Understanding the naturalization of Eucalyptus globulus in Portugal: a comparison with Australian plantations

Despite the potential utility of a biogeographical approach to understanding the naturalization of exotic species, studies using this approach are scarce. <em>Eucalyptus globulus</em> is an economically important Australian tree species that has become naturalized in a number of countries where it was introduced. Portugal is an ideal territory to study the naturalization of <em>E. globulus</em> owing to: a long introduction history, the antipodal location compared to Australia and the large cultivated area. Wildling density was assessed in 116 <em>E. globulus</em> plantations in central Portugal through 213 transects established along plantation borders. Boosted regression trees were used to model the influence of plantation-scale variables. Results from this survey were compared with data obtained in plantations from seven Australian regions, where a similar sampling protocol had been used. In Portugal, wildlings were more abundant in plantations that were: located in moist aspects, coppiced, with older tree stems and corresponding to intermediate site growth indexes. The overall density (127 plants ha⁻¹) was 14.9 times higher than in the Australian estate, but this ratio was reduced to 3.1 in a more comparable subset of unburnt, first rotation plantations. A generalized linear model fitted using a dataset combining the two surveys showed that country influenced wildling density, together with plantation rotation and stem age. These results provide insights into the naturalization of a widely cultivated tree species, pointing to a fundamental role of the introduction history, possibly acting along with the biogeographical characteristics of the introduced range.