Predicting the impact of climatic warming on the carbon balance of the moss Sanionia uncinata on a maritime Antarctic island

The effects of climatic factors, especially those of temperature, on the carbon balance of the moss Sanionia uncinata were examined on King George Island in the maritime Antarctic. Net photosynthesis (P(n)) and dark respiration rates of two colonies (A and B) were measured with a portable infrared gas analyzer. Colony A showed small P(n) compared with its dark respiration rates throughout the growing season. Colony B showed much higher net photosynthetic rates, but the dark respiration rates of the two colonies did not differ significantly. Net photosynthetic rate determined at light saturation was almost constant over a wide temperature range, from 5 degrees to 15 degrees C, while the dark respiration was strongly affected by temperature. To assess the impact of warming on the carbon balance of the moss, cumulative carbon gain of the moss was calculated using a simulation model for the main part of the growing season. The results suggest that climatic warming may cause a reduction of carbon gain in some relatively photosynthetically inactive moss colonies.     

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%.     

Seasonal patterns of CO2 and water vapor exchange of three salt marsh succulents

Summary Diurnal carbon dioxide exchange patterns of three salt marsh succulents, Borrichia frutescens, Batis maritima and Salicornia virginica, were determined on a seasonal basis in the marsh at Sapelo Island, Georgia. Year-round photosynthetic activity was observed in these species though winter rates of CO₂ exchange were reduced. Net primary productivity, estimated using gas exchange techniques, agreed with previously reported harvest data. The role of light and temperature in the control of seasonal photosynthetic changes was investigated. A similar variation in light utilization with season was found in all three species, while seasonal temperature acclimation was species dependent. Less than 20% of fixed CO₂ was lost through dark respiration in any of the species.Water use in the salt marsh succulents was found to be relatively inefficient. High rates of transpiration were observed both summer and winter in the succulents.The succulents were judged to be C₃ plants on the basis of several criteria.Contribution No. 391 from the University of Georgia Marine Institute     

Field measurement of net photosynthesis of mosses at Langhovde,East Antarctica

Net photosynthesis and dark respiration (CO₂ flux) of Antarctic mosses were measured at Langhovde, East Antarctica, from 9 to 17 January 1988. Moss blocks were taken from communities in the Yukidori Valley (69°14′30″S, 39°46′00″E) at Langhovde. Each block was composed ofCeratodon purpureus andBryum pseudotriquetrum, orB. pseudotriquetrum. The upper part of the block was used to measure net photosynthesis and dark respiration. The net photosynthesis of each sample was measured in the field for one or three days with two infrared CO₂ gas analyzers and an assimilation chamber. The relationships of net photosynthetic rate and dark respiration rate, to the water content of the sample, the intensity of solar radiation and the moss temperature were estimated from the field data. The maximum rate of net photosynthesis was about 4 μmol CO₂ m⁻²s⁻¹ at saturating radiation intensity and at optimum temperature, about 10°C. Environmental features of moss habitats in the Yukidori Valley are discussed in relation to these results.     

Sensitivity analysis of ecosystem CO<Subscript>2</Subscript> exchange to climate change in High Arctic tundra using an ecological process-based model

Arctic terrestrial ecosystems are extremely vulnerable to climate change. A major concern is how the carbon balance of these ecosystems will respond to climate change. In this study, we constructed a simple ecological process-based model to assess how the carbon balance will be altered by ongoing climate change in High Arctic tundra ecosystems using in situ observations of carbon cycle processes. In particular, we simulated stand-level photosynthesis, root respiration, heterotrophic respiration, and hence net ecosystem production (NEP) of a plant community dominated by vascular plants and mosses. Analyses were carried out for current and future temperature and precipitation conditions. Our results showed that the tundra ecosystem was a CO₂ sink (NEP of 2.3–18.9 gC m^?2 growing season^?1) under present temperature conditions. Under rising temperature (2–6 °C), carbon gain is significantly reduced, but a few days’ extension of the foliage period caused by their higher temperatures compensated for the negative effect of temperature on NEP. Precipitation is the major environmental factor driving photosynthetic productivity of mosses, but it had a minor influence on community-level NEP. However, NEP decreased by a maximum 15.3 gC m^?2 growing season^?1 under a 30-day prolongation of the moss-growing season, suggesting that growing season extension had a negative effect on ecosystem carbon gain, because of poorer light conditions in autumn. Because the growing season creates a weak CO₂ sink at present, lengthening of the snow-free season coupled with rising temperature could seriously affect the future carbon balance of this Arctic tundra ecosystem.     

