Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ

Summary Carbon dioxide concentrations were elevated in three estuarine communities for an entire growing season. Open top chambers were used to raise CO₂ concentrations ca. 336 ppm above ambient in monospecific communities of Scirpus olneyi (C₃) and Spartina patens (C₄), and a mixed community of S. olneyi, S. patens and Distichlis spicata (C₄). Nitrogen and carbon concentration (% wt) of aboveground tissue was followed throughout growth and senescence. Green shoot %N was reduced and %C was unchanged under elevated CO₂ in S. olneyi. This resulted in a 20%–40% increase in tissue C/N ratio. There was no effect of CO₂ on either C₄ species. Maximum aboveground N (g/m²) was unchanged in S. olneyi, indicating that increased productivity under elevated CO₂ was dependent on reallocation of stored N. There was no change in the N recovery efficiency of S. olneyi in pure stand and a decrease in the mixed community. Litter C/N ratio was not affected by elevated CO₂ suggesting that decomposition and N mineralization rates will also remain unchanged. Continued growth responses to elevated CO₂ could, however, be limited by the ability of S. olneyi to increase the total aboveground N pool.     

Growth and senescence in plant communities exposed to elevated CO2 concentrations on an estuarine marsh

Summary Three high marsh communities on the Chesapeake Bay were exposed to a doubling in ambient CO₂ concentration for one growing season. Open-top chambers were used to raise CO₂ concentrations ca. 340 ppm above ambient over monospecific communities of Scirpus olneyi (C₃) and Spartina patens (C₄), and a mixed community of S. olneyi, S. patens, and Distichlis spicata (C₄). Plant growth and senescence were monitored by serial, nondestructive censuses. Elevated CO₂ resulted in increased shoot densities and delayed sensecence in the C₃ species. This resulted in an increase in primary productivity in S. olneyi growing in both the pure and mixed communities. There was no effect of CO₂ on growth in the C₄ species. These results demonstrate that elevated atmospheric CO₂ can cause increased aboveground production in a mature, unmanaged ecosystem.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO₂ concentrations (C_a) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO₂ between the atmosphere and the biosphere (NEE), under both ambient and elevated C_a. Here we report data on annual sums of CO₂ (NEE_net) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C₃ photosynthetic pathway)‐ and Spartina patens (C₄ photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C_a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C_a on seasonally integrated CO₂ assimilation (NEE_net = NEE_day + NEE_night, kg C m^?2 y^?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE_net averaged 1.9 kg m^?2 y^?1 in ambient C_a and 2.5 kg m^?2 y^?1 in elevated C_a, for the C₃‐dominated community. Similarly, elevated C_a significantly (P < 0.01) increased carbon uptake in the C₄‐dominated community, as NEE_net averaged 1.5 kg m^?2 y^?1 in ambient C_a and 1.7 kg m^?2 y^?1 in elevated C_a. This resulted in an average CO₂ stimulation of 32% and 13% of seasonally integrated NEE_net for the C₃‐ and C₄‐dominated communities, respectively. Increased NEE_day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C_a, while decreased NEE_night was associated with lower canopy nitrogen content. These results suggest that rising C_a may increase carbon assimilation in both C₃‐ and C₄‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.     

Insects and fungi on a C3 sedge and a C4 grass exposed to elevated atmospheric CO2 concentrations in open-top chambers in the field

The effects of elevated atmospheric CO₂ concentration on plant-fungi and plant-insect interactions were studied in an emergent marsh in the Chesapeake Bay. Stands of the C₃ sedge Scirpus olneyi Grey, and the C₄ grass Spartina patens (Ait.) Muhl. have been exposed to elevated atmospheric CO₂ concentrations during each growing season since 1987. In August 1991 the severities of fungal infections and insect infestations were quantified. Shoot nitrogen concentration ([N]) and water content (WC) were determined. In elevated concentrations of atmospheric CO₂, 32% fewer S. olneyi plants were infested by insects, and there was a 37% reduction in the severity of a pathogenic fungal infection, compared with plants grown in ambient CO₂ concentrations. S. olneyi also had reduced [N], which correlated positively with the severities of fungal infections and insect infestations. Conversely, S. patens had increased WC but unchanged [N] in elevated concentrations of atmospheric CO₂ and the severity of fungal infection increased. Elevated atmospheric CO₂ concentration increased or decreased the severity of fungal infection depending on at least two interacting factors, [N] and WC; but it did not change the number of plants that were infected with fungi. In contrast, the major results for insects were that the number of plants infected with insects decreased, and that the amount of tissue that each insect ate also decreased.     

