microclimates and water budget of algae, lichens and a moss on some nunataks in queen maud land

Int J Biometeorol (1989) 33:272-281

meteorology

Microclimates and water budget of algae, lichens and a moss on some nunataks in Queen Maud Land

Yngvar Gjessing 1 and Dag Olav Ovstedal 2 1 Department of Meteorology, University of Bergen, N-5007 Bergen, Norway 2 ARBOHA, University of Bergen, N-5007 Bergen, Norway

Abstract. During the Norwegian Antarctic Expedi- tion of 1984-1985, land parties worked in the area of Mfihlig-Hofmannfjella and Gjelsvik0ella in Queen Maud Land (5~ 1~ 1000-1600 m a.s.1.). The nunataks in this area, which represents one of the climatic limits for terrestrial life on earth, is among those areas absorbing the highest quantity of shortwave radiation during a period of 24 h in summer. In spite of this the air temperature never, or very seldom, exceeds 0 ~ C. The limiting factor for photosynthesis over most of the summer was water availability. Melt-water plays an important role in spring. During rest of the growth season, water from condensation is probably the most important water source for plants. On calm nights the rate of condensation during 6 h may reach 0.5 mm, which constitutes only 10%-30% of daily potential evapotranspiration. Plants situated in narrow clefts or shielded by stone blocks have the highest rate of potential photosynthesis. These locations are shielded from direct solar radiation most of the time, but the radiation from surrounding stone surfaces is higher than from the atmosphere and heat loss by turbulence is smaller than for exposed locations. These locations also probably have the highest rate of actual photosynthesis.

Key words: Microclimate - Water budget - Vegeta- tion - Antarctica

Introduction

During the Norwegian Antarctic expedition of 1984-1985, land parties worked in the area of

Offprint requests to : Y. Gjessing

Mfihlig-Hofmannfjella and Gjelsvikfjella (5~ to 1~ 72~ (Fig. 1) in Queen Maud Land. This area is situated 1000-1600 m above sea level (a.s.1.) and about 200 km from the open sea. The nunataks in this area and other nunataks surrounding the Antarctic plateau represent one of the climatic limits for terrestrial life on earth.

The sun is continually under the horizon for about 21/2 months during mid-winter and is continually above the horizon from the middle of No- vember to the end of January.

Due to its inland locality this area is only slightly affected by the cyclones that buffet the coast from northeast. The climate here is therefore little influenced by heat advected from the ocean, and radiation from the sun and the atmosphere is the main energy source. Loewe (1970) found a good relationship between snow temperature at 10 m depth and mean annual screen temperature at 2 m above the snow surface. Measurements of snow temperature in the area at 10 m depth indicate a mean annual air temperature of - 2 6 ~ C.

Several colonies of the Antarctic Petrel Thalas- soia antarctica are known from this area. The larg- est colony at Svarthammaren was estimated at 207 500 breeding pairs (Mehlum et al. 1988). Rela- tively little is known about the invertebrate fauna of these remote areas. In Queen Maud Land studies of terrestrial invertebrates have so far been concentrated on mites and insects (Block /984; Somme 1985).

Two bryophytes, a number of algae as yet only determined to genus level (Engelskjon 1985, 1987) and 33 lichens (Ovstedal, unpublished) occur in the area. A description of the plant communities is provided by Engelskjon (1985, 1987). The main rock types are coarse- and medium-grained char- nockitoids and small amounts of xenoliths. Banded gneisses, biotite amphibolites and granites of the

273

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amphibol i te facies minera logy are included in the charnocki toids . The slopes are covered by stone blocks and decomposed feltspathic sand (Y. Ohta, personal communica t ion) .

The purpose o f the present s tudy was three- fold: (i) to give a general descript ion o f the microclimate at the nunataks in this area; (ii) to measure, estimate and discuss the water budget for plants; and (iii) to estimate the growth condit ions and growth period.

