Seasonal pattern of leaf growth and senescence of Nardus stricta and responses of tussocks to differing severity, timing and frequency of defoliation.
A field study of the seasonal growth of Nardus stricta showed that leaf turnover was slow, averaging 2.5 leaves per tiller over the growing season. Periods of lamina extension growth and senescence were generally separated in time; fastest growth, at 4-5 mm per tiller per day, occurred in June-July and the senescence rate was low until early autumn when it rose to 6 mm per tiller per day. Sheathing tiller bases, already well developed in May, increased in weight by 27% between May and October; total water-soluble carbohydrate (TWSC) concentration increased by 150% (from 75 to over 165 g per kg dry weight) while N, P and K concentrations declined over this period. A pot study showed that the age class of inter-connected tillers on rhizome tips greatly influenced growth; mean rates of lamina extension and production of daughter tillers were higher in younger tillers while the likelihood of flowering was greater for older (3-4 leaf) tillers. The pattern of growth provided evidence for acropetal movement of assimilate. Field cutting experiments showed that Nardus was relatively insensitive to defoliation in the short term. Removal of 50% or 100% of the length of the leaf lamina in June only, July only or in both June and July resulted in a temporary reduction in lamina extension rate per tiller but did not significantly affect the weight of leaf produced in the first season. There was a non-significant trend for the size of tiller base to be reduced as the severity of cutting was increased. Concentrations of TWSC and N, P, and K in tiller bases showed no variation attributable to cutting. In a long-term cutting experiment lasting 4 years, Nardus was cut so as to leave 1, 4 or 8 cm of lamina in June only, July only, or in June and July. Two seasons elapsed before lamina extension was reduced on cut plants. The weights of clippings were greatly reduced over successive years when tillers were cut to leave only 1 cm of lamina but were little affected when 8 cm of lamina remained, with the 4-cm treatment intermediate; the reduction was greatest with plants cut in both June and July and least with plants cut in June only with those cut only in July being intermediate. Tussock biomass, assessed after 4 years of cutting and a season of uninterrupted growth, was decreased as the height of cut was reduced and by repeated cutting; timing (June compared with July) was unimportant. Both tiller numbers and mean weight per tiller were reduced. Flowering vigour was increased by cutting. Timing was important, with plants cut in July only or in both June and July having more flowers than those cut in June only. The amounts of TWSC and N, P and K reserves were reduced by cutting with reduction in tiller base size being more important than effects on concentration. In a comparative field study, the rate of lamina extension in Nardus averaged less than half that of Agrostis capillaris or A. canina growing in the same sward. This underlines the importance of selective grazing as an influence on species balance in acid grasslands.