Annals of Botany 78 : 719–728, 1996
Seasonal Variation in Respiratory and Photosynthetic Parameters in Three Mosses
from the Maritime Antarctic
M A R T I N C. D A V E Y and P E T E R R O T H E R Y*
British Antarctic SurŠey, Natural EnŠironment Research Council, High Cross, Madingley Road,
Cambridge CB3 0ET, UK
Received : 26 March 1996
Accepted : 25 June 1996
Carbon fixation under controlled conditions was measured in three mosses from the maritime Antarctic using an
infra-red gas analysis system. Gas exchange parameters were determined during each season in 1993 and 1994 using
the Arrhenius equation and a hyperbolic tangent function applied to respiration and photosynthesis, respectively.
Environmental data was collected in 1994 for comparison. All seasonal variations were greater in Brachythecium than
in the species from less hydric habitats. Respiration rates were highest in summer and lowest in winter at all
temperatures in Brachythecium, but there was little change in Chorisodontium or Andreaea. There was some seasonal
variation in the initial slope (Kp) of the photosynthesis-irradiance curve in all species, although the environmental
data suggested that this was of little ecological importance. In all species seasonal changes in the maximum rates of
photosynthesis (GPmax, NPmax) were observed, generally with a pattern of summer maxima, although there were some
interannual differences. These changes are considered to be the most important in affecting the overall annual
productivity of the mosses. There were no seasonal variations in the optimum temperatures for either gross or net
photosynthesis, or for the irradiance at the onset of light saturation (Ik). The results have important implications for
the use of models to estimate the productivity of the Antarctic flora based upon present or predicted climate data.
# 1996 Annals of Botany Company
Key words : Antarctica, bryophytes, mosses, carbon exchange, irradiance, photosynthesis, productivity models,
respiration, seasonal variation, temperature.
INTRODUCTION
The productivity of a plant is increased if it is able to
acclimatize to regular or long-term changes in its habitat.
Seasonal variations in environmental factors are the most
predictable of these changes, and it is to be expected that the
greatest seasonal variations in plant physiological processes
will be observed in those ecosystems exposed to the largest
range of environmental conditions.
Plants living in the polar regions are exposed to large
variations in both irradiance and temperature, two primary
factors controlling carbon fixation. Irradiance varies
seasonally, as well as daily, as a result of changes in the
angle of solar elevation (Gates, 1962) and both the reflecting
and attenuating effects of snow cover, which itself varies
both spatially and temporally (Rosenberg, 1974 ; Walton,
1984 ; Davey, Pickup and Block, 1992). Temperatures of
plant material vary widely on both a daily and seasonal
basis. Annual ranges of up to 50 °C are commonly recorded
and similar daily ranges may also occur (Walton, 1982 ;
Smith, 1986 ; Davey et al., 1992). In addition, differences in
water availability occur seasonally and between habitats
and are believed to be important in determining species
distribution and the composition of communities in these
ecosystems (Smith, 1972, 1984 ; Kennedy, 1993).
* Present address : Institute of Terrestrial Ecology, Monks Wood,
Abbots Ripton, Huntingdon, Cambs PE17 2LS, UK.
0305-7364}96}120719­10 $25.00}0
The plant communities in the maritime Antarctic are
largely cryptogamic, with mosses providing the greatest
phytomass in many areas (Smith, 1972, 1984, 1996 ;
Longton, 1988). Seasonal variation in both temperature
optima and maximum rates of net photosynthesis have been
demonstrated in Arctic mosses (Hicklenton and Oechel,
1976 ; Oechel and Sveinbjo$ rnsson, 1978). The absence of
variation in respiration in some cases implies that changes in
gross photosynthesis also occurred (Oechel and
Sveinbjo$ rnsson, 1978). Studies on Antarctic mosses have
suggested that acclimation to temperature may occur in
some, but not necessarily all, species (Collins, 1977). Less
information is available on acclimatization in response to
changes in irradiance, although optima and compensation
points may vary seasonally in Arctic mosses (Hicklenton
and Oechel, 1976 ; Oechel and Sveinbjo$ rnsson, 1978). Most
studies on Antarctic bryophytes have been based on singleseason (usually summer) measurements ; seasonal variation
in carbon exchange parameters has not been considered.
