process unless some other influence intervenes. Initial size difference may arise
from slightly different germination times or genetic differences between
individuals in their vigor of growth but this will be accentuated by competition
once they start interacting.
In general, a plant population is affected by many processes that are indepen-
dentof a plant’s density such as frosts or floods at particular times and in
density-dependent interactions with members of its own and other species.Plants that spread vegetativelycan regulate the density of their own shoots,
although there can be competition between shoots as described for non-clonal
species. An entire ‘population’ can be made up of just one genetic individual
and this may be true of a hillside covered with bracken, Pteridium aquilinum, or a
reed bed dominated by Phragmites australis. Clonal plants may grow densely to
dominate an area or as long strands invading new places, and these invaders
can tap the resources of the mother plant giving them a great advantage over
seedling invaders. The invaders can change their ‘behavior’ (in a plant,
behavior means changes in growth form) in response to local conditions,
growing densely where there are many nutrients, but sparsely between rich
patches.
There appears superficially to be such a great advantage to clonal spread that
it is surprising that, although it is common, more plants do not spread this way.
The main disadvantage lies in the fact that clones are all identical genetically.
This leaves them susceptible to insect attack or disease since, if a predator or
pathogen can overcome the resistance of one member of the clone, all will be
susceptible. This was shown graphically in Europe in the 1970s with the
outbreak of elm disease attacking all the clonal elm species, e.g. Ulmus procera,
but only some individuals of the mainly non-clonal Ulmus glabra. In agriculture,
clonally produced crops require greater uses of pesticides.Populations of
clonal plants
K4 – Populations 179