Forest migration is the movement of large seed plant dominated communities in geographical space over time.
The emphasis of forest migration is placed on the movement of the populations that make up the forest community. Though an individual tree is permanently fixed in a location, tree populations may migrate over the landscape through successful dispersal and establishment into new regions and/or a lack of regeneration in a portion of its previous habitat range over the course of generations. Tree migration is controlled by two overlying forces: environmental suppression and dispersal capacity of the population by seed. Though the true rate of forest expansion is difficult to quantify, efforts are being made to evaluate and predict past, current, and future rates and extents of forest movements.
Forces controlling forest migrations
Forest migration happens by the occurrence of two processes: population expansion into new habitat range and population retreat from historical habitat range. These processes are governed by two competing forces. The positive force of forest migration, plant population expansion, is governed by the seed dispersal capacity of the tree species' population and seedling establishment success. The population expansion limiting force, negative force, is the suppression by the environment of species' success in an area. Suppression by the environment could include human land use, disturbance, unfulfilled species-specific resource needs, and/or climatic stress.
These two major forces compete and change through time causing advances and retreats in the borders of plant populations' regions. An advance in the range border of a tree population occurs when environmental suppressive forces beyond the historical range fall below the population's dispersal and establishment potential, thus allowing for seedling success in new territory. This creates a 'leading edge' of the tree population habitat range.
Range border contractions occur when environmental suppressive forces increase to a point where seedling success is limited in the current range. Regeneration failure in a portion of a species' habitat range creates a lagging or 'trailing edge'. Though dispersal and environmental suppressive forces continually act, a static range boundary may occur when there is no change in the rate of these two factors.
Zones within a plant population
There are three basic zones within each plant population; the reproductive core, the marginal establishment zone, and the outer seed shadow. The reproductive core of the plant population is the area in which sexually mature parental plants are present. This is the established reproductive source that provides the positive force for the population's expansion. The second region is the marginal establishment zone. In this region, seeds are successful and plants establish. The plants in this region have yet to reach reproductive maturity, thus they do not contribute to the seed dispersal potential of the population. The final region is the seed shadow region. In this region, inflow of seeds from the reproductive core is occurring, but because of environmental conditions germination or seedling survival is repressed causing an absence of species representatives in this region. This region is controlled by the negative force of the environment to the extent of zero success of the population.
Rapid plant migration
There has been debate over how plant populations move under rapid climate change situations. This debate stems from an issue called "Reid's paradox of rapid plant migration". After the last glacial period, tree species spread to recover the newly exposed land. Through studies, it was calculated that this expansion occurred faster than perceived possible. The two explanations for this rapid movement of forest populations across the landscape that came to the forefront were the retention of low-density founder populations and long-distance migration.