Interactions between C? and C? Salt Marsh Plant Species during Four Years of Exposure to
Elevated Atmospheric CO2
Author(s): W. J. Arp, B. G. Drake, W. T. Pockman, P. S. Curtis, D. F. Whigham
Reviewed work(s):
Source: Vegetatio, Vol. 104/105, CO? and Biosphere (Jan., 1993), pp. 133-143
Published by: Springer
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Vegetatio 104/105: 133-143, 1993. 
J. Rozema, H. Lambers, S.C. van de Geyn and M.L. Cambridge (eds). CO2 and Biosphere 133 
? 1993 Kluwer Academic Publishers. Printed in Belgium. 
Interactions between C3 and C4 salt marsh plant species during four 
years of exposure to elevated atmospheric C02 
W. J. Arp* B. G. Drake, W. T. Pockman, P. S. Curtis & D. F. Whigham 
Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037, USA; *Present 
address: Department of Ecology, Faculty of Biology, Vrije Universiteit, De Boelelaan 1087, 1081 HV, 
Amsterdam, The Netherlands 
Keywords: Growth, Water relations, Competition, Distribution 
Abstract 
Elevated atmospheric C02 is known to stimulate photosynthesis and growth of plants with the C3 
pathway but less of plants with the C4 pathway. An increase in the CO2 concentration can therefore be 
expected to change the competitive interactions between C3 and C4 species. The effect of long term 
exposure to elevated CO2 (ambient CO2 concentration + 340 ,umol CO2 mol 1) on a salt marsh vege 
tation with both C3 and C4 species was investigated. Elevated CO2 increased the biomass of the C3 sedge 
Scirpus olneyi growing in a pure stand, while the biomass of the C4 grass Spartinapatens in a monospecific 
community was not affected. In the mixed C3/C4 community the C3 sedge showed a very large relative 
increase in biomass in elevated CO2 while the biomass of the C4 species declined. 
The C4 grass Spartina patens dominated the higher areas of the salt marsh, while the C3 sedge Scirpus 
olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate el 
evations. Scirpus growth may have been restricted by drought and salt stress at the higher elevations, 
while Spartina growth at the lower elevations may be affected by the higher frequency of flooding. Ele 
vated CO2 may affect the species distribution in the salt marsh if it allows Scirpus to grow at higher 
elevations where it in turn may affect the growth of Spartina. 
Nomenclature. Radford, A. E., Ahles, H. E. & Bell, C. R. (1968). Manual of the vascular flora of the 
Carolinas, University of North Carolina Press, Chapel Hill. 
Introduction 
The atmospheric CO2 concentration is increasing 
due to the burning of fossil fuels and deforesta 
tion, and is expected to double in the next century 
relative to pre-industrial levels (Strain & Cure 
1985). The present atmospheric CO2 concentra 
tion (ca 353 4mol mol 1) is a limiting factor to 
photosynthesis of plants with the C3 pathway. 
Consequently, an increase in the CO2 concentra 
tion enhances photosynthesis and productivity in 
these plants (Cure & Acock 1986). Because of the 
CO2 concentrating mechanism of the C4 path 
way, photosynthesis of C4 plants is much less 
limited by the ambient CO2 concentration. All 
plants can benefit by elevated CO2 by regulating 
the stomatal conductance and limiting water loss 
through evapotranspiration. In plant communi 
ties containing both C3 and C4 species the rela 
tive advantage of the C3 species in high CO2 may 
134 
alter the competitive balance in favor of the C3 
species, as long as other abiotic factors (e.g. tem 
perature) do not change. An increased competi 
tive ability of C3 species growing with C4 species 
has been found in competition experiments (Cart 
er & Peterson 1983; Bazzaz & Carlson 1984; 
Patterson etal. 1984; Zangerl & Bazzaz 1984; 
Marks & Strain 1989), although an increase in the 
competitiveness of the C4 species in elevated CO2 
has also been reported (Bazzaz et al. 1989). Not 
all C3 species show the same response to elevated 
C02, and therefore changes in the competition 
between C3 species in high CO2 may also occur 
(Garbutt & Bazzaz 1984; Tolley & Strain 1984a; 
Tolley & Strain 1984b; Overdieck & Reining 
1986). 
