Plant community structure in calcareous fens: Effects of competition, soil environment, and clonal growth architecture.

Abstract

I investigated whether plants with phalanx (clumper) and guerilla (runner) clonal architecture experience trade-offs between colonization ability and competitive ability and their consequences for species distributions along environmental gradients in calcareous fens in southeastern Michigan, USA. Fens were excellent study systems because most species are clonal and distinct vegetation zones occur along natural productivity gradients. In two fens, I quantified variation in soil parameters and plant productivity with species composition along gradients from marl to peat soils. Peat had higher organic matter content, nutrient availability, and plant productivity than marl. Clumpers dominated peat, but runners dominated marl. To test whether competition and edaphic conditions influence these species distributions, I planted isolated ramets of eight species in two fens with and without all naturally occurring vegetation in marl, marl-peat mixtures, and peat. Although competition occurred, neither neighbor vegetation nor edaphic conditions precluded the survival of any species at the productive or unproductive extremes of its distribution. I examined trade-offs between runners and clumpers and their consequences for species distributions in two ways. I grew six species alone and in pairs in pots of marl and peat to quantify their colonization ability and competitive ability. In 150 fen mesocosms, I also used a community density series to quantify the community-level consequences of competition in assemblages of seven species on marl and peat. Runners had superior colonization ability in marl and peat. Clumpers were superior competitors in both soil types due to their superior ability to suppress the clonal growth of competitors, tolerate such suppression by competitors, and/or prevent invasion by competitors. Clumpers significantly reduced the growth and relative biomass of runners in both soil types. However, rates of species replacement were slower in marl because lower resource availability and/or high spatial heterogeneity decreased the clonal growth and competitive advantage of clumpers. Initial conditions and the degree of intraspecific aggregation of superior competitors also strongly influenced rates of species replacement in both soil types. In fens, the relative biomass of clumpers and runners in productive peat and unproductive marl environments is strongly positively correlated with competitive ability and maximum clonal growth rates, respectively.