Functional
Ecology 1987,
1, 179-194
In situ photosynthetic responses to light,
temperature and carbon dioxide in herbaceous
plants from low and high altitude
Ch.KORNER and M. DIEMER
tional limits, which vary regionally and inter-
Institut far Botanik, Universitdt Innsbruck,
specifically. It is not clear which environmental
Sternwartestraf3e 15, A-6020 Innsbruck, Austria
constraints or plant processes are important or
decisive for plant success at high elevation. The
majority of alpine plants tolerate site-specific frost
Abstract. Net CO2 assimilation (A) was analysed
in situ in 12 pairs of altitudinally separated,
herbaceous plant species in the Austrian Alps at
600 and 2600m. Both groups of species show a
similar average response to light, saturating at
quantum flux densities (400-700mm) (QFD) of
more than 1200 VLmol m-2 sol. Temperature optimum of QFD-saturated A differs little (3K) and
corresponds to the median of air temperature at
leaf level for hours with rate-saturating light conditions and not to mean air temperature which
differs by 10K. Species with an exclusive high
altitude distribution show steeper initial slopes
and higher levels of saturation of the response of A
to internal partial pressure of CO2 (CPI) than low
elevation species. Mean A at local ambient partial
pressure (CPA) does not differ between sites (c. 18
VLmol m-2 s-1), despite the 21% decrease in
atmospheric pressure. Plants at high altitude oper-
ate at mean CPJ of 177 debar as compared to 250
debar at low altitude. The higher ECU (efficiency of
carbon dioxide uptake [linear slope of A/CPJ
curve]) as well as the steeper CO2 gradient between
mesophyll and ambient air of alpine plants are
explained by (1) greater leaf and palisade layer
thickness and (2) greater nitrogen (protein) content
per unit leaf area. We hypothesize that alpine
plants profit more from enhanced CO2 levels than
lowland plants (Fig 7).
Key-words: Microclimate, gas exchange, carboxylation,
elevated CO2, nitrogen, anatomy, season, alpine
Introduction
179
conditions within a substantial leeway towards
lower temperatures (cf. Pisek, Larcher & Unterholzner, 1967; Larcher, 1980, 1985; Sakai & Larcher, 1987). A number of other factors may be
limiting, including the ability to establish
seedlings, the availablity of water, carbon dioxide
and mineral nutrients and the maintenance of
essential metabolic and biosynthetic processes
under adverse conditions. Our present contribution to the understanding of high elevation
plant functioning aims to elucidate the carbon
dioxide question in perennial herbaceous plants.
The history of photosynthesis research in mountain plants can be traced back almost a century
(Pisek, 1960; Billings & Mooney, 1968). The major-
ity of these investigations, revealed relatively high
photosynthetic rates. A number of publications
compared low and high elevation plants from the
same climatic region. Maximum rates of C02-assimilation in forest trees from different altitudes
measured at, or converted to, natural local CO2
partial pressures, were found to be equal (Beneke
et al., 1981) or lower at high elevation (Pisek &
Winkler, 1958; Slatyer & Morrow, 1977; Tranquillini & Havranek, 1985). No significant differ-
ences were found amongst species from an altitu-
dinal moisture gradient in California (Mooney &
West, 1964; Mooney, Wright & Strain 1964; Chabot
& Billings, 1972) or amongst subalpine and subniveal plant communities in the central Caucasus
(Nakhutsrishvili, 1974). Tussocks of the same
grass species from different elevations in New
Zealand (Greer, 1984) and Taraxacum officinale
aggr. Weber from lowland and upland seed sour-
Alpine plants live in an environment commonly
ces in the Rocky Mountains (Oulton, Williams &
described as cold and windy, with high radiation,
May, 1979) both grown and measured at low
reduced partial pressures of 02 and CO2 and only
altitude did not differ in their rate of CO2 assimi-
brief periods supportive of growth and develop-
lation either.
ment of organisms (e.g. Billings & Mooney, 1968;
Conversely, Michler & Ndsberger (1977) found
Franz, 1979; Larcher, 1983). These increasingly
that white clover clones from high elevation exhi-
adverse conditions restrict higher plants to eleva-
bited higher rates of CO2 assimilation under equal
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this correlates with the microclimatic conditions;
180
greenhouse conditions than low elevation prov-
Ch. Korner &
enances from the same area. Similar conclusions
as well as (2) how efficiently carbon dioxide is
M. Diemer
were reached by Woodward (1986) for Vaccinium
utilized in groups of closely related plant species
myrtillus L.
at different altitudes.
Thus, no clear consensus on effects of altitude
on CO2 assimilation rates has been reached. The
major reason for this uncertainty has been the
Sites and plant species
absence of concurrent comparative field studies of
Both the low and high elevation experimental sites
typical high and low altitude species. A review of
are in the vicinity of Innsbruck (47 0N 11 0E). Some
the literature on herbaceous plants indicates a
relevant site characteristics are summarized in
great disparity between the abundance of informa-
Table 1. Plant species investigated here are listed
tion on the photosynthetic behaviour of alpine
in Table 2.
versus native wild plants from low, non-arctic
The low elevation plant species were studied in
and/or non-water stressed, unshaded environ-
two locations within the suburban belt of Inns-
ments. Except for forage grasses little is known
bruck. Plots were weeded, so that leaves of the
about low altitude herbs. In addition, problems
experimental plants developed under full sunlight
arise when comparisons are attempted between
as they do at high elevation. The soil, loamy brown
results from early alpine studies and data obtained
earth derivative with pH between 6 and 7, was
subsequently at low elevation, since equipment
moist throughout the study. The species employed
and plant material (mostly crop plants) differed
here comprise typical elements of the meadow and
substantially.
forest-edge flora of the Inn river valley. The
In the present analysis, we investigated plants of
populations studied consist of individuals that
similar life form, within the same geographic
germinated spontaneously in the test area and
region, at similar phenological stage, without
individuals that were transplanted from a wet
interference from water stress and within their
meadow in sod blocks 2-5 yr prior to the exper-
specific elevational center of abundance. No
iments.
attempts were made to investigate intraspecific
High elevation plants were studied on Mount
altitudinal differences, since this necessarily
Glungezer, one of the peaks around Innsbruck that
would include comparisons at sites of optimal and
protrudes into the subniveal zone. Here popu-
marginal life conditions of every species, which
lations of about 80 species of phanerogams that
was not the aim of this paper. The same methods
grow throughout the rock- and fellfields of the
and instruments were used, at both high and low
Central Alps are present (Bahn & Kdrner, 1987).
altitude, within a short period of time, to further
The site was first used for gas exchange studies by
reduce experimental noise. The purpose of this
Cartellieri (1940) and provides a variety of expo-
investigation was to ascertain in the field, whether
sures ranging from the edge of a permanent snow
and how: (1) photosynthetic capacity and photo-
bank to thermally favoured South flanks, within
synthetic light and temperature responses differ in short distance of a permanent field station. Soils
low and high altitude herbaceous plants and how
are derived from silicaceous schist and amphibo-
Table 1. Macroclimate and phenological dates for the study sites.