Arctic browning: Impacts of extreme climatic events on heathland ecosystem CO2 fluxes

Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as “Arctic browning.” These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO₂ balance. However, there is little understanding of the impacts on CO₂ fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO₂ fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO₂ fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (R_eco) and soil respiration (R_soil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO₂ fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO₂ balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO₂ fluxes. The scale of reductions in ecosystem CO₂ uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO₂ balance will respond to continuing climate change.     

Predicting ecosystem carbon balance in a warming Arctic: the importance of long‐term thermal acclimation potential and inhibitory effects of light on respiration

The carbon balance of Arctic ecosystems is particularly sensitive to global environmental change. Leaf respiration (R), a temperature‐dependent key process in determining the carbon balance, is not well‐understood in Arctic plants. The potential for plants to acclimate to warmer conditions could strongly impact future global carbon balance. Two key unanswered questions are (1) whether short‐term temperature responses can predict long‐term respiratory responses to growth in elevated temperatures and (2) to what extent the constant daylight conditions of the Arctic growing season inhibit leaf respiration. In two dominant Arctic species E riophorum vaginatum (tussock grass) and B etula nana (woody shrub), we assessed the extent of respiratory inhibition in the light (R _L/R _D), respiratory response to short‐term temperature change, and respiratory acclimation to long‐term warming treatments. We found that R of both species is strongly inhibited by light (averaging 35% across all measurement temperatures). In E . vaginatum both R _L and R _D acclimated to the long‐term warming treatment, reducing the magnitude of respiratory response relative to the short‐term response to temperature increase. In B . nana, both R _L and R _D responded to short‐term temperature increase but showed no acclimation to the long‐term warming. The ability to predict plant respiratory response to global warming with short‐term temperature responses will depend on species‐specific acclimation potential and the differential response of R _L and R _D to temperature. With projected woody shrub encroachment in Arctic tundra and continued warming, changing species dominance between these two functional groups, may impact ecosystem respiratory response and carbon balance.     

A hemiparasite in the forest understorey: photosynthetic performance and carbon balance of Melampyrum pratense

• Melampyrum pratense is an annual root‐hemiparasitic plant growing mostly in forest understorey, an environment with unstable light conditions. While photosynthetic responses of autotrophic plants to variable light conditions are in general well understood, light responses of root hemiparasites have not been investigated.
• We carried out gas exchange measurements (light response and photosynthetic induction curves) to assess the photosynthetic performance of M. pratense in spring and summer. These data and recorded light dynamics data were subsequently used to model carbon balance of the hemiparasite throughout the entire growth season.
• Summer leaves had significantly lower rates of saturated photosynthesis and dark respiration than spring leaves, a pattern expected to reflect the difference between sun‐ and shade‐adapted leaves. However, even the summer leaves of the hemiparasite exhibited a higher rate of light‐saturated photosynthesis than reported in non‐parasitic understorey herbs. This is likely related to its annual life history, rare among other understorey herbs. The carbon balance model considering photosynthetic induction still indicated insufficient autotrophic carbon gain for seed production in the summer months due to limited light availability and substantial carbon loss through dark respiration.
• The results point to potentially high importance of heterotrophic carbon acquisition in M. pratense, which could be of at least comparable importance as in other mixotrophic plants growing in forests – mistletoes and partial mycoheterotrophs. It is remarkable that despite apparent evolutionary pressure towards improved carbon acquisition from the host, M. pratense retains efficient photosynthesis and high transpiration rate, the ecophysiological traits typical of related root hemiparasites in the Orobanchaceae.

     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Thermal acclimation of photosynthesis: on the importance of adjusting our definitions and accounting for thermal acclimation of respiration

While interest in photosynthetic thermal acclimation has been stimulated by climate warming, comparing results across studies requires consistent terminology. We identify five types of photosynthetic adjustments in warming experiments: photosynthesis as measured at the high growth temperature, the growth temperature, and the thermal optimum; the photosynthetic thermal optimum; and leaf-level photosynthetic capacity. Adjustments of any one of these variables need not mean a concurrent adjustment in others, which may resolve apparently contradictory results in papers using different indicators of photosynthetic acclimation. We argue that photosynthetic thermal acclimation (i.e., that benefits a plant in its new growth environment) should include adjustments of both the photosynthetic thermal optimum (T _opt) and photosynthetic rates at the growth temperature (A _growth), a combination termed constructive adjustment. However, many species show reduced photosynthesis when grown at elevated temperatures, despite adjustment of some photosynthetic variables, a phenomenon we term detractive adjustment. An analysis of 70 studies on 103 species shows that adjustment of T _opt and A _growth are more common than adjustment of other photosynthetic variables, but only half of the data demonstrate constructive adjustment. No systematic differences in these patterns were found between different plant functional groups. We also discuss the importance of thermal acclimation of respiration for net photosynthesis measurements, as respiratory temperature acclimation can generate apparent acclimation of photosynthetic processes, even if photosynthesis is unaltered. We show that while dark respiration is often used to estimate light respiration, the ratio of light to dark respiration shifts in a non-predictable manner with a change in leaf temperature.     

