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Functional
Ecology 1987, 1, 179-194
179
In situ photosynthetic responses to light,
temperature and carbon dioxide in herbaceous
plants from low and high altitude
Ch.KORNER and M. DIEMER Institut far Botanik, Universitdt Innsbruck,
Sternwartestraf3e 15, A-6020 Innsbruck, Austria
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 opti-
mum of QFD-saturated A differs little (3K) and
corresponds to the median of air temperature at
leaf level for hours with rate-saturating light con-
ditions 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
Alpine plants live in an environment commonly
described as cold and windy, with high radiation,
reduced partial pressures of 02 and CO2 and only
brief periods supportive of growth and develop-
ment of organisms (e.g. Billings & Mooney, 1968;
Franz, 1979; Larcher, 1983). These increasingly
adverse conditions restrict higher plants to eleva-
tional limits, which vary regionally and inter-
specifically. It is not clear which environmental
constraints or plant processes are important or
decisive for plant success at high elevation. The
majority of alpine plants tolerate site-specific frost
conditions within a substantial leeway towards
lower temperatures (cf. Pisek, Larcher & Unter-
holzner, 1967; Larcher, 1980, 1985; Sakai & Lar-
cher, 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 contri-
bution 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 moun-
tain 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-as-
similation 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; Tran-
quillini & 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 sub-
niveal 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-
ces in the Rocky Mountains (Oulton, Williams &
May, 1979) both grown and measured at low
altitude did not differ in their rate of CO2 assimi-
lation either.
Conversely, Michler & Ndsberger (1977) found that white clover clones from high elevation exhi-
bited higher rates of CO2 assimilation under equal
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180
Ch. Korner &
M. Diemer
greenhouse conditions than low elevation prov-
enances from the same area. Similar conclusions
were reached by Woodward (1986) for Vaccinium
myrtillus L.
Thus, no clear consensus on effects of altitude
on CO2 assimilation rates has been reached. The
major reason for this uncertainty has been the
absence of concurrent comparative field studies of
typical high and low altitude species. A review of
the literature on herbaceous plants indicates a
great disparity between the abundance of informa-
tion on the photosynthetic behaviour of alpine
versus native wild plants from low, non-arctic
and/or non-water stressed, unshaded environ-
ments. Except for forage grasses little is known
about low altitude herbs. In addition, problems
arise when comparisons are attempted between
results from early alpine studies and data obtained
subsequently at low elevation, since equipment
and plant material (mostly crop plants) differed
substantially.
In the present analysis, we investigated plants of
similar life form, within the same geographic
region, at similar phenological stage, without
interference from water stress and within their
specific elevational center of abundance. No attempts were made to investigate intraspecific
altitudinal differences, since this necessarily
would include comparisons at sites of optimal and
marginal life conditions of every species, which
was not the aim of this paper. The same methods
and instruments were used, at both high and low
altitude, within a short period of time, to further
reduce experimental noise. The purpose of this
investigation was to ascertain in the field, whether
and how: (1) photosynthetic capacity and photo-
synthetic light and temperature responses differ in low and high altitude herbaceous plants and how
this correlates with the microclimatic conditions;
as well as (2) how efficiently carbon dioxide is
utilized in groups of closely related plant species
at different altitudes.
Sites and plant species
Both the low and high elevation experimental sites
are in the vicinity of Innsbruck (47 0N 11 0E). Some
relevant site characteristics are summarized in
Table 1. Plant species investigated here are listed
in Table 2.
The low elevation plant species were studied in
two locations within the suburban belt of Inns-
bruck. Plots were weeded, so that leaves of the
experimental plants developed under full sunlight
as they do at high elevation. The soil, loamy brown
earth derivative with pH between 6 and 7, was
moist throughout the study. The species employed
here comprise typical elements of the meadow and
forest-edge flora of the Inn river valley. The
populations studied consist of individuals that
germinated spontaneously in the test area and
individuals that were transplanted from a wet
meadow in sod blocks 2-5 yr prior to the exper-
iments.
High elevation plants were studied on Mount
Glungezer, one of the peaks around Innsbruck that
protrudes into the subniveal zone. Here popu-
lations of about 80 species of phanerogams that
grow throughout the rock- and fellfields of the
Central Alps are present (Bahn & Kdrner, 1987). The site was first used for gas exchange studies by
Cartellieri (1940) and provides a variety of expo- sures ranging from the edge of a permanent snow
bank to thermally favoured South flanks, within
short distance of a permanent field station. Soils
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.
Materials and methods
Climate. A portable, automatic climate station
(Micromet-1, G. Cernusca, Innsbruck) was
installed during the main growth period at each
elevation. Air, soil and canopy temperature, as
well as quantum flux density [QFD (400-700nm)]
were measured in 2 min intervals and recorded as
hourly means. The CO2 content of the air was
recorded continuously at each elevation for 10
days in June and July with an infrared gas analyser
(225 MK3, ADC, Hoddesdon, England) and a chart
recorder. Means for daylight hours were deter-
mined from integrals between 0500 and 1900h.