Innsbruck Glungezer
(=100%)
Altitude
600
m
2600m
Mean atmospheric pressure (mbar) 946 5 749 5 (-20 8%)
SD from 60 observations over two summers 4 1 3 3
Mean air temperature estimated from data of
nearest meteorological stations (QC).
annual average 8 0 -2 0 (-10OK)
warmest month (July) 18 0 5-0 (-13 0K)
Mean annual precipitation (mm) 870 >1000
Mean number of days with snow cover c 80 c 220 (+175%)
Vegetative active period (months) c 6 c 3 (-50%)
(April-September) (mid June-mid September)
Period of highest biological activity mid May-June July-mid August
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181
Photosynthesis
at high altitude
Table 2. List of plant species (nomenclature follows Flora Europaea).
Plant
family
Low
altitude
High
altitude
I. Pairs of species investigated for CO2 response of photosynthesis (* indicates species examined for light or
temperature response as well).
Ranunculaceae pooled data for * Ranunculus glacialis
* Ranunculus acris
R. aconitifolius
R. ficaria
R. nemorosus
R. repens
Rosaceae
*
Geum
rivale
*
Geum
reptans
* Potentilla anserina * Potentilla crantzii
P. verna
Polygonaceae * Polygon um bistorta * Polygonum viviparum
Fabaceae Trifolium repens Trifolium thalii
Apiacae * Daucus carota * Ligusticum mutellina
Primulaceae Primula elatior Primula glutinosa
Asteraceae
Erigeron
acre
Erigeron
uniflorus
* Taraxacum officinale Leontodon pyrenaicus ssp. helveticus
Taraxacum alpinus
(= T. officinale aggr.)
Achillea millefolium * Achillea erba-rotta ssp. moschata
Cyperaceae Carex acutiformis Carex curvula
II. Additional species examined for temperature or light response only:
Geum urban um Oxyria digyna
Homogyne alpina
Doronicum clusii
Geum montanum
Poa alpina
lite (pH near 4 7) and are generally moist, at least in
the deeper root zones.
ADC, Hoddesdon, England) serves as a null point
device (differential mode) and, alternatively, to
check CO2 concentrations of gas mixtures (abso-
Materials and methods
Climate. A portable, automatic climate station
lute mode) generated by a MFC-controlled gas
mixing unit. The photosynthetic response to inter-
nal CO2 concentration (CPI) was investigated at
(Micromet-1, G. Cernusca, Innsbruck) was
concentrations between 50 and 1500 vll 1-1, begin
installed during the main growth period at each
ning at low concentrations. The same bottle of
elevation. Air, soil and canopy temperature, as
primary calibration gas was used throughout the
well as quantum flux density [QFD (400-700nm)]
study at both elevations. Calibration of MFC's was
were measured in 2 min intervals and recorded as
achieved with an electronic film flow meter
hourly means. The CO2 content of the air was
(SF-101, STEC Inc., Kyoto, Japan) corrected for
recorded continuously at each elevation for 10
pressure and temperature.
days in June and July with an infrared gas analyser
Dry air flow in the gas exchange system was
(225 MK3, ADC, Hoddesdon, England) and a chart
adjusted, to maintain a cuvette vapour pressure
recorder. Means for daylight hours were deter-
deficit of 0.86 ? 0.2 kPa. Somewhat higher deficits
mined from integrals between 0500 and 1900h.
occurred only at the highest temperature range
Gas exchange studies. All measurements were
during the measurement of temperature response
made in the field, using a portable, steady-state gas
curves. Unless temperature response was studied,
exchange system (FG-02, Armstrong Enterprises,
leaf temperature was maintained within 2K of
Palo Alto, California) as described by Field, Berry
optimum leaf temperature for CO2 uptake at
& Mooney (1982) and Atkinson, Winner & Mooney
saturating light conditions. Leaf temperatures
(1986). In this system, the rates of transpiration
were measured with two independent Cu/constan-
and C02-uptake are balanced by separate flows of
tan thermocouples mounted on the abaxial leaf
dry air and 1 % C02 in air, controlled by electronic
surface by small pieces of porous fiber tape (Leu-
mass flow controllers (MFC). The IRGA (225 MK3,
copor no 2471, Beiersdorf, Hamburg, Federal
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182
Republic of Germany) that covered not more than
rate of transpiration will be higher at high eleva-
Ch. Kbrner &
5% of the leaf surface. The diffusive conductance
tions due to the higher diffusivity of water vapour
M. Diemer
of this tape is equal to or higher than maximum leaf
in air. However, at equal mole fraction difference,
conductance. Depending on leaf size, boundary
temperature and stomatal aperture, the rate of
layer conductance for water vapour varied bet-
transpiration will be the same at all elevations and
ween 1 and 2 mol m-2 s-1, which is about two to
so will the conductance, as defined by Cowan
six times larger than maximum stomatal con-
(1977). Thus, any observed differences in conduc-
ductance.
tance are due to differences in the dimensions of
Light was supplied by a 'Multi-mirror reflector'
the diffusion path.
halogen lamp (Type EYC, 12V/75W General Elec-
Since we attempted to compare leaves at their
tric, Cleveland, Ohio) in connection with a 45
physiological optimum, it was imperative to select
degree cold mirror (45? CM-Pyrex-wit, Ocli, Santa
mature fully developed leaves. At low altitude
Rosa, California) to reduce heat input. With the
visual estimation of leaf age is much easier than at
cold mirror attached, we measured peak intensity
high elevation. Colour and lustre of leaves undergo
(100%) between 660 and 680nm. Beyond 690nm
more dramatic changes and are more conspicuous
intensity declines sharply to 20%. Quantum flux
in low altitude herbs. Leaves selected to be fully
areal density (QFD) was measured with a quantum
expanded and just mature varied little in ECU.
