Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L.) Gaertn. under elevated carbon dioxide

Short-term studies of tree growth at elevated CO₂ suggest that forest productivity may increase as atmospheric CO₂ concentrations rise, although low soil N availability may limit the magnitude of this response. There have been few studies of growth and N₂ fixation by symbiotic N₂-fixing woody species under elevated CO₂ and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO₂ on growth, tissue N accretion, and N₂ fixation of nodulated Alnus glutinosa (L.) Gaertn. grown under low soil N conditions for 160 d. Root, nodule, stem, and leaf dry weight (DW) and N accretion increased significantly in response to elevated CO₂. Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta-¹⁵N analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO₂ grown plants fixed approximately 40% more N than did ambient CO₂ grown plants. Leaves from both CO₂ treatments showed similar relative declines in leaf N content prior to autumnal leaf abscission, but total N in leaf litter increased 24% in elevated compared to ambient CO₂ grown plants. These results suggest that with rising atmospheric CO₂ N₂-fixing woody species will accumulate greater amounts of biomass N through N₂ fixation and may enhance soil N levels by increased litter N inputs.     

Performance and allocation patterns of the perennial herb,Plantago lanceolata,in response to simulated herbivory and elevated CO2 environments

Summary We tested the prediction that plants grown in elevated CO₂ environments are better able to compensate for biomass lost to herbivory than plants grown in ambient CO₂ environments. The herbaceous perennial Plantago lanceolata (Plantaginaceae) was grown in either near ambient (380 ppm) or enriched (700 ppm) CO₂ atmospheres, and then after 4 weeks, plants experienced either 1) no defoliation; 2) every fourth leaf removed by cutting; or 3) every other leaf removed by cutting. Plants were harvested at week 13 (9 weeks after simulated herbivory treatments). Vegetative and reproductive weights were compared, and seeds were counted, weighed, and germinated to assess viability.Plants grown in enriched CO₂ environments had significantly greater shoot weights, leaf areas, and root weights, yet had significantly lower reproductive weights (i.e. stalks + spikes + seeds) and produced fewer seeds, than plants grown in ambient CO₂ environments. Relative biomass allocation patterns further illustrated differences in plants grown in ambient CO₂ environments. Relative biomass allocation patterns further illustrated differences in plant responses to enriched CO₂ atmospheres: enriched CO₂-grown plants only allocated 10% of their carbon resources to reproduction whereas ambient CO₂-grown plants allocated over 20%. Effects of simulated herbivory on plant performance were much less dramatic than those induced by enriched CO₂ atmospheres. Leaf area removal did not reduce shoot weights or reproductive weights of plants in either CO₂ treatment relative to control plants. However, plants from both CO₂ treatments experienced reductions in root weights with leaf area removal, indicating that plants compensated for lost above-ground tissues, and maintained comparable levels of reproductive output and seed viability, at the expense of root growth.     

Root restriction as a factor in photosynthetic acclimation of cotton seedlings grown in elevated carbon dioxide

Interactive effects of root restriction and atmospheric CO₂ enrichment on plant growth, photosynthetic capacity, and carbohydrate partitioning were studied in cotton seedlings (Gossypium hirsutum L.) grown for 28 days in three atmospheric CO₂ partial pressures (270, 350, and 650 microbars) and two pot sizes (0.38 and 1.75 liters). Some plants were transplanted from small pots into large pots after 20 days. Reduction of root biomass resulting from growth in small pots was accompanied by decreased shoot biomass and leaf area. When root growth was less restricted, plants exposed to higher CO₂ partial pressures produced more shoot and root biomass than plants exposed to lower levels of CO₂. In small pots, whole plant biomass and leaf area of plants grown in 270 and 350 microbars of CO₂ were not significantly different. Plants grown in small pots in 650 microbars of CO₂ produced greater total biomass than plants grown in 350 microbars, but the dry weight gain was found to be primarily an accumulation of leaf starch. Reduced photosynthetic capacity of plants grown at elevated levels of CO₂ was clearly associated with inadequate rooting volume. Reductions in net photosynthesis were not associated with decreased stomatal conductance. Reduced carboxylation efficiency in response to CO₂ enrichment occurred only when root growth was restricted suggesting that ribulose-1,5-bisphosphate carboxylase/oxygenase activity may be responsive to plant source-sink balance rather than to CO₂ concentration as a single factor. When root-restricted plants were transplanted into large pots, carboxylation efficiency and ribulose-1,5-bisphosphate regeneration capacity increased indicating that acclimation of photosynthesis was reversible. Reductions in photosynthetic capacity as root growth was progressively restricted suggest sink-limited feedback inhibition as a possible mechanism for regulating net photosynthesis of plants grown in elevated CO₂.     