Inter‐annual changes in detritus‐based food chains can enhance plant growth response to elevated atmospheric CO2

Elevated atmospheric CO₂ generally enhances plant growth, but the magnitude of the effects depend, in part, on nutrient availability and plant photosynthetic pathway. Due to their pivotal role in nutrient cycling, changes in abundance of detritivores could influence the effects of elevated atmospheric CO₂ on essential ecosystem processes, such as decomposition and primary production. We conducted a field survey and a microcosm experiment to test the influence of changes in detritus‐based food chains on litter mass loss and plant growth response to elevated atmospheric CO₂ using two wetland plants: a C₃ sedge (Scirpus olneyi) and a C₄ grass (Spartina patens). Our field study revealed that organism's sensitivity to climate increased with trophic level resulting in strong inter‐annual variation in detritus‐based food chain length. Our microcosm experiment demonstrated that increased detritivore abundance could not only enhance decomposition rates, but also enhance plant growth of S. olneyi in elevated atmospheric CO₂ conditions. In contrast, we found no evidence that changes in the detritus‐based food chains influenced the growth of S. patens. Considered together, these results emphasize the importance of approaches that unite traditionally subdivided food web compartments and plant physiological processes to understand inter‐annual variation in plant production response to elevated atmospheric CO_2.     

Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations

The effects of elevated atmospheric CO₂ (ambient + 340 μmol mol^–1) on above-ground litter decomposition were investigated over a 6-week period using a field-based mesocosm system. Soil respiratory activity in mesocosms incubated in ambient and elevated atmospheric CO₂ concentrations were not significantly different (t-test, P > 0.05) indicating that there were no direct effects of elevated atmospheric CO₂ on litter decomposition. A study of the indirect effects of CO₂ on soil respiration showed that soil mesocosms to which naturally senescent plant litter had been added (0.5% w/w) from the C₃ sedge Scirpus olneyi grown in elevated atmospheric CO₂ was reduced by an average of 17% throughout the study when compared to soil mesocosms to which litter from Scirpus olneyi grown in ambient conditions had been added. In contrast, similar experiments using senescent material from the C₄ grass Spartina patens showed no difference in soil respiration rates between mesocosms to which litter from plants grown in elevated or ambient CO₂ conditions had been added. Analysis of the C:N ratio and lignin content of the senescent material showed that, while the C:N ratio and lignin content of the Spartina patens litter did not vary with atmospheric CO₂ conditions, the C:N ratio (but not the lignin content) of the litter from Scirpus olneyi was significantly greater (t-test;P < 0.05) when derived from plants grown under elevated CO₂ (105:1 compared to 86:1 for litter derived from Scirpus olneyi grown under ambient conditions). The results suggest that the increased C:N ratio of the litter from the C₃ plant Scirpus olneyi grown under elevated CO₂ led to the lower rates of biodegradation observed as reduced soil respiration in the mesocosms. Further long-term experiments are now required to determine the effects of elevated CO₂ on C partitioning in terrestrial ecosystems.     

Interactions between C3 and C4 salt marsh plant species during four years of exposure to elevated atmospheric CO2

Elevated atmospheric CO2 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 µmol CO₂ mol^-1) on a salt marsh vegetation with both C₃ and C₄ species was investigated. Elevated CO₂ increased the biomass of the C₃ sedgeScirpus olneyi growing in a pure stand, while the biomass of the C₄ grassSpartina patens 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 grassSpartina patens dominated the higher areas of the salt marsh, while the C₃ sedgeScirpus olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate elevations.Scirpus growth may have been restricted by drought and salt stress at the higher elevations, whileSpartina growth at the lower elevations may be affected by the higher frequency of flooding. Elevated CO₂ may affect the species distribution in the salt marsh if it allowsScirpus to grow at higher elevations where it in turn may affect the growth ofSpartina.     