sensor (LI-190S, Licor) at leaf level. Sunlight was
However, at high altitude post-maturity stages are
always screened off. For light response curves the
often difficult to detect visually. A further problem
leaf-lamp distance was altered. During all
arises at high altitude from after-effects of frost.
measurements other than light response QFD was
Freezing temperatures without snow or snow
kept at or above saturating densities.
cover of several days depress ECU on successive
Calculations of gas exchange parameters were
warm days and this becomes more pronounced
based on equations from Von Caemmerer & Farqu-
later in the season. In order to delineate peak
har (1981) and Field et a]. (1982), using Cowan's
capacity, seasonal changes of photosynthetic
(1977) system of molar units. An additional
behavour were monitored in alpine taxa.
routine was incorporated to account for the differ-
For CO2 response three pairs of species (Ranun-
ences in boundary layer conductance due to differ-
culus, Geum and Polygon um) have been studied in
ences in leaf width. A constant cuticular conduc-
more detail, with a total of six to 10 response
tance to water vapour of 15 mmol m-2 s-1 was
curves from different individuals pooled for
assumed, based on the results of desiccation
species comparisons. Additional comparisons for
experiments (unpublished data). The efficiency of
all other species pairs were based on two to five
carbon dioxide uptake (ECU), often inade-
curves each. Temperature and light response was
quately termed carboxylation efficiency, was
determined from two samples per species. All data
derived from the initial linear slope of the A/CPI
are expressed per unit projected leaf area.
Leaf anatomy. Experimental leaves were inves-
curves.
A major aspect in the treatment of data in this
study is the influence of atmospheric pressure.
tigated for overall thickness and thickness of
palisade layer by light microscopy.
The use of mole fraction units for gas concentra-
Leaf nitrogen content. Approximately 100cm2
tions (for all practical purposes, the partial pres-
of leaf laminae (15-50 leaves, depending on leaf
sure of a gas species divided by total pressure)
size) comparable to those subject to photosyn-
leads to pressure independent expressions of leaf
thesis measurements were collected and mean
conductance (G) - that is to say, the conductance
total Kjeldahl nitrogen was determined from
of pores of a particular size does not depend on the
ground subsamples.
pressure at which the measurement is made.
However, for the comparison of ECU at different
elevations, it is important to know the actual
partial pressure of CO2 (CPI) present at the meso-
Results
Environmental studies
phyll surface. This is derived by multiplying the
At low elevation the first and most productive
mole fraction of CO2 inside the leaf by total
phase of growth and development for the majority
ambient (local) pressure.
of herbaceous phanerogams ends in mid-June.
Similar considerations apply to the estimation
Subsequent and generally less productive cycles
and a moisture gradient. At equal vapour pressure
(second flush or new generations) are disregarded
here. There is only one growth cycle at the high
difference, temperature and stomatal opening, the
elevation site due to the short snow-free period.
of transpiration rates from leaf conductances (G)
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183
Since the end of the first growth period at low
Photosynthesis
elevation and the beginning of the annual growth
at high altitude
Glungezer 2600 m Innsbruck 600 m
cycle at high elevation (snow melt) fall in the
1
period of summer solstice, the mean solar angle is
20
/
4
0/
similar in both periods. Results of the microcli-
mate studies over the main growing period at each
elevation, i.e. the period from 1 April to 21 June at
C
low elevation and from 23 June to 8 September at
I
high elevation covering about 1800hours from
each altitude are shown in Table 3 and Figs 1-3.
Plant canopy temperature (CC)
Fig. 2. Frequency distribution of air temperature within
20 aGlungezer 2600 m
the plant canopy during the main growth period at each
0 Innsbruck 600 m
altitude (width of classes 4K in the range from -5 to
+39QC). Hours with QFD > 30 Lmol m-2 s-' only.
10
and QFDs > 2000 LmoI m-2 S-1 occurred more
frequently at high elevation (Table 3). However,
0 400 800 1200 1600 2000 2400
Quantum flux density (kmot
2s)frequency distribution of QFD based on
them
overall
hourly means was very similar (Fig. 1). This
Fig. 1. Frequency distribution of QFD > 30 P'mol m-2 s-1
mediation is attributable to the higher frequency of
during the main growth period at each altitude (width of
convective cloud accumulation in the summit
classes 200 Pmol m-2 s-1).
area, which virtually equalized the effect of
reduced atmospheric absorbance at high altitude.
Light climate. QFD does not differ substantially
This observation is in accordance with data from
between the two elevations. Daily totals of quan-
Moser et a]. (1977) and Rott (1976) for the summer
tum input in the 400-700nm range were 15%
period in this region of the Alps. The higher
greater at high elevation only on completely
frequency of hours in the 30-200 Lmol m-2 s-1
cloudless days, which are very rare. In addition,
class at low altitude is largely due to the screening
short-term (2 min) maxima of QFD were higher
of the horizon by mountains, which extends
Table 3. Microclimate at the study sites during the 1986 season.
Low altitude High altitude
Light climate
Mean daily quantum flux density (QFD)
during the period of highest biological
activity (cf. Table 1), mmol m-2 day1,
(only hours with QFD > 0.03) 11-1 ? 4 2 11 7 ? 5 2 (+5 4%)
Frequency of days with 2-min periods of
QFD > 2 mmol m-2 s-1 (% of all days) 61 70 (+13%)
Mean maximum (and absolute maximum)
of QFD on these days (Lmol m-2 s-1) 2281 (2578) 2467 (3020) (+8.2%)
Temperature climate
Mean temperatures for the study sites
during hours with QFD > 0-03 mmol
m-2 s-' in the main growing period (0C).
1 April - 21 June at low elevation.
24 June - 8 September at high elevation.
2
m
above
Canopy
Soil
(15
cm
ground
air
depth,
15
all
15
9
3
8
7
118
hours)
12
8
(-6-6K)
(-4.1K)
7
1
(-5.7K)
Atmospheric CO2 level
Mixing
Mean
ratio
partial
(pl
l-1)
pressure
335
5
(>bar)
335
317
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?
251
3
184
QFD<500 LOmoL m-2 s-' QFD > 1500 Lmol m-2 s-'
Ch. Karner &
2600 m
M. Diemer
40-
30
10
07~
0~
2
0
1
0
~
3
20
30600
0m
2600Gm
0
2
Pl
0
4
ant
canopy
temperature
(?C
)
Fig. 3. Frequency distribution of air temperature
with low (left) and high (right) QFD. Width of cl
extends periods with low QFD in the morning and
which occurs when direct midday sunlight com-
evening hours.
bines with diffuse light from bright surrounding
Temperature. Ambient temperatures for the
clouds. Thus, these species (Ranunculus glacialis
periods considered here differed less than long-
L. and Ligusticum mutellina [L.] Crantz) are
term annual differences (compare Table 1 and
always light-limited.
