Pugnaire F.I. Valladares F. Functional Plant Ecology

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there is little evidence for this and the depression could easily represent increased PR as in

higher plants. Many mosses show negative NP at higher temperatures, over about 208C

(Gannutz 1970; Longton 1988; Kappen 1989; Noekes and Longton 1989a; Wilson 1990;

Convey 1994, Davey and Rothery 1997). At 08C NP can still be substantial but is always

very low by 58C and ceases at around 78C (Kappen et al. 1989, Pannewitz et al. 2005).

Data on the photosynthetic response to WC for Antarctic mosses are not extensive but it

seems unlikely that they will differ greatly from temperate mosses. Most data are for species in

the northern, maritime Antarctic (Davey 1997b) but little difference was found between these

plants and others studied in temperate or northern tundra areas (Fowbert 1996). At Cape

Hallett (728 S) maximal WC for B. subrotundifolium and B. pseudotriquetrum were around

750% and 600% d.wt., respectively; there was a depression in NP at WC above that optimal

for NP (about 400% and 300%, respectively. Thallus water contents for half maximal NP for

the two species were 150% and 120%. These values for B. pseudotriquetrum fall in the mid-

range of those found for the same species in wet and dry sites at Windmill Islands (668 17

Robinson et al. 2000). In their study on Grimmia. antarctici, B. pseudotriquetrum, and

C. purpureus, Robinson et al. (2000) found correlations between water loss rate under

standard conditions, maximal water content, WC for half maximal photosynthetic efficiency,

desiccation tolerance, and water status of the habitat. Samples and species from the drier

habitat lost water less rapidly, had a lower WCmax, and remained photosynthetically active

to lower WC than those from wetter sites.

A strong relationship is reported between moss species distribution and water flow for

several species (Schwarz et al. 1992, Okitsu et al. 2004, Robinson et al. 2003), and Kappen et al.

(1989) found photosynthetic differences between mesic and xeric ecodemes of Schistidium

(Grimmia) antarctici Card.in the Windmill Islands. However, more detailed investigations

are needed because the moss species appear to vary in this characteristic as well. At Granite

Harbour, C. purpureus occupies the wetter sites, the exact opposite as that found for Windmill

Islands. Additionally, drying rates, water contents, and NP are commonly expressed on a dry

weight basis and it is an unfortunate fact that a large proportion of the samples can be

inorganic, for example, Pottia heimii samples that came from an apparently clean moss

hummock in the Dry Valleys had an inorganic content of a surprising 66% (unpublished

results). Response to CO

concentration has been little investigated for Antarctic mosses but

appears to be similar to that found for other C

species with saturation occurring at levels well

above ambient (Pannewitz et al. 2005). In studies at Cape Hallett at 208C, B. pseudotriquetrum

was not CO

saturated at 2000 ppm (Figure 13.4) compared with B. subrotundifolium that was

saturated at about 1000 ppm. The actual CO

concentration around the mosses is a matter of

debate but very high values, several times normal ambient levels, have been found in Grimmia

antarctici in continental Antarctica, (Tarnawski et al. 1992) and in B. subrotundifolium at Cape

Hallett (Green et al. 2000). The source of the high CO

, its seasonal change and its effect on

overall productivity are not yet known. B. subrotundifolium measured at normal (360 ppm) and

2000 ppm CO

under identical conditions showed a 60%–80% increase in daily carbon gain so

carbon budgets modeled on normal ambient levels may be in error (Pannewitz et al. 2005).

Lichens

Considerably more work has been done on Antarctic lichens than the mosses (Kappen 1993a,

2000). However, as they are poikilohydric the lichen and moss photosynthetic responses are

similar in form although differing in detail (see Longton 1988a). PPFD required for satur-

ation can be very high, often around 1000 mmol m

2

1

, and light compensation values

are dependent on the thallus temperature, for example: saturation at 1300 mmol m

2

1

above 18C and compensation from 5 mmol m

2

1

at 28Cto128mmol m

2

1

at 208C

for Leptogium puberulum Hue (Figure 13.5 and see Schlensog et al. 1997a). Sun and shade

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400 Functional Plant Ecology

forms of Usnea sphacelata R. Br. differed little in the form of their photosynthetic responses

to PPFD at identical temperatures although the sun form had considerably larger NP at

higher temperatures (Kappen 1983). Topt tends to be lower and more consistent than for

mosses and values around 58C158C are common (Longton 1988a, Kappen 1993a).

