Increase of atmospheric CO2 promotes phytoplankton productivity

It is usually thought that unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric CO₂. Here we suggest that this view may be biased by a neglect of the effects of carbon (C) assimilation on the pH and the dissociation of the C species. We show that under eutrophic conditions, productivity may double as a result of doubling of the atmospheric CO₂ concentration. Although in practice productivity increase will usually be less, we still predict a productivity increase of up to 40% in marine species with a low affinity for bicarbonate. In eutrophic freshwater systems doubling of atmospheric CO₂ may result in an increase of the productivity of more than 50%. Freshwaters with low alkalinity appeared to be very sensitive to atmospheric CO₂ elevation. Our results suggest that the aquatic C sink may increase more than expected, and that nuisance phytoplankton blooms may be aggravated at elevated atmospheric CO₂ concentrations.     

Effects of elevated CO2 on growth,photosynthesis and respiration of sweet chestnut (Castanea sativa Mill.)

Two year old sweet chestnut seedlings (Castanea sativa Mill) were grown in pots at ambient (350 µmol·mol^–1) and double (700 µmol·mol^–1) atmospheric CO₂ concentration in constantly ventilated greenhouses during entire growing seasons. CO₂ enrichment caused either no significant change or a decrease in shoot response, depending on yearly weather conditions. Similarly, leaf area was either reduced or unchanged under elevated CO₂. However, when grown under controlled conditions in a growth chamber, leaf area was enlarged with elevated CO₂.The CO₂ exchanges of whole plants were measured during the growing season. In elevated CO₂, net photosynthetic rate was maximum in May and then decreased, reaching the level of the control at the end of the season. End of night dark respiration of enriched plants was significantly lower than that of control plants; this difference decreased with time and became negligible in the fall. The original CO₂ level acted instantaneously on the respiration rate: a double concentration in CO₂ decreased the respiration of control plants and a reduced concentration enhanced the respiration of enriched plants. The carbon balance of a chestnut seedling may then be modified in elevated CO₂ by increased carbon inputs and decreased carbon outputs.     

Plant responses to atmospheric carbon dioxide enrichment: interactions with some soil and atmospheric conditions

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration. Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels. Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation. Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.     

Freshwater biota and rising pCO2?

Rising atmospheric carbon dioxide (CO₂) has caused a suite of environmental issues, however, little is known about how the partial pressure of CO₂ (pCO₂) in freshwater will be affected by climate change. Freshwater pCO₂ varies across systems and is controlled by a diverse array of factors, making it difficult to make predictions about future levels of pCO₂. Recent evidence suggests that increasing levels of atmospheric CO₂ may directly increase freshwater pCO₂ levels in lakes, but rising atmospheric CO₂ may also indirectly impact freshwater pCO₂ levels in a variety of systems by affecting other contributing factors such as soil respiration, terrestrial productivity and climate regimes. Although future freshwater pCO₂ levels remain uncertain, studies have considered the potential impacts of changes to pCO₂ levels on freshwater biota. Studies to date have focused on impacts of elevated pCO₂ on plankton and macrophytes, and have shown that phytoplankton nutritional quality is reduced, plankton community structure is altered, photosynthesis rates increase and macrophyte distribution shifts with increasing pCO₂. However, a number of key knowledge gaps remain and gaining a better understanding of how freshwater pCO₂ levels are regulated and how these levels may impact biota, will be important for predicting future responses to climate change.     

Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle

Terrestrial higher plants exchange large amounts of CO₂ with the atmosphere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial-plant photosynthesis each year, with an about equal amount returned to the atmosphere as CO₂ in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO₂ content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO₂ concentration might influence the rate of atmospheric CO₂ increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO₂ concentration may modulate plant and ecosystem responses to CO₂ concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO₂ concentration may be as significant to plant and ecosystem C balance as CO₂ concentration itself. Moreover, human activities such as deforestation and livestock grazing can have impacts on the C balance and structure of individual terrestrial ecosystems that far outweigh effects of increasing CO₂ concentration and climatic change. In short-term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO₂ concentration affects terrestrial higher plants in several ways. Elevated CO₂ can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO₂ concentration. Acclimation and adaptation of photosynthesis to increasing CO₂ concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO₂ concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO₂ concentration. Plant respiration may be inhibited by elevated CO₂ concentration, and although a naive C balance perspective would count this as a benefit to a plant, because respiration is essential for plant growth and health, an inhibition of respiration can be detrimental. The net effect on terrestrial plants of elevated atmospheric CO₂ concentration is generally an increase in growth and C accumulation in phytomass. Published estimations, and speculations about, the magnitude of global terrestrial-plant growth responses to increasing atmospheric CO₂ concentration range from negligible to fantastic. Well-reasoned analyses point to moderate global plant responses to CO₂ concentration. Transfer of C from plants to soils is likely to increase with elevated CO₂ concentrations because of greater plant growth, but quantitative effects of those increased inputs to soils on soil C pool sizes are unknown. Whether increases in leaf-level photosynthesis and short-term plant growth stimulations caused by elevated atmospheric CO₂ concentration will have, by themselves, significant long-term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO₂ concentration is a matter for speculation, not firm conclusion. Long-term field studies of plant responses to elevated atmospheric CO₂ are needed. These will be expensive, difficult, and by definition, results will not be forthcoming for at least decades. Analyses of plants and ecosystems surrounding natural geological CO₂ degassing vents may provide the best surrogates for long-term controlled experiments, and therefore the most relevant information pertaining to long-term terrestrial-plant responses to elevated CO₂ concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO₂ concentrations near vents is limited. On the whole, terrestrial higher-plant responses to increasing atmospheric CO₂ concentration probably act as negative feedbacks on atmospheric CO₂ concentration increases, but they cannot by themselves stop the fossil-fuel-oxidation-driven increase in atmospheric CO₂ concentration. And, in the very long-term, atmospheric CO₂ concentration is controlled by atmosphere-ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO₂ concentration.     

Elevated CO2 and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Plants often respond to elevated atmospheric CO₂ levels with reduced tissue nitrogen concentrations relative to ambient CO₂-grown plants when comparisons are made at a common time. Another common response to enriched CO₂ atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO₂-grown plants made at a common time may not demonstrate CO₂-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO₂ environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C₃ photosynthetic pathway) and Amaranthus retroflexus (C₄ photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO₂ levels of 350 mol·mol^–1 or 700 mol·mol^–1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO₂-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO₂-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO₂ level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO₂ availability, but was altered by nutrient availability. These results indicate that a CO₂-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.     

沉水植物茎叶微界面特性研究进展

沉水植物茎叶-水界面是浅水湖泊的重要界面之一,对湖泊生物地球化学循环和水环境质量具有重要影响。富营养化水体中,大量的附着物常富集在沉水植物茎叶表面,形成了特殊的生物-水微界面。对该微界面特性进行深入研究,有助于揭示沉水植物在微环境层面对富营养化水体中物质循环的调控过程和机制。沉水植物茎叶微界面具有促进水体养分转化、改变环境因子及可溶性物质的空间分布,增加物质运输的阻力和距离、降低植物光合作用、调控重金属等生态功能;微界面结构及环境因子受水体营养盐浓度、沉水植物种类及生长阶段等因素的影响。对微界面结构功能的主要研究方法进行了分析总结,并对沉水植物茎叶微界面的研究前沿进行了展望。     

洱海沉水植物的光合特性与分布水深的关系

为研究沉水植物光合特性与其分布水深的关系,选取黑藻(Hydrilla verticillata)、苦草(Vallisneria natans)、水蕴草(Egeria densa)、大茨藻(Najas marina)、微齿眼子菜(Potamogeton maackianus)、光叶眼子菜(Potamogeton lucens)和穿叶眼子菜(Potamogeton perfoliatus)等15种洱海常见沉水植物,测定其光合作用参数。结果表明:光合速率为2.8—18.1μmol O₂/(g DW·h)、暗呼吸速率为0.3—2.0μmol O₂/(g DW·h)、光补偿点为6.3—63.8μE/(m²·s)、光饱和点为55.6—441.5μE/(m²·s),不同沉水植物间光合作用参数存在显著差异。结合洱海全湖沉水植物分布水深调查结果,沉水植物的光补偿点和光饱和点与分布水深呈显著负相关;苦草与其他物种比较具有更低的光补偿点6.3μE/(m²·s)、光饱和点55.6μE/(m^2...     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.     

Respiration and photosynthesis in cones of Norway spruce (Picea abies (L.) Karst.)

Summary Dark respiration and photosynthetic carbon dioxide refixation in purple and green Picea abies cones were investigated from budbreak to cone maturity. The rate of dark respiration per unit dry weight and CO₂ refixation capacity decreased during cone maturation. At the beginning of the growing season, photosynthetic CO₂ refixation could reduce the amount of CO₂ released by respiration in green and purple cones by 50% and 40%, respectively. The seasonal performance of the components of the cone carbon balance was calculated using information on the seasonal course of respiration, refixation capacity and the light response curves of cone photosynthesis, as well as the actual light and temperature regime in the field. The daily gain of CO₂ refixation reached 28%–34% of respiration in green and 22%–26% in purple cones during the first month of their growth, but decreased later in the season. Over the entire growth period refixation reduced carbon costs of cone production in both cone colour polymorphs by 16%–17%.     

