The effect of Pittosporum undulatum on the native vegetation of the Blue Mountains of Jamaica



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The effect of Pittosporum undulatum on the native

vegetation of the Blue Mountains of Jamaica

May 1997


T. Goodland and J.R. Healey, School of Agricultural and Forest

Sciences, University of Wales, Bangor, LL57 2UW, U.K. Contact

email t.goodland@bangor.ac.uk

Research supported by the U.K. Overseas Development

Administration and the U.K. Department of the Environment Darwin

Initiative programme


1.

Introduction

1.1

Background

The Australian tree Pittosporum undulatum Vent. was introduced to the Blue Mountains of Jamaica in 1883.

Sixty-six years later this bird-dispersed tree had become “perhaps the commonest tree” in secondary forest

around the Cinchona Botanic Gardens, its place of introduction (Bengry & Serrant 1949).  Previous research

projects have identified its competitive success against native tree species (Healey 1990) and the low density

and species richness of native vegetation beneath dense stands of the alien (Goodland 1990).  The density of



P. undulatum seedlings in areas of previously uninvaded forest greatly increased following the disturbance

created by Hurricane Gilbert in 1988 (Bellingham 1993), and the species has now spread throughout at least

1,300 ha of primary and secondary montane forest (Healey & Goodland 1995).  We have estimated that the

area of the potential range of the species in the Blue Mountains could be as high as 44,000 hectares (Goodland

& Healey 1996), seriously threatening the high biodiversity of the mountain range.  There are about 275 species

of flowering plants in the Blue and John Crow Mountains National Park endemic to Jamaica (Grubb & Tanner

1976, Bellingham 1993, Muchoney et al. 1994).

These facts have led to concern amongst many scientists that the spread of P. undulatum in the Blue Mountains

may lead to the competitive exclusion of many native plant species.  The former Park Manager considered the

need for reliable information about the impact of the invasion to be one of the highest priorities for the national

park (D. Lee, pers. comm., 1991).  This is because past work had not provided sufficient evidence of the

severity of the threat to convince donor agencies of the need to provide necessary funds for a control

programme, or to form the basis for a management plan when funding is available.

This report, therefore, investigates the effects of P. undulatum on the native plant biodiversity of forests of the

Blue Mountains.  Given sufficient time (and without man’s intervention) it seems probable that all the montane

forests of the Blue Mountains would become invaded by P. undulatum.  The lower altitudinal limit of P.



undulatum is poorly known (it is probably between 600-1000 m), but most primary, and therefore diverse, forest

below 1000 m has been cleared in the last two centuries or so.  The report deals only briefly with “time-

dependent” issues, (such as the current extent of P. undulatum, the rate of spread, population changes in

permanent sample plots, possible limiting factors to its range), or the ability of P. undulatum to grow outside the

forest (on deforested slopes or landslides), subjects dealt with in Goodland & Healey (1996).  It concentrates on

the immediate and long-term effects of the introduced species once it has already arrived at a site, a small area

of forest such as a permanent sample plot.

Possible effects on other aspects of the ecology of the Blue Mountains (non-vascular plants, animals, the

nutrient and hydrological cycles), and on humans, were considered in Goodland & Healey (1996).

The factors that determine the effect of P. undulatum on native plants in the Blue Mountains can be broken

down into the rate of dispersal, the ability of the species to capture land and resources once it arrives at a site

(its competitve ability) and its persistence (whether it is eventually replaced by other species at the site).

Competitive 

success 


The impact of Pittosporum undulatum

Rate of 


dispersal

Persistence of 

the population


Figure 1.  Factors that determine the success and impact of an invasive plant such as P. undulatum

The main factors that control the rate of dispersal of P. undulatum are shown in the figure below.  In summary:

• 

P. undulatum was introduced to the Cinchona Botanic Gardens in 1883

• 

the species may have been planted by man outside the gardens



• 

in the few decades after introduction most of the land around Cinchona which had been under coffee and



Cinchona plantations was gradually abandoned and reverted to secondary forest

• 

P. undulatum juveniles start to produce seed when about 5-6 years old

• 

seed production is high, a regression relationship between DBH and seed production in 1992 giving a mean of



37,500 seeds for a 8 cm DBH tree

• 

seed production has been at least fairly high for every year from 1992 to 1996



• 

there are at least six common native bird species that eat and presumably disperse P. undulatum seeds

• 

vegetative spread, through mechanisms such as suckering or layering, are not important to the rate of invasion



