Pugnaire F.I. Valladares F. Functional Plant Ecology

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productivity. However, it appears certain that the severity of any effect depends on the

normal habitat of the plant, in particular how xeric it is.

PHOTOSYNTHESIS AT SUBZERO TEMPERATURES

Lichens have long been known to carry out photosynthesis at subzero temperatures. Lange

(1965) surveyed a wide range of species including some from Antarctica and found photo-

synthesis to 248CbyC. alcornis, a temperate lichen. Other species, Umbilicaria decussata

(Vill.) Zahlbr., and P. caesia (Hoffm.) Fuernr. (Parmelia coreyi in article) from Cape Hallett,

had limits of 118C and 148C, respectively, values that are similar to species from hot

deserts (Lange and Kappen 1972). Schroeter et al. (1994) have since measured photosynthesis

of U. aprina to 178C in the field. In their analysis of the response of NP to subzero

temperatures they proposed that tolerance of subzero temperatures had the same physio-

logical basis as tolerance to low water potentials. This impression was gained from the very

similar response shown by photosynthesis to low water potentials whether generated by

subzero temperatures or by equilibration with low atmospheric humidity (Kappen 1993b).

In both cases water is removed from the cells either to intercellular ice, lichens have external

ice-nucleating agents with freezing initiating at around 58C (Ashworth and Kieft 1992,

Schroeter and Scheidegger 1995), or to the atmosphere. Schroeter et al. (1994) suggested

that this explained the absence of cyanobacterial lichens in continental Antarctica because

they cannot photosynthesize in equilibrium with humid air but need liquid water (Lange et al.

1988, Schroeter 1994). In addition, Schroeter and Scheidegger (1995) demonstrated convin-

cingly that lichen thalli could rehydrate at subzero temperatures only in the presence of ice.

Thallus water content depended on the temperature being only 7% of maximal WC at 218C,

18% at 4.58C, and nearly 100% at 88C (Figure 13.13). Concurrent use of a low temperature

scanning electron microscope showed the progressive refilling of the algal and fungal cells as

the temperature was increased with the water coming, via the atmosphere, from extracellular

ice (Schroeter and Scheidegger 1995). At the same time photosynthesis commenced and

increased in line with thallus WC.

Water content (% d.wt.)

–4 –6 –8 –10 –12

Thallus temperature (⬚C)

–14 –16 –18 –20 –22

FIGURE 13.13 The relationship between thallus water content (% d.wt.) and thallus temperature for

Umbilicaria aprina (Schroeter and Scheidegger 1995). Dry lichen thalli where equilibrated in the dark

for 24 h at the selected temperature and in the presence of ice.

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410 Functional Plant Ecology

A common mechanism would also explain the correlation between cold resistance and dry

habitats, such as deserts, found for lichens. The concept has even more far-reaching interest

when it is realized that their distribution pattern of Antarctic plants better reflects their rank

order for subzero photosynthesis than their desiccation or cold tolerance (Table 13.3). For

instance, whereas cyanobacterial lichens have very good resistance to desiccation and to low

temperatures, they have nearly no ability to photosynthesize at subzero temperatures, and are

excluded from continental Antarctica.

PHOTOSYNTHESIS UNDER SNOW

Kappen (1989) demonstrated that rehydration and reactivation of U. sphacelata occurred in

the field under snow and at subzero temperatures in the maritime Antarctic. This was an

important observation because, taken with the later studies of Schroeter and Scheidegger

(1995) it appeared that green algal lichens can reactivate photosynthesis under snow without

liquid water present that is, without the need of a thaw cycle. The influence of snow cover has

been reviewed by Kappen (1993b, 2000). Snow can provide an efficient insulation against wind

and extreme temperatures. Considerable light, equivalent to around 10%–30% of incident

values, can penetrate to the base of a 15 cm snow pack (Kappen and Breuer 1991), and this is

more than sufficient to saturate NP at the low temperatures. The performances of northern

maritime Antarctic mosses under snow (Collins and Callaghan 1980) and Cetraria nivalis in

Sweden (Kappen et al. 1996) were modeled and considerable photosynthetic production was

found. It seems almost certain that substantial photosynthesis can occur long before the

temperatures reach above freezing and while the lichens are still covered with snow.

