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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.|
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).|
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:
|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
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
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.
The questions concerned with the effect of P. undulatum that HIFE was designed to answer are:
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 1212 metres, surrounded by a 9 m guard area (giving a 3030 m treatment area). Twenty 11 m sub-plots were randomly selected within each plot. Woody plants over 3 m tall were enumerated within the 1212 m plot, those less than 3m, in the twenty 1 m2 sub-plots. Each individual was identified to species level wherever possible and labelled with an aluminium identification tag. There were four treatments:
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
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).
The long-term objectives of SIFE are to investigate the following:
Population dynamics of the invasion
Figure 3. Plan of a Slightly Invaded Forest Experiment plot
Differences between SIFE and HIFE
The similarities between the two experiments include, of course, the central interest in the effect that P. undulatum has on the forest, the identical height/girth thresholds used to define the different categories of plants, and the presence in SIFE of all the species present in HIFE. The most notable differences are:
The experiment has a simple randomised block design, with six blocks and a single replicate of each treatment in each block, randomly assigned. Each 2415 m plot is staked out in 33 m cells, with eight 315 m strata. In the centre of two cells in each strata a 1.21.2 m sub-plot has been established in which all tree seedlings were enumerated; all seedlings over 100 cm tall and all saplings (300 cm tall to 10 cm GBH) were enumerated in the remainder of the 33 m cell.
Trees (defined as those woody plants >3m tall). We identified and measured the GBH of all trees within the 30 x 21m plot. All individuals were tagged with aluminium tags (except plants >3m tall to 10 cm GBH within the 16 selected cells in each plot, which had their coordinates within the cell measured instead). Thirty-five individuals have not been identified yet, and a further 24 individuals have only tentatively been identified to specific level. Another 240 individuals belong to groups of closely related species either difficult to distinguish without fertile material or of doubtful status as separate species.
Large seedlings (defined as woody plants >1m tall). We identified and measured the height of all woody plant seedlings >1m tall in the sixteen 33 m cells. The spatial coordinate of each within the cell (to the nearest 5cm) was recorded.
Small seedlings (defined as woody plants <1m tall) occuring in the 1.2x1.2 m sub-plots were similarly measured, although the spatial coordinate of each was recorded to the nearest centimetre. Where a particular species occurred at a high density, seedlings were marked with aluminium tags.
There have been three enumerations so far, all carried out in the month of July:
1994 The full pre-treatment enumeration.
1995 We measured the height of P. undulatum seedlings only; we removed P. undulatum from half the plots as planned.
1996 We measured the
height of all seedlings in 12 sub-plots, 6 beneath P. undulatum
trees and 6 which had never been beneath P. undulatum trees. After
an initial analysis it was decided that a full re-enumeration would be
Figure 4. Relationship between the basal area of P. undulatum (m2 ha-1) and the mean seedling density (m-2, on the y-axis) of seven important native species, and all native species combined, in 37 plots (HIFE and SIFE). Each plot has been put into one of three classes of past human disturbance. Note the different scale in the bottom two graphs to the rest of the graphs.
There is a definite relationship between the basal area of P. undulatum and the density of all native seedlings combined. The relation is best described as linear if the very high values in some of the primary forest plots are excluded - these values are largely due to the shade-tolerant Eugenia virgultosa, (the species occurs at a much higher density (in primary forest) that any other native species in our permanent sample plots). The relationship indicates that the density of native seedlings falls to near zero in the most heavily invaded forest.
Two reasons for the fairly large amount of scatter in the relationship is firstly due to spatial variation in the density of P. undulatum trees in some plots, especially the large SIFE plots. Another reason may be the use of basal area and the way it increases so rapidly with DBH. For example, the basal area of the largest P. undulatum tree we have found is 3380 cm2, 478 times that of a 3 cm DBH tree (the bottom of the tree size class). Given the usually dense crown of smaller P. undulatum trees and the often thinning crowns of larger trees it seems unlikely that there could be such a difference in the effect of trees of these different sizes.
Guarea glabra and Cinnamomum
montanum are two shade-tolerant species common in primary forest, but
rare as trees in secondary forest. Both species show signs of re-invading
older secondary forest, however neither species appear to show an ability
to grow into larger size classes beneath dense P. undulatum. Maytenus
jamaicensis is a shade-tolerant species typically confined to primary
forest (with only one secondary or intermediate plot with the species present
in as a seedling) so making it very difficult to determine what effect
undulatum has on the regeneration of the species. Eugenia virgultosa
is the only species (for which we have sufficient data) that has significant
numbers of seedlings reaching large seedling size in heavily invaded secondary
forest. Psychotria corymbosa is a species (rarely exceeding 6 m
in height) fairly common as a seedling in secondary as well as primary
forest. However mortality of small (<20 cm) seedlings is high and there
is very little recruitment of the species in heavily invaded forest. Alchornea
latifolia and Clethra occidentalis are gap demanding/benefitting
species rare in forest with more than about 1 m2 of P. undulatum
per hectare; adult trees of both species are common in secondary forest
so seed input is unlikely to be limiting.
Table 2. Mean dates of enumerations of seedlings and trees in the HIFE plots.
t0 t1 t2 t3
Seedlings 12/08/91 22/08/92 12/01/94
Trees 12/08/91 24/12/93 08/07/96
Figure 5. The relationship between the basal area of P. undulatum and dominance of seedlings of native trees at three enumerations in the ten Undisturbed Control plots in HIFE., (a) absolute dominance values, (b) relative dominance values. Note that in all plots (except plot 15 where P. undulatum was not present) the basal area of P. undulatum increased between each enumeration
The relationship between the basal area of P. undulatum and absolute dominance of native seedlings at the three enumerations (Figure 5(a)) is not clear. Four plots experienced an increase in the dominance of the seedling layer in absolute terms, all plots where P. undulatum had not achieved great dominance, with the disturbance caused by Hurricane Gilbert in 1988 probably still having an effect.
The relative dominance of native seedlings shows a clearer relation with the dominance of P. undulatum. There is a general tendency for there to be a diminishing dominance of native seedlings with more P. undulatum. One notable aspect of this relationship is the presence of a clear boundary to the maximum dominance (or density or diversity) of native vegetation. This suggests that, though many factors (such as disturbance history, soil type and depth, and slope steepness) influence native vegetation, P. undulatum appears to be a clear limiting factor. It is not possible to say whether the species acts directly (by allelopathy for example) or indirectly (by depriving understorey plants of resources). Time does not appear to equate with space; if it did, one might expect the mean slope of the relationship for each plot through time to be about the same as that of the slope between plots at one point in time. Eight of the ten plots showed a decline in the relative dominance between t0 and t2 but the situation was complex - five plots showed a decline in both intervals, three plots showed a decline then increase, one plot showed an increase then decline and one an increase in both intervals.
Analyses of the effect of differing amounts of P. undulatum on the growth and survival of native species did not give clear results. There was a tendency for the growth rate and survivorship of Eugenia virgultosa and all native species combined to decrease with increasing P. undulatum, but there was much variation between plots. The problems of carrying out this investigation in forest with so few native seedlings was apparent.
In July 1996 we carried out a preliminary enumeration of SIFE, exactly one year after the removal of P. undulatum. It did not seem likely that there would be a major effect, as P. undulatum had not been dominant in any of the plots. The cover of native trees was sufficient to prevent a very marked increase in light levels following P. undulatum removal. The table below shows the results from the enumeration. These results have been shown only to give an indication of the situation in 1996, they are not full statistically valid, because the data was collected from only two plots.
Table 3. Number of seedlings, mean absolute height increment (cm) and standard error of the mean (SEM), of Eugenia virgultosa, other native species and P. undulatum in SIFE. Results are shown from six sub-plots (in plot 1) beneath the crowns of at least one P. undulatum tree, and six sub-plots (in plot 2) that have never experienced shading from P. undulatum.
