Effects of Differential Defoliation on Shoot Growth, Density and Phytomass of Three Graminoids in a Calcareous Fen

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Abstract
Phytomass, density, growth and turnover of shoots were measured in three perennial graminoids in response to regular, single or no defoliation. Shoot demography of the two species with the shortest tiller longevities, Molinia caerulea and Schoenus ferrugineus, was readily modified by defoliation. The lack of responses in Carex fusca may be attributable to morphological constraints associated with longer-lived shoot systems. Shoot leaf areas of all three species were reduced under regular mowing, and with the exception of Carex, individual shoot phytomass declined as well. A single mowing of formerly unmowed swards caused an increase in biomass of Schoenus and Molinia, to values characteristic of unmowed plots. Shoot densities independent of treatment were highest in Schoenus, and lowest in Carex. Defoliation, irrespective of periodicity, resulted in highest shoot densities of Schoenus. Density changes in response to mowing were less pronounced in Molinia and absent in Carex. Reproductive output increased in all three species after the single mowing, thereby significantly exceeding the respective values of Molinia and Schoenus in regularly mowed plots. Reproductive output of Carex was related to age structure, rather than presence or absence of mowing. Detailed mortality studies of Schoenus showed that shoot death was highest in dense, regularly defoliated swards. Tiller dynamics of Schoenus may indicate self-thinning. The observed increases in density, reproductive output and phytomass associated with infrequent mowing suggest that aperiodical defoliation represents a compromise between accelerated shoot turnover associated with regular (annual) mowing on one hand, and inhibition due to self-shading in unmowed swards on the other.