• 

the species is able to establish itself in all habitat types in the western Blue Mountains, though with difficulty in very

undisturbed forest, in Mor Ridge forest or on landslides

• 

Hurricane Gilbert in particular and probably hurricanes in general have played a major role in facilitating the



establishment of the species in otherwise undisturbed forest

Rate of dispersal

Dispersal

 success

Frequency

Distance

Ability to establish 

in diverse 

environments

Age to 

maturity


History

Chance


Hurricanes

Initial dist- 

ribution

Number of seeds

Attractiveness to 

birds


Land use

Figure 2.  Factors determining the rate of spread of P. undulatum in the Blue Mountains

This report will focus on the competitive suppression by P. undulatum of native plant species, as the

competition for light and below-ground resources are the most obvious mechanisms by which P. undulatum may

affect native species.  P. undulatum trees have a dense crown, so shade probably accounts for a large part of the

suppressive effect of the species (though we cannot determine to what extent the dense canopy has its effect

because of a reduction in light or a probable reduction in throughfall).  We have some evidence that the below-

ground competitive ability of P. undulatum is high in comparison with the native species, though we have no


experimental evidence on the relative importance of above- and below-ground competition.  We give our best

assessment of the persistence of P. undulatum in the Discussion chapter at the end.

There are several other possible mechanisms by which P. undulatum may affect native plant species, briefly

discussed below, though little is known about many of these.

1. Allelopathy.  Allelopathy has been suggested as a factor depressing the number of native seedlings

beneath scattered P. undulatum trees in the Blue Mountains where the light levels would have indicated a

higher seedling density (J. Dalling, pers. comm., 1991).  In Australia, Gleadow and Ashton (1981) found that

leachates from P. undulatum leaves appeared to inhibit the germination (expressed as a percentage of

control) of several Eucalyptus species; for example, germination of E. obliqua, E. melliodora and E.

gonocalyx was 47.1, 8.1 and 48.3% of untreated seeds.  However, they stated that no inhibitory effects,

other than that expected from deep shade, have been shown under canopies in the field.  In South Africa,

Richardson & Brink (1985) found no seedlings of P. undulatum or native species beneath established P.

undulatum trees and thought that this was due to an allelopathic effect from P. undulatum foliage.

 

2. P. undulatum trees as a habitat.  The greatest effect that P. undulatum may have as a different habitat to



native trees is on animals, but the structure of its crown or nature of its bark may have an effect on epiphytic

plants, independent from the density of its foliage.  A large proportion of the non-woody plant species in the

Blue Mountains are epiphytic (P.J. Bellingham, pers. comm., 1994), but the effect of P. undulatum on

epiphytes was not specifically addressed in the study, mainly because of the great difficulty of seeing

through dense canopies of P. undulatum trees.  Our observations and data collected by Mitchell (1989)

suggest that the numbers of epiphytes are much reduced both in the crowns of P. undulatum trees in

comparison with native trees of similar size and on native species beneath dense P. undulatum canopies.

This could be due to many factors such as reduced light levels and rain throughfall, the upright growth habit,

different branch arrangement and bark characteristics of P. undulatum trees, allelopathic leachates from P.

undulatum foliage and the faster growth rate of P. undulatum trees (hence less time for establishment).

 

3. Effects on animals.  The most obvious example of the effect of P. undulatum on a native plant caused



indirectly by the effect on an animal is the effect the alien may have on pollinators and seed dispersers of

native plants.  If P. undulatum is relatively successful in attracting pollinators and dispersers, and if those

tree species that are less attractive are neglected as a consequence, these native species could find their

regeneration threatened.  These indirect effects could be very important, but they are hard to determine.

Conversely, as native trees become isolated in heavily invaded forest, their predation by native, co-evolved

pests and pathogens is likely to decline as they become harder to find.

 

4. Effects on susceptibility of native species to windthrow.  Another possible mechanism by which P.



undulatum could affect native trees is by changing their allometry through greater competition, thus changing

their vulnerability to windthrow.  Studies of the impact of Hurricane Gilbert show that hurricanes play a major

role in the forest dynamics of the Blue Mountains.  Also, P. undulatum trees do not get covered with lianes

as frequently as native species (though this could be because most climber species seem to be restricted to

primary and therefore less invaded forest), so would not pull down other trees when blown down, (though we

have no evidence as to the importance of this phenomenon in the Blue Mountains).

