Singapore’s Garden City And Invasive Tree Species: How The Forests Are Coping

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Introduction

Trees received relatively little attention in the invasive plants literature until rec

ently [1]

They are

slower growing and have longer life

–

cycles than most shrubs and herbs, making potential tree invaders

harder to detect and study

, and th

ere tends to be a long lag time between introduction and invasion

[2,

. When exotic tree species do spread beyond their point of introduction, however, they have the

potential to be more damaging than smaller plants

[1]

. They create shade, which is a bar

rier to the re

–

establishment of many native species

[4, 5]

;

they can dominate nutrient cycling, potentially changing the

site in a more permanent way

[6, 7, 8]

;

and they can make the invaded habitat

less

suitable

for

arthropods

]

and native vertebrates

[

10]

Other

reported impacts of some tree invasions include

allelopathic suppression of competitors

[11, 12]

changes in the local fire r

egime

[13]

, and redu

ced

streamflow [14]

Invasive species are under

–

studied in tropical Asia compared to other parts o

f the world

[

]

, with plant

invasions

and impacts on natural ecosystems,

in particular

receiving little recent attention in the

literature

[16

]

A recent review estimated the total economic and environmental cost of invasive aliens

in Southeast Asia as U

S$33.5 billion a year, but nearly 90% of this was for the agricultural sector

[17

]

lack of

attention may

, in part, reflect a lack of concern, since intact tropical forests on the Asian

mainland and continental

–

shelf islands appear to be relatively res

istant to b

h plant and animal

invasions [16, 18

]

. However, Asian tropical forests are currently under extreme stress from clearance,

fragmentation, l

ging, and rapid urbanization [16

]

all factors that are expected to make them more

vulnerable to invasi

on.

e investigated

invasive trees in Singapore, a densely populated equatorial island nation in Southeast

Asia

, separated by a narrow strait

from the Asian mainland. In Europe, national wealth and human

population density are the strongest predictors of

biological invasions

[19

]

;

tropical

Asia

Singapore

ranks highest

in both

parameters

. Singapore

has suffered the same impacts as the rest of the region, but

up to a century earlier

, so it can act as an early warning system for the region as a whole

[20

]

Forest

conversion to monoculture crop plantations and timber extraction, which are now the major threats to

tropical forests worldwide, had already transformed Singapore’s lands

cape by the 1880s

. Today, only

0.3% of the original primary forest remains,

surrounded by a larger area of secondary forests of various

ages

[21

]

e Singapore government has

very active

import

exotic

plant materials, starting with mostly

economic plants in the nineteenth century,

and

dominated by ornamental species more re

cently,

particularly after the Garden City campa

ign was launched in 1967

[22

]

largely un

regulated

commercial trade also imports numerous alien plants, although relatively few trees.

Nowadays, 44% of

the total vascular flora in Singapore is exotic

[23

]

Most of these species grow only in cultivation, but

18% occur spontaneously in the wild

and

12% are considered fully

naturalized

therefore

asked the

following questions

Which exotic tree species are naturalized in Singapore and which of these are

inv

asive?

How well do the invasive species survive and grow in shaded and open sites?

How do the

invasive exotic

trees

differ from the native pioneer trees with which they coexist

and from the widely

planted exotic species that have failed to naturalize?

And

, finally, are invasive tree species an actual or

potential conservation problem in Singapore and SE Asia?

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ethods

Study area

The Republic of

Singapore

consists of one main island and several smaller ones, with a total land area of

718 km

. It

situa

ted at the end of the Malay Peninsula, from which it is separated by

shallow straits

less than 1 km wide at the narrowest point. Singapore is

7 km north of the equator

and

has a climate

of extreme equability, with negligible seasonality in temperature

a mean of >100 mm rainfall

every month.

The human population consists of 5.3 million people with a per capita gross national

product of US$62,000.

Survey

Possible invasive tree species were identified from the literature on naturali

d plant species

Singapore [23

]

and

from

the opinions of local botanists. All vegetation types capable of supporting

spontaneous tree populations were then surveyed in 2008

–

10, including open areas, primary and

secondary forests, urban habitats, and coastal vegetation.

Singapore’s small size, dense road network,

and large number of active ecologists make it unlikely that any significant invaders were overlooked.

