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Accueil du site → Doctorat → Australie → The ecological impacts of invasive Pinus radiata in eucalypt vegetation : pattern and process

University of Sydney (2008)

The ecological impacts of invasive Pinus radiata in eucalypt vegetation : pattern and process

Williams, Moira Caroline

Titre : The ecological impacts of invasive Pinus radiata in eucalypt vegetation : pattern and process

Auteur : Williams, Moira Caroline

Université de soutenance : University of Sydney.

Grade : Doctor of Philosophy PhD 2007

Résumé partiel
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 vi 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

Subject words : propagule pressure • time-lag • litterfall • recruitment • fire • spatial variation • landscape

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Page publiée le 6 mars 2009, mise à jour le 16 juillet 2017