Introduction

Seeds and phytoplankton can be extremely mobile, but usually we are interested in surveying plants that cannot move. Sessile plants are usually arranged over a substrate (soil, sediment, etc.) and can be found, identified and examined at leisure. This characteristic means that, in many ways, it is much easier to census plants than it is to census other organisms and estimates of, e.g., density, species number and composition and distribution of a species are more accurate for plants. A second characteristic of plants, however, causes problems in deciding how best to characterise the abundance of species. Plant species, and even individuals within a species, in a community can differ enormously in size. An English wood may contain oak trees 30 m tall and with a canopy diameter of 40 m, in contrast to herbs, grasses and oak seedlings in the understorey, which are only a few centimetres in height. Even in a grassland where all plants are a few centimetres tall, there will be huge differences in the horizontal spread of individuals, from a few millimetres to several metres. While the standard measure of abundance of animals, a count of individuals, can be used for plants, this variety in plant size will mean that counts ignore a large amount of information about the community. For instance, there may be equal numbers of individuals of two species in your study area but the species with a larger average size will have a greater importance for the ecological processes.