Flowering regimes of terrestrial orchids: unpredictability or regularity?

Empirical data on many species of terrestrial orchids suggest that their fl owering pattern over the years is extremely irregular and unpredictable. A long search for the reason has hitherto proved inconclusive. Irregular flow ering was attributed to costs associated with sexual reproduction, to herbi vory, or to the chaotic behaviour of the system represented by difference e quations describing growth of the vegetative and reproductive organs. Data on the seasonal growth of leaves and inflorescence of Dactylorhiza maj alis are used here to test alternative explanations of the irregular flower ing patterns of orchids. These patterns are found to be extremely rare in o ur data set. Neither costs of reproduction nor grazing seem to explain the rare events of a transition from flowering one year to sterility or absence the next year. These transitions are almost exclusively characteristic of one of four experimental sites, the only unmown site, where the mean leaf a rea and incidence of flowering in the whole population is also in decline. It is therefore hypothesized that irregular flowering regimes may be charac teristic of sites with temporarily or steadily declining populations and th at they are usually not present in prosperous ones - at least for D. majali s.