Reduction In Carbon Gain Research Articles

A moving target: trade-offs between maximizing carbon and minimizing hydraulic stress for plants in a changing climate.

Observational evidence indicates that tree leaf area may acclimate in response to changes in water availability to alleviate hydraulic stress. However, the underlying mechanisms driving leaf area changes and consequences of different leaf area allocation strategies remain unknown. Here, we use a trait-based hydraulically enabled tree model with two endmember leaf area allocation strategies, aimed at either maximizing carbon gain or moderating hydraulic stress. We examined the impacts of these strategies on future plant stress and productivity. Allocating leaf area to maximize carbon gain increased productivity with high CO2, but systematically increased hydraulic stress. Following an allocation strategy to avoid increased future hydraulic stress missed out on 26% of the potential future net primary productivity in some geographies. Both endmember leaf area allocation strategies resulted in leaf area decreases under future climate scenarios, contrary to Earth system model (ESM) predictions. Leaf area acclimation to avoid increased hydraulic stress (and potentially the risk of accelerated mortality) was possible, but led to reduced carbon gain. Accounting for plant hydraulic effects on canopy acclimation in ESMs could limit or reverse current projections of future increases in leaf area, with consequences for the carbon and water cycles, and surface energy budgets.

Novel small molecules targeting bZIP23 TF improve stomatal conductance and photosynthesis under mild drought stress by regulating ABA.

Drought-induced abscisic acid (ABA) accumulation plays a key role in plant water relations by regulating stomatal movements. Although ABA helps in the survival of the plants, reduced carbon gain affects plant productivity. To improve crop productivity under mild drought stress conditions, it is necessary to manipulate ABA responses. Other research groups have used forward chemical genomics for the identification of ABA agonists and antagonists aiming to manipulate ABA biosynthesis and signalling. In the present study, we identified indolyl-ethyl amine and serotonin small molecules using a reverse chemical genomics approach, with these acting as potent inhibitors of ABA biosynthesis through transient regulation of bZIP23 transcription factor activity. In rice, wheat and soybean, each of the small molecules enhanced the germination of seeds, even in the presence of ABA. These molecules nullified the effect of ABA on intact and detached leaves, resulting in higher photosynthesis. Furthermore, these small molecules effectively reduced the transcription levels of bZIP23 targeting NCED4, PP2C49 and CO3 genes. Rice plants treated with the small molecules were found to have improved stomatal conductance, spikelet fertility and yield compared to untreated plants under mild drought stress conditions. Our results suggest that indolyl-ethyl amine and serotonin small molecules could be utilized to improve yield under mild drought conditions.

Optimization theory explains nighttime stomatal responses.

Nocturnal transpiration is widely observed across species and biomes, and may significantly impact global water, carbon, and energy budgets. However, it remains elusive why plants lose water at night and how to model it at large scales. We hypothesized that plants optimize nighttime leaf diffusive conductance (gwn ) to balance potential daytime photosynthetic benefits and nocturnal transpiration benefits. We quantified nighttime benefits from respiratory reductions due to evaporative leaf cooling. We described nighttime costs in terms of a reduced carbon gain during the day because of water use at night. We measured nighttime stomatal responses and tested our model with water birch (Betula occidentalis) saplings grown in a glasshouse. The gwn of water birch decreased with drier soil, higher atmospheric CO2 , wetter air, lower leaf temperature, and lower leaf respiration rate. Our model predicted all these responses correctly, except for the response of gwn to air humidity. Our results also suggested that the slow decrease in gwn after sunset could be associated with decreasing leaf respiration. The optimality-based nocturnal transpiration model smoothly integrates with daytime stomatal optimization approaches, and thus has the potential to quantitatively predict nocturnal transpiration across space and time.