M e t h o d s

M i c r o c l i m a t i c m e a s u r e m e n t s . Microcl imat ic m e a s u r e m e n t s were carried ou t on the top o f a smal l n u n a t a k (1500 m a.s.1.) on Gjelsvikfjella, in the bird co lony on the no r theas t slope o f S v a r t h a m m a r e n , over a hor izon ta l surface o f snow and on top o f an app rox ima te ly hor izonta l s tone block m e a s u r i n g 6 x 8 m and of 1.5 m in he ight s i tua ted in a blue-ice area.

Globa l r ad ia t ion and total i ncomi ng rad ia t ion were recorded 1.5 m above the g r o u n d by Mol l -Corczynsk i so lar imeter and Simen Ersk ing hemispher ica l rad iometer , respectively. Inte- gra ted values were recorded every 15 m on A a n d e r a a da ta log- gers. The a lbedos for the different loca t ions were de te rmined for different s u n al t i tudes and the reflected sho r twave rad ia t ion was de te rmined for the different locat ions and slopes. The surface t empera tu res were m e a s u r e d by infrared t h e r m o m e t e r

( P R T 5) 20 to 30 t imes du r ing the day. As a supp lement , " s u r - face t e m p e r a t u r e s " were recorded by pla t ina resis tance e lements where the active par t was approx ima te ly 1 m m 3. The ou tgo ing longwave radia t ion was calculated by the Stefan B o l t z m a n n equa t ion with an emissivi ty o f 1.00 based on the P R T 5 mea- su remen t s and for in te rpola t ion between these m e a s u r e m e n t s were used as the regis t ra t ions o f surface tempera tures . Ai r tempera tu re was m e a s u r e d by shielded pla t ina resis tance e lements ; wind velocity with cup a n e m o m e t e r s ; and t empera tu re in lichen, moss and soil with p la t ina resis tance e lements o f the same type as those used for surface tempera tures . The fetch, i.e. the shor- test hor izonta l d is tance to snow/ice surface, was approx. 50 m for the small n u n a t a k and in the bird colony. For the measure - men t s on the s tone block the fetch was 2 m.

The solar imeters were caIibrated by c o m p a r i s o n over several days with an Eppley PCP, which was reguiar ly checked aga ins t an Eppley self-cal ibrat ion pyrhel iometer . The the rmo- pile can be considered independen t o f t empera ture , solar altitude and a z i m u t h (Rad ia t ion Y e a r b o o k 1980). Cal ibra t ions o f the rad iomete r were m a d e at the Rad ia t i on Obse rva to ry in Ber- gen.

W a t e r b u d g e t . The water budge t o f one species o f m o s s and several species o f l ichens was s tudied us ing small , s imple lysimeters . Eight different species, and two samples o f each, were placed in petri dishes o f 50 cm 2 in d iameter and a vertical edge o f 5 m m . The dishes were comple te ly full and were weighed 5 to 8 t imes du r ing the day. One sample o f each species was suppl ied with snow 1 to 2 t imes a day while the o ther samples were not suppl ied with any artificial water supply. The accuracy

274

of the balance was • g, equivalent to • m m evaporation/condensation.

After the field experiments the plants were taken back to the laboratory in Bergen in a freeze-box. Here they were weighed in the air-dried condition and dried at 80 ~ C. In those cases where it was impossible to free them by hand from sand and gravel, they were burnt in an oven at 550 ~ C and the residue weighed. In the cases of the mosses, the area of the tuft was measured, the number of shoots/cm z was calculated, and sub- samples were taken to estimate biomass.

Results

The radiation budget Radiation exchange at the earth's surface can be expressed as

Q*=K+-aK$ + L~,-~ T)~ (1)

where Q* is net radiation (W m-a) , K+ i n c o m i n g shortwave radiation, a the albedo of the surface,

L $ longwave radiation from the atmosphere, cr the Stefan Boltzmann's constant, and T~ the surface temperature, K.

Due to the low water vapour content, little pol- lution in the atmosphere, and low air pressure due to high altitude, the incoming shortwave radiation on a horizontal surface on clear days in summer can exceed 40 MJ m - ~ day-~.