In mosses, Topt declines with fall in PPFD below saturation (Figure 13.6) (Schroeter et al. 1995,

Thallus temperature (°C)

−5 0 5 10152025

LCP

(µmol m

−2

−1

)

100

200

300

400

500

600

Leptogium puberulum

Parmelia saxatilis

Umbilicaria aprina

Usnea antarctica

FIGURE 13.5 The typical rise in light compensation point with temperature for four lichens measured in

the field in Antarctica. The most continental species, Umbilicaria aprina, shows the least change. (Data

from Schlensog, M., Schroeter, B., Sancho, L.G., Pintado, A., and Kappen, L., Secc. Biol. 93, 107, 1997a

[Leptogium puberulum]; Sancho, L.G., Parmelia, A., Valladares, F., Schroeter, B., and Schlensog, M., Bibl.

Lichenol. 67, 197, 1997 [Parmelia saxatilis], Schroeter, B., Kappen, L., and Moldaenke, C., Lichenologist,23,

253, 1991 [Usnea antarctica], and Schroeter, B., Grundlagen der Stoffproduktion von Kryptogamen unter

besonderer Beru

cksichtigung der Flechten—eine Synopse—Habilitationsschrift der Mathematisch—

Naturwissenschaftliche Fakulta

t der Christian—Albrechts—Universita

t zu Kiel, 1997 [U. aprina].)

PPFD (µmol m

−2

−1

)

0 250 500 750 1000 1250 1500

Topt

(°C)

−5

Leptogium puberulum

Parmelia saxatilis

Umbilicaria nylanderiana

Usnea antarctica

FIGURE 13.6 The typical change in Topt (optimal temperature for net photosynthesis) with increase

in irradiance (PPFD in mmol Photons m

2

1

) for four lichens measured in the field in Antarctica.

Sources as for Figure 13.5 except Umbilicaria nylanderiana. (From Sancho, L.G., Pintado, A.,

Valladares, F., Schroeter, B., and Schlensog, M., Bibl. Lichenol., 67, 197, 1997.)

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Plant Life in Antarctica 401

Sancho et al. 1997). In complete contrast to mosses, positive NP has been found to

temperatures as low as 178C for U. aprina Nyl. measured in the field (Schroeter et al.

1994) although this ability is not confined to Antarctic lichens, for example: 248C for

Cladonia alcornis (Lightf.) Rabh. measured in the laboratory (Lange 1965).

Fewer studies of the effect of WC on photosynthesis exist but those that do show the

characteristic low WC range (200%–400% maximal WC) compared with mosses and often

with depressed NP at high WC due to increased CO

diffusion resistances (Kappen 1985b,

Harrisson et al. 1989, Kappen and Breuer 1991, Schroeter 1991). Lichens in the Antarctic

normally occupy much drier habitats than mosses and are not found in wet areas unless on

the drier tops of mosses. Response to CO

concentration seems to have not been measured but

would be expected to be similar to that of temperate lichens that is, not saturated at ambient

levels. The increased ambient CO

reported around mosses would not be expected

around lichens because of their elevation and lack of substantial organic substrates below

them (Tarnawski et al. 1992).

MAXIMAL RATES OF PHOTOSYNTHESIS

Longton (1988a) gives an extensive list of maximal NP and Topt for a wide range of polar

bryophytes and lichens although only a few are from the areas covered in this article. Kappen