Trees were defined as vascular plants exceeding 5 m height at maturity

and their exotic status was

determine

d from an up

–

date checklist

[23

]

Two species that are native to the region but whose

status in Singapore is uncertain,

Morinda citrifolia

(noni)

and

Pterocarpus indicus

(Burmese rosewood)

were excluded. Conversely,

Pipturus argenteus

(white mulberry),

which may be native in the region

around Singapore, was included because it is a well

–

documented recent arrival here.

Ricinus communis

(castor oil plant)

naturalized

in Singapore but, unlike in

some

drier parts of the region, does not reach

tree height

Following Pyšek

et al.

[24

]

, a species was considered naturalized if there was at least one self

–

replacing

population

and naturalized species were considered invasive if they had spread

far

from where they

had been introduced. In the absence of long

–

erm observations, an uneven

–

aged population structure

with some reproductive individuals was taken as evidence for

naturalization

and the presence of many

individuals at least 1 km from the point(s) of introduction as evidence for invasiveness. In practic

e, the

invasive species were easily distinguished, while there was a continuum between naturalized and casual

(i.e.

not self

–

replacing) species

, with the role of continuing propagule subsidies from cultivated plants

hard to assess

Further

studies

were th

erefore restricted to the

well

–

defined

invasive group.

Growth experiments

Growth rate appears to be the single best predictor of invasiveness in exotic trees

[25

]

. To assess the

invasive potential of the r

ecently

arrived (> c. 1960) invasive tree species

that

may not have

had time to

reach their

potential maximum extent

and

compare them with the well

–

established species, the

survival and growth of eight species (all those identified as invasive except the

relatively

uncommon

Pipturus

argenteus

) were i

nvestigated experimentally

at two sites selected to represent the expected

extremes of the invasibility spectrum i

n Singapore: a species

–

rich,

–

year old,

secondary forest in the

Central Catchment Nature Reserve

and an exposed area of

mown

grassland s

urrounded by alien

–

dominated woody vegetation on the campus of the National University of Singapore. Seedlings were

raised in a shade house from seeds or, where these were insufficient, wild

–

collected

small

seedlings

(

Acacia auriculiformis

[ear

–

leaf acacia

]

and

Falcataria moluccana

[Moluccan albizia]

)

stem cuttings

(

Manihot

carthaginensis

[Ceara rubber tree]

were

hardened

and then planted out when 20

–

40 cm tall.

Five seedlings of each species were sacrificed for measurement of initial biomass.

At each

site,

–

seedlings of each species were planted interdispersed

with one another, 75

–

100 cm apart.

In the forest,

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Rarotonga Island in the Cook Islands

[30

]

Another

Cecropia

species,

C. pelta

was introduced to the

Singapore

Botanic Gardens

in 1902

and there were spontaneous plants in the

vicinity by the 1980s

[31

;

RTC

personal observations]

, but

this species has shown

no sign of spreading further.

Table 2. Invasive tree species in Singa

pore with first official record, source(s) of seeds/plants

introduced, purpose of introduction, dispersal mode in Singapore, native distribution range, and

other SE Asian locations where the species is at least naturalized.

Species

Common

name

First

recor

Source

Purpose

Dispersal

mode

Native range

Naturalized

in SE Asia

Acacia auriculiformis

Benth.

ear

–

leaf

acacia

1890

Darwin,

Australia

ornamental

bird

N. Australia,

New Guinea

widely

Falcataria moluccana

(Miq.) Barneby &

J.W.Grimes

Moluccan

albizia

190

Bangalore,

India

afforestation

wind

E. Indonesia

to Solomon

Is.

widely

Cecropia

pachystachya

Trécul

trumpet

tree

1967

unknown

ornamental?

bird/bat

South

America

not reported

elsewhere

Leucaena

leucocephala

(Lam.)

de Wit

leucaena

1888

Florida,

USA

rnamental?

mechanical

Central and

South

America

widely

Manihot

carthaginensis

(Jacq.)

Müll.Arg.

Ceara

rubber tree

1879

many

sources

rubber

mechanical

South

America

locally

Muntingia calabura

Jamaican

cherry tree

1895

unknown

fruits

bird/bat

Central

and

South

America

widely

Piper aduncum

spiked

pepper

2003

Malaysia?

spontaneous

bird/bat

Central and

South

America

widely

Pipturus argenteus

(G.