Global climate models predict increases in temperature as
a result of rising concentrations of greenhouse gases in the
atmosphere (Mitchell et al., 1990) that may lead to changes
in irradiance through the effects of varying cloud cover
(Maxwell, 1992). The biological consequences of climate
change are the subject of considerable investigation
(reviewed in Chapin et al., 1992 ; Fautin, Futuyma and
James, 1992 ; Kennedy, 1995). Climate change models
predict that the largest changes will occur in high latitude
# 1996 Annals of Botany Company
720
DaŠey and Rothery—Bryophyte Gas Exchange
ecosystems, hence it has been suggested that the biological
consequences will be greater and be manifested earlier in
these areas (Callaghan, Sonesson and Sømme, 1992 ; Chapin
et al., 1992). Hence, the ability of the plants growing in
polar regions to acclimatize to environmental change could
have significant consequences for the productivity and
composition of these ecosystems. A knowledge of the effects
of acclimatization is required if moss productivity is to be
estimated from either present or predicted microclimate
data.
This paper describes seasonal variations in the respiratory
and photosynthetic gas exchange parameters for three
maritime Antarctic mosses over 2 years. The magnitudes of
interspecific, interseasonal and interannual differences are
described, and the results compared to measurements of
environmental factors. The implications of the results for
predictions of moss productivity are considered.
MATERIALS AND METHODS
EnŠironmental data
Moss temperatures at a depth of 1 cm were recorded
throughout 1994 at 10 min intervals using micro-thermistors
(3 mm diameter, accuracy ³0±2 °C) attached to Grant
Squirrel data loggers (Smith, 1988 ; Davey et al., 1992).
Vertical temperature gradients in mosses from these habitats
are small, and measurements at this depth give results that
are representative of the samples collected (Davey et al.,
1992). Incident irradiance at ground level was recorded
using a Skye Instruments PAR sensor attached to one of the
loggers. Depth of snow coverage was measured at varying
intervals using preplaced snowpoles. Moss water contents
were measured at weekly intervals during snow-free periods.
Five samples of 20 mm depth and 0±1–1 g dry weight of each
species were collected. These were weighed, dried at 105 °C
for 24 h, reweighed, ashed at 550 °C for 24 h, reweighed and
water content calculated relative to the ash-free dry weight
(afdw) of the sample.
Sample preparation
The mosses selected for study came from a range of
habitats on Signy Island, South Orkney Islands. Samples of
Andreaea depressinerŠis Card. (xeric) and Chorisodontium
aciphyllum (Hook. f. et Wils.) Broth. (mesic) were collected
from the slope behind the British Antarctic Survey station,
and Brachythecium austro-salebrosum (C.Muell.) Kindb.
(hydric) from the foot of the slope to the south of Factory
Cove. Samples were collected during summer (Jan.), autumn
(Apr.), winter (Jul.) and spring (Nov.) 1993 and 1994.
For each set of observations six replicates of 50 mm
diameter and 20 mm depth were collected and returned to
the laboratory. Smaller samples of 10 mm diameter of
Brachythecium were also collected for use under conditions
of high carbon flux. The samples were placed in 50 or
10 mm diameter polypropylene dishes, water added if
necessary, and the samples left for 3 d at 10 °C and constant
illumination of 150 µmol m−# s−" supplied by fluorescent
tubes, to acclimatize. Any excess water was removed by
tamping with tissue paper 2 h prior to the start of the
observations.
At the completion of the observations the samples were
dried at 105 °C for 24 h, weighed, ashed at 550 °C for 24 h,
reweighed and afdw calculated.
Carbon flux measurements
Gas exchange was measured using a Binos II infra-red gas
analyser (IRGA) in open differential mode. Both analysis
and reference chambers were of 40 cm$ capacity and
incorporated small fans to ensure thorough mixing of the
airstream. Air flow rates of 500 ml min−", controlled using
Platon GT rotameters, were used throughout. Samples were
placed in the analytical chamber for 2 min, sufficient time to
reach equilibrium, and the carbon dioxide differential noted.
Net carbon flux was calculated using the formula appropriate to rotameters given by Jana! c, Catsky and Jarvis
(1971) (formula 3.21, p. 163).
All measurements were made in a constant temperature
room at nominal temperatures of 0, 2, 5, 10, 15 and 20 °C
(³1±5 °C) ; the actual temperature of the moss being
recorded after each measurement. Irradiances between 0
and 150 µmol m−# s−" were supplied by fluorescent tubes
and those between 100 and 700 µmol m−# s−" by a high
intensity mercury vapour lamp (measurements made using
both light sources at similar irradiances were similar). The
actual irradiances used varied as the output of the fluorescent
tubes varied with temperature, but at 10 °C were
approximately 0, 18, 38, 78, 100, 260 and 700 µmol m−# s−".
