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,itismucheasiertocensusplantsthanitistocensusotherorganismsandestimatesof,e.g., density,speciesnumberandcompositionanddistributionofaspeciesaremoreaccurateforplants. 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 diameterof40m,incontrasttoherbs,grassesandoakseedlingsintheunderstorey,whichareonly afew 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. Clonal plants present another problem. They grow as a set of connected shoots or ramets (e.g. grass tillers). The connections may be buried and ramets of various plants may be intermingled. So it is impossible to distinguish the complete individual (the genet). It is sensible and usual in this case to count ramets rather than genets and this has the added benefit that ramets will exhibit less size variation.