Growth,nutrient dynamics,and efficiency responses to carbon dioxide and phosphorus nutrition in soybean

Plant mineral nutrients such as phosphorus may exert major control on crop responses to the rising atmospheric carbon dioxide (CO₂) concentrations. To evaluate the growth, nutrient dynamics, and efficiency responses to CO₂ and phosphorus nutrition, soybean (Glycine max (L.) Merr.) was grown in controlled environment growth chambers with sufficient (0.50 mM) and deficient (0.10 and 0.01 mM) phosphate (Pi) supply under ambient and elevated CO₂ (aCO₂, 400 and eCO₂, 800 µmol mol^?1, respectively). The CO₂ × Pi interaction was detected for leaf area, leaf and stem dry weight, and total plant biomass. The severe decrease in plant biomass in Pi-deficient plants (10–76%) was associated with reduced leaf area and photosynthesis (P_net). The degree of growth stimulation (0–55% total biomass) by eCO₂ was dependent upon the severity of Pi deficiency and was closely associated with the increased phosphorus utilization efficiency. With the exception of leaf and root biomass, Pi deficiency decreased the biomass partitioning to other plant organs with the maximum decrease observed in seed weight (8–42%) across CO₂ levels. The increased tissue nitrogen (N) concentration in Pi-deficient plants was accredited to the lower biomass and increased nutrient uptake due to the larger root to shoot ratio. The tissue P and N concentration tended to be lower at eCO₂ versus aCO₂ and did not appear to be the main cause of the lack of CO₂ response of growth and P_net under severe Pi deficiency. The leaf N/P ratio of >16 was detrimental to soybean growth. The tissue P concentration needed to attain the maximum productivity for biomass and seed yield tended to be higher at eCO₂ versus aCO₂. Therefore, the eCO₂ is likely to increase the leaf critical P concentration for maximum biomass productivity and yield in soybean.     

Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]

An important question with respect to plant performance in future climatic scenarios is whether the offspring of mature trees that have experienced lifelong exposure to elevated [CO₂] show altered physiological responses to elevated [CO₂] compared with those originating from current ambient CO₂ concentrations. To investigate this question, acorns were collected from two seed sources, denoted as ‘control’ and ‘spring’, from Quercus ilex mother trees grown at ambient (36 Pa) and at about twice ambient CO₂ concentrations, respectively, close to a natural CO₂ spring, Laiatico, central Italy. The seedlings were raised for 8 months under controlled conditions at ambient and elevated [CO₂] in a reciprocal experimental design and were used for the determination of biomass, photosynthesis and foliar carbohydrate concentrations, as well as the accumulation of structural biomass and lignin during leaf maturation. Under ambient [CO₂], biomass and foliar carbon acquisition in control progeny were not significantly different from spring progeny. However, under elevated [CO₂], spring seedlings showed less CO₂ acclimation than control seedlings but no significant differences in non‐structural carbohydrate concentrations and structural biomass per unit leaf dry mass. Developmental lignin accumulation in leaves was delayed under elevated [CO₂] compared with ambient [CO₂], but only in control progeny. Under elevated [CO₂], whole‐plant biomass, leaf area and stem diameter were significantly increased in Quercus ilex seedlings from both seed sources but with a higher stimulation of above‐ground biomass in spring than in control seedlings and a higher stimulation of below‐ground biomass in control seedlings. These results indicate that life history and/or progeny may determine the species‐specific CO₂ response and suggest that positive CO₂ acclimation is possible.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Root-zone CO2 and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

