The Population Biology of Plants with Clonal Growth: I. The Morphology and Structural Demography of Carex Arenaria

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(1) The structural demography of the rhizomatous sand sedge, Carex arenaria L., was studied in two dune-slack systems on Anglesey, North Wales. Carex arenaria was chosen as a model species, since any study of rhizomes is easier when plants grow in loose-textured substrates such as dune sand. (2) A distinctive architectural feature of this plant is the longevity of the rhizome system. Clonal growth by C. arenaria results in subpopulations of shoot modules (tillers) which may be separated into distinct growth phases in the field (juvenile, adolescent, mature, senile and slack). (3) Field plots were established in the mature, senile and slack phases of natural populations; half the plots received a massive nutrient application of NPK fertilizer. The shoot populations within permanent quadrats were mapped at frequent intervals. (4) Recruitment and death of tillers of C. arenaria occurred throughout the year, although the birth and death rates were both highest during the summer. The spring and summer cohorts were generally larger and had lower survivorship. Most of the flowering shoots were derived from shoot cohorts produced during the previous spring and summer. (5) Life expectancy of tillers differed little between the mature and senile phases, but was considerably less in the slack phase. (6) Nutrient application increased the density of shoot populations in the mature and senile phases, but even more striking was the speeding-up of the flux of shoot turnover. The mortality risk within individual cohorts was increased by adding nutrients. Survivorship curves were generally negatively skewed (Deevey Type I) though some maintained a constant death risk (Deevey Type II). Added nutrients changed the age structure of the shoot population to a more juvenile-dominated state. (7) The flux in the shoot populations in the slack phase was sluggish in response to applications of nutrients, probably because this phase was associated with abundant vegetation of other species which absorbed the applied nutrients.