Beneath P. undulatum Not beneath P. undulatum
No. MAI SEM No. MAI SEM
Eugenia virgultosa 72 2.27 0.27 68 2.56 0.39
Other native species 48 4.63 0.67 51 4.75 0.72
P. undulatum 38 6.47 1.05 44 7.38 1.13
The growth rate was not significantly
different (at the 5% level) for any of the three species groups between
the two treatments, though was significantly different for the three species
Figure 6. Variation in the numbers of seedling recruits (log scale) between 1991 and 1993 of three species groups (P. undulatum, Shade-intolerant native species and Shade-tolerant native species) per plot, with basal area (cm2 m-2) of P. undulatum. The results of three treatments are shown, each being the sum of the number of recruits at the two post-treatment enumerations. For the Undisturbed Control treatment (10 plots) the recruitment is plotted against the basal area of P. undulatum in 1991. For the Remove P. undulatum Trees and Remove all P. undulatum treatments (5 plots each) the recruitment is plotted against the basal area of P. undulatum removed in 1991.
Overall, the graph on the left (Undisturbed Control) shows that the number of recruits declined with increasing amounts of P. undulatum, whilst the two other graphs show that the number of recruits increased with increasing amounts of P. undulatum removed in 1991. Results are shown separately for the six commonest shade-tolerant species and the six commonest shade-intolerant species, commonest meaning as new recruits at the t1 enumeration. The recruitment of shade-tolerant species was higher in the Undisturbed Control treatment though declined at about the same rate as the recruitment of shade-intolerant species declined with increasing P. undulatum. The recruitment of shade-intolerant species was on average greater following the removal of all P. undulatum compared with the removal of only P. undulatum tree, whereas there was no significant difference with shade-tolerant species.
There was no recruitment of P. undulatum in seven of the Undisturbed Control plots. In both the Remove all P. undulatum and Remove P. undulatum Trees treatments the recruitment of P. undulatum increased greatly with increasing amounts of P. undulatum removed, reaching 2144 recruits in plot 20 (107.2 seedlings m-2).
Analyses of the effect of P. undulatum removal on the growth and survival of those seedlings already present at the pre-treatment enumeration ("advance regeneration") show much less clear results. In brief, those few seedlings that were beneath dense P. undulatum were shade-tolerant species, mostly Eugenia virgultosa, and these showed little sign of increased growth; indeed in one plot (plot 20, south-eastern aspect, thin soils) from which all P. undulatum was removed (comprising 17.5% of the total basal area) most of the advance regeneration either died or died back. In less heavily invaded forest there was a greater diversity of advance regeneration but the removal of P. undulatum led to a lesser opening up of the canopy, so effects were slight.
Figure 7. Factors determining the impact of P. undulatum
Table 4. Number of plots, total area (ha), and mean enumeration dates and intervals of the three series of plots
number area t0 t1 years
E.V.J. Tanner plots1 40 0.400 14/03/91 08/08/94 3.405
P.J. Bellingham stratified plots 15 0.300 10/08/90 10/09/94 4.089
HIFE Undisturbed Control plots 10 0.144 14/08/91 09/07/96 4.905
1 Col, Mull Ridge, Wet slope
The tree threshold size is 3 cm DBH (7.0685 cm2 BA). The relative basal area increment (RBAI) was calculated as follows:
RBAI = (lnBAt1 - lnBAt0) / t1-t0
where BA is in m2, and t is time in years.
All results are given by size class. Five size classes were chosen to contain approximately the same number of individuals in each class, the size thresholds are:
Size class BA (cm2)
As well as analysing data for individual species we grouped species into regeneration classes (mostly based on a number of independent field studies (Sugden et al. 1985, Healey 1990, Vernon 1991, Dalling 1992)):
GD gap-demanding - gaps or severe canopy disturbance essential for germination and recruitment
GB gap-benefitting - some disturbance necessary for germination and recruitment
SGP slow-growing pioneer - species with regeneration usually confined to habitats such as landslides
ST shade-tolerant - species relatively more successful as seedlings in undisturbed conditions than gaps
- minor species for which we do not have sufficient information to confidently
During July 1995 we started two studies of the ability of common species to exploit and explore the above-ground environment. Both studies were carried out in forest that showed a range of disturbance from moderately disturbed (mostly by Hurricane Gilbert seven years previously) to very undisturbed. It is important to note that areas of severe or recent disturbance (or well-defined gaps) were not included. Some bias is likely, as those more gap-benefitting species were more likely to be growing in areas that experienced higher light levels in the past, even if the light levels were fairly uniform throughout the plots in 1995.
The first study was directed at looking at the effect of P. undulatum trees on the above-ground growth form of large seedlings (between 1 and 3 metres tall) of common native species and P. undulatum itself. The species were chosen for commoness and from three "regeneration groups" - Gap demanding, Gap benefitting and Shade tolerant. The seedlings were sampled in the Undisturbed Control and Remove all P. undulatum treatments of SIFE, all beneath the crowns of P. undulatum saplings (which were subsequently removed of course in the latter treatment). The seedlings were not confined to the plot itself but were often in the treatment area, and were tagged and flagged for later relocation. The parameters measured were:
Figure 8. Method of measuring branch extension. The maximum distance that any living part of any branch reaches to the NE, SE, SW and NW from the stem was measured, such as distance a in the NE quadrant.
In the second study we investigated the same aspects of the growth of a slightly wider range of species, this was possible because of the much larger area within the SIFE plots not beneath the crowns of P. undulatum. The differences were that:
Data were collected from a total of 18 species in the two studies, but in the figures, results are just shown for 15 of these, the five commonest gap-demanding, gap-benefitting and shade-tolerant species.
Maximum DBH of each species
The maximum DBH of any individual stem in any plot at any enumeration for each of 119 species (those present in at least one of the 144 permanent sample plots) was calculated. The heights of trees have not been recorded in most of the plots in the Blue Mountains and even when they have been the measurements have often not been accurate (P.J. Bellingham, pers. comm., 1993), so we do not provide an analysis of maximum heights here.
m = 1-(N1/N0)1/t
where N0 and N1 are population counts at the beginning and end of the measurement interval, and t is in years (Sheil et al. 1995).
Maximum population density. The maximum seedling density (m-2) per sub-plot in the 25 HIFE plots and 12 SIFE plots (692 sub-plots altogether) reached by each of the 119 species occurring in at least one of the 144 permanent sample plots; and the maximum tree density (m-2) reached by the same species in any one of the 144 plots at any enumeration, was calculated.
The summed heights of all seedlings in a plot or sub-plot is a useful measure
of dominance (Healey 1990). The heights of all seedlings in the 692 sub-plots
in the 37 HIFE and SIFE plots at the pre-treatment enumeration was summed,
and expressed as a density per square metre. The maximum value for all
89 species occuring as a seedling in either HIFE or SIFE was calculated.
Figure 9. Mean (and standard error of mean) relative basal area increment of five size classes of different regeneration classes between 1991 and 1994 in the Tanner Col, Mull ridge and Wet slope plots, Bellingham stratified plots and the Undisturbed Control plots of HIFE.
The RBAI declines with increasing size as would be expected, with an increase from Size class 2 to 3 for the Slow growing pioneer class, largely due to the RBAI of Cyrilla racemiflora, shown in Figure 10). The similarity in RBAI between the gap-benefitting, shade-tolerant and slow growing pioneers trees is interesting, given the very different growth rates of seedlings of these classes. There are few small stems of gap-demanding species, mostly because of the lack of disturbance of these forests between Hurricane Hazel in 1951 and H. Gilbert in 1988. No individuals of gap-demanding species recruited as a result of H. Gilbert had reached tree size by 1991.
The RBAI between 1991 and
1994 of P. undulatum and the nine commonest native species (all
species with >100 stems) in the same Tanner (Col, Mull Ridge and North
slope) plots, Bellingham stratified plots and HIFE Undisturbed Control
plots is shown in Figure 10.
