The extent to which a single species utilises a resource, be it space or anything else which may be required by the plant, is termed the niche breadth.
Competition for space and other resources compresses the fundamental niche of the competitors, promoting resource partitioning and hence greater specialisation.
Generalist species are characterised by a broad niche whilst specialist species would be expected to occupy a narrow niche.
Species with similar patterns of resource utilisation would experience a high degree of niche overlap whilst species with dissimilar resource utilisation curves would have a less niche overlap.