The area is covered with snow except for small areas of bare rock or blue-ice. Due to l o w temperatures the s n o w n e v e r mel t s , a n d therefore the albedo is high (80%-95%). In Fig. 2A the diurnal variation of the parameters in Eq. I is given for a horizontal snow surface during calm weather and with clear sky in the middle of January. Apparent- ly, only 10% of the incoming shortwave radiation is absorbed at the surface. There is a net loss to the atmosphere of longwave radiation for 24 h a

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Fig. 2A-D. Radiation budget for selected days in January 1985 at Svarthammaren in Dronning Maud Land. �9 �9 �9 Incoming shortwave radiation; o o o reflected shortwave radiation; x x x longwave atmospheric radiation; - - - outgoing terrestrial radiation; - - net radiation

day. Except for 7-8 h during the middle of the day, there is a net loss of radiation to the atmosphere in spite of an incoming shortwave radiation of 38 MJ m-2 day-1. The ratio of daily sum of global radiation for clear sky received at the surface to the sum of extraterrestrial radiation was 0.86 which corresponded to the value of 0.84 found at Plateau Station (79 ~ 15' S) for similar conditions (Kuhn et al. 1972).

Parallel studies were carried out over the surface of a stone block situated in an area of blue-ice. The stone surface was 6 x 8 m and approximately horizontal. In Fig. 2 B the diurnal variation of the same parameters shown in Fig. 2A is given for the stone surface on a clear day in January. The incoming shortwave radiation is approximately the same as in Fig. 2A; however, the net radiation is 20 MJ m - 2 day -~. This surface absorbed 83% of the incoming shortwave radiation. During calm periods at midday the surface temperature was close to 30 ~ C, and the corresponding longwave radiation from the surface to the atmosphere was about 450 W m-2.

The hillside at Svarthammaren where the large colony of Antarctic Petrel nested sloped 20 ~ to- wards the northeast. The mean albedo here was 11%. Figure 2 C shows the diurnal variation of the same parameters as those in Fig. 2B, but for a gravel surface on this hillside on January 20. This inclined surface received more shortwave radiation before noon than a comparable horizontal surface. The surface also received shortwave radiation reflected from adjacent snow and ice surfaces. The total amount of shortwave energy received by this surface was 37 MJ m -2 day -1. The surface temperature on gravel and stone occasionally exceeded 30 ~ C at midday under clear and calm conditions. This led to a longwave radiation loss to the atmosphere of about 500 W m - 2.

The South pole during a clear day at the end of December is that place on earth receiving the highest quantity of shortwave radiation on a horizontal surface during a 24-h periods. However, about 90% of this energy will be reflected. Areas of bare rock on nunataks in Queen Maud Land facing north will, during clear sky at the end of December, be among those areas on earth absorbing the highest quantity of shortwave radiation during a 24-h period.

The diurnal variation of the same parameters on an overcast and windy day are shown in Fig. 2 D. Again, during this 24-h period the sum of net radiation was positive and the outgoing longwave radiation, o-T~, was reduced compared to a calm day with clear sky.

275

Soil temperatures and heat transport to the subsurface

Recordings of air temperature at a height of 1.5 m and of soil temperatures at different depths were made in the sandy soil of the hillside of Svartham- maren during a period of 30 days beginning on January 15. Figure 3 A shows the diurnal variation during a calm day with clear sky. During the night the area was affected by katabatic winds from inland. Recordings were taken every 15 rain, so the hourly values shown in Fig. 3 represent the mean of 4 values.

The air temperature reached - 5 ~ C at midday and fell to - 15 ~ C during the night. Soil temperature at depths of 0.25 m and 0.13 m was constantly above 0 ~ C and showed a daily amplitude of 1.8 ~ and 3.7 ~ C, respectively. The maximum temperature at 0.25 m occurred about 3 h later than at 0.13 m, and 8 h later than at 2 mm. The temperature at 2 mm reached 13 ~ C during the day on Jan- uary 15 and dropped to - 1 5 ~ C at night. All measurements were carried out on a gravel area with comparatively free exposure.

On a windy and overcast day the daily amplitude and local differences were lower. Neverthe- less, the maximum temperature at 2 mm depth at the best location reached 6 ~ C.