Figure 10. Mean (and standard error of mean) relative basal area increment of five size classes of ten species between 1991 and 1994 in the Tanner Col, Mull ridge and Wet slope plots, Bellingham stratified plots and the Undisturbed Control plots of HIFE. The species are divided into (a) gap demanding or benefitting species, and (b) shade-tolerant or slow-growing pioneer species
The most striking result is the high RBAI of some gap demanding or benefitting species compared with the dominant tree species Clethra occidentalis, Podocarpus urbanii, Eugenia virgultosa, and Vaccinium meridionale. Hedyosmum arborescens was the only species of these nine to have a growth pattern similar to that of P. undulatum, although the growth rate of P. undulatum was higher than H. arborescens for all size classes, particularly the middle size classes. H. arborescens is a relatively short-lived, though medium sized, tree. A higher proportion of H. arborescens trees were killed by Hurricane Gilbert than any other of the 47 commonest tree species (Bellingham et al. 1995).
The high variability of growth of small stems of Alchornea latifolia and Cyrilla racemiflora is partly due to the common occurrence of large sprouts from the trunk of large trees of these species. These sprouts can sometimes grow very fast, significantly faster than individual small trees of the same species and size in the same environment. But this is not so for all species which produce many sprouts. For example Ilex macfadyenii produces a higher number of sprouts (that reach 3 cm DBH) than any other species, but sprouts as well as main stems grow slowly.
The results of an analysis of P. undulatum and native seedling growth (not presented here) show a similar pattern, with P. undulatum consistently having a faster mean growth than almost all native species. In four experimentally created gaps however a small number of native species (particularly Brunellia comocladiifolia and Miconia dodecandra) grew faster as juveniles and small trees than P. undulatum. Both of these species can be described as pioneer trees typical of lowland tropical rain forest. B. comocladiifolia can become a large tree (>50 cm DBH) but its regeneration is confined to areas of high disturbance and is uncommon in forest (occurring at a mean density of only 1.59 stems per hectare in our permanent sample plots). M. dodecandra is a small, even less common tree, and is similarly confined to disturbed areas.
The relationship between the height and the mean crown extension of the large seedlings is shown in Figure 11 overleaf.
Figure 11. Relationship between the height (cm, on the x-axis) and the mean branch extension (cm, on the y-axis) of large (100-300 cm) seedlings of 15 species. Blue diamonds represent individuals not growing directly beneath the crown of P. undulatum trees, whereas red crosses represent individuals beneath P. undulatum trees. The top five species are gap demanding, the middle five are gap benefitting and the bottom five are shade tolerant. Mean branch extension on the y-axis, height on the x-axis, both in cm.
It is evident that the steepness of the regression relationship between height and mean crown extension is greater for P. undulatum than for any native species and that the relationship is a relatively close one (the data has not yet been statistically analysed). Urbananthus critoniforme (a small short-lived near-pioneer tree) is the only one of these native species to show a similar increase in branch extension with increasing size, though unfortunately it was not possible to find more individuals in the SIFE plots to make the relationship clearer. Overall there is no obvious difference between gap-demanding, gap-benefitting and shade-tolerant species. Note that the gap-benefitting class includes species with a wide degree of response to gap formation, from species clearly greatly favoured by disturbance (for example Hedyosmum arborescens) to species apparently little affected (for example Ilex harrisii). Of the shade-tolerant species three of the commonest (Eugenia virgultosa, E. monticola and Guarea glabra) show a noticeably similar relationship. Cinnamomum montanum has a particularly extensive crown.
Myrsine coriacea shows a much greater degree of variability in mean branch extension for a given height than P. undulatum or native species such as Hedyosmum arborescens or Alchornea latifolia. M. coriacea is a species that has branches that fail to grow if the light intensity from the side, or if inter-plant competition, is intense (leafless dying-back branches are common). In these situations M. coriacea seedlings appear (above-ground) to put all their resources into height growth.
Table 5. Mean crown area (m2); maximum branch extension as a percentage of the individual's height for any individual; mean of the coefficient of variation (CV) of branch extension for each individual; mean of the height:crown diameter ratio for each individual; mean Leaf Area Index (LAI); and mean crown volume (m3) for the 18 species
|Species||Mean crown area (m2)||Max. branch extension as % of height||Mean CV of branch extension||Mean height/ crown diam. ratio||Mean LAI||Mean crown volume (m3)|
The mean branch extension of P. undulatum was 4 cm greater than the mean branch extension of any native species, and 14.6 cm (54.3%) greater than the mean for all native species. The maximum branch extension of P. undulatum was 25 cm longer than that of any branch of any native species, or when extensions are expressed as a percentage of seedling height (column 3), over 10% greater than that of any native species. The height:crown diameter ratio (column 5) of P. undulatum was 2.29, just above Cinnamomum montanum, i.e. both these species have a very broad crown for a given height. The mean L.A.I. of P. undulatum was near the average for all species, the high leaf area per individual (Fig. 13) being distributed over a larger crown area than native species (column 2). The high crown area of P. undulatum combined with its deep crown (Fig. 12) to give a mean crown volume over three times the mean for native species combined.
Variability of branch extension
The ability to efficiently exploit above-ground resources depends on the degree to which plants can increase their lateral growth towards areas of higher resource availability. The light environment in many areas of the Blue Mountains has been highly spatially variable since Hurricane Gilbert, with many gaps created by the fall of large branches or trees, or the later death of standing damaged trees (though most of these gaps have now "filled in"). As the distance of branching was measured in four directions for each seedling we have been able to calculate the coefficient of variation of branch length for each individual plant and derive the mean for each species, shown in Table 5 (column 4).
P. undulatum had the
third highest value, lower than the two gap demanding or benefitting species
alpina and Psychotria corymbosa. The mean branch extension of
both these native species is only about 60% that of P. undulatum
and their high degree of variability in branch extension is partly due
to a failure of some seedlings to produce branches in some directions.
Whether this was a failure to produce a branch, mechanical damage to a
developing branch (P. corymbosa is very weak stemmed) or a "tactical"
exploitation of resources by the plant (effected by a diversion of resources
into those branches produced on the side of the plant experiencing higher
light levels) we cannot say. Such a tactical explanation seems more likely
with P. alpina than with P. corymbosa, and very likely with
Crown depth of large seedlings
The crown depth of the seedlings
is shown in Figure 12 on the next page. It would be expected that, for
individuals with equal leaf area, those with deep crowns would collect
less light from vertically above and more sidelight, hence deep crowns
would be more prevalent in shade-tolerant species. For these 15 species
the relationships are not at all clear, though there appears to be some
evidence to support this hypothesis. Shade-tolerant Eugenia monticola
had a consistently deep crown, whereas the gap-demanding Alchornea latifolia
had a consistently shallow crown. The deep crowns of Sapium harrisii
and Palicourea alpina are rather deceptive, as both species can
retain leaves produced on the main stem early in growth, and whether the
crown can really be said to extend this low is questionable. Hedyosmum
arborescens had a very similar regression relationship to P. undulatum,
both having a rather consistently deep crown. Both Eugenia virgultosa
(probably the most shade-tolerant of these species) and Myrsine coriacea
(one of the least shade-tolerant species) had very variable crown depths.
Figure 12. The relationship between seedling height (cm, on the x-axis) and the crown depth (cm, on the y-axis), for each large seedling of the 15 species. Crown depth was calculated as total height minus height to the lowest living leaf.
Leaf area of large seedlings
The relationship between the height of the saplings and the total leaf area is shown in Figure 13.
Figure 13. The relationship between the height (cm, on the x-axis) and the total leaf area (m2, on the y-axis) of large seedlings of 15 species. Blue diamonds represent individuals not growing directly beneath the crown of P. undulatum trees, whereas red crosses represent individuals growing beneath P. undulatum trees. The top five species are gap demanding, the middle five are gap benefitting and the bottom five are shade tolerant.
In general the mean leaf area
per species increased in the order gap-demanding < gap-benefitting <
shade-tolerant, with much variation between species within class. The leaf
area of P. undulatum was strikingly higher than any native species
(again with the possible exception of Urbananthus critoniforme).
