Inverse yield–density relationships were used to evaluate how competitive balances in mixed plant
species associations may influence productivity, as measured by three indices : Relative Yield Total
(RYT), Relative Land Output (RLO), and Total Land Output (TLO). Parameters of the inverse
relationships, which express competitive influences and plant growth potential in the absence of
competition, were incorporated into expressions used to predict RYT, RLO and TLO. Initial
parameter values were derived from 25 experiments on binary species mixtures, and parameter values
were systematically varied over a 100-fold range. Response to parameter manipulation was also
investigated in five specific binary associations representing a broad range of relative mixture
productivity. As indicated by RLO or RYT, and in accord with ecological concepts of niche
differentiation, high productivity of mixtures relative to monocultures occurred if between-species
competition was low. As indicated by TLO, the total combined productivity of the mixed species was
enhanced by higher potential growth per plant in the absence of competition. Lower within- and
between-species competition also increased TLO. There was a significant positive correlation between
RYT and RLO. Relative and total measures of mixture productivity, however, showed different
responses to parameter manipulation, and were not correlated.