The potential effectiveness of four different options to reduce environmental impacts of grazed pastures. A model-based assessment

Pastoral agriculture can have negative environmental impacts. These include greenhouse gas emissions (such as enteric CH4 and soil N2O emissions), NH3 volatilisation, and leaching of nitrogen into waterways. We used a modelling approach to assess the effect of three plant traits and one management practice on N2O emissions, nitrogen (N) losses via leaching and NH3 volatilisation, pasture production and soil organic carbon (SOC) changes (as applicable). The aim was to identify traits/management practices that could potentially be environmentally beneficial and could then inform future research to find or breed plants with those traits.The traits and practice investigated were: 1) N content in animal feed; 2) plant-excreted nitrification inhibitors; 3) deep rooting; and 4) frequency of pasture renewal. Of these, the N content in animal feed provided the most promising results, with low N content resulting in lower urine N excretion, and consequently reduced leaching losses and emissions of N2O and NH3. Modelling the effect of plant-excreted nitrification inhibitors showed mixed results, with reduced leaching rates but increased NH3 emissions. N2O emissions could be reduced if nitrification rates were strongly reduced. However, at lower rates of inhibition, there was little effect on N2O emissions. In the deep rooting scenarios, we found that N leaching losses were minimised if roots grew predominantly in upper soil layers where any mineral N was likely to spend more time before being leached. Nitrogen was, therefore, more effectively intercepted and prevented from leaching by greater root proliferation in the upper rather than deeper layers. For pasture renewal, we unexpectedly found that SOC could be increased by more frequent pasture renewal. However, this would come at the cost of reduced pasture production as renewal events reduced grazing off-takes proportionately more than the reduction in carbon gain by pasture plants. A renewal period of about 10 years was found to be optimal for pasture production and could be maintained with minimal SOC changes.

Impacts of growth temperature, water deficit and heatwaves on carbon assimilation and growth of cotton plants (Gossypium hirsutum L.)

Increased variability in growing season climates continues to threaten the growth and yields of many crops. The impacts of individual climate stress conditions on crops has been documented frequently, yet how crops respond to multiple abiotic stress components is less well understood. Here, we report on the main and interactive effects of growth temperature, water deficit and a heatwave on leaf physiology and biomass production of cotton plants (Gossypium hirsutum L.). Plants were raised under two day/night growth temperature regimes (28/18 °C and 32/22 °C) and their corresponding nocturnal warming (+4 °C) scenarios (i.e. 28/22 °C and 32/26 °C). Following the emergence of the first square (flower bud), plants were subjected to two water treatments (well-watered and water deficit) until the beginning of the flowering stage, and then half of the plants in all temperature treatments were exposed to a 5-day heatwave treatment (40/26 °C). We found that elevated growth temperature increased growth rate (as defined by plant height) and leaf-level carbon gain, but decreased total aboveground biomass. Water deficit stress decreased leaf level carbon gain and biomass, but these impacts were generally less pronounced. Nocturnal warming moderately decreased leaf carbon gain for plants grown under the cool temperature regime (i.e. 28/18 °C), but not the warm temperature regime (i.e. 32/22 °C), and its impacts on biomass were also thermal regime specific. In contrast, leaf carbon gain was promoted by the heatwave under the cool daytime temperature treatment, but not the warm daytime temperature treatment. However, total aboveground biomass was less affected by the heatwave due to high resilience of gas exchange, although there was decreased fruit biomass. Overall, both short- and long-term increases in daytime temperature decreased cotton fruit biomass, while nocturnal warming had limited capacity to buffer that impact. Moderate soil water deficit will not strongly reduce carbon gain and growth. This study adds to the knowledge regarding the response of cotton plants to climate change and underscores the complexity of plant response to multiple environmental factors.

