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Factors Affecting the Density Response of Vulpia Fasciculata

A. R. Watkinson
Journal of Ecology
Vol. 70, No. 1 (Mar., 1982), pp. 149-161
DOI: 10.2307/2259870
Stable URL: http://www.jstor.org/stable/2259870
Page Count: 13
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Factors Affecting the Density Response of Vulpia Fasciculata
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Abstract

(1) The effects of germination time, drought and thinning on the density response of Vulpia fasciculata were examined in glasshouse experiments. (2) The growth in dry weight of individual plants in populations which were watered regularly and not thinned was markedly affected by both 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. (3) 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. (4) The populations which were thinned by half at various stages of the life-cycle showed a similar weight-density response to that shown by the unthinned populations, except that for the corresponding density at maturity the plants were smaller. How much smaller depended on the time of thinning. For example, 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. (5) A discrete (difference) population dynamics model is used to show how the time at which individuals die can affect population size. Using parameter estimates from the experiments it is shown that the time of thinning may reduce population size by up to 57%. (6) Drought during flowering reduced seed set, produced a more marked density-dependent reduction in fecundity, and masked any differences between the five thinning treatments in their response to density.

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