 

5. Change in disturbance regime.  As it is likely that P. undulatum trees are blown over at a smaller size than



the average for native species (Healey & Goodland 1995), the advanced stage of invasion would see, after

hurricanes, a high proportion of the area in gaps.  Because of the sparse understorey beneath dense P.



undulatum (few P. undulatum seedlings as well as few native seedlings) a few highly gap demanding native

species, such as Bocconia frutescens and Brunellia comocladiifolia, (as well as alien weeds like P. undulatum

and Polygonum chinense) may benefit.

1.2


Report structure

The report has two main chapters.  The first examines the direct evidence for the effects of P. undulatum.  The

only way to do this is examine differences in the performance of native species with varying amounts of P.

undulatum.  We have tried three approaches:

1. Correlation between the dominance of P. undulatum and native vegetation in many plots at one point in

time.

2. Correlation between the change in dominance of P. undulatum and native vegetation through time, in a



smaller number of plots.

3. Experimental removal of P. undulatum.

We have not tried the fourth possibility, the experimental addition of P. undulatum, because of time, and ethical,

constraints.

The second chapter considers the reasons for any competitive effects, though our understanding of causal

mechanisms are not well advanced, and are partly conjecture.  The project has not had the explicit objective of

discovering the causes for any effects P. undulatum may be having.  Despite this, data and observations

collected from the Blue Mountains, together with information from other invasions, have been enough to

provide strong indications on the underlying mechanisms.

We examine likely reasons for the competitive success and hence supression by P. undulatum of native

species in three categories, in each case comparing P. undulatum with native species.  These three categories

are not true causes, in the sense that they themselves are the result of more underlying physiological or

ecological mechanisms.  We have discovered many facts about the biology of P. undulatum necessary to an

understanding of its success without (mainly through time constraints) being able to find out similar information

for native species - aspects such as age to reproductive maturity, seed production per individual.  Full

information on P. undulatum is given in Goodland & Healey (1996).  In this report we focus on those important

aspects of the invasion for which we have information on native species as well as P. undulatum.  This list of

questions and subsequent analyses are not intended to be comprehensive, but only to address the more

important factors.

1. Growth rate of individuals.  How fast do P. undulatum individuals grow?

2. Growth form of individuals.  What is the growth form of P. undulatum individuals?  How large do P.

undulatum trees get?

3. Population density.  What is the survival rate of P. undulatum juveniles? How dense can P. undulatum

populations become?

The results from the two main chapters are discussed in the last chapter.

A full list of all woody species occurring in the Blue Mountains permanent sample plots, together with the 6-

letter codes used in some figures, is given in an appendix.



2.

Direct evidence for the effect of 

Pittosporum undulatum

The most important direct evidence for the effects of P. undulatum on native plants come from two removal

experiments established in forest near Cinchona (the place of introduction).  More data and analyses are

presently available from the first of these, Heavily Invaded Forest Experiment - its methods are described in

detail in Healey et al. (1992), only briefly here.  The second removal experiment, the Slightly Invaded Forest

Experiment, is described fully, although the experiment is still at an early stage.



2.1

Methods

2.1.1

Heavily Invaded Forest Experiment

Pittosporum undulatum usually occurs on steep hillsides with thin and rocky soils and is typically associated with

a sparse and depauperate understorey.  To understand the effect P. undulatum is having on the native

vegetation it is necessary to experimentally remove it.  It is not sufficient to rely on simple correlations between

P. undulatum dominance and the native vegetation alone, as P. undulatum might be largely restricted to

unstable and often human disturbed forests, which may naturally have an understorey of low diversity.  A

removal experiment was established in forests with a range of degrees of invasion in the western end of the

Blue Mountains during the latter half of 1991.



Specific questions

The questions concerned with the effect of P. undulatum that HIFE was designed to answer are:

1. How close is the apparent correlation between P. undulatum dominance and the recruitment, survival and

growth of native vegetation?

2. Will there be a decline in the dominance of native species between the first and subsequent enumerations in

the Undisturbed control treatment?

3. After P. undulatum removal, to what extent will the diversity and density of native species in the understorey

increase?

4. What is the relative effect of P. undulatum trees and P. undulatum seedlings on native vegetation?

Methods

A randomised block design was used, with five blocks, five plots within each block and four treatments.  Each

block contained two replicates of the undisturbed control and one of the other three treatments.  The design was

partially orthogonal.  Each plot was 12

×

12 metres, surrounded by a 9 m guard area (giving a 30



×

30 m treatment

area).  Twenty 1

×

1 m sub-plots were randomly selected within each plot.  Woody plants over 3 m tall were



enumerated within the 12

×

12 m plot, those less than 3m, in the twenty 1 m



2

 sub-plots.  Each individual was

identified to species level wherever possible and labelled with an aluminium identification tag.  There were four

treatments:

1. Undisturbed Control (UC).  No treatment.

2. Remove P. undulatum Trees (RPT).  P. undulatum trees (plants >3m) were cut, the stumps were not treated

with a herbicide, but the resprouts were removed three times after cutting in an attempt to kill them.

