Demography of Short-Lived Plants in Chalk Grassland. I. Life Cycle Variation in Annuals and Strict Biennials

Abstract

(1) Detailed demographic observations were made in permanent quadrats on three hemiparasitic summer annuals (Euphrasia pseudokerneri, E. nemorosa and Rhinanthus minor) and two strict biennials (Linum catharticum and Gentianella amarella) at three chalk grassland sites in England from April 1979 to October 1981. All species flowered in the autumn and germinated in the spring. (2) Variation over three years in the exact time of germination was correlated with temperature in E. pseudokerneri and Gentianella. Linum germinated at the same time every year, even at sites in different countries; daylength may be a controlling factor. (3) In 1981, early-germinating seedlings had lower survival to flowering in Linum and Gentianella, but in E. pseudokerneri germination date had no effect on survival. (4) In summer 1980, both biennials had a sizeable seed bank (600-2500 m-2), but E. pseudokerneri had virtually none. For all three species, most seeds which germinate apparently do so in the first spring after seedfall. In the field, 14% of seeds sown in autumn 1980 germinated the following spring for Euphrasia, 24% for Linum and 30% for Gentianella. (5) Survival from germination to flowering varied significantly between years by as much as 30-fold. There was less difference between years in the pattern of mortality through the growing season. Survival to flowering ranged from 43% to 0-8% in the annuals and from 12% to 6% in the biennials. (6) Many flowering plants (up to 38%) and many flowers (up to 46%) set no seed. Seed outputs per fruiting plant were very low (10-25 for the Euphrasia spp. and Linum, and 3090 for Rhinanthus and Gentianella), and varied between sites and years. Estimates of seed output were an order of magnitude lower than those of Salisbury (1942). (7) Both Linum and Gentianella were strict biennials (flowering only in the second year) in Cambridgeshire. Both may have annual forms elsewhere but theoretical considerations suggest that the chalk grassland sites are so low in nutrients and so densely vegetated that Linum and Gentianella cannot reach reproductive size in one year. Work on Linum in Holland, where annual forms are found, supports this view.