Interactions between inorganic phosphate (Pi) assimilation,photosynthesis and respiration in the Pi-limited green alga Selenastrum minutum*

Inorganic phosphate (P_i)-limited chemostat cultures of the green alga Selenastrum minutum were employed to investigate interactions between P_i assimilation, respiration and photosynthetic processes. Changes in net and gross gas exchange rates indicated that O₂ evolution decreases during photosynthetic P_i assimilation. Room temperature and 77K Chi a fluorescence measurements revealed that this photosynthetic suppression is correlated with a transition from state 1 to state 2. Substantial photosynthetic P_i uptake rates occur in the presence of DCMU and KCN. Additionally, the cellular ratio of ATP:NADPH increases following P_i enrichment, suggesting that the ratio of cyclic to linear electron flow is enhanced in response to the high energy requirements of P_i uptake. Net starch degradation was observed during photosynthetic P_i assimilation and the cellular pool size of 3-phosphoglycerate increased; however, gross gas exchange parameters and cellular metabolite pool sizes indicated that mitochondrial respiration plays a smaller role during P_i assimilation in the light than it does in the dark. These observations were used to formulate a model depicting possible interactions between photosynthetic electron flow, photosynthetic and respiratory carbon metabolism and metabolite exchange between the chloroplast, cytosol and mitochondrion during photosynthetic P_i assimilation.     

Evaluating the carbon balance estimate from an automated ground‐level flux chamber system in artificial grass mesocosms

Measuring and modeling carbon (C) stock changes in terrestrial ecosystems are pivotal in addressing global C‐cycling model uncertainties. Difficulties in detecting small short‐term changes in relatively large C stocks require the development of robust sensitive flux measurement techniques. Net ecosystem exchange (NEE) ground‐level chambers are increasingly used to assess C dynamics in low vegetation ecosystems but, to date, have lacked formal rigorous field validation against measured C stock changes. We developed and deployed an automated and multiplexed C‐flux chamber system in grassland mesocosms in order rigorously to compare ecosystem total C budget obtained using hourly C‐flux measurements versus destructive net C balance. The system combines transparent NEE and opaque respiration chambers enabling partitioning of photosynthetic and respiratory fluxes. The C‐balance comparison showed good agreement between the two methods, but only after NEE fluxes were corrected for light reductions due to chamber presence. The dark chamber fluxes allowed assessing temperature sensitivity of ecosystem respiration (R_eco) components (i.e., heterotrophic vs. autotrophic) at different growth stages. We propose that such automated flux chamber systems can provide an accurate C balance, also enabling pivotal partitioning of the different C‐flux components (e.g., photosynthesis and respiration) suitable for model evaluation and developments.     

Modelled net carbon gain responses to climate change in boreal trees: Impacts of photosynthetic parameter selection and acclimation

Boreal forests are crucial in regulating global vegetation‐atmosphere feedbacks, but the impact of climate change on boreal tree carbon fluxes is still unclear. Given the sensitivity of global vegetation models to photosynthetic and respiration parameters, we determined how predictions of net carbon gain (C‐gain) respond to variation in these parameters using a stand‐level model (MAESTRA). We also modelled how thermal acclimation of photosynthetic and respiratory temperature sensitivity alters predicted net C‐gain responses to climate change. We modelled net C‐gain of seven common boreal tree species under eight climate scenarios across a latitudinal gradient to capture a range of seasonal temperature conditions. Physiological parameter values were taken from the literature together with different approaches for thermally acclimating photosynthesis and respiration. At high latitudes, net C‐gain was stimulated up to 400% by elevated temperatures and CO₂ in the autumn but suppressed at the lowest latitudes during midsummer under climate scenarios that included warming. Modelled net C‐gain was more sensitive to photosynthetic capacity parameters (V_cmax, J_max, Arrhenius temperature response parameters, and the ratio of J_max to V_cmax) than stomatal conductance or respiration parameters. The effect of photosynthetic thermal acclimation depended on the temperatures where it was applied: acclimation reduced net C‐gain by 10%–15% within the temperature range where the equations were derived but decreased net C‐gain by 175% at temperatures outside this range. Thermal acclimation of respiration had small, but positive, impacts on net C‐gain. We show that model simulations are highly sensitive to variation in photosynthetic parameters and highlight the need to better understand the mechanisms and drivers underlying this variability (e.g., whether variability is environmentally and/or biologically driven) for further model improvement.     

