Invasive potential of Eucalyptus globulus and Pinus radiata into native eucalypt forests in Western Australia
Place of Publication
School of Science
Tree plantations are an important cause of plant invasions worldwide, being a serious concern for biodiversity conservation. Pinus and Eucalyptus are the two most important genera for the forestry industry worldwide, with Eucalyptus globulus and Pinus radiata among the most widespread and invasive. In this study we analyse the establishment of wildlings of these two species in native eucalypt forests (karri and jarrah forests) of SW Australia, a global biodiversity hotspot. We selected boundaries between plantations and adjoining native forests (26 for E. globulus and 25 for P. radiata) and established in each a plot 100 m long (aligned to outer edge of forest) × 100 m or 50 m deep into the native forest (for boundaries with P. radiata and E. globulus plantations, respectively), that was thoroughly searched for wildlings. Eucalyptus globulus wildlings were only found in 3 of the 26 boundaries studied, 6 individuals in total, all within first 10 m from the forest edge. Pinus radiata wildlings were found in all boundaries and at all distances inspected. Wildling density was greater in jarrah forest compared to karri forest (mean 70.7 and 24.7 wildlings ha−1; respectively); it was negatively correlated with understorey cover (r = −0.501, F = 7.72, p = 0.011) and the density of older wildlings (20 + years) was positively correlated with time since plantation establishment (r = 0.512; F = 8.16; P = 0.009). Invasion of native eucalypt forests by alien E. globulus does not seem currently of concern in SW Australia. The scheduled short rotations (8–10 years), which means that trees are mostly harvested before reaching their full reproductive potential, lower dispersal ability than P. radiata and the customary establishment of cleared margins around plantations (10 m wide on average) all contribute to reduce the propagule pressure in native forests and with it the invasion potential. Nevertheless, attention should be paid to abandoned plantations left unharvested for longer, which may increase the invasion risks. In contrast, P. radiata plantations do constitute a threat to native vegetation for the invasion capacity of this species, as indicated by the presence of wildlings in all native forests studied, together with the relatively high density of old trees with high reproductive capacity in the forests interior. Awareness about wilding pine spread is urgently needed in SW Australia, in order to minimise the risks.