If elevated CO2 can prevent the C3 species from 
declining during drought stress relative to the C4 
species, then the relative advantage of elevated 
CO2 to C3 species may be as large during drought 
stress as during periods with sufficient rainfall. 
Most studies on the effect of elevated CO2 on 
competition between C3 and C4 species have been 
conducted in greenhouse or growth chamber, and 
were limited to one season or less. Growing plants 
in small pots may affect the competition experi 
ments because this affects the response of plants 
to elevated CO2 by limiting the sink for photo 
synthates (Arp 1991). Long term CO2 exposure 
experiments on natural vegetation containing C3 
and C4 species have not yet been described. This 
paper describes the effects of four years of expo 
sure to elevated CO2 on the competition between 
C3 and C4 perennial species in a natural salt marsh 
ecosystem. 
In this ecosystem species are distributed in very 
distinct patterns. The C3 sedge Scirpus olneyi and 
the C4 grass Spartina patens occur in almost pure 
stands, as well as in a mixed community with the 
C4 grass Distichlis spicata. Interspecific relation 
ships and the effects of abiotic factors on species 
distribution need to be understood before predic 
tions can be made on the impact of elevated CO2 
on the distribution of the species in the salt marsh 
(Rozema et al. 1991b; Rozema et al. 1988). The 
elevation of the marsh may be the most important 
abiotic variable affecting species distribution be 
cause plants at higher elevations are more subject 
to drought and fluctuations in salinity. At lower 
elevations the increased frequency and duration 
of flooding results in more anaerobic conditions 
and lower redox potentials in the soil, which can 
reduce the growth of plants not equipped with a 
well-functioning aerenchyma system (Rozema 
etal. 1988; Van Diggelen 1988; Ernst 1990). 
Materials and methods 
CO2 exposure system 
The effects of long term exposure to elevated CO2 
on three salt marsh communities were investi 
gated in a subestuary of the Chesapeake Bay, 
USA. The first community was dominated by the 
C3 sedge Scirpus olneyi, the second community 
was a monospecific stand of the C4 grass Spartina 
patens, and the third community consisted of a 
mixture of Scirpus, Spartina and another C4 grass, 
Distichlis spicata. Ten open top chambers were 
placed in each community. In five chambers 
plants were exposed to normal ambient CO2 con 
centration (350 imol CO2 mol' 1 air), and in the 
other five chambers the CO2 concentration was 
raised 340 iimol mol- 1 above the ambient CO2 
concentration. The plants were exposed to ele 
vated CO2 from 1987 to 1990. The CO2 exposure 
started when plants emerged in the spring and 
ended after total senescence in the fall. Five sites 
without chambers in each community were used 
as controls. For a detailed description of the CO2 
exposure and measurement system see Drake 
etal. (1989). 
Biomass estimation 
Aboveground biomass of Scirpus, Spartina and 
Distichlis in all chambers and control sites was 
estimated in June 1986 before the CO2 treatment 
started, as well as seven times during the grow 
ing season of 1987, five times during 1988, and 
once at peak biomass in 1989 and 1990. Scirpus 
shoots are unbranched and lack leaves, allowing 
for non-destructive biomass estimation using the 
relationship between shoot length and dry weight. 
A small sample of shoots ( ? 3 O) was harvested 
to determine this relationship, while the length of 
all other shoots in the chambers was measured. 
