T. Goodland and J.R. Healey, School of Agricultural and Forest Sciences, University of Wales, Bangor, LL57 2UW, U.K.
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The Australian tree Pittosporum undulatum Vent. was introduced to the Blue Mountains of Jamaica in 1883. This bird dispersed tree has so far spread throughout at least 1000 ha of primary and 300 ha of secondary montane forest and has accelerated its rate of invasion following Hurricane Gilbert in 1988. We estimate that the potential range of the species in the Blue Mountains could be as high as 40,000 hectares, very seriously threatening the high biodiversity of the range. This report gives our latest information and recommendations on the control of P. undulatum in the Blue Mountains. It is in five parts.
Assessing the distribution of Pittosporum undulatum
Present extent and density
Our estimate is that P. undulatum occurs in 330 four hectare cells, ie. in a total of 1320 hectares, see Figure 1. Note that the class of a given cell does not necessarily equate with density in that cell, as some cells on the forest boundary are only part forest so may be in a low class even if very heavily invaded. Of the 330, we estimate that 108 are predominantly in secondary forest, some of it old and not necessarily heavily invaded. The positive correlation between past forest disturbance, mostly around Cinchona and the Cinchona plantations in the Sir Johns Peak-Bellevue Peak area, and the density of P. undulatum is very striking. Concentrations also occur far down the north slopes, in steep landslide-prone areas. However, these calculations should be taken as an approximation only, bearing mind that isolated P. undulatum seedlings may occur several hundreds of metres beyond the boundary. And whilst P. undulatum seedlings are generally common within the boundary, some areas (of up to several hectares) of mostly undisturbed forest will be free of P. undulatum seedlings. It is quite possible that there are isolated trees or clumps of trees well outside the area, on the north slopes, in addition to P. undulatum spreading from the Whitfield Hall and the Hardwar Gap P. undulatum invasion.
|To put this area
in perspective, we have estimated the total area of forest in the Blue
Mountains that we think is invadable by P. undulatum. For this estimation
we assume that all forest above 600 m, the lowest height that P. undulatum
known to occur at in Jamaica (Adams 1972), is invadable. The forest area
used is that shown on the 1984 edition of Map 13 of Jamaica's 1:50,000
series, based on aerial photographs taken in 1979-80. It is still reasonably
accurate as to the extent of forest cover, most deforestation since then
having being small scale or below 600 m. The area does not include the
adjoining limestone massif of the John Crow Mountains. The planar area
is thus estimated to be 28,140 ha of forest. An alternative method is to
use the areas of different forest types given in Muchoney et al.
(1994) and assume that all non-limestone montane primary and modified forest
and scrub is invadable - a total of about 54,000 hectares. We estimate
that a figure about halfway between these, approximately 40,000 hectares,
is invadable, which would mean that about 3.3% of P. undulatum's
potential range has already been "invaded".
Assessing the distribution more accurately1.2.1
Visibility of P. undulatum
Finding P. undulatum during clearance operations
Manual methods of killing Pittosporum undulatum plants
The desirable qualities of
manual control are that it should be cost-effective, with minimal impact
on the environment and workers, therefore we have explored every possibility
of killing the species by physical means alone, as well as by using chemical
methods. We have tried the following combinations of physical and chemical
Choice of herbicide
Woody plants are commonly treated by cutting then spraying the regrowth a few months later, thereby getting larger quantities of translocatable herbicides such as glyphosate into the plant, as with Rhododendron ponticum in the UK (Forestry Commission 1990). This method has limited applicability in the Blue Mountains as many of the cut stumps would be widely scattered throughout remote forest and therefore would be very hard to find again, although in very heavily invaded forest regrowth spraying could be a useful technique.
Time needed to eradicate plants
The time, based on extensive experience, needed to treat various sizes of P. undulatum in different ways is shown below. They do not include any time taken to locate the plants.
|Treatment and size||Size of plant||Estimated time (secs)||Comments|
|Uproot seedlings||<2 m tall||10 to 30||Uprooted seedlings should be either kept off the ground surface or collected and piled|
|Cut||>2 m tall||20 to 80||Using machete|
|Girdle||>6 m tall||30 to 150||Using machete, frilling as well|
|Hole drilling||Large tree||100 to 200||Using hand brace|
|Application of herbicide||Any cut or girdled surface||30 to 70||Time for transporting and handling herbicide not included|
|Strip bark from 30 cm to soil||Over 3 m tall||50 to 150||All bark must be removed|
Environmental impacts and safety of manual control
There are serious worries about the environmental impact and persistence of Tordon. Picloram, a principal active ingredient of Tordon, was still present in soil twelve months after application in a tropical rain forest in Puerto Rico (reaching a maximum of 0.05 ppm at 12-30 cm deep) (Dowler & Tschirley 1970). As glyphosate appears to be effective, and has a very low toxicity to humans (Grossbard & Atkinson 1984), there seems to be little advantage in using Tordon, apart from its low cost. In Mauritius there is a large ongoing programme using herbicides to control many invasive species, and herbicide safety has become an important issue. Because of the strenuous work in humid heat it has not been possible to ensure that workers wear their protective clothing all the time, therefore glyphosate has found a greater role, in preference to more hazardous herbicides (W. Strahm, pers. comm., 1993). Some men in the Blue Mountains are familiar with the application of herbicides, because of their extensive use on the coffee plantations, and realise the potential danger to human health. We think that although herbicides may have significant environmental impact, the alternative of letting the invasion proceed could have considerably greater impact on the environment. The use of herbicides to kill woody plants is common in areas protected for nature conservation, such as national parks in the USA (OTA 1993).