Rising CO2 Levels Will Intensify Phytoplankton Blooms in Eutrophic and Hypertrophic Lakes

Harmful algal blooms threaten the water quality of many eutrophic and hypertrophic lakes and cause severe ecological and economic damage worldwide. Dense blooms often deplete the dissolved CO₂ concentration and raise pH. Yet, quantitative prediction of the feedbacks between phytoplankton growth, CO₂ drawdown and the inorganic carbon chemistry of aquatic ecosystems has received surprisingly little attention. Here, we develop a mathematical model to predict dynamic changes in dissolved inorganic carbon (DIC), pH and alkalinity during phytoplankton bloom development. We tested the model in chemostat experiments with the freshwater cyanobacterium Microcystis aeruginosa at different CO₂ levels. The experiments showed that dense blooms sequestered large amounts of atmospheric CO₂, not only by their own biomass production but also by inducing a high pH and alkalinity that enhanced the capacity for DIC storage in the system. We used the model to explore how phytoplankton blooms of eutrophic waters will respond to rising CO₂ levels. The model predicts that (1) dense phytoplankton blooms in low- and moderately alkaline waters can deplete the dissolved CO₂ concentration to limiting levels and raise the pH over a relatively wide range of atmospheric CO₂ conditions, (2) rising atmospheric CO₂ levels will enhance phytoplankton blooms in low- and moderately alkaline waters with high nutrient loads, and (3) above some threshold, rising atmospheric CO₂ will alleviate phytoplankton blooms from carbon limitation, resulting in less intense CO₂ depletion and a lesser increase in pH. Sensitivity analysis indicated that the model predictions were qualitatively robust. Quantitatively, the predictions were sensitive to variation in lake depth, DIC input and CO₂ gas transfer across the air-water interface, but relatively robust to variation in the carbon uptake mechanisms of phytoplankton. In total, these findings warn that rising CO₂ levels may result in a marked intensification of phytoplankton blooms in eutrophic and hypertrophic waters.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.     

Carbon Assimilation Characteristics of the Aquatic CAM Plant, Isoetes howellii

The relationship between malic acid production and carbon assimilation was examined in the submerged aquatic Crassulacean acid metabolism (CAM) plant, Isoetes howellii Engelmann. Under natural conditions free-CO₂ level in the water was highest at 0600 hours and ¹⁴CO₂ assimilation rates in I. howellii were also highest at this time. After 0900 hours there was a similar pattern in (a) rate of free-CO₂ depletion from the water, (b) reduction of carbon assimilation rates, and (c) rate of deacidification in leaves. Rates of daytime deacidification increased under CO₂-free conditions and as irradiance intensity increased. Nighttime CO₂ uptake was estimated to contribute one-third to one-half of the total daily gross carbon assimilation. CO₂ uptake, however, accounted for only one-third to one-half of the overnight malic acid accumulation. Internal respiratory CO₂ may be a substrate for a large portion of overnight acid accumulation as leaves incubated overnight without CO₂ accumulated substantial levels of malic acid. Loss of CAM occurred in emergent leaf tips even though submerged bases continued CAM. Associated with loss of CAM in aerial leaves was an increase in total chlorophyll, a/b ratio, and carotenoids, and a decrease in leaf succulence. δ¹³C values of I. howellii were not clearly distinguishable from those for associated non-CAM submerged macrophytes.     

Effects of daytime carbon dioxide concentration on dark respiration in rice

Rising atmospheric carbon dioxide concentration ([CO₂]) has generated considerable interest in the response of agricultural crops to [CO₂]. The objectives of this study were to determine the effects of a wide range of daytime [CO₂] on dark respiration of rice (Oryza sativa L. cv. IR-30). Rice plants were grown season-long in naturally sunlit plant growth chambers in subambient (160 and 250), ambient (330), or super-ambient (500, 660 and 900 μmol CO₂ mol^?1 air) [CO₂] treatments. Canopy dark respiration, expressed on a ground area basis (R_d) increased with increasing [CO₂] treatment from 160 to 500 μmol mol^?1 treatments and was very similar among the superambient treatments. The trends in R_d over time and in response to increasing daytime [CO₂] treatment were associated with and similar to trends previously described for photosynthesis. Specific respiration rate (R_dw) decreased with time during the growing season and was higher in the subambient than the ambient and superambient [CO₂] treatments. This greater R_dw in the subambient [CO₂] treatments was attributed to a higher specific maintenance respiration rate and was associated with higher plant tissue nitrogen concentration.     

