Using upper boundary constraints to quantify competitive response of desert annuals

Using a target-neighborhood approach with six annual dicot species in the N egev Desert of Israel, we tested whether neighborhood biomass constrained t he upper limit of plant performance and if the slope of the upper boundary was correlated to species trait means, such as relative growth rate (RGR) a nd seed mass. Target individuals were measured in early spring and then col lected at the onset of the dry season along with all naturally occurring ne ighbors within a 5-cm radius of the target. Using a minimum of 50 samples f or each of the six target species, we found no significant relationships be tween target performance and either density or aboveground biomass of neigh bors, when including all target individuals in the regressions. However, ab oveground neighbor biomass did influence the maximum potential size of targ ets. When regressions were restricted to the largest target plant within cl asses of neighbor biomass, significant negative relationships were found fo r all six species, with neighbor biomass explaining 55 to 94% of the varian ce in maximum target biomass. The slopes of the regressions were used as an index of competitive response. The correlations between competitive respon se and estimates of species traits were not significant with the possible e xception that species with lower RGR,,, may be better response competitors (i.e. were less sensitive to competition). These findings indicate the usef ulness of the boundary regression technique for describing competitive inte ractions among neighbors, particularly in low productivity environments.