The heat transport to the subsurface can be expressed as :

dT QG = C Ks dz (2)

Here C is the volumetric heat capacity of the subsurface and Ks is the thermal diffusivity. In the gravel soil the water content of the upper 0.25 m was insignificant and the volumetric heat capacity was estimated to 1.3 x 10 6 J m - 3 K -1 , K S can be determined by the formula"

[ P11/2 = Z l ) " ( 3 )

Here t 2 - tl is the time difference for the appearance of the maximum temperature at depth Z2 and Z1, respectively, and P is the period. For Z2 and Z~, respectively 0.13 m and 0.002 m, t 2 - ta=3 h, and Z2-Z1=0.13 m, Eq. 3 gives Ks=0.26 10 - 6 m - 2 s - 1 . It follows from Fig. 3 that there is a downward flux for 24 hours a day between 0.13 m and 0.25 m. The heat fluxes were 0.35 MJ m-2 day - ~ and 0.26 MJ m - 2 day - ~ for days with clear and overcast sky, respectively. From Fig. 2 C, D it follows that the corresponding net radiation during these two days was, respectively, 21 MJ m -2 day -1 and 14 MJ m -2 day -~. Consequently

276

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Fig. 3. Tempera tu re s in S v a r t h a m m a r e n petrel colony. M e a s u r e m e n t s were t aken in the air and in the soil on a ca lm day with clear sky, and a windy day with overcas t sky, respectively, x x x Air t empera tu res 1.5 m above g round ; �9 �9 �9 soil t empera tu res 0.25 m dep th ; - - - soil t empera tu res 0.13 m dep th ; - - soil t empera tu res 2 m m dep th

less than 2% of the net radiation is transported to the subsurface and for rock and soil surfaces the rate of evaporation is insignificant most of the time. Probably more than 90% of the available net radiation is transported back to the atmosphere as sensible heat.

Temperatures in lichen and moss at different exposures

At the nunatak in the Gjelsvikfjella at 1500 m a.s.l., temperature recordings in different species of lichen and moss with different exposures were carried out during a period of 11 days. The temperature censors with an active part of about I mm 3 were placed about 2 mm under the surface. Fig- ure 4 shows the temperatures for the different locations and species for a day with clear sky and only slight winds and for an overcast, windy day. Re- cordings were taken every 15 rain, so the hourly values shown in Fig. 4 represent a mean of 4 values. There are steep vertical gradients in temperature in the upper soil layer. Therefore the vertical position of the temperature sensors is crucial, and some of the variation shown in Fig. 4 for temperatures at 2 mm may be due to small inaccuracies in vertical sensor positioning.

Figure 4 shows that the air temperature has amplitudes of, respectively, 8 ~ and 4~ on a day with clear sky and on a cloudy day, while the corresponding highest temperature amplitudes in the vegetation are, respectively, 24 ~ and 4 ~ C. As expected there is a time lag in the appearance of the maximum temperature between the south and north slopes of about 8 h. The temperature in sheltered locations, curves 2 and 4, were above air temperature all day and showed relatively small daily amplitudes. These locations are shielded from direct solar radiation most of the time. On the other hand the turbulent heat fluxes here are smaller than for exposed locations, and the absorp- tion of longwave radiation from surrounding stone surfaces is considerably higher than that from the atmosphere.

From a biological point of view the temperatures at the vegetative part of the species are of special interest. Here the daily amplitudes are markedly higher than those some 2 mm below. Manual measurements with an infrared thermome- ter indicated maximum temperatures above 40 ~ C and minimum temperatures 15o-20 ~ C lower than of the air. At exposed locations the temperature of the vegetative part of moss and lichen is above

277

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Description of the locations:

i. South slope, horizontal, freely exposed. (Umbilicaria aprinal,

2. South slope, bottom 30 cm narrow cleft.(Xanthoria candelaria)

3. North slope, horizontal, some shelter.(Umbilicaria aprina 1

4. In between large blocks of stone. (Grimmia lawiana)

5. N - S cleft, 5cm wide, 20 cm deep. (Usnea sphacelata)

6. Top of nunatak, presumeable "best location".(umbilicaria aprina)

7. Air temperature 1.5 m above the ground

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Fig. 4A, B. Temperatures for different locations and species at 2 mm under the surface. A Clear sky and only slight winds. B Overcast and windy day. See Table 2 for a description of locations/species 1-7

278

0 ~ C for 3 to 6 h on sunny days with calm or light wind in summer. On windy, overcast days the temperature never exceeded 0 ~ C. For biotops shielded by blocks or situated in small clefts the temperature of the vegetative part of moss and lichen may exceed 0~ for longer periods than at more exposed locations and may exceed 0~ even on cloudy days. The daily amplitude at such locations, however, is only 5~ ~ C.

