Programmed Tiller Differentiation, Intraclonal Density Regulation and Nutrient Dynamics in Carex Bigelowii

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Abstract
The internal controls of tiller dynamics in three populations of Carex bigelowii along an altitudinal gradient in Swedish Lapland were examined by mapping the aboveground shoots for four years, after which the tiller systems were excavated and rhizome connections recorded. Tillers could be divided into three categories depending on their position on the parental tiller: ventral, lateral and dorsal tillers. The first daughter tiller to be produced was usually ventral. These generally had long rhizomes (guerilla), while dorsally produced tillers had shorter rhizomes or no rhizome at all (phalanx). Rhizomes of laterally produced tillers were of intermediate length. Bud dormancy varied between zero and four years. The mean time between the emergence aboveground of a parent and its first daughter was at least 2.2 yr. Ventral tillers had more vegetative offspring, while dorsal tillers were more likely to flower. Large tillers, irrespective of tiller type, tillered more and were more likely to flower. After the production of a daughter tiller, the parent shoot gradually declined in vigour and finally died. A fertilizing experiment revealed a strong response of C. bigelowii to increased levels of soil nutrients. Offspring production increased by 50%, and the proportion of dorsal tillers increased from 20% to 36%. In natural conditions, the amount of resorption from the leaf fraction into the tiller system was high for the macro-nutrients, and ranged from 42% for N to 92% for K. Ventral tillers were costlier to produce than dorsal tillers in terms of nutrients and carbon, due to their biomass. The relative cost of a ventral tiller was ecpecially high in terms of P and K, but lower for N, Mg, and Ca. Intra-clonal density in C. bigelowii is regulated both by the programmed mortality caused by the production of offspring and flowering, and by the shift in balance between guerilla and phalanx growth form controlled by soil nutrient availability. The first daughter tiller being of a guerilla type maximizes the forward 'search' for favourable patches. Should these be encountered, the increased nutrient availability increases the production of offspring, these being mainly of the phalanx type, thus efficiently exploiting the favourable micro-site. The phalanx type tillers are more likely to flower, while not producing vegetative offspring, using the extra nutrients for the requirements of the maturing seeds.