Elevated atmospheric carbon dioxide concentrations ([CO 2 ]) generally increase plant photosynthesis in C 3 species, but not in C 4 species, and reduce stomatal conductance in both C 3 and C 4 plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO 2 , particularly in C 3 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 (C 3 sedge) community, a Spartina patens-dominated (C 4 grass) community and a C 3 -C 4 -mixed species community exposed to ambient and elevated (ambient 1 340 ppm) atmospheric [CO 2 ] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO 2 ] in the S. olneyi-dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO 2 ] was found for total biomass in the C 4 -dominated community. We found a significant decline in C 4 biomass (correlated with rising sea level) and a concomitant increase in C 3 biomass in the mixed community. This shift from C 4 to C 3 was accelerated by the elevated [CO 2 ] treatment. The elevated [CO 2 ] 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 À1 but in the three driest years (1995, 1999, 2002), it was 1.2 t ha À1 . Elevated [CO 2 ] 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 [CO 2 ] stimulation of carbon accumulation in this ecosystem.
Increased atmospheric CO 2 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 CO 2 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 CO 2 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 (r 2 5 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.
Acclimation of photosynthesis and respiration in shoots and ecosystem carbon dioxide fluxes to rising atmospheric carbon dioxide concentration (Ca) was studied in a brackish wetland. Open top chambers were used to create test atmospheres of normal ambient and elevated Ca(= 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 Ca 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 Ca increased carbohydrate and reduced Rubisco and soluble protein concentrations as well as photosynthetic capacity(A) and dark respiration (Ra; dry weight basis) in excised shoots and canopies (leaf area area basis) ofScirpus olneyi. Nevertheless, the rate of photosynthesis was stimulated 53% in shoots and 30% in canopies growing in elevated Ca compared to normal ambient concentration. Elevated Ca inhibited Rd measured in excised shoots (-19 to -40%) and in seasonally integrated ecosystem respiration (Re; -3 6 to -57%). Growth of shoots in elevated Ca was stimulated 14-21%, but this effect was not statistically significant at peak standing biomass in midseason. Although the effect of elevated Ca 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 Ca increased net ecosystem production (NEP = GPP -Re) 59% in 1993 and 50% in 1994. While this study consistently showed that elevated Ca produced a significant increase in NEP, we have not identified a correspondingly large pool of carbon below ground.
Wetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems. However, the effects of elevated atmospheric carbon dioxide (C0 2 ) concentration (C a ) on wetland evapotranspiration (ET) are largely unknown. Here, we present data on 12 years of measurements of ET, net ecosystem C0 2 exchange (NEE), and ecosystem water use efficiency (EWUE, i.e. NEE/ET) at 13:00-15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient + 340 pmol mol x ) C a in a Chesapeake Bay wetland. Although a decrease in stomatal conductance at elevated C a in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extend, ET was still reduced by 19% on average over 12 years. In the S. patens community, LAI was not affected by elevated C a and the reduction of ET was 34%, larger than in the S. olneyi community. For both communities, the relative reduction in ET by elevated C a was directly proportional to precipitation due to a larger reduction in stomatal conductance in the control plants as precipitation decreased. NEE was stimulated about 36% at elevated C a in the S. olneyi community but was not significantly affected by elevated C a in S. patens community. A negative correlation between salinity and precipitation observed in the field indicated that precipitation affected ET through altered salinity and interacted with growth C a . This proposed mechanism was supported by a greenhouse study that showed a greater C a effect on ET in controlled low salinity conditions compared with high salinity. In spite of the differences between the two communities in their responses to elevated C^ EWUE was increased about 83% by elevated C a in both the S. olneyi and S. patens communities. These findings suggest that rising C a could have significant impacts on the hydrologic cycles of coastal wetlands.
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