Whole-plant Carbon Balance Research Articles

Photosynthesis and whole-plant carbon balances of warm affinity Halophila nipponica and cold affinity Zostera marina in relation to water temperature rise: implication for future geographic distribution

Climate change associated sea surface temperature rise could significantly affect the physiology, and consequently geographic distribution of seagrasses. Photosynthetic and respiratory responses of warm affinity Halophila nipponica and cold affinity Zostera marina to a change in water temperature (7–26°C) were investigated to estimate whole-plant carbon balance for prediction of the future geographic distributions of these seagrasses in the northwestern Pacific region. It was hypothesized that H. nipponica and Z. marina showed distinctly different patterns of carbon balance with changes in water temperature. Photosynthetic and respiratory rates of H. nipponica and Z. marina generally increased with increasing water temperature, except for Ic and Ik of H. nipponica and α of Z. marina. H. nipponica showed high Pmax and α values, which could support a greater respiratory demand related to the higher proportion of non-photosynthetic tissues (~67%), compared with Z. marina (~34%). The whole-plant carbon balance of Z. marina remained positive throughout the experiment, although it decreased to nearly zero during the high water-temperature period. H. nipponica exhibited a negative carbon balance during winter and early spring, which corresponded with severely limited growth at water temperatures< 15°C. These results suggest that increases in water temperature, particularly during winter, in relation to continuous climate change, could induce substantial changes in the seagrass ecosystem structure and corresponding changes in coastal ecosystem services in the temperate coastal waters of the northwestern Pacific region. The results of this study will provide valuable information on the effective management and conservation of coastal and estuarine ecosystems under ongoing climate change.

Contrasting transcriptomic patterns reveal a genomic basis for drought resilience in the relict fir Abies pinsapo Boiss.

Climate change challenges the adaptive capacity of several forest tree species in the face of increasing drought and rising temperatures. Therefore, understanding the mechanistic connections between genetic diversity and drought resilience is highly valuable for conserving drought-sensitive forests. Nonetheless, the post-drought recovery in trees from a transcriptomic perspective has not yet been studied by comparing contrasting phenotypes. Here, experimental drought treatments, gas-exchange dynamics and transcriptomic analysis (RNA-seq) were performed in the relict and drought-sensitive fir Abies pinsapo Boiss. to identify gene expression differences over immediate (24h) and extended drought (20days). Post-drought responses were investigated to define resilient and sensitive phenotypes. Single nucleotide polymorphisms (SNPs) were also studied to characterize the genomic basis of A. pinsapo drought resilience. Weighted gene co-expression network analysis showed an activation of stomatal closing and an inhibition of plant growth-related genes during the immediate drought, consistent with an isohydric dynamic. During the extended drought, transcription factors, as well as cellular damage and homeostasis protection-related genes prevailed. Resilient individuals activate photosynthesis-related genes and inhibit aerial growth-related genes, suggesting a shifting shoot/root biomass allocation to improve water uptake and whole-plant carbon balance. About, 152 fixed SNPs were found between resilient and sensitive seedlings, which were mostly located in RNA-activity-related genes, including epigenetic regulation. Contrasting gene expression and SNPs were found between different post-drought resilience phenotypes for the first time in a forest tree, suggesting a transcriptomic and genomic basis for drought resilience. The obtained drought-related transcriptomic profile and drought-resilience candidate genes may guide conservation programs for this threatened tree species.

The Importance of Stem Photosynthesis for Two Desert Shrubs Across Different Groundwater Depths.

Water availability could alter multiple ecophysiological processes such as water use strategy, photosynthesis, and respiration, thereby modifying plant water use and carbon gain. However, a lack of field observations hinders our understanding of how water availability affects stem photosynthesis at both organ and plant levels of desert shrubs. In this study, we measured gas exchange and oxygen stable isotopes to quantify water sources, stem recycling photosynthesis, and whole-plant carbon balance in two coexisting Haloxylon species (Haloxylon ammodendron and Haloxylon persicum) at different groundwater depths in the Gurbantonggut Desert. The overall aim of the study was to analyze and quantify the important role of stem recycling photosynthesis for desert shrubs (Haloxylon species) under different groundwater depths. The results showed that (1) regardless of changes in groundwater depth, H. ammodendron consistently used groundwater and H. persicum used deep soil water as their main water source, with greater than 75% of xylem water being derived from groundwater and deep soil water for the two species, respectively; (2) stem recycling photosynthesis refixed 72–81% of the stem dark respiration, and its contribution to whole-plant carbon assimilation was 10–21% for the two species; and (3) deepened groundwater increased stem water use efficiency and its contribution to whole-plant carbon assimilation in H. persicum but not in H. ammodendron. Our study provided observational evidence that deepened groundwater depth induced H. persicum to increase stem recycling photosynthetic capacity and a greater contribution to whole-plant carbon assimilation, but this did not occur on H. ammodendron. Our study indicates that stem recycling photosynthesis may play an important role in the survival of desert shrubs in drought conditions.