Forst.) Wedd.

white

mulberry

1979

unknown

spontaneous

bird

Australia to

SE Asia.

not reported

elsewhere

Spathodea

campanulata

P.Beauv.

African

tulip tree

1909

Bangalore,

Penang

ornamental

wind

West Africa

widely

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Seven

additional

ree species were classified

somewhere on the casual

–

naturalized

continuum,

but not

invasive

in Singapore

. These spec

ies vary greatly in

their current

abundance

and distribution

Alstonia

macrophylla

(deviltree)

widely

naturalized in semi

–

shaded sites on Sentosa

Island, just south of

Singapore

, but the species has been in Singapore since 1879 and does not appear to be

spreading much

beyond the area where it was planted.

Hevea brasiliensis

(Para rubber tree)

is the most

abundant and

widespread of the additional species

, but although the shade

–

tolerant saplings dominate the

understorey of some areas of secondary forest,

they are rarely found beyond the crown

mature

trees

and

most

or all

of these

pear to be remnants of the very

extensive commercial plantings in the

first half of the twentieth century. Similarly,

Andira inermis

is widespread as spontaneous, relatively

shade

–

tolerant

saplings, but all or almost all these are within bat

–

dispersal distance

(< 100 m

for large

seeds

[32

]

)

of mature planted trees.

Ptychosperma

macarthurii

(Macarthur palm)

is better established,

but the s

pontaneous population is still

subsidi

zed by seeds dispersed from extensive ornamental

plantings

and further study is need

to assess its current status

[33

]

Elaeis guineensis

(oil palm)

has

een widely planted in the past and

apparently spontaneous individuals

are widespread in the south of

Singapore

. Seeds

are dispersed by crows (

Corvus

spp.)

, which carry ripe fruits to nearby trees for

processing (RTC, personal observations)

, but there is no

strong

evidence for a self

–

replacing wild

population. Finally,

Ficus religiosa

(bo tree)

is abundan

t as seedlings and saplings

on artificial structures,

but spontaneous adults are rare and

the seed rain is dominated by

planted individuals.

Eleven additional species are classified as naturalized trees in the 2009 Singapore checklist

[23

], but no

eviden

ce for a persistent, self

–

replacing population was found for any of these during this study.

One of

these,

Flacourtia jangomas

(coffee plum)

is also listed in the latest global checklist of invas

ive alien trees

and shrubs [

] on the basis of a record from

Singapore, but the only reported self

–

replacing

population

, on the island

of Pulau Ubin

and Pulau Tekong, seem

to be in decline.

An earlier list of

naturalized species [

] also included

Anacardium occidentale

(cashew tree),

which was a rare casual

spec

ies in our survey,

and

Morinda citrifolia

which

is largely coastal and may be

native.

Table 3. Means and standard deviations of growth rates of the invasive tree species

tested at the open site in Singapore.

Species

Common name

Height growth

Relative g

rowth rate

(cm.week

–

)

(mg.g

–

day

–

)

Acacia auriculiformis

ear

–

leaf acacia

4.8 (0.9)

27.1 (3.9)

Cecropia pachystachya

trumpet tree

2.3 (1.2)

13.0 (3.4)

Falcataria moluccana

Moluccan albizia

4.1 (1.8)

26.7 (5.7)

Leucaena leucocephala

leucaena

8.6 (2.

34.5 (4.4)

Manihot carthaginensis

Ceara rubber tree

2.8 (0.8)

23.2 (3.0)

Muntingia calabura

Jamaican cherry tree

4.2 (2.0)

17.3 (1.3)

Piper aduncum

spiked pepper

1.6 (0.8)

7.6 (3.2)

Spathodea campanulata

African tulip tree

2.0 (1.4)

5.6 (5.3)

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Growth experiments

In the forest

experimental

site, mortality was high (75

–

100%) and height growth of the survivors low (

–

0.4 cm wk

–

) in all species.

Manihot

(all dead by week 9),

Muntingia

(all by week 10), and

Falcataria

(all

by week 17) performed

worst and

Spathodea

best (25% survival at week 54, but

with

only 8 cm mean

height growth). The other species with survivors at week 54 were

Leucaena

(3),

Cecropia

(2), and

Piper

(2).

