PHYSIOLOGIA PLANTARUM 122: 21–27. 2004 doi: 10.1111/j.1399-3054.2004.00348.x Printed in Denmark – all rights reserved Copyright # Physiologia Plantarum 2004 How long must a desiccation-tolerant moss tolerate desiccation? Some results of 2 years’ data logging on Grimmia pulvinata Michael C. F. Proctor Department of Biological Sciences, University of Exeter, Washington Singer Laboratories, Perry Road, Exeter, Devon. EX4 4QG, UK E-mail: [email protected] Received 17 December 2003; revised 24 February 2004 Grimmia pulvinata (Hedw.) Sm. is a common desiccation- average factor of 1.7, and the number of dry–wet cycles less tolerant moss of wall tops in Britain. Readings were recorded at than the number of precipitation events by a factor of 3 or 30-min intervals from sensors measuring duration of precipita- more, especially during autumn and winter. There was little tion, ‘moss wet’, solar irradiance, net radiation, air temperature indication of hydration following dew-fall. The proportion of and moss temperature. The duration of both ‘moss-dry’ and the time the moss was wet in daylight at any time of year was ‘moss-wet’ periods varied widely, but generally approximated nearly proportional to the fraction of the 24 h between sunrise to a log-normal frequency distribution. The median length of and sunset. Most growth took place in autumn and early winter dry and wet periods was relatively short, generally between 5 when (with relatively low irradiance) the moss was wet for long and 15 h. The longest dry periods, recorded in early summer, periods and the weather was still mild. Desiccation-tolerant were 15 and 17 days, and the longest continuously wet period bryophytes (and lichens) are pre-eminently organisms adapted (almost 28 d), in the unusually wet autumn of 1988. The moss to frequent, and often short, dry–wet cycles. This should be a was generally wet for longer than the duration of rain by an prime focus of research on their physiology. Introduction Bryophytes are well known for their desiccation toler- in approaching some of these questions, and their ance, and there is abundant experimental evidence that physiological implications for desiccation-tolerant bryo- many will survive and recover after drying to very low phytes, this paper presents some results of 2 years’ data water content for weeks, months or even years (Maheu logging on Grimmia pulvinata (Hedw.) Sm., a small 1922, Abel 1956, Hosokawa and Kubota 1957, Keever desiccation-tolerant cushion-forming moss, common on 1957, Dilks and Proctor 1974). However, there is little walls in Britain and neighbouring parts of continental information on the length of the dry and wet periods that Europe. bryophytes actually experience in the field. Twenty-two ‘rainless’ periods ranging in length from 37 to 73 days were recorded in the British Isles from the 1890s to the Materials and methods 1990s (Dukes and Ede 1997), but this defined as ‘rainless’ a day with 0.2 mm of measurable precipitation, so The measurements were made on a population of Grimmia many of these dry periods were probably not truly rain- pulvinata on a mortared hard sandstone garden wall less. Further, really long dry periods are clearly rare, and about 1.7 m high near Morchard Bishop in mid-Devon, most dry periods in the British climate must be very south-west England (Lat. 50510 N, Long. 3450 W; much shorter. Dry periods will tend on average to be OSGB ref. SS 773 066), 145 m above sea level. Mean shorter in high-rainfall oceanic climates, and longer monthly temperature in January is approximately 4C, in climates that are more arid and more seasonal, as in and in July approximately 15C. Average annual rainfall the Mediterranean and subtropics. As a starting point is approximately 1000 mm; monthly rainfall during the Abbreviations – OSGB, Ordnance Survey of Great Britain. Physiol. Plant. 122, 2004 21 observation period at Kennerleigh, approximately 5 km with repeated short showers the factor may sometimes be east of the recording site and at slightly lower altitude, much greater than this, and occasionally light but detect- is shown in Fig. 1. able rainfall failed to moisten the moss at all, the rain- The equipment consisted of two Grant ‘Squirrel’ drops remaining poised on the hair-points at the tips of recorders (Grant Instruments, Shepreth, Cambs., UK). the leaves. Only rarely was there any indication of hydration One recorded moss temperature and air temperature (in following dew-fall, probably partly because the wall a well-ventilated, cylindrical, bright-metal radiation was an effective heat store or sink, and partly because of screen) from thermistors. The other recorded mV out- relatively free convective heat exchange with the air. The puts from a tube solarimeter and a tube net radiometer ‘duration of precipitation’ measured was always substan- (Type TRM; Delta-T Devices Ltd, Burwell, Cambs., tially greater than that indicated by the Meteorological UK), and from two resistance probes (made from a Office figures for duration of rainfall (about 800 h per pair of platinum wires tightly wound figure-of-eight- year, or approximately 10% of the time for this site), wise with polyester thread and sealed at the ends with which refer to ‘measurable’ rainfall. epoxy resin), approximately 1 cm long, connected to a The data also show clearly that the frequency of dry– simple resistor network powered by a 1.5-V battery to wet cycles was in general less (by a median factor of give an output between À0.4 mV (dry) and 11.4 mV 3.29), and often much less, than the frequency of discrete (saturated). One of these probes was half-immersed ‘precipitation events’ recorded (Fig. 3). The effect was across the surface of a Grimmia cushion (measuring very much more pronounced in autumn and winter ‘moss wet’), and the other was freely exposed to the air than in spring and summer; apart from one peak in (measuring ‘duration of precipitation’). The threshold August the highest quotients are all in the winter half value between ‘dry’ and ‘wet’ was arbitrarily taken as of the year between September and March (Fig. 3B). The 0.0 mV for both probes. Because the resistance ranges so monthly number of dry–wet cycles showed a well marked widely between wet and dry conditions the output concentration in the spring and early summer months, approximates to an all-or-nothing curve, and the exact with the exception of May 1989 in which very little rain threshold value chosen makes little difference to the fell at all. results. Readings were recorded at 30-min intervals The foregoing results leave aside the length of individ- from early May 1988 to April 1990. The record is essen- ual dry and wet periods. Plotting the measured length of tially continuous over this period, with only a few gaps wet and dry periods as cumulative frequencies showed a of up to about a week, mostly due to equipment mal- fair approximation to a set of cumulative normal distri- functions. bution curves on a logarithmic time scale. This is illus- trated by the data for the 3 months from June to August Results 1988 in Fig. 4. For description and comparison the cumulative frequency plots are usefully fitted by sigmoid The measurements show that, as would be expected, the logistic curves. These give a good fit to most of the data moss remained wet for substantially longer than the time sets, except for the winter wet-period measurements, precipitation was actually falling (Fig. 2A); capillary where 3 months is probably too short a time to sample water storage in the moss cushion generally increased potential long wet periods adequately, and there is a the time the moss was hydrated by a factor of about similar tendency to truncate the curves for dry periods 1.7. However, as shown in Fig. 2B, in unsettled periods in late summer (Fig. 5). However, these curves show very Fig. 1. Monthly rainfall at Kennerleigh, Devon, UK (Lat. 50510 N, Long. 3410 W; OSGB ref. SS 819 074, alt. 120 m), approximately 5 km E of the measurement site, during the period of observation. Rainfall at Lapford, approximately 5 km W of the measurement site was closely similar. 22 Physiol. Plant. 122, 2004 Fig. 2. A, Percentage of time with precipitation (.) and percentage of time moss wet (*) in arbitrary recording periods (generally 5–8 days). Note that the recorded ‘time with precipitation’ will depend on the sensitivity of the sensor, and is inherently not sharply bounded. B, Quotient of ‘time moss wet’ and ‘time with precipitation’. clearly the shift in the relative lengths of wet and dry that the moss was dry for substantial periods at all periods through the seasons, and the tendency of the times of year. curves for both dry and wet periods to flatten with As Fig. 7 shows, the proportion of time the moss was increasing average time dry or wet. The period of obser- wet in daylight at any time of year was roughly propor- vation happened to embrace a particularly dry summer tional to the fraction of the 24 h between sunrise and and an exceptionally wet autumn and winter; the data sunset. It was depressed only slightly by higher tempera- for the driest and wettest 3 months show the seasonal ture and greater net radiation income in summer, and the contrast close to its most extreme (Fig. 6). longer daylight hours at that season more than compen- Themedianlengthofbothdryandwetperiodsat sated for this. all seasons was notably short – in most cases between The radiation and temperature measurements will not be 5 and 15 h, the single exception being the 25 h median considered further in this paper; however, it may be noted dry period in late summer 1989 (Table 1).