The situation, however, appears to be different in continental Antarctica. In an innovative

experiment Pannewitz et al. (2003b) constructed fiber optics to measure lichen and moss

activity in late summer at Granite Harbour (778 S) and left them to be covered by winter

snow. In the following early summer it was then possible to follow the chlorophyll a

fluorescence activity of the samples under the snow without disturbance. In a complete

contrast to the maritime Antarctic there was no evidence of reactivation until liquid water

formed when temperatures reached freezing point (Figure 13.14). Temperatures under the

snow had, over the long winter, equilibrated with those above the snow and the insulating

TABLE 13.3

A Summary of Physiological Abilities of Cyanobacterial Lichens, Green Algal Lichens,

Liverworts, and Mosses with Respect to Photosynthetic Performance at Subzero

Temperatures

Plant Type

Cyanobacterial

Lichens Liverworts Mosses

Green Algal

Lichens

Distribution Maritime only Mainly maritime Throughout Antarctica

Subzero photosynthesis to 2



C Little to 8



Cto24



Positive NP In humid air None ? Little ability Above 80% rh

Desiccation=

Cold tolerance: WET

High Low Low=medium High

DRY Very high Low High Very high

Note: Positive NP in humid air refers to the ability of the group to attain positive net photosynthesis from a dry

condition only in the presence of humid (90%–95% relative humidity) air. Desiccation=cold tolerances are given in

two rows, the upper when wet and the lower when dry.

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Plant Life in Antarctica 411

properties of the snow then prevented rapid warming in the early summer of the buried

plants. Far from providing a suitable environment for production the snow delayed any

activity until it was completely melted. Increases in snow fall could, therefore, have a very

negative effect on lichen and moss biodiversity by shortening the productive season. The

effect of large snow banks is clearly visible at Cape Hallett, where there is high lichen

abundance at the outer edges of the snow banks but a zone with no lichens appears when

the snow bank edge retreats later in the season.

REVERSE DIEL CYCLE OF PHOTOSYNTHESIS AND HIGH LIGHT STRESS

Being poikilohydric, lichens and mosses depend on water to rehydrate and become active.

Typically, even in Antarctica, this means that liquid water is required (Hovenden and Seppelt

1995); the exception would be the rehydration by lichens at subzero temperatures. For

mosses, in particular, liquid water is certainly required and the larger biomasses are found

where consistent water flows occur. In temperate zones water is normally provided by rain,

which typically occurs at cloudy times. Mosses and lichens rapidly dry out as soon as brighter

light conditions and full sunshine reoccur. Desiccation has often been given as one of

the means used by poikilohydric plants to avoid high light and high temperature stress

(Kappen 1988).

The situation in the Antarctic tends to be the opposite, melt water only occurs when

insolation and temperatures are high so that the lichens and mosses are active when the light

is brightest. This can be seen in Figure 13.10 where the photosynthetic activity of B. frigida

was monitored using chlorophyll a fluorescence (Schroeter et al. 1997). Initially when sunlight

reaches the thalli in the early morning they rapidly dry and become inactive. Then, about

noon, the snowmelt rehydrates the thalli and maximal activity occurs. The thalli then remain

photosynthetically active, even when frozen overnight, until drying again occurs at sunrise the

next morning. Mosses also display this reverse pattern since water flow from snow or glacier

melt is always at its maximum during the day and when irradiance is greatest.

Date (2000)

10/11 15/11 20/11 25/11 30/11 5/12

Temperature (°C)

–15

–10

–5

–10

–5

Candelariella flava

Physcia dubia

FIGURE 13.14 Insulating effect of a snow bank at Cape Geology, Granite Harbour (778 S): chlorophyll

a fluorescence (solid line) and thallus temperature (dotted line) were measured for the two lichens

Candelariella flava (upper panel) and Physcia dubia (lower panel) using probes put in place the previous

year and allowed to become naturally covered with snow. Note how the thallus temperature slowly rises

(ambient temperatures were around zero degrees) showing a slight daily fluctuation due to incident

radiation. Photosynthetic activity does not start until the temperature is close to zero and free water

appears.

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412 Functional Plant Ecology

High light stress is, therefore, of unexpected importance in Antarctica. The severity of the

conditions can be seen in Table 13.4 that shows the mean conditions when the lichens studied

were active. There is a major difference between the continental site at Granite Harbour,

mean PPFD about 55% of full sunlight, and the temperate and maritime sites, mean PPFD

about 5% of full sunlight.

A combination of cold temperatures and high light has been found to be particularly

likely to cause photoinhibition in some higher plants and a similar response is sometimes

suggested for Antarctic mosses and lichens. In mosses, photoinhibition has, indeed, been

reported several times in studies in both the maritime and continental Antarctica. A midday

depression in NP was consistently found under high light by Collins (1977) and was built into

models of photosynthetic production by Collins and Callaghan (1980) and Davis (1983).