Leaf photosynthesis and simulated carbon budget of Gentiana straminea from a decade-long warming experiment

Aims Alpine ecosystems may experience larger temperature increases due to global warming as compared with lowland ecosystems. Information on physiological adjustment of alpine plants to temperature changes can provide insights into our understanding how these plants are responding to current and future warming. We tested the hypothesis that alpine plants would exhibit acclimation in photosynthesis and respiration under long-term elevated temperature, and the acclimation may relatively increase leaf carbon gain under warming conditions.Methods Open-top chambers (OTCs) were set up for a period of 11 years to artificially increase the temperature in an alpine meadow ecosystem. We measured leaf photosynthesis and dark respiration under different light, temperature and ambient CO₂ concentrations for Gentiana straminea, a species widely distributed on the Tibetan Plateau. Maximum rates of the photosynthetic electron transport (J max), RuBP carboxylation (V c max) and temperature sensitivity of respiration Q 10 were obtained from the measurements. We further estimated the leaf carbon budget of G. straminea using the physiological parameters and environmental variables obtained in the study.Important findings1)?The OTCs consistently elevated the daily mean air temperature by ~1.6°C and soil temperature by ~0.5°C during the growing season. 2)?Despite the small difference in the temperature environment, there was strong tendency in the temperature acclimation of photosynthesis. The estimated temperature optimum of light-saturated photosynthetic CO₂ uptake (A max) shifted ~1°C higher from the plants under the ambient regime to those under the OTCs warming regime, and the A max was significantly lower in the warming-acclimated leaves than the leaves outside the OTCs. 3)?Temperature acclimation of respiration was large and significant: the dark respiration rates of leaves developed in the warming regime were significantly lower than leaves from the ambient environments. 4)?The simulated net leaf carbon gain was significantly lower in the in situ leaves under the OTCs warming regime than under the ambient open regime. However, in comparison with the assumed non-acclimation leaves, the in situ warming-acclimated leaves exhibited significantly higher net leaf carbon gain. 5)?The results suggest that there was a strong and significant temperature acclimation in physiology of G. straminea in response to long-term warming, and the physiological acclimation can reduce the decrease of leaf carbon gain, i.e. increase relatively leaf carbon gain under the warming condition in the alpine species.     

Microclimate and production of peat moss Sphagnum palustre L. in the warm‐temperate zone

Sphagnum palustre L. is one of the few Sphagnum species distributed in the warm‐temperate zone. To elucidate the mechanisms that enable S. palustre to maintain its productivity under warm climatic conditions, we examined the temperature conditions and photosynthetic characteristics of this species in a lowland wetland in western Japan. Moss temperatures during the daytime were much lower than the air temperature, particularly during summer. The optimum temperature for the net photosynthetic rate was approximately 20°C, irrespective of the season, but summer and autumn samples maintained high rates at higher temperatures as well. The net photosynthetic rate at near light saturation was much higher during summer–autumn than during spring–winter. A model estimation in which net production was calculated from the photosynthetic characteristics and microclimatic data showed that both the low temperature of the moss colony and the seasonal shift in photosynthetic characteristics are among the mechanisms that enable this species to maintain its productivity under warm climatic conditions.     

Effects of an Experimental Increase of Temperature and Drought on the Photosynthetic Performance of Two Ericaceous Shrub Species Along a North–South European Gradient

Plant ecophysiological changes in response to climatic change may be different in northern and southern European countries because different abiotic factors constrain plant physiological activity. We studied the effects of experimental warming and drought on the photosynthetic performance of two ericaceous shrubs (Erica multiflora and Calluna vulgaris) along a European gradient of temperature and precipitation (UK, Denmark, The Netherlands, and Spain). At each site, a passive warming treatment was applied during the night throughout the whole year, whereas the drought treatment excluded rain events over 6–10 weeks during the growing season. We measured leaf gas exchange, chlorophyll a fluorescence, and leaf carbon isotope ratio (¹³C) during the growing seasons of 1999 and 2000. Leaf net photosynthetic rates clearly followed a gradient from northern to southern countries in agreement with the geographical gradient in water availability. Accordingly, there was a strong correlation between net photosynthetic rates and the accumulated rainfall over the growing season. Droughted plants showed lower leaf gas exchange rates than control plants in the four sites. Interestingly, although leaf photosynthetic rates decreased along the precipitation gradient and in response to drought treatment, droughted plants were able to maintain higher leaf photosynthetic rates than control plants in relation to the accumulated rainfall over the months previous to the measurements. Droughted plants also showed higher values of potential photochemical efficiency (F _v/F _m) in relation to controls, mainly at midday. The warming treatment did not affect significantly any of the studied instantaneous ecophysiological variables..     

