Factors Affecting Frequency Distributions of Plant Mass: The Absence of Dominance and Suppression in Competing Monocultures of Festuca Paradoxa

In even-aged monocultures, distributions of seedling mass shift from symmetric to right-skewed as plants grow. One explanation for the shift is that numerous small individuals and few large ones reveal underlying dominance and suppression caused by depletion of resources. A second explanation is that the asymmetry represents a shift from normal to lognormal mass distri- butions because of variance in exponential growth rate. As a consequence of the first mechanism but not the second, the presence of resource depletion should increase both the skewness and the variance of distributions of individual plant mass. Even-aged populations of the prairie grass Festuca paradoxa were grown in the greenhouse for 44 d with and without competition (dense flats or one individual to a container), and with and without augmented nutrients. At each harvest, plants grown densely were smaller than isolated plants, dem- onstrating that resource depletion occurred. Distributions of mass of isolated plants skewed first and remained more highly skewed throughout the experiment. At a given mean mass, the standard de- viation of mass for crowded and isolated plants did not differ. At a given mean mass, the skewness of mass for crowded plants was not greater than for isolated ones. The second explanation is sup- ported, and shifts in distribution shape can be attributed to growth alone, in the absence of dominance and suppression. Competition in this case did not promote skewing but actually retarded its appear- ance. Consistency of these results with previous findings is discussed.