Effects of elevated root-zone (RZ) CO₂ concentration (RZ [CO₂]) and RZ temperature (RZT) on photosynthesis, productivity, nitrate (NO₃ ^?), total reduced nitrogen (TRN), total leaf soluble and Rubisco proteins were studied in aeroponically grown lettuce plants in a tropical greenhouse. Three weeks after transplanting, four different RZ [CO₂] concentrations (ambient, 360 ppm, and elevated concentrations of 2,000; 10,000; and 50,000 ppm) were imposed on plants at 20°C-RZT or ambient(A)-RZT (24–38°C). Elevated RZ [CO₂] resulted in significantly higher light-saturated net photosynthetic rate, but lower light-saturated stomatal conductance. Higher elevated RZ [CO₂] also protected plants from both chronic and dynamic photoinhibition (measured by chlorophyll fluorescence Fv/Fm ratio) and reduced leaf water loss. Under each RZ [CO₂], all these variables were significantly higher in 20°C-RZT plants than in A-RZT plants. All plants accumulated more biomass at elevated RZ [CO₂] than at ambient RZ [CO₂]. Greater increases of biomass in roots than in shoots were manifested by lower shoot/root ratios at elevated RZ [CO₂]. Although the total biomass was higher at 20°C-RZT, the increase in biomass under elevated RZ [CO₂] was greater at A-RZT. Shoot NO₃ ^? and TRN concentrations, total leaf soluble and Rubisco protein concentrations were higher in all elevated RZ [CO₂] plants than in plants under ambient RZ [CO₂] at both RZTs. Under each RZ [CO₂], total leaf soluble and Rubisco protein concentrations were significantly higher at 20°C-RZT than at A-RZT. Our results demonstrated that increased P _Nmax and productivity under elevated [CO₂] was partially due to the alleviation of midday water loss, both dynamic and chronic photoinhibition as well as higher turnover of Calvin cycle with higher Rubisco proteins.     

Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO₂ concentrations ([CO₂]) of 360 μmol mol⁻¹ (ambient) and 720 μmol mol⁻¹ (elevated) and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO₂] and temperature, and to assess whether elevated [CO₂] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO₂] had higher midday photosynthetic CO₂ exchange rate (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO₂]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO₂]. Declines in Rubisco under elevated growth [CO₂] were 27–30% for initial activity, 5–12% for total activity, and 9–20% for protein content. Although Rubisco protein content and activity were down-regulated by elevated [CO₂], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO₂] plants was 1.3- to 1.9-fold greater than that of the ambient-[CO₂] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO₂] were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient [CO₂]. Under elevated [CO₂], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO₂] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyrophosphorylase, was up-regulated under elevated growth [CO₂]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO₂] and temperature as predicted for this century.     

Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO₂ plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO₂ partial pressures were maintained at ambient CO₂ (Amb) and elevated CO₂ (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO₂ under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO₂ produced 90% more biomass after four growing seasons than did trees grown in ambient CO₂. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO₂. Although there was not a sustained effect of elevated CO₂ on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO₂ as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO₂ on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO₂, but these differences were greater in the summer (60–130% increase) than in the winter (14–44% increase), reflecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO₂ will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO₂ compared with ambient CO₂. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then these trees could be a large sink for fossil carbon emitted to the atmosphere and produce a negative feedback on atmospheric CO₂.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

Biomass production in a nitrogen-fertilized,tallgrass prairie ecosystem exposed to ambient and elevated levels of CO2