The shade-tolerant Maytenus jamaicensis had a noticeably high leaf
area. Although the slopes of the regression relationship of Alchornea
latifolia and Sapium harrisii are similar to Palicourea alpina,
corymbosa and Myrsine coriacea the leaves of both species are
significantly larger, partly explaining why their slopes intercept the
y-axis at a higher level than the other three gap favoured species. The
difference between the congenerics Eugenia monticola and E. virgultosa
is very noticeable. The two species had almost identical mean branch extensions,
whereas E. monticola had a consistently deeper crown than E.
virgultosa, but the most noteworthy difference between them is the
different leaf sizes (a mean of 0.0004 m2 for
and 0.0018 m2 for E. monticola).
Maximum DBH of each species
The maximum DBH of any tree for each of the 116 species occurring as a tree in at least one of the 144 plots permanent sample plots in the Blue Mountains is shown in Table 6. The small size of the trees compared with lowland tropical rain forest is obvious but is typical of montane forests (Grubb & Tanner 1976). Larger trees occur outside plots of course, the largest typically-shaped native tree (a Sapium harrisii) so far encountered having a DBH of 97 cm. The largest P. undulatum had a DBH of 41.8 cm, the 21st in rank, and the DBH of the largest measured P. undulatum outside plots was 65.6 cm. Judged by the size of crowns we think that a few significantly larger P. undulatum trees occur on a remote hillside which we have never been able to visit.
Table 6. The maximum DBH (cm) of any tree for each of the 116 species occurring as a tree in at least one of the 144 plots permanent sample plots in the Blue Mountains.
|Jun luc||75.0||Eug mon||30.5||Cya woo||14.1||Cli ter||6.4|
|Alc lat||71.9||Rha sph||29.2||Den pen||13.9||Bid shr||6.3|
|Sid mon||71.3||Den nut||28.3||Psy cor||13.8||Con mon||5.7|
|Tur occ||68.1||Clu hav||28.1||Cal rig||13.7||Tou gla||5.5|
|Hae inc||61.0||Sym oct||27.8||Lyo jam||13.0||Sch inv||5.5|
|Pod urb||58.2||Cle the||27.5||Psy slo||12.5||Mal arb||5.3|
|Vac mer||57.1||Ile mac||27.1||Cle oxa||12.2||Boc fru||5.1|
|Gor hae||55.8||Myr cor||26.7||Xyl nit||12.0||Oss asp||5.1|
|Cyr rac||55.4||Hed arb||26.6||Tre flo||11.8||Pip arb||5.0|
|Gua gla||52.8||Vib spp||26.5||Ges alp||11.4||Ure ela||4.7|
|Sol pun||52.7||B1 mel||25.4||Vib vil||11.3||Bla tri||4.6|
|Cle occ||52.5||Ile har||25.2||Per alp||11.0||Aca vir||4.3|
|Sap har||52.3||Ile obc||24.2||Mar bro||10.8||Lob ass||3.8|
|Myr cer||51.1||Den p/n||24.0||Cri par||10.4||Wei pin||3.7|
|Bru com||50.1||Ile vac||23.8||Mec pur||10.4||Wal faw||3.6|
|Den arb||47.7||Eug har||23.8||Mer leu||10.2||Dur ere||3.4|
|Cha glo||44.7||Cin pub||23.7||Cya con||9.9||Bes lut||3.4|
|Cin mon||44.6||Gar fad||23.1||Bru jam||9.8||Sal sca||3.1|
|May jam||44.1||Eug mar||23.0||Ces hir||9.4||Lob mar||3.0|
|Lyo oct||43.0||Sch sci||21.4||Mic rig||9.1||Pip fad||3.0|
|Pit und||41.8||Eug vir||20.4||Phy arb||8.8||Mic dod||2.5|
|Cle ale||38.5||Mer pur||20.1||Boe cau||8.3||Ver plu||1.9|
|Cit cau||37.2||Wal cal||19.7||Pic ant||7.9||Phe hir||1.1|
|Ile nit||36.6||Eug bra||18.9||Urb cri||7.7||Koa har||1.0|
|Myr acr||34.4||Cin off||17.8||Wal cra||7.6||Met spp||0.9|
|Vib alp||33.6||Mic the||17.4||Cas vim||7.5||Smi bal||0.8|
|Myr fra||33.6||Cya fur||17.2||Pit vir||7.1||Mik max||0.7|
|Oco pat||33.3||Mic qua||17.1||Odo fad||6.6||Com cli||0.6|
|Cya pub||31.8||Pru occ||16.7||Cal fer||6.5||Pas pen||0.5|
The mortality of P. undulatum and native species classified into regeneration groups is shown in Figure 14 and is shown for all native species with over ten individuals in Table 7.
7. Annual mortality rate of five size classes of P.
undulatum, all individual native species with over ten individuals and
four regeneration groups of native species (in bold) in the ten Undisturbed
Control plots in HIFE between 1991 and 94; results are in the sequence
of decreasing overall mortality. RG = regeneration group. The five size
classes are, in cm: 1 <10, 2 10-19, 3 20-49, 4
50-99, 5 >100 cm.
|Number of seedlings in 1991||Annual mortality rate|
Figure 14. Annual mortality rate between 1991-93 of seedlings of P. undulatum, and those of native species classified into four regeneration groups, in 10 Undisturbed Control plots in HIFE; see text for an explanation of how annual mortality was calculated.
The mortality rate for smaller seedlings was greater than that for larger seedlings for P. undulatum and all regeneration groups. Shade-tolerant species generally had lower mortality, though two climber/scramblers classified as gap benefitting (Cassia viminea and Malvaviscus arboreus) had a lower overall mortality than the mean for all shade-tolerant species. The mortality rate of Eugenia monticola was only about one-tenth that of E. virgultosa. There was a low density of gap-demanding species (many of those that were present were recruited by Hurricane Gilbert); there were no seedlings of the gap-demanding class in the largest size class.
The mortality rate for all
size classes of P. undulatum was very similar to gap-benefitting
native species, the group which P. undulatum would be placed into,
based on growth rate criteria. For the largest three size classes the mortality
of P. undulatum was less than that of the mean for shade-tolerant
species. Of the 24 native tree species with >10 individuals, all those
classified as gap demanding or gap benefitting (and some classified as
shade-tolerant) had a higher overall mortality than P. undulatum.
Maximum population density
The maximum seedling density
(m-2) per sub-plot in the 37 HIFE and SIFE plots reached by
each of the 119 species occurring in at least one of the 144 permanent
sample plots; and the maximum tree density (m-2) reached by
the same species in any one of the 144 plots at any enumeration is shown
in Table 8.
Table 8. The maximum seedling density (m-2) per sub-plot in the 37 HIFE and SIFE plots reached by each of the 119 species occurring in at least one of the 144 permanent sample plots in the western Blue Mountains; and the maximum tree density (m-2) reached by the same species in any one of the 144 plots at any enumeration. The species are arranged in order of decreasing seedling density within each column.
