Changing Plant Distributions and Abundances

Abstract

Changing limits of plant distributions are difficult to study directly, particularly for long-lived species like trees and shrubs. The good regional coverage of pollen diagrams in many parts of the world makes it possible to study changes in distribution and abundance of many, mainly wind-pollinated, trees on timescales of hundreds to thousands of years over vast areas. Subcontinental syntheses of pollen diagrams show that species did not stay together in fixed combinations through time but that species distributions expanded and contracted independently of each other. Recent investigations indicate that the spreading of plants and the expansion of populations have to be viewed as two separate mechanisms and that changes in past plant abundance may not necessarily coincide with changes in the distribution limits of that species or vice versa. The rapid spread of temperate and boreal trees during the Late Glacial and Holocene must have occurred through frequent long-distance dispersal jumps. In many cases, the initial spread occurred at low population density and was followed by population expansion. Holocene climate variation, competition, and human activity are important factors that are likely to have influenced Holocene spread of trees and changes in abundance, but their relative influence on the individualistic spread and abundance changes is still debated.