In 1989 and 1990 the regression was improved by 
measuring both shoot length and shoot width at 
a height of 40 cm. Total biomass was calculated 
by applying the regression to the length measure 
ments of all shoots in the chambers. From 1986 
to 1988 Spartina and Distichlis biomass was esti 
mated by harvesting 5 quadrats of 5 x 5 cm2 from 
each chamber to establish the dry weight per 
shoot, and counting the shoots in 5 quadrats of 
10 x 10 cm2 to estimate the density of the shoots. 
Total biomass was calculated by extrapolating 
the shoot density and shoot weight measure 
ments. In 1989 and 1990 biomass was estimated 
by harvesting 10 to 12 quadrats of 5 x 5 cm2 from 
each chamber and extrapolating the results to 
total biomass per meter squared. Senescence and 
percentage of shoots which were reproductive 
were also recorded for all species. 
Water relations 
Midday and pre-dawn water potential measure 
ments were made several times during the grow 
135 
ing season using a Scholander pressure bomb. 
Soil salinity measurements were made with a re 
fractometer, using interstitial water obtained from 
pvc pipes 15, 30, 50 and 100 cm deep, which were 
installed in six chambers of each community in 
1987. Rainfall and temperature data were re 
corded at the Smithsonian Environmental Re 
search Center, approximately 1.5 km from the 
study site. 
Marsh survey 
The elevation of the marsh was measured in 1990 
by establishing the height of points in a grid over 
a 130 x 80 m area using a surveyers level (Keuffel 
& Esser). A 5 x 5 m grid was laid out over the 
study site and 3 measurements were taken at each 
point, 50 cm apart. The composition of the veg 
etation and the average height of the Scirpus 
shoots were also estimated at these points. 
To determine the effect of Scirpus canopy on 
the light environment of Spartina, photosynthetic 
photon flux (PPF) was measured using a sunfleck 
septometer (Decagon) at 25, 50, 75, 100 and 
125 cm above the surface of the marsh over an 
10 m transect extending from the Spartina to the 
Scirpus community. Scirpus shoot density and 
shoot height were also recorded over this transect. 
soil water potential plant water soil water potential plant water 
cm potential cm potential 
15 30 50 100 elev. amb. 15 30 50 100 elev. amb. 
MPa 
-1.5 
-2.0 Scirpus olneyi Spartina patens 
-2.5 - t pre-dawn 
A LI mid-day B 
Fig. 1. Soil water potential at four different depths, and plant water potential measured mid-day and pre-dawn for plants grown 
at elevated and ambient CO2. A: Scirpus olneyi, B: Spartina patens. 
136 
Results and discussion 
Water relations of Scirpus olneyi and Spartina 
patens 
Pre-dawn and mid-day plant water potentials of 
Scirpus and Spartina were measured six times in 
the period June to August 1988, and mean values 
for plants grown in elevated and ambient CO2 are 
presented in Fig. 1. Water potential of both Scir 
pus and Spartina grown in elevated CO2 was 
+ 0.6 MPa higher (less negative) than plants 
grown in ambient CO2 when measured in the 
afternoon. Similar results have been reported for 
the European salt marsh species Scirpus maritimus 
(C3), Elymus athericus (C3) and Spartina anglica 
(C4) (Lenssen & Rozema 1990; Lenssen et al. 
1991; Rozema et al. 1991a; Rozema et al. 199lb). 
A smaller difference was observed between the 
pre-dawn water potentials. The mean salinity of 
interstitial water in the soil at four different depths 
is shown in the same figure. Soil salinity was 
highest in the top 30 cm of the marsh, and low 
est at a depth of 100 cm. The pre-dawn water 
potential of Scirpus equilibrated at a level equal to 
the soil water potential at 100 cm, indicating that 
this species is able to tap the relatively fresh water 
from this level. Spartina pre-dawn water potential 
is equivalent to the soil water potential at the 
surface of the marsh. Spartina has a shallow root 
ing system while roots of Scirpus extend much 
deeper into the marsh (Curtis et al. 1990). 
