Relationships between Height, Diameter and Weight Distributions of Chenopodium album Plants in Stands: Effects of Dimension and Allometry

Abstract

Frequency distributions of height (H), stem diameter (D) and dry weight (W) of Chenopodium album plants in even-aged monocultures at the initial plant densities of 400, 800 and 1200 plants m-2 (400-, 800- and 1200-plots) were compared. The height distributions at the end of the growing season were bimodal for the 400-plot but L-shaped for the 800- and 1200-plots. The distributions of diameter and dry weight were L-shaped for all the plots. When the size measures were expressed on the log scale so as to eliminate effects of the differences in dimension, frequency distributions of log W and log H for the 400-plot were bimodal. In contrast, the log D distributions were bell-shaped for all the plots.To elucidate factors responsible for the differences in frequency distributions among log size measures, we examined allometric relationships between log size measures. They were all non-linear all non-linear mainly because the allometric relationships were markedly different between the plants forming the upper-most leaf layer ('upper' plants) and the suppressed plants ('lower' plants). A striking feature was that the heights of the 'upper' plants were almost uniform in spite of their varied diameters. Continuous monitoring of growth of the individual plants in the stand revealed that the height of the 'upper' plants became uniform just after the canopy closure, at about 30 d after emergence, and that uniform growth in height of these plants continued for about 70 d until the end of the growing season. On the other hand, growth of the suppressed 'lower' plants terminated at various stages before flowering. The 'height convergence' of the 'upper' plants was thus identified as a key factor for the segregation of the plants into the two groups of distinctly different allometries.Since the segregation of the plants into 'upper' and 'lower' plants was reflected in their dry weights, frequency distributions of log W were similar to those of log H. The bell-shaped distributions of log D were attributed to the overlapping of the diameters of slender 'upper' plants with those of larger 'lower' plants. Significance of 'height convergence' is discussed.