Table 3). This is even more pronounced in plant
Plants attained 95% saturation of photosyn-
canopy temperatures. However, evaluation of tem-
thetic capacity at widely differing QFDs with no
peratures with respect to photosynthetic activi-
clear elevational differences. Extremes at high
ties, requires incorporation of the concurrent
elevation are exemplified by R. glacialis and L.
radiation regime. Enhanced radiant heating at high
mutellina which require full sunlight and Doroni-
elevation causes a pronounced asymmetry of the
cum clusii (All.) Tausch which requires only 25%
temperature distribution, whereas the distribution
of the above level. At low elevation interspecific
at low altitude is almost perfectly normal (Fig. 2).
variation is less. Geum exhibits comparatively low
At QFD below 500 Lmol m-2 s-1 temperatures
light requirements at both elevations. For the
between 15 and 20'C are five times more frequent
alpine species, our results confirm trends
at low elevation (Fig. 3). However, at QFD above
observed by Cartellieri (1940) and Moser (1965).
1500 Lmol m-2 s-1 the frequency ratio for such
Alpine species require about one tenth of full
temperatures between the low and high elevation
sunlight to reach 50% of photosynthetic capacity,
site is only 3:2, and the median of temperatures in
which is less than required by low elevation
this range differs by only 3 3 K among sites.
species. Due to the restricted number of species
C02-concentration. Mean mixing ratios of CO2
and the incorporation of such different response
between 0500 and 1900h in June and July did not
types as R. glacialis and D. clusii, the altitudinal
differ between sites and are in full accordance with differences obtained here are statistically insignirecent studies at the swiss 'Jungfrauj och' in 3500 m
ficant.
altitude (334.8 Vl 1-1 for July 1980-1982, Zumbrunn et a]. 1983).
Photosynthetic response to light
Under optimum temperature conditions, saturat-
Photosynthetic response to temperature
Altitudinal 'differences in the temperature
response of photosynthesis are small (Table 5).
The mean difference among sites amount to only
ing QFD of photosynthesis was reached only above 2-3 K but are statistically significant. Alpine
species occupying warm rocky slopes like
1200 Imol m-2 s-1 in all species (Table 4). Some
Achillea moschata (Wulfen) I.B.K. Richardson,
species required at least 2000 Lmol m-2 s-1, which
exhibit higher temperature optima than some of
represents the clear day maximum of QFD. Thickthe low elevation species. Species with lower
leaved alpine species show increasing photo-
synthetic rates up to 3000 Lmol m-2 s-1, the
absolute maximum QFD recorded in the field,
saturating QFD like Oxyria digyna (L.) Hill, the
three Geum species and Doronicum clusii exhibit
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185
Table 4. The photosynthetic response to quantum flux
lower temperature optima. This corresponds with
Photosynthesis
density (pmol photons m-2 s-1).
their preference for microsites less exposed to
at high altitude
Percentage of saturation
of A
99% 95% 50%
direct sunlight. The major difference among sites
occurs at very low temperatures. Alpine species
maintain 50% of photosynthetic capacity at tem-
peratures around 40C, compared with 80C in the
Low Elevation
Geum urbanum 1500 800 280
lowland species. Pisek et a]. (1967) and Larcher &
Geum rivale 1200 820 230
Wagner (1976) showed that the low temperature
Ranunculus acris 1800 1060 390
minima for positive net photosynthesis of alpine
Taraxacum officinale 1800 1150 370
plants range from -2 to -60C. Comparable herba-
Polygonum bisorta 2000 1400 500
Mean
(?SE)
1600
?140
1046
354
?111
?47
High Elevation
Doronicum clusii 1200 530 170
Geum reptans 1500 770 170
Oxyria digyna 1500 1100 140
Polygonum viviparum 2000 1030 200
ceous species from low elevations have not been
studied in this temperature range.
Photosynthetic responses to carbon dioxide
Diurnal changes of ECU. No changes of ECU were
found in any experiment within the normal 2-3 h
Ligusticum mutellina 3000 1900 520
of leaf enclosure in the constant cuvette environ-
Ran unculus glacialis 3000 2000 210
ment. However, prolonged exposure of leaves to
Mean
(?SE)
2033
?323
1222
235
saturating light conditions and optimum tem-
?255
?58
peratures under high humidity caused a slight
Level of significance (t-test) 0 353 0-563 0-154
(i.e. NS)
decline in ECU, A and G after 4-5 h. Since such
prolonged optimal conditions are rare in nature, it
appears unlikely that time dependent reductions
will exert substantial limitations to daily carbon
Table 5. Temperature response of photosynthesis under
saturating light conditions (the last two columns show
the temperatures at which either 95 or 50% of the rate
found at optimum temperature is reached [?C]).
gain.
Seasonal changes of ECU. Fig. 4 shows ex-
amples for the seasonal change of ECU at the
alpine site in 1986. Substantial variation is
apparent, although none of these curves was
Optimum temperature
100% 95% 50%
obtained immediately after a period of sub-zero
temperatures or snow. From visual detection all
Low elevation (600 m)
Geum urbanum 20-0 17-5-23-0 c 5-5
samples appeared non-senescent, except those of
Ranunculus repens 22-0 18-5-
Ranunculus from 13 September. Leaf unfolding in
Geum rivale 23-0 18-5-27 0 7-5
Daucus carota 23-5 19 0-28-5 9 5
the alpine species occurs rather quickly after snow
Polygon um bistorta 24-5 18 0-30-0 c 7-5
melt (early June to early July) and leaf maturation
Potentilla anserina 25-0 18-5-31 5 6-5
was completed after mid-July. The data from 25
Taraxacum officinale 26 0 21 0-32-5 8-5
June and 5 July for developing, but expanded
Ranunculus acris 27-0 20-0-32-5 8-0
Mean
23-9
18-9-29
3
7
5
(?SE) (0-8) (0-4) (1-2) (0.5)
High elevation (2600m)
leaves of Polygonum viviparum L. indicate the
pre-maturation changes in ECU. A period with
stable values was reached by all species between
Oxyria digyna 18-0 13 5-22-7 2-7
mid-July and early August. By mid-August ECU
Geum reptans 19-5 14-5-23-7 3-2
generally declined. This suggests that many alpine
Poa alpina 20-0 14 0-25 5 c 2-0
plants will not profit from prolonged growth
Geum montanum 20 5 13-0-275 -
periods and undergo metabolic senescence either
Doronicum clusii 20-5 15-5-235 -
Ranunculus glacialis 20-8 14-8-27-7 3-5
autonomically or induced by shorter day length or
Potentilla crantzii 21-5 17-0-26-5 3-5
lower night temperatures. The two Rosaceae
Polygonum viviparum 22-0 17-0-27-3 -
species Geum reptans L. and Potentilla crantzii
Ligusticum mutellina 23 0 16-5-29-0 4-5
Erigeron uniflortrm 23 5 19 0-28-0 6-5
Achillea moschata 25 0 20-5-28 0 c 9-0
Mean
212
15-8-26-1
4-2
(?SE) (0 6) (0.7) (0.7) (0-8)
Difference
low-high altitude 2 7 3-1 3-2 3-3
Level of significance
(t-test) 0 010 0.001 0020 0 004
(Crantz) G. Beck ex Fritsch may be exceptions, as
they continue to produce new leaves with high A
and ECU until very late in the season. Such
behaviour was documented by Johnson & Caldwell (1974) for Geum rosii (R.Br.) Ser. in the Rocky
Mountains.