Spartina patens and Distichlis spicata have salt 
glands in the leaves with which they can excrete 
excess salt and these species can tolerate high 
salinity levels. Scirpus olneyi does not have salt 
glands and must rely on restricting salt uptake by 
the roots. Runoff from the surrounding forest may 
create a flow of fresh water under areas of the salt 
marsh which may be available to Scirpus, but no 
data are available. At the higher elevations of the 
marsh growth of Scirpus may be limited because 
salt and drought stress are likely to be greater 
than at the lower elevations during periods of 
drought. 
If elevated CO2 can enhance the water use ef 
ficiency of Scirpus, then this might improve growth 
and competitive ability of Scirpus. Salt tolerance 
of C3 species may be enhanced at elevated CO2 
by an increase in available photosynthates and by 
improved water status (Bowman & Strain 1987; 
Rozema et al. 1990). During dry years both C3 
and C4 species can benefit from reduced evapo 
transpiration at elevated CO2, and the relative 
advantage of the C3 species in high CO2 may be 
reduced. However, drought stress is likely to en 
hance the competitive ability of C4 species at nor 
mal atmospheric CO2 concentration because of 
their inherent higher water use efficiency, and el 
evated CO2 may reduce this advantage of the C4 
species during drought by increasing the water 
use efficiency of the C3 species. 
The effect of elevated CO2 on growth of Scirpus 
olneyi 
A summary of the effects of elevated CO2 on 
Scirpus olneyi growth in pure and mixed commu 
nities is given in Fig. 2. The data are expressed as 
percent increase in plants grown in elevated CO2 
when compared with plants grown in ambient 
CO2 at peak biomass for four years. Shoot den 
sity increased in all four years and in both com 
munities, but the increase was much larger in the 
mixed community. Shoot height decreased in the 
C3 community and increased in the mixed com 
munity. The dry weight per unit shoot length was 
slightly enhanced during the first years but de 
creased in the last year. Changes in shoot density, 
shoot height and weight per unit shoot length were 
reflected in the change in total biomass. Biomass 
was enhanced in the C3 community in 1987 and 
1988, but showed only a small increase in 1989 
and 1990. In the mixed community a large in 
crease in Scirpus biomass was found in all four 
years, but the response in 1990 was smaller than 
in the other years. Elevated CO2 reduced senes 
cence of Scirpus, with the exception of the last two 
years in the mixed community. The percentage 
shoots which were flowering was reduced in ele 
vated CO2 during the first two years, but was 
enhanced in the last two years. 
A large interaction of community type with el 
137 
Scirpus in C3 community 
40, A ll lll 
20 
0 
-20 
-40 
87 8889 90 187 8889 90 187 8889 90 187 8889 90 1 87 8889 90 
shoot shoot weight I total senescence flowering 
density height cm shoot biomass shoots 
Scirpus in mixed community 
300 B 
~200 
100 
87 88 89 90 
0 F 
87 88 8990 87 88 89 90 87 88 89 90 87 8889 90 87 88 8990 
shoot shoot weight / total senescence flowering 
density height cm shoot biomass shoots 
Fig. 2. Percentage increase in shoot density, shoot height, weight per unit shoot length, total biomass, senescence and reproduction 
of Scirpus olneyi plants exposed to elevated C02, as compared with plants grown in normal ambient CO2 for four years. A: Pure 
Scirpus olneyi (C) community, B: mixed (C3-C4) community. 
evated CO2 was observed, with Scirpus in the 
mixed community responding much stronger to 
elevated CO2 in shoot density, shoot height and 
biomass (Fig. 2). A possible explanation is that 
Scirpus growing in the mixed community consti 
tutes only a small portion of the biomass and is 
able to expand, but when growing in the pure 
Scirpus community, the increase in biomass is 
restricted by intraspecific competition and self 
shading. This response is similar to the findings 
of Carter and Peterson (1983) where Sorghum 
responded to elevated CO2 in the high light en 
vironment of the mixed community, but was lim 
ited in its response by self-shading in the unmixed 
culture. 
