Density-dependent mortality,growth and size inequality in roadside populations of the annual herbGalium aparine L.

Biomass, plant size, plant density and the inequality of sizes were assessed for autumn-emerging roadside populations dominated byGalium aparine during early stages of growth in two independent studies. A third data set dealt with the survival of labelled seedlings belonging to different cohorts of emergence. One data set showed that the slope of the log-log size/density relationship for all plant species present in the samples was closer to −1.5 and that forG. aparine was closer to −1.0 in five separate populations. Biomass increase and density decrease was not found to take place in any of these simultaneously. The size inequality ofG. aparine tended to increase or to remain constant during periods of high mortality, and in the early harvests it was negatively related to population density. The second data set revealed simultaneous decreases of both biomass and density ofG. aparine and of all plant species during a period of a month soon after emergence, and a higher size inequality ofG. aparine in those patches where plant density (and that ofG. aparine) was lower. The labelling of seedlings indicated density-dependent mortality and a higher probability of survival for seedlings emerging very early. The size/density relationship of roadside populations dominated byG. aparine may follow a trajectory over time similar to that predicted by the 3/2 power law of self-thinning, but this species seems to have a weak size hierarchy development and limited individual growth at high population densities. The importance of plant architecture in relation to this response is discussed.