Factors Affecting the Density Response of Vulpia Fasciculata

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Abstract
The effects of germination time, drought and thinning on the density response of V. fasciculata were examined in glasshouse experiments. The growth in dry weight of individual plants in populations which were watered regularly and not thinned was markedly affected by density and date of germination. By the final harvest, total yield was largely independent of the number of seeds sown and the date of germination. There was a simple allometric relationship between the number of seeds produced per plant and shoot dry weight. The same relationship did not apply to spikelet number; fine-tuning of the allometric response was through the number of seeds set per spikelet. The populations which were thinned by half at various stages of the life-cycle had a similar weight-density response to that of the unthinned populations, except that for the corresponding density at maturity the plants were smaller. Those individuals in populations thinned at the time of inflorescence development were on average 37% smaller at maturity than those grown for the full duration of the experiment at the new lower density. A discrete (difference) population dynamics model was used to show how the time at which individuals die can affect population size. The time of thinning may reduce population size by up to 57%. Drought during flowering reduced seed set, produced a more marked density-dependent reduction in fecundity and masked any differences between the 5 thinning treatments in their response to density.

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