Extended leaf phenology has limited benefits for invasive species growing at northern latitudes

Many understory woody invasive plants in North America leaf out earlier or retain leaves later than their native associates. This extended leaf phenology is thought to grant invasive species an advantage over native species because spring and fall are crucial times for light access and carbon acquisition in understory habitats. However, it is unclear whether this advantage persists at northern latitudes where freezing temperatures constrain growing season length and low light levels reduce carbon gain. To investigate the costs and benefits of extended leaf phenology at northern latitudes, we observed leaf phenology, estimated total carbon gain, measured growth, and tested susceptibility to freezing temperatures for four native and four invasive woody shrubs in a disturbed forest in northern Minnesota, USA. We found that the invaders leafed out simultaneously with the natives in the spring but retained their leaves later in the autumn than native species. This extended fall phenology did not enable greater total carbon gain for the invaders because they assimilated less carbon earlier in the year than the natives. There was also no significant difference between native and invasive species in their susceptibility to freezing temperatures. Instead freezing tolerance related more to native range then leaf phenology. Our results suggest that freezing temperatures do not limit invasive species’ northern expansion and instead indicate that at the northern edge of their ranges, these species may lose any competitive advantage granted by extended leaf phenology over their native associates. This study demonstrates the importance of considering latitude and forest structure when investigating phenology and growth.

Daily heliotropic movements assist gas exchange and productive responses in DREB1A soybean plants under drought stress in the greenhouse

Drought stress is one of the most severe environmental constraints on plant production. Under environmental pressures, complex daily heliotropic adjustments of leaflet angles in soybean can help to reduce transpiration losses by diminishing light interception (paraheliotropism), increase diurnal carbon gain in sparse canopies and reduce carbon gain in dense canopies by solar tracking (diaheliotropism). The plant materials studied were cultivar BR 16 and its genetically engineered isoline P58, ectopically overexpressing AtDREB1A, which is involved in abiotic stress responses. We aimed to follow the movements of central and lateral leaflets in vegetative stages V7-V10 and reproductive stages R4-R5, integrating the reversible morphogenetic changes into an estimate of daily plant photosynthesis using three-dimensional modeling, and to analyze the production parameters of BR 16 and P58. The patterns of daily movements of central leaflets of BR 16 in V7-V10 and R4-R5 were similar, expressing fewer diaheliotropic movements under drought stress than under non-limiting water conditions. Daily heliotropic patterns of lateral leaflets in V7-V10 and R4-R5 showed more diaheliotropic movements in drought-stressed P58 plants than in those grown under non-limiting water conditions. Leaf area in R4-R5 was generally higher in P58 than in BR 16. Drought significantly affected gas exchange and vegetative and reproductive architectural features. DREB1A could be involved in various responses to drought stress. Compared with the parental BR 16, P58 copes with drought through better compensation between diaheliotropic and paraheliotropic movements, finer tuning of water-use efficiency, a lower transpiration rate, higher leaf area and higher pod abortion to accomplish the maximum possible grain production under continued drought conditions.

Three-dimensional plant architecture and sunlit-shaded patterns: a stochastic model of light dynamics in canopies.

Background and AimsDiurnal changes in solar position and intensity combined with the structural complexity of plant architecture result in highly variable and dynamic light patterns within the plant canopy. This affects productivity through the complex ways that photosynthesis responds to changes in light intensity. Current methods to characterize light dynamics, such as ray-tracing, are able to produce data with excellent spatio-temporal resolution but are computationally intensive and the resulting data are complex and high-dimensional. This necessitates development of more economical models for summarizing the data and for simulating realistic light patterns over the course of a day.MethodsHigh-resolution reconstructions of field-grown plants are assembled in various configurations to form canopies, and a forward ray-tracing algorithm is applied to the canopies to compute light dynamics at high (1 min) temporal resolution. From the ray-tracer output, the sunlit or shaded state for each patch on the plants is determined, and these data are used to develop a novel stochastic model for the sunlit–shaded patterns. The model is designed to be straightforward to fit to data using maximum likelihood estimation, and fast to simulate from.Key ResultsFor a wide range of contrasting 3-D canopies, the stochastic model is able to summarize, and replicate in simulations, key features of the light dynamics. When light patterns simulated from the stochastic model are used as input to a model of photoinhibition, the predicted reduction in carbon gain is similar to that from calculations based on the (extremely costly) ray-tracer data.ConclusionsThe model provides a way to summarize highly complex data in a small number of parameters, and a cost-effective way to simulate realistic light patterns. Simulations from the model will be particularly useful for feeding into larger-scale photosynthesis models for calculating how light dynamics affects the photosynthetic productivity of canopies.

