SEASONAL PATTERN OF LEAF GROWTH AND SENESCENCE OF NARDUS-STRICTA AND RESPONSES OF TUSSOCKS TO DIFFERING SEVERITY, TIMING AND FREQUENCY OF DEFOLIATION

1. A field study of the seasonal growth of Nardus stricta showed that leaf turnover was slow, averaging 2.5 leaves per tiller over the growi ng season. Periods of lamina extension growth and senescence were gene rally 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 aut umn when it rose to 6 mm per tiller per day. Sheathing tiller bases, a lready well developed in May, increased in weight by 27% between May a nd October; total water-soluble carbohydrate (TWSC) concentration incr eased by 150% (from 75 to over 165 g per kg dry weight) while N, P and K concentrations declined over this period. 2. A pot study showed tha t the age class of inter-connected tillers on rhizome tips greatly inf luenced growth; mean rates of lamina extension and production of daugh ter tillers were higher in younger tillers while the likelihood of flo wering was greater for older (3-4 leaf) tillers. The pattern of growth provided evidence for acropetal movement of assimilate. 3. Field cutt ing experiments showed that Nardus was relatively insensitive to defol iation 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 n ot significantly affect the weight of leaf produced in the first seaso n. There was a non-significant trend for the size of tiller base to be reduced as the severity of cutting was increased. Concentrations of T WSC and N,P, and K in tiller bases showed no variation attributable to cutting. 4. In a long-term cutting experiment lasting 4 years, Nardus was cut so as to leave 1, 3 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 lami na but were little affected when 8 cm of lamina remained, with the 4-c m treatment intermediate; the reduction was greatest with plants cut i n both June and July and least with plants cut in June only with those cut only in July being intermediate. 5. Tussock biomass, assessed aft er 4 years of cutting and a season of uninterrupted growth, was decrea sed 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 cut ting. Timing was important, with plants cut in July only or in both Ju ne and July having more flowers than those cut in June only. The amoun ts of TWSC and N, P and K reserves were reduced by cutting with reduct ion in tiller base size being more important than effects on concentra tion. 6. In a comparative field study, the rate of lamina extension in Nardus averaged less than half that of Agrostis capillaris or A. cani na growing in the same sward. This underlines the importance of select ive grazing as an influence on species balance in acid grasslands.