|Eugenia virgultosa||215.97||0.347||Wallenia calyptrata||3.00||0.050||Cionosicys pomiformis||0.69||0.000|
|Pittosporum undulatum||197.00||0.306||Malvaviscus arboreus||3.00||0.030||Gonolobus jamaicensis||0.69||0.000|
|Maytenus jamaicensis||47.22||0.076||Miconia theaezans||3.00||0.008||Gonolobus stapelioides||0.69||0.000|
|Myrcianthes fragrans||43.00||0.042||Passiflora penduliflora||2.78||0.000||Ilex vaccinoides||0.69||0.030|
|Clethra occidentalis||40.28||0.175||Ilex obcordata||2.78||0.080||Marcgravia brownei||0.69||0.024|
|Psychotria corymbosa||29.00||0.095||Symplocos octopetala||2.78||0.020||Conostegia montana||0.69||0.020|
|Eugenia monticola||26.00||0.192||Acalypha virgata||2.78||0.017||Persea alpigena||0.69||0.020|
|Alchornea latifolia||22.92||0.110||Wallenia fawcettii||2.78||0.010||Odontocline fadyenii||0.69||0.011|
|Guarea glabra||18.75||0.165||Sideroxylon montanum||2.08||0.050||Brunellia comocladiifolia||0.69||0.008|
|Psychotria sloanei||16.00||0.125||Ilex harrisii||2.08||0.049||Duranta erecta||0.69||0.007|
|Palicourea alpina||15.28||0.165||Tournefortia glabra||2.08||0.003||Lyonia octandra||0.00||0.370|
|Eugenia marchiana||15.28||0.049||Calyptranthes rigida||2.00||0.060||Cyathea pubescens||0.00||0.173|
|Mecranium purpurascens||11.11||0.080||Miconia quadrangularis||2.00||0.050||Clethra alexandra||0.00||0.130|
|Cassia viminea||9.72||0.020||Urbananthus critoniformis||2.00||0.050||Dendropanax pendulus||0.00||0.120|
|Myrsine coriacea||9.00||0.069||Dendropanax arboreus||2.00||0.040||Cyrilla racemiflora||0.00||0.117|
|Koanophyllon hardwarense||7.64||0.000||Bidens shrevei||2.00||0.021||Cyathea furfuracea||0.00||0.110|
|Hedyosmum arborescens||7.64||0.150||Callicarpa ferruginea||2.00||0.020||Juniperus lucayana||0.00||0.100|
|Sapium harrisii||7.64||0.010||Rhamnus sphaerospermus||2.00||0.015||Boehmeria caudata||0.00||0.083|
|Turpinia occidentalis||6.94||0.083||Wallenia crassifolia||2.00||0.015||Miconia rigida||0.00||0.070|
|Mannetia lygistum||6.25||0.000||Dendropanax pen/nut||1.39||0.000||Cleyera theaoides||0.00||0.045|
|Phyllanthus arbuscula||6.25||0.051||Phenax hirtus||1.39||0.000||Myrica cerifera||0.00||0.042|
|Cinnamomum montanum||6.25||0.021||Vaccinium meridionale||1.39||0.326||Bocconia frutescens||0.00||0.033|
|Meriania purpurea||5.56||0.085||Solanum punctulatum||1.39||0.050||Mecranium virgatum||0.00||0.033|
|Piper arboreum||5.56||0.020||Haenianthus incrassatus||1.39||0.030||Eugenia alpina||0.00||0.030|
|Salmea scandens||4.17||0.000||Gordonia haematoxylum||1.39||0.021||Schradera involucrata||0.00||0.025|
|Eugenia brachythrix||4.17||0.017||Cestrum hirtum||1.39||0.017||Cinchona pubescens||0.00||0.021|
|Lobelia assurgens||4.17||0.005||Besleria lutea||1.00||0.000||Cyathea woodwardioides||0.00||0.020|
|Smilax balbisiana||4.00||0.000||Cissampelos pareira||1.00||0.000||Lyonia jamaicensis||0.00||0.017|
|Podocarpus urbanii||4.00||0.180||Daphnopsis americana||1.00||0.000||Gesneria alpina||0.00||0.015|
|Eugenia harrisii||4.00||0.132||Cinchona officinalis||1.00||0.195||Urera elata||0.00||0.015|
|Citharexylum caudatum||4.00||0.058||Ilex macfadyenii||1.00||0.192||Clibadium terebinthinaceum||0.00||0.014|
|Brunfelsia jamaicensis||4.00||0.044||Chaetocarpus globosus||1.00||0.150||Cyathea concinna||0.00||0.010|
|Myrsine acrantha||4.00||0.035||Clusia havetiodes||1.00||0.120||Lobelia martagon||0.00||0.010|
|Ocotea patens||4.00||0.035||Schefflera sciadophyllum||1.00||0.083||Viburnum villosum||0.00||0.010|
|Prunus occidentalis||4.00||0.021||Garrya fadyenii||1.00||0.076||Weinmannia pinnata||0.00||0.010|
|Vernonia pluvialis||3.47||0.000||Viburnum alpinum||1.00||0.063||Ilex sideroxyloides||0.00||0.007|
|Picramnia antidesma||3.47||0.028||Critonia parviflora||1.00||0.060||Pittosporum viridiflorum||0.00||0.007|
|Piper fadyenii||3.47||0.005||Meriania leucantha||1.00||0.040||Ossaea asperifolia||0.00||0.005|
|Xylosma nitida||3.00||0.066||Ilex nitida||1.00||0.020||Dendropanax nutans||0.00||0.002|
|Blakea trinerva||1.00||0.010||Trema floridanum||0.00||0.002|
was the only native species to occur at a similar high density to P.
undulatum. Of the other common species, Maytenus jamaicensis
is another shade-tolerant species common in primary forest; and Myrcianthes
fragrans is a species with an unusually clumped distribution - the
mean density in one plot was 8.05 seedlings m-2 compared with
a mean density in the other HIFE and SIFE plots of 0.21 seedlings m-2.
Recruitment of Clethra occidentalis is almost confined to the lower
stems of Cyathea tree ferns (Newton & Healey 1989), where it
can be dense. All the sub-plots in which seedlings were enumerated occured
in forest that was relatively undisturbed, and densities of several species
were much higher in gaps (Healey 1990). P. undulatum was one of
only four species to have a maximum tree density of over 200 stems per
hectares; Eugenia virgultosa is the commonest tree in the Blue Mountains,
whilst Vaccinium meridionale and especially Lyonia octandra
are abundant in the localised Mor Ridge forest, where both occur typically
as multi-stemmed individuals. Twelve of the 14 species occurring as seedlings
but not as trees were climbers that never reach 3 cm DBH. The maximum density
of saplings (stems >3 m high but <3 cm DBH) of P. undulatum was
0.605 m-2, nearly twice that of Eugenia virgultosa, the
densest native species - this data is not presented in full as saplings
were enumerated in only 59 of the 144 plots.
Table 9. The maximum seedling dominance (M.D.) per sub-plot (in terms of summed heights (m m-2)) of all 89 species occuring as seedlings in at least one of the 692 sub-plots in HIFE or SIFE. The species are arranged in order of decreasing seedling dominance within each column.
|Pittosporum undulatum||52.04||Ilex harrisii||3.02||Eugenia brachythrix||1.82|
|Alchornea latifolia||20.47||Dendropanax arboreus||2.97||Brunfelsia jamaicensis||1.81|
|Koanophyllon hardwarense||16.88||Citharexylum caudatum||2.95||Turpinia occidentalis||1.76|
|Eugenia virgultosa||15.35||Tournefortia glabra||2.93||Daphnopsis americana||1.60|
|Mecranium purpurascens||14.97||Mannetia lygistum||2.84||Passiflora penduliflora||1.60|
|Eugenia monticola||8.01||Myrsine coriacea||2.80||Persea alpigena||1.59|
|Piper arboreum||7.95||Psychotria sloanei||2.80||Ilex macfadyenii||1.54|
|Psychotria corymbosa||7.95||Chaetocarpus globosus||2.75||Ilex obcordata||1.54|
|Guarea glabra||7.63||Clusia havetiodes||2.70||Phyllanthus arbuscula||1.53|
|Maytenus jamaicensis||6.88||Malvaviscus arboreus||2.70||Viburnum alpinum||1.48|
|Acalypha virgata||6.79||Urbananthus critoniformis||2.70||Conostegia montana||1.24|
|Hedyosmum arborescens||6.60||Meriania purpurea||2.64||Odontocline fadyenii||1.19|
|Palicourea alpina||5.78||Cinnamomum montanum||2.62||Haenianthus incrassatus||1.11|
|Smilax balbisiana||5.30||Eugenia marchiana||2.51||Ilex nitida||1.05|
|Clethra occidentalis||5.12||Wallenia calyptrata||2.50||Cestrum hirtum||1.02|
|Ocotea patens||4.87||Picramnia antidesma||2.33||Cionosicys pomiformis||0.67|
|Cassia viminea||4.84||Miconia quadrangularis||2.30||Bidens shrevei||0.60|
|Eugenia harrisii||4.76||Myrsine acrantha||2.23||Gonolobus jamaicensis||0.60|
|Prunus occidentalis||4.66||Critonia parviflora||2.22||Solanum punctulatum||0.56|
|Wallenia fawcettii||4.58||Podocarpus urbanii||2.22||Besleria lutea||0.54|
|Salmea scandens||4.44||Sideroxylon montanum||2.17||Cissampelos pareira||0.51|
|Wallenia crassifolia||4.31||Xylosma nitida||2.16||Blakea trinerva||0.48|
|Piper fadyenii||4.11||Gordonia haematoxylum||2.16||Lobelia assurgens||0.45|
|Sapium harrisii||3.86||Schefflera sciadophyllum||2.15||Vaccinium meridionale||0.39|
|Phenax hirtus||3.75||Brunellia comocladiifolia||1.99||Marcgravia brownei||0.26|
|Symplocos octopetala||3.49||Miconia theaezans||1.95||Garrya fadyenii||0.25|
|Calyptranthes rigida||3.44||Callicarpa ferruginea||1.92||Ilex vaccinoides||0.18|
|Vernonia pluvialis||3.30||Cinchona officinalis||1.90||Gonolobus stapelioides||0.13|
|Myrcianthes fragrans||3.15||Rhamnus sphaerospermus||1.90||Meriania leucantha||0.11|
The maximum dominance of P. undulatum as a seedling can be very great, 52 m m-2, 2.5 times the dominance of the highest native species, Alchornea latifolia. A. latifolia is a species whose germination and recruitment is greatly enhanced by disturbance and after Hurricane Gilbert achieved dominance, over large areas in the more disturbed areas, to a much greater degree than any native species. Koanophyllon hardwarense is a climber that forms clumps, one clump dominating a single sub-plot in SIFE, giving the species such a high value. Eugenia virgultosa is the commonest understorey species and can achieve dominance of the seedling layer except under the densest shade. Mecranium purpurascens is a species that produces suckers, and can achieve high local dominance by that means.