Shade tree species affect gas exchange and hydraulic conductivity of cacao cultivars in an agroforestry system.

Shade tolerance is a widespread strategy of rainforest understory plants. Many understory species have green young stems that may assimilate CO2 and contribute to whole-plant carbon balance. Cacao commonly grows in the shaded understory and recent emphasis has been placed on diversifying the types of trees used to shade cacao plants to achieve additional ecosystem services. We studied three agricultural cacao cultivars growing in the shade of four timber species (Cedrela odorata L., Cordia thaisiana Agostini, Swietenia macrophylla King and Tabebuia rosea (Bertol) A.D.C.) in an agroforestry system to (i) evaluate the timber species for their effect on the physiological performance of three cacao cultivars; (ii) assess the role of green stems on the carbon economy of cacao; and (iii) examine coordination between stem hydraulic conductivity and stem photosynthesis in cacao. Green young stem photosynthetic CO2 assimilation rate was positive and double leaf CO2 assimilation rate, indicating a positive contribution of green stems to the carbon economy of cacao; however, green stem area is smaller than leaf area and its relative contribution is low. Timber species showed a significant effect on leaf gas exchange traits and on stomatal conductance of cacao, and stem water-use efficiency varied among cultivars. There were no significant differences in leaf-specific hydraulic conductivity among cacao cultivars, but sapwood-specific hydraulic conductivity varied significantly among cultivars and there was an interactive effect of cacao cultivar × timber species. Hydraulic efficiency was coordinated with stem-stomatal conductance, but not with leaf-stomatal conductance or any measure of photosynthesis. We conclude that different shade regimes determined by timber species and the interaction with cacao cultivar had an important effect on most of the physiological traits and growth variables of three cacao cultivars growing in an agroforestry system. Results suggested that C. odorata is the best timber species to provide partial shade for cacao cultivars in the Barlovento region in Venezuela, regardless of cultivar origin.

Structural complexity governs seagrass acclimatization to depth with relevant consequences for meadow production, macrophyte diversity and habitat carbon storage capacity

Analyses of the integrated seagrass response to depth support the previously documented low plasticity and consistent shade-adapted leaf physiology of a habitat-builder that dominates well-illuminated reef environments. Two structural responses, “canopy-opening” and “below-ground-mass-depletion”, govern the photoacclimatory response and facilitate, respectively, light penetration within the canopy and functional adjustments in whole-plant carbon balances. Conversely, “canopy-closing” may also explain dense canopies formed close to the waterline, as they provide shade and photoprotection to a susceptible leaf physiology under high-light. Canopy light attenuation is primarily regulated by the leaf area index (LAI), which is governed by changes in shoot size and density. Shoot density diminishes non-linearly with depth, while shoot size increases to a maximum followed by a decline. The initial increase in shoot size, which resembles a self-thinning response, increases LAI and meadow production in shallow depths. These seagrass structural adjustments have relevant ecological implications. Canopy-thinning allows macrophyte diversity to increase with depth, while seagrass production and carbon storage diminish exponentially, and are maximal only in a shallow coastal fringe. The results support the universality of plant self-thinning, from phytoplankton to complex canopies, likely the consequence of simple physical laws related to light limitation and pigment self-shading within photosynthetic structures and communities.

Diurnal periodicity of assimilate transport shapes resource allocation and whole-plant carbon balance.