Adamson et al. (1988) demonstrated severe photoinhibition in Grimmia (Schistidium) antarctici.

Depressed photosynthesis occurred after 100 min at 500 mmol m

2

1

and CO

exchange was

negative after 150 min at 1000---1800 mmol m

2

1

and there was a corresponding fall in F

the variable fluorescence.

Later studies have tended not to confirm these earlier results and especially not so in

continental Antarctica. Lovelock et al. (1995a,b) demonstrated the occurrence of photoin-

hibition, measured with chlorophyll a fluorescence, during freeze=thaw cycles. Plants that

were originally under snow showed severe photoinhibition if the snow was removed and they

became exposed to full irradiance. Lovelock et al. (1995a,b) showed full recovery to occur

under warmer temperatures and under low light, and they suggested that photoinhibition was

more of a protective process than one of damage. Post et al. (1990) showed similar photo-

inhibition and recovery for C. purpureus. Post (1990) also showed that the ginger pigment

found in exposed plants of C. purpureus appeared to serve a protective function against high

irradiance. In studies on sun- and shade-adapted B. subrotundifolium, Green et al. (2000)

showed that only the shade-adapted form, which was bright green, was susceptible to high

light. It appeared that the silvery shoot points and nonphotochemical quenching within the

photosystems could protect the sun form against several hours of full sunlight.

Although Kappen et al. (1991) reported photoinhibition, indicated by depressed NP, for

U. sphacelata under high light, no signs of photoinhibition were found when U. aprina was

monitored for 2 days under full natural sunlight even though the thalli had been dug from

under 70 cm of snow before receiving any sunlight in that summer (Kappen et al. 1998a).

Similarly, Schlensog et al. (1997b) found no signs of photoinhibition for L. puberulum after

TABLE 13.4

Mean Values for Incident Radiation and Thallus Temperature for Three Species of Lichens

in Spain, the Maritime Antarctic and the Continental Antarctic

Spain, Guadarrama

Summit, Madrid,

(2000 m) 418 N

Lasallia hispanica

Antarctica

Livingston I.

(10 m) 628 S

Usnea

aurantiaco-atra

Antarctica Granite

Harbour (10 m) 778 S

Umbilicaria aprina

Radiation mean

PAR (mol m

2

day

1

)

All values 17.5 9.8 38

Active only 2.7 3.8 77

Mean temperature (8C) All values 9.7 2.2 9.74

Active only 4.5 1.1 2.9

Note: The mean values have been calculated in each case either for the entire measuring period (1, 14, and 3 years,

respectively) and for times when the lichens were active (Active only).

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Plant Life in Antarctica 413

extensive treatment with high light (Figure 13.15). It seems that pigments in lichens act to

reduce the light level within the thallus and thus protect the photobionts (Schroeter et al.

1992, Buedel and Lange 1994, Rikkinen 1995, Schlensog et al. 1997b).

UV RADIATION

There has been considerable interest in the possible effects of UV (especially UV-B) radiation

on Antarctic plants since the presence of the ozone hole in spring over Antarctica since the

1980s (Robinson et al. 2003, Convey and Smith 2006). Many experiments have been carried

out usually involving the removal of UV-B by filters (Robinson et al. 2003). The typical

response is subtle and by changed growth suggesting reallocation of resources (Ruhland and

Day 2000, Lud et al. 2002, Robinson et al. 2005). Despite earlier worries that Antarctic plants

would suffer extensively because they had evolved under very low UV-B levels (not actually

correct because levels are high around midday) and because they were simple in structure

(Gehrke 1998, Gwynn-Jones et al. 1999) recent studies indicate otherwise. Newsham et al.

(2002) have shown that several bryophytes on the Antarctic Peninsula can rapidly (within a

day) adjust their UV-B protection to match UV-B incidence. Green et al. (2005) have shown

that the sun form of B. subrotundifolium is well protected against incident UV-A and that,

when this protection is lowered by shading, it is reinstated within a few days following

reexposure with little obvious effect on plant performance (Figure 13.16). Studies on DNA

damage by the formation of cyclobutyl pyrimidine dimer formation showed that the moss

Sanionia uncinata (Hedw.) at Leonie Island in the maritime Antarctic (678 35

S) showed that

any negative effects were transitory and that the species appeared to be well adapted to

ambient levels of UV-B radiation (Lud et al. 2002, 2003). When these results from mosses are

coupled with the extreme protection to incident radiation possessed by lichens, no effect of

full sunlight for 2 days was found on U. aprina samples that were immediately exposed after

uncovering from snow following winter (Kappen et al. 1998a), it must now be concluded that

UV radiation is unlikely to be a stress for these plants except for manipulated situations where

UV levels have been rapidly changed.