There is the possibility that cutting rather than girdling P. undulatum trees could increase the probability of alien weeds establishing, but this much less likely in primary forest than in heavily invaded secondary forest. In primary forest P. undulatum trees tend to be smaller and to have a dense layer of native seedlings and saplings beneath, in contrast to heavily invaded forest. Additionally, the alien seed input is likely to be lower because of the distance from the highly invaded and disturbed land on the south slopes.
The second main way in which
undulatum may be controlled is biologically, though the use of fire
is briefly discussed later. Another method of control often mentioned in
the literature is environment manipulation, though this is not relevant
to the primary forests of the Blue Mountains, which are natural ecosystems
essentially unaffected by human activities.
Biological control against vegetative growth of P. undulatum
|Table 3. Number of dead seedlings 12 months after complete defoliation|
|The ability of
undulatum trees to resprout after being cut four times in HIFE indicates
that large reserves in the stem or roots of the plant is probably a reason
for its ability to produce new leaves following defoliation. Because of
the ability of Pittosporum trees to produce resprouts after damage,
and of small seedlings to resprout following decapitation (Gleadow 1982),
it seems unlikely that the species could be controlled by an animal that
just ate its leaves.
Predation in Australia
The following is a list of all the agents known to attack Pittosporum undulatum (or the genus more generally), none of them attacking flowers or fruit. Sources, in addition to the publications, are personal communications from Robin Buchanan, Peter Myerscough and Stefan Neser.
undulatum agents outside Australia
Biological control against reproduction of Pittosporum undulatum
Monitoring of fecundity
All P. undulatum plants >2 m tall but <25 cm GBH either in, with at least half their crown in, each gap were tagged and flagged for later relocation. For each of these, a P. undulatum in the understorey was selected. Understorey P. undulatum trees tend to have a different growth form, as well as a slower growth rate, from those growing in gaps, so are only comparable in a loose sense. They were selected by, firstly, subjectively choosing a nearby point in the understorey (one that had a sufficient density of P. undulatum, and a similar aspect and slope to the gap); and then selecting, for each of the P. undulatum trees in the gap, the nearest P. undulatum to that point within +/- 20% of its GBH (Gilbert gaps) or +/- 10% of its GBH (Juniper gaps). For each individual the following were measured:
No. capsules Mean SEM Gap 66 33.1 0.71 Understorey 72 30.3 0.80 All 138 31.7 0.75
There was no significant difference in the number of seeds per capsule between trees in the understorey and in gaps. There was no sign of any insects having bored into capsules or any other loss due to biotic factors, though capsules have been found in other locations that had been gnawed on the ground (probably by rodents) and partially eaten on trees (probably by birds).
relationship between the DBH of P. undulatum trees and their fecundity
(number of seeds, on the y-axis) for five successive years, 1992-96, and
the effect of gap creation on fecundity. The data for the Gilbert and Juniper
gaps have been combined.
seed production was high, the regression relationship between DBH and fecundity
giving a mean of 37,500 seeds for a 8 cm DBH tree in 1992, though there
was very large variability in fecundity amongst trees of a similar size.
It varied substantially through the five year period, so that in 1993 only
one tree produced more than 7,000 seeds, there being no discernible relationship
between DBH and fecundity. After five years the pattern of fecundity can
be described thus: very high in 1992, only about half that in 1993, the
seed production building up gradually between 1994-1996 to near 1992 levels.
Whether this pattern is governed by extrinsic factors (such as weather)
or intrinsic factors (for example within-plant food reserves) is not known.
The seed production of trees in the generally more open Gilbert gaps was
slightly but not significantly greater than in the Juniper gaps, likewise
in the understorey of both areas. For the Gilbert sites, understorey trees
were more likely to bear few or no cymes, compared with gap individuals.
Individuals at the Juniper site started producing cymes at a larger size
than those at the Gilbert sites. This may be because before August 1992
the forest was relatively tall and undisturbed, and therefore the smaller
undulatum trees would have been growing in a relatively low light environment.