Springtime photoinhibition constrains regeneration of forest floor seedlings of Abies sachalinensis after a removal of canopy trees during winter

A clear-cutting of canopy trees during winter often causes severe foliar damage during the following spring in forest floor seedlings of Abies sachalinensis, a typical shade-tolerant evergreen coniferous species. The maximum photochemical efficiency of photosystem II after an overnight dark adaptation showed a temporary decrease immediately before budbreak in 1-year-old shoots of A. sachalinensis seedlings grown under full sunlight in a nursery, suggesting “springtime photoinhibition” related to the phenology of evergreen coniferous species. In the field, a greater rate of canopy tree cutting during winter was associated with more severe photoinhibition in the following spring, immediately before budbreak, which subsequently resulted in a reduction in carbon gain in 1-year-old shoots, and consequently suppressed the growth of current-year shoots. Although photoinhibition under low temperature is a well-known factor to determine the survival rate of tree seedlings during winter in cool regions, the present study additionally proposes that the temporary increase in the susceptibility to photoinhibition in springtime i.e. “springtime photoinhibition” would be a constraint for the regeneration of coniferous seedlings especially when the canopy trees are removed during winter.

The Impacts of Fluctuating Light on Crop Performance.

Rapidly changing light conditions can reduce carbon gain and productivity in field crops because photosynthetic responses to light fluctuations are not instantaneous. Plant responses to fluctuating light occur across levels of organizational complexity from entire canopies to the biochemistry of a single reaction and across orders of magnitude of time. Although light availability and variation at the top of the canopy are largely dependent on the solar angle and degree of cloudiness, lower crop canopies rely more heavily on light in the form of sunflecks, the quantity of which depends mostly on canopy structure but also may be affected by wind. The ability of leaf photosynthesis to respond rapidly to these variations in light intensity is restricted by the relatively slow opening/closing of stomata, activation/deactivation of C3 cycle enzymes, and up-regulation/down-regulation of photoprotective processes. The metabolic complexity of C4 photosynthesis creates the apparently contradictory possibilities that C4 photosynthesis may be both more and less resilient than C3 to dynamic light regimes, depending on the frequency at which these light fluctuations occur. We review the current understanding of the underlying mechanisms of these limitations to photosynthesis in fluctuating light that have shown promise in improving the response times of photosynthesis-related processes to changes in light intensity.

Contrasting changes in above- and below-ground biomass allocation across treeline ecotones in southeast Tibet

Under conditions of a warmer climate, the advance of the alpine treeline into alpine tundra has implications for carbon dynamics in mountain ecosystems. However, the above- and below-ground live biomass allocations among different vegetation types within the treeline ecotones are not well investigated. To determine the altitudinal patterns of above-/below-ground carbon allocation, we measured the root biomass and estimated the above-ground biomass (AGB) in a subalpine forest, treeline forest, alpine shrub, and alpine grassland along two elevational transects towards the alpine tundra in southeast Tibet. The AGB strongly declined with increasing elevation, which was associated with a decrease in the leaf area index and a consequent reduction in carbon gain. The fine root biomass (FRB) increased significantly more in the alpine shrub and grassland than in the treeline forest, whereas the coarse root biomass changed little with increasing altitudes, which led to a stable below-ground biomass (BGB) value across altitudes. Warm and infertile soil conditions might explain the large amount of FRB in alpine shrub and grassland. Consequently, the root to shoot biomass ratio increased sharply with altitude, which suggested a remarkable shift of biomass allocation to root systems near the alpine tundra. Our findings demonstrate contrasting changes in AGB and BGB allocations across treeline ecotones, which should be considered when estimating carbon dynamics with shifting treelines.

Costs of leaf reinforcement in response to winter cold in evergreen species.