(1988, 1993a) gives additional, similar data for lichens. The brevity of both lists indicates the

lack of knowledge that exists at present. Considerable variation in rates certainly exists for

lichens, from 0:08 mg CO

1

for Rhizoplaca melanophthalma (Ram.) Leuck and Poelt,

to 0:8mgCO

1

for U. aprina. Although it is occasionally suggested that the abun-

dance of particular lichen species may be related to photosynthetic performance we do not

really yet have the data to be certain. Lechowicz (1982) has analyzed the relationship between

several photosynthetic parameters and latitude for the Northern Hemisphere. From his

analysis maximal NP are certainly substantially lower at latitudes greater than 658 N and

these far-northern values are similar to those found in the Antarctic. Kappen (1988) states

that maximal rates of NP are lower for lichens in continental Antarctic than in the maritime

zone. No similar analysis exists for mosses but, in general, the rates found seem to be close to

the higher rates reported elsewhere (1---2 mg CO

g:d:wt:

1

, Rastorfer 1972, Longton

1988a, Kappen et al. 1989) and no depression is obvious.

DIEL AND LONG-TERM PHOTOSYNTHETIC PERFORMANCE

Continuous measurement of photosynthetic performance over a day, or several days, has only

recently become common in Antarctica and a few extensive studies have been made in the

northern maritime (e.g., Kappen et al. 1991, Schroeter 1991, Schroeter et al. 1991, Sancho

et al. 1997 for lichens, Collins 1977 for mosses). A more common approach to obtain an

estimate of seasonal production is to log microclimate parameters and to interpolate plant

performance from photosynthetic response to PPFD and temperature (e.g., Kappen et al.

1991, Schroeter 1991, Schroeter et al. 1995). The present situation is probably a reflection of

the difficulties of working in Antarctica with large and expensive equipment. Kappen et al.

(1991) obtained eight diurnal records for U. sphacelata near Casey Station and recorded

positive photosynthesis through the entire light period of the day even though temperatures

were mostly below 08C. They were able to demonstrate relatively good agreement between

their model, based on PPFD and temperature, and in situ rates of photosynthesis. Sancho

et al. (1997) obtained diurnal courses for three cosmopolitan lichen species, Parmelia saxatilis

(L.) Ach., Pseudophebe pubescens (L.) M. Choisy, and Umbilicaria nylanderiana (Zahlbr.)

H. Magn., over three months of the summer, 1995, in the maritime Antarctic (Livingston

Island) and found activity only for 182 h. Typical examples of diel patterns for two lichens

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402 Functional Plant Ecology

(P. saxatilis and U. aprina) and one moss (B. subrotundifolium) are given in Figure 13.7,

Figure 13.8, and Figure 13.9. The two examples from continental Antarctica (Figure 13.8 and

Figure 13.9) show positive NP 24 h a day, whereas the maritime example (P. saxatilis, Figure

13.7) had negative NP overnight and the daily carbon balance was often negative as, also, was

the cumulative balance calculated over 3 months (Sancho et al. 1997).

Schroeter et al. (1991) demonstrated that basal chlorophyll a fluorescence could be used

to monitor photosynthetic activity of Usnea antarctica Du Rietz. Photosynthetic carbon

fixation could be estimated using microclimate records of temperature and PPFD (Schroeter

1991, Schroeter et al., 1997a,b). Schroeter et al. (1992) demonstrated that the photosynthetic

activity of the crustose lichen, B. frigida (Darb.) Dodge at Granite Harbour could easily be

monitored with a chlorophyll a fluorescence system and this has now become a common

investigative method. Concurrent measurements of CO

exchange and chlorophyll a fluores-

cence revealed complex relationships between ETR (relative electron transport rate calculated

from the fluorescence signal) and photosynthetic rate for lichens, and simpler but still not

linear relationships for mosses (Green et al. 1998). Schroeter et al. (1997c) measured fluores-

cence activity of B. frigida over several days at Granite Harbour. The studies revealed the

extremely erratic nature of thallus moistening, which depended on small-scale topography,

proximity to snow, degree of snow melt, and snow fall (Figure 13.10). Even though air

500

1000

1500

2000

100

200

300

400

−2

−1

612180

Local time (Feb. 8/9, 1995)

Parmelia saxatilis

PPFD (µmol m

−2

−1

)

WC (% d.wt.)