Control of irradiance was achieved by altering the
positioning of the light sources and by the use of black cloth
or black netting. Irradiances were measured using a Skye
Instruments PAR sensor.
Observations started at 20 °C in darkness, measurements
made for all six replicates, irradiance increased and the
measurements repeated. This process was continued to the
maximum irradiance at which point the samples were
returned to darkness and the temperature lowered. Periods
of 30 min for short-term acclimatization to irradiance
changes, and 60 min for short-term acclimatization to
temperature changes, were allowed. Initial observations
suggested that these periods were more than sufficient for
equilibrium to be achieved.
Due to variations in the temperature of the experimental
environment the temperatures of the sample during gas
exchange measurements differed from the nominal
temperatures. The results were therefore corrected to those
for the required temperatures by interpolation. In practice
these corrections were small (34 % zero, 48 % less than the
resolution of the IRGA, 18 % greater than the resolution of
the IRGA) and had little effect on the results obtained.
Dark respiration (R, µg C g−" afdw h−") and temperature
(T, °C) were related using the Arrhenius equation (Gates,
1980) :
R ¯ Rmax exp [®k}(T­273)]
where Rmax is the maximum rate of dark respiration and k
determines the rate of increase with temperature. However,
over the range of experimental temperatures (0–20 °C), no
721
DaŠey and Rothery—Bryophyte Gas Exchange
Irradiance (µmol m–2 s–1)
2000
A
1600
1200
800
400
0
B
20
10
0
–10
–20
C
Temperature (°C)
30
20
10
0
–10
–20
D
20
10
0
–10
Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
1994
F. 1. Daily ranges of physical factors during 1994 at the sample collection sites. A, Irradiance ; B, Andreaea temperature ; C, Chorisodontium
temperature ; D, Brachythecium temperature.
information was available to allow Rmax to be estimated, so
an equivalent form of the equation was used :
R ¯ R exp [(273 b T)}R (273­T)]
!
!
where R is the intercept and b the slope, both at T ¯ 0.
!
Values for respiration obtained from this formula were
used to convert the results obtained for net photosynthesis
(NP, µg C g−" afdw h−") to an estimation of gross photosynthesis (GP, µg C g−" afdw h−"), assuming that the effects
of photorespiration are negligible, as follows :
GP ¯ NP®R
These figures for GP were then plotted against irradiance
(I, µmol m−# s−") and modelled for each temperature using a
722
DaŠey and Rothery—Bryophyte Gas Exchange
Irradiance (µmol m–2 s–1)
1000
800
600
400
200
0
8
6
Moss temperature (°C)
4
2
0
–2
–4
–6
–8
–10
–12
–14
0
4
8
12
16
20
24
0
4
Local time (h)
8
12
16
20
24
F. 2. Mean hourly irradiances and Andreaea temperatures at a depth of 10 mm for each month of 1994. The values represent the mean for each
full hour completed and are plotted on the half-hour following the start of each hour. (E), January ; (+), February ; (_), March ; (y), April ;
(U), May ; ([), June ; (D), July ; (*), August ; (^), September ; (x), October ; (V), November ; (W), December.
form of the hyperbolic tangent function (Chalker, 1980 ;
Geider and Osborne, 1992) :
GP ¯ GPmax²(exp [2αI]®1)}(exp [2αI]­1)´
where α ¯ (Kp}GPmax). Values for GPmax, (the maximum
rate of gross photosynthesis) and Kp (the initial slope of the
curve) could then be determined for each temperature. This
model was found to provide a better fit to the data than
three other models that were tested : the rectangular
hyperbola (Michaelis–Menten curve), the modified rectangular hyperbola (Gates, 1980) and the asymptotic
exponential (Mauzerall and Greenbaum, 1989).
In most cases the model equations used fitted the data
well (the percentage variation accounted for generally
exceeded 90 %). Examination of the residuals from the fitted
models did not reveal any systematic lack of fit.
Throughout the above analysis curves were fitted separately to each of the six replicate samples. Parameter
estimates for each temperature were not independent
because they were based on the same samples and this was
allowed for in subsequent analyses in testing for trends with
temperature. In an alternative analysis (not reported in full)
estimated values of GPmax and Kp were obtained by fitting
curves to pooled data. The results by both methods were
very similar, but the first is preferred as it provides an
estimate of the variation between replicate samples.