Yearly differences in the response to elevated 
CO2 were found. The increase in biomass of Scir 
pus in the pure community was much larger in the 
first two years, while reproduction was enhanced 
in the last two years. In the mixed community 
senescence was reduced in 1987 and 1988, and 
reproduction was increased in 1989 and 1990. A 
138 
700 
600 
500 1989 
co 
co 400 
1990 
>~ 300 
servd fom 988to 990.Thetemerau1987r 
200 
100 1988 
0 May June July August 
Fig. 3. Cumulative rainfall for May through August for 1987, 
1988, 1989 and 1990. 
drop in shoot density and biomass was also ob 
served from 1988 to 1990. The temperature dur 
ing the growing seasons of 1987 and 1988 was 
higher than during the last two years (mean max 
imum daytime temperature May-August: 1987: 
28.8, 1988: 28.4, 1989: 26.6, 1990: 26.6 ?C), and 
long periods of drought occurred in the first two 
years (Fig. 3). Rainfall deficit during the growing 
season has been correlated with variation in peak 
biomass of salt marsh plants, through its impact 
on soil salinity and soil moisture content (De 
Leeuw et al. 1990). In dry years, elevated CO2 
may prevent the reduction of biomass through 
drought, resulting in a large relative increase in 
biomass compared with years of abundant rain 
fall. In the mixed community the correlation be 
tween drought and the effect of elevated CO2 on 
biomass is less clear. It is likely that the large 
increase in biomass in 1988 carries over into the 
biomass increase found in 1989. Because the ef 
fect of elevated CO2 on growth is strongly corre 
lated with air temperature (Idso et al. 1987), 
higher temperatures in 1987 and 1988 may also 
have contributed to the larger CO2 effect. 
250 1986 1987 1988 1989 1990 
200 -~~~~~~ 
S 200* elevated 
m9 0 ambient 
11 control S150 -tS 
S 0
0~~~~~~~~~~~ 
,?'' 100 DE n 
50 - 0 
00 
0 
o.o01 0 chamber effect . 
* co 2 ef fect 
0.001 
Fig. 4. A: Total biomass of Scirpus olneyi in the mixed community of Scirpus, Spartina and Distichlis for all harvests from 1986 
to 1990. CO2 treatment started in 1987. Values shown are mean values for 5 chambers. B: p-values for the difference between 
elevated and ambient chambers (treatment effect) and between ambient chambers and control sites (chamber 
effect). The hori 
zontal line represents the 0.05 level. 
The effect of elevated CO2 on interaction between 
the C3 and C4 species 
A five year record of biomass of Scirpus growing 
in the mixed community is presented in Figure 4. 
The mean biomass for 5 chambers of each treat 
ment is given for every harvest since the start of 
the experiment. From the first year of exposure 
until 1990, the biomass of Scirpus grown in ele 
vated CO2 has been higher than the biomass of 
plants grown in ambient C02, and this difference 
became statistically significant in late .1987. A 
large decrease and biomass of Scirpus in ambient 
chambers and unchambered control sites of the 
mixed community was found from 1987 to 1988. 
This may have been the result of the continued 
drought in 1988 (Fig. 3). The decrease is smaller 
in Scirpus grown in elevated C02, resulting in a 
large relative increase in density and biomass in 
1988 (Fig. 2). Biomass of Scirpus plants not ex 
posed to elevated CO2 recovered during 1989 and 
139 
1990 and the relative increase in density and bio 
mass of plants grown in elevated CO2 declined 
since 1988. However, due to a smaller variation 
in the data the level of significance has increased 
since the start of exposure to CO2 for both shoot 
density and biomass (Fig. 4). 
The effects of elevated CO2 on the biomass of 
the C4 grasses are presented in Figure 5. Biomass 
and shoot density of the C4 species combined 
were lower in the elevated CO2 chambers than in 
the ambient chambers in 1989 and 1990, and the 
effect on biomass was significant at the 0.05 level 
in 1990. No effect of the ambient CO2 chamber 
on density or biomass was found when compared 
with control sites without chambers. 