Altitudinal differences of the response to carbon
dioxide. The rate of CO2 uptake at different ambi-
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186
Ch. K rner & Geum reptans RPnunculus qiacla/is PotentI/la crantz01
M.
Diemer
E
4
June
25
E~~~~~~~
IE40- ~ 0
~ un
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',20
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0 ~ ~ ~~ 0
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7
4
27
4.Sesonl
I I I I I I I 60 8l I I I
I
leaves
o
S
fe2
5a
aritin
Ii
T
A nenlprilpesr fC2(ubr
o 200 400 600
been ollowd fro prmauitltyatriy
800
fivepecishaebeeinvstigtedu frmute//in developmernt unatef/aorum Phelyasnu vpeiev/olygoum
ent CO2 concentrations for paired species is depic-
the selection of related pairs of species may justify
ted in Figs 5 and 6. In nine of the 12 pairs (Fig. 5)
a pooled comparison of the two groups for a first
both ECU and the rate of CO2 saturated photo-
approximation at the community level. The results
synthesis are significantly higher in the high
are shown in Table 7. It becomes evident that A at
altitude species. All these species are restricted in
local CPA does not differ among the two altitudi-
their natural abundance to either high or low
nally separated groups although CPA is 21% lower
elevation. The remaining three pairs of species
at high altitude. Among several pairs of species the
(Fig. 6) did not exhibit pronounced differences in
high altitude representative exhibits significantly
ECU. The high altitude species of this group are
higher rates (e.g. Ranunculus, Polygonum,
not exclusively alpine and may be called
Erigeron, Geum and Primula) while in other cases,
'ubiquists'. P. crantzii grows also on rock crevices
represented by the 'ubiquists' Potentilla and
around Innsbruck and Trifolium and Taraxacum
Taraxacum, in situ rates are lower at high eleva-
species cover a wide elevational span. The latter
tion. If the comparison were restricted to those
species is only vaguely separated taxonomically
pairs containing distinct high elevation taxa, then
from the Taraxaxum officinalis aggregate
the high altitude group would yield significantly
occurring at lower altitudes. Table 6 summarizes
higher rates of maximum A at local CPA. The
photosynthetic rates at local partial pressure of
particularly low A in the genus Primula corre-
CO2 in the air surrounding the leaf (CPA).
sponds to results obtained by Whale (1983).
Although the species investigated here rep-
An explanation for this compensation or, even,
of the elevational decline of
resent only a small fraction of the respective overcompensation
floras,
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187 Table 6. Rates of photosynthesis, leaf and palisade layer thickness and nitrogen contents.
Photosynthesis
at
high
altitude
A
LTH
PTH
SLA
NLA
Low altitude
Taraxacum officinale 22-7 (2-3) 187 (15) 78 ( 6) 3-00 (0-13) 86 (13)
Potentilla vern. & ans. 22 5 (2 1) 159 (25) 75 ( 9) 1-74 (0-21) 121 ( 5)
Carex acutiformis 20-8 (1 6) 144 (18) 43 ( 6) 1-86 - 121 Erigeron acre 20-5 (2 4) 328 (24) 154 ( 4) 1 89 - 136 Daucus carota 19-0 (1-1) 208 ( 7) 82 ( 4) 2 64 - 95 Polygon um bistorta 18-0 (1 8) 258 (17) 96 ( 9) 2-16 (021) 133 (26)
Achillea millefolium 17-9 (0-9) 288 (35) 135 (34) 1.42 (0.20) 193 (13)
Trifolium repens 16 3 (2-3) 159 (16) 55 ( 8) 2-95 - 109 Ranunculus, 5 species 15 7 (1-3) 316 (23) 111 ( 9) 2-12 (0-16) 104 (11)
Geum rivale 12 7 (0-5) 144 ( 5) 46 ( 3) 2-27 (0.15) 88 ( 4)
Primula elatior 10 7 (0-9) 113 (11) 26 ( 5) 2-39 - 54 High altitude
Ligusticum mutellina 23-9 (3.8) 297 ( 9) 130 ( 6) 1-34 (0-08) 201 ( 4)
Erigeron uniflorus 22-5 (3-0) 289 ( 8) 122 ( 3) 1-75 (0-13) 141 ( 3)
Leontodon helveticus 20-6 (1-8) 234 ( 9) 98 (10) 2-18 (0.14) 96 ( 6)
Polygonum viviparum 20-2 (2-3) 311 (14) 124 (10) 1-74 (0.07) 179 ( 9)
Ran unculus glacialis 19-1 (1-9) 566 (20) 228 ( 5) 1-42 (0-08) 157 ( 7)
Trifolium thalii 16-9 (0-9) 220 (14) 131 (10) 1-65 - 153 Achillea moschata 16-4 (0-9) 497 (13) 301 (20) 1-96 - 168 Potentilla crantzii 16-4 (3.0) 246 ( 8) 115 (13) 1-38 (0-11) 162 ( 5)
Carex curvula 15-8 (3-6) 304 ( 7) 130 ( 7) 1-08 (0-08) 159 (15)
Geum reptans 13-7 (2-1) 284 ( 6) 133 ( 3) 1-36 (0-07) 151 (19)
Primula glutinosa 12-3 (1-3) 598 (38) 224 (29) 1-27 (0-03) 139 ( 3)
Taraxacum alpinum 10 6 (2.2) 289 (14) 115 (14) 2-74 - 77 -
A (p'mol m-2 s-1), maximum A at local CPA derived from CPI/CPA
LTH and PTH (pm), leaf and palisade layer thickness ? SE of 3-7 (m
SLA and NLA (diM2 g-1; mmol N m-2), specific leaf area and leaf nitr
of 15-50 leaves each. In cases where no SE is presented only one
Table 7. Statistical analysis of altitude specific differences.