Chlorophyll analysis
At the same time as the samples for carbon flux
measurements, ten samples of approximately 50 mg afdw
were collected. These were stored at ®80 °C to maximize
chlorophyll extraction (Hansson, 1988). In five of these the
wet weight was measured, afdw determined as above and
afdw per unit wet weight calculated. In the other five, wet
weight was measured, 5 ml of 95 % methanol added, the
sample ground and left overnight at 4 °C for extraction to
take place. The samples were then centrifuged and chloro-
723
DaŠey and Rothery—Bryophyte Gas Exchange
phyll determined spectrophotometrically using the
equations of Talling, Marker and Toms (1978). Chlorophyll
per unit wet weight and hence chlorophyll per unit afdw
were calculated.
RESULTS
EnŠironmental data
Daily ranges of irradiance and moss temperatures are
shown in Fig. 1. Mean hourly irradiances and temperatures
were calculated for each calendar month and those for
Andreaea are shown in Fig. 2 ; results for the other two
species were broadly similar. Moss water contents and
depth of snow cover at the three sites are shown in Fig. 3.
These data indicate that the experimental regimes
employed were relevant to the habitat of the bryophytes.
The temperature range of 0–20 °C covered almost all the
summer period. Although daily maximum irradiances in
summer often exceeded the experimental maximum of
700 µmol m−# s−" this was restricted to a limited period
around midday.
There are two anomalies in these data. Firstly, the mean
moss temperatures for Oct. were lower than those for Sep. ;
this reflected low mean air temperature in Oct. (®7±8 °C)
compared to Sep. (®1±4 °C) and Aug. (®5±7 °C). Secondly,
mean irradiance was lower in Dec. than in Nov., particularly
around the midday period ; this was caused by a 45 % fall
in the number of hours of sunshine between the two months.
Dark respiration
Seasonal variations in the two respiratory parameters are
shown in Fig. 4. Results for Andreaea varied little and in no
clear seasonal pattern. There was some indication in
Chorisodontium that maximum values of R occurred in
!
autumn and of b in summer, although inter annual
variations were large. The clearest trends occurred in
Brachythecium, where summer maxima and winter minima
in R and the reverse situation in b were recorded. However,
!
much inter annual variation was again observed. Plots of
respiration against temperature for Brachythecium based
upon the mean values of R and b for each season are given
!
in Fig. 5. These demonstrate that, over the temperature
range tested, rates of respiration at a given temperature in
Brachythecium were higher in summer than spring or
autumn than winter.
Photosynthesis
Seasonal variation in Kp for gross photosynthesis is
shown in Fig. 6. The trends observed were very similar to
those for the respiratory parameter R . There was no clear
!
pattern of change in Andreaea. In Chorisodontium maxima
Snow depth (cm)
10
0
10
0
20
10
Water content (g g–1 afdw)
20
15
10
5
0
Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
1994
Aug.
Sep.
Oct.
Nov.
Dec.
F. 3. Snow depth at sample collection sites and water content of mosses during snow-free periods during 1994. (D) Andreaea ; (E)
Chorisodontium ; (*) Brachythecium.
724
DaŠey and Rothery—Bryophyte Gas Exchange
7
50
Kp [(µg C g–1 afdw h–1) (µmol m–2 s–1)–1]
R0 (µg C g–1 afdw h–1)
60
40
30
20
10
0
0.30
b (°C–1)
0.25
6
5
4
3
2
1
0.20
0
0.15
Sum. Aut. Win. Spr. Sum. Aut. Win. Spr.
1993
1994
0.10
F. 6. Seasonal changes in the initial slope of the photosynthesisirradiance curve (Kp). Results given as the means of all experimental
temperatures. Symbols and error bars as Fig. 4.
0.05
0.00
Sum. Aut. Win. Spr. Sum. Aut. Win. Spr.
1994
1993
250
F. 4. Seasonal changes in respiratory parameters. Symbols as Fig. 3.
Error bars indicate standard errors ; where not shown these were
smaller than the symbols used.