The competitive equilibrium between deciduous and evergreen plant species to a large extent depends on the intensity of the reduction in carbon gain undergone by evergreen leaves, associated with the leaf traits that confer resistance to stressful conditions during the unfavourable part of the year. This study explores the effects of winter harshness on the resistance traits of evergreen leaves. Leaf mass per unit area (LMA), leaf thickness and the concentrations of fibre, nitrogen (N), phosphorus (P), soluble protein, chlorophyll and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined in three evergreen and two deciduous species along a winter temperature gradient. In the evergreen species, LMA, thickness, and P and structural carbohydrate concentrations increased with the decrease in winter temperatures. Nitrogen and lignin concentrations did not show definite patterns in this regard. Chlorophyll, soluble proteins and Rubisco decreased with the increase in winter harshness. Our results suggest that an increase in LMA and in the concentration of structural carbohydrates would be a requirement for the leaves to cope with low winter temperatures. The evergreen habit would be associated with higher costs at cooler sites, because the cold resistance traits imply additional maintenance costs and reduced N allocation to the photosynthetic machinery, associated with structural reinforcement at colder sites.

High-Resolution Three-Dimensional Structural Data Quantify the Impact of Photoinhibition on Long-Term Carbon Gain in Wheat Canopies in the Field.

Photoinhibition reduces photosynthetic productivity; however, it is difficult to quantify accurately in complex canopies partly because of a lack of high-resolution structural data on plant canopy architecture, which determines complex fluctuations of light in space and time. Here, we evaluate the effects of photoinhibition on long-term carbon gain (over 1 d) in three different wheat (Triticum aestivum) lines, which are architecturally diverse. We use a unique method for accurate digital three-dimensional reconstruction of canopies growing in the field. The reconstruction method captures unique architectural differences between lines, such as leaf angle, curvature, and leaf density, thus providing a sensitive method of evaluating the productivity of actual canopy structures that previously were difficult or impossible to obtain. We show that complex data on light distribution can be automatically obtained without conventional manual measurements. We use a mathematical model of photosynthesis parameterized by field data consisting of chlorophyll fluorescence, light response curves of carbon dioxide assimilation, and manual confirmation of canopy architecture and light attenuation. Model simulations show that photoinhibition alone can result in substantial reduction in carbon gain, but this is highly dependent on exact canopy architecture and the diurnal dynamics of photoinhibition. The use of such highly realistic canopy reconstructions also allows us to conclude that even a moderate change in leaf angle in upper layers of the wheat canopy led to a large increase in the number of leaves in a severely light-limited state.

Physiological responses to soil lime in wild grapevine (Vitis vinifera ssp. sylvestris)

Lime-induced chlorosis is a widespread nutritional disorder affecting grapevines cultivated in calcareous soils. A greenhouse experiment was conducted to investigate the response of Vitis vinifera ssp. sylvestris to soil lime by evaluating the effects of a range of soil CaCO3 contents (0–60%) on plant growth, nutrient content (iron, potassium, nitrogen and phosphorus) and photosynthetic performance (gas exchange, chlorophyll fluorescence parameters and photosynthetic pigments). The highest soil CaCO3 concentration induced nutrient imbalances and significantly inhibited photosynthetic function, causing a reduction in carbon gain and consequently, a drastic growth reduction and high mortality. However, all the plants survived external CaCO3 contents of up to 40%, and reduction in growth at 20% CaCO3 was slightly lower than that recorded in several previously studied lime-tolerant varieties of grapevine. Plants grown at 20% CaCO3 maintained net photosynthesis values of around 6μmolm−2s−1, a similar chlorophyll content to that of the control plants and dawn Fv/Fm values close to the optimal values for unstressed plants. Up to the 40% CaCO3 treatment, the study species was capable of maintaining Fe uptake by the roots and translocation to leaves, while controlling the nutritional status of N and P. Our study indicates that the studied population of V. vinifera ssp. sylvestris could provide a source of genetic diversity for lime tolerance improvement in grapevine.

