9624656 Casper This study will examine the combined effect of aboveground and belowground competition on plant performance in an oldfield plant community. Current literature assumes that their combined effect is simply the sum of each effect by itself. However, belowground competition is likely to affect plant response to aboveground competition and vice versa. Whether their combined effect is greater than, equal to, or less than the sum of each separate effect may depend on the relative strength of the aboveground versus belowground interactions. These ideas will be tested in a field experiment in which gradients in the intensity of aboveground and belowground competition will be crossed in a full factorial design at two levels of soil fertility. The growth response of target individuals of the annual plant, Abutilon theophrasti, will be used as the dependent variable. The belowground competition gradient will be created through the use of soil partitions with different degrees of accessibility by neighboring roots. Aboveground competition will be varied by pulling back portions of neighboring shoots. This research will be the first to test directly the additivity of competitive forms in a field experiment. It will also be able to test whether belowground competition decreases in importance with increased soil resources.
