The Population Biology of Plants with Clonal Growth: III. Analysis of Tiller Mortality in Carex Arenaria

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(1) Tiller survival in the rhizomatous sedge, Carex arenaria, was studied in two sand dune-slack systems on Anglesey, North Wales. (2) Field plots were established in the mature, senile and slack phases of natural populations where half of the plots received a large application of NPK fertilizer. The tiller populations within permanent quadrats were mapped at intervals of 1 or 2 months for 16 months. (3) The application of fertilizer led to a large increase in the numbers of shoots that emerged for the mature and senile phases but only a modest increase for the slack phase. (4) Because of the complexity of the survivorship data gathered there was considerable difficulty in determining whether or not the apparent differences in cohort survivorship were statistically significant. (5) Therefore, tiller counts were analysed by a non-parametric method which did not require that individual tillers within the same cohort die independently of each other, nor that they have an equal probability of death in a given time interval. (6) Separate statistical models were fitted to the mortality data from both control and fertilized quadrats, so that the main features of the data could be described together with the variability that was present. The residual variation was much greater for the fertilized quadrats, due to the heavy fertilizer application greatly increasing tiller population flux. (7) The mortality rates for three of the five cohorts (April, June and September), in both control and fertilized quadrats did not differ greatly for the time intervals that they had in common but differed significantly from the mortality rates of the other two cohorts (July and January). The cohorts from the mature and senile phases had similar mortality rates except for the fertilized quadrats at one of the two sites. The application of fertilizer led to higher mortality for the mature and senile phases but this was not the case for the slack phase. (8) The mortality distributions of the fertilized shoots with time were of a different shape from those for the controls.