Whole-plant carbon balance comprises diurnal fluctuations of photosynthetic carbon gain and respiratory losses, as well as partitioning of assimilates between phototrophic and heterotrophic organs. Because it is difficult to access, the root system is frequently neglected in growth models, or its metabolism is rated based on generalizations from other organs. Here, whole-plant cuvettes were used for investigating total-plant carbon exchange with the environment over full diurnal cycles. Dynamics of primary metabolism and diurnally resolved phloem exudation profiles, as proxy of assimilate transport, were combined to obtain a full picture of resource allocation. This uncovered a strong impact of periodicity of inter-organ transport on the efficiency of carbon gain. While a sinusoidal fluctuation of the transport rate, with minor diel deflections, minimized respiratory losses in Arabidopsis wild-type plants, triangular or rectangular patterns of transport, found in mutants defective in either starch or sucrose metabolism, increased root respiration at the end or beginning of the day, respectively. Power spectral density and cross-correlation analysis revealed that only the rate of starch synthesis was strictly correlated to the rate of net photosynthesis in wild-type, while in a sucrose-phosphate synthase mutant (spsa1), this applied also to carboxylate synthesis, serving as an alternative carbon pool. In the starchless mutant of plastidial phospho-gluco mutase (pgm), none of these rates, but concentrations of sucrose and glucose in the root, followed the pattern of photosynthesis, indicating direct transduction of shoot sugar levels to the root. The results demonstrate that starch metabolism alone is insufficient to buffer diurnal fluctuations of carbon exchange.

A dichotomy resolved: Plant growth can control the rate of starch accumulation.

A dichotomy resolved: Plant growth can control the rate of starch accumulation.

The optimal lateral root branching density for maize depends on nitrogen and phosphorus availability.

Observed phenotypic variation in the lateral root branching density (LRBD) in maize (Zea mays) is large (1-41 cm(-1) major axis [i.e. brace, crown, seminal, and primary roots]), suggesting that LRBD has varying utility and tradeoffs in specific environments. Using the functional-structural plant model SimRoot, we simulated the three-dimensional development of maize root architectures with varying LRBD and quantified nitrate and phosphorus uptake, root competition, and whole-plant carbon balances in soils varying in the availability of these nutrients. Sparsely spaced (less than 7 branches cm(-1)), long laterals were optimal for nitrate acquisition, while densely spaced (more than 9 branches cm(-1)), short laterals were optimal for phosphorus acquisition. The nitrate results are mostly explained by the strong competition between lateral roots for nitrate, which causes increasing LRBD to decrease the uptake per unit root length, while the carbon budgets of the plant do not permit greater total root length (i.e. individual roots in the high-LRBD plants stay shorter). Competition and carbon limitations for growth play less of a role for phosphorus uptake, and consequently increasing LRBD results in greater root length and uptake. We conclude that the optimal LRBD depends on the relative availability of nitrate (a mobile soil resource) and phosphorus (an immobile soil resource) and is greater in environments with greater carbon fixation. The median LRBD reported in several field screens was 6 branches cm(-1), suggesting that most genotypes have an LRBD that balances the acquisition of both nutrients. LRBD merits additional investigation as a potential breeding target for greater nutrient acquisition.

Leaf shedding and whole-plant carbon balance(<Feature 1>Leaf lifespan: recent advances and future perspectives)

Leaf shedding and whole-plant carbon balance(<Feature 1>Leaf lifespan: recent advances and future perspectives)

Gas exchange and leaf aging in an evergreen oak: causes and consequences for leaf carbon balance and canopy respiration

Leaves of Mediterranean evergreens experience large variations in gas exchange rates over their life span due to aging and seasonally changing environmental conditions. Accounting for the changing respiratory physiology of leaves over time will help improve estimations of leaf and whole-plant carbon balances. Here we examined seasonal variations in light-saturated net CO(2) assimilation (A(max)), dark respiration (R(d)) and the proportional change in R(d) per 10 °C change in temperature (Q(10) of R(d)) in previous-year (PY) and current-year (CY) leaves of the broadleaved evergreen tree Quercus ilex L. A(max) and R(d) were lower in PY than in CY leaves. Differences in nitrogen between cohorts only partly explained such differences, and rates of A(max) and R(d) expressed per unit of leaf nitrogen were still significantly different between cohorts. The decline in A(max) in PY leaves did not result in the depletion of total non-structural carbohydrates, whose concentration was in fact higher in PY than CY leaves. Leaf-level carbon balance modeled from gas exchange data was positive at all ages. Q(10) of R(d) did not differ significantly between leaf cohorts; however, failure to account for distinct R(d) between cohorts misestimated canopy leaf respiration by 13% across dates when scaling up leaf measurements to the canopy. In conclusion, the decline in A(max) in old leaves that are close to or exceed their mean life span does not limit the availability of carbohydrates, which are probably needed to sustain new growth, as well as R(d) and nutrient resorption during senescence. Accounting for leaf age as a source of variation of R(d) improves the estimation of foliar respiratory carbon release at the stand scale.

