Functional Interactions among Traits that Determine Reproductive Success in a Native Annual Plant

We grew cocklebur, Xanthium strumarium L. (Compositae), in an experimental garden and monitored a variety of morphological, physiological, and phenological traits known to have a significant influence on plant productive success. We used a hierarchical series of multiple regression analyses to generate a path diagram explaining the interacting contributions of individual traits to the numbers and size of fruits produced by individual plants. Fruit production was strongly positively correlated with plant vegetative biomass, and fruit size was negatively correlated with the total number of fruits produced. Although larger plants produced more fruit, there was also a significant direct effect of the timing of anthesis on total fruit numbers; plants in this determinate species that flowered later produced more fruit. The influence of all other traits on reproductive success was indirect through their influence on plant size. Plants with higher water—use efficiencies early in the season produced leaves more quickly, while plants which emerged earlier grew more rapidly in height. These faster growing plants had a higher vegetative biomass and consequently a greater reproductive success. Latter emergence also had a direct positive influence on plant size, demonstrating that a single trait can have significant but counteracting influences on final reproductive success. These results demonstrate the utility of an inductive approach in analyzing the actual functional pathways by which diverse traits influence growth and reproduction at the whole plant level.