Parameter Low altitude High altitude Significance
Mean ? SE Mean ? SE
A (Pimol m-2 s-1) 18-2 1.1 17-4 1-1 0-58 n.s.
At (pmol m-2 S-1) 14-1 0-8 17-4 1-1 0-031 *
Slope A = f (CPI)'
in the linear range
(mmol m-2 s-1 Pl 1-1) 83-1 6-1 117-4 9-1 0-005 **
Slope CPI = f(CPA) 0-79 0-02 0-69 0-02 0-001
CPI (at normal local CPA, pA 1-1 250-3 6-7 177-0 4-5 < 0.001 *
SLA (g dm-2) 2-29 0-15 1-65 0-14 0.005 **
NLA (mmol N m2) 110-6 10-1 148-6 9 8 0-012 *
NDW (% dry wt) 3-37 0-20 3-30 020 0-785 n.s.
Leaf thickness (pm) 207 20 343 38 0-006 **
Palisade layer thickness (pm) 81 11 143 14 0.003 **
aCo2 compensation point equals 31 ?5 ill- 1 at 22 ?C for both e
determined by Bauer & Martha (1981).
Number of species in each group = 12; differences of variances between groups are not significant at the 5% level
(F-test). Significance of group differences was tested by Student t-test.
At = estimated rate of photosynthesis of species from low elevation under normal ambient CPA of high elevation with
all other variables the same.
NLA = nitrogen content per unit leaf area (m mol m-2)
NDW = nitrogen content in percent of dry weight
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188
Ch. Korner & Ronunc/us gqoc/olis Polygonum
M.
Diemer
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ai)
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elation
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0 200 400 600 800 0 200 400 600 8000 200 400 600 800
Internal partial pressure of CO2 (pbar)
Fig. 5. In situ efficiency of carbon dioxide uptake (ECU) in nine pairs of taxonomically related plant species with
distinct altitudinal ranges of distribution. Arrows indicate the rate of CO2 uptake at normal local partical pressure of
CO2 (marked by triangles at the abscissa) commonly termed operating point. The results shown here comprise
measurements in six to 10 individuals for the genera Ranunculus, Geum and Polygonurn and two to five individuals in
all other species. The scatter is the result of intraspecific variation, since data for individual leaves show hardly any
deviation from a 'smooth response curve'.
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189
Photosynthesis
Potenfi/la crantzli Trifo//um tha/il Taraxacum a/pi;wm
LO
040
at high altitude
-
o
30
3
20-
2
(n
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~
~
20
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0)
~
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1
20
6e0
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8
40060
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4~~~ntralprta
Fig 6 AsinFig 5bu fo treeparsof peieswih reperatudnslrne 7eere toff/c/na/qist'ithte.
Fi.6 si i.5btfrtrepisofseiswt ie liuia ag, eerdt s'bqit'i h et
p
CPA lies in the mesophyll and not the stomatal
achieves higher A by virtue of a pronounced
diffusion path. Under the given experimental
increase of ECU.
conditions maximum G averages around 200
mmol m-2 s-' at both sites. Again interspecific
variation is large and 'ubiquists' at high elevation
tend to exhibit lower Gmax than exclusive alpine
Morphological and nutritional differences
Any change in ECU may have structural or speci-
species. For Ranunculus, the most intensively
fically biochemical reasons. A detailed casual
investigated genus, mean Gmax is 203 mmol m-2
analysis of both these relations is currently in
s51 (?19 SE) at low elevation and 213 (?22 SE) at
progress. At this state we can provide information
high elevation. According to Farquhar & Sharkey
on global structural parameters and on nutritional
(1982) the relative stomatal limitation (SL) of A
status of leaves (Table 6). Both, overall leaf
can be expressed by the equation SL = (A0-A)/A0.
thickness and palisade layer thickness increase by
A0 is C02 uptake derived from the A/CPI curve,
66 and 76% respectively with altitude (Table 7).
assuming CPI equals CPA (i.e. zero stomatal resist-
Preliminary results of a broad survey of mesophyll
ance). This yields a relative stomatal limitation of
surface:leaf area ratios indicate significantly
17% at low elevation and 30% at high elevation,
higher ratios (+30%) for high elevation plant
which is largely attributable to the different curva-
communities. Mean total leaf nitrogen content per
ture of the A/CPI responses. If expressed as the
unit leaf area is higher by 34% in alpine taxa but is
ratio of stomatal versus total (stomatal plus meso-
similar on a dry weight basis. Inversely correlated
phyll) resistance at the operating point, the differ-
to the increase in thickness is specific leaf area
ence in the relative gas phase limitation among
(SLA), which is reduced by 28% at high altitude.
sites is smaller: 23% at low altitude and 28% at
Pooled correlations between these parameters for
high altitude. Since the latter approach is devoid
mountain and lowland species are depicted in
of extrapolation to higher CPI, it may be more
Table 8. ECU correlates well with SLA and NLA
realistic (Jones, 1985).
As summarized in Table 7 the initial slope of the
(nitrogen content per unit leaf area). The correlations between ECU and thickness parameters are
A/CPI curve is about 40% steeper at high elevation.
significant on the 5% level but scatter is sub-
CPI under mean local CPA (251 VFbar at high
stantial.
elevation and 317 VFbar at low elevation with 335
VL 1-1 at both sites) is about 30% lower at high
altitude; a reduction exceeding that in CPA. This
Discussion
difference is reflected in the altered slope of the
This investigation has revealed a number of
CPI/CPA regression. The ratio is 11% smaller at
physiological characteristics of herbaceous peren-
high altitude and scatter in the separate regres-
nial plants from low and high altitudes. Our major
sions for each altitude is very small. In conclusion,
concern was to follow a comparative approach and
the investigated group of high altitude plant
eliminate bias by variables related not a priori to
species on average operates at lower CPI than
the problem of altitudinal differentiation of plant
might be expected from the pressure decline and
functioning, e.g. moisture stress. An important
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Tr
190
Table 8. Linear regression analysis of factorial corre-
conclusions for alpine plants. In arctic taxa direct
Ch. Korner &
lations.
effects of temperature do not influence carbon
M. Diemer
Y-X b (slope) a r for b Significance
of b
A-ECU 0 0548 11-919 0-534 0-005 **
ECU-SLA -0-0292 0-164 -0 562 0-005 **
yield significantly either (Chapin, 1983).