Seasonal gas exchange and water relations in juveniles of two evergreen Neotropical savanna tree species with contrasting regeneration strategies

Colonization dynamics of woody species into grasslands in Neotropical savannas are determined by two main factors: plant-available moisture and fire. Considering seasonality of precipitation and high fire frequency in these ecosystems, vegetative reproduction has been suggested as the main regeneration strategy in woody species. This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species with contrasting regeneration strategies: Palicourea rigida (sexual reproduction) and Casearia sylvestris (asexual reproduction). The studied species showed similar transpiration rates to deep-rooted adult evergreen tree species during the rainy period, suggesting little water availability limitations on surface soil layers. P. rigida juveniles significantly decreased their leaf water potentials from wet to dry seasons. In C. sylvestris resprouts, there were no seasonal differences in their predawn water potentials and gas exchange parameters, indicating a water deficit avoidance characteristic derived from their connections to deep-rooted adult counterparts allowing access to moist soil at depth even during the drought period. P. rigida rely on strict control of water losses and turgor maintenance through elastic cell walls during the dry season. The iso-hydric behavior of gas exchange and most water relations parameters in C. sylvestris enable turgor maintenance during the dry season which also gives the possibility to achieve foliar expansion under water-stressed conditions for shallow-rooted plants. Nevertheless, in absence of water deficits, P. rigida had the advantage to be physiologically independent individuals, showing an equal or even superior photosynthetic performance that eventually could be translated into a more favorable whole-plant carbon balance and higher growth rates in wet habitats.

Ontogenetic variation in light requirements of juvenile rainforest evergreens

1 Although shade tolerance is often assumed to be a fixed trait of species, recent work has reported size-related changes in the relative and absolute light requirements of woody taxa. We hypothesized that, in evergreen forests, light requirements of shade-tolerant species that accumulate multiple foliage cohorts will be more stable during juvenile ontogeny than those of intolerant species with short leaf lifetimes. 2 We quantified the light environments occupied by three size classes of 13 coexisting evergreens in a temperate rainforest, to determine how size influenced their relative shade tolerance. Minimum light requirements (MLRs) of species were estimated by computing the 10th percentile of the distribution of juveniles in relation to percentage canopy openness, for each size class. Leaf life span in low light (2%–5% canopy openness) was estimated by recording survival of marked leaves over 12 months, or retrospectively on species with clearly discernible foliage cohorts. 3 Agreement of ranks of species’ MLR across size classes was significant, although not strong (Kendall's W = 0·159, P = 0·02). MLRs of the most shade-tolerant species changed little between size-classes, whereas those of most of the less-tolerant species rose with increasing size. 4 Shift in MLR across size-classes was negatively correlated with leaf life span, possibly because of the effects of leaf life span on biomass distribution and whole-plant carbon balance. Survival of light-demanding species with short leaf lifetimes may thus depend on their encountering increasing light levels as they grow taller, whereas progressive accumulation of an extensive leaf area by late-successional taxa enables them to continue to tolerate low light despite increasing size. 5 Results suggest that shade-tolerance differences between evergreens become increasingly apparent with increasing size. In identifying a relationship with leaf life span, this work also provides a basis for predicting changes in species’ light requirements during juvenile ontogeny.

