The processes of height-rank determination among individuals and neighbourhood effects in Chenopodium album L. stands

Abstract

The height of Chenopodium album L. plants grown in monoculture at three different densities was followed throughout the growing season to examine size-rank determination processes with special reference to the effects of neighbourhood conditions. Changes in height rank of plants in the stands were assessed by the rank correlation between final height and the height at each measurement during the growing season. The height ranks of plants were almost fixed 1-2 weeks after canopy closure when the stand height was 10-20 % of final stand height, and fixation occurred earlier in the denser plot. At each measurement, the effects of neighbourhood were evaluated as the partial correlation coefficient between height growth and neighbourhood index with height held constant (r(GN.H)), in which competitive asymmetry was incorporated. During the early period of the growing season, r(GN.H) was non-significant or positive (plants with taller and/or closer neighbours elongated faster), indicating no local competition. Just after canopy closure, r(GN.H) became negative, indicating local competition. A plant's rank changed only in an initial short period of the competition. Plants occupying the upper canopy of stands at the end of the growing season were distinguished by greater height growth during the initial short period of competition after canopy closure, although these plants were not necessarily taller before the onset of local competition. These results suggest that the fate of a plant in a crowded stand is determined in the early stage of stand development.