How long must a desiccation-tolerant moss tolerate desiccation?: Some results of 2 years' data logging on Grimmia pulvinata

Grimmia pulvinata (Hedw.) Sm. is a common desiccation-tolerant moss of wall tops in Britain. Readings were recorded at 30-min intervals from sensors measuring duration of precipitation, 'moss wet', solar irradiance, net radiation, air temperature and moss temperature. The duration of both 'moss-dry' and 'moss-wet' periods varied widely, but generally approximated to a log-normal frequency distribution. The median length of dry and wet periods was relatively short, generally between 5 and 15 h. The longest dry periods, recorded in early summer, were 15 and 17 days, and the longest continuously wet period (almost 28 d), in the unusually wet autumn of 1988. The moss was generally wet for longer than the duration of rain by an average factor of 1.7, and the number of dry-wet cycles less than the number of precipitation events by a factor of 3 or more, especially during autumn and winter. There was little indication of hydration following dew-fall. The proportion of the time the moss was wet in daylight at any time of year was nearly proportional to the fraction of the 24 h between sunrise and sunset. Most growth took place in autumn and early winter when (with relatively low irradiance) the moss was wet for long periods and the weather was still mild. Desiccation-tolerant bryophytes (and lichens) are pre-eminently organisms adapted to frequent, and often short, dry-wet cycles. This should be a prime focus of research on their physiology.