3. Remove all P. undulatum (RAP).  All P. undulatum seedlings over 50 cm tall were killed (pulled up whenever

possible) in the 30

×

30 m treatment area and all other P. undulatum seedlings were killed within the central



14

×

14 m area (ie. at least 2 m from the nearest sub-plots).  All P. undulatum recruits in all the sub-plots have



been removed on three occasions since the original treatment.

4. Remove Equivalent Native Trees (RENT).  In this treatment native trees were removed until the same total

GBH was removed as that of P. undulatum in the RPT treatment in that block.


Table 1.  Enumeration activity, size class, treatment dates and months from the pre-treatment enumeration, in

the Heavily Invaded Forest Experiment.

Activity

Size class

Date

Months

Pre-treatment enumeration (t0) 

Trees and seedlings

July-August 1991

0

Imposition of treatments



Sept.-Oct. 1991

2

First post-treatment enumeration (t1)



Seedlings

August 1992

12

Second post-treatment enumeration (t2)



Trees and seedlings

December 1993

28

Partial enumeration



Dead seedlings only

June 1995

46

Full enumeration of trees



Trees

July 1996

59

2.1.2

Slightly Invaded Forest Experiment

The Heavily Invaded Forest Experiment is providing valuable information on the effects of P. undulatum on

native vegetation.  However, HIFE has a major limitation in its ability to provide proof of the effects of P.

undulatum in that, by necessity, it was carried out in highly disturbed secondary forest in which P. undulatum is

abundant, (P. undulatum probably regenerates after near total clearance, cultivation and then abandonment) -

the prime objective of the experiment had been to provide information on the management of P. undulatum in

heavily invaded forest.  Another characteristic of this secondary forest on the southern slopes of the mountain

range is that it has a lower species diversity than primary and old secondary forest (T. Goodland, unpublished

data).  Therefore the capacity of native species to respond to the removal of P. undulatum is, as expected,

limited.  The primary factor is likely to be a lack of propagules, as there are no nearby potential seed parent

trees of many species that would be expected to occur in primary forest on such sites.  In addition, there may be

characteristics of the soil in such secondary forest that limit the capacity of native species to colonize (Dalling

(1992) studied the extreme case of landslides where all top soil had been lost from much of their surface).

However, we have no evidence that the soil conditions in the HIFE plots do limit the establishment of native

species that would occur in primary forest in these sites.

The only way to objectively determine the effect that P. undulatum has is to follow the whole invasion process in

permanent plots, paired with plots in comparable forest from which P. undulatum is removed as a seedling or

small tree.  Therefore during June to September 1994 we established a second removal experiment.  The

experiment had the specific objective of investigating the effect of P. undulatum on native plants and so was set

up in diverse primary forest only slightly invaded by P. undulatum.  It is called the Slightly Invaded Forest

Experiment (SIFE).



Objectives

The long-term objectives of SIFE are to investigate the following:



Population dynamics of the invasion

• 

Performance (recruitment, growth and survival) of P. undulatum and native species in gaps and understorey



• 

Relationship between P. undulatum seedling performance and distance to the nearest mature P. undulatum

tree (for which an assessment of the fecundity, and position relative to the plot, of all P. undulatum trees in

the treatment area or vicinity will be made)

• 

Micro-site distribution of P. undulatum and common native seedlings; to what extent is the distribution



clumped, because of bird dispersal patterns or preferential establishment on certain substrates?

 

 Effect of P. undulatum on the native community



• 

Performance of native tree seedlings, either by plot level correlations or by proximity to individual P.



undulatum saplings and trees

• 

Growth form (eg. degree of branchiness, leaf area) of those native species able to grow in both treatments

• 

Distribution of herbs, climbers, epiphytes, and non-vascular plants



 

 Effect of P. undulatum on community level productivity

• 

Community productivity, in terms of basal area increment, biomass increment (perhaps by means of limited



destructive harvesting in the treatment area or surrounding forest), and litterfall (quantity and quality)

• 

Soil chemistry and nutrient status



• 

Effect of P. undulatum on forest microclimate and hydrology, e.g. throughfall, stemflow, soil moisture

• 

Light regimes, as characterised by hemispherical photographs, and/or PAR quantum sensors



 

 

 



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