Benefits and risks of biotic exchange between Eucalyptus plantations and native Australian forests

Australia is unique in having two highly diverse plant genera, Eucalyptus a nd Acacia, that dominate the vegetation on a continent-wide scale. The rece nt shift in plantation forestry away from exotic Pinus radiata to native Eu calyptus species has resulted in much more extensive exchange of biota betw een native forest and plantation ecosystems than exchange in the past with plantations of exotic species. Growing numbers of hectares are being plante d to Eucalyptus globulus across Australia, and plantations are providing re sources and corridors for native biota. The present paper focuses on both t he benefits and risks of having large-scale forestry plantations of native species that are closely related to dominant native taxa in local forests. At least 85 species of insects have been recorded as pests of Eucalyptus pl antations around Australia; the vast majority of these have been insects us ing the same host species, or closely related taxa, in native forests. Plan tations of native species may also benefit from closely related local fores ts through the presence of: (i) the diverse array of ectomycorrhizal fungi favourable for tree growth; (ii) natural enemies harboured in native habita ts; and (iii) recruitment of other important mutualists, such as pollinator s. Exchanges work in two directions: plantations are also likely to influen ce native forests through the large amount of insect biomass production tha t occurs in outbreak situations, or through the introduction or facilitatio n of movements for insects that are not native to all parts of Australia. F inally, older plantations in which trees flower may exchange genes with sur rounding forest species, given the high degree of hybridization exhibited b y many Eucalyptus species. This is an aspect of exchange for which few data have been recorded. In summary, because of Australia's unique biogeography , plantation forestry using eucalypt species entails exchanges with natural habitats that are unparalleled in scale and diversity in any other part of the world. More exchanges are likely as plantations occupy greater area, a nd as the time under cultivation increases.