofthis ubiquitous urban phenomenonand disproportionately make thiscondition apparent. Some underusedbut very promising street trees thattolerate soil pHs in the 7.5 - 8.5range or even higher are Tiliatomentosa, Quercus macrocarpazurz muhlenbergii, and CorylusA
Salt tolerance both aerial and soilborne, is a feature of Acer
Table 3 Species Percentages From a 1989 Sample InventoryCompared With a Full 1978 Inventory of Syracuse, NY
SPECIES 1989 SAMPLE 1978 INVENTORY*Acer platanoides 3 6
Acer saccharinum 16.1%Acer saccharum 7:1; 7.8%Gleditsia triacanthos 8.1% 5.1%Malus sp. 5.7% 4.9%Tilia cordata 7.4% 3.0%Platanus x acerifolia 2.9% 2.8%Picea sp. 0.6% 2.5%Fraxinus sp. 5.1% 2.5%Acer rubrum 5.6% 2.3%Acer negundo 1.4% 2.2%
zzzztza0.9% -0.9% 1.7%
Aesculus sp. 0.2% 1.1%Exotic maples 0.8% 1.0%(A. campestre, A. ginnala)Tilia americana 0.2% 1.0%Prunus sp. 0.8% 0.9%Celtis occidentalis 0.7% 0.8%Catalpa sp. 1.0% 0.8%Ulmus sp. 0.2% 0.8%Populus sp. 0.2% 0.8%Juniperus sp. 0.05% 0.8%crataegus sp. 0.5% 0.8%carpinus sp. 0.9% 0.7%Ginkgo biloba 1.2% 0.7%Betula sp. 0.4% 0.5%Liriodendron tulipifera 0.6% -Quercus sp. 0.6% -Ostrya sp. 0.4%salix sp. 0.2% 0.596.Sophora japonica 1.2% 0.5%Sorbus sp. 0.2% 0.2%Platanus occidentalis 0.1% 4.7%Other 1.4% 4.7%Total Number of Trees 33,453 39,030
* N. Richards, Syracuse University, Syracuse, NY
Table 4. Species percentages from a 1990 sample inventorycompared with a full 1987 inventory of Ithaca, NY
SPECIESAcer platanoidesAcer saccharumGleditsia triacanthosAcer saccharinurnAcer rubrumMalus spp.Platanus x acerifoliaGinkgo bilobaFraxinus pennsylvanicaP!c$l&yana
Quercus rubraQuercus palustrisZelkova serrataAcer negundoAesculus hippocastanumAilanthus altissimaOthersTotal Number of Trees
1990 SAMPLE 1987 INVENTORY33.7% 33.1%17% 19.0%9.4% 8.8%6.0% 5.9%5.6% 5.2%2.5% 2.5%1.2% 0.3%2.7% 2.2%1.8% 1.8%1.9% 1.5%0.9% 1.7%1.6% 1.7%1.5% 0.7%0.6% 0.7%0.6% 0.7%0.6% 0.5%0.1% 0.2%13.3% 11.0%5700 5541
pseudoplatanus and Robiniapseudoacacia. Robinia has many goodfeatures which help it to overcomeurban stresses. Once established it isboth drought and flooding tolerant,fixes its own nitrogen and is tolerantof high pH soils. However, itsassociation with borers makes it anunpopular choice. Three namedcultivars of Robinia pseudoacacia wereselected by the United States SoilConservation Service for borerres i s t ance . Up until now, lack ofpropagation success has limited theirdissemination. In the spring of 1990,we successfully rooted softwoodcuttings of these clones and hope totest them for wider distribution in thenear future.
WHAT ARE THE LIMITS OFPLANT SELECTION?
There is one problem, however,that we cannot select for and that islack of rooting space. Because of theway in which sidewalks and roads areconstructed, their base materials areseverely compacted making street treeroot growth often contained within thetypically 4 x 4 opening in which theyare planted. Recent work has begunto show how much rooting space isnecessary for tree growth. However,the reality of urban construction oftenprecludes this with notable exceptions.
Where landscape architects haveplanted trees in large open volumes of
Table 5. Species percentages from a 1990 sample inventory comparedwith a full 1988 inventory of Rochester, NY
SPECIESAcer platanoidesAcer platanoides ColumnareAcer platanoides Crimson KingAcer platanoides SchwedleriFraxinus pennsylvanicaGleditsia triacanthosTilia cordataAcer saccharumAcer saccharinumPlatanus x acerifoliaLiquidambar styracifluaPyrus calleryanaMalus spp.Acer rubrumSophora japonicaTilia americanaGinkgo bilobaQuercus rubraOthersTotal Number of Trees
1988 PARTIAL1990 SAMPLE INVENTORY27.5% 26 62.2% 0:8it3.8% 4.5%1.7% 2.5%
11.0% 11.3%10.9% 10.7%8.1% 7.9%4.6% 4.0%4.1% 6.2%3.2% 3.8%2.4% 1.9%2.3% 1.2%2.1% 0.6%1.7% 1.9%1.4% 2.6%1.3% 0.1%0.9% 1.5%0.9% 1.8%9.9% 11.1%
48,000 No Estimate
soil with plenty of shared rootingspace, tree growth and health havebeen dramatically superior to trees incontained root zones (Kuhns, L.J.1985.). These empirical examplespoint out the need for engineering forrooting space within the urban sidewalkenvironment. Our latest project servesto address this need: how to meetengineers requirements yet still providea medium that allows for root growth,air and water movement into the soil.
There appear to be four factors thatare needed to assure street treesuccess: site assessment, plantselection, site modification wherenecessary and proper plantingtechniques. By not following throughwith any one of these, the entire
planting may be in jeopardy.However, by zeroing in on thesefactors, success in urban greeningcould become a more plausible andcommon reality.
Bassuk N.L. and Jaenson, R.J.1988. Ithaca Street Tree Survey.Department of Public works, Ithaca,NY.77pp.
Dreistadt, S.H., Dahlsten, D.L. andFrankie, G.W. 1990. UrbanForests and Insect Ecology. BioScience 40(3) 192-198.
Eaton, S.W. and Schrot, E.F. 1987.A Flora of the Vascular Plants ofCattaraugus County, NY. Bulletinof The Buffalo Society of NaturalSciences. Vol 31.
Kuhns, L.J. 1985. Creative SitePreparation. METRIA: 5.Selecting and Preparing Sites forurban trees. pp. 92-94.