Components of Plant Competition Along an Experimental Gradient of Nitrogen Availability
Experiment Id

This experiment was set up in Field C, on top of the already existing experiment C011. For a description of the plot layout, see E011. For a list of fertilizer treatments for each plot, see file trmte11 and fertilization details. The object of this research was to measure the intensity of above and below-ground competition along a gradient of soil resource availability. Neighbor effects were quantified by measuring the biomass, growth, morphology and survivorship of three species transplanted into vegetation along a gradient of soil nitrogen availability. Transplants were used to test for an inverse relationship between above and below-ground competition, variation in the intensity of competition at different levels of nutrient availability, and reversal of competitive hierarchies. Eight plots of each nitrogen treatment were randomly selected and used for competition experiments. Three competition treatments were used to measure the above and below-ground effects of neighbors on transplanted seedlings at each rate of nitrogen supply. Transplants were grown in subplots with no neighbors, or in subplots with the roots but not shoots of neighbors, or in subplots with all above- and below-ground parts of neighbors present. No neighbor treatments were imposed by driving a ring of galvanized sheet steel (50cm in diameter, 15cm tall) into the ground until the top of it was at the soil surface and then killing all the plants in the enclosed circle with a systemic, rapidly decomposing herbicide (Roundup). Transplants were grown with only the roots of neighbors using white plastic garden net (50x50cm; mesh:1x2cm) to tie back the shoots and leaves of neighbors so that a transplant in the center of the net was not shaded by neighbors but was surrounded by neighbor roots. The all neighbors treatment consisted of a transplant grown in the center of a 50cm x 50cm subplot of undisturbed vegetation. Treatments were replicated five times for each of three transplanted species in each of the 8 replicate plots for each of the three levels of the experimental nitrogen gradient. Thus, the 24 plots of the nitrogen gradient (3 nitrogen levels x 8 replicates used in the competition experiment) each contained 45 subplots(3 competition treatments x 3 transplanted species x 5 replicates), each subplot comprising one transplant of a single species in one competition treatment. A total of 1080 seedlings were transplanted. Competition treatments and species were assigned randomly to subplots within each plot. Transplants were of three grass species, Schizachyrium scoparium, Poa pratensis and Agropyron repens, selected for their different distributions along the experimental nitrogen gradient. One transplant was planted in the center of each subplot.