Our data show that interspecific and/or micro-
site differences in temperature and light response
are much larger than mean altitude-specific differ-
ECU-NLA 0 4278 47-92 0 549 0 006 **
ences, which defies any predictions for individual
ECU-LTH 0 1318 68 48 0-465 0 019 *
species. The temperature optima of A at both
ECU-PTH 0-2250 77 09 0-461 0 020 *
elevations cover a range of 7 K. The ranges
SLA-LTH -0 002433 2 698 -0 456 0 021 *
observed at a 500 m higher mountain site by Moser
Dimensions and statistics as in Table 7.
et a]. (1977) are even larger. These are striking
A = net rate of photosynthetic uptake
examples of the high variability in response pat-
ECU = efficiency of CO2 uptake
SLA = specific leaf area
NLA = nitrogen content per unit leaf area
terns in plants from high mountains and support
Larcher's (1980) view that fluctuations in time and
LTH = leaf thickness
space of both plant and environmental factors play
PTH = palisade layer thickness
a key role in the understanding of alpine plant life.
Therefore, broad experimental screening appears
indispensable in alpine ecology.
element of this approach was to compare species
with distinct ranges of elevational distribution, at
their respective peaks of photosynthetic activity.
Temperature and light response - no differentiation with altitude?
We find that differences in photosynthetically
Photosynthetic capacity and internal CO2 level in
high altitude plants
The rates of net CO2 uptake at ambient CPA at high
elevation reported here (Tables 6 and 7) are greater
than most reported in the earlier literature. Apart
from subsequent increases of atmospheric CO2 by
effective radiation and temperature regimes at the
30-40ppm, this is probably due to the fact that
two altitudes are much less than commonly
data were often not corrected for the effects of CO2
assumed (cf. Kdrner & Cochrane, 1983). This is
depletion in gas exchange cuvettes used in differ-
reflected in observed photosynthetic responses.
ential measurement systems. The observation that
The temperature and light responses of A indicate
mean maximum A at local ambient CPA is similar
that plants at either elevation are well adapted to
at both elevations, confirms trends observed in
utilize the warmest and brightest periods rather
several of the earlier studies within temperate
than average conditions, although the former
latitudes. However, at equal cuvette-CPA alpine
comprise only 25-30% of all daylight hours at both
species exhibit significantly higher rates of A (At
elevations. Relatively high temperature optima of
in Table 7). It is possible to estimate a mean
A for alpine herbaceous plants have previously
difference of + 20% for alpine taxa if CPA were 251
been described by Mooney and Billings (1961),
VFbar at both elevations (which equals present
Moser (1970), K6rner (1982) and others. These
ambient CPA at 2600 m altitude). For extrapolation
results are in contrast to observations in forest
to higher CO2 levels the curvature of the A/CPI
trees. Since trees are more closely coupled to
curve needs to be taken into consideration.
ambient temperature, they exhibit steeper elevational gradients of optimum temperature for
CO2 saturated A (CPI > 500 VFbar) is approximately 50% higher in alpine versus low elevation
photosynthesis (e.g. Fryer and Ledig, 1972; Slatyer
species. This confirms estimates of elevational
and Morrow, 1977). Chapin and Oechel (1983)
differences of A at 1% C02 by Nakhutsrishvili and
report similar observations for an arctic tundra
co-workers (Nakhutsrishvili, 1974) in the Central
sedge transplanted to thermally different environ-
Caucasus. When exposed to high CO2 for 5-10 min
ments. In a quantitative analysis of the relative
sub-niveal plants exhibited 70% higher CO2
importance of temperature and QFD for annual
saturated A than sub-alpine plants (n = 11, P <
carbon yield of Carex curvula All., Korner (1982)
0 05). A at ambient CPA did not differ significantly
showed that QFD is much more important than
in their study. Schulze et al. (1985) investigated
temperature. Suboptimal QFD at leaf level restricts
the CO2 response of afro-alpine giant rosettes in
yield by 40%, compared to 8% as a result of
4200m altitude (CPA = 188 VFbar). In accordance
suboptimal leaf temperatures. Moser (1970) and
with our observations they found leaves to operate
Scott, Hillier & Billings, (1970) arrived at similar
well down in the linear range of the A/CPI curve.
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191
thicker leaves, one would expect similarly posiCPI at local CPA was near 100 vFbar and stomatal
Photosynthesis
limitation reached 42% when maximum A was
tive effects on A in arctic plants which is not the
at high altitude
achieved.
case (Mooney & Billings, 1961; Pisek, 1960;
Billings
The CPI of 250 ? 23 VFbar (? SD) of the low
elevation plants at present ambient CO2 level (335
& Mooney, 1968; and others).
The remarkable constancy of nitrogen content
per unit dry weight suggests a uniform protein
Vd 1-1) is similar to the mean value of 247 ? 12
derived from a literature review by Yoshie (1986)
volume density in the leaves of herbaceous plants,
for many different species and life forms adapted
irrespective of elevation. The elevational increase
to 328 vl 1-1. Yoshie's original number is in Vd 1-1 of nitrogen content per unit leaf area appears to be
but if most of these data were obtained close to sea
largely attributable to increases in leaf thickness
level, this figure would correspond to partial
which corresponds to reduced SLA. This confirms
pressure.
observations by Korner, Bannister & Mark (1986)
It is not certain whether these altitudinal differ- in the mountains of New Zealand. However, the
ences in photosynthetic capacity are of genotypic
possibility that the specific carboxylation effici-
origin. Billings, Clebsch & Mooney (1961) demon-
ency at the biochemical level among low and high
strated that seedlings of alpine ecotypes of 0.
elevation taxa is altered still exists. Von Caemerer
digyna show higher A at any given CPA below
& Farquhar (1981) and others showed that ECU is
ambient pressure than arctic ecotypes when both
largely controlled by the activity of RUBP-
groups of plants were grown at the same CO2 level. carboxylase per unit leaf area. Leaves developing
This suggests ecotypical differentiation but not
at lower temperatures tend to exhibit higher speci-
necessarily in response to a long history of growth
fic Rubisco activity per unit protein (Bjbrkman,
under different CPA since both environments
Badger & Armond, 1978) as well as per unit leaf
area (Bunce, 1986). Pandey, Bhadula & Purohit
differ in other respects as well. In addition CPI and
hence ECU were not known for Oxyria. No differ-
(1984) observed higher Rubisco activity in high
ences in A at equal CPA were found following
altitude than in lowland samples of the perennial
short term exposure of three alpine and one desert
forb Selinum vaginatum Clarke in the Himalaja
species to contrasting elevations (Mooney, Strain
front range. Exposure to altered CO2 levels can also
& West, 1966). In accordance with the latter
influence Rubisco activity. The majority of recent
observation we found no difference in ECU
studies on the effect of increased CO2 during leaf
between 0. digyna in our mountain site and in
expansion indicate that Rubisco activities decline
alpine ecotypes grown for several years in the
both per unit soluble protein and per unit leaf area
Botanical garden in Innsbruck (unpublished data).
