a framework for street tree planning in urban areas in israel

Landscape and Urban Planning, 19 ( 1990) 203-2 12 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

203

A Framework for Street Tree Planning in Urban Areas in Israel

S. AMIR and A. MISGAV

Technion-Israel Institute of Technology, Faculty ofArchitecture and Town Planning, Haifa 32000 (Israel)

(Accepted for publication 18 January 1990)

ABSTRACT

Amir, S. and Misgav, A., 1990. A frameworkfor street tree planning in urban areas in Israel. Landscape Urban Plann., 19: 203-212.

This article presents the methodological aspects of a proposed planning process for an urban street tree system. The process specifies goals and objectives for street tree planning, criteria for evaluation of process output, develop ment stages in the preparation of conceptual and detail planting plans and a case study for dem- onstrating the use of an analysis method to de-

termine the suitability of site/tree projile of a given street.

Based on the use of subjective and objective data sources, the process could be adapted for use in the planning of other urban vegetation areas. The contribution of such a process to decision making depends on availability of data on site conditions and on species selection choices that are provided by the list being used.

INTRODUCTION

The increased intensity in development of urban areas often creates environmental conditions which will not support the continuous existence of street trees (Hubbes, 1975 ) . Past studies often show that there is a need for a conscious planning policy and management of street trees to prevent the persistent and ongoing destruction of this resource.

Clark ( 1978) indicates that trees generally have a low priority among decision makers. Public authorities often view tree planting

largely as a visual enhancement effort. They often do not take into consideration the broader environmental and functional roles that such trees possess (Rousakis, 1978).

Several studies have examined the functions of urban trees (Brown, 1983; Yahav, 1986) and Robinette ( 1972) classified the functions of trees as architectural, functional, engineering, climatic and aesthetic. Economic values were studied by Payne ( 1978) and Johnson et al. ( 1982) analysed their social and psychological functions. Pitt et al. ( 1979) dealt with climatic and engineering functions, while Appleyard

0169-2046/90/$03.50 0 1990 Elsevier Science Publishers B.V.

204 S. AMIR AND A. MISGAV

AND OBJECTIVES

EVELOPHENT kF ti ONCEPNAL

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Fig. 1. Street tree planning process.

URBAN STREET TREE PLANNING IN ISRAEL 205

( 1978) investigated trees as an aesthetic as- pect of the urban environment. Environmen- tal functions of trees were studied by Rich ( 1970) for air pollution and amelioration, and by Schiller ( 1984) for noise control.

Street tree resource management has lacked a long-range and coordinated approach that would ensure the biological needs of trees. Un- like most of the urban infrastructure, trees are living units and are the concern of many professionals. Andresen ( 1975 ) indicated that in many cities management tools that go be- yond the day-to-day problem-solving type are lacking. Growth and the enduring viability of trees require a management system that can assess the impact of daily actions on the continuous existence of a tree population. What is often lacking is a clear and predetermined policy that identifies urban trees as a living resource and one that fulfils multiple functions. Such a policy necessitates an ongoing planning and management system whose goal is to ensure the existence of trees and their development over time.

This article presents the content and stages of a proposed planning process for an urban street tree system in Israel.

Past studies have suggested various contents and structures for street tree management systems and the means for their implementation. Grey and Deneke ( 1986), International City Management Association ( 1982)) Howard and Hudson ( 1984), Tate ( 1984), Johnson et al. ( 1982 ) all identified the importance of includ- ing the following three elements as a part of any management system: inventory of the resource, plan for new planting and a strategy for resource maintenance. In addition to such professional elements, several administrative aspects have been suggested for inclusion such as long-range planning for manpower, and budget, as management tools which indirectly impact the well-being of the resource.

The process described below is one of three issues that emerged in the original study, (see Fig. 1): ( 1) development of methods for in-

ventory of the street tree population; (2) design of a planning process and of its update for planting street trees; (3) development of a maintenance program for urban trees. Details of the descriptive model and proposed maintenance program are not included here, but are described in Amir and Misgav ( 1988 ).

In preparing the plan, we followed a four- stage process: ( 1) specification of the problem through the identification of goals and objectives; (2) development of evaluation criteria needed to assess the quality of the results; (3) generation of development and detailed street tree plans; (4) demonstration of the use of the process in a case study.

GOALS AND OBJECTIVES FOR URBAN STREET TREE PLAN

This stage provided the conceptual basis of the plan that is based on preference by decision makers, on past studies and on research generated from case studies. This stage was divided into four elements: goals, objectives, means and criteria. The subjects divided into three groups (spatial-visual, function and biological-physiological aspects) and are presented in Fig. 2 in an hierarchical order, from the general to the specific.