Gas exchange studies. All measurements were
made in the field, using a portable, steady-state gas
exchange system (FG-02, Armstrong Enterprises,
Palo Alto, California) as described by Field, Berry
& Mooney (1982) and Atkinson, Winner & Mooney
(1986). In this system, the rates of transpiration
and C02-uptake are balanced by separate flows of
dry air and 1 % C02 in air, controlled by electronic
mass flow controllers (MFC). The IRGA (225 MK3,
ADC, Hoddesdon, England) serves as a null point
device (differential mode) and, alternatively, to
check CO2 concentrations of gas mixtures (abso-
lute mode) generated by a MFC-controlled gas
mixing unit. The photosynthetic response to inter-
nal CO2 concentration (CPI) was investigated at
concentrations between 50 and 1500 vll 1-1, begin- ning at low concentrations. The same bottle of
primary calibration gas was used throughout the
study at both elevations. Calibration of MFC's was
achieved with an electronic film flow meter
(SF-101, STEC Inc., Kyoto, Japan) corrected for
pressure and temperature.
Dry air flow in the gas exchange system was
adjusted, to maintain a cuvette vapour pressure
deficit of 0.86 ? 0.2 kPa. Somewhat higher deficits
occurred only at the highest temperature range
during the measurement of temperature response
curves. Unless temperature response was studied,
leaf temperature was maintained within 2K of
optimum leaf temperature for CO2 uptake at
saturating light conditions. Leaf temperatures
were measured with two independent Cu/constan-
tan thermocouples mounted on the abaxial leaf
surface by small pieces of porous fiber tape (Leu-
copor no 2471, Beiersdorf, Hamburg, Federal
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182
Ch. Kbrner &
M. Diemer
Republic of Germany) that covered not more than
5% of the leaf surface. The diffusive conductance
of this tape is equal to or higher than maximum leaf
conductance. Depending on leaf size, boundary
layer conductance for water vapour varied bet-
ween 1 and 2 mol m-2 s-1, which is about two to
six times larger than maximum stomatal con-
ductance.
Light was supplied by a 'Multi-mirror reflector'
halogen lamp (Type EYC, 12V/75W General Elec-
tric, Cleveland, Ohio) in connection with a 45
degree cold mirror (45? CM-Pyrex-wit, Ocli, Santa
Rosa, California) to reduce heat input. With the
cold mirror attached, we measured peak intensity
(100%) between 660 and 680nm. Beyond 690nm
intensity declines sharply to 20%. Quantum flux
areal density (QFD) was measured with a quantum
sensor (LI-190S, Licor) at leaf level. Sunlight was
always screened off. For light response curves the
leaf-lamp distance was altered. During all
measurements other than light response QFD was
kept at or above saturating densities.
Calculations of gas exchange parameters were
based on equations from Von Caemmerer & Farqu-
har (1981) and Field et a]. (1982), using Cowan's
(1977) system of molar units. An additional
routine was incorporated to account for the differ-
ences in boundary layer conductance due to differ-
ences in leaf width. A constant cuticular conduc-
tance to water vapour of 15 mmol m-2 s-1 was
assumed, based on the results of desiccation
experiments (unpublished data). The efficiency of
carbon dioxide uptake (ECU), often inade-
quately termed carboxylation efficiency, was
derived from the initial linear slope of the A/CPI
curves.
A major aspect in the treatment of data in this
study is the influence of atmospheric pressure.
The use of mole fraction units for gas concentra-
tions (for all practical purposes, the partial pres-
sure of a gas species divided by total pressure)
leads to pressure independent expressions of leaf
conductance (G) - that is to say, the conductance
of pores of a particular size does not depend on the
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-
phyll surface. This is derived by multiplying the
mole fraction of CO2 inside the leaf by total
ambient (local) pressure.
Similar considerations apply to the estimation of transpiration rates from leaf conductances (G)
and a moisture gradient. At equal vapour pressure
difference, temperature and stomatal opening, the
rate of transpiration will be higher at high eleva-
tions due to the higher diffusivity of water vapour
in air. However, at equal mole fraction difference,
temperature and stomatal aperture, the rate of
transpiration will be the same at all elevations and
so will the conductance, as defined by Cowan
(1977). Thus, any observed differences in conduc- tance are due to differences in the dimensions of the diffusion path.
Since we attempted to compare leaves at their
physiological optimum, it was imperative to select mature fully developed leaves. At low altitude visual estimation of leaf age is much easier than at
high elevation. Colour and lustre of leaves undergo
more dramatic changes and are more conspicuous
in low altitude herbs. Leaves selected to be fully
expanded and just mature varied little in ECU.
