A neighbourhood approach to self-thinning.

The fate of individual plants of Helianthus annuus was monitored in populations sown at a density of 650 seeds m. Ninety six percent of the seeds germinated but only fifty two percent of the seedlings survived to the end of the experiment due to self-thinning. Whilst the seedlings emerged over only four days the cotyledons then took up to eight days to emerge from the seed case and open fully. The survival of individuals depended primarily on cotyledon-opening time but plants which occupied very small areas (<900 mm) at germination were also less likely to survice. The increased likelihood of mortality amongst the late germinating individuals was probably as a result of shade from older, larger neighbours. It is suggested that one-sided interference for light during self-thinning will increase the importance of early establishment relative to neighbour effects in determining the survival of plants except at very high levels of crowding. The complex of environmental and genetic factors acting to maximise the growth rate of the very young seedling relative to its neighbours will clearly play an important part in determining the position of a plant in the size hierarchy of a population.