TT (°C)

NP (µmol CO

−1

)

612180

FIGURE 13.7 Diel pattern of net photosynthesis (upper panel, mmol CO

1

), water content

(middle panel in % d.wt.) and thallus temperature and irradiance (lower panel, PPFD in

mmol Photons m

2

1

) for Parmelia saxatilis on February 8 and 9, 1995 measured at Livingston Island.

Because this is a maritime site there is a brief period of darkness each day leading to a period of negative

NP. Measurements were made in the field at a natural growth site using a porometer system and the

lichen was rewetted on the 8th by rainfall. (From Sancho, L.G., Pintado, A., Valladares, F., Schroeter, B.,

and Schlensog, M., Bib. Lichenol, 67, 197, 1997.)

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Plant Life in Antarctica 403

temperatures remained between 7.58C and 12.08C lichen thallus temperatures were

much higher, often around 158C higher, even when wet. Thus, most of the photosynthetic

activity actually occurred at positive temperatures, but was detectable down to 88C, and no

saturation was evident at 1500 mmol m

2

1

. These results, which were made in November

at 778 S, indicate that the growing season could be much longer than previously thought, even

at these high latitudes. The introduction of imaging systems for chlorophyll a fluorescence has

allowed even finer analysis of the responses to water and drying (Barta

k et al. 2005).

A feature of all the diurnal studies is the elevation of thallus temperatures above ambient

air temperatures because of the absorbed irradiation. Kappen (1993a) noted that thallus

temperatures of wet lichens were often about 88C–108C at many maritime and continental

sites as a result of shelter and sun exposure, although B. frigida at Granite Harbour could

reach 158C (Schroeter et al. 1997c, Kappen et al. 1998b). Monitoring of the photosynthetic

activity of P. heimii on the Canada Glacier flush (Taylor Valley) showed that, in late

−15

−10

−5

0.0

0.5

1.0

1.5

2.0

Umbilicaria aprina

400

800

1200

1600

61218

Local time (Nov. 19/20, 1994)

PPFD (µmol m

−2

−1

)

TT (°C)

0 6 12 18 0

NP (µmol CO

−1

)

FIGURE 13.8 Diel pattern of net photosynthesis (upper panel, mmol CO

2

1

), thallus temperature

(middle panel, 8C), and irradiance (lower panel, PPFD in mmol Photons m

2

1

) for Umbilicaria aprina

on November 19 and 20, 1994 at Botany Bay, Granite Harbour. The lichen was rewetted at the points

marked with arrows in the upper panel. Although this is early in the season at a continental site (778 S)

the irradiance reaches high values each day and there is no period of complete darkness. Hence positive

NP can occur through the full 24 h and also, on the 19th at subzero thallus temperatures. (Data from

Schroeter, B., Grundlagen der Stoffproduktion von Kryptogamen unter besonderer Beru

cksichtigung

der Flechten—eine Synopse—Habilitationsschrift der Mathematisch—Naturwissenschaftliche Fakulta

der Christian—Albrechts—Universita

t zu Kiel, 1997.)

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404 Functional Plant Ecology

December, the plant was continuously active and did not face subzero temperatures. The wet

plants were buffered from the colder air by ice formation at zero degrees (Figure 13.12).

NUTRITION EFFECTS

The role of nutrients in the abundance and diversity of Antarctic plants is not well known

and requires more detailed studies. Elemental analyses for mosses (Smith 1996) show consider-

able variation depending on proximity to birds and species. Total nitrogen ranged from a low

0.55%–0.77% for P. alpestre to over 3% for Drepanocladus (Sanionia) uncinatus (Hedw.)

Loesk. Rastorfer (1972) found 2.95%, 2.75%, 1.50%, and 2.35% for Calliergidium austrostra-

mineum, Drepanocladus uncinatus, Polytrichum strictum Brid. (syn. P. alpestre), and Pohlia

nutans (Hedw.) Lindb., respectively. These values would not seem to be limiting and are not

likely to be the major control on production. Lichens, in contrast, can have very low

nitrogen contents, for example: 0.36% and 0.83% for U. sphacelata and Usnea aurantiaca-

atra (Jacq.) Bory, respectively (Kappen 1985b). Lichens with green algal photobionts seem

to almost always have below 1% total nitrogen (Green et al. 1980) and fruticose species are

the lowest (Green et al. 1997). However, chlorophyll content, which can be related to

