TREE MIGRATION/SPREADING

Tree migration describes the spread of tree populations across a landscape. Land plants, such as trees, are not able to move in response to a change in environmental conditions and their survival options are either to adapt or to migrate; if they fail to do either, they will undergo extirpation or extinction. Migration of plant populations refers to their spreading outside their former geographical range, via propagules (sexual or vegetative), which establish and reach maturity under favourable environmental conditions. For any given species, migration tends to occur in response to an environmental change, which results in either unfavourable climatic conditions locally or suitable climatic conditions in an area previously unfavourable. As conditions become favourable for colonisation outside the range of a population, new individuals will establish themselves by dispersal into the area or spread by vegetative growth. Under these conditions, an advancing range margin will develop. In contrast, if unfavourable conditions develop and established plants die, they will not be replaced by further seedlings or vegetative spread. Under these conditions, a retreating range margin develops. The relative speeds of the retreating and advancing range margins of a plant population in a changing environment determine whether survival or extinction of that population occurs.

Due to the long generation time of most tree species (decades to centuries), tree migration cannot be observed directly and retrospective studies are required. Various approaches can be used to examine the spread of tree populations over time, including the approaches used in palaeoecology (e.g. obtaining pollen records from a number of sites distributed across a region), dendroecology (e.g. the examination of tree remains, which can indicate either when a species first became established within a region or the distribution of that species relative to its current range), historical evidence (e.g. aerial photography showing range expansion or contraction), and ecological studies (e.g. the examination of seedling dynamics at the migration front, and of established trees within the range boundaries). Only the stages of migration (the establishment and growth of seedlings and saplings away from the parent population) can be studied by ecologists and foresters. The study of plant migration is essential to aid in the prediction of the abundance and distribution of tree populations in response to future global warming.