Rising sea level,temperature, and precipitation impact plant and ecosystem responses to elevated CO2 on a Chesapeake Bay wetland: review of a 28‐year study

An ongoing field study of the effects of elevated atmospheric CO₂ on a brackish wetland on Chesapeake Bay, started in 1987, is unique as the longest continually running investigation of the effects of elevated CO₂ on an ecosystem. Since the beginning of the study, atmospheric CO₂ increased 18%, sea level rose 20 cm, and growing season temperature varied with approximately the same range as predicted for global warming in the 21st century. This review looks back at this study for clues about how the effects of rising sea level, temperature, and precipitation interact with high atmospheric CO₂ to alter the physiology of C3 and C4 photosynthetic species, carbon assimilation, evapotranspiration, plant and ecosystem nitrogen, and distribution of plant communities in this brackish wetland. Rising sea level caused a shift to higher elevations in the Scirpus olneyi C3 populations on the wetland, displacing the Spartina patens C4 populations. Elevated CO₂ stimulated carbon assimilation in the Scirpus C3 species measured by increased shoot and root density and biomass, net ecosystem production, dissolved organic and inorganic carbon, and methane production. But elevated CO₂ also decreased biomass of the grass, S. patens C4. The elevated CO₂ treatment reduced tissue nitrogen concentration in shoots, roots, and total canopy nitrogen, which was associated with reduced ecosystem respiration. Net ecosystem production was mediated by precipitation through soil salinity: high salinity reduced the CO₂ effect on net ecosystem production, which was zero in years of severe drought. The elevated CO₂ stimulation of shoot density in the Scirpus C3 species was sustained throughout the 28 years of the study. Results from this study suggest that rising CO₂ can add substantial amounts of carbon to ecosystems through stimulation of carbon assimilation, increased root exudates to supply nitrogen fixation, reduced dark respiration, and improved water and nitrogen use efficiency.     

Seventeen years of elevated CO2 exposure in a Chesapeake Bay Wetland: sustained but contrasting responses of plant growth and CO2 uptake

Increased atmospheric CO₂ concentration (Ca) produces a short‐term stimulation of photosynthesis and plant growth across terrestrial ecosystems. However, the long‐term response remains uncertain and is thought to depend on environmental constraints. In the longest experiment on natural ecosystem response to elevated Ca, we measured the shoot‐density, biomass and net CO₂ exchange (NEE) responses to elevated Ca from 1987 to 2003 in a Scirpus olneyi wetland sedge community of the Chesapeake Bay, MD, USA. Measurements were conducted in five replicated open‐top chambers per CO₂ treatment (ambient and elevated). In addition, unchambered control plots were monitored for shoot density. Responses of daytime NEE, Scirpus plant biomass and shoot density to elevated Ca were positive for any single year of the 17‐year period of study. Daytime NEE stimulation by elevated Ca rapidly dropped from 80% at the onset of the experiment to a long‐term stimulation average of about 35%. Shoot‐density stimulation by elevated Ca increased linearly with duration of exposure (r²=0.89), exceeding 120% after 17 years. Although of lesser magnitude, the shoot biomass response to elevated Ca was similar to that of the shoot density. Daytime NEE response to elevated Ca was not explained by the duration of exposure, but negatively correlated with salinity of the marsh, indicating that this elevated‐Ca response was decreased by water‐related stress. By contrast, circumstantial evidence suggested that salinity stress increased the stimulation of shoot density by elevated Ca, which highlights the complexity of the interaction between water‐related stresses and plant community responses to elevated Ca. Notwithstanding the effects of salinity stress, we believe that the most important finding of the present research is that a species response to elevated Ca can continually increase when this species is under stress and declining in its natural environment. This is particularly important because climate changes associated with elevated Ca are likely to increase environmental stresses on numerous species and modify their present distribution. Our results point to an increased resilience to change under elevated Ca when plants are exposed to adverse environmental conditions.     

Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ

Summary Mono-specific communities of the C₃ sedge, Scirpus olneyi and the C₄ grass, Spartina patens, were exposed to normal ambient or elevated CO₂, (ca. 680 l l^–1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C₃ plants grown in ambient or elevated CO₂ showed an increased photosynthetic rate when tested at elevated CO₂ for both seasons. This increase in photosynthetic response in the C₃ species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO₂ appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C₃ species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO₂ conditions continued until October. In contrast to the C₃ sedge, the C₄ grass showed no significant photosynthetic increase to elevated CO₂ except at the beginning of the 1988 season.     

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N₂ fixation to elevated CO₂ was measured in Scirpus olneyi, a C₃ sedge, and Spartina patens, a C₄ grass, using acetylene reduction assay and ¹⁵N₂ gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO₂ significantly (P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in ¹⁵N incorporated by Scirpus olneyi. In Spartina patens, elevated CO₂ (660 ± 1 μ mol mol ^{− 1}) increased nitrogenase activity and ¹⁵N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N₂ fixation in Scirpus olneyi under elevated CO₂ was 611 ± 75 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} compared with 367 ± 46 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} in ambient CO₂ plants. In Spartina patens, however, the rate of N₂ fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} for elevated and ambient CO₂, respectively. Heterotrophic non-symbiotic N₂ fixation in plant-free marsh sediment also increased significantly (P < 0·05) with elevated CO₂. The proportional increase in ¹⁵N₂ fixation correlated with the relative stimulation of photosynthesis, in that N₂ fixation was high in the C₃ plant in which photosynthesis was also high, and lower in the C₄ plant in which photosynthesis was relatively less stimulated by growth in elevated CO₂. These results are consistent with the hypothesis that carbon fixation in C₃ species, stimulated by rising CO₂, is likely to provide additional carbon to endophytic and below-ground microbial processes.     

Acclimation of photosynthesis in relation to Rubisco and non-structural carbohydrate contents and in situ carboxylase activity in Scirpus olneyi grown at elevated CO2 in the field

Stands of Scirpus olneyi, a native saltmarsh sedge with C₃ photosynthesis, had been exposed to normal ambient and elevated atmospheric CO₂ concentrations (C_a) in their native habitat since 1987. The objective of this investigation was to characterize the acclimation of photosynthesis of Scirpus olneyi stems, the photosynthesizing organs of this species, to long-term elevated C_a treatment in relation to the concentrations of Rubisco and non-structural carbohydrates. Measurements were made on intact stems in the Held under existing natural conditions and in the laboratory under controlled conditions on stems excised in the field early in the morning. Plants grown at elevated C_a had a significantly higher (30–59%) net CO₂ assimilation rate (A) than those grown at ambient C_a when measurements were performed on excised stems at the respective growth C_a. However, when measurements were made at normal ambient C_a, A was smaller (45–53%) in plants grown at elevated C_a than in those grown at ambient C_a. The reductions in A at normal ambient C_a, carboxylation efficiency and in situ carboxylase activity were caused by a decreased Rubisco concentration (30–58%) in plants grown at elevated C_a; these plants also contained less soluble protein (39–52%). The Rubisco content was 43 to 58% of soluble protein, and this relationship was not significantly altered by the growth CO₂ concentrations. The Rubisco activation state increased slightly, but the in situ carboxylase activity decreased substantially in plants grown at elevated C_a. When measurements were made on intact stems in the field, the elevated C_a treatment caused a greater stimulation of,A (100%) and a smaller reduction in carboxylation efficiency (which was not statistically significant) than when measurements were made on excised stems in the laboratory. The possible reasons for this arc discussed. Plants grown at elevated C_a contained more non-structural carbohydrates (25–53%) than those grown at ambient C_a. Plants grown at elevated C_a appear to have sufficient sink capacity to utilize the additional carbohydrates formed during photosynthesis. Overall, our results are in agreement with the hypothesis that elevated C_a leads to an increased carbohydrate concentration and the ensuing acclimation of the photo-synthetic apparatus in C₃ plants results in a reduction in the protein complement, especially Rubisco, which reduces the photosynthetic capacity in plants grown at elevated C_a, relative to plants grown at normal ambient C_a. Nevertheless, when compared at their respective growth C_a, Scirpus olneyi plants grown at elevated C_a in their native habitat maintained a substantially higher rate of photosynthesis than those grown at normal ambient C_a even after 8 years of growth at elevated C_a.     