0.0

0.2

0.4

0.6

0.8

400

800

1200

61218

Local time (h) (Jan. 19/20, 1992)

PPFD (µmol m

−2

−1

)

TT (°C)

0 6 12 18 0

NP (mg CO

g Chl

−1

)

FIGURE 13.9 Diel pattern of net photosynthesis (upper panel mg CO

Chl

1

), thallus temperature

(middle panel, 8C), and irradiance (lower panel, PPFD in mmol Photons m

2

1

) for moist Bryum

subrotundifolium on January 19 and 20, 1992 at Botany Bay, Granite Harbour. This is just after mid-

summer and NP is positive through the entire period. Thallus temperatures are almost always positive and

are linked to the irradiance. (From Schroeter, B., Grundlagen der Stoffproduktion von Kryptogamen

unter besonderer Beru

cksichtigung der Flechten—eine Synopse—Habilitationsschrift der Mathematisch—

Naturwissenschaftliche Fakulta

t der Christian—Albrechts—Universita

t zu Kiel, 1997.)

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Plant Life in Antarctica 405

nitrogen content (Green et al. 1997), is much lower in Antarctic than in temperate species

(Sancho et al. 1997). In essence, the values are little different to those found for temperate

lichens and mosses.

Nutrient supply, especially nitrogen and phosphorous, is strongly influenced by the

presence of birds and other animals. Leishman and Wild (2001) showed that soil nutrient

were enriched near to sea bird nests and declined rapidly with increasing distance. The

number of lichen species and mean lichen species abundance increased with soil nutrient

levels, with total P having a stronger influence than total N. In contrast, moss diversity and

abundance showed little correlation with soil nutrient levels but was positively correlated with

soil water content. Although nutrient deficiency has rarely been shown (Kappen and Schroeter

PPFD (µmol m

−2

−1

) Temperature (°C) ETR (∆F/F

⫻ PPFD)

500

1000

1500

2000

−15

−10

−5

200

400

600

17 18 19 20 21

November 1994

22 23 24 25 26

FIGURE 13.10 Diel pattern of relative electron transport rate through Photosystem II, ETR, (upper

panel, mmol e



2

1

), thallus and air temperatures (middle panel, 8C), and irradiance (lower panel,

PPFD in mmol Photons m

2

1

) for a Buellia frigida thallus over a 10 day period, November 16–25, 1994,

at Botany Bay, Granite Harbour. Although air temperatures remain around 108C through the whole

period the thallus temperature is positive at times of high irradiance and several degrees above air

temperature during the night because of heat storage in the rock surface. ETR depends on both irradiance

and moistening of the thalli. Initially the lichen received water from melting snow on the rock but, as

the snow patch retreated away from the lichen, it was active only after snow falls (indicated by the stars in

the upper panel). ETR activity is least or zero in the late morning when the lichen has dried out, highest in the

afternoon, after wetting by melt, and continues overnight at subzero temperatures—the so-called reverse

diel pattern (see text). (From Schroeter, B., Grundlagen der Stoffproduktion von Kryptogamen

unter besonderer Beru

cksichtigung der Flechten—eine Synopse—Habilitationsschrift der Mathematisch—

Naturwissenschaftliche Fakulta

t der Christian—Albrechts—Universita

t zu Kiel, 1997.)

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406 Functional Plant Ecology

2002), addition of nutrients causes increased photosynthetic rates, ETR (relative electron

transport rate from chlorophyll fluorescence), and chlorophyll content (Smith 1993, Wasley

et al. 2006). Although it is normally claimed that this indicates nutrient limitation, it may be

that the plants are not carbon limited and the enhancement may represent utilization of storage

materials over several years. Longer-term increases in nutrient supply lead to changes in

community structure and biodiversity rather than just increased growth. This can be clearly

seen for mosses in the Ross Sea region. At low nutrient sites such as Taylor Valley, the moss

patches contain only two main species. At a more nutrient rich site, Granite Harbour or

Edmonson Point, four or five species can be common. Finally, where there is an overwhelming

presence of birds, Cape Hallett or Beaufort Island, then only B. subrotundifolium is found.