100

GP (% maximal rate)

0 50 100

Before strong light treatment

Directly after strong light treatment

After 24 h of recovery

PPFD (µmol m

–2

–1

)

500 1000

FIGURE 13.15 The almost complete lack of photoinhibition in the cyanobacterial lichen Leptogium

puberulum despite treatment with strong light (3 h at 1600 mmol m

2

1

PPFD). Immediately after the

treatment there was a slight but insignificant decline in NP at intermediate irradiances but no change in

apparent quantum efficiency (initial slope of the response to irradiance). (Modified from Schlensog, M.,

Schroeter, B., Sancho, L.G., Pintado, A., and Kappen, L., Bibliotheca Lichenologica, 67, 235, 1997b.)

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414 Functional Plant Ecology

ENDOLITHIC LICHEN COMMUNITY

Under extremely dry conditions in Antarctica, and in other deserts, lichens can adopt an

endolithic growth form where they live within the pores of rocks composed of materials like

sandstone and limestone (Friedmann and Galun 1974). The most studied endoliths are the

‘‘cryptoendoliths’’ that penetrate to 2 cm in the Beacon Sandstone of the Dry Valleys,

Southern Victoria Land, 778 S, (Friedmann 1982, Kappen 1988). Other species such as

Lecidea phillipsiana also grow widely in East Antarctica in granites where they produce a

prominent brown color on the surface and cause extensive rock flaking (Hale 1987). The

lichens form a layered structure within the rock with the upper layer, containing fungal

hyphae and the lichen green algal symbiont Trebouxia, that had a dark color, possibly to

reduce light intensities. The endoliths grow only on the faces of rocks where higher insolation

is received, north-facing or horizontal. On a sunny day the temperature of the rock can

reach þ88C, whereas the air temperature is still lower than 58C below freezing (Kappen et al.

1981). Humidity within the rock is around 80%–90%, which is considerably higher than air,

which is normally around 30%–40% rh. Because the temperature is so strongly dependent on

insolation it can fluctuate markedly during a day and freeze=thaw transitions are common.

A typical daily pattern of the internal rock environment is shown in Figure 13.17 and a

freeze=thaw or wet=dry transition is expected to occur at least once on all days when

metabolic activity occurs, around 120–150 per year. Water is provided to the rock by blowing

snow which then melts into the rock when it is warmed by sunlight. The endoliths are wetted

either by equilibration with the high humidity within the rock or by direct moisture uptake

after snow melt. The latter method is thought to be the most important in the Dry Valley

region (Friedmann 1978).

Carbon metabolism (incorporation of H

) was saturated at 150 mmol m

2

1

08C, and had an optimum at 158C (Vestal 1988). Measurements of CO

exchange showed

it to be maximal at 38 C68C and to be still positive at 108 C. In a classic long-term study

of the nanoclimate using automated recording systems reporting over satellites, 3 years

UV-A Protection (%)

23 24 25 26 27 28 29

January

Sun form

Shade form

FIGURE 13.16 Changes in UV-A protection (% depression of chlorophyll a fluorescence) of the moss

Bryum subrotundifolium measured in situ with a UV-A PAM chlorophyll a fluorometer (Gademann.

Instruments, Wu

rzburg, Germany). The UV-A protection of the sun form of the moss remained

constant, whereas the protection of a shade form (created by shading the moss for 10 days) rapidly

increased back to normal over 6 days. (Modified from Green, T.G.A., Kulle, D., Pannewitz, S., Sancho,

L.G., and Schroeter, B., Pol. Biol., 28, 822, 2005.)

