FLOWERING DYNAMICS OF ORCHIS-MORIO L. AND HERMINIUM-MONORCHIS (L.) R.BR. AT 2 SITES IN EASTERN ENGLAND

Information taken from two long-term demographic studies on Orchis mor io L. and Herminium mnorchis (L.) R.Br. is used to explore some of the factors which influence flowering. The proportion of plants which flo wered each year varied considerably between species, flowering in O. m orio exceeding 40% in all years except one over an 18 year period; ove r a 30 year period (1966-95), the number of plants of Herminium in flo wer never exceeded 36% of: the population and no inflorescences were p roduced in 1977 and 1991. The relationship between flowering in Hermin ium in a given year and the monthly rainfall and temperature for the c urrent and 3 precious years was analysed using logistic regression. Be st fits were obtained using data for the summer months in the previous year, with an increasing flowering rate with rainfall and a decline w ith temperature. It is hypothesized that drought and high temperatures in the summer reduce leaf area and cause premature senescence and the death of leaves, with the result that not enough carbohydrates are st ored to enable plants to support or initiate inflorescences the follow ing year. For species such as Orchis morio which produce leaves in the autumn and remain green, summer drought causes no problems as they ha ve no above ground organs. Factors which influence flowering in this s pecies are as yet unknown. (C) 1998 The Linnean Society of London.