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Urban Ecology, 7 (1982/1983) 159. 171 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
159
DIVERSITY AND STABILITY IN A STREET TREE POPULATION
N .A. RICHARDS
College of Environmental Science and Forestry, School of Forestry, State University of New York, Syracuse Campus, Syracuse, NY 13210 (U.S.A.)
(Accepted 5 May 1982)
ABSTRACT
Richards, N.A., 1983. Diversity and stability in a street tree population. Urban Ecol., 7: 159.. 171.
The relationship of species diversity to the stability of a street tree population is ex- plored, using data from Syracuse, NY. Because streetside spaces are complexly-stressed environments, one generally observes that relatively few species prove wide adaptation and good longevity in a particular community, and that there is greater species diversity among recently planted trees than among older trees in the population. The few species surviving to be well represented among the older trees are likely to be better prospects for contributing to population stability in the uncertain future than are short-lived, ill- adapted, or little-tested species that may be added to increase diversity. Population stability depends on species adaptation to the diversity of streetside conditions in a com- munity over time, rather than on species diversity per se. Good age diversity, to provide adequate successful replacements, is essential for population stability. Undue emphasis on species diversity in replacement plantings may further threaten stability by causing in- adequate replacement of the proven adapted species in the older population.
INTRODUCTION
A common tenet of popular ecology is that high species diversity con- tributes to the stability of ecosystems by reducing hazards of catastrophic loss of a particular species. This is often translated to street tree populations as concern that the older trees in a population are predominantly of only a few species, and that greater diversity is needed in replacement plantings. A survey of street tree management throughout New York in 1979 found several com- munities emphasizing new species in their replacement rather than their pre- dominant species in the older population, because of the presumed desirability of greater diversity (Richards and Giedraitis, 1980). Also following this premise, Sanders (1981) devised a method for monitoring species diversity, using the street trees of Syracuse, NY as a case study. There is much evidence from plant ecological studies that relationships between diversity and stability cannot be as simply expressed as this premise suggests (e.g. Whittaker, 1975, pp. 42-54). This paper examines these relationships as they pertain to street tree popula- tions, primarily using the same case study population employed by Sanders.
0304-4009/83/0000---0000/$03.00 o 1983 Elsevier Scientific Publishing Company
160
STABILITY IN A STREET TREE POPULATION
The term “stability” has many meanings, but in general terms, the stabil- ity of a population can be defined as its resistance to internal or external events causing extreme fluctuations in numbers, mass, or other qualities of the population. For a street tree population, I would define stability as a low probability that the number of functional trees will decline over the foresee- able future to the point of disrupting both the values of the population and the management allocations needed for removal and replacement of trees.
A community’s street tree population differs from a natural tree popula- tion primarily in the role of human selection and, consequently, the relation- ship of the older trees to replacements in the population. The older street trees, which are mostly survivors of previous human selection, contribute progeny for replacements only when their seedlings are transplanted or per- mitted to grow as street trees. Most selected replacements are not progeny of the older trees, and may or may not reflect the existing species composition in the population.
The stability of natural tree populations depends on the long-term success, or longevity, of individuals and also on the success of species reproduction. In street tree populations, stability depends primarily on the longevity of in- dividual trees and sufficient numbers of successful planted replacements. Each individual is important in a street tree population, so we try to avoid the high early mortality common in natural populations. If street trees fail at an early age, the need for frequent replacement is disruptive to population management. On the other hand, if street trees live for several decades, the timing of their eventual failure has relatively little effect on the annual rate of replacement needed to maintain the population (Richards, 1979). However, high losses of older trees within a short period can be disruptive to population values and management. Therefore, the stability of a street tree population depends both on the species being adapted for long-term success on their particular sites, and on an age distribution that assures continuation of the adapted species.