Acclimation of photosynthesis,respiration and ecosystem carbon flux of a wetland on Chesapeake Bay,Maryland to elevated atmospheric CO2 concentration

Acclimation of photosynthesis and respiration in shoots and ecosystem carbon dioxide fluxes to rising atmospheric carbon dioxide concentration (C _a ) was studied in a brackish wetland. Open top chambers were used to create test atmospheres of normal ambient and elevated C _a (=normal ambient + 34 Pa CO₂) over mono-specific stands of the C₃ sedge Scirpus olneyi, the dominant C₃ species in the wetland ecosystem, throughout each growing season since April of 1987. Acclimation of photosynthesis and respiration were evaluated by measurements of gas exchange in excised shoots. The impact of elevated C _a on the accumulation of carbon in the ecosystem was determined by ecosystem gas exchange measurements made using the open top chamber as a cuvette.Elevated C _a increased carbohydrate and reduced Rubisco and soluble protein concentrations as well as photosynthetic capacity(A) and dark respiration (R _d ; dry weight basis) in excised shoots and canopies (leaf area area basis) of Scirpus olneyi. Nevertheless, the rate of photosynthesis was stimulated 53% in shoots and 30% in canopies growing in elevated C _a compared to normal ambient concentration. Elevated C _a inhibited R _d measured in excised shoots (–19 to –40%) and in seasonally integrated ecosystem respiration (R _e ; –36 to –57%). Growth of shoots in elevated C _a was stimulated 14–21%, but this effect was not statistically significant at peak standing biomass in midseason. Although the effect of elevated C _a on growth of shoots was relatively small, the combined effect of increased number of shoots and stimulation of photosynthesis produced a 30% stimulation in seasonally integrated gross primary production (GPP). The stimulation of photosynthesis and inhibition of respiration by elevated C _a increased net ecosystem production (NEP=GPP–R _e ) 59% in 1993 and 50% in 1994. While this study consistently showed that elevated C _a produced a significant increase in NEP, we have not identified a correspondingly large pool of carbon below ground.     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Gas exchange responses of Chesapeake Bay tidal marsh species under field and laboratory conditions

Summary Laboratory and field gas exchange measurements were made on C₃ (Scirpus olneyi Gray) and C₄ (Spartina patens (Ait.) Mahl., Distichlis spicata (L.) Green) species from an irregularly flooded tidal marsh on the Chesapeake Bay. Laboratory measurements were made on plants grown from root stocks that were transplanted to a greenhouse and grown under high light and high nutrient conditions. The two C₄ species were similar in their laboratory gas exchange characteristics: both had higher net carbon exchange rates, higher mesophyll conductances, higher photosynthetic temperature optima and lower leaf conductances than the C₃ species. The laboratory photosynthetic water use efficiency of the C₄ species was approximately three times that of the C₃ species.Field gas exchange responses of the above species were measured in situ a Chesapeake Bay tidal marsh. Despite differences in biological potential measured in the laboratory, all three species had similar in situ carbon exchange rates on a leaf area basis. On a dry weight basis, leaves of the two C₄ species had about 1.4 times higher light saturated CO₂ assimilation rates than the C₃ species. Light saturation of CO₂ exchange occurred at photosynthetic photon flux densities of 80 n Einstein cm^-2s^-1, compared with 160 n Einstein cm ^-2s^-1 in the laboratory grown plants. Spartina patens and Scirpus olneyi had similar daily CO₂ assimilation rates, but the daily transpiration rate of the C₃ species was almost twice that of the C₄ species. Spartina patens showed greater seasonal decrease in photosynthesis than Distichlis spicata and Scirpus olneyi. The two C₄ grass species maintained higher mesophyll conductances and photosynthetic water use efficiencies than the C₄ sedge.     