However, at high altitude post-maturity stages are often difficult to detect visually. A further problem
arises at high altitude from after-effects of frost. Freezing temperatures without snow or snow
cover of several days depress ECU on successive warm days and this becomes more pronounced later in the season. In order to delineate peak
capacity, seasonal changes of photosynthetic behavour were monitored in alpine taxa.
For CO2 response three pairs of species (Ranun- culus, Geum and Polygon um) have been studied in
more detail, with a total of six to 10 response curves from different individuals pooled for
species comparisons. Additional comparisons for all other species pairs were based on two to five
curves each. Temperature and light response was determined from two samples per species. All data
are expressed per unit projected leaf area.
Leaf anatomy. Experimental leaves were inves-
tigated for overall thickness and thickness of
palisade layer by light microscopy.
Leaf nitrogen content. Approximately 100cm2
of leaf laminae (15-50 leaves, depending on leaf
size) comparable to those subject to photosyn- thesis measurements were collected and mean
total Kjeldahl nitrogen was determined from ground subsamples.
Results
Environmental studies
At low elevation the first and most productive
phase of growth and development for the majority of herbaceous phanerogams ends in mid-June. Subsequent and generally less productive cycles (second flush or new generations) are disregarded here. There is only one growth cycle at the high elevation site due to the short snow-free period.
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183
Photosynthesis
at high altitude
Since the end of the first growth period at low
elevation and the beginning of the annual growth
cycle at high elevation (snow melt) fall in the
period of summer solstice, the mean solar angle is
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
low elevation and from 23 June to 8 September at
high elevation covering about 1800hours from
each altitude are shown in Table 3 and Figs 1-3.
20 aGlungezer 2600 m 0 Innsbruck 600 m
10
0 400 800 1200 1600 2000 2400
Quantum flux density (kmot m 2s)
Fig. 1. Frequency distribution of QFD > 30 P'mol m-2 s-1 during the main growth period at each altitude (width of
classes 200 Pmol m-2 s-1).
Light climate. QFD does not differ substantially
between the two elevations. Daily totals of quan-
tum input in the 400-700nm range were 15%
greater at high elevation only on completely
cloudless days, which are very rare. In addition,
short-term (2 min) maxima of QFD were higher
Glungezer 2600 m Innsbruck 600 m
1 20 / 4
0/
C I
Plant canopy temperature (CC)
Fig. 2. Frequency distribution of air temperature within the plant canopy during the main growth period at each altitude (width of classes 4K in the range from -5 to
+39QC). Hours with QFD > 30 Lmol m-2 s-' only.
and QFDs > 2000 LmoI m-2 S-1 occurred more frequently at high elevation (Table 3). However,
the overall frequency distribution of QFD based on
hourly means was very similar (Fig. 1). This
mediation is attributable to the higher frequency of
convective cloud accumulation in the summit
area, which virtually equalized the effect of
reduced atmospheric absorbance at high altitude.
This observation is in accordance with data from
Moser et a]. (1977) and Rott (1976) for the summer
period in this region of the Alps. The higher
frequency of hours in the 30-200 Lmol m-2 s-1
class at low altitude is largely due to the screening
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 ground 15 3 8 7 (-6-6K)
Canopy air 15 9 118 (-4.1K)
Soil (15 cm depth, all hours) 12 8 7 1 (-5.7K)
Atmospheric CO2 level
Mixing ratio (pl l-1) 335 5 335 ? 3 Mean partial pressure (>bar) 317 251
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184
Ch. Karner &
M. Diemer
QFD<500 LOmoL m-2 s-' QFD > 1500 Lmol m-2 s-'
2600 m
40-
30 10 20 30600 0m
07~ ~ 2600Gm
0~ 2 0 1 0 3 0 2 0 4
Pl ant canopy temperature (?C )
Fig. 3. Frequency distribution of air temperature within the plant canopy for daylight hours (QFD > 30 pmol m-2 s-) with low (left) and high (right) QFD. Width of classes 5K in the range from -5 to +40?C.
extends periods with low QFD in the morning and evening hours.
Temperature. Ambient temperatures for the periods considered here differed less than long- term annual differences (compare Table 1 and Table 3). This is even more pronounced in plant canopy temperatures. However, evaluation of tem- peratures with respect to photosynthetic activi- ties, requires incorporation of the concurrent radiation regime. Enhanced radiant heating at high elevation causes a pronounced asymmetry of the temperature distribution, whereas the distribution at low altitude is almost perfectly normal (Fig. 2). At QFD below 500 Lmol m-2 s-1 temperatures between 15 and 20'C are five times more frequent at low elevation (Fig. 3). However, at QFD above 1500 Lmol m-2 s-1 the frequency ratio for such temperatures between the low and high elevation site is only 3:2, and the median of temperatures in
this range differs by only 3 3 K among sites.