Dominance of trees as expressed
by basal area is less illuminating, as P. undulatum is present in
most plots only as a small tree, because of the early stage of the invasion.
In one plot P. undulatum comprised 68% of the total plot basal area,
though in most plots in heavily invaded forest, larger native trees, possibly
left when the original forest was cleared, dominate in terms of basal area.
An interesting question that we have not addressed in this study is where in the Blue Mountains species diversity is highest. A preliminary inspection of the data suggests it may not always be in primary forest, as sometimes old secondary forest (little invaded by P. undulatum) can have a high diversity. It is possible that the density and diversity of the understorey in secondary forest would decline (perhaps in the short-term only) even without P. undulatum, as the native shade-tolerant species that are dominant in primary forest invade. Some of them (for example Guarea glabra and Dendropanax arboreus) can have crowns that are about as dense as those of P. undulatum trees (T. Goodland, unpublished data). But we do not have sufficient data from old secondary forest, where this re-invasion appears to be happening, to draw firm conclusions.
In the study of the growth form of P. undulatum and native species (section 3.2.2) a visual examination of the data shows that for some species (such as Eugenia virgultosa, Guarea glabra) there seems to be a significant effect of P. undulatum on leaf area, and for one species (E. virgultosa) P. undulatum appears to be affecting mean branch extension, though we have not yet carried out any statistical analyses. There seems to be little effect of P. undulatum adults on crown depth, so overall it is clear that the small P. undulatum trees in the SIFE plots are not yet having a major impact on native regeneration.
There does not appear to be any relationship between the number of P. undulatum and native trees (results not given here). This is probably because when the forest started to regrow following clearance, P. undulatum was not dominant as a species, but the scattered trees that did establish have now lead to dense regeneration of the species, which now seems to be suppressing the growth of smaller native plants.
All the correlations presented here suffer from confounding, and this is particularly true when interpreting the results for individual species. Several species occur commonly as seedlings in primary forest but are rare or absent from secondary forest. We cannot say whether this is due to the effects of P. undulatum or the fact that adult trees of most of the species were eliminated when the original forests were cut down, (21 (84%) of the HIFE plots are in forest that is definitely secondary). The recruitment and growth of P. undulatum has been markedly increased by the past human disturbance in these forests. Also the disturbance created by Hurricane Gilbert has complicated the results of HIFE as the density of advance regeneration at the initial enumeration was probably significantly higher in some plots than would have been the case without the hurricane. Another complication is that all the HIFE plots are on the southern slopes of the Blue Mountains, whilst four out of the 12 SIFE plots were either on the north slopes or Grand Ridge, areas which tend to have a different forest composition, perhaps because of higher rainfall.
Ideally an experimental
approach is needed if the objective is to find out what is limiting the
re-invasion of these secondary forests by primary forest species, planting
seedlings of those common primary forest species absent or rare in secondary
forest, beneath dense stands of P. undulatum and beneath stands
of native trees in univaded secondary forest. The majority of native species
in the Blue Mountains either require disturbance for their recruitment
or benefit from it. Of the 27 most studied species the recruitment of 22
was increased by disturbance (Sugden et al. 1988, Healey 1990, Vernon
1991, Dalling 1992). Therefore the effect of P. undulatum on recruitment
following disturbance is probably crucial to a majority of species.
One way in which species may differ in their response to a low light environment is their ability to position their whole axis towards higher light levels. In very low light levels (usually beneath dense stands of P. undulatum trees) P. undulatum seedlings are often leaning or prostrate. This is probably a sign of stress (indeed where this occurs in the most heavily invaded HIFE plots a number of these seedlings have died-back, and even died) but they do tend to be oriented towards higher light levels. This is occasionally seen in native species, though not so often (T. Goodland, pers. obs.).
We have some intriguing evidence
that the below-ground competitive ability of P. undulatum is very
high, but further research would be needed to provide a clearer picture.
The evidence that we do have comes from measurements of the root system
of six seedlings of P. undulatum and eight native species (Goodland
& Healey, unpublished data). In summary, the root system of P. undulatum
was comparatively extensive, usually shallow, and with individual roots
sometimes longer than the height of the stem. In Australia the root system
of P. undulatum seedlings was highly variable, depending on soil
type (Gleadow & Ashton 1981).
In HIFE following the removal of all existing P. undulatum the recruitment of P. undulatum could be very high. In plot 20 at t0 (before treatment, but after the effects of Hurricane Gilbert) the density of P. undulatum seedlings had been 105.6 m-2 whilst the density of P. undulatum recruits was 155.3 m-2 at t1 and 161.9 m-2 at t2, a combined recruitment density of 317.2 m-2. The t1 P. undulatum recruitment density of 155.3 m-2 compares with a total recruitment of native species of only 6.9 seedlings m-2 in that plot. We do not have data from HIFE or SIFE on seedling recruitment in primary forest so cannot make a quantitative comparison between secondary and primary forest, but the density of recruitment of some species (for example Eugenia virgultosa, Guarea glabra and Prunus occidentalis) can be high (>50 seedlings m-2) in primary forest (T. Goodland, personal observations).
The survivorship of P.
undulatum is also surprisingly high for a species whose recruitment
is so affected by degree of disturbance, although mortality of small seedlings
can be very high. For example, in one sub-plot in HIFE that had experienced
quite severe disturbance by Hurricane Gilbert, then heavy shading by P.
undulatum saplings, of 115 P. undulatum seedlings 20 cm high
in 1991, 109 had died by 1995, a mortality of 94%.
Figure 15. Factors determining the persistence of P. undulatum
As the invasion progresses, the reaction of P. undulatum trees to future hurricanes will become more important. Useful data is now available on the effects of H. Gilbert on P. undulatum and native trees in mostly primary forest (Bellingham 1993). The effect of H. Gilbert was assessed in 91 plots totalling 1.10 ha between February 1989 and August 1990, i.e. 5-23 months after the hurricane; namely the E.V.J. Tanner (Tanner 1977); J.R. Healey plots (Healey 1990); 26 non-bounded plots along a transect in the Mabess River valley and the 16 systematically placed plots of P.J. Bellingham (Bellingham 1993). A total of 5242 native and 53 P. undulatum trees were sampled.
