Predicting competitive ability from plant traits: A comparative study of 63 terrestrial herbaceous plant species.

There is critical need for studies on interspecific competition which enable general principles to be deduced that apply beyond the species and conditions of a particular study or site. Studies on plant traits are a key part of this search for general principles. I measured relative competitive ability in 63 terrestrial herbaceous plant species using the phytometer Trichostema brachiatum, to test whether competitive ability can be predicted from simple measurable plant traits. The test species utilised in this experiment represented a wide array of terrestrial vegetation types (e.g. rock. barrens, alvars, old fields), and an array of growth forms, from small rosette species (e.g. Saxifraga virginiensis) to large clonal graminoids (e.g. Agropyron repens). The experiment was carried out under both a "normal" and a "drought" treatment. Multiple linear regression showed that there was a strong relationship between plant traits and competitive ability (Normal treatment--r$\sp2$ = 0.54; Drought--treatment r$\sp2$ = 0.55). Total plant biomass explained 34% of the variation in competitive ability in the normal treatment and below-ground biomass explained 35% of the variation in the drought treatment. Leaf shape explained most of the residual variation. The competitive hierarchy for plants in the normal and drought treatment was compared. Rankings for individual species varied between treatments, however, when all species were compared simultaneously, competitive hierarchies in both treatments were highly correlated (r$\sb{\rm s}$ = 0.91). This suggests that invariant and variant views of competitive hierarchies are not mutually exclusive but instead depend on the scale at which competition is being addressed. On the broad scale, relative competitive abilities appear consistent across different environments. Thus, both traits and hierarchies show general repeatable patterns that allow us to generalize from one set of circumstances to another.