Additional data collected in 1995 showed that the fecundity often falls
to very low levels on larger P. undulatum trees.
Flower and fruit development
To assess the number of flowers that survive to produce capsules, in February 1993 we counted the number of flowers on six flower bearing cymes on 39 P. undulatum trees <25 cm GBH (20 in a gap and 19 in the understorey). The trees were revisited in early December 1993 and the number of capsules in each marked cyme was recorded. In summary, overall 17.0% of the flowers produced capsules, with the mean number of flowers surviving slightly higher in gaps than the understorey. Twenty one of the trees produced no capsules at all. Observations made of two branches on separate trees during January 1994 showed that P. undulatum flowers took between 2 and 3 weeks between flower opening and "withering" after fertilisation. Some of the flowers had already been eaten, probably by birds; the Greater Antillean Bullfinch, Loxigilla violacea, has been seen eating P. undulatum flowers (T. Goodland, pers. obs.).
Type of biological control
Introducing an exotic agent to Jamaica inevitably involves some risk to native species, though release of biological control agents is nowadays undertaken only after a rigorous screening programme. There are no native members of the Pittosporaceae in Jamaica which may mean that the risks of any introduced agent attacking native plants is reduced (S. Neser, pers. comm., 1993). However, if introduced agents did kill all P. undulatum it is possible that the agent(s) would attack native species, especially as the population of P. undulatum declined. Evidence from South Africa indicates that the disease which is attacking P. undulatum there does not affect other species, however this would clearly need to be fully tested in Jamaica before any introduction of it to Jamaica.
A major problem with the intentional or accidental release of an effective lethal agent in Jamaica is that the sudden death of all P. undulatum in heavily invaded forest could have serious ecological consequences. The light levels on the forest floor would be increased substantially, almost certainly enhancing the growth of Hedychium gardneranum and probably facilitating the germination and growth of Polygonum chinense. The large quantity of dead wood could, during an exceptionally dry period, leave these areas vulnerable to fire, either accidentally started, or by people deliberately taking advantage of the opportunity to clear forest for farming with the minimum of effort. The consequences would be very serious, with fire resistant weeds, such as Melinis minutiflora, benefitting. Perhaps a lethal, vegetative blight could be introduced eventually, once the dominance (and fuel load) of P. undulatum in heavily invaded forest had been reduced through management.
In summary, it may be able to reduce the rate of P. undulatum spread, and its final density, but would be very unlikely to eliminate P. undulatum from the Blue Mountains altogether. In addition, it may well be politically unacceptable to introduce a disease that kills P. undulatum, as any proposal that may lead to the loss of forest cover would be hard to justify in a country that has lost so much. The use of biological rather than manual control favours scientists and technicians, many from richer countries, instead of local people who would be employed to carry out manual eradication. Therefore, careful political and socio-economic analysis of this option is required and it is crucial that the decision is taken at an appropriate local level. Contact should be maintained with researchers in South Africa, because of the potential of the unidentified agent currently killing P. undulatum in the Cape Province to be used as a biological control agent in Jamaica. However, should further research into the potential for biological control become necessary full attention should be given to an assessment of its risks and the costs of a proper screening programme before any decisions are taken.
The use of fire as a way of controlling P. undulatum in the Blue
If the primary objective is
to slow or stop an invasion, modelling highlights the importance of eradicating
small outlying populations before starting to eradicate the main original
population, even if it is expanding slightly faster than the satellite
populations (Moody & Mack 1988). Although it is less urgent to remove
undulatum from heavily invaded forest than from lightly invaded forest,
it should still be a long term objective, not least because of the value
of P. undulatum wood. However, it is very important to discover
ways of removing the species without leading to the forest becoming even
more heavily invaded by alien weeds. The very high recruitment of
undulatum and the generally low recruitment of other species following
the removal of P. undulatum trees has been examined in the accompanying
The Blue Mountains are different from almost all other areas where much effort has been expended on the control of alien plants in that there are many people who live locally who would be willing and able to work on a manual control programme. Elsewhere labour has either been expensive (such as Hawaii or New Zealand) or scarce (such as the Galapagos Islands or Mauritius, where labour is only available when the sugar cane is not being harvested (W. Strahm, pers. comm., 1993)). A manual control programme would be labour intensive and therefore clearly beneficial in an area with high unemployment and under-employment (R. Kerr, pers. comm., 1993). Rather than hiring local people on a casual basis as labourers, it would be preferable to build up a committed team of people and train them rather like forest rangers, so that they will be able to take a wide responsibility for protecting the forest and promoting it amongst local people.