Acacia

was planted late and had to be harvested at week 17, when one unh

ealthy seedling survived.

In the open site, mortality was low (0

–

25%) and height growth rapid (

1.6

–

8.6 cm wk

–

)

(Table 3)

. Relative

biomass

growth rates ranged from 7.6 mg g

–

day

–

Piper

to 34.5 mg g

–

day

–

Leucaena

and were

highly correlated

among

pecies

ith height growth (

Spearman’s

= 0.87, P < 0.001).

Flowering

occurred in

Manihot

(105 days after planting),

Leucaena

(114 days),

Muntingia

(140 days), and

Piper

(225 days) before the end of the experiment.

Trait comparisons

The only trait differ

ence between native and invasive pioneers that was significantly different at the

0.05 level w

as the dispersal syndrome (

Chi

–

square test;

p = 0.049)

(Table 4

)

: none of the native

pioneers

are wind dispersed

while two invasive species (

Falcataria

and

Spath

odea

) a

re. Invasive species also

tended to be shorter (

Mann

–

Whitney U test;

p = 0.067) and to have less dense wood (

p =

0.082) than

coexisting natives.

In contrast, there were highly significant differences between invasive and non

–

invasive

exotics in wood

density (

Mann

–

Whitney U test;

p < 0.0001) and seed size (p = 0.007), with

invasive species having only 54% o

f the mean wood density and 19

% of the dry seed mass of non

–

invasive species.

However, there is considerable overlap between the two groups and the

two

characters are not sufficient to identify p

otential invasives

Flower sexual system also differed between

the two groups

(Chi

–

square test;

p = 0.039

)

, but in the opposite direction to

that which was expected: all

but one of the non

–

invasive species

have hermaphrodite flowers while two invasive species are

dioecious and one monoecious. Invasive

exotic

species were significantly f

rther from their native

ranges than non

–

invasive species (

Mann

–

Whitney U test;

p = 0.015).

Table 4. Means and standard de

viations of quantitative traits and percentages of qualitative traits for native

pioneer trees, invasive alien trees, and non

–

invasive alien trees. The bottom two rows give p

–

values for pair

–

wise Mann

–

Whitney tests for quantitative traits and chi

–

square te

sts for qualitative tests; p < 0.05 in bold.

Height

(m)

Wood

density

(kg m

–

)

Seed mass

(mg)

Distance to

natural range

(km)

Residence

time (yr)

Sexual system

Dispersal

mode

Native

18.5

(6.8)

501.2

(197.7)

400.6

(1186.5)

47% herm.

42% dioec.

11% mo

noec.

90% flying

0% wind

10% other

Invasive

13.9

(11.8)

383.3

(125.0)

82.5

(211.6)

10,800

(7,100)

85.6

(46.4)

67% herm.

22% dioec.

11% monoec.

56% flying

22% wind

22% other

Non

–

invasive

19.1

(8.6)

709.5

(178.9)

426.9

(409.1)

4,300

(6,000)

97.5

(35.

96% herm.

4% monoec.

28% flying

50% wind

22% other

Native

vs.

invasive

0.067

0.082

0.290

0.576

0.049

Invasive

non

–

invasive

0.092

< 0.001

0.014

0.030

0.450

0.039

0.236

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iscussion

Singapore’s small si

may tend

inflate the fraction

species

treated as exotic

relative to larger,

continental areas

[36

]

, but

even allowing for this,

some sites in Singapore have more coexisting invasive

tree species than anywhere else we have seen

in the humid tropics

between northern Australia and

sout

hern China

(personal observations)

. However

, the

total

number of invasive tree species

represents

only 1.1

of the exotic tree species

recorded in

Singapore

[23

]

suggest

ing

that the risk of

a particular

exotic tree

species becoming invasive is

low

, i

agr

eement with other studies [37

]

Strikingly, none of

the five most widely

grown

exotic tree species

in Singapore

(

Albizia

saman

Swietenia macrophylla

Khaya senegalensis

Tabebuia rosea

, and

Xanthostemon ch

ysanthus

), each with >

10,000 individuals

planted

[26

]