C02-concentration. Mean mixing ratios of CO2 between 0500 and 1900h in June and July did not
differ between sites and are in full accordance with recent studies at the swiss 'Jungfrauj och' in 3500 m
altitude (334.8 Vl 1-1 for July 1980-1982, Zum- brunn et a]. 1983).
Photosynthetic response to light
Under optimum temperature conditions, saturat-
ing QFD of photosynthesis was reached only above 1200 Imol m-2 s-1 in all species (Table 4). Some
species required at least 2000 Lmol m-2 s-1, which represents the clear day maximum of QFD. Thick- leaved alpine species show increasing photo- synthetic rates up to 3000 Lmol m-2 s-1, the absolute maximum QFD recorded in the field,
which occurs when direct midday sunlight com-
bines with diffuse light from bright surrounding
clouds. Thus, these species (Ranunculus glacialis
L. and Ligusticum mutellina [L.] Crantz) are
always light-limited. Plants attained 95% saturation of photosyn-
thetic capacity at widely differing QFDs with no
clear elevational differences. Extremes at high
elevation are exemplified by R. glacialis and L.
mutellina which require full sunlight and Doroni-
cum clusii (All.) Tausch which requires only 25%
of the above level. At low elevation interspecific
variation is less. Geum exhibits comparatively low
light requirements at both elevations. For the
alpine species, our results confirm trends
observed by Cartellieri (1940) and Moser (1965).
Alpine species require about one tenth of full
sunlight to reach 50% of photosynthetic capacity, which is less than required by low elevation
species. Due to the restricted number of species
and the incorporation of such different response
types as R. glacialis and D. clusii, the altitudinal
differences obtained here are statistically insigni-
ficant.
Photosynthetic response to temperature
Altitudinal 'differences in the temperature
response of photosynthesis are small (Table 5).
The mean difference among sites amount to only
2-3 K but are statistically significant. Alpine
species occupying warm rocky slopes like
Achillea moschata (Wulfen) I.B.K. Richardson,
exhibit higher temperature optima than some of
the low elevation species. Species with lower
saturating QFD like Oxyria digyna (L.) Hill, the three Geum species and Doronicum clusii exhibit
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185
Photosynthesis
at high altitude
Table 4. The photosynthetic response to quantum flux
density (pmol photons m-2 s-1).
Percentage of saturation
of A
99% 95% 50%
Low Elevation
Geum urbanum 1500 800 280
Geum rivale 1200 820 230
Ranunculus acris 1800 1060 390
Taraxacum officinale 1800 1150 370
Polygonum bisorta 2000 1400 500
Mean 1600 1046 354
(?SE) ?140 ?111 ?47
High Elevation
Doronicum clusii 1200 530 170
Geum reptans 1500 770 170
Oxyria digyna 1500 1100 140 Polygonum viviparum 2000 1030 200
Ligusticum mutellina 3000 1900 520
Ran unculus glacialis 3000 2000 210 Mean 2033 1222 235
(?SE) ?323 ?255 ?58
Level of significance (t-test) 0 353 0-563 0-154
(i.e. NS)
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]).
Optimum temperature
100% 95% 50%
Low elevation (600 m) Geum urbanum 20-0 17-5-23-0 c 5-5
Ranunculus repens 22-0 18-5-
Geum rivale 23-0 18-5-27 0 7-5
Daucus carota 23-5 19 0-28-5 9 5
Polygon um bistorta 24-5 18 0-30-0 c 7-5
Potentilla anserina 25-0 18-5-31 5 6-5
Taraxacum officinale 26 0 21 0-32-5 8-5
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)
Oxyria digyna 18-0 13 5-22-7 2-7
Geum reptans 19-5 14-5-23-7 3-2
Poa alpina 20-0 14 0-25 5 c 2-0
Geum montanum 20 5 13-0-275 -
Doronicum clusii 20-5 15-5-235 -
Ranunculus glacialis 20-8 14-8-27-7 3-5 Potentilla crantzii 21-5 17-0-26-5 3-5
Polygonum viviparum 22-0 17-0-27-3 -
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
lower temperature optima. This corresponds with
their preference for microsites less exposed to
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
lowland species. Pisek et a]. (1967) and Larcher &
Wagner (1976) showed that the low temperature
minima for positive net photosynthesis of alpine
plants range from -2 to -60C. Comparable herba-
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
of leaf enclosure in the constant cuvette environ-
ment. However, prolonged exposure of leaves to
saturating light conditions and optimum tem-
peratures under high humidity caused a slight
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
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
obtained immediately after a period of sub-zero
temperatures or snow. From visual detection all
samples appeared non-senescent, except those of
Ranunculus from 13 September. Leaf unfolding in