SPECIES DIVERSITY AND STREETSIDE CONDITIONS
Many ecological studies document relatively low plant species diversity associated with highly stressed or unpredictable environments, due to few species being adapted to the stresses present. Low community stability com- monly associated with such sites is more often a result of the environmental stresses than of the low species diversity. On the other hand, high species diversity tends to be a fragile characteristic because at least some of the species contributing to diversity are likely to be dependent on specific site or community conditions for their survival. This is evident in the special care often required to maintain high diversity, and especially rare species, in communities. Diversity and stability thus appear to be rather separate at-
161
tributes of plant communities, often inter-related but not causally linked in any consistent manner.
Streetside spaces are well-known as complexly stressed environments, due to cultural variables of limited space, compacted soil, chemical impacts, and mechanical damage, in addition to natural variables; occurring in varying degrees and combinations over space and time. Thus, of the many species that might be chosen for planting, relatively few are likely to demonstrate broad and long-standing adaptation to the range of streetside conditions in a particular community. The introduction of additional species to street tree populations is not a new practice; but one generally finds greater species diversity among the recently planted trees than among the older, surviving trees in the population because of loss of the less-adapted species. Solotaroff (1911) reported that experimentation with some 30 varieties (sic) of street trees in Washington, DC since 1872 had resulted in finding only ten or twelve desirable street tree varieties for that city. He also observed that the street trees of Paris, France, included only eleven species that were represented by more than 100 specimens each. It was not surprising to him that only a limit- ed number of species could withstand city conditions, were long-lived, and were otherwise suitable as street trees. The few species predominant among the older street trees of a community have proven their adaptation to diverse conditions, and have provided an element of population stability over time.
One can argue that past adaptation does not assure the adaptation of these species to unknown future conditions; but use of this argument to justify replacing proven street trees with other species must be questioned. Francis (1915) described physical impacts on streetside sites and street trees in New York communities much as they can be described today. Chemical impacts have certainly changed since 1915, but there appear to have been few new chemical impacts on street trees in New York during the last two decades. Because trees tend to lose resilience as they mature, the older street trees are likely to have been severely tested by recent impacts as well as by accumulat- ed impacts over their lives. Also, one would hope that control of future chemical impacts is a more urgent concern for environmental quality than is finding street trees that might withstand further environmental deterioration. In trying to anticipate future physical and chemical environments for street trees, it therefore seems logical that species with proven broad and longstand- ing adaptation to the conditions of a particular community are more likely to be successful under future conditions there than would species that have not been proven to date.
THE “MONOCULTURE” HAZARD
Street tree species rarely occur as a monoculture to the extent found in agricultural crops or forest plantations; nor would monocultures be logical over the diversity of streetside conditions in a community. However, the disease and insect hazards of monoculture are often called up in objection to a
162
few species dominating a street tree population. The Dutch Elm Disease (DED) catastrophe is commonly used as an example for street tree populations, so is a good one to examine critically. The case of Syracuse provides a good example.
In 1951 at the start of DED in Syracuse, native elms, primarily UZmus
americana, constituted 41% of the city’s 47,000 street trees and probably over half of the street tree biomass. Initial control attempts through sanita- tion cutting were lauded in Carson’s Silent Spring (1962, pp. 116-117), but these soon failed due to inadequate funding and poor control over the much larger off-street elm population in the city. Thus, the losses became cata- strophic during the 1960’s, so only about 100 pre-1951 elms remained on Syracuse streets in 1978 (Richards and Stevens, 1979).
The native elms had been heavily planted in Syracuse because of their proven adaptation and function as street trees. Their loss was catastrophic because of their preponderance among the larger size classes of the popula- tion. We now recognize that too-close spacing of the street elms, often two or more per lot front, had hastened disease spread and added costs of removing trees that had not been needed in the population. Also, more effective sanita- tion, and other disease management methods now known, could have spread losses over a longer period and thus reduced the impact on values and manage- ment of the total population (Strobe1 and Lanier, 1981).