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Plant Life in Antarctica 415

of continuous data were obtained by Friedmann et al. (1987) allowing modeling of aspects

like the thermal environment (Nienow et al. 1988). This was then connected to the CO

exchange studies to produce estimates by modeling of the community productivity through-

out the year (Friedmann et al. 1993). The cryptoendoliths showed positive photosynthesis

for around 13 h day

1

with annual totals reaching around 1000 h depending on aspect

and slope (Kappen and Friedmann 1983). Net productivity was greatest around 2

to þ28C with significant gains down to 8to108C. Horizontal surfaces proved more

productive than north-facing sloping surfaces, although colder, because of better water

relations. One unusual feature of the community is the relative unimportance of respiratory

carbon loss during the dark winter, the system is simply too cold. Despite the long productive

periods and the low respiration the estimated carbon gain of 106 mg C m

2

year

1

only

translates into an actual gain of around 3mgCm

2

year

1

(estimated from carbon dating

Local time (h) (Dec. 9, 1979)

In rock

Ambient

In rock

Ambient

In rock

Ambient

12 18 24

PPFD in rock

400

PPFD ambient

Temperature (⬚C)

rH (%)

800

1200

1600

–15

–10

–5

100

FIGURE 13.17 An example of the diel pattern of humidity (upper panel, % RH), temperature (middle

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

2

1

) both within the rock and in

the surroundings of a north-facing endolithic lichen community on December 9, 1979 at Linnaeus

Terrace, Aasgard Range, McMurdo Dry Valleys. (Modified from Kappen, L., Friedmann, E.I., and

Garty, J., Flora 171, 216, 1981.) In the lower panel note the very different scales for the incident PPFD

(left-hand axis) and PPFD within the rock (right-hand axis).

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416 Functional Plant Ecology

techniques and metabolic turnover rates) probably because of the high stresses including

possible loss of metabolites in the freeze=thaw or wet=dry cycles (Greenfield 1988). Because

the endoliths can grow on almost all north-facing rock faces (porous rock types), where

epilithic species are excluded because of the low temperatures and, in particular the abrasive

wind, they are the most common terrestrial vegetation community in the Dry Valleys

(Friedmann 1982). Endolithic communities are one of the better demonstrations of the

importance of shelter, aspect, and water supply in continental Antarctica. As a contrast, it

appears that endolithic communities may also contribute extensively to weathering of their

host rock by dissolving the silica matrix by the production of locally high pH during

photosynthesis (Buedel et al. 2004).

More recently, studying the lithobiontic microorganisms has concentrated in some

ultrastructural aspects. It has been demonstrated that these lithobiontic communities can

be defined as complex biofilms (Figure 13.18) exhibiting a high diversity and different

ecological requirements (De los Rı

os et al. 2005b). Accurate identification of lithobiontic

microorganisms was possible by means of molecular techniques (De los Rı

os et al. 2005a)

and unexpectedly high genetic diversities have been found (De la Torre et al. 2003). It was also

demonstrated that some minerals in Antarctic rocks are biogenically transformed to

generate inorganic biomarkers—traces left by living microorganisms due to their biological

activity (Wierzchos and Ascaso, 2001, Wierzchos et al., 2003, 2006). Further, in situ

microscopy studies have for the first time demonstrated the presence of microbial fossils

within Antarctic sandstone rocks from the Dry Valleys (Ascaso and Wierzchos, 2003,

Wierzchos et al. 2005, Ascaso et al. 2005), which has important implications for the detection

of endolithic microfossils everywhere in the world or even in extraterrestrial probes (Friedmann

et al. 2001, Ascaso and Wierzchos 2002, Wierzchos and Ascaso 2002). These studies have

been concentrated in the McMurdo Sound to date. However, the lithobiontic habitats of

many important Antarctic areas, such as the Antarctic Peninsula and Transantarctic

Mountains and some ice-free oasis on land remain almost completely unexplored.

~200 µm

FIGURE 13.18 SEM–BSE image of a transverse section of granite from Granite Harbour showing an

algae-rich biofilm colonizing the rock fissures. Scale ¼200 mm.

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Plant Life in Antarctica 417

ACTIVE VERSUS INACTIVE

Typically, when looking at performance by plants across Antarctica, it is necessary to use

normal meteorological data that do not reflect local microclimates. In addition, even when

microclimate data are available, all lichens and mosses in Antarctica are poikilohydric and are

inactive for variable, but often large, periods and, at those times, are almost totally pro-

tected from the extremes of climate. Data sets are now available that have recorded activity

of lichens using chlorophyll a fluorescence techniques as well as thallus parameters such as

temperature and incident PPFD. It is now possible to make preliminary analyses of the

conditions when these organisms are active and to see how substantially they differ from

the annual means that include inactive periods as well. Mean annual temperature ranges

from þ9.78C, at the summit of Guadarrama Mountains near Madrid, to 9.78C Granite

Harbour, at a range of 19.4 K. In contrast, the mean temperatures during the active periods

show only a 3.4 K range with the coldest being the maritime Antarctic site, Livingston Island,

and the difference between the Guadarrama summit and Granite Harbour being only 1.6 K.