Whole-plant carbon relations and root respiration associated with root tolerance to high soil temperature for Agrostis grasses

Plant tolerance to high soil temperature may be related to the adjustment in carbon production and utilization. The objective of this study was to determine changes in whole-plant carbon balance and root respiration rate in relation to root tolerance to high soil temperature for two Agrostis grass species varying in heat tolerance. Plant tolerance to high soil temperature was compared between Agrostis scabra, a thermal grass species adapted to chronic high-temperature soils in the geothermal areas in Yellowstone National Park, and creeping bentgrass ( Agrostis stolonifera), a cultivated grass species adapted to cool climatic regions. Plant roots were exposed to low soil temperature (20 °C) or high soil temperature (37 °C) for 17 days in water baths placed in a controlled-environment growth chamber. Root biomass and cell membrane stability were determined to evaluate root thermotolerance of both species. Canopy photosynthetic rate (Pn), whole-plant respiration rate, root respiration rate, and total non-structural carbohydrate (TNC) content were measured to assess changes in carbon production and utilization in response to high soil temperature. Root biomass and cell membrane stability declined with increasing soil temperature, but the decline was much less for A. scabra than A. stolonifera, suggesting that roots of A. scabra were more tolerant to heat stress. Canopy Pn decreased and whole-plant respiration rate increased for A. stolonifera, but canopy Pn and respiration rate were unchanged for A. scabra in response to increasing soil temperature. After 17 days of high soil temperature treatment, A. stolonifera exhibited carbon deficit at the whole-plant level, whereas A. scabra maintained positive carbon gain. Root respiration of plants previously grown at 20 °C increased after a short-term treatment (24 h) at 37 °C, but the increase was significantly lower for A. scabra than for A. stolonifera. TNC content in roots did not show response to short-term (24 h) changes in temperature and did not exhibit species variations. Leaves of A. scabra, however, maintained TNC content under both low and high temperature regimes. Our results suggest that root thermotolerance of cool-season grasses could be related to the maintenance of positive whole-plant carbon balance, and down-regulation of whole-plant and root respiration rates in response to increasing soil temperature.

Overview of the physiological ecology of carbon metabolism in seagrasses.

The small but diverse group of angiosperms known as seagrasses form submersed meadow communities that are among the most productive on earth. Seagrasses are frequently light-limited and, despite access to carbon-rich seawaters, they may also sustain periodic internal carbon limitation. They have been regarded as C3 plants, but many species appear to be C3-C4 intermediates and/or have various carbon-concentrating mechanisms to aid the Rubisco enzyme in carbon acquisition. Photorespiration can occur as a C loss process that may protect photosynthetic electron transport during periods of low CO(2) availability and high light intensity. Seagrasses can also become photoinhibited in high light (generally>1000 µE m(-2) s(-1)) as a protective mechanism that allows excessive light energy to be dissipated as heat. Many photosynthesis-irradiance curves have been developed to assess light levels needed for seagrass growth. However, most available data (e.g. compensation irradiance I(c)) do not account for belowground tissue respiration and, thus, are of limited use in assessing the whole-plant carbon balance across light gradients. Caution is recommended in use of I(k) (saturating irradiance for photosynthesis), since seagrass photosynthesis commonly increases under higher light intensities than I(k); and in estimating seagrass productivity from H(sat) (duration of daily light period when light equals or exceeds I(k)) which varies considerably among species and sites, and which fails to account for light-limited photosynthesis at light levels less than I(k). The dominant storage carbohydrate in seagrasses is sucrose (primarily stored in rhizomes), which generally forms more than 90% of the total soluble carbohydrate pool. Seagrasses with high I(c) levels (suggesting lower efficiency in C acquisition) have relatively low levels of leaf carbohydrates. Sucrose-P synthase (SPS, involved in sucrose synthesis) activity increases with leaf age, consistent with leaf maturation from carbon sink to source. Unlike terrestrial plants, SPS apparently is not light-activated, and is positively influenced by increasing temperature and salinity. This response may indicate an osmotic adjustment in marine angiosperms, analogous to increased SPS activity as a cryoprotectant response in terrestrial non-halophytic plants. Sucrose synthase (SS, involved in sucrose metabolism and degradation in sink tissues) of both above- and belowground tissues decreases with tissue age. In belowground tissues, SS activity increases under low oxygen availability and with increasing temperatures, likely indicating increased metabolic carbohydrate demand. Respiration in seagrasses is primarily influenced by temperature and, in belowground tissues, by oxygen availability. Aboveground tissues (involved in C assimilation and other energy-costly processes) generally have higher respiration rates than belowground (mostly storage) tissues. Respiration rates increase with increasing temperature (in excess of 40 degrees C) and increasing water-column nitrate enrichment (Z. marina), which may help to supply the energy and carbon needed to assimilate and reduce nitrate. Seagrasses translocate oxygen from photosynthesizing leaves to belowground tissues for aerobic respiration. During darkness or extended periods of low light, belowground tissues can sustain extended anerobiosis. Documented alternate fermentation pathways have yielded high alanine, a metabolic 'strategy' that would depress production of the more toxic product ethanol, while conserving carbon skeletons and assimilated nitrogen. In comparison to the wealth of information available for terrestrial plants, little is known about the physiological ecology of seagrasses in carbon acquisition and metabolism. Many aspects of their carbon metabolism - controls by interactive environmental factors; and the role of carbon metabolism in salt tolerance, growth under resource-limited conditions, and survival through periods of dormancy - remain to be resolved as directions in future research. Such research will strengthen the understanding needed to improve management and protection of these environmentally important marine angiosperms.

Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 187:121-131 (1999) - doi:10.3354/meps187121 Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina Teresa Alcoverro1,*, Richard C. Zimmerman2, Donald G. Kohrs2, Randall S. Alberte3 1Centre d'Estudis Avançats de Blanes (CSIC), Carretera Sta. Bàrbara s/n, 17300 Blanes, Girona, Spain 2Moss Landing Marine Laboratories, San Jose State University Foundation, PO Box 450, Moss Landing, California 95039, USA 3Areté Associates Inc. 1725 Jefferson Davis Hwy, Arlington, Virginia 22202, USA *E-mail: teresa@ceab.csic.es ABSTRACT: This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation. A quantitative understanding of the mobilization process and its limitations is essential to the development of robust models predicting minimum light levels required to maintain healthy seagrass populations. Plants were grown in running seawater tanks under 2 light regimes. One treatment was provided with 2 h irradiance-saturated photosynthesis (Hsat) to produce severe light limitation, while control plants were grown under 7 h Hsat, simulating the typical wintertime condition in Monterey Bay, California, USA. Although plants maintained under 2 h Hsat were more severely carbon limited than plants grown under 7 h Hsat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both Hsat treatments. The eelgrass studied here responded to negative carbon balances by suppressing the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of carbon stress. The 7 h Hsat plants survived the 45 d course of the experiment while the plants grown under 2 h Hsat died within 30 d, even though one-third of their carbon reserves remained immobilized in the rhizome. Thus, extreme light limitation can prevent full mobilization of carbon reserves stored in below-ground tissues, probably through the effects of anoxia on translocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatments. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines. KEY WORDS: Seagrass · Carbon balance · Resource allocation · Photosynthesis · Light Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 187. Publication date: October 14, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Light and carbon balance in the seagrass Thalassia testudinum : evaluation of current production models

The production dynamics and carbon balance of Thalassia testudinum in the lower Laguna Madre, Texas, USA, were examined during the 1995 summer period based on in situ photosynthesis vs irradiance (PI) measurements and continuous measurements of underwater photon-flux density (PFD). The validity of applying the H sat model, used to calculate production for Zostera marina as the product of the maximum rate of photosynthesis (P max) and daily hours of saturating irradiance (H sat) was assessed for T. testudinum by comparison with integrated production estimates derived through numerical integration. Gross integrated production values were combined with dark-respiration measurements of photosynthetic (PS) and non-photosynthetic (NPS) tissues and areal biomass to generate daily whole-plant carbon balance. Production and whole-plant carbon balance are discussed in relation to surface and underwater PFD measurements, biomass and other physical and chemical parameters collected during a 1 yr period from January to December 1995. The H sat model significantly underestimated production during all summer months, averaging 70% of integrated production over the entire study period. Gross integrated production ranged between 11.5 mg C g−1 leaf dry wt d−1 in June (during a period of unseasonably low PFDs caused by a drift-alga mat covering the seagrass bed) to 26.7 mg C g−1 leaf dry wt d−1 in July. Modeled net carbon gain was highest in July at 454 mg C m−2 d−1 (1.4 g dry wt m−2 d−1), sufficient to account for measured rates of leaf production in the study area and representative of T. testudinum populations of low productivity. During part of the summer period, however, the population was in negative carbon balance. The relatively low productivity of this population and the periods of negative carbon balance are attributed to low net photosynthesis:dark respiration (P net:R d) ratios, sporadic low-light periods, the small fraction of PS tissue relative to whole-plant biomass (5 to 13%) and nutrient limitation. Production models are sensitive to both light availability and the proportion of PS tissue supporting NPS biomass as reflected in whole-plant P net:R d ratios.