The water budget

The absolute air humidity in this area is less than 1 x 10 -3 kg m -3 except for periods with fog in summer. The difference in water vapour pressure between an air layer just above a wet moss or lichen surface with a surface temperature of about 20~ and the air is about 2 kPa. These small patches of vegetation (typical area 0.01 m 2) func- tion as a micro-oasis. To model the turbulent fluxes from such a small area is very complicated and requires input parameters that are not available. The lysimeter experiments described above are, in spite of many sources of error, preferable.

Figure 5 shows the daily variation in the mean rate of evaporation for the different species during a period of 8 days. The maximum rate of presumed potential evaporation was about 0.2 mm h - t and the rate of condensation which occurred at low sun altitudes was about 0.05 mm h-1. The actual rate of evaporation in the middle of the day ac-

counted for approximately 25% of the evaporation from the samples that were supplied with snow 1 to 2 times a day.

Condensation takes place when the temperatures of the vegetation due to a net heat loss from longwave radiation reach the dew point temperature of the air just above the surface. Katabatic winds from the antarctic plateau, which were chan- nelled through a valley glacier 5 km from our camp, regularly began blowing just before mid- night and lasted for about 6 h. This led to turbulent transport of sensible heat to the surface caus- ing the condensation to cease. Most condensation therefore took place between 6 p.m. and 12 p.m. Assuming a mean rate of condensation of 0.05 mm h- 1, plants at those locations not exposed to these winds could be expected to experience total condensation during the night of the order of 0.5 mm.

Discussion

Kappen et al. (1987) studied the temperature de- pendent rates of net photosynthesis for Usneafas- ciala, Hirnantormia lugubris and Caloplaca regalis in field experiments on the South Shetland Islands. The samples were soaked with water, and the curve describing the relationship between temperature and net photosynthesis peaked at about 150-20 ~ C.

Table 2 lists the mean and extreme temperatures for the 6 locations in Fig. 4. Net potential

Nosfor sp. Umbilicario decussofo

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A

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ACTUAL EVAPORATION A POTENTIAL EVAPORATION �9 ACTUAL EVAPORATION o POTENTIAL EVAPORATION �9

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A �9 �9 k O �9 �9

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Fig. 5. Mean rate of measured actual evapotranspiration and presumed potential evapotranspiration for 2 different species during the period Feb. 2-11, 1985

279

Table 1. Dry weight of plants used in the lysimeters and estimated maximum condensation in one night during the experi- ment

Species Dry Surface Max con- Max con- weight area densation densation/

(mm dry weight (g) (cm 2) night - 1) (%)

Nostoc sp. 3.50 58 0.35 31 J(anthoria elegans 2.81 58 0.30 28 Prasiola crispa 3.08 58 0.30 29 Umbilicaria decussata 2.05 58 0.33 41 Umbilicaria aprina 1.96 58 0.30 45 Xanthoria elegans 1.98 58 0.30 40 Usnea sphacelata 32.3 187 0.15 4

Table 2. Mean and extreme temperatures and net potential photosynthesis for locations with different exposure on the top of a nunatak in Gjelsvikfjella

Loca- Temperatures (~ C) Net potential tion ~ photo-

A B synthesis (mg CO2

T ~ T~ Tra Tmean Tx T m g dry wt-1

Day- 1

A B

1 -4.0 5 .0 -10 .0-4 .2 -1.8 -8.2 1.32 1.06 2 1.0 2.4 -1.1-1.0 0.6 -2.3 2,98 2.53 3 -1.2 21.0 -12 .0 -6 .2 -3.2 -9.9 2.00 0.52 4 2.3 7.8 -2.0 1.1 3.5 -1.7 3.28 3.04 5 -5.0 -0 .8 - -11 .0 -6 .0 -4.3 -10.3 1.14 0.39 6 4.7 27.0--9.0 2.0 5.7 -8.8 2 .07 ~.82 7 -4.8 - 0 . 9 - 9 . 6 -6.3 -3.3 -10.8