Effects of a three-year exposure to ambient ozone on biomass allocation in poplar using ethylenediurea

We examined the effect of ambient ozone on visible foliar injury, growth and biomass in field-grown poplar cuttings of an Oxford clone sensitive to ozone (Populus maximoviczii Henry × berolinensis Dippel) irrigated with ethylenediurea (EDU) or water for three years. EDU is used as an ozone protectant for plants. Protective effects of EDU on ozone visible injury were found. As a result, poplar trees grown under EDU treatment increased leaves, lateral branches and root density in the third year, although no significant enhancement of stem height and diameter was found. Ambient ozone (AOT40, 24.6 ppm h; diurnal hourly average, 40.3 ppb) may finally reduce carbon gain by reducing the number of branches, and thus sites for leaf formation, in ozone-sensitive poplar trees under not-limiting conditions.

Physiological responses to salinity in the yellow-horned poppy, Glaucium flavum

Glaucium flavum Crantz. is a short-lived perennial herb found in coastal habitats in southern Spain growing under a wide range of interstitial soil salinity levels, from that of fresh water up to the high concentration typical of sea water. An experiment was designed to investigate the effect of exposure to this range of salinity on the photosynthetic apparatus, growth and reproduction of G. flavum, by measuring relative growth rate, percentage of dead leaves, seed production, leaf relative water content, chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. We also determined total sodium, potassium, calcium, magnesium, and nitrogen concentrations. G. flavum survived at NaCl concentrations as high as 300mM, although the excess of NaCl resulted in a biomass reduction of between 26 and 76% (in 60 and 300mM NaCl treatments, respectively). The long-term effects of salinity on the growth and reproduction of G. flavum were mainly linked to an overall reduction in carbon gain as a result of stomatal conductance regulation. Also, the excess of salt caused a reduction in pigment concentrations, as well as Ca-, Mg- and N-uptake. The results indicate that, in the presence of excess soil-water salinity, G. flavum sustains little overall effects on the photochemical (PSII) apparatus, and is capable of tolerating a very high and continued exposure to salinity by maintaining low levels of net photosynthesis.

Foliage profiles of individual trees determine competition, self-thinning, biomass and NPP of a Cryptomeria japonica forest stand: A simulation study based on a stand-scale process-based forest model

A simulation study was carried out to investigate simultaneously the effects of eco-physiological parameters on competitive asymmetry, self-thinning, stand biomass and NPP in a temperate forest using an atmosphere–vegetation dynamics interactive model (MINoSGI). In this study, we selected three eco-physiological relevant parameters as foliage profiles (i.e. vertical distribution of leaf area density) of individual trees (distribution pattern is described by the parameter η), biomass allocation pattern in individual tree growth ( χ) and the maximum carboxylation velocity ( V max). The position of the maximal leaf area density shifts upward in the canopy with increasing η. For scenarios with η < 4 (foliage concentrated in the lowest canopy layer) or η > 12 (foliage concentrated in the uppermost canopy layer), a low degree of competitive asymmetry was produced. These scenarios resulted in the survival of subordinate trees due to a brighter lower canopy environment when η < 4 or the generation of spatially separated foliage profiles between dominant and subordinate trees when η > 12. In contrast, competition between trees was most asymmetric when 4 ≤ η ≤ 12 (vertically widespread foliage profile in the canopy), especially when η = 8. In such cases, vertically widespread foliage of dominant trees lowered the opportunity of light acquisition for subordinate trees and reduced their carbon gain. The resulting reduction in carbon gain of subordinate trees yielded a higher degree of competitive asymmetry and ultimately higher mortality of subordinate trees. It was also shown that 4 ≤ η ≤ 12 generated higher self-thinning speed, smaller accumulated NPP, litter-fall and potential stand biomass as compared with the scenarios with η < 4 or η > 12. In contrast, our simulation revealed small effects of χ or V max on the above-mentioned variables as compared with those of η. In particular, it is notable that greater V max would not produce greater potential stand biomass and accumulated NPP although it has been thought that physiological parameters relevant to photosynthesis such as V max influence dynamic changes in forest stand biomass and NPP (e.g. the greater the V max, the greater the NPP). Overall, it is suggested that foliage profiles rather than biomass allocation or maximum carboxylation velocity greatly govern forest dynamics, stand biomass, NPP and litter-fall.