Goals and objectives are concerned with two uses: ( 1) as a broad and general system which is relevant to any city-scale street tree planting plan; (2 ) as a framework that defines the content of a specific plan prepared for smaller areas such as a neighbourhood or the individual street. The goals-objectives framework relates to specific characteristics of a site and its conditions.

The proposed street tree plan has four major goals. The most important goal is the selection of tree species with the greatest suitability for a given location. The second goal deals with the achievement of a major function of trees in cities as providers of a green living environment. Trees give special landscape character to a place. A third concerns trees’ contribution to

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ee

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th

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te

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te

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Phys

ical

and

biolog

ical

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Tree

Tr

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Tr

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Func

tiona

l Bo

th

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Bo

th

Site

Si

te

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size

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wn

Heig

ht

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for

m

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tex

ture

Tr

ee c

olou

r: Le

aves

Bl

osso

m

Crow

n de

nsity

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g de

nsity

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antin

g ar

rang

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t Co

nstru

ctio

n de

tails

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reet

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nitur

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ound

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ws

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t va

riety

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atic

regio

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th

rate

to:

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uous

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the aesthetic, climatic, environmental, traffic and safety qualities of the city. The fourth goal concerns the contribution of trees to the reduc- tion of conflict between conditions needed for their long-range survival and human activities in their immediate environment (Fig. 2).

Six objectives were identified that provide a basis for management (Fig. 2 ) . They are intended to achieve maximum suitability between trees and their site environment in three areas: the spatial-visual, biological-physical and the functional. The objective of the process is to arrive at an optimum among these areas. To make the framework operational, a set of criteria and means was developed. Cri- teria were used to evaluate the degree to which a given street tree design achieved the stated objectives.

Three sets of criteria were developed to re- late to two types of tree planning tasks. The first is concerned with tree selection, the second with site suitability, the third with both tasks. Visual-spatial criteria are intended for use in the evaluation of the degree to which a tree is suitable for a site with a given visual quality. The second group is concerned with evaluation of tree suitability for a site with a given growth and physical constraint. The third type evaluates the degree to which a tree is likely to fulfil a function that would ameliorate the climatic or other environmental conditions. Cri- teria and measures for the evaluation of trees and sites, or both, are identified by subject and type of task in Table 1.

PROCESS FOR GENERATING A CONCEPTUAL STREET TREE PLAN

The plan-making process is based on the content developed in the goals-objective stage. This includes the preparation of two plans: one for general guidelines and another for detailed street planting.

The conceptual stage includes design guidelines and the preparation of a city-wide plan. An important function of the latter is to pro-

vide a comprehensive decision-making framework. It makes the implementation of design guidelines in local plans feasible. One of the plan’s objectives is to develop agreeable visual and landscape identities for various parts of the city. It is intended to secure a given visual and spatial form in the organization of new planting.

In the preparation of the conceptual plan, decisions were based on three design elements: spatial arrangements of trees, their size and form. Three types of spatial arrangement were identified for use: the formal arrangement, non-formal and a combination of the two (see Table 1). Tree size is concerned with the di- mension of the diameter of the crown and the height of the tree. For planning objectives, trees were divided into three groups: small, medium and large. The third design element is tree form which defines the space of streets, the type of care that is needed by the tree and the functions that a tree could fulfil.

As shown in Fig. 1, the first stage in plan preparation is specification of measures, as de- fined in Table 1. The three elements are used in the following sequence: arrangement, size and form. To develop a tree profile, the elements are combined through two matrices, arrangement/size, with the resulting combinations brought together with form. This results in the specification of a wide range of tree profiles. Each profile with its characteristics con- stitutes the basis for evaluation of the suitability of trees for various site conditions.

The profile data are used with a set of predetermined design principles to develop a general planting plan, as is demonstrated for the case study below. The final product, at this stage, does not name tree species, but specifies design principles and tree profiles to be ap plied in each street. It is intended that the conceptual stage will provide sufficient flexibility to enable the completion of detailed design for various sites. Such design could take into consideration additional tree-specific and site-related aesthetic and functional factors.


DETAILED STREET PLANTING PLAN

Once tree arrangement, size and form for a given street type are specified, it is possible to complete the preparation of a detailed street planting plan. This stage includes detailed studies of site characteristics and the selection of tree species. Site evaluation is made on the basis of three groups of criteria: spatial-visual, biological and functional (Table 1). Site suit- abilities and limitations on tree growth and function were identified. This information was then used as a basis for the selection of species suitable for a site. The tree species selection process includes three stages: ( 1) development of a street tree list; (2) selection criteria ordering; (3) selection of species.

A general tree list was based on existing lists and on other written sources that helped verify the suitability of species as street trees. Data describing the trees in the list were classified according to size and to four groups of site characteristics: physical, tree growth conditions, site maintenance factors and site aesthetic qualities. Table 2 presents a sample tree profile.