Overall, the major determinant for the occurrence of mosses, in particular, and lichens is the

availability of liquid water. Extra water may not enhance photosynthesis as much as when

nutrients are also present (Wasley et al. 2006). Finally, there are also suggestions that salt can

control distributions close to the coast (Broady 1989).

SURVIVING ANTARCTICA’S EXTREMES

There are few articles, and possibly even fewer grant applications, written that do not mention

some extreme aspects of Antarctica, usually the climate, and the accepted fact that the

organisms show adaptations to the stresses that they face. Of course the Antarctic climate

is extreme to us, but is it extreme to the organisms that survive there, and do they show special

adaptations; these are important questions in our understanding of potential responses to

change.

DESICCATION

All Antarctic terrestrial biota face a long period of cold, darkness, and desiccation over the

winter. Conditions are particularly extreme in continental Antarctica (Table 13.1) and there

can be little doubt that this limits the vegetation to mainly species of lichens and bryophytes.

Both groups are poikilohydric, meaning that their thallus hydration tends to equilibration

with the water status of the environment (Green and Lange 1994). Water uptake tends, with

the exception of the endohydric mosses like Polytrichum, to occur over the entire surface.

Typically, these groups also show exceptional resistance to desiccation and can survive very

low water contents for long periods. Such resistance is, however, not inevitable and some

lichens, inhabitants of consistently moist rainforest, are very sensitive to desiccation (Green

et al. 1991). The Antarctic liverwort M. berteroana (see earlier Section ‘‘Liverworts’’) is also

very sensitive (Davey 1997a).

Schlensog et al. (2004) studied the ability of lichens and mosses to recover from desicca-

tion after an Antarctic winter at Granite Harbour (778 S). They found that, although all

species showed recovery starting within minutes as measured by chlorophyll fluorescence, the

lichen U. aprina, from open rock surfaces, reached full photosynthetic activity in just over an

hour, whereas the mosses, B. subrotundifolium and Hennediella heimii from running water

sites, took almost 24 h (Figure 13.11). This difference cannot be generalized to all bryophytes

as various mosses from xeric environments are known to reactivate within minutes (Proctor

2000). Moreover, the lichen P. caesia, characteristic of submerged water channel sides, took

considerably longer than U. aprina to recover (Figure 13.11). The results fit with the concept

from non-Antarctic bryophytes that the tolerance and activation of poikilohydric organisms

can be correlated to their normal active environment. That is, plants from xeric environments

are strongly desiccation-tolerant and activate rapidly, and the opposite occurs for plants from

consistently wet environments.

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Plant Life in Antarctica 407

EXTREME COLD

In the dry state, poikilohydric organisms tend to be resistant to environmental extremes. Dry

lichens all survived liquid nitrogen temperatures (1968C, Kappen 1973) and moist thalli of

Antarctic species (Xanthoria candelaria (L.) Th. Fr., R. melanophthalma) fully tolerated slow

or rapid freezing to 1968C (Kappen and Lange 1972). Extended periods of cold and dryness

also had little effect, Alectoria ochroleuca (Hoffm.) Massal recovered totally after 3.5 years at

608C (Larson 1978) and the moss Schistidium antarctici withstood 18 months at 188C

(Kappen et al. 1989). Despite the impressive figures given above it is equally clear that

considerable differences do exist between species, both lichens and mosses, in their abilities

to withstand cold. Liverworts, based on the evidence from M. berteroana, seem to be excluded

from continental Antarctica, because of their lack of cold tolerance, as also are cyanobacterial

lichens (Lange 1965, Schroeter et al. 1994). Lichens also show a range of abilities to photo-

synthesize below 08C (Lange 1965).

There are, however, several reports of damage through subzero temperatures to young

shoot apices in mosses (Longton and Holdgate 1967, Collins 1976) and it does appear that

these highly hydrated shoots are at risk. Generally, however, it must be accepted that the

plants in Antarctica can survive the winter cold of their habitat.

