in situ photosynthetic responses to light, temperature and carbon

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

This content downloaded from 38.125.197.2 on Wed, 26 Oct 2016 17:56:16 UTCAll use subject to http://about.jstor.org/terms

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


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, beginning 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


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 conductance 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 photosynthesis 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.


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


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 temperatures with respect to photosynthetic activities, 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 photosynthetic 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


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-


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 depicted 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


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


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'.


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


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 environments. 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 approximately 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.


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 differences 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 pronounced increases in tree ring width at high

elevations where CO2 limitation is supposed to be

greater than at low elevation. Against this expla-


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.


in situ photosynthetic responses to light, temperature and carbon

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