Criteria classification provided a basis for the description of trees in the general list (Ta- ble 1). The use of the list for the selection of the most suitable tree for a site required the ordering of criteria according to importance.

Criteria were classified into those of primary and secondary importance (Table 2). The first included all those that are mandatory for the continuous existence of a street tree. The second group were considered not mandatory. Ten types of criteria were used in the case study. The primary upper, the more important group, are concerned with visual, bio-physical, and maintenance subject, and the secondary with visual and functional criteria. Criteria were also ordered within the group as indicated by the numbers l-3 (Table 2 ) .

Street trees were selected from a general list through a two-stage elimination process. In the first stage, a list of potential tree selections for

a street is evaluated according to the primary criteria. Only those trees that satisfied all the primary criteria were retained on the list. The new list is then evaluated using the secondary criteria. In this stage, the less suitable trees are not dropped from the list, but are placed in a less preferable position. This order provides the designer with greater flexibility in making choices among trees.

SELECTION OF A TREE FOR A SITE - A CASE STUDY

The selection of the most suitable tree species for a place is based on site and tree suitability characteristics. These are detailed in the lower part of Fig. 1 where site evaluation and tree species selection factors are presented. Each factor is detailed by selected characteristics of a site or trees to describe the nature of suitability.

The content and sequence of tree species screening involves five succeeding matrices (Table 2 ). Each matrix concerns different combinations of site/tree characteristics. The first three matrices are concerned with primary criteria, the latter two with secondary criteria.

To demonstrate the screening process, we used a general list of 14 different trees indicated by number from 1 to 14 (Table 2 ) . Tree names are indicated in footnotes. Trees were put on the list after pre-screening of a larger list for proper sizes and forms. The ( - ) symbol indicates that the tree does not satisfy the required level of a mandatory criterion specified in the corresponding row and that the tree is not suitable for the site in question. In the second phase, screening continues with the secondary criteria.

To demonstrate the screening process, we chose a street in the study area. Its main function is transportation within a residential area. Street characteristics and functions were used to design the preferred profile. Tree and site

TABL

E 2

Tree

list

scr

eenin

g pr

oces

s -

a ca

se st

udy

Type

of

Orde

r Tr

ee/s

ite

decis

ion

Tree

cha

rac-

Ex

istin

g Pr

efer

red

leve

ls Tr

ee s

pecie

s lis

t’ cr

iteria

fa

ctor

s m

atric

es

teris

tic

site

for

tree

in st

reet

co

nditio

ns

1 2

3 4

5 6

7 8

9 10

11

12

13

14

Prim

ary

I cr

iteria

2

Tree

size

an

d fo

rm/-

site

phys

ical

limita

tion

Site

gro

wth

cond

itions

/- tre

e en

viron

men

t su

itabi

lity

3 Tr

ee

main

tena

nce

requ

irem

ents

/- sit

e ph

ysica

l- sp

atia

l ch

arac

teris

tics

Seco

ndar

y 1

crite

ria

Tree

ae

sthe

tics

2 Tr

ee

cont

ribut

ion

to e

nviro

nmen

t/-

site

fimct

iona

l pr

oblem

s

Size

Form

Extre

me

tem

pera

ture

Vu

lnera

bility

to

air

pollu

tion

Decid

uous

/- ev

ergr

een

Grow

th

patte

rn

Root

Br

owth

Gr

owth

ra

te

Life

expe

ctan

cy

Crow

n de

nsity

Co

lour

Text

ure

Parti

cles

abso

rptio

n

Noise

ins

ulatio

n

Narro

w M

ediu

m

(The

list

was

pre

-scr

eene

d fo

r siz

e an

d fo

rm)

sidew

alk

size

Elec

tric

A+B

lines

gr

oups

2 M

ount

ain

Low

mou

ntain

a3

.

clim

ate

clim

ate

Pollu

ted

Resis

tanc

e to

air

air

poll

ution

’

Expo

sed

site

Narro

w pa

ssag

e Pa

ved

sidew

alk

Norm

al

Ever

gree

n .

0 l

Decu

rrent

Not

aggr

essiv

e ro

ot

Med

ium

l

.

.

Norm

al

Med

ium-lo

ng

l 0

.

Not

appli

cable

No

t ap

plica

ble

Not

appli

cable

Ai

r po

llutio

n

Mini

ma1

Grey

or

dark

gre

en

Med

ium

Data

not

av

ailab

le’

Tran

spor

t Da

ta n

ot

avail

able

0 .

l

‘List

of

14 m

ediu

m-s

ized

trees

: 1.

Acac

ia d

ealab

aita;

2.

Ace

r ob

tusif

oliu

m;

3. A

kctry

on

tom

enta

sum

; 4.

Albi

zzia

julibr

issin;

5.