(e.g. Downton, Bjbrkman & Pike, 1980; Wong,
1980; Von Caemerer & Farquhar, 1984). Perhaps
Structural and functional reasons for increased
ECU
the opposite effect holds for long-term exposure to
reduced CO2.
At this stage, our findings do not allow an interpreElevated atmospheric C02 levels - more effective
tation of the observed changes of leaf character-
istics towards a 'low CO2 adaptation'. Exposure of
for high altitude plants?
plants to low CPA reduced leaf thickness of
The question whether plant life at high altitude is
greenhouse plants (Madsen, 1973). However,
particularly limited by CO2 supply has attracted
alpine plants exhibit thicker assimilatory tissues
ecophysiological researchers for many years
(Decker, 1959; Billings et al. 1961; Milner, Hiesey
(high in nitrogen) which favour higher rates of CO2
uptake per unit leaf area (Nobel & Walker, 1985).
& Nobs, 1963; Mooney et al. 1966; Gale, 1973).
Mechanisms leading to this morphological expres-
Mainly technical constraints made it difficult to
sion are still largely speculative. Differences in
approach this problem in the past with adequate
QFD between sites are too small to account for the
accuracy in the field. The data presented here
substantial differences in leaf structure observed
permit an experimental verification of theoretical
at both elevations. However, there may be a
considerations by Gale (1973) and Cooper, Gale &
genotypic selection for thinner leaves in low
LaMarche (1986). The latter authors discussed the
elevation plants since competition for light is
hypothesis of LaMarche et al. (1984) that increased
CO2 levels in the atmosphere could explain progreater at low altitude than at high altitude. In the
nounced increases in tree ring width at high
short-term ultraviolet also does not appear to affect
alpine plant structure (Caldwell, 1968). If low
mean temperatures alone were responsible for
elevations where CO2 limitation is supposed to be
greater than at low elevation. Against this expla-
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192
nation it was argued that negative effects of altitu-
Ch. Karner &
dinally decreasing CPA on A will be diminished or
M. Diemer
offset by reduced gas diffusive resistance at lower
40
C02-level 435.t L' , m
total pressures. Hence, the question was whether
(1) CPI decreases with increasing altitude proportionally to the decline in CPA or (2) remains at
comparatively higher level because CO2 influx is
facilitated.
+ 1%
20 +3 21/
600 M
Our data suggest a third alternative, namely a
reduction in CPI relative to CPA largely due to an
increased efficiency of carbon dioxide uptake by
the mesophyll. The possibility of physiological or
?10 +- / X i
0Q //335 FL( FI
present C02 -level
morphological alteration in the assimilatory tissue
was not taken into account in the above-
mentioned discussion. Although our study
0 100 200 300 400 500
revealed a relative constancy of A over a gradient
of 2000m of elevation, this was not due to the
diffusion effects stressed by Gale (1973) and in
Cooper et a]. (19,86) but rather to changes within
the plant. Even if such modifications were not
present, the stomatal resistance of C3 plants,
including many conifers, averages at about one
fifth of residual resistance when environmental
Internal partial press
Fig. 7. Average shape of response characteristics of
photosynthetic CO2 uptake and CP1 at the two altitudes.
Shaded areas indicate the estimated increase of CO2
uptake when global atmospheric CO2 level would
increase from 335 pAl-1 at present to 435 pl-P1, assum
plant response characteristics remain the same.
long-term they may be altered by acclimative
conditions are optimal (Kbrner, Scheel & Bauer,
modifications and by progressive imbalances
1979). A global survey of carbon isotope discrimi-
between carbon and nutrient relations. Also, the
nation in plants from high altitude provides
comparison is only valid as long as water stress
further support for the hypothesis that the propordoes not interfere. Extrapolations for altitudes
tion of mesophyll bound limitations to CO2 uptake
beyond the ones studied here are not justified
declines with altitude (Ch. Kbrner, G.D. Farquhar
because the morphological changes observed over
& Z. Roksandic, in preparation).
this elevational range do not proceed predictably
The question, whether the present increase of
with increasing elevation (unpublished data).
global CO2 will be particularly favourable for CO2
However, our estimates indicate - at least trend-
assimilation in plants from high elevation, appears
wise - that mountain plants should profit more
in a new light if our data are representative of
from increased global CO2 levels than lowland
altitudinal phenomena. The shape of the CO2
plants.
response curves allows us to predict what would
happen if C02 level were to increase by 100 P1-1
and plant, as well as environmental, conditions
other than CO2 remain unchanged. An increase of
Acknowledgements
This research was funded by the Fonds zur For-
the atmospheric mole fraction of C02 by 100 pA 11
derung der Wissenschaftlichen Forschung
would cause CPA to rise by 95 pLbar at our 600m
(Vienna) project P5597. We are grateful to I. Cowan
site and by 75 pLbar at our 2600m site. Mean CPI
for revising the paragraph on pressure effects and
from our regression over CPA would then amount
conductance units. M.M. Caldwell and A. Cernu-
to 227 pLbar at high and 325 pLbar at low altitude.
sca provided logistic support on computation of
The estimated gain in A under the given assump-
gas exchange and climate data. W. Seidenbusch
tions would amount to 21% at low elevation and
facilitated spectral analysis of our light source. W.
31% at high elevation (Fig. 7). For higher incre-
Larcher and two referees contributed valuable
ments in CPA the relative increase in A of alpine
comments to the manuscript.
versus lowland species becomes larger because the
response in low elevation species levels off earlier.
An additional 100 p 1-1 of C02 would increase A
by 9% at low elevation and 21% at high elevation.
It is necessary to emphasize that these are purely
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