Increased biomass production in terrestrial ecosystems with elevated atmospheric CO₂ may be constrained by nutrient limitations as a result of increased requirement or reduced availability caused by reduced turnover rates of nutrients. To determine the short-term impact of nitrogen (N) fertilization on plant biomass production under elevated CO₂, we compared the response of N-fertilized tallgrass prairie at ambient and twice-ambient CO₂ levels over a 2-year period. Native tallgrass prairie plots (4.5 m diameter) were exposed continuously (24 h) to ambient and twice-ambient CO₂ from 1 April to 26 October. We compared our results to an unfertilized companion experiment on the same research site. Above- and belowground biomass production and leaf area of fertilized plots were greater with elevated than ambient CO₂ in both years. The increase in biomass at high CO₂ occurred mainly aboveground in 1991, a dry year, and belowground in 1990, a wet year. Nitrogen concentration was lower in plants exposed to elevated CO₂, but total standing crop N was greater at high CO₂. Increased root biomass under elevated CO₂ apparently increased N uptake. The biomass production response to elevated CO₂ was much greater on N-fertilized than unfertilized prairie, particularly in the dry year. We conclude that biomass production response to elevated CO₂ was suppressed by N limitation in years with below-normal precipitation. Reduced N concentration in above- and belowground biomass could slow microbial degradation of soil organic matter and surface litter, thereby exacerbating N limitation in the long term.     

Reversibility of photosynthetic acclimation of swiss chard and sugarbeet grown at elevated concentrations of CO2

Although leaf photosynthesis and plant growth are initially stimulated by elevated CO₂ concentrations, increasing insensitivity to CO₂ (acclimation) is a frequent occurrence. In order to examine the acclimation process, we studied photosynthesis and whole plant development in swiss chard (Beta vulgaris L. Koch ssp. ciela) and sugarbeet (Beta vulgaris L. ssp. vulgaris) grown at either ambient or twice ambient concentrations of CO₂. In an initial controlled environment study, photosynthetic acclimation to elevated CO₂ levels was observed in both subspecies 24 days after sowing (DAS) but was not observed at 42 and 49 DAS for sugarbeet or at 49 DAS for swiss chard. Although sugarbeet and swiss chard differed in root size and morphology, this was not a factor in the onset of photosynthetic acclimation. The reversal of photosynthetic acclimation that was observed in older plants grown at elevated CO₂, concentrations was associated with a rapid increase in root development (i.e. increased root: shoot [R/S] ratio), increased sucrose levels in sinks (roots) and no differences in total soluble leaf protein of either subspecies relative to the ambient CO₂ condition. In a second set of experiments, swiss chard and sugarbeet were grown in outdoor Plexiglass chambers at different times of the year (i.e. summer and early fall). Average 24-h temperature was 30.7 and 19.4°C for the summer and fall plantings, respectively. In agreement with the controlled environment study, lack of photosynthetic acclimation, determined from the response of photosynthesic rate to internal CO₂ concentration, was correlated with increased root biomass and sucrose concentration relative to the ambient condition. However, photo-synthetic acclimation was observed depending on the season, i.e. summer (swiss chard) or fall (sugarbeet), suggesting that acclimation was affected by environmental factors, such as temperature. Data from both experiments suggest that continued long-term photosynthetic stimulation may be dependent upon the ability of increased CO₂ to stimulate new sink development which would allow full utilization of the additional carbon made available in a high CO₂ environment.     

Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition

Small birch plants (Betula pendula Roth.) were grown from seed for periods of up to 70d in a climate chamber at optimal nutrition and at present (350 μmol mol^?1) or elevated (700 μmol mol^?1) concentrations of atmospheric CO₂. Nutrients were sprayed over the roots in Ingestad-type units. Relative growth rate and net assimilation rate were slightly higher at elevated CO₂, whereas leaf area ratio was slightly lower. Smaller leaf area ratio was associated with lower values of specific leaf area. Leaves grown at elevated CO₂ had higher starch concentrations (dry weight basis) than leaves grown at present levels of CO₂. Biomass allocation showed no change with CO₂, and no large effects on stem height, number of side shoots and number of leaves were found. However, the specific root length of fine roots was higher at elevated CO₂. No large difference in the response of carbon assimilation to intercellular CO₂ concentration (A/C_i curves) were found between CO₂ treatments. When measured at the growth environments, the rates of photosynthesis were higher in plants grown at elevated CO₂ than in plants grown at present CO₂. Water use efficiency of single leaves was higher in the elevated treatment. This was mainly attributable to higher carbon assimilation rate at elevated CO₂. The difference in water use efficiency diminished with leaf age. The small treatment difference in relative growth rate was maintained throughout the experiment, which meant that the difference in plant size became progressively greater. Thus, where plant nutrition is sufficient to maintain maximum growth, small birch plants may potentially increase in size more rapidly at elevated CO₂.     