Data on the 47 commonest species were analysed. P. undulatum was one of nine species that had no stems killed by the hurricane. P. undulatum was also one of only five species which had no stems that were completely defoliated and no stems broken. However 11.4% of P. undulatum were uprooted (the ninth highest species percentage, the mean for all species was 0.49%). Bellingham (1993) classified all the species into five categories of resistance to the hurricane according to levels of non-fatal damage and mortality. P. undulatum was placed into the most resistant category, though the relatively small number of stems (53) makes the classification tentative. In contrast to all the other species in the resistant category, P. undulatum is readily recruited into hurricane caused gaps. Because of this, Bellingham (1993) considered the species to have no ecological analogue in the native tree flora.
P. undulatum should
also be considered a resilient species, in the sense that if damaged (for
example, blown down or snapped) it shows a great ability to survive. Trees
that have been blown down often put up many vertical ("epitrophic") sprouts
along the fallen trunk. These sprouts can become very large (>25 cm DBH)
and would indicate a prolonged life perhaps similar to that of the long-lived
native tree Cyrilla racemiflora. Cut stems of P. undulatum
produced a much greater biomass of resprouts than all native species except
macfadyenii. After 27 months P. undulatum produced about ten
times the mean biomass of all native species combined (Healey et al.).
P. undulatum is rather poor at recruiting beneath dense canopies of P. undulatum trees. But, given the high seed production and soil seed bank, and the requirement for only slight disturbance for germination and recruitment, there usually are some P. undulatum seedlings of a range of size classes beneath all but the densest P. undulatum stands. It is likely that these have been recruited after sporadic, usually hurricane-caused, disturbance events.
There are native species able
to grow in less disturbed conditions that P. undulatum, species
that may have a higher chance of growing up beneath mature P. undulatum
trees than P. undulatum itself. As the crowns of large P. undulatum
trees rise above ground level, the light levels on the forest floor seems
to increase (and the crown itself sometimes appears to thin). It is possible
that light levels would be significantly raised beneath a stand of uniformly
large and tall P. undulatum, but we know of no such stands at present,
large P. undulatum trees are still scattered either in otherwise
lightly invaded forest or amongst smaller P. undulatum regeneration.
From what we know it seems highly unlikely that any native shade-tolerant
species could start to replace P. undulatum, though the most shade-tolerant
species such as Eugenia virgultosa or Guarea glabra may be
able to survive in a Blue Mountains completely invaded by P. undulatum.
The future biotic relations of P. undulatum could be very important, and are very unpredictable. For example in the British Isles sycamore (Acer pseudoplatanus) is a common invader of ash woodlands but once it has achieved dominance it fails to regenerate beneath its own canopy, whilst ash does (P. Savill, pers. comm., 1994). A possible explanation for this is that the litter layer builds up beneath a sycamore canopy providing shelter for slugs from frost during the winter; sycamore seedlings are vulnerable to slug damage whilst ash seedlings are not (P. Binggeli, pers. comm.). It is this type of unexpected interaction with native organisms that could provide an effective limit on the density, if not distribution, of P. undulatum. P. undulatum does suffer some herbivory in the Blue Mountains. So far we have identified seven different patterns of damage that we suspect are caused by seven distinct agents (Goodland & Healey 1996). Three of the types of damage were very localised (several square metres) in which all P. undulatum individuals were damaged, suggesting the possibility of future spread. All the responsible pest and pathogen species are most likely to be local "generalist" species and none have lead to such extensive defoliation that death seems likely. It is much too early to say what the ultimate population density of P. undulatum might be, when in equilibrium with native plants, pests and pathogens.
Weedy species usually have a depauperate genetic structure (Burdon & Marshall 1981) and this can be particularly pronounced when introduced to a new location in small numbers because of a "bottleneck effect" (Harper 1977). We think that P. undulatum was introduced to Jamaica in very small numbers, so presumably the population is likely to have a narrow genetic range. We do not know to what extent P. undulatum will change genetically now that it has been introduced to the Blue Mountains.
This publication is an output from a research project partly funded by the United Kingdom Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of DFID. R4742 Forestry Research Programme. The work was co-funded by the Darwin Initiative of the United Kingdom Department of Environment, Transport and the Regions.
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Appendix. Woody plant species occurring in permanent sample plots in the western Blue Mountains.
Status: BME - Endemic to the BMts; JE - Jamaican endemic; N - Native to Jamaica; I - Introduced.
|Code||Full species name||Family||Local name||Status|
|Aca vir||Acalypha virgata L.||Euphorbiaceae||JE|
|Alc lat||Alchornea latifolia Sw.||Euphorbiaceae||Womanwood||N|
|Bes lut||Besleria lutea L.||Gesneriaceae||N|
|Bid shr||Bidens shrevei Britton||Asteraceae||JE|
|Bla tri||Blakea trinerva L.||Melastomataceae||N|
|Boc fru||Bocconia frutescens L.||Papaveraceae||John Crow bush||N|
|Boe cau||Boehmeria caudata Sw.||Urticaceae||N|
|Bru com||Brunellia comocladiifolia Humb. & Bonpl.||Brunelliaceae||Sumach||N|
|Bru jam||Brunfelsia jamaicensis (Benth.) Griseb.||Solanaceae||BME|
|Cal fer||Callicarpa ferruginea Sw.||Verbenaceae||N|
|Cal rig||Calyptranthes rigida Sw.||Myrtaceae||N|
|Cas vim||Cassia viminea L.||Caesalpiniaceae||Treeribs; fourvine||JE|
|Ces hir||Cestrum hirtum Sw.||Solanaceae||N|
|Cha glo||Chaetocarpus globosus (Sw.) Fawcett & Rendle||Euphorbiaceae||N|
|Cin mon||Cinnamomum montanum (Sw.) Bercht.& Presl.||Lauraceae||Wild cinnamon||N|
|Cin off||Cinchona officinalis L.||Rubiaceae||I|
|Cin pub||Cinchona pubescens Vahl.||Rubiaceae||I|
|Cio pom||Cionosicys pomiformis Griseb.||Cucurbitaceae||Duppy apple||JE|
|Cis par||Cissampelos pareira L.||Menispermaceae||N|