Christopher Paul Varty

<h3>Abstract</h3> <h3>Background and Aims</h3> Diurnal changes in solar position and intensity combined with the structural complexity of plant architecture result in highly variable and dynamic light patterns within the plant...

Modelling net ecosystem carbon and water exchange of a temperate Eucalyptus delegatensis forest using multiple constraints

This study examined the carbon budget of a Eucalyptus delegatensis forest over four years under contrasting weather conditions by using a comprehensive forest-growth model (CenW 3.1). Model parameterisation was constrained through multiple measurements, including daytime eddy flux measurements of CO 2 and water vapour exchange, estimates of above- and below-ground biomass pools, growth rates measured through stem-diameter increments and measurement of specific ecosystem processes, such as litter fall and soil respiration rates. The resultant estimates of net ecosystem exchange ranged from an uptake of 4.0 t C ha −1 year −1 in a year with adequate rainfall to a loss of 0.4 t C ha −1 year −1 in a year affected by drought and insect damage. The simulations indicated that this reduction in carbon gain was primarily due to insect damage rather than a direct consequence of water shortage. Under good growing conditions, carbon gain accumulated equally in stem wood and other biomass pools while soil organic carbon and woody litter were gradually decreasing following harvesting that had occurred 20 years earlier. Under more stressful conditions, net biomass increments decreased, while soil organic carbon and woody litter increased because of greater litter inputs and because drier conditions inhibited decomposition. Modelled estimates of photosynthesis and net ecosystem exchange were similar to estimates from eddy flux observations alone provided that it used a novel routine for deriving night-time carbon fluxes. Estimates of net ecosystem exchange derived in the present work were, however, substantially lower than previous estimates that had been derived using a more traditional analysis of eddy flux data. This study showed that detailed physiological modelling is a valuable technique for combining all available site information as well as further constraints based on broader scientific principles and considerations of the conservation of mass. It can thus provide a powerful constraint on the overall site carbon budget of an ecosystem.

Refugial forests of the southern Appalachians: photosynthesis and survival in current-year Abies fraseri seedlings

Fraser fir (Abies fraseri (Pursh) Poiret) is an endemic, high-elevation conifer confined to six relict mountaintop communities in the southern Appalachian Mountains, USA. High adult mortality has occurred over the past 50 years, possibly the result of an introduced insect (Adelges piceae Ratzeburg), air pollution, or both. Knowledge of the mechanisms of and limitations to seedling establishment may allow reestablisment and perpetuation of this unique community type, notwithstanding global climate change. We monitored seedling emergence and mortality in relation to photosynthetic performance and water relations in microsites differing in canopy openness (sunlight exposure) over the summer of 2004. Abundance of cotyledonous seedlings in early summer was 2.3 times greater (849 versus 366 seedlings m(-2)) in microsites with lower sky exposure (greater canopy closure) than in microsites with greater sky exposure (greater canopy openness). In contrast, late-season abundance and survival were greater in areas beneath more open canopies than in areas beneath less open canopies (3.3 times and 11.7 times greater, respectively). However, newly emerged seedling survival in a completely open site (no overhead canopy) was zero, despite an initial density of 124 seedlings m(-2). Seedling water status was similar in open- and closed-canopy sites (-0.52 and -0.74 MPa, respectively). Photosynthetic carbon gain was higher in newly emerged seedlings at open canopy than at closed canopy sites, especially during early morning. Based on photosynthetic light response curves and measured sunlight regimes, seedlings in open canopy sites were estimated to assimilate 3.3-4.5 times more carbon than seedlings at closed sites. Reductions in carbon gain of closed-site seedlings, primarily a result of limited sunlight, corresponded to substantial increases in seedling mortality (98 versus 79% in open canopy sites). Thus, sunlight exposure, which reflects overstory canopy structure, appears to be an important factor influencing newly emerged seedling survival and distribution.