the alpine species occurs rather quickly after snow
melt (early June to early July) and leaf maturation
was completed after mid-July. The data from 25
June and 5 July for developing, but expanded leaves of Polygonum viviparum L. indicate the
pre-maturation changes in ECU. A period with
stable values was reached by all species between mid-July and early August. By mid-August ECU
generally declined. This suggests that many alpine
plants will not profit from prolonged growth
periods and undergo metabolic senescence either
autonomically or induced by shorter day length or lower night temperatures. The two Rosaceae
species Geum reptans L. and Potentilla crantzii
(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 & Cald- well (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
',20 0 Oc2 / IE40- ~ ~ un 29 5Juy2
6 Aug 9 +* ; July
/ I I I I I I I I 60 8l I I I Ii
E~~~~~~~
40 (Aug.I ) 7 4 27
Fig 4.Sesonl aritin of , SC ttephghat.td 9 ie Ecdaerpsntmauemensi w qaleeoe
20.0
.30 *\* Aug213
leaves o S fe2 5a T
0 ~ ~ ~~ 0
fivepecishaebeeinvstigtedu frmute//in developmernt unatef/aorum Phelyasnu vpeiev/olygoum vvprm a
o 200 400 600 800
A nenlprilpesr fC2(ubr
been ollowd fro prmauitltyatriy
ent CO2 concentrations for paired species is depic- ted in Figs 5 and 6. In nine of the 12 pairs (Fig. 5)
both ECU and the rate of CO2 saturated photo-
synthesis are significantly higher in the high altitude species. All these species are restricted in
their natural abundance to either high or low elevation. The remaining three pairs of species (Fig. 6) did not exhibit pronounced differences in
ECU. The high altitude species of this group are
not exclusively alpine and may be called
'ubiquists'. P. crantzii grows also on rock crevices
around Innsbruck and Trifolium and Taraxacum
species cover a wide elevational span. The latter species is only vaguely separated taxonomically from the Taraxaxum officinalis aggregate
occurring at lower altitudes. Table 6 summarizes
photosynthetic rates at local partial pressure of CO2 in the air surrounding the leaf (CPA).
Although the species investigated here rep-
resent only a small fraction of the respective floras,
the selection of related pairs of species may justify
a pooled comparison of the two groups for a first
approximation at the community level. The results
are shown in Table 7. It becomes evident that A at
local CPA does not differ among the two altitudi-
nally separated groups although CPA is 21% lower
at high altitude. Among several pairs of species the
high altitude representative exhibits significantly
higher rates (e.g. Ranunculus, Polygonum,
Erigeron, Geum and Primula) while in other cases,
represented by the 'ubiquists' Potentilla and
Taraxacum, in situ rates are lower at high eleva-
tion. If the comparison were restricted to those
pairs containing distinct high elevation taxa, then
the high altitude group would yield significantly higher rates of maximum A at local CPA. The
particularly low A in the genus Primula corre-
sponds to results obtained by Whale (1983).
An explanation for this compensation or, even,
overcompensation of the elevational decline of
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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 and A/CPI regression (95% confidence limit). LTH and PTH (pm), leaf and palisade layer thickness ? SE of 3-7 (mostly 4) samples with 5-10 determinations each. SLA and NLA (diM2 g-1; mmol N m-2), specific leaf area and leaf nitrogen content ? SE of 2-5 (mostly 3) mixed samples of 15-50 leaves each. In cases where no SE is presented only one mixed sample has been analysed.
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 elevations and corresponds to the average for C3-plants as 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 4Geum repfns
40 - em etn
.00 @
30~~~~~~~ 300 ;./. 0 ;/. 0 .X#0. 0
20 O oc/ 0 Pbs/or/a
EfGCarla*0
R nernorosus - G rivale
0 t A 1A n tt /u A IA^ II I I I 7 1* I I I I I
(n~~~~~~~~~~~~~~~~~~~~~~~0
C
~~~~~~~~~~L /9zsf icum (I mufte//mas
E40 Crex curvule Leonloodon he/vel/cus E40 L 0
E
-uI- 30- -. f; U,
(, ai)
c 20 - /$ C. Gcu1/form5s > 20 ; i/ ./ 0 TarGXGcum U,
o 0 Daucus carofa officInale 0
z ? I A it I * I I I I A lA I
Achi//ea erba -rot/a Erlgeron unlf/orum
40-
30 - PriMu/a gluf/noso
20 - ri folo E acre
f ~ ~ ~ ~ ~ ~ ~ ~ ~ , p elation
10~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I A I I I I A l I I A I I A A I I I I I
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
at high altitude
Potenfi/la crantzli Trifo//um tha/il Taraxacum a/pi;wm LO
040 - o
30
3 20- 20 Pa0 6e0 8
Fig 6 AsinFig 5bu fo treeparsof peieswih reperatudnslrne 7eere toff/c/na/qist'ithte.