A, Clear sky, light winds; B, Claudy day, moderate winds Tm,~n : Daily mean temperature Tx: Daily maximum temperature Tin: Daily minimum temperature

a Description of the locations: 1. South slope, horizontal, freely exposed (Umbilicaria aprina); 2. South slope, bottom 30 cm deep narrow cleft (Xanthoria candelaria) ; 3. North slope, horizontal, some shelter (U. aprina); 4. In between large blocks of stone (Grimrnia lawiana); 5. North-south cleft, 5 cm wide, 20 cm deep (Usnea sphaee- lata); 6. Top of nunatak, presumed "best location" (U. aprina); 7. Air temperature 1.5 m above the ground

photosynthes is , NPS, was calculated on the basis o f the results o f K a p p e n et al. (1987) for Usnea fasciata. These results f r o m U. fasciata f r o m a coasta l a rea are not strictly re levant for U. sphacelata on inland nuna taks . They belong however to the same genus and have the same g rowth form, so a c o m p a r i s o n is assumed to be meaningful .

Loca t ions 2 and 4 had the highest sums o f daily NPS on sunny, ca lm days as well as on a cloudy,

windy day. These locat ions were shielded f rom direct sunshine for mos t o f the day, but the shield also reduced the heat loss by turbulence. As the sky view area was small the net heat loss f r o m longwave rad ia t ion was reduced. Loca t ions 1 and 5 had the lowest daily sums o f NPS. These loca- t ions were s o m e w h a t sheltered and this reduced the incoming shor twave radia t ion, but the shelter effect was not effective in reducing turbulen t heat loss. These locat ions also had compara t i ve ly large sky view areas.

Loca t ions 3 and 6 had the highest mean and m a x i m u m tempera tu res and were therefore selected as the p resumed " b e s t loca t ions" . The daily sums o f NPS for these locations, however , were marked ly lower than for locat ions 2 and 4. The m a x i m u m tempera tu res exceeded 20 ~ C, but ac- cord ing to K a p p e n et al. (1987) the ra te o f N P S has its m a x i m u m at a b o u t 14 ~ C and becomes neg- at ive when T > 28 ~ C. These locations, which had some shelter, had an op t imal exposi t ion for shortwave rad ia t ion leading to high m a x i m u m tempera - tures. On the other hand the sky view area was high and there was no p ro tec t ion for tu rbu len t heat loss.

K a p p e n s exper iments were done on species f r o m the coastal area o f Anta rc t i ca under condi- t ions with no shor tage o f water . The vegeta t ion in the coasta l area o f Antarc t ica is f requent ly mois tened by bo th falling and drif t ing snow. The Mi ih l ig -Hofmannf je l l a and Gjelsvikfjella owing to their inland localities are only slightly affected by the cyclones tha t buffe t the coas t f rom the nor th- east. Both the rate o f prec ip i ta t ion and f requency of drif t ing snow are cons iderably lower here than at the coast.

The t e rm potent ia l evapo t ransp i ra t ion , PET, in t roduced by P e n m a n (1948) is defined as water v a p o u r loss f r o m a large, level and un i fo rm vegeta- t ion (grass) with unres t r ic ted water supply:

P E T = A ( Q * - Q 6 ) + 1 6 . 6 7(1 + 0 . 5 4 u) ( e e - e ) (4) L (A + 7)

Here L is the la tent hea t o f vapor i za t ion o f water , 7 the p syc rome te r constant , d the slope o f the sa tu ra t ion pressure curve at t empera tu re T, u hor izonta l wind veloci ty and e and es are, respectively, actual water v a p o u r pressure and sa tu ra t ion wa te r v a p o u r pressure of the air. Due to b o u n d a r y effects the evapo ra t i on f rom small oases are con- s iderably higher than P E T as defined by Eq. 4.

