Assessing the extent and the environmental drivers of Eucalyptus globulus wildling establishment in Portugal: results from a countrywide survey

Tasmanian blue gum (Eucalyptus globulus) has been increasingly used in forestry outside its native range, and is nowadays one of the most important pulpwood species in the world. E. globulus has great economic importance in many countries, and in Portugal it has recently become the most widespread tree species. However, there is also an increasing concern about the potential ability of eucalypts to naturally establish from seed (wildling establishment), because of negative ecological and economic impacts this could cause. The natural establishment of this fast-growing exotic species may have undesirable consequences, but little is known about its distribution, or which are the factors influencing its occurrence. In order to investigate these issues, we characterized wildling occurrence and abundance along 3111 roadside transects adjacent to eucalypt plantations distributed throughout continental Portugal. Eucalypt wildlings were found in 60 % of the sampled transects and across all natural regions, with densities ranging from 0 to 10,000 plants ha⁻¹ (mean = 277 plants ha⁻¹). The potential influence of environmental variables on wildling establishment from plantations was assessed using boosted regression trees. The abundance of wildlings was found to be primarily affected by precipitation and distance from the sea (used as a surrogate of thermal amplitude), although topography, frost occurrence and soil type also played a significant role. Plant density peaked at around 1500 mm of annual precipitation and it decreased with both decreasing and increasing precipitation, reaching the lowest values below 800 mm and above 2400 mm. Eucalypt wildlings were also more abundant in areas with milder temperatures, namely closer to the sea (with lower thermal amplitude) and with lower number of frost days. Finally, plant density also seemed to be favoured in areas with intermediate elevation, higher slope and with certain soil types (namely Cambisols and Podzols). Knowing the regions with higher wildling density and understanding the factors influencing plant establishment may help managers to establish and prioritize eventual control plans in regions with higher probability of recruitment.