Elevated CO<Subscript>2</Subscript> mitigates the adverse effects of drought on daytime net ecosystem CO<Subscript>2</Subscript> exchange and photosynthesis in a Florida scrub-oak ecosystem

Drought is a normal, recurrent feature of climate. In order to understand the potential effect of increasing atmospheric CO₂ concentration (C _a) on ecosystems, it is essential to determine the combined effects of drought and elevated C _a (EC) under field conditions. A severe drought occurred in Central Florida in 1998 when precipitation was 88 % less than the average between 1984 and 2002. We determined daytime net ecosystem CO₂ exchange (NEE) before, during, and after the drought in the Florida scrub-oak ecosystem exposed to doubled C _a in open-top chamber since May 1996. We measured diurnal leaf net photosynthetic rate (P _N) of Quercus myrtifolia Willd, the dominant species, during and after the drought. Drought caused a midday depression in NEE and P _N at ambient CO₂ concentration (AC) and EC. EC mitigated the midday depression in NEE by about 60 % compared to AC and the effect of EC on leaf P _N was similar to its effect on NEE. Growth in EC lowered the sensitivity of NEE to air vapor pressure deficit under drought. Thus EC would help the scrub-oak ecosystem to survive the consequences of the effects of rising atmospheric CO₂ on climate change, including increased frequency of drought, while simultaneously sequestering more anthropogenic carbon.     

Edaphic diatom communities associated with Spartina alterniflora and S. patens in New Jersey

Edaphic diatoms inhabiting the sediments beneath dwarf Spartina alterniflora Loisel. and S. patens (Ait.) Muhl. in Great Bay salt marsh, Tuckerton, New Jersey were collected from 24 September 1974 through 20 August 1975. Of the 91 taxa encountered, 8 were endemic to the dwarf S. alterniflora habitat and 42 endemic to the S. patens habitat. The edaphic diatom community associated with S. patens was comprised of a much greater number of taxa and possessed higher values for species diversity (H') and evenness (J') than the community associated with dwarf S. alterniflora. The salinity of the marsh surface showed a completely opposite trend, being greatly reduced at the S. patens habitat. A highly significant relationship (P < 0.001) between the number of diatom taxa and marsh surface salinity at the S. patens habitat was demonstrated by a least squares regression. This finding led to the conclusion that the dissimilarity in the structure of the two edaphic diatom communities was primarily due to the very low marsh surface salinities at the S. patens habitat from January through June, and that this sustained. low-salinity regime allowed a very large number of taxa to coexist only in the S. patens community. Comparison of the diatom flora of Great Bay salt marsh with that of a Delaware marsh studied previously by the author showed that 67.0% of the 91 taxa encountered in New Jersey also occur on the Delaware marsh.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Direct and indirect effects of elevated CO2 on transpiration from Quercus myrtifolia in a scrub-oak ecosystem

Elevated atmospheric carbon dioxide (C_a) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant‐power sap flow gauges, we measured transpiration from Quercus myrtifolia Willd., the dominant species of the Florida scrub‐oak ecosystem, which had been exposed in situ to elevated C_a (350 µmol mol ^{? 1} above ambient) in open‐top chambers since May 1996. Elevated C_a reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the C_a treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated C_a. However, there was also an apparent indirect effect of C_a on transpiration: when transpiration in all plants was measured under common C_a, transpiration in elevated C_a‐grown plants was lower than that in plants grown in normal ambient C_a. Results from measurements of stomatal conductance (g_s), leaf area index (LAI), canopy light interception and correlation between light and g_s indicated that the direct, reversible C_a effect on transpiration was due to changes in g_s caused by C_a, and the indirect effect was caused mainly by greater self‐shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self‐shading at elevated C_a decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self‐shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated C_a.