Recent theoretical and experimental studies have addressed whether the rel- ative importance of aboveground and belowground competition changes along gradients of biomass productivity. Results have been contradictory, with some researchers finding a decrease in the importance of belowground competition and an increase in aboveground competition with increased productivity, and others finding either no relationship, or a positive correlation between the various factors. Belowground competitive intensity (BCI), resulting from root interactions, and total competitive intensity (TCI), resulting from both root and shoot interactions, have usually been measured as the proportional growth reduction due to competition (relative to growth without competition). Instead of direct measurement, aboveground competitive intensity (ACI) has been estimated by assuming that aboveground competition and belowground competition do not interact to affect plant growth, and there- fore ACI ? BCI ? TCI. In this study, Abutilon theophrasti was used as a focal species to determine whether an interaction between the two competitive forms could exist. Target plants were grown with varying degrees of interaction with the roots of neighboring plants, through the use of modified root exclusion tubes, and by tying back the aboveground neighboring vegetation. In total, 16 combinations of varying intensities of aboveground and belowground interactions with neighbors were created at each of two fertilization levels. The strength of belowground competition decreased with fertilization, while neither above- ground competition nor total competition (occurring both above- and belowground simul- taneously) varied among fertilization treatments. Not only was there evidence for an in- teraction between above- and belowground competition, the form of interaction varied with productivity, switching from no interaction in the unfertilized block to a positive interaction in the fertilized block. With fertilization, belowground competition decreased a plant’s ability to compete in asymmetric competition for light. These results contrast with existing models of the role of competition in plant communities, and a new model is presented. In order to understand the role of aboveground and belowground competition in plant com- munities, the potential for interactions between the two competitive forms must be con- sidered in future studies.
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