However, popular hindsight places most blame on too-heavy use of the species per se. This ignores the fact that most of the street elms had served well for over 50 years before succumbing to a disaster, and therefore had been a major element in the stability of the population throughout that period. Even with the DED, the elms were successful street trees, and the previous heavy dependence on them cannot be fairly termed a mistake in retrospect. If an equally adapted and functional species faced no greater hazard of loss over a half-century or more than that encountered by the elms, it might well be reasonable to depend fairly heavily on such a species. The alternative of rely- ing more on less adapted or untested species would probably be more threat- ening to population stability over future decades. Species that are poorly adapted to the abiotic conditions of streetsides in a community are more like- ly to encounter serious disease or insect damage, and good local knowledge of a species is needed to develop effective disease or insect management practices, should a serious problem arise.
SPECIES ADAPTATION
From the foregoing, it is apparent that few inferences about population stability can be made from simple measures of species diversity in a street tree population. Rather, we must examine the biotic and cultural adaptation of the individual species making up the population. Also, we must examine the age distribution of each species, fairly easily inferred from diameter of relatively opengrown street trees. Particular attention should be given to
163
those species in the older population that have proven their contribution to population stability in the past, while species present only in recent plantings should be somewhat discounted.
Table I enumerates the more common species, or in some cases genera, in the street tree population of Syracuse in 1978 (Richards and Stevens, 1979); expanding on the data presented by Sanders (1981). Four general groups of species can be identified in terms of their proven adaptation and function in the street tree population, and thus their probable continuing contribution to population stability. The findings on species in this case study are given only for illustration, but the general groups and process of species evaluation should be applicable elsewhere.
A. Predominant, successful species
Three maple species, Norway, silver and sugar, constitute 55% of the total population and 84% of the street trees over 40 cm diameter; thus demonstrat- ing their long-term success. Of these species, only Norway maple continues to be planted in substantial numbers. This locally naturalized species has proven widely adapted to Syracuse streetsides, although it has functional defects: excessive crown density and leaf volume, relatively decay-prone wood, and poor response to crown pruning for wire clearance. Silver maple has proven highly adapted and long-lived in Syracuse, but its excessive size for limited streetside spaces has discouraged recent planting. Sugar maple is a choice street tree, and many have persisted for over 80 years in Syracuse. However, its greater sensitivity to site conditions than Norway and silver maple is in- dicated by a higher mortality rate over the years (Richards, 1979). Widespread publicity on sugar maple problems has now greatly reduced its planting, even on suitable sites. All three of these species therefore have limitations that need to be considered in site selection, but their proven adaptation and longevity make them logical species to continue in replacement plantings where appropriate in Syracuse.
B. Less common successful species
Several other species, planted less in the past than these maples, are now more common in the larger size classes than in the total population. This suggests good adaptation and longevity, but less recent planting for various reasons. Box-elder, horsechestnut and catalpa have shown good adaptation and longevity in Syracuse, but are disfavored in horticultural literature as rather unrefined species. The planting of poplars and willows is now dis- couraged due to aggressive top and root growth and weak wood. American linden, white ash and American planetree, or sycamore, have proven at least selectively adapted and long-lived on Syracuse streets, but have now been passed over in favor of other currently-promoted species in their genera. Col- lectively, the group B species now constitute less than 4% of the street trees
TA
BL
E
I
Syr
acu
se s
tree
t tr
ee p
opu
lati
on
- 10
0% i
nve
nto
ry
1978
a
Ada
ptat
ion
S
peci
es o
r ge
nu
s gr
oupb
S
peci
es p
erce
nt
of D
BH
cla
ss
Nu
mbe
r of
_.
. O
-20
cm
20-4
0 cm
40
-60
cm
>
60 c
m
All
DB
H
tree
s
A
A
A
C
C
D
D
C
C
C
B
D
B
D
B
Nor
way
m
aple
(A
cer
plat
anoi
des)
S
ilve
r m
aple
(A
cer
socc
hari
num
) S
uga
r m
aple
19.4
1.9
2.4
(Ace
r sa
ccha
rum
) H
oney
locu
st
9.1
(Gle
dits
ia
tria
cant
hos)
App
le
10.9
(M
alu
s sp
.)
Lit
tlel
eaf
lin
den
6.
9 (T
ilia
co
rdat
a)
Lon
don
pl
anet
ree
5.7
(X P
lata
nus
acer
ifol
ia)
Spr
uce
3.
