Above- and belowground competition intensity in two contrasting wetland plant communities

A fundamental question in plant ecology is if and how the intensity of competition changes with productivity. This question has been the source of considerable discussion during the last two decades, yet few experiments have tested whether competition intensity changes with productivity in nature. Even fewer studies have separated competition into its above- and belowground components in the field. We used a field experiment to measure total competition intensity and its above- and belowground components in two wetlands that represent extremes in habitat productivity: an infertile sandy shoreline and a fertile bay. Transplants of Lythrum salicaria and Carer crinita were grown with no neighbors, with roots of neighbors only, and with roots and shoots of neighbors; their growth rates were used to measure competition intensity (CI). The experiment was carried out to answer the following main questions: (1) Is there a difference in total, above- and ground competition intensity in two wetlands that differ in standing crop? and (2) Is there an effect of standing crop on total, above- and belowground competition intensity when the data from the two wetlands are combined? Results based on the average of both species show that total and aboveground competition intensity were greater in the high standing crop wetland, but belowground competition did not differ between wetlands (CITOTAL: P < 0.00001, CIABOVE: P = 0.0013, CIBELOW:P = 0.58). Total and aboveground competition intensity were significantly affected by standing crop in the wetlands studied but belowground competition intensity was not (Cl-TOTAL: P = 0.0001, CIABOVE: P = 0.0001, CIBELOW: P = 0.89). Results based on the two species separately show that species vary in their sensitivity to competition in wetland communities.