Ecology and genetics of tree invasions:: from recent introductions to Quaternary migrations

The use of fossil sediments and genetic markers has allowed reconstruction of range shifts of forest trees induced by past climatic changes. Independently, studies of the ecology and population biology of human-induced tree invasions have started to appear. Here, we review the literature on the ecology and the genetics of past and contemporary invasions of forest trees and argue that population research dealing with past (natural) or with ongoing (human-induced) invasions should be better integrated. In particular, we suggest using identical terms to describe artificial and natural invasions, as this should help integrate the two fields. We then discuss the modalities of introduction of tree species, the life history traits associated with invasiveness, the importance of long-distance seed dispersal, the establishment of small populations, the role of biotic interactions and the specific contribution of palaeoecology to the study of tree invasions. Next, we discuss genetic and evolutionary aspects, including the genetic characteristics of invasive woody species, the genetic consequences of dispersal (founder effects and admixture), shifts in dispersal-related traits and in mating system and the importance of hybridization and introgression during invasion. We then review models of the genetic consequences of tree invasions, examine the possibility for rapid adaptation of trees to new environmental conditions and discuss the relative importance of environmental versus genetic factors in controlling tree invasions. We conclude by proposing solutions for two central "genetic paradoxes" of invasions. The first one is the necessity for invasive species to adapt to new conditions, which seems difficult to reconcile with the expected loss of diversity through founder events. The second is the common observation of invasive species outcompeting native ones, which does not seem to fit with the belief in widespread local adaptation. (C) 2004 Elsevier B.V. All rights reserved.