Growth and onto-morphogenesis of soybean (Glycine max Merril) in an open, naturally CO2-enriched environment

Springs emitting carbon dioxide are frequent in Central Italy and provide a way of testing the response of plants to CO₂ enrichment under natural conditions. Results of a CO₂ enrichment experiment on soybean at a CO₂ spring (Solfatara) are presented. The experimental site is characterized by significant anomalies in atmospheric CO₂ concentration produced by a large number of vents emitting almost pure CO₂ (93%) plus small amounts of hydrogen sulphide, methane, nitrogen and oxygen. Within the gas vent area, plants were grown at three sub-areas whose mean CO₂ concentrations during daytime were 350,652 and 2370 μmol mol^-1, respectively. Weekly harvests were made to measure biomass growth, leaf area and ontogenetic development. Biomass growth rate and seed yield were enhanced by elevated CO₂. In particular, onto-morphogenetic development was affected by elevated CO₂ with high levels of CO₂ increasing the total number of main stem leaf nodes and the area of the main stem trifoliolate leaves. Biochemical analysis of plant tissue suggested that there was no effect of the small amounts of H₂S on the response to CO₂ enrichment. Non-protein sulphydryl compounds did not accumulate in leaf tissues and the overall capacity of leaf extracts to oxidize exogenously added NADH was not decreased. The limitations and advantages of experimenting with crop plants at elevated CO₂ in the open and in the proximity of carbon dioxide springs are discussed.     

A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology

Quantitative integration of the literature on the effect of elevated CO₂ on woody plants is important to aid our understanding of forest health in coming decades and to better predict terrestrial feedbacks on the global carbon cycle. We used meta-analytic methods to summarize and interpret more than 500 reports of effects of elevated CO₂ on woody plant biomass accumulation and partitioning, gas exchange, and leaf nitrogen and starch content. The CO₂ effect size metric we used was the log-transformed ratio of elevated compared to ambient response means weighted by the inverse of the variance of the log ratio. Variation in effect size among studies was partitioned according to the presence of interacting stress factors, length of CO₂ exposure, functional group status, pot size, and type of CO₂ exposure facility. Both total biomass (W _T) and net CO₂ assimilation (A) increased significantly at about twice ambient CO₂, regardless of growth conditions. Low soil nutrient availability reduced the CO₂ stimulation of W _T by half, from +31% under optimal conditions to +16%, while low light increased the response to +52%. We found no significant shifts in biomass allocation under high CO₂. Interacting stress factors had no effect on the magnitude of responses of A to CO₂, although plants grown in growth chambers had significantly lower responses (+19%) than those grown in greenhouses or in open-top chambers (+54%). We found no consistent evidence for photosynthetic acclimation to CO₂ enrichment except in trees grown in pots <0.5 l (−36%) and no significant CO₂ effect on stomatal conductance. Both leaf dark respiration and leaf nitrogen were significantly reduced under elevated CO₂ (−18% and −16% respectively, data expressed on a leaf mass basis), while leaf starch content increased significantly except in low nutrient grown gymnosperms. Our results provide robust, statistically defensible estimates of elevated CO₂ effect sizes against which new results may be compared or for use in forest and climate model parameterization. Received: 16 May 1997 / Accepted: 9 September 1997     

Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO₂, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO₂ enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol^?1) and enriched (700 μmol mol^?1) levels of atmospheric CO₂. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO₂‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO₂ effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO₂ treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO₂ enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO₂‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO₂ enrichment.     

