ABSTRACT
Spatially segregated root systems have been
documented among conspecifics and among species at the scale of whole
root systems and
individual fine roots. Root segregation is often caused by
architectural constraints, proliferation in particular microsites, and
plastic responses to competition for resources, but there is also
evidence to suggest that allelopathy
and non-toxic signals contribute to active root segregation. Root
segregation
appears to provide competitive advantages for water and nutrients for
some
species, as well as advantages of space itself. Plant growth and
photosynthesis
decreases when space is physically restricted, even when other
resources
are abundant. Moreover, plants appear to be able to compete for
space
independently of nutrient, water, or light resources. Species
that
utilize resources efficiently and conservatively may particularly
benefit
from active root segregation because more profligate neighboring
species
would not be able to take resources that were being utilized slowly.
Stressful conditions, produced by adverse
physical conditions
and herbivory, have been shown to enhance the production of secondary
metabolites
and increase root exudation, mechanisms that can affect spatial root
segregation.
Resource availability may also determine the relative importance of
root
segregation in plant communities. A large portion of the evidence
for
root segregation comes from arid and semi-arid environments, where
resources
are often low. In resource-rich communities the defense of space
may
be less important.
For animals, the defense and exclusive use of
space
is considered to be evidence for territoriality and suggests that
organisms
that exhibit such behavior are avoiding the costs and uncertainties of
“scramble”
competition. Active root segregation and the defense of space by
plants
indicates that plants also may be territorial and opens the possibility
of
a level of taxonomic generality in population biology that is not
currently
recognized.