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iForest -Biogeosciences and Forestry
IntroductionDuring the early 19 th century, the practice of planting non-native tree species in Lithuania was initiated with the primary goal of increasing wood productivity. It was unknown at the time certain species would successfully undergo naturalization, and subsequently exhibit a widespread geographic distribution, threatening natural ecosystems. Vitousek & Walker (1989) characterized the alterations of the community stability and structure due to non-native tree species, which successfully proliferated and became invasive community members, limiting native plant growth (Lichstein et al. 2004). Boxelder (Acer negundo L.), black locust (Robinia pseudoacacia L.), and black cherry (Prunus serotina Ehrh.) are included in the list of invasive species in Lithuania, and additional alien species are currently being considered for invasive status. One of the more problematic non-native species in Lithuania is the sycamore maple (Acer pseudoplatanus L., Sapindaceae), characterized by rapid growth (when grown on suitable sites) and potentially high timber prices (Hein et al. 2009). Sycamore maple is mostly a central-European, mountain species reaching its northeastern range limit in Poland. Sycamore is rarely found in pure stands covering small areas on steep slopes on limestones where it forms its own communities (Acerion pseudoplatani alliance -Jones 1945). More frequently it grows in mixed broad-leaved forests of the Carpinion and Alno-Padion alliances (Boratynski 1999). In Lithuania, the species grows in fertile moist sites and in secondary forests. Moreover, it is shade and frost tolerant (Navasaitis 2004), and in naturally regenerated stands often grows mixed with ash (Binggeli 1992). Typically, the species begins bearing fertile seeds at approximately 20 to 30 years of age (Burschel & Huss 1997, Navasaitis 2004.Sycamore maple is well adapted to the current climatic conditions of central Europe, and is expected to expand beyond its current distributional limits (Kölling & Zimmermann 2007, Kölling 2007. This species has become naturalized in many countries, including Spain, Turkey, Ireland, Sweden, North and South America, India, and New Zealand (Binggeli 1992, Rusanen & Myking 2003. In Lithuania, sycamore maple may reach up to 24 m of height and 1 m of stem diameter (Januškevičius et al. 2006), and is mostly found in parks and gardens, with only few forest plantations.The question we pose is whether sycamore maple may be considered an invasive nonnative tree in Lithuania. Williamson & Fitter (1996) estimated that only approximately 10% of naturalized plant species become invasive and produce significant economic and ecological impacts. Historical records indicated sycamore maple was introduced to the Vilnius University Botanical Garden in 1802 (Skridaila 2001), and since then it readily spreads into local forest stands. Informed decisions must be made to control invasive trees in forests, which requires establishing the relative importance of each alien tree.The principal...