Effects of Flowering on the Shoot Dynamics of Carex Bigelowii Along an Altitudinal Gradient in Swedish Lapland

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Abstract
The main role of flowering in plant population dynamics has been assumed to be the production of seeds, which in many populations provide the main input of new individuals. However, flowering may also have a considerable influence on the shoot dynamics of many clonal plants, as the growth of an individual shoot is terminated with the production of an inflorescence by the apical meristem. The aboveground shoot dynamics of a long-lived clonal sedge, Carex bigelowii, were investigated by comparing three populations along an altitudinal gradient for five years. The roles of flowering, tiller size and spatial pattern in determining shoot dynamics were investigated. No correlations were found between shoot survival, shoot emergence or proportion of shoots flowering and the density or aggregation of shoots. All new shoots were vegetatively produced, emerging mainly in early or mid summer. At the most severe upper site, survival of the younger age classes was highest, but lower for the older age classes, resulting in the fastest turnover rates at the upper site. Responsive death (not including the death of flowering shoots) in winter and summer was about equally important at the upper and intermediate sites, whereas 82% of the shoots at the lower site died during winter. Flowering varied greatly between years, especially at the upper site, where 32% of the shoots flowered in 1986, compared to between 1% and 6% in the other four years. There was also a seasonal trend in that shoots arising early in the season had a much greater probability of flowering early in their life than shoots emerging later. At all sites, flowering was followed by a corresponding decrease in shoot numbers, due to the programmed death of the flowering shoots. The fate of an individual shoot was highly correlated with its size. The mean size in one year of shoots which died in the following year was less than that for shoots remaining vegetative, whereas those which flowered in the following year were larger than vegetative shoots. Floral initiation in a shoot is dependent on shoot size and a developmental period of some years. After flowering, the shoots die, population size decreases and then gradually increases until the next flowering event, such as in 1986. This indicates that population size changes in a cyclical way. The extent of flowering, therefore, directly controls the growth and performance of the plant by restricting the number of meristems available for the production of photosynthetic tissue. This demonstrates the importance of taking the developmental morphology of the plant into account when studying the dynamics of monocarpic clonal plants.

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