While density and biomass of the two C4 
grasses combined were uniform throughout the 
community, the distribution of each separate spe 
cies in the mixed community was highly variable, 
obscuring any effects of elevated CO2 on density 
and biomass of Spartina and Distichlis separately. 
1250. 
1986 1987 1988 1989 1990 
cv 
~~~~~~~~~~0 
1000 
750 - 
o 
C) 500 D 
w 250 1 * elevated chamber 0 
w4~) S 0 ambient chamber 
El control (no chamber) 
0 
0.1 0 I___0_* 
; 0.01 0 chamber effect 
0 co2 effect 
0.001 
Fig. 5. A: Total biomass of the C4 grasses Spartina patens and Distichlis spicata in the mixed community of Scirpus, Spartina and 
Distichlis for all harvests from 1986 to 1990. C02 treatment started in 1987. Values shown are mean values for 5 chambers. B: 
p-values for the difference between elevated and ambient chambers (treatment effect) and between ambient chambers and con 
trol sites (chamber effect). The horizontal line represents the 0.05 level. 
140 
However, the decline of the C4 species in elevated 
CO2 appears to be at the expense of Spartina, 
which experienced a relative decline in the biom 
ass in elevated chambers, while the Distichlis bio 
mass increased in elevated CO2 in the first three 
years (Fig. 6). The reduction of C4 biomass in the 
elevated chambers is larger than the increase in 
Scirpus biomass in those chambers, resulting in a 
lower total biomass in the elevated chambers. It 
has been found before that production of mixed 
stands may not increase with elevated C02, be 
cause the increase in C3 biomass can be offset by 
a decrease in the C4 biomass (Carter & Peterson 
1983). With the exception of the year 1990, ele 
vated CO2 did not reduce the biomass of Spartina 
growing in the pure C4 community (Fig. 6), sug 
gesting that the decline in the mixed community 
may have been the result of increased competition 
by Scirpus. 
The possible effects of elevated CO2 on species dis 
tribution 
The studied plant communities, while irregularly 
shaped, formed well defined patches. Although 
only a 15 cm difference was found between the 
elevations where Spartina and Scirpus were most 
abundant, the distribution was highly correlated 
with the elevation of the marsh (Fig. 7). Scirpus 
olneyi reached its highest density at the lower el 
evations of the study site, while Spartina patens 
dominated the higher elevations. No correlation 
of Distichlis spicata with elevation was found. 
Typha angustifolia and Iva frutescens (data not 
shown) also occurred at lower elevations. Spartina 
patens is well adapted to periods of high salinity 
at the higher elevations, but appears to be sensi 
tive to frequent flooding at the lower areas (Bert 
ness 1991 a; Bertness 199 1b). The well developed 
aerenchyma of Scirpus olneyi provides oxygen in 
the waterlogged soils and allows it to grow in the 
lower areas of the marsh. 
The effect of marsh elevation on Scirpus shoot 
height is displayed in Figure 8. A marked decline 
in Scirpus height occurred at the same elevation 
where biomass and shoot density of Scirpus were 
reduced and where Spartina became the domi 
nant species (Fig. 7). Under favorable conditions 
Scirpus grows much taller than Spartina and Dis 
tichlis and may shade these grasses. A 10 m 
transect from the Spartina to the Scirpus commu 
N 
0 40 
0 
co 
> 20 C) 
CD 
C.) 
u-20 
CD 
4u 87 888 9 90 87 88 89 90 87 s8 89 90 878 8990 
Spartina Spanlina Distichlis total 
C4 mixed mixed mixed 
Fig. 6. Percentage increase in total biomass of plants grown in elevated CO2 as compared with plants grown in ambient CO2 for 
four years, for Spartina patens in the pure Spartina community (C4), and for Spartina, Distichlis spicata and both C4 species in the 
mixed (C3-C4) community. 