Landscape and Ecosystem-Level Controls on Net Carbon Dioxide Exchange along a Natural Moisture Gradient in Canadian Low Arctic Tundra

Our understanding of the controls and magnitudes of regional CO₂ exchanges in the Arctic are limited by uncertainties due to spatial heterogeneity in vegetation across the landscape and temporal variation in environmental conditions through the seasons. We measured daytime net ecosystem CO₂ exchange and each of its component fluxes in the three major tundra ecosystem-types that typically occur along natural moisture gradients in the Canadian Low Arctic biweekly during the full snow-free season of 2004. In addition, we used a plant-removal treatment to compare the contribution of bulk soil organic matter to total respiratory CO₂ loss among these ecosystems. Net CO₂ exchange rates varied strongly, but not consistently, among ecosystems in the spring and summer phases as a result of ecosystem-specific and differing responses of gross photosynthesis and respiration to temporal variation in environmental conditions. Overall, net carbon gain was largest in the wet sedge ecosystem and smallest in the dry heath. Our measures of CO₂ flux variation within each ecosystem were frequently most closely correlated with air or soil temperatures during each seasonal phase. Nevertheless, a particularly large rainfall event in early August rapidly decreased respiration rates and stimulated gross photosynthetic rates, resulting in peak rates of net carbon gain in all ecosystems. Finally, the bulk soil carbon contribution to total respiration was relatively high in the birch hummock ecosystem. Together, these results demonstrate that the relative influences of moisture and temperature as primary controls on daytime net ecosystem CO₂ exchange and its component fluxes differ in fundamental ways between the landscape and ecosystem scales. Furthermore, they strongly suggest that carbon cycling responses to environmental change are likely to be highly ecosystem-specific, and thus to vary substantially across the low arctic landscape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Thermal plasticity of photosynthesis: the role of acclimation in forest responses to a warming climate

The increasing air temperatures central to climate change predictions have the potential to alter forest ecosystem function and structure by exceeding temperatures optimal for carbon gain. Such changes are projected to threaten survival of sensitive species, leading to local extinctions, range migrations, and altered forest composition. This study investigated photosynthetic sensitivity to temperature and the potential for acclimation in relation to the climatic provenance of five species of deciduous trees, Liquidambar styraciflua, Quercus rubra, Quercus falcata, Betula alleghaniensis, and Populus grandidentata. Open‐top chambers supplied three levels of warming (+0, +2, and +4 °C above ambient) over 3 years, tracking natural temperature variability. Optimal temperature for CO₂ assimilation was strongly correlated with daytime temperature in all treatments, but assimilation rates at those optima were comparable. Adjustment of thermal optima was confirmed in all species, whether temperatures varied with season or treatment, and regardless of climate in the species' range or provenance of the plant material. Temperature optima from 17° to 34° were observed. Across species, acclimation potentials varied from 0.55 °C to 1.07 °C per degree change in daytime temperature. Responses to the temperature manipulation were not different from the seasonal acclimation observed in mature indigenous trees, suggesting that photosynthetic responses should not be modeled using static temperature functions, but should incorporate an adjustment to account for acclimation. The high degree of homeostasis observed indicates that direct impacts of climatic warming on forest productivity, species survival, and range limits may be less than predicted by existing models.     

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.     

Photosynthetic responses of C3 and C4 species from cool shaded habitats in Hawaii

Summary The C₄ species, Euphorbia forbesii, and the C₃ species, Claoxylon sandwicense, occupy cool, shaded habitats in Hawaii. Both of these species exhibit the photosynthetic characteristics of typical shade plants: low light-saturated photosynthetic rates, low dark respiration rates, low light levels for saturation of photosynthesis, and low light compensation points. In addition, the quantum yields of the two species are similar at leaf temperatures near 22°C, reflecting a significant increase in the quantum yield of E. forbesii over that of C₄ species from open habitats. C. sandwicense has a lower dark respiration rate than E. forbesii. Hence, since the quantum yields of the two species are similar at cool temperatures, C. sandwicense has a higher photosynthetic rate than E. forbesii at low incident photon flux densities. As a consequence, C. sandwicense should have a greater carbon gain than E. forbesii under the diffuse radiation conditions of their native habitat. However, since E. forbesii has a higher light-saturated photosynthetic rate than C. sandwicense, E. forbesii may have a greater carbon gain than C. sandwicense during sunflecks.