5 (P
icea
sp
.)
Gre
en a
sh
5.6
(Fra
xinu
s pe
nnsy
lvan
ica
var.
) R
ed m
aple
3.
8 (A
cer
rubr
um)
Box
elde
r 1.
1 (A
cer
negu
ndo)
Z
elk
ova
4.0
(Zel
kova
se
rrat
a)
Hor
sech
estn
ut
0.2
(Aes
culu
s hi
ppoc
asta
num
) E
xoti
c m
aple
s 2.
3 (A
cer
cam
pest
re,
A.
ginn
ala,
A
. ps
eudo
plat
anus
) A
mer
ican
lin
den
0.
1 (T
ilia
am
eric
ana)
47.0
45
.8
12.4
26
.1
9.5
14.0
5.2
0.3
1.4
0.1
0.3
<O
.l
1.9
0.1
4.4
0.2
0.3
0.2
1.9
0.8
2.9
3.1
0.3
0
1.5
2.0
0.1
0
0.9
1.9
12.5
31
.2
1219
4
55.3
16
.1
6274
12.1
7.
9 30
70
0.2
5.1
2000
0 4.
9 19
19
0 3.
0 11
64
co.1
2.
8 11
07
co.1
2.
5 98
3
0.1
2.5
964
0.3
2.3
880
3.3
2.2
878
0 1.
7 67
1
1.3
1.1
413
0 1.
0 39
4
2.4
1.0
375
C
1.7
0.4
0.2
<O
.l
0.9
D
1.8
0.3
<O
.l
<O
.l
0.8
B
0.5
1.5
0.9
0.5
0.8
C/B
1.
0 0.
6 0.
5 0.
9 0.
8
B
0.3
0.2
0.5
4.2
0.8
C
1.4
0.8
<O
.l
<O
.l
0.8
C
1.7
0.1
co.1
0
0.8
D
1.8
0 0
0 0.
7
D
1.6
0.1
co.1
0.
1 0.
7
D
1.2
0.2
0 0
0.5
C
1.0
0.5
co.1
0
0.5
A
0.1
0.3
0.2
2.8
0.5
B
0.1
0.7
0.6
1.0
0.5
D
1.0
0.1
0 0
0.5
C
1.1
0.1
0 0
0.2
B
co.1
0.
1 0.
3 1.
1 0.
2
All
oth
er s
peci
es
6.8
4.0
2.0
1.8
4.7
Tot
al n
um
bers
in
DB
H c
lass
16
357
9068
92
05
4400
39
030
Per
cen
t of
tot
al i
n D
BH
cla
ss
41.9
23
.2
23.6
11
.3
100.
0
Vpe
cies
or
gen
era
over
1%
of
anv
DB
H c
lass
are
lis
ted.
Dat
a ad
ante
d fr
om
Ric
har
ds a
nd
Ste
ven
s (1
979)
.
Ch
erry
, P
lum
(P
run
us
sp.)
H
ack
berr
y (C
elti
s oc
cide
ntal
is)
Cat
alpa
(C
atal
pa s
p.)
Nat
ive
elm
(U
lmus
am
eric
ana,
U
. ru
bra)
P
opla
r (P
opul
us
sp.
and
hyb
rids
) “C
edar
s”
(Th
uja
sp.
, Ju
nipe
rus
sp.)
H
awth
orn
(C
rata
egus
sp
.)
Hor
nbe
am
(Car
pinu
s sp
.)
Gin
kgo
(G
ingk
o bi
loba
) M
orai
ne
ash
(F
raxi
nus
holo
tric
ha)
Bir
ch
(Bet
ula
sp.
) W
illo
w
(Sal
ix s
p.)
Wh
ite
ash
(F
raxi
nus
amer
ican
a)
Sop
hor
a (S
opho
ra
japo
nica
) M
oun
tain
ash
(S
orb
us s
p.)
