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| Title: | The ecological impacts of invasive Pinus radiata in eucalypt vegetation: pattern and process |
| Authors: | Williams, Moira Caroline |
| Keywords: | propagule pressure
time-lag
litterfall
recruitment
fire
spatial variation
landscape |
| Issue Date: | 20-Aug-2007 |
| Publisher: | School of Biological Sciences. University of Sydney. |
| Abstract: | Early recognition of plant invaders is key to their successful management. Yet
knowledge of the ecological impacts of species before they become widespread is poor.
This thesis examines the ecology of invasive Pinus radiata, a species which is known to
spread from introduced plantings in Australia but is currently a low profile invader.
Pinus invasions are considered major ecological problems in New Zealand and South
Africa where wildlings are beginning to dominate natural areas and suppress native
vegetation. Invasion success elsewhere and the large softwood estate in Australia
suggest that pines may begin to dominate native eucalypt forests bordering large
commercial plantations. This research focused on three components of impact of P.
radiata; extent, abundance and effect per individual. The borders of 29 P. radiata
plantations in NSW were surveyed in order to quantify the current level of invasion and
to identify factors facilitating pine spread. Of particular interest was the role of
propagule pressure, vegetation type and fire in the invasion process.
The area of land in NSW currently invaded by P. radiata was estimated at
almost 4 500 ha, although this is likely to be an underestimate due to an inability to
detect wildlings (self-sown pines) at long distances from the plantation. Twenty six of
the 29 plantations produced wild pines, however most of the sites are in the very early
stages of invasion. Noticeable wildling populations were recorded at nine sites
indicating that P. radiata is capable of establishing within native vegetation. Pine
spread was most severe in the world heritage listed Blue Mountains region where pine
densities reached up to 2000 per hectare in areas adjacent to the plantation and isolated
pines were recorded up to 4 km from the source. The presence of isolated pines within
intact native vegetation suggests that disturbance is not required for pine establishment
in forested environments. Furthermore, high pine emergence and survival rates in
eucalypt woodland and evidence of self reproduction by wildlings suggest that in the
absence of adequate control measures pines may become established invaders in the
Australian landscape.
While low levels of current invasion at many sites hindered the ability to
examine the factors facilitating invasion some variables that appear to be driving pine
success were identified. At the landscape scale plantation size and residence time were
significant predictors of the level of invasion at a site. Areas of native vegetation
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adjacent to plantations less than 40 years experienced very low levels of invasion
suggesting a lag period between plantation establishment and invasion. However, pines
with diameters up to 60 cm were observed growing adjacent to plantations younger than
40 years implying that the first colonisers are capable of establishing soon after
plantation trees become reproductive. Propagule pressure was also found to have a
strong influence on invasion success on a smaller scale manifesting in a significant
positive relationship between the age of a plantation compartment and the likelihood of
invasion.
A negative relationship between plantation size and level of invasion was a
surprising result and was influenced by just two large sites that happened to be located
in areas of high rainfall. All sites receiving more than 1300 mm annual rainfall
experienced low levels of invasion suggesting that this is a limiting factor for pine
spread in NSW. There were significant differences in the level of invasion between
vegetation types implying that some communities are more susceptible to invasion.
Patterns of spread confirmed ideas regarding the facilitative effect of disturbance in the
invasion process and the resistance of wet sclerophyll forest to invasion in Australia. An
absence of wildlings in cleared land and areas of remnant bushland was attributed to
high levels of grazing pressure. Wind direction did not appear to influence the
distribution of pines close to the plantation, but evidence of long distance wind dispersal
of pines was provided by an investigation of pine spread from the air at one site where
large pines were found growing 10 km downwind from a mature plantation.
Fire was found to have both a positive and negative influence on the invasion
process. High intensity wildfires are capable of destroying large pines with diameters
exceeding 50 cm. However, fire can stimulate seed release from cones resulting in large
post-fire recruitment pulses. Seedling densities of up to 3050 per hectare were recorded
almost 3 years after wildfire, suggesting that follow up control prior to recruits reaching
coning age, i.e. within 5 years, would be beneficial. Surveys of wildling pines exposed
to low intensity hazard reduction burns suggest that the majority of pines greater than 3
m in height and with a diameter of more than 10 cm will survive the fire. Low intensity
prescribed fires that are carried out after pines have reached this size will fail to control
wildling populations.
To examine the influence of P. radiata once it has established in the native
community this study focused on two mechanisms of impact, the addition of pine litter
and increased shade due to an increase in canopy cover. Collection of pine litterfall
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within an invaded eucalypt woodland over a 2 year period recorded rates of up to 1400
kg/ha/year in the most heavily invaded area with a pine basal area of 11.3m2/ha. More
than 70 % of pine litter fell directly below the pine canopy suggesting that the most
severe litter effects will be limited to these areas. Glasshouse and field experiments
were conducted to examine the influence of this increased litter load on the emergence
of P. radiata and two native species. Recruitment of native plant species was impeded
by litter levels of 6000 kg/ha, the equivalent of approximately 4 years of pine litterfall.
Both P. radiata and the two natives responded similarly to pine and eucalypt litter
suggesting the two litter types are influencing the recruitment phase equally. However,
where pines are added to the system, increased litterfall rates could potentially result in
the doubling of the litter load and hence a greater barrier to seedling establishment. Pine
invaded eucalypt woodlands are also subjected to three fold increases in canopy cover.
Trends in reduced emergence of native species under a pine canopy suggest that the
addition of pines to eucalypt forests is likely to have a negative influence on native
recruitment and may result in a shift towards a shade tolerant community. However,
reversal of trends in emergence below pine canopy between seasons implies that
quantifying invasion impacts requires a consideration of temporal variation.
Increased levels of disturbance, forest fragmentation and an increasing pine
estate are likely to lead to the infestation of new areas. Furthermore the lag phase
associated with pine spread means that even if no new plantations are established the
number of invasion events will increase. This study has identified a number of risk
factors that can be used to guide plantation establishment and the management of
invasion events. Minimising disturbance at plantation borders and increasing the ‘no
planting’ zone will help to reduce the impacts of pines. Where possible new plantations
should be established upwind of cleared land or at least, wet sclerophyll forest. Frequent
monitoring of the borders of plantations yet to source invasions, particularly those
greater than 40 years of age, will help identify problem areas before control becomes
difficult and costly. Maps of the 29 plantations marked with areas of pine infestation
will help prioritise sites for control and provide base level knowledge for future
monitoring of pine spread. Stringent legislation that binds plantation managers to
control wildlings beyond their boundaries is critical for the effective management of
pine invasions.
With infinite numbers of invaders and limited funds to dedicate to their control,
a method of triaging species for management is critical. This is particularly difficult
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when information is typically biased towards invaders that are already widespread. By
focussing on the ecological impacts of invaders it becomes possible to rank species on
the basis of the threat they pose to native communities. Ecological research is capable of
providing the knowledge to quantify invasion impacts and must remain at the centre of
policy decisions. |
| Description: | Doctor of Philosophy (PhD) |
| URI: | http://hdl.handle.net/2123/2761 |
| Appears in Collections: | Sydney Digital Theses
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