Seed traits,seed‐reserve utilization and offspring performance across pre‐industrial to future CO2 concentrations in a Mediterranean community

Atmospheric CO₂ enrichment can affect plants directly via impacts on their performance, and indirectly, by environment‐specific traits passed down from the mother plant to the offspring. Such maternal effects can significantly alter plant species composition, especially in annual ecosystems where the entire community is recruited from seeds each year. This study assessed impacts of future, high CO₂ (440 and 600 ppm) and pre‐industrial, low CO₂ (280 ppm) on seed traits and offspring performance in three plant functional groups (grasses, legumes, forbs) comprising 17 annual species of a semi‐arid Mediterranean community. In grasses, seed size and seed‐reserve utilization as expressed by root elongation tended to be higher at high than at low maternal CO₂, but total seed protein concentration and protein pool decreased with increasing maternal CO₂. The response of seed size to high CO₂ increased with increasing leaf‐mass fraction in grasses, and decreased with decreasing concentration of leaf non‐structural carbohydrates in legumes. Offspring development was studied at ambient CO₂, and showed reduced emergence success of high‐CO₂ progeny compared with low‐CO₂ progeny in forbs. Total biomass was lower in high‐CO₂ than in low‐CO₂ offspring across all functional groups. The biomass response to high maternal CO₂ in legume offspring correlated inversely with seed size, resulting in up to 25% lower biomass in large‐seeded species. Under the scenario of maternal effects combined with projected changes in biomass and seed production under direct exposure to high CO₂, legumes might gain and forbs and grasses might lose from future CO₂ enrichment. Most changes in seed traits and offspring performance were greater between pre‐industrial and near‐future CO₂ than between near‐ and remote‐future CO₂ concentrations. Hence, maternal effects of increasing CO₂ may contribute to current changes in plant productivity and species composition, and they need to be considered when predicting impacts of global change on plant communities.     

Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO₂ on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C₃) grown in an elevated CO₂ atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO₂ should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO₂ concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO₂ with no open top chamber, ii) at ambient CO₂ in an open top chamber, and iii) at twice-ambient CO₂ in an open top chamber. Plants were fertilized with 4.5 g N m⁻² over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO₂ effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO₂ treatments. Rates of photosynthesis in plants grown at elevated CO₂ were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO₂. Total and belowground biomass were significantly greater at elevated CO₂. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO₂ compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO₂ concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Impact of arbuscular mycorrhizal fungi (AMF) and atmospheric CO2 concentration on the biomass production and partitioning in the forage legume alfalfa

The influence of mycorrhizal symbiosis, atmospheric CO₂ concentration and the interaction between both factors on biomass production and partitioning were assessed in nodulated alfalfa (Medicago sativa L.) associated or not with arbuscular mycorrhizal fungi (AMF) and grown in greenhouse at either ambient (392 μmol?mol^?1) or elevated (700 μmol?mol^?1) CO₂ air concentrations. Measurements were performed at three stages of the vegetative period of plants. Shoot and root biomass achieved by plants at the end of their vegetative period were highly correlated to the photosynthetic rates reached at earlier stages, and there was a significant relationship between CO₂ exchange rates and total nodule biomass per plant. In non-mycorrhizal alfalfa, the production of leaves, stems and nodules biomass significantly increased when plants had been exposed to elevated CO₂ concentration in the atmosphere for 4 weeks. Regardless CO₂ concentration at which alfalfa were cultivated, mycorrhizal symbiosis improved photosynthetic rates and growth of alfalfa at early stages of the vegetative period and then photosynthesis decreased, which suggests that AMF shortened the vegetative period of the host plants. At final stages of the vegetative period, AMF enhanced both area and biomass of leaves as well as the leaves to stems ratio when alfalfa plants were cultivated at ambient CO₂. The interaction of AMF with elevated CO₂ improved root biomass and slightly increased the leaves to stems ratio at the end of the vegetative growth. Therefore, AMF may favor both the forage quality of alfalfa when grown at ambient CO₂ and its perennity for next cutting regrowth cycle when grown under elevated CO₂. Nevertheless, this hypothesis needs to be checked under natural conditions in field.