|Cit cau||Citharexylum caudatum L.||Verbenaceae||Fiddlewood||N|
|Cle ale||Clethra alexandra Griseb.||Clethraceae||BME|
|Cle occ||Clethra occidentalis (L.) Kuntze||Clethraceae||Soapwood||N|
|Cle the||Cleyera theaoides (Sw.) Choisy||Theaceae||N|
|Cli ter||Clibadium terebinthinaceum (Sw.) DC.||Asteraceae||N|
|Clu hav||Clusia havetiodes (Griseb.) Planch. & Triana||Guttiferae||Fan fan; wild mango||JE|
|Con mon||Conostegia montana (Sw.) DC.||Melastomataceae||BME|
|Cri par||Critonia parviflora DC.||Asteraceae||JE|
|Cya con||Cyathea concinna (Baker ex Jenman) Jenman||Cyatheaceae||BME|
|Cya fur||Cyathea furfuracea Baker||Cyatheaceae||N|
|Cya pub||Cyathea pubescens Mettenius ex Kuhn||Cyatheaceae||BME|
|Cya woo||Cyathea woodwardioides Kaulf.||Cyatheaceae||N|
|Cyr rac||Cyrilla racemiflora L.||Cyrillaceae||Beetwood||N|
|Dap ame||Daphnopsis americana (Mill.) J.R.Johnston||Thymelaeaceae||N|
|Den arb||Dendropanax arboreus (L.) Decne & Planch.||Araliaceae||Manjack||N|
|Den nut||Dendropanax nutans (Sw.) Decne & Planch.||Araliaceae||Manjack||BME|
|Den p/n||Dendropanax pen/nut||Araliaceae|
|Den pen||Dendropanax pendulus (Sw.) Decne & Planch.||Araliaceae||Manjack||JE|
|Dur ere||Duranta erecta L.||Verbenaceae||N|
|Eug alp||Eugenia alpina (Sw.) Willd.||Myrtaceae||BME|
|Eug bra||Eugenia brachythrix Urban||Myrtaceae||BME|
|Eug har||Eugenia harrisii Krug & Urban||Myrtaceae||Rodwood||JE|
|Eug mar||Eugenia marchiana Griseb.||Myrtaceae||JE|
|Eug mon||Eugenia monticola (Sw.) DC||Myrtaceae||Rodwood||N|
|Eug vir||Eugenia virgultosa (Sw.) DC||Myrtaceae||Rodwood||JE|
|Gar fad||Garrya fadyenii Hook.||Garryaceae||N|
|Ges alp||Gesneria alpina (Urb.) Urb||Gesneriaceae||BME|
|Gon jam||Gonolobus jamaicensis Rendle||Asclepiadaceae||BME|
|Gon sta||Gonolobus stapelioides Desv.||Asclepiadaceae||JE|
|Gor hae||Gordonia haematoxylum Swartz||Theaceae||Bloodwood||JE|
|Gua gla||Guarea glabra Vahl||Meliaceae||Broadleaf: alligator wood||JE|
|Hae inc||Haenianthus incrassatus (Sw.) Griseb||Oleaceae||BME|
|Hed arb||Hedyosmum arborescens Sw.||Chloranthaceae||Headache bush||N|
|Hed nut||Hedyosmum nutans Sw.||Chloranthaceae||Headache bush||N|
|Het umb||Heterotrichum umbellatum (Mill) Urb.||Melastomataceae||N|
|Ile har||Ilex harrisii Loes.||Aquilfoliaceae||JE|
|Ile mac||Ilex macfadyenii (Walp.) Rehder||Aquilfoliaceae||Black tea||N|
|Ile nit||Ilex nitida (Vahl) Maxim||Aquilfoliaceae||N|
|Ile obc||Ilex obcordata Sw.||Aquilfoliaceae||BME|
|Ile sid||Ilex sideroxyloides (Sw.) Griseb.||Aquilfoliaceae||N|
|Ile vac||Ilex vaccinoides Loes.||Aquilfoliaceae||BME|
|Jun luc||Juniperus lucayana Britton||Cuppressaceae||Juniper||N|
|Koa har||Koanophyllon hardwarense (Proctor ex C.Adams) R.King & H.Robinson||Asteraceae||BME|
|Lob ass||Lobelia assurgens L.||Campanulaceae||Fat & borrow; milkbush||N|
|Lob mar||Lobelia martagon (Griseb.) Hitchc.||Campanulaceae||BME|
|Lyo jam||Lyonia jamaicensis (Sw.) D.Don||Ericaceae||N|
|Lyo oct||Lyonia octandra (Sw.) Griseb.||Ericaceae||JE|
|Mal arb||Malvaviscus arboreus Cav.||Malvaceae||N|
|Man lyg||Mannetia lygistum (L.) Sw.||Rubiaceae||BME|
|Mar bro||Marcgravia brownei (Triana & Planch.) Krug & Urban||Marcgraviaceae||JE|
|May jam||Maytenus jamaicensis Krug & Urban||Celastraceae||Sweetwood||N|
|Mec pur||Mecranium purpurascens (Sw.) Triana||Melastomataceae||JE|
|Mel Bl1||Unidentified Melastome species in block 1 of HIFE||Melastomataceae|
|Mer leu||Meriania leucantha (Sw.) Sw.||Melastomataceae||JE|
|Mer pur||Meriania purpurea (Sw.) Sw.||Melastomataceae||N|
|Met atr||Metastelma atrorubens Schltr.||Asclepiadaceae||N|
|Met har||Metastelma harrisii Schltr.||Asclepiadaceae||BME|
|Mic dod||Miconia dodecandra (Desr.) Cogn.||Melastomataceae||N|
|Mic qua||Miconia quadrangularis (Sw.) Naud.||Melastomataceae||N|
|Mic rig||Miconia rigida (Sw.) Triana||Melastomataceae||N|
|Mic the||Miconia theaezans (Bonpl.) Cogn.||Melastomataceae||N|
|Mik max||Mikania maxonii Proctor||Asteraceae||BME|
|Myr acr||Myrsine acrantha Krug & Urban||Myrsinaceae||N|
|Myr cer||Myrica cerifera L.||Myricaceae||Waxwood||N|
|Myr cor||Myrsine coriacea (Sw.) R.Br. ex Roem.& Schult.||Myrsinaceae||N|
|Myr fra||Myrcianthes fragrans (Sw.) McVaugh||Myrtaceae||N|
|Oco pat||Ocotea patens (Sw.) Nees||Lauraceae||Sweetwood||N|
|Odo fad||Odontocline fadyenii (Griseb.) B.Nord.||Asteraceae||JE|
|Oss asp||Ossaea asperifolia (Naud.) Triana||Melastomataceae||N|
|Pal alp||Palicourea alpina (Sw.) DC.||Rubiaceae||N|
|Pas pen||Passiflora penduliflora Bert. ex DC.||Passifloraceae||N|
|Per alp||Persea alpigena (Sw.) Spreng.||Lauraceae||Wild Pear||JE|
|Phe hir||Phenax hirtus (Sw.) Wedd.||Urticaceae||N|
|Phy arb||Phyllanthus arbuscula (Sw.) J.F. Gmelin||Euphorbiaceae||N|
|Pic ant||Picramnia antidesma Sw.||Simaroubaceae||N|
|Pip arb||Piper arboreum Aublet||Piperaceae||N|
|Pip fad||Piper fadyenii C.DC.||Piperaceae||JE|
|Pit und||Pittosporum undulatum Vent.||Pittosporaceae||Wild coffee; mock orange||I|
|Pit vir||Pittosporum viridiflorum Sims vel.aff.||Pittosporaceae||Wild coffee; mock orange||I|
|Pod urb||Podocarpus urbanii Pilger||Podocarpaceae||Fineleaf; yucca||N|
|Pru occ||Prunus occidentalis Sw.||Rosaceae||N|
|Psy cor||Psychotria corymbosa Sw.||Rubiaceae||JE|
|Psy slo||Psychotria sloanei Urban||Rubiaceae||BME|
|Rha sph||Rhamnus sphaerospermus Sw.||Rhamnaceae||Buckthorn||N|
|Sal sca||Salmea scandens (L.) DC.||Asteraceae||N|
|Sap har||Sapium harrisii Urban ex Pax||Euphorbiaceae||Milkwood||JE|
|Sch inv||Schradera involucrata (Sw.) K.Schum.||Rubiaceae||JE|
|Sch sci||Schefflera sciadophyllum (Sw.) Harms||Araliaceae||Old name=woman wood||JE|
|Sid mon||Sideroxylon montanum (Swartz) Pennington||Sapotaceae||Bulletwood||JE|
|Smi bal||Smilax balbisiana Kunth||Smilacaceae||Chainy root||N|
|Smi dom||Smilax domingensis Willd.||Smilacaceae||N|
|Sol pun||Solanum punctulatum Dunal||Solanaceae||BME|
|Sym oct||Symplocos octopetala Sw.||Symplocaceae||JE|
|Tou gla||Tournefortia glabra L.||Boraginaceae||N|
|Tre flo||Trema floridanum Britton||Ulmaceae||N|
|Tur occ||Turpinia occidentalis (Sw.) G.Don||Staphyleaceae||Candlewood||N|
|Urb cri||Urbananthus critoniformis (Urban) R.King||Asteraceae||BME|
|Ure ela||Urera elata (Sw.) Griseb.||Urticaceae||JE|
|Vac mer||Vaccinium meridionale Sw.||Ericaceae||Bilberry||N|
|Ver plu||Vernonia pluvialis Gleason||Asteraceae||BME|
|Vib alp||Viburnum alpinum Macf. ex Britton||Caprifoliaceae||Blackwattle||N|
|Vib vil||Viburnum villosum Sw.||Caprifoliaceae||N|
|Wal cal||Wallenia calyptrata Urban||Myrsinaceae||BME|
|Wal cra||Wallenia crassifolia Mez||Myrsinaceae||BME|
|Wal faw||Wallenia fawcettii Mez||Myrsinaceae||BME|
|Wei pin||Weinmannia pinnata L.||Cunoniaceae||N|
|Xyl nit||Xylosma nitida (Hellenius) I.Gray ex Griseb.||Flacourtiaceae||JE|