Cel

tis a

ustra

liq

6. C

eltis

ges

her

aziv;

7. E

ryth

rina

cora

ioak

mod

rum

; 8.

Gra

villea

ru

bust

a; 9

. Jun

eper

us;

10. L

oges

trom

ia sp

ecio

sa;

11. O

lea e

urop

aea;

12

. Que

rcus

ilex

, 13

. Sch

inus

tere

binth

i- fo

liw,

14. T

amar

ix ar

ticul

ata

‘Pre

ferre

d tre

e fo

rms:

A =

roun

d ty

pe;

B =

colum

nar

and

palm

s.

‘* in

dica

tes

the

tree

is no

t su

itable

for

the

pre

ferre

d cr

iteria

. ‘L

ack

of v

alida

ted

data

on

thes

e fa

ctor

s in

the

case

stu

dy m

ade

their

inc

lusion

in

the

evalu

ation

im

poss

ible

.


characteristics were evaluated with preferred measures. The measures indicate levels in each criterion that were preferred in the street tree screening process.

Screening was carried out in five levels. (a) The primary physical conditions of the site, with narrow sidewalks and overhead power lines, limited the type of arrangement, size and form of the trees that could be considered to those of medium size and of the round and col- umnar form groups. For the same reason, the proposed formal arrangement type was chosen. (b) In the second matrix, the lack of suitable growth conditions in the site eliminated four trees ( 1, 7, 13, 14). They were not suitable to the mountain climate prevailing on the site. (c) In the last level of primary criteria screening, unsuitable visual and functional characteristics eliminated an additional eight species. Levels (d) and (e) dealt with secondary evaluation criteria. There were no limitations on the use of the remaining two species because visual-functional characteristics and environmental problems were non-mandatory requirements.

Two species, Nos. 2 and 11, Acer obtusifol- ium and Olea europaea, respectively, satisfied the required profile for the chosen site. The analysis indicated that the two species were equally preferred, both satisfied the secondary aesthetic factors in the fourth matrix and data on site/tree environmental constraints were not available. Data on the last two matrices would have shown differences in suitability, the species that would satisfy criteria of higher importance would be selected.

DISCUSSION AND CONCLUSIONS

This article deals with methodological aspects of the street tree planning process, but not with those aspects which are related to the development of resource description, and inventory and tree management processes.

A strong relationship exists among the three parts of the whole process, in content, reason-

ing and in the method of analysis. However partial description of one stage, as in this pa- per, could not provide a full explanation or sufficient details to consider the many solu- tions presented in the planning stage. This pre- sentation provides a description of the ideas and the skeleton of a process for making decisions rather than fully documenting a “how to do it” process.

The methods for data processing described above apply objective data, particularly concerned with site characteristics. On the other hand, content dealing with visual-design aspects of the environment is largely subjective in nature. Regardless of the degree of data objectivity, implementation processes will be highly influenced by ideas and design concepts chosen by various decision makers during planning and implementation. They will deter- mine the final quality and degree of objectivity of the plan. The processes and content suggested above should be seen as subjective/objective bases on which the final plan will be produced.

The process is described as a static condi- tion, but in reality the development of a street tree plan is a dynamic endeavour which continues for many years and in ever-changing en- vironments. In the future, it will be important to adopt decision-making processes to changes in site conditions. This could be done either through re-evaluation of the suitability of criteria and measures, or by reconsideration of the levels of importance and re-evaluation of the effect of implementation of a given planning stage in the future.

The process is suitable for implementation in cities of different scales. Several changes could be made in the type and intensity of the inventory to suit the size, lay-out and a given combination of characteristics. A city with a large existing street tree system, but with no new trees to be planted, would be likely to invest its resources in planning for maintenance. A new location which is expecting additional physical growth would invest more in the de-


velopment of a plan for new planting. The proposed process could be adapted to suit different scales and characteristics of various systems.

The process could also be used for managing other vegetation resources found in public parks, private yards and other urban green open spaces. This would require some adjust- ment of the descriptive model and criteria, and resources to suit vegetation dispersal patterns in the open spaces.

The experience with the case study has shown that for the process to be fully taken ad- vantage of, it is important to improve data availability for a site for which a plan is being prepared. It is important to develop data on the functional-environmental characteristics of the site. In our case, data were not available on air pollution and noise levels. Also needed is a broader tree list that contains a variety of species. Such additional data will help justify the detailed list of criteria provided for in the process.

Clearly, where data are not available and the number of criteria that could be used is re- stricted, the level of details that a planting plan could arrive at would be confined and the potential contributions of the overall process will be limited.

ACKNOWLEDGEMENTS

This research was funded partly by the KKL Forestry Department, to which the researchers are grateful.

REFERENCES

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a framework for street tree planning in urban areas in israel

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