2 0) 0 0
(n ~ ~ ~ ~ 1 20 40060 0
0 S- 4~~~ntralprta p Tressure of CO2(,Ltbar) Fi.6 si i.5btfrtrepisofseiswt ie liuia ag, eerdt s'bqit'i h et
CPA lies in the mesophyll and not the stomatal
diffusion path. Under the given experimental
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
species. For Ranunculus, the most intensively
investigated genus, mean Gmax is 203 mmol m-2
s51 (?19 SE) at low elevation and 213 (?22 SE) at high elevation. According to Farquhar & Sharkey
(1982) the relative stomatal limitation (SL) of A
can be expressed by the equation SL = (A0-A)/A0.
A0 is C02 uptake derived from the A/CPI curve,
assuming CPI equals CPA (i.e. zero stomatal resist-
ance). This yields a relative stomatal limitation of
17% at low elevation and 30% at high elevation,
which is largely attributable to the different curva-
ture of the A/CPI responses. If expressed as the
ratio of stomatal versus total (stomatal plus meso-
phyll) resistance at the operating point, the differ-
ence in the relative gas phase limitation among
sites is smaller: 23% at low altitude and 28% at
high altitude. Since the latter approach is devoid
of extrapolation to higher CPI, it may be more
realistic (Jones, 1985).
As summarized in Table 7 the initial slope of the
A/CPI curve is about 40% steeper at high elevation.
CPI under mean local CPA (251 VFbar at high 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
difference is reflected in the altered slope of the
CPI/CPA regression. The ratio is 11% smaller at high altitude and scatter in the separate regres-
sions for each altitude is very small. In conclusion,
the investigated group of high altitude plant
species on average operates at lower CPI than
might be expected from the pressure decline and
achieves higher A by virtue of a pronounced
increase of ECU.
Morphological and nutritional differences
Any change in ECU may have structural or speci-
fically biochemical reasons. A detailed casual
analysis of both these relations is currently in
progress. At this state we can provide information
on global structural parameters and on nutritional
status of leaves (Table 6). Both, overall leaf
thickness and palisade layer thickness increase by
66 and 76% respectively with altitude (Table 7).
Preliminary results of a broad survey of mesophyll
surface:leaf area ratios indicate significantly
higher ratios (+30%) for high elevation plant
communities. Mean total leaf nitrogen content per
unit leaf area is higher by 34% in alpine taxa but is
similar on a dry weight basis. Inversely correlated
to the increase in thickness is specific leaf area
(SLA), which is reduced by 28% at high altitude.
Pooled correlations between these parameters for
mountain and lowland species are depicted in
Table 8. ECU correlates well with SLA and NLA
(nitrogen content per unit leaf area). The corre-
lations between ECU and thickness parameters are
significant on the 5% level but scatter is sub-
stantial.
Discussion
This investigation has revealed a number of
physiological characteristics of herbaceous peren-
nial plants from low and high altitudes. Our major
concern was to follow a comparative approach and
eliminate bias by variables related not a priori to
the problem of altitudinal differentiation of plant
functioning, e.g. moisture stress. An important
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190
Ch. Korner &
M. Diemer
Table 8. Linear regression analysis of factorial corre-
lations.
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 **
ECU-NLA 0 4278 47-92 0 549 0 006 **
ECU-LTH 0 1318 68 48 0-465 0 019 *
ECU-PTH 0-2250 77 09 0-461 0 020 *
SLA-LTH -0 002433 2 698 -0 456 0 021 *
Dimensions and statistics as in Table 7.
A = net rate of photosynthetic uptake
ECU = efficiency of CO2 uptake
SLA = specific leaf area
NLA = nitrogen content per unit leaf area
LTH = leaf thickness
PTH = palisade layer thickness
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 differenti-
ation with altitude?
We find that differences in photosynthetically
effective radiation and temperature regimes at the
two altitudes are much less than commonly
assumed (cf. Kdrner & Cochrane, 1983). This is
reflected in observed photosynthetic responses.
The temperature and light responses of A indicate
that plants at either elevation are well adapted to
utilize the warmest and brightest periods rather
than average conditions, although the former
comprise only 25-30% of all daylight hours at both
elevations. Relatively high temperature optima of
A for alpine herbaceous plants have previously
been described by Mooney and Billings (1961),
Moser (1970), K6rner (1982) and others. These
results are in contrast to observations in forest
trees. Since trees are more closely coupled to
ambient temperature, they exhibit steeper eleva-
tional gradients of optimum temperature for
photosynthesis (e.g. Fryer and Ledig, 1972; Slatyer
and Morrow, 1977). Chapin and Oechel (1983)
report similar observations for an arctic tundra
sedge transplanted to thermally different environ- ments. In a quantitative analysis of the relative
importance of temperature and QFD for annual
carbon yield of Carex curvula All., Korner (1982)
showed that QFD is much more important than
temperature. Suboptimal QFD at leaf level restricts
yield by 40%, compared to 8% as a result of
suboptimal leaf temperatures. Moser (1970) and Scott, Hillier & Billings, (1970) arrived at similar
conclusions for alpine plants. In arctic taxa direct
effects of temperature do not influence carbon
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-
ences, which defies any predictions for individual
species. The temperature optima of A at both
elevations cover a range of 7 K. The ranges
observed at a 500 m higher mountain site by Moser
et a]. (1977) are even larger. These are striking
examples of the high variability in response pat-
terns in plants from high mountains and support
Larcher's (1980) view that fluctuations in time and
space of both plant and environmental factors play
a key role in the understanding of alpine plant life.