141 
Scirpus olneyi 
75 
50 
25 
" 
~ Spartina patens 
~75 
50 
25 
Distichlis spicata 
75 
50 
25 
40 45 50 55 60 65 
relative marsh elevation (cm) 
Fig. 7. Percentage of total biomass of Scirpus olneyi, Spartina patens and Distichlis spicata as a function of marsh elevation. Iva 
frutescens and Typha angustifolia were also found at the lower elevations. 
nity revealed that light levels at the height of the 
Spartina canopy ( + 25 cm above the surface) de 
clined to about 20% full sunlight in the Scirpus 
community. Light intensity is likely to have a large 
effect on the growth of these C4 species in the 
marsh because plants with the C4 pathway can 
benefit more from high light levels (Ehleringer 
1978). Shading by Juncus gerardi affected growth 
of Spartina patens in a New England salt marsh 
(Bertness 1991a). It can be speculated that in a 
high CO2 environment an increased carbon up 
take and improved salt and drought tolerance of 
the C3 species Scirpus olneyi will allow it to ex 
pand towards higher elevations of the marsh, 
which may lead to a decrease of the C4 species 
due to increased shading by Scirpus. 
Conclusions 
Elevated CO2 increased the shoot density and 
total biomass of the C3 sedge Scirpus olneyi in 
both mixed and pure communities, but the in 
crease was much larger in the mixed community 
with Spartina patens and Distichlis spicata. The 
absence of a large increase in biomass in the pure 
C3 community may be explained by an increase 
in self shading, counteracting the effects of ele 
vated CO2. This negative feedback was absent in 
the mixed community where Scirpus constituted 
only 5 to 10% of the total biomass and was able 
to expand. 
It has been suggested that during periods of 
drought stress both C3 and C4 species are ex 
pected to benefit from elevated CO2 by reducing 
water loss, resulting in a smaller relative benefit 
for the C3 species (Pearcy & Bjorkman 1983; 
Bazzaz et al. 1985). In this study however, ele 
vated CO2 offered only a minor benefit for the 
drought and salt tolerant C4 species during 
drought stress, but it had a large impact on the C3 
species. At high CO2 the C3 sedge was able to 
maintain its position in the community, while bio 
142 
120 
110 
~90 
80 
40 45 50 55 60 65 
relative marsh elevation (cm) 
Fig. 8. Mean shoot height of Scirpus olneyi as a function of marsh elevation. 
mass was reduced by drought at ambient CO2 
levels. As a result drought stress amplified the 
difference between C3 and C4 species in response 
to elevated CO2. A similar response to drought 
was found in the pure C3 community where ele 
vated CO2 increased biomass in the two dry years, 
but in the years with abundant rainfall biomass 
was only marginally increased by elevated CO2. 
The increase of Scirpus olneyi biomass in the 
elevated chambers in the mixed community co 
incided with a reduction of Spartina patens bio 
mass after four years of exposure. Distichlis spicata 
showed a positive response to elevated CO2 dur 
ing the first three years of exposure. No negative 
effect of elevated CO2 was found on the pure 
Spartina community, suggesting that the reduc 
tion of Spartina patens in the mixed community 
may be due to shading as a result of an increase 
in biomass of Scirpus olneyi. 
Scirpus olneyi favors the lower elevations of the 
marsh (Fig. 7). Density and shoot height are re 
duced at the higher elevations of the marsh, which 
may be the result of increased salt or drought 
stress. Spartina patens, which is more salt toler 
ant, is most abundant at the higher elevations. 
Growth of Spartina may be affected by the higher 
frequency of flooding and by the low light levels 
in the pure Scirpus community at the lower ele 
vations. In a high CO2 environment growth of 
Scirpus olneyi may be expected to expand at the 
higher elevations of the marsh, at the expense of 
the C4 grasses. 
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