Am
eric
an
plan
etre
e (P
lata
nus
occi
dent
alis
)
335
330
322
320
301
300
293
292
265
215
214
192
186
178
102 79
1820
166
under 20 cm diameter. Some of these species, planted on appropriate sites, are logical candidates for increased use in replacement plantings.
C. Less successful species
On the other hand, several species that have long been planted, have not persisted to the larger size classes; but are prominent in the smallest size class due to continued planting. Some of these, especially honeylocust, apple, cherry and plum, and mountain ash, normally mature early and at small diameters on Syracuse streets. Heavy planting of these short-lived species automatically reduces population stability because of need for frequent re- placement, as described earlier. Spruce, “cedars” and birch are poorly suited for streetsides in Syracuse, but continue to be planted there by adjacent residents. Green ash and red maple are native species that have proven relative- ly short lived on Syracuse streets, but cultivars of these species continue to be planted. Collectively, the group C species now constitute about 42% of the Syracuse street trees under 20 cm diameter. These species appear to con- tribute little to stability of the street tree population, so should only be used where their specific characteristics of form or color compensate for their generally poor longevity.
The native elm species must currently be placed between groups B and C in this classification. These species are no longer being planted, but a number of young volunteers are present on neglected streetsides and many more are elsewhere in the city. Small numbers persist in all larger size classes of street trees, resisting or escaping disease to date. If the continuing population of wild, volunteer elms in Syracuse can be reduced so that reasonable disease management practices could provide a good probability of at least a 40-year life expectancy of elm street trees, selective planting of elms might once again contribute to population stability. The elms would be particularly valuable for highly stressed inner-city streetsides where they were previously successful and few other species have proven as well adapted.
D. Recent species
Finally, several species recently planted in Syracuse contribute to the diversity of the smaller size classes, but their adaptation over the longer run is uncertain. Littleleaf linden, hackberry and ginkgo have previously been planted on streets or elsewhere in Syracuse, but recent plantings have been large enough to more fully test them. These species appear to have at least selective adaptation to Syracuse streetsides, but they are slowstarting and thus require more intensive early care than young street trees have usually received here. London planetree and zelkova are showing generally poor biotic adaptation to Syracuse streetsides, apparently due to local climate con- ditions. The recently planted exotic maples and Moraine ash are already prov- ing poor street trees in Syracuse, and are rapidly dropping from the popula-
16’7
tion. Sophora is starting well, but is still too recent an introduction to pro- vide a verdict. There are also several recent introductions among the many species that occur in too small numbers to identify separately in Table I.
Collectively, the group D species constitute about 28% of the street trees under 20 cm diameter and, as a group, probably threaten more than con- tribute to future stability of the street tree population. From Francis (1915) - written in Syracuse - and other local historical information, I conclude that most species that have recently been added to the street tree population have probably been tried here before, and would now be represented among the older trees if they had been successful. The few new species being added must also be expected to have a low probability of long-term success.
TESTING STREET TREE SPECIES
Known limitations to virtually every tested species prompt continuing search for good street trees. However, caution is required in transferring in- formation on street trees tested elsewhere. For example, the lo-year results of a shade tree testing program in Ohio (Chapin and Kozel, 1975) are now being used quite widely in species and cultivar selection for street tree plant- ings. Several of the species or cultivars recommended from the Ohio tests are proving poor street trees in Syracuse, in spite of superficial similarities between the areas. New species need to be tested under the local biotic and management conditions to identify their adaptation to specific site and cul- tural variables in a community. Test species should collectively be a small enough proportion of the replacement plantings so that their failures would have little effect on the stability of the total street tree population.
Because of the diverse streetside situations and high values associated with street trees, the characteristics of individual genotypes may be as important as those of species. Attention to branch and crown form, and other specific features important to the function of street trees, has encouraged selection and use of cultivars - clones or narrowly selected seed sources - to gain greater genetic control of desired characteristics. Many, or most, street tree species are now being planted as a relatively few cultivars, distributed widely; whereas the older trees in the population are more commonly of seedling origin, probably with greater genetic diversity but including more local selec- tions. Therefore, the use of cultivars may further reduce the potential of new plantings to contribute to population stability. Reviewing recent plantings in Syracuse, I suspect that some of the unsatisfactory performance we are ob- serving in honeylocust, green ash, and some other species, may be due to the particular cultivars rather than to the species. Even for species already proven in the older population, cultivars selected from elsewhere must be regarded as unproven until they have been adequately tested under the local site and cultural conditions.