Heavily invaded forest
Eradicate entirely from the Blue Mountains area
Removal of P. undulatum from heavily invaded forest is less urgent, but harvesting could start also on a limited scale. Because of the very high recruitment of P. undulatum following canopy disturbance and the commoness of other serious weeds in more accessible forest, great care needs to be taken in any control or harvesting. It would be very difficult to manage forest for timber production which is threatened by three alien weeds with such different characteristics. If the overriding objective is to leave the slopes of the Blue Mountains forested there is the real possibility that not clearing P. undulatum from heavily invaded forest would be the best policy.
A moderately high priority is an investigation into ways to deal with large P. undulatum trees in heavily invaded forest. They are vulnerable to windthrow and when the next hurricane strikes this could provide many nuclei for the further spread of Polygonum chinense and Hedychium gardneranum. Harvesting the P. undulatum trees for timber may have a similar effect. Thus, if P. undulatum is to be controlled or harvested in heavily invaded forest, every effort must be taken to maximise the regeneration success of native species in the disturbed areas, rather than creating the conditions that favour exotic species. Therefore, the effect on the regeneration of exotic and native species of killing large P. undulatum trees by felling versus killing them while still standing should be experimentally tested. If the timber is harvested the effect on the regeneration of the resulting soil disturbance should also be investigated. We suggest an experiment with three control treatments: · cut and treat stump with glyphosate · girdle and treat girdle with glyphosate · undisturbed control and two levels of soil disturbance. At least six large (>25 cm DBH) P. undulatum trees should be included in each treatment. The most suitable area would probably be upper Clydesdale but other areas should be considered. The seedling bank around each tree should be recorded before and after treatment (but this need not be taken to the level of labelling of individuals). This would be suitable for an MSc project or similar.
Kill all P. undulatum (cut, harvest and apply herbicide) (or for larger badly formed trees girdle and apply herbicide); also remove larger seedlings (>2m tall) at same time. Go back in about 5-10 years, just before those P. undulatum seedlings left have started to produce seeds.
Planting trees should be a policy of last resort for the restoration of natural forest cover following control of P. undulatum because of the likely cost. However, should it be necessary, species suitable for planting in areas cleared of P. undulatum should preferably be native, fast growing and with a dense crown enabling the suppression of exotic invasive species. They would probably be a subset of the species that are successful in gaps in the natural forest. A wide range should be tested initially, the actual species selected for a particular area should be largely determined by the degree of canopy opening caused by the removal of P. undulatum. We have made an initial list, ordered in approximate order of decreasing requirement for gaps for establishment: Brunellia comocladiifolia, Alchornea latifolia, Turpinia occidentalis, Clethra occidentalis, Symplocos octapetala, Juniperus lucayana, Podocarpus urbanii, Guarea glabra. The ease of propagation of bare root cuttings, survival after transplanting and subsequent growth of a number of these species is currently being tested at Cinchona Botanic Garden by M.A. McDonald in ODA FRP project R6290 and the results of this trial will provide further criteria for species selection.
In order to determine whether commercial use of harvested P. undulatum timber could make a contribution towards the costs of control in heavily invaded forest, more information is needed on the working properties, acceptability and market value of P. undulatum wood. Logs of P. undulatum should be distributed to a cross section of Kingston wood processors to obtain this information. It would also be valuable to obtain a better idea of the economic viability and environmental impact of making charcoal from P. undulatum than has been obtained so far; this could readily be carried out in the Cinchona area.
Management of other invasive species
Tests should be made of the effect of physical and chemical treatments on the control of Polygonum chinense and Hedychium gardneranum. These two invasive species should be eradicated from the Vinegar Hill Trail (and any other trails opened up). Existing contacts should be maintained, and new ones developed, with other organisations concerned with the control of H. gardneranum (e.g. in New Zealand, South Africa, Azores, Madeira and Réunion) and P. chinense and any other invasive species found to be problematic in the Blue Mountains. The two known Syzygium jambos populations should be eradicated because of the extent to which this species has become invasive in other countries.
Use of Pittosporum undulatum
outside the forest
So far, our research on the
impact of P. undulatum has concentrated on its effects on terrestrial
plant biodiversity. New research is needed on the interaction of the invasion
and animal biodiversity, especially birds. The attractiveness of P.
undulatum fruit to native bird species has played an important role
in the spread of the invasion. On the other hand, forest heavily invaded
by P. undulatum has a greatly reduced diversity of types of fruit
and a reduced structural complexity (which may greatly reduce the diversity
of epiphytes and of invertebrates) with potentially serious negative effects
for bird biodiversity. In the wake of the pioneering research work of Dr
Andrew Lack, the Jamaican bird fauna (including many endemic species) has
received international conservation interest. Dr Nigel Varty of Birdlife
International carried out detailed studies of the impact of Hurricane Gilbert
on the bird fauna of the Blue Mountains. These contacts should be developed
to enable the necessary research to be carried out on the interaction of
undulatum and animal biodiversity.
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