F o r 5 h in the middle o f a day with no clouds the potent ia l evapo t ransp i ra t ion , calculated on the basis o f Eq. 4 and Fig. 4, is app rox ima te ly 0.2 m m h - i which is close to the highest rate o f evapo ra - t ion given in Fig. 5. This indicates tha t in spite

280

of a water content that exceeds the dry weight of the plants by a factor of 2-3, the evapotranspiration is restrained probably because the water transport through the plants fails.

Plants need light, heat and water for photosynthesis, and these demands must be fulfilled simulta- neously. A chain is as strong as the weakest link, and the weakest link for photosynthesis on the nunataks in this area is water. Five different mech- anisms can provide water to the plants in this area: precipitation, drifting snow, melt-water from snow above the location, water from clouds, and condensation from water vapour in the air on the surface.

In previous works, for example Kappen and Redon (1987), the assumption was made that the contribution from melt-water and precipitation were the most important water sources for plants in this area. The rate and frequency of precipitation during the summer season in this area is low. A rough estimate based on 5 summer expeditions to this area is that less than 10% of the days have precipitation. The prevailing winds in this area are katabatic winds from the interior at night. These winds may cause drifting snow but normally do not reach higher than about I m above the snow surface. During spring melt-water is an important water source for plants. The north slopes of the nunataks are normally snow-free from the middle of December. For the rest of the growth period water from fog and condensation is probably the most important water source for plants, with condensation dominating.

The lysimeter experiments indicate 0.05 mm h-1 as a maximum rate of condensation. During a clear calm night the rate of condensation may reach 0.5 ram.

The amount of condensed water, expressed as percentage of plant biomass (dry wt) is found in Table 1. Condensation contributed a water mass of up to about 40% of the biomass of some species. The field conditions did not allow an investigation of the total water content of the thalli; thus we do not know the degree of saturation of the species during the investigation period. Samples air-dried (180-20 ~ C) in the laboratory showed a water content of 6.3%-7.5% of that of the thalli dried at 85~ but this is definitely not representative of Antarctic field conditions. Thus we cannot state that conditions were sufficient for photosynthesis. The conclusion is that the only water supply available during the observation period was from condensation, and that most likely the "thallus- water" and condensation water together made it possible for the thallus to photosynthesize. Typical

rates of measured "potential evaporation" before noon were 0.10-15 mm h- 1

This leads to the conclusion that photosynthesis will start in the morning after vegetation temperature has risen above about - 5 ~ C. Normally it will take only a few hours before the water supply from condensation has completely evaporated leading to a drastic reduction in the rate of photosynthesis.

Concluding remarks

The nunataks surrounding the Antarctic plateau represent one of the climatic limits for terrestrial life on earth. In summer these locations are among the places on earth absorbing the highest quantity of shortwave radiation during a 24-h period. In spite of this the air temperature never, or very seldom, exceeds 0 ~ C.

Plants need heat, light and water for photosynthesis, and in this area water is the limiting factor for most of the summer. The contribution from snowfall and drifting snow is of minor importance, while melt-water plays an important role in spring. For the rest of the growth period water from condensation is probably the most important water source for plants. During a calm night the rate of condensation may reach 0.5 mm. This contrib- utes only about 10%-30% of the potential evapotranspiration during the day.

The plants situated in narrow clefts or shielded by stone blocks have the highest rate of potential photosynthesis. Such locations are shielded from direct solar radiation for most of the time. On the other hand, radiation from surrounding stone surfaces is higher than from the atmosphere and the heat loss by turbulence is smaller than for exposed locations. The rate of condensation is lower at the shielded locations compared to the exposed area. This difference is probably compensated by reduced turbulent flux of latent heat from the shielded locations. These shielded locations which have the highest rate of potential photosynthesis have, therefore, probably also the highest rate of actual photosynthesis.

Acknowledgement. We are grateful to T. Engelskj6n who contributed to the collection of plants and to H. Parker for improv- ing the English.

References

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Received July 4, 1988; revised June 26, 1989 Accepted July 6, 1989

microclimates and water budget of algae, lichens and a moss on some nunataks in queen maud land

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