168
AGE AND SPECIES DIVERSITY
In contrast to possible but largely controllable risks from low species divers- ity among the older trees in a street tree population, the consequence of poor diversity among age classes is quite certain. Trees ultimately die, and their ex- pected longevity can be estimated from experience for any given species and growth situation. Predominance of the major species in mature age classes guarantees a destabilizing effect on the population when many of them die in the near future.
Returning to the Syracuse data, Table I, diameter classes can be used to roughly approximate age classes. Most street trees in Syracuse under 20 cm diameter are under 20 years age since planting; those over 60 cm are generally over 60 years old and range to over 100 years. Age distribution to assure reasonable stability of the street tree population must reflect the various types of losses that can be expected over the life cycle: establishment related, age- independent, and senescence-related losses (Richards, 1979). For adapted, long-lived species under existing site conditions and management in Syracuse, it appears that a good age distribution for population stability would be about 40% trees under 20 cm diameter, 30% 20-to 40-cm trees in the early func- tional stage, 20% 40-to 60cm functionally mature trees, and 10% older trees with most of their functional life behind them. The high proportion needed in the replacement class could be reduced by improved species siting and early care, but unavoidable losses would still require a large proportion of young, developing trees to maintain a stable population over the future.
Using my approximate guidelines for adapted, long-lived species in Syracuse, the age distribution of the total street tree population superficial- ly appears favorable for future population stability. However, I have rated about 70% of the replacement plantings under 20 cm diameter as short-lived, ill-adapted, or inadequately tested species; groups C and D in Table I. Con- versely, of the three maple species that dominate the older population, the recent plantings of Norway maple will not maintain its population, and the very low replacement of silver and sugar maples assures their future loss as major species. It therefore appears likely that the street tree population of Syracuse will be destabilized in the near future; and that the number of func- tional trees in the population will remain low thereafter, unless more atten- tion is given now to replacement with proven adapted species.
The recent survey of community tree management throughout New York indicated that many communities are faced with a preponderance of older, maturing street trees, especially maples (Richards and Giedraitis, 1980). As a result, tree removal costs typically dominate their limited tree budgets at the expense of tree replacement. On the other hand, most communities that experienced catastrophic loss of elm street trees, as did Syracuse, have since increased their priorities for replacement. However, several communities reported emphasis of recent plantings on relatively short-lived species, such as apples and honeylocust. In some cases, this is a conscious decision to achieve
169
population stability by frequent replacement rather than by species adapta- tion and longevity; but it will substantially change the functional values of the street tree population.
The recent or present unstable state of the street tree populations in many communities responding to our New York survey appears to cause a cyclic focus in their street tree management at any time. Tree removal is emphasized for a few years, followed by a concentrated replacement program; with pos- sibly some care and maintenance before removals must be emphasized again. In smaller communities, a degree of cyclic emphasis may have some advan- tages for organizing funding and management; and may not significantly affect the population if the cycle of emphasis is completed within each decade or so. The disadvantage of the cyclicity is that it may separate replacement decisions from consideration of the total population function over time. This appears to be the case in several communities where recent experience with expensive removal of large numbers of older trees has biased replacement decisions against these long-serving species, in favor of a diversity of other species that are unlikely to serve as well. An integrated program of replace- ment, care, and removal may be more likely to identify factors contributing to population stability over the long run.
CONCLUSION
If we define stability in a street tree population as a low probability that the number of functional trees will decline in the foreseeable future to the point of disrupting values and management of the population, species adapta- tion is much more critical than is species diversity. Species diversity should be related to the diversity of site conditions and functional requirements on a community’s streets, and not to preset ideas of the value of diversity per se. Because streetsides are usually complexly-stressed environments, one general- ly observes that relatively few of the many species planted in a particular com- munity prove broad and long-standing adaptation as street trees there. The few species predominating in the older population have proven their adapta- tion, and thus are logical species to choose for replacement plantings wherever they are appropriate.
I find little ecological basis for the premise, implied in recent replacement practices of Syracuse and other communities, that the future stability of a street tree population can be enhanced by replacing the proven adapted species predominant among the older trees with a high diversity of other species. Most of the additional species are unlikely to prove as widely adapted to the range of physical and chemical conditions already experienced by the older trees, and there is no reason to expect that they might be better adapted to future changes in these conditions. Risks of catastrophic disruption of the population by disease or insects, often unduly attributed to species per se, are best reduced directly by proper site selection, good planting and early care, and hazard control through damage or stress control and pest manage-
170
ment. As all of these are better accomplished with well-known species, un- proven species are a greater risk in this respect.
The preponderance of a very few species among the older street trees of Syracuse and many other communities in New York is undesirable on func- tional grounds, because at least a moderate diversity of species can provide greater benefits over the diverse streetside situations in a community. The most logical candidates for a moderate increase in the number of dominant species in a population are the less common species among the older trees, that have proven at least selectively adapted to a community’s streetside con- ditions. There is also an appropriate, but probably limited, place in most com- munities for short-lived species that have proven useful in particular streetside situations. Unproven species, and also new cultivars of proven species, should be treated as tests, rather than presumed to be replacements in the popula- tion. Those that prove adapted and functional in particular situations can then be brought into the replacement program.
Because good age diversity is essential for future population stability, in- adequate replacement of the species predominant and proven adapted in the older age classes is a more certain threat to future stability than is low species diversity among the older trees. If emphasis on species diversity among re- placements causes inadequate replacement to maintain proven adapted spcies, it is a misuse of ecological concepts. Species diversity contributes to the stability of a street tree population only to the extent that individual species or cultivars prove successful, so must be used selectively to maintain a population likely to be adapted to the diverse and changing streetside en- vironments of a community. The maintenance of stability in natural forest stands by large numbers of replacement trees, compensating for high mortal- ity, is not an option for street tree populations. The limited numbers of satisfactory planting sites and the high costs of planting street trees require that replacement be achieved by a minimum number of trees, carefully selected, sited and maintained to assure a high success rate. Even with good management, the probability of some uncontrollable losses requires a re- placement rate that taxes most street tree management programs. Misdirected emphasis on species diversity in replacement plantings can add further stress by reducing replacement success over the long run.
REFERENCES
Carson, R., 1962. Silent Spring. Houghton Mifflin, Boston, 363 pp. Chapin, R. and Kozel, P., 1975. Shade tree evaluation studies at the Ohio Agricultural Re-
search Center. Res. Bull. 1074, Ohio Agricultural and Development Center, Wooster, OH, 46 pp.
Francis, H.R., 1915. Suggestions for proper procedure in systematic street tree planting for towns and villages of New York. Bull. No. 4, New York State College of Forestry, Syracuse, NY, 56 pp.
Richards, N., 1979. Modeling survival and consequent replacement needs in a street tree population. J. Arboric., 5 (11): 251-255.
171
Richards, N. and Giedraitis, J., 1980. Community tree management in New York, 1979. ESF Publ. 80-002, S.U.N.Y. College of Environmental Science and Forestry, Syracuse, NY, 42 pp.
Richards, N. and Stevens, J., 1979. Streetside space and street trees in Syracuse, 1978. Occasional publication of S.U.N.Y. College of Environmental Science and Forestry, Syracuse, NY, 73 pp.
Sanders, R., 1981. Diversity in the street trees of Syracuse, New York. Urban Ecol., 5: 3343.
Solotaroff, W., 1911. Shade-Trees in Towns and Cities. John Wiley, New York, 287 pp. Strobe& G.A. and Lanier, G.N., 1981. Dutch elm disease. Sci. Am., 245 (2): 5666. Whittaker, R., 1975. Communities and Ecosystems. 2nd Edition, Macmillan, New York,
385 pp.