200
150
100
400
50
GPmax (µg C g–1 afdw h–1)
Respiration (µg C g–1 afdw h–1)
A
300
200
0
250
B
200
150
100
50
0
C
1600
1200
100
800
400
0
0
5
10
15
Temperature (°C)
20
F. 5. Relationships between respiration and temperature for
Brachythecium based upon mean respiratory parameters for each
season. (D) summer ; (E) autumn ; (*) winter ; (+) spring.
in autumn and minima in spring were observed, although
these changes were small. The largest changes occurred in
Brachythecium, where summer maxima were followed by a
decline through the winter. In 1993 this continued to a
spring minimum, but in 1994 a winter minimum was
followed by recovery in spring.
Sum. Aut. Win. Spr. Sum. Aut. Win. Spr.
1993
1994
F. 7. Seasonal changes in the maximum rate of gross photosynthesis
(GPmax) at (D) 0 ; (E) 2 ; (*) 5 ; (+) 10 ; (^) 15 and (_) 20 °C. A,
Andreaea ; B, Chorisodontium ; C, Brachythecium. Error bars as Fig. 4.
Seasonal variations in GPmax are shown in Fig. 7. Cyclic
changes are seen in all species, although these are not always
clear at the lower temperatures where rates of photosynthesis
were low. Some interannual differences were observed. In
Andreaea summer maxima occurred at most temperatures,
but there was no clear pattern to the results for the rest of
the year. In Chorisodontium summer or autumn maxima
DaŠey and Rothery—Bryophyte Gas Exchange
100
afdw. However, there were no seasonal patterns to these
differences and regression analysis indicated that there was
no statistically significant correlation between chlorophyll
content and either Kp or GPmax.
50
DISCUSSION
A
150
NPmax (µg C g–1 afdw h–1)
725
0
B
150
100
50
0
C
1200
800
400
0
Sum. Aut. Win. Spr. Sum. Aut. Win. Spr.
1993
1994
F. 8. Seasonal changes in the maximum rate of net photosynthesis
(NPmax) at (D) 0 ; (E) 2 ; (*) 5 ; (+) 10 ; (^) 15 and (_) 20 °C. A,
Andreaea ; B, Chorisodontium ; C, Brachythecium. Error bars as Fig. 4.
were followed by a decline to spring minima. Summer or
autumn maxima were also seen in Brachythecium. As for
Kp, decline in GPmax in Brachythecium continued to a spring
minimum in 1993, but in 1994 a winter minimum was
followed by recovery in spring. Temperature optima for
GPmax were generally & 20 °C for Andreaea and
Chorisodontium, and 15–20 °C for Brachythecium.
Seasonal variation in NPmax (Fig. 8) was more complicated
as it combined the effects of both respiration and gross
photosynthesis. This again assumed that photorespiration
was absent, although in the opposite calculation (GP to NP,
as opposed to the reverse) so cancelling any possible errors
due to the assumption. Compared to the results for GPmax,
this resulted in a reduction and broadening of the
temperature optima to 10–20 °C, usually centred at 15 °C,
for all species. The patterns of summer or autumn maxima
and spring or winter minima were similar to those for
GPmax.
Values for Ik, the irradiance at the onset of light saturation,
were calculated as GPmax}Kp. There were no statistically
significant differences in the results either between species or
between seasons. Ik varied with temperature in a manner
similar to that of GPmax, increasing through the temperature
range investigated ; mean (s.e.) overall results were 175
(10) µmol m−# s−" at 10 °C and 273 (14) µmol m−# s−" at
20 °C.
Chlorophyll content
Marked intersample differences in chlorophyll content
occurred in all three species : Andreaea 0±1–0±5,
Chorisodontium 0±03–0±11, Brachythecium 1±1–2±0 mg chl g−"
The environmental data collected demonstrate that the three
species studied were subject to different micrometeorological
regimes. Summer temperature maxima tended to be lower in
Brachythecium than the other two species due to the
buffering effects of the higher water content of the hydric
species. Winter temperature minima were lower and daily
variation greater in Andreaea than Chorisodontium than
Brachythecium, a trend related to the insulating effect of
increasing snow cover (Leinaas, 1981 ; Davey et al., 1992).
The clearest differences occurred in summer water content.
Brachythecium remained at high, though variable, water
contents throughout the ice-free period, being supersaturated following periods of snow thaw. Water contents
in Chorisodontium were lower, but very stable around
3 g g−" afdw due to a combination of its rhizoid system and
the water holding capacity of the underlying moss peat
(Gimingham and Smith, 1971). Andreaea was subject to
wider variations in water content, from dehydration to
water saturation, dependent upon the supply of external
water, and experienced regular drying events during the
summer.