, is

invasive,

although casual seedlings of

saman

K. senegalensis

, and

T. rosea

are fairly

common

[38

]

. This shows

that propagule pressure alone is not sufficient

to drive invasion

Conversely,

the widely planted forestry tree

Acacia mangium

, whi

ch is naturalizing throughout the humid tropics of

SE Asia

on poor soils

[39

is only casual in Singapore, where it has rarely been planted. Other

naturalization failures are likely to reflect Singapore’s extremely equable equatorial climate, which lacks

the seasonal cues on which tropical plant reproduction often depends. Possible candidates

for this

barrier to

naturalization include

the

globally widespread tropical invasive trees

Castilla

elastica

Miconia

calvescens

Psidium cattleianum

, and

Syzygium j

ambos

which have all

been cultivated in Singapore, but

only

C. elastica

and

S. jambos

produce

the occasional self

–

sown sapling.

he nine invasive tree species in Singapore differ considerably in their biological characteristics and also

in their pathway

s to invasion success, suggesting that predicting which tree species will become invasive

will be difficult.

Coutts

et al.

[40

]

show that the rate of spread of an invasive species is expected, in

general, to be driven primarily by dispersal ability, with m

ean dispersal distances

the best single

predictor.

However, the small size of Singapore in relation to normal seed dispersal distances, and the

wide initial planting of some species, may make this factor less important.

carthaginensis

depends on

ballist

ic seed dispersal (with possibly secondary dispersal by ants) and

moluccana

and

campanulata

have relatively large, wind

–

dispersed seeds, but all three were widely planted in the past, so their

distributions within Singapore do not appear to be disper

sal

–

limited today.

leucocephala

also

has

limited natural dispersal

by mechanical means,

but its persistent seed bank

[41

]

probably ensures

dispersal with soil

and it is common around construction sites. All the other invasive species have small

seeds i

n fleshy fruits that are readily swallowed by ubiquitous open

–

country birds (

yellow

–

vented bulbul,

Pycnonotus goiavier

mynas,

Acridotheres

spp., and others) and, in some species, by fruit bats

(

Cynopterus

brachyotis

). This should permit dispersal distance

1 km per generation

[32

]

, which is

consistent with the rapid spread of the newest invaders, none of which were widely planted. On the

other hand,

Cecropia

Muntingia

Piper

, and

Pipturus

all have considerably smaller seeds than most

native pioneers

which

may limit their ability to invade natural gaps in native forests, as suggested for

Cecropia peltata

in Malaysia [31

]

Implications for conservation

Both the survey and the planting experiment suggest that none of the

currently

invasive tree species

Singapore

a threat to the integrity of

native

closed

–

canopy forest.

Spathodea

appears to have the

most shade

–

tolerant saplings and is present in the margins of the Nature Reserves, but these incursions

appear to be a reflection of past disturbance an

d there is no sign of active invasion into closed forest.

However, two of the species considered naturalized but not (yet) invasive,

Hevea

brasiliensis

and

Andira

inermis

, potentially are

a threat

because of the

shade

–

tolerant saplings.

Hevea

is likely t

o be

permanently constrained by poor seed dispersal, but

A. inermis

is dispersed well by

fruit bats and is of

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greater concern. Removal of all adult trees of both species from the nature reserves and their

immediately vicinity would be a sensible precaution

. The populations of

the

other species

listed above

as potentially invasive

should be monitored

. The trait

comparisons suggest that

the currently

invasive

exotic trees are distinguished by smaller seeds and less dense wood―presumably proxies for the

pioneer traits of good dispersal and rapid height growth.

However, the opposite traits―large seeds and

denser wood―

are likely to be associated with pote

ial for invading closed

–

canopy

forests

[42, 43

]

and

these species

may be

of greater concern in the long run

[44

Fortunately, any invasion by such species is

likely to be slow and should be detectable by surveying the

fringes of the reserves at

regular

intervals for

shade

–

tolerant saplings of additional exotic species.