Nutrition and the ozone sensitivity of birch (Betula pendula)

Cuttings of a single birch clone (Betula pendula) were grown in field fumigation chambers throughout the growing season in either filtered air (control) or 90/40 nl O 3 1 -1 (day/night), both regimes being split into high and low nutrient supply. High nutrition was neither advantageous for maintaining the photosynthetic capacity and life span of the leaves (see Maurer et al. 1997) nor for limiting the productive loss of the whole plant under O 3 stress relative to low-fertilized (LF) plants. However, nutrition determined, through carbon allocation and leaf turn-over, the way plants coped with O 3 impact. High leaf turn-over under O 3 stress related the carbon gain of high-fertilized (HF) plants to the photosynthesis of newly formed, intact leaves, although the foliage area remained reduced (shedding of 03-injured leaves, inhibited branching). In contrast, the low leaf turn-over of LF plants reflected the maintenance of the 03-injured leaves, causing high respiratory costs in the whole-plant carbon balance and a root/shoot biomass ratio as low as in the HF plants. Within the root system, the carbon allocation was determined by nutrition rather than ozone, whereas the water-use efficiency of the whole-plant carbon increment was lowered by ozone in both nutrient regimes. The relationship between biomass production and nutrient levels in the whole plant was hardly affected by ozone, with only the range of interaction being narrowed. Conditions requiring the maintenance of foliage rather than favoring the replacement of 03-injured leaves may render trees more susceptible to shifts in the carbon allocation.

GePSi: A generic plant simulator based on object-oriented principles

The Generic Plant Simulator (GePSi) is a physiologically-based model that combines modules for canopy, root environment, water relations, and potential growth to generate whole-plant carbon, nitrogen, and water balances. The version presented here is coded in the object-oriented programming (OOP) language, C + +, to enhance the implementation of modularity. In the aboveground aerial environment, the Weather module defines the weather conditions above a canopy, and Micro Weather defines the vertical profiles of micro-meterological variables in a canopy. The belowground soil environment contains the SoilProperty modules, which define vertical profiles of physical and chemical variables in a soil column. The ‘part-of’ hierarchy in GePSi follows the structure of a real plant: the Plant module calls canopy and root system modules; the Canopy module, in turn, calls leaf, stem and fruit modules; and the RootSystem module calls coarse and fine root modules, etc. Our long-term goal is for GePSi to serve as a template for building a plant growth simulator by simply selecting appropriate modules for the question being asked. We are building a suite of plant modules (and their interfaces) based on general principles that are fundamentally similar for different kinds of plants. This includes photosynthesis, growth, nutrient and carbon allocation, water uptake, etc. These modules can be parameterized for specific species, related groups of species, life-forms, or broader groups depending on how variable the processes are across the groupings and the amount of unexplained variability that is acceptable for the question being investigated. Our modular-based approach has numerous advantages, including improving the understanding of the model, reducing duplication of effort, and facilitating the adaptation of the model for different sites and ecosystems.

Models of sapling mortality as a function of growth to characterize interspecific variation in shade tolerance of eight tree species of northwestern British Columbia

We have developed models of sapling mortality for the eight dominant tree species of northwestern British Columbia in order to better understand forest community dynamics and succession in this important forest region. The species-specific models characterize an individualis probability of mortality as a function of recent growth (a surrogate for whole-plant carbon balance). Interspecific comparisons of survival under low growth rates (i.e., suppression) provide a quantitative measure of the effective shade tolerance of these tree species. In particular, the more shade tolerant species exhibited more rapid decays in mortality probability with increased radial growth. The broad and continuous range in shade tolerance ranking was as follows: Thuja plicata Donn ex D. Don > Tsuga heterophylla (Raf.) Sarg. = Abies lasiocarpa (Hook.) Nutt. > Picea glauca (Moench) Voss × Picea sitchensis (Bong.) Carrire > Pinus contorta Dougl. ex Loud. var. latifolia Engelm. > Populus tremuloides Michx. > Populus balsamifera ssp. trichocarpa (Torr. & A. Gray) Brayshaw = Betula papyrifera Marsh. At low growth rates, mortality varied between T. plicata and B. papyrifera by more than an order of magnitude. For some species, the three replