FORESTRY TREES AS INVASIVE ALIENS

Some alien tree species used in commercial forestry and agoforestry ca use major problems as invaders of natural and seminatural ecosystems. The magnitude of the problem has increased significantly over the past few decades, with a rapid increase in afforestation and changes in la nd use. Trends can be explained by analyzing natural experiments creat ed by the widespread planting of a small number of species in differen t parts of the world. The species that cause the greatest problems are generally those that have been planted most widely and for the longes t time. The most affected areas have the longest histories of intensiv e planting. Pinus spp. are especially problematic, and at least 19 spe cies are invasive over large areas in the southern hemisphere, where s ome species cause major problems. The most invasive Pinus species have a predictable set of life-history attributes, including low seed mass , short juvenile period, and short interval between large seed crops. Pine invasions have severely impacted large areas of grassland and scr ub-brushland in the southern hemisphere by causing shifts in life-form dominance, reduced structural diversity, increased biomass, disruptio n of prevailing vegetation dynamics, and changing nutrient cycling pat terns. The (unavoidable) negative impacts of forestry with alien speci es are thus spilling over into areas set aside for conservation or wat er production. There is an urgent need to integrate the various means available for reducing the negative impacts of current invaders and to implement protocols to regulate the translocation of species that are known to be invasive.