A key strategy for reducing the risk from alien plant invasions is to prevent their introduction by a formal

risk assessment of species before they are imported [

]. The most widely use

d risk assessment for

plants is the Australian Weed Risk Assessm

ent System (A

–

WRA)

. A

test of the transferability of A

–

WRA

scores

from

other tropical localities showed that these can be used directly in Singapore, reducing the

number of species that would

require new assessments [

]. However, Singapore is a tiny, trade

–

dependent economy, protective of

its number one global ranking

for both ‘ease of doing business’ and

‘trading across borders’ (World Bank Group;

www.doingbusiness.org/rankings

)

. The introduction of new

border controls

would

not be

seen as

compatible with this.

A more practical alternative may be to assess

alien

species

after

they have become established in Singapore, either in cultivation

or in the wild.

This

has the additional advantage of including

the many

accidental introductions, which would not be

prevented by pre

–

border screening. A focus on species that can potentially invade the closed

–

canopy

native forests in Singapore’s Nature

Reserves would further reduce the burden

of enforcement

, and

eliminating high

–

risk species from a

–

km width

quarantine

zone around the Reserves would

be a

practical way to

minimize invasion risk.

Moreover, most tree planting in Singapore is by government

agencies and a small number of private developers, so a voluntary policy might work.

In contrast

to the situation in native, closed

–

canopy

forests,

nvasive exotics dominate all stages of plant

succession on

most

land that has been abandoned in recent de

cades

. W

hen protected from further

disturbance these sites eventually develop an invasive

–

dominated closed

–

canopy forest. This contrasts

strikingly with the native secondary forest that developed on land abandoned from the late nineteenth

cen

tury through t

o around the middle of the

twentieth century [29

]

This pattern resemb

les that

reported for

New York, where exotic trees dominate most ‘regenerated patches’, on sites that had been

cleared for urban use and then abandoned, but native trees continue to domi

nate in areas that were

forested when incorporat

ed into the city [47

]

Three

factors may account for these

two

very different

successional

trajectories

in Singapore

, but the evidence is insufficient at present to

assess their relative

importance

. One

possi

bility is that it

reflects

differences in

soil conditions: earlier sites were

simply

abandoned after farming to exhaustion, while more recent sites were generally bulldozed clear of a

mixture of small settlements

, tree crops,

and vegetable farms.

An altern

ative or additional explanation is

that the earlier sites were all within dispersal distance of native forests

while more recent sites have

typically been isolated from native forest

and

exposed to a rain of invasive tree seeds.

A third possibility

is tha

t invasive species are superior to native pioneers on sites where propagules of both are available.

This could occur if the invasive trees had advantageous traits that are not present in the native flora:

two

possibilities suggested by this study are

nitro

gen fixation (three invasives, but no native pioneers)

and wind

–

ispersed seeds (two invasives, but very

few native pioneers, none common).

Moreover,

elease from natural enemies could create an alien advantage even in the absence of di

fferences in

other t

raits

[16

]

. T

his was not

directly assessed in this study, but the observation that invasive exotic

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–

Tropical Cons

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–

, 2015

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–

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211

species are significantly f

rther from their native ranges than non

–

invasive

exotic

species is consistent

with enemy release being a factor

and this needs f

urther study.

key question for the future of Singapore’s vegetation is: what happens next

to these alien

–

dominated

forests?

Will

the invasives gradually displace the

remaining

native species?

Will

the natives gradually

replace the invasives? Or,

will

ingapore end up with ‘novel forests’ in which native and invasive species

coexist?

arallel situations can be found on oceanic islands, such as

Puerto Rico

[48

]

Hawaii

[4]

, and the

Ogasawara (Bonin) Islands

[49

]

but

Singapore has a hyperdiverse continent

al rainforest flora, with

860

native

tree species, including a diverse pioneer flora.

In this situation,

the high diversity of species and

functional groups

would be expected to

capture most available resources and thus minimize the

potential for plant inv

asions

[16, 18

Yet this is clearly not the case.

If Singapore is vulnerable to invasive

trees

preempting forest recovery

then so, potentially,

is everywhere else in the increasingly human

–

dominated continental tropics.

The

same invasive tree species, wi

several

hers, dominate increasing

areas

on the urban fringes

of Asian cities and scattered patches in rural areas (personal observations)

Scale is important, however, and dispersal limitation greatly reduces the immediate threat to

most

non

–

urban are

as, except from

widely grown exotic plantation trees, including several species of

Acacia

Finally,