A clear trend of increasing seasonal variation in respiratory parameters with increasing water availability in the
moss habitat was observed. In the xeric moss, Andreaea,
there was no seasonal pattern, and in the mesic moss,
Chorisodontium, the changes were small. As the two
parameters varied in opposite directions their cumulative
effect on respiration by the mosses was reduced. However,
in the hydric moss, Brachythecium, the seasonal changes
were large, and led to large variations in the respiratory
rates of the moss (seasonal means : 1–49 µg C g−" afdw h−"
at 0 °C and 209–421 µg C g−" afdw h−" at 20 °C) between
winter minima and summer maxima. These variations are
sufficient to have an important impact on the net productivity of the moss, although the effects of interannual
variation were also large. As a result, the extrapolation of
summer measurements of respiratory parameters to wholeyear estimates of carbon exchange would lead to a large
overestimation of respiration by Brachythecium.
A winter reduction in the rates of carbon loss through
respiration, even if accompanied by general reductions in
metabolic rates including photosynthesis, is of possible
benefit to the moss. During the winter irradiances are low
(Fig. 1) due to the effects of both reduced solar elevation
(Gates, 1962) and snowcover (Fig. 3) (Walton, 1984), and,
as a result, predicted net productivity by bryophytes is
negative (Collins and Callaghan, 1980 ; Davis, 1983). Hence,
a reduction of all metabolic activity during this period could
lead to an increase in the total annual production.
There have been few previous reports of seasonal variation
in respiration by polar bryophytes. Oechel (1976) recorded
a slight depression of respiration during early- compared to
mid- or late-summer in Dicranum spp. (to which
726
DaŠey and Rothery—Bryophyte Gas Exchange
Chorisodontium is closely related, and is morphologically
almost identical), but no changes in Calliergon sarmentosum.
Oechel and Sveinbjo$ rnsson (1978) found a small decrease in
respiration by the mesic moss Polytrichum alpinum during
the growing season. Seasonal variation in respiration by the
hydric moss Brachythecium rutabulum from a harsh temperate environment was of a similar magnitude to that
reported here, although the maximum occurred in earlyrather than late-summer (Kershaw and Webber, 1986).
These studies were in general agreement with the results
presented here. However, marked seasonal respiration
changes have been recorded in lichens [reviewed in Kershaw
(1985)], and bryophytes might be expected to behave in a
similar manner.
Interseasonal differences in Kp were also greatest in
Brachythecium. Since the initial slope of the photosynthesisirradiance curve (Kp) is a function of the chlorophyll
content and its absorption characteristics (and hence the
number of reaction centres) (Pre! zlin, 1981) it would be
expected that the seasonal changes in Kp would occur as a
result of changing chlorophyll concentrations in the moss.
However, there was no evidence that this was the case, as
chlorophyll concentrations did not vary in any regular
seasonal pattern or in relation to the measured values of Kp,
and the saturation irradiances (Ik) did not vary seasonally.
As mosses are considered physiologically to be shade
plants (Green and Lange, 1995), it is not clear whether the
observed seasonal changes in Kp will have any great
practical effect on the overall net productivity of the mosses.
For most of any day irradiances are either zero or above
that at which photosynthesis is saturated (compare Fig. 2
and values for Ik) ; the periods during which the initial slope
of the photosynthesis-irradiance curve is applicable are
short. Again there is a dearth of other studies on seasonal
variation in Kp to which these results can be compared.
Although not directly calculated, the results of Kershaw
and Webber (1986) for Brachythecium rutabulum suggest
small seasonal Kp variation, despite large changes in
chlorophyll content and chlorophyll a}b ratios.
The observed seasonal changes in GPmax and NPmax are
likely to be of far greater ecological significance. In all
seasons for all temperatures rates of photosynthesis were
greater in Brachythecium than in the species from less hydric
habitats, following the pattern of other reports (Collins,
1977). Studies on seasonal acclimation in some polar mosses
have recorded a change in the optimum temperatures for
photosynthesis (Hicklenton and Oechel, 1976 ; Oechel, 1976 ;
Collins, 1977 ; Oechel and Sveinbjo$ rnsson, 1978), while in
others, as in the present study, no consistent change has
been found (Oechel, 1976 ; Collins, 1977). However, all of
the aforementioned have reported an increase in the
photosynthetic capacity of the mosses (usually presented as
NPmax) during the summer.