Therefore, broad experimental screening appears
indispensable in alpine ecology.
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
30-40ppm, this is probably due to the fact that
data were often not corrected for the effects of CO2
depletion in gas exchange cuvettes used in differ-
ential measurement systems. The observation that
mean maximum A at local ambient CPA is similar
at both elevations, confirms trends observed in
several of the earlier studies within temperate
latitudes. However, at equal cuvette-CPA alpine
species exhibit significantly higher rates of A (At in Table 7). It is possible to estimate a mean
difference of + 20% for alpine taxa if CPA were 251
VFbar at both elevations (which equals present ambient CPA at 2600 m altitude). For extrapolation
to higher CO2 levels the curvature of the A/CPI
curve needs to be taken into consideration.
CO2 saturated A (CPI > 500 VFbar) is approxi- mately 50% higher in alpine versus low elevation
species. This confirms estimates of elevational differences of A at 1% C02 by Nakhutsrishvili and co-workers (Nakhutsrishvili, 1974) in the Central
Caucasus. When exposed to high CO2 for 5-10 min sub-niveal plants exhibited 70% higher CO2 saturated A than sub-alpine plants (n = 11, P < 0 05). A at ambient CPA did not differ significantly in their study. Schulze et al. (1985) investigated
the CO2 response of afro-alpine giant rosettes in
4200m altitude (CPA = 188 VFbar). In accordance with our observations they found leaves to operate well down in the linear range of the A/CPI curve.
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191
Photosynthesis
at high altitude
CPI at local CPA was near 100 vFbar and stomatal limitation reached 42% when maximum A was
achieved.
The CPI of 250 ? 23 VFbar (? SD) of the low elevation plants at present ambient CO2 level (335
Vd 1-1) is similar to the mean value of 247 ? 12 derived from a literature review by Yoshie (1986)
for many different species and life forms adapted
to 328 vl 1-1. Yoshie's original number is in Vd 1-1 but if most of these data were obtained close to sea
level, this figure would correspond to partial
pressure.
It is not certain whether these altitudinal differ-
ences in photosynthetic capacity are of genotypic
origin. Billings, Clebsch & Mooney (1961) demon-
strated that seedlings of alpine ecotypes of 0.
digyna show higher A at any given CPA below ambient pressure than arctic ecotypes when both
groups of plants were grown at the same CO2 level. This suggests ecotypical differentiation but not necessarily in response to a long history of growth
under different CPA since both environments
differ in other respects as well. In addition CPI and
hence ECU were not known for Oxyria. No differ- ences in A at equal CPA were found following
short term exposure of three alpine and one desert
species to contrasting elevations (Mooney, Strain & West, 1966). In accordance with the latter observation we found no difference in ECU
between 0. digyna in our mountain site and in
alpine ecotypes grown for several years in the Botanical garden in Innsbruck (unpublished data).
Structural and functional reasons for increased ECU
At this stage, our findings do not allow an interpre-
tation of the observed changes of leaf character-
istics towards a 'low CO2 adaptation'. Exposure of
plants to low CPA reduced leaf thickness of
greenhouse plants (Madsen, 1973). However,
alpine plants exhibit thicker assimilatory tissues
(high in nitrogen) which favour higher rates of CO2 uptake per unit leaf area (Nobel & Walker, 1985). Mechanisms leading to this morphological expres- sion are still largely speculative. Differences in
QFD between sites are too small to account for the substantial differences in leaf structure observed at both elevations. However, there may be a
genotypic selection for thinner leaves in low elevation plants since competition for light is greater at low altitude than at high altitude. In the short-term ultraviolet also does not appear to affect
alpine plant structure (Caldwell, 1968). If low mean temperatures alone were responsible for
thicker leaves, one would expect similarly posi-
tive effects on A in arctic plants which is not the
case (Mooney & Billings, 1961; Pisek, 1960;
Billings & Mooney, 1968; and others).