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Temperate forests of North America are thought to besignificant sinks of atmospheric CO₂. Wedeveloped a below-ground carbon (C) budget forwell-drained soils in Harvard Forest Massachusetts, anecosystem that is storing C. Measurements of carbonand radiocarbon (¹⁴C) inventory were used todetermine the turnover time and maximum rate ofCO₂ production from heterotrophic respiration ofthree fractions of soil organic matter (SOM):recognizable litter fragments (L), humified lowdensity material (H), and high density ormineral-associated organic matter (M). Turnover timesin all fractions increased with soil depth and were2–5 years for recognizable leaf litter, 5–10 years forroot litter, 40–100+ years for low density humifiedmaterial and >100 years for carbon associated withminerals. These turnover times represent the timecarbon resides in the plant + soil system, and mayunderestimate actual decomposition rates if carbonresides for several years in living root, plant orwoody material.Soil respiration was partitioned into two componentsusing ¹⁴C: recent photosynthate which ismetabolized by roots and microorganisms within a yearof initial fixation (Recent-C), and C that is respiredduring microbial decomposition of SOM that resides inthe soil for several years or longer (Reservoir-C).For the whole soil, we calculate that decomposition ofReservoir-C contributes approximately 41% of thetotal annual soil respiration. Of this 41%,recognizable leaf or root detritus accounts for 80%of the flux, and 20% is from the more humifiedfractions that dominate the soil carbon stocks.Measurements of CO₂ and ¹⁴CO₂ in thesoil atmosphere and in total soil respiration werecombined with surface CO₂ fluxes and a soil gasdiffusion model to determine the flux and isotopicsignature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cmof the soil (O + A + Ap horizons). The average residencetime of Reservoir-C in the plant + soil system is8±1 years and the average age of carbon in totalsoil respiration (Recent-C + Reservoir-C) is 4±1years.The O and A horizons have accumulated 4.4 kgC m^–2above the plow layer since abandonment by settlers inthe late-1800's. C pools contributing the most to soilrespiration have short enough turnover times that theyare likely in steady state. However, most C is storedas humified organic matter within both the O and Ahorizons and has turnover times from 40 to 100+ yearsrespectively. These reservoirs continue to accumulatecarbon at a combined rate of 10–30 gC m^{minus 2}yr^–1. This rate of accumulation is only 5–15% of the total ecosystem C sink measured in this stand using eddy covariance methods.     

Effects of phytoplankton on the distribution of submerged macrophytes in a small canal

Submerged macrophytes have been disappearing from the Kanto Plain, Japan since the 1960s. This disappearance is usually attributable to the interaction between macrophytes and phytoplankton. Phytoplankton contributes to shading of the available light and changes the availability of inorganic carbon from free CO₂ to HCO ₃ ^? for use in photosynthesis. However, limited information is available about the interaction between carbon fraction and submerged macrophytes through phytoplankton abundance. In this short note, we observe the distribution of submerged macrophytes and phytoplankton in a small canal. We found that, despite high photosynthetically active radiation (PAR) in the downstream region, low free CO₂ concentration through phytoplankton abundance can deplete free CO₂ for submerged macrophytes. In contrast, the upstream region exhibited macrophytes in an environment with high free CO₂ concentration. The stable carbon isotope ratio of submerged macrophytes follows this pattern, with more positive values occurring in the downstream region and more negative values in the upstream region. It has been reported that phytoplankton limits light availability for submerged macrophytes, but carbon availability could also be a factor in the distribution of submerged macrophytes. Because the source of water for submerged macrophytes is groundwater, its preservation possibly plays a key role for the restoration of submerged macrophytes.     

Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2 and temperature regimes

Increases in the concentration of atmospheric carbon dioxide may have a fertilizing effect on plant growth by increasing photosynthetic rates and therefore may offset potential growth decreases caused by the stress associated with higher temperatures and lower precipitation. However, plant growth is determined both by rates of net photosynthesis and by proportional allocation of fixed carbon to autotrophic tissue and heterotrophic tissue. Although CO₂ fertilization may enhance growth by increasing leaf-level assimilation rates, reallocation of biomass from leaves to stems and roots in response to higher concentrations of CO₂ and higher temperatures may reduce whole-plant assimilation and offset photosynthetic gains. We measured growth parameters, photosynthesis, respiration, and biomass allocation of Pinus ponderosa seedlings grown for 2 months in 2×2 factorial treatments of 350 or 650 bar CO₂ and 10/25° C or 15/30° C night/day temperatures. After 1 month in treatment conditions, total seedling biomass was higher in elevated CO₂, and temperature significantly enhanced the positive CO₂ effect. However, after 2 months the effect of CO₂ on total biomass decreased and relative growth rates did not differ among CO₂ and temperature treatments over the 2-month growth period even though photosynthetic rates increased 7% in high CO₂ treatments and decreased 10% in high temperature treatments. Additionally, CO₂ enhancement decreased root respiration and high temperatures increased shoot respiration. Based on CO₂ exchange rates, CO₂ fertilization should have increased relative growth rates (RGR) and high temperatures should have decreased RGR. Higher photosynthetic rates caused by CO₂ fertilization appear to have been mitigated during the second month of exposure to treatment conditions by a 3% decrease in allocation of biomass to leaves and a 9% increase in root:shoot ratio. It was not clear why diminished photosynthetic rates and increased respiration rates at high temperatures did not result in lower RGR. Significant diametrical and potentially compensatory responses of CO₂ exchange and biomass allocation and the lack of differences in RGR of ponderosa pine after 2 months of exposure of high CO₂ indicate that the effects of CO₂ fertilization and temperature on whole-plant growth are determined by complex shifts in biomass allocation and gas exchange that may, for some species, maintain constant growth rates as climate and atmospheric CO₂ concentrations change. These complex responses must be considered together to predict plant growth reactions to global atmospheric change, and the potential of forest ecosystems to sequester larger amounts of carbon in the future.     

Effects of substrate and shading on the growth of two submerged macrophytes

Excessive nutrient loading may cause a shift from submerged macrophyte dominance to free-floating macrophyte dominance. Tolerance and persistence of submerged plants in response to shade may be key characteristics in determining when/if such a shift occurs in shallow eutrophic lakes. This study examines how the cover of floating macrophyte (Lemna minor) and shade of dark mesh affect the growth and photosynthetic efficiency of two submerged plants (Vallisneria natans and Myriophyllum spicatum) on different nutrient substrates. We found that low- and mid-cover intensities generally enhanced the leaf/shoot growth of both submerged plants under all cover and substrate types. The relative growth rates (RGR) were slightly enhanced under the treatment of Lemna with low- and mid-intensity cover on both nutrient-rich substrates. The leaf/shoot growth and RGR of both submerged macrophytes generally increased more under Lemna cover than mesh cover. The photosynthetic efficiency (F _v/F _m value) typically increased with the duration of treatment and the cover densities. In addition, these two macrophytes with contrasting growth forms have markedly different growth and survival strategies in response to covers. These results strengthen the hypothesis that submerged plants can successfully develop under a low-intensity cover of floating vegetation on nutrient-rich substrate.     

Atmospheric CO2 mole fraction affects stand‐scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon‐use‐efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂. Sunflower stands were grown at low (200 μmol mol^?1) or high CO₂ (1000 μmol mol^?1) in controlled environment mesocosms. CUE of stands was measured by dynamic stand‐scale ¹³C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand‐scale respiratory metabolism at high CO₂. Two main processes contributed to the reduction of CUE at high CO₂: a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.     

Rapidly diminishing mangrove forests in Myanmar (Burma): a review

IPCC predictions indicate an increase in temperatures by 1.5–7°C in some Amazonian regions during the twenty-first century. These changes could disrupt the present distribution patterns of organisms, including wetland plant species. In this work, we determined in microcosms the effects of scenarios combining elevated temperature and atmospheric CO₂ concentration on the germination and initial growth of the arborescent Amazonian aquatic macrophyte Montrichardia arborescens. Seeds were germinated, and seedlings produced were monitored over a 5-month period in four microcosms: Control: ambient temperature and CO₂ level; Mild: Control + 1.5°C and + 200 ppm CO₂; intermediate: control + 2.5°C and + 400 ppm CO₂; Extreme: Control + 4.5°C and + 850 ppm of CO₂. Rapid light response curves and Fv/Fm values taken in seedlings showed a decrease in electron transportation rate with CO₂ and temperature elevation. Mild and Intermediate treatments stimulated biomass production; Extreme treatment and Control produced similar results. The severe climatic changes expected in the future may negatively influence carbon accumulation in M. arborescens. Since aquatic macrophytes in Amazonian wetlands and wetlands worldwide are key plant species, further studies are needed to predict their fate in a global change perspective.