The possible effects of photorespiration on the recorded
rates of gas exchange, and hence on the derived values for
GPmax, have not been determined. Therefore, the values of
GPmax presented here may represent overestimates, particularly as photorespiration is likely to be significant in C
$
plants such as mosses (Edwards and Walker, 1983).
However, gross photosynthesis and photorespiration gen-
erally vary in a similar manner in response to irradiance and
temperature (Edwards and Walker, 1983), and hence the
observed trends are unlikely to be affected.
Although some interspecific and inter annual variation in
trends was observed it is clear that values of both GPmax and
NPmax were higher in the summer than during other times of
the year. Hence, studies in which the values of GPmax are
extrapolated to calculate productivity for the whole year
[e.g. Longton (1974) for mosses, Schroeter et al. (1995) for
lichens] could lead to overestimates of total net productivity.
Some studies have attempted to overcome this problem by
preconditioning of samples to ‘ warm ’ or ‘ cold ’ regimes
(Collins and Callaghan, 1980 ; Davis, 1983) before determination of photosynthetic parameters, but the results
are less applicable than the use of the full range of seasonal
variations based on field material. Given the intraseasonal
temperature dependence and the large interseasonal variability of GPmax, this parameter is likely to be the most
important in determining the overall productivity of the
mosses, with the effects of respiratory changes being an
important secondary factor. The results presented here
provide the basis for an important refinement to models of
bryophyte productivity in Antarctic ecosystems.
GPmax is controlled by the rate constant of the ratedetermining step(s) of the Calvin cycle (Pre! zlin, 1981) and is
therefore temperature dependent, but can also be affected
by the density of photosynthetic units (which is related to
chlorophyll content) (Kershaw and Webber, 1986). However, this investigation provides no indication of either an
increase in chlorophyll concentration during the summer, or
a correlation of GPmax (or NPmax) and chlorophyll content,
that would account for the observed increase in
photosynthetic capacity.
It appears that the degree of seasonal variation in
respiratory and photosynthetic parameters is correlated
with water availability within the moss habitat ; the greatest
variation is recorded in the wettest habitats. This is the
opposite to that which may have been expected if these
changes were related to the ranges of environmental factors
to which the mosses were exposed. The xeric moss, Andreaea,
was subject to the widest range of temperatures, due to the
lack of buffering from snowcover in winter and water
content in summer, and also to regular summer drying
events (Fig. 3). One major factor that has not been taken
into account is the amount of viable plant material within
the samples ; should this be reduced in winter then the
observed patterns of carbon exchange would be explained.
There is some evidence that hydric bryophytes are less able
to survive freezing [or the indirect drying of tissues caused
by freezing (Burke et al., 1976)] than the more xeric species
(Longton and Holdgate, 1967 ; Longton, 1988), and this
could lead to cell damage and a fall in carbon exchange
capacity during winter.
Although seasonal changes in many of the gas exchange
parameters have been recorded it is not yet known to what
extent these are significant in determining the overall annual
productivity of the mosses. Clearly the direct effects of
irradiance and temperature are likely to be greater than the
indirect effects through (often contradictory) changes in
respiratory and photosynthetic parameters. However, until
DaŠey and Rothery—Bryophyte Gas Exchange
the results are tested through their application to recorded
environmental data their possible importance should not be
underestimated.
The results have important implications for the prediction
of Antarctic plant productivity from either known or
predicted environmental data. As the net and gross
photosynthetic temperature optima were above the
temperatures of the mosses for most of the year any increase
in temperature, as predicted by global climate models
(Mitchell et al., 1990) will lead to an increase in productivity,
despite an increase in respiratory activity. This increase
could be enhanced by an increased photosynthetic capacity
of the mosses during periods of increased temperature. As
the bryophytes are not irradiance limited during most of the
day, an accompanying increase in cloud cover causing a
reduction in irradiance (Maxwell, 1992) should not have a
great effect on productivity, although an increase in
precipitation and water availability could be ecologically
significant. In all predicted scenarios, the greatest benefits
should accrue to the species from the hydric habitats.
A C K N O W L E D G E M E N TS
Thanks are due to the British Antarctic Survey for providing
facilities for this work. Dr H. J. Peat helped with handling
of the micrometeorological data. J. D. Shanklin supplied
the macrometeorological data. Dr R. I. Lewis Smith provided advice throughout.
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