FREEZE–THAW CYCLES

Freeze–thaw cycles can be extremely common in Antarctica with up to 110 in 1 year recorded

in the northern maritime (Longton 1988a). The cycles, although rarely falling to temperatures

more than a few degrees below freezing point, are thought to provide a severe stress to the plants

through intracellular freezing damaging tissue, extracellular freezing disrupting structure,

dehydration by withdrawal of water to external ice and phase changes in membranes leading

to loss of cell contents. Lovelock et al. (1995a,b) studied the effects of freeze=thaw cycles on the

photosynthesis of G. antarctici as monitored using chlorophyll a fluorescence. Every subzero

cycle caused decreased photosynthetic efficiency but recovery at low light was also rapid.

The most thorough analysis has been by Kennedy (1993a) for a northern maritime

species, P. alpestre, an endohydric moss in contrast to the entirely exohydric species of the

Elapsed time after sprayin

(h)

0 20 40 60 80

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Bryum subrotundifolium

Umbilicaria aprina

Physcia caesia

FIGURE 13.11 Recovery of F

(optimal quantum efficiency of Photosystem II measured after

darkening) for specimens that had been rehydrated after collection in a dehydrated state under

snow in early summer. Note the rapid recovery of Umbilicaria aprina (rock surface lichen), slow recovery

of Bryum subrotundifolium (moss from continually wet areas), and intermediate rate for Physcia caesia

(a lichen species that borders intermittent water flows and can be submerged for some time each day).

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408 Functional Plant Ecology

continental Antarctic. The species proved to be sensitive to freeze=thaw cycles with reduction

in GP almost directly proportional to the temperature reached and with no recovery at 58C

or below. Almost all damage occurred on the first freeze cycle and plants with lower thallus

water contents were less sensitive. The mechanism of damage was not clarified but could well

have been membrane disruption since increased nitrogen loss from the plants followed the

freezing (Greenfield 1988). Kennedy (1993b) was of the opinion that freeze= thaw cycles could

easily limit species distribution, however, it must be questioned as to how severe this stress

actually is, especially to exohydric mosses. Continual recordings of photosynthetic activity of

P. heimii in Taylor Valley (Pannewitz et al. 2003a) showed that freezing of surrounding water

buffered the moss so that it faced temperatures only a few tenths of a degree below freezing

(Figure 13.12). Actual freezing temperature of the moss initiates about 1.88C (personal

research observations). It appears that, for much of the summer season, mosses in wet areas

are rarely exposed to subzero temperatures and that freeze–thaw events may be confined to

drier environments.

Wetting=drying cycles, also common in the Antarctic, are also thought to be disruptive

and release of carbohydrates has been detected (Davey 1997b, Greenfield 1993, Melick and

Seppelt 1992). Friedmann et al. (1993) estimated that examples of the cryptoendolithic

community in the Ross Desert would pass through an active=inactive cycle (either wet=dry

or freeze=thaw) a minimum of 120–150 days each year with substantial loss of metabolites

(Greenfield 1988). The stresses associated with these cycles contributed to the difference

between a modeled net production of 106 mg C m

2

year

1

and actual growth, estimated

using a variety of techniques, to be 3mgCm

2

year

1

Studies on U. aprina, going through a regular daily wetting=drying event near a retreating

snow patch showed no deleterious effects on the lichen (Schroeter et al. 1997c). A fuller

understanding of these transients is still needed before we can better gauge their effects on

2000/2001

16 18 20 22 24 26 28 30 1 3

MOSS temperature (°C)

–5

–10

–5

December January

AIR temperature (°C)

FIGURE 13.12 Uncoupling of thallus temperature (lower panel, 8C) from air temperature (upper panel,

8C) for Pottia heimii measured over an 18 day period in mid-summer at the Canada Glacier flush, Taylor

Valley, 778 S. Note how the air temperature is substantially negative each day, whereas the wet and

active moss never goes subzero. The temperature maxima and minima for air=moss were 4.0=10.0

and 7.0=0.18C, respectively. (Data modified from Pannewitz, S., Green, T.G.A., Scheidegger, C.,

Schlensog, M., and Schroeter, B., Pol. Biol., 26, 545, 2003a.)

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