The importance of woody plant introduction for forest trees improvement.
The history of woody plant introduction is closely linked with that of transportation and the European exploration of the planet (16th-19th centuries). Each colonial power established major botanical gardens and experimental stations in various parts of the world. By the 20th century, the purpose of introductions shifted from food plants to timber and other species yielding non-agricultural products. Finally, during the latter part of the 20th century the importance of ornamental species increased dramatically, especially to the more developed and wealthier regions. Over the past two centuries many species have started to spread in their introduced ranges. Until relatively recently the majority of introduced woody species have been highly beneficial, if not essential, to humanity's development, but now ever-increasing numbers of species are becoming detrimental to the maintenance of the earth's biodiversity and to the well-being of human societies. Throughout the 19th and specially in 20th century the large-scale planting of trees for timber production has been one of the main reasons for the introductions of a large number of species, especially conifers e.g. Pinus, Picea, Pseudotsuga and Poplars and Willows species. By using methods of mass and individual selection and by establishing of provenances tests, as well as by half and full sib lines of selected exotics test trees, genetical potential productivity and adaptability of introduced species have been tested in numerous experimental plots in areas where introduced have been done. This paper attempts to unravel the relationships between humans and woody plants by looking at the changes in the introduction of species, the way they are perceived by different human groups and the impact these non native species have on forest trees improvement and other human activities.