For total daily radiation during the active period the Guadarrama and maritime sites are

practically identical, whereas the Granite Harbour site stands out with an exceptionally high

value equivalent to a mean instantaneous rate of about 55% full sunlight. This confirms the

high light stress that comes from the reverse diel pattern (Section ‘‘The Reverse Diel Cycle of

Photosynthesis and High Light Stress’’) but shows that it is a feature of continental Antarc-

tica and not of the maritime Antarctic.

The net result is that the habitats of the active plants are by no means as extreme as the

ambient conditions would suggest and, in fact, may be remarkably constant over large

latitudinal ranges, the relative constancy of habitat conditions, first demonstrated by Poelt

(1987) with lichens growing from the Mediterranean to Greenland. If this concept proves

correct for Antarctica then it means that restriction to suitable habitats controls distributions

and adaptation may not play as major role a originally expected. This is certainly an area

requiring future investigation.

METABOLIC AGILITY

The majority of the vegetation in Antarctica is composed of lichens and bryophytes, which

have considerably simpler morphology than higher plants. However, it is a mistake to equate

simpler morphology with simpler metabolism as has been done in the past, for example:

for UV-B protection (Gehrke 1998, Gwynn-Jones et al. 1999). There is growing evidence

that lichens and mosses can be agile in their metabolism. The rapid change in protection

against UV-B found by Newsham et al. (2002) and Green et al. (2005) are two examples. The

moss B. subrotundifolium, and no doubt other mosses like C. purpureus, are capable of

changing from sun to shade forms and back, again within days to weeks. Lichens are

known to be able to alter their dark respiration rate at such a rate that they are almost

fully acclimated (Lange and Green 2005). This agility poses obvious problems if samples

are taken from the field and kept in some form of storage before use. The extreme shade

response to PPFD of photosynthesis by the mosses studied by Rastorfer (1970) is a probable

result of prestorage of the material in the laboratory before use.

INTEGRATING PERFORMANCE

NNUAL PRODUCTIVITY

Considerable efforts have been made to obtain values for the productivity, the seasonal net

carbon gain, for Antarctic plants. Unfortunately most estimates have been made in the

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418 Functional Plant Ecology

northern maritime area (Signy Island). However, since no species have had their photosyn-

thesis monitored for complete years the estimates have been produced by the application of

models constructed by linking CO

exchange and microclimate data sets. The two largest data

sets are those for the cryptoendolithic community in the Dry Valleys (see Section ‘‘The

Endolithic Lichen Community’’) and for Usnea aurantiaco-atra on Livingston Island in the

South Shetlands where microclimate, CO

exchange and activity (from chlorophyll fluores-

cence) data sets have been linked (Schroeter et al. 1991, 1997a). In the latter case productivity

gains are predicted throughout the year with spring and autumn with the higher rates and

winter and summer limited by cold and drought, respectively. Considerable variation in

annual total production was also found from year to year (Schroeter et al. 1997a). When

modeled CO

gain is plotted against actual PPFD and temperature data it is clear that the

lichen is rarely active under conditions suitable for optimal photosynthesis but is usually

limited by drying out at high PPFD and high temperatures (Figure 13.19). Most other

productivity estimates are from small data sets and with many and varied assumptions.

One common assertion is that increased winter temperatures will reduce productivity because

of greater respiration rates. However, despite substantial winter warming in the northern

Antarctic Peninsula there appears to have been no effect on lichen growth rates, in fact some

seem to have slightly increased (Sancho and Pintado. 2004). It is known from a temperate

study that some lichens can fully acclimate their respiration to seasonal temperature and it

appears that this may also be happening in Antarctica (Lange and Green 2005).

1000

-exchange (mg ChI

–1

)

–3

–2

–1

800

600

PPFD (µ

mol m

–2

–1

)

400

200

TT (

⬚C)

–5

FIGURE 13.19 The relationship between thallus temperature, PPFD and CO

exchange for Usnea

aurantiaco-atra at Livingston Island, maritime Antarctica. Thallus temperature (8C), PPFD

(mmol Photons m

2

1

), and photosynthetic activity (from chlorophyll a fluorescence measurements)

were recorded at 10 min intervals for 1 year. Combinations of temperature and PPFD when the lichen

was active were then plotted on a NP response surface to PPFD and temperature generated in the

laboratory. The lichen was rarely active under optimal conditions for PPFD and temperature. (Modified

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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Plant Life in Antarctica 419