The remarkable constancy of nitrogen content
per unit dry weight suggests a uniform protein
volume density in the leaves of herbaceous plants,
irrespective of elevation. The elevational increase
of nitrogen content per unit leaf area appears to be
largely attributable to increases in leaf thickness
which corresponds to reduced SLA. This confirms
observations by Korner, Bannister & Mark (1986)
in the mountains of New Zealand. However, the
possibility that the specific carboxylation effici-
ency at the biochemical level among low and high
elevation taxa is altered still exists. Von Caemerer
& Farquhar (1981) and others showed that ECU is
largely controlled by the activity of RUBP-
carboxylase per unit leaf area. Leaves developing
at lower temperatures tend to exhibit higher speci-
fic Rubisco activity per unit protein (Bjbrkman,
Badger & Armond, 1978) as well as per unit leaf
area (Bunce, 1986). Pandey, Bhadula & Purohit
(1984) observed higher Rubisco activity in high
altitude than in lowland samples of the perennial
forb Selinum vaginatum Clarke in the Himalaja
front range. Exposure to altered CO2 levels can also
influence Rubisco activity. The majority of recent
studies on the effect of increased CO2 during leaf
expansion indicate that Rubisco activities decline
both per unit soluble protein and per unit leaf area
(e.g. Downton, Bjbrkman & Pike, 1980; Wong, 1980; Von Caemerer & Farquhar, 1984). Perhaps
the opposite effect holds for long-term exposure to
reduced CO2.
Elevated atmospheric C02 levels - more effective
for high altitude plants?
The question whether plant life at high altitude is
particularly limited by CO2 supply has attracted ecophysiological researchers for many years (Decker, 1959; Billings et al. 1961; Milner, Hiesey & Nobs, 1963; Mooney et al. 1966; Gale, 1973).
Mainly technical constraints made it difficult to approach this problem in the past with adequate accuracy in the field. The data presented here
permit an experimental verification of theoretical considerations by Gale (1973) and Cooper, Gale &
LaMarche (1986). The latter authors discussed the
hypothesis of LaMarche et al. (1984) that increased CO2 levels in the atmosphere could explain pro- nounced increases in tree ring width at high
elevations where CO2 limitation is supposed to be
greater than at low elevation. Against this expla-
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192
Ch. Karner &
M. Diemer
nation it was argued that negative effects of altitu-
dinally decreasing CPA on A will be diminished or
offset by reduced gas diffusive resistance at lower
total pressures. Hence, the question was whether
(1) CPI decreases with increasing altitude propor-
tionally to the decline in CPA or (2) remains at
comparatively higher level because CO2 influx is facilitated.
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
morphological alteration in the assimilatory tissue
was not taken into account in the above-
mentioned discussion. Although our study 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
conditions are optimal (Kbrner, Scheel & Bauer, 1979). A global survey of carbon isotope discrimi-
nation in plants from high altitude provides
further support for the hypothesis that the propor- tion of mesophyll bound limitations to CO2 uptake
declines with altitude (Ch. Kbrner, G.D. Farquhar & Z. Roksandic, in preparation).
The question, whether the present increase of
global CO2 will be particularly favourable for CO2 assimilation in plants from high elevation, appears in a new light if our data are representative of
altitudinal phenomena. The shape of the CO2 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 the atmospheric mole fraction of C02 by 100 pA 11 would cause CPA to rise by 95 pLbar at our 600m
site and by 75 pLbar at our 2600m site. Mean CPI from our regression over CPA would then amount to 227 pLbar at high and 325 pLbar at low altitude.
The estimated gain in A under the given assump- tions would amount to 21% at low elevation and
31% at high elevation (Fig. 7). For higher incre-
ments in CPA the relative increase in A of alpine 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 physiological estimates derived from instanta- neous plant response characteristics. Over the
40
C02-level 435.t L' , m
+ 1% 600 M 20 +3 21/
?10 +- / X i 0Q //335 FL( FI
present C02 -level
0 100 200 300 400 500
Internal partial pressure of CO2 (/ibar)
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, assuming plant response characteristics remain the same.
long-term they may be altered by acclimative
modifications and by progressive imbalances
between carbon and nutrient relations. Also, the comparison is only valid as long as water stress
does not interfere. Extrapolations for altitudes
beyond the ones studied here are not justified because the morphological changes observed over
this elevational range do not proceed predictably
with increasing elevation (unpublished data). However, our estimates indicate - at least trend-
wise - that mountain plants should profit more
from increased global CO2 levels than lowland
plants.
Acknowledgements
This research was funded by the Fonds zur For-
derung der Wissenschaftlichen Forschung (Vienna) project P5597. We are grateful to I. Cowan
for revising the paragraph on pressure effects and
conductance units. M.M. Caldwell and A. Cernu-
sca provided logistic support on computation of gas exchange and climate data. W. Seidenbusch
facilitated spectral analysis of our light source. W. Larcher and two referees contributed valuable
comments to the manuscript.
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Received 10 March 1987; revised 12 May 1987; accepted
13 May 1987.
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