Fertilization effects on interactions between above- and belowground competition in an old field

Recent theoretical and experimental studies have addressed whether the rela tive importance of aboveground and belowground competition changes along gr adients of biomass productivity. Results have been contradictory, with some researchers finding a decrease in the importance of belowground competitio n and an increase in aboveground competition with increased productivity, a nd others finding either no relationship. or a positive correlation between the various factors. Belowground competitive intensity (BCI), resulting fr om root interactions, and total competitive intensity (TCI), resulting from both root and shoot interactions, have usually been measured as the propor tional growth reduction due to competition (relative to growth without comp etition). Instead of direct measurement, aboveground competitive intensity (ACI) has been estimated by assuming that aboveground competition and below ground competition do not interact to affect plant growth, and therefore AC I + BCI = TCI. In this study, Abutilon theophrasti was used as a focal spec ies to determine whether an interaction between the two competitive forms c ould exist. Target plants were grown with varying degrees of interaction wi th the roots of neighboring plants, through the use of modified root exclus ion tubes, and by tying back the aboveground neighboring vegetation. In tot al, 16 combinations of varying intensities of aboveground and belowground i nteractions with neighbors were created at each of two fertilization levels . The strength of belowground competition decreased with fertilization, whi le neither aboveground competition nor total competition (occurring both ab ove- and belowground simultaneously) varied among fertilization treatments. Not only was there evidence for an interaction between above- and belowgro und competition, the form of interaction varied with productivity, switchin g from no interaction in the unfertilized block to a positive interaction i n the fertilized block. With fertilization, belowground competition decreas ed a plant's ability to compete in asymmetric competition for light. These results contrast with existing models of the role of competition in plant c ommunities, and a new model is presented. In order to understand the rule o f aboveground and belowground competition in plant communities, the potenti al for interactions between the two competitive forms must be considered in future studies.