Abstract:

Many examples have proved that exotic trees can produce great ecological,economic and social benefits.However,biological invasion of exotic trees has drawn increasing attention.The potential risk of biological invasion of exotic trees should not be overlooked in any tree introduction program.Correct understanding of the concepts of exotics,invasiveness of exotic trees and risk of invasion is needed to develop sound strategies for preventing and controlling potential biological invasion of exotic trees.Many studies have been carried out in many countries to assess invasiveness of exotic plants,patterns of biological invasion and consequences of invasion,to predict potential invasion risk and to develop strategies for prevention and control of invasive alien plants.However,much fewer studies were concentrated on exotic tree species due to scarce literature available on invasive trees.Similar to alien invasive plants,invasive exotic trees can have negative impacts in multiple aspects on environment,most severely on biodiversity and ecosystem functions,which are usually in large-scale,long term and irreversible.Based on literature review of 1 211 exotic tree species,443 species are invasive or potentially invasive,accounting for 39% of the total number.282 invasive tree species have been used in forestry operations.Invasion usually occurs in steps of introduction,establishment and rapid spread.Risk assessment should be conducted for any future tree introduction program.Trees have much longer life cycle compared to other types of biological organisms such as insects and grasses,therefore the time-lag of invasion is long,making it more difficult to predict the potential of invasion.Moreover,most of exotic trees have great values for its timber and can be utilized once found to be invasive.These unique features of exotic trees should be considered in developing strategies for invasion control.Criteria and indicators should be developed to assess invasiveness and ecological impacts of exotic trees.Risk assessment of biological invasion should be conducted prior to any tree introduction.Invasiveness can be assessed according to the biological characteristics of alien tree species.However,ecological impacts can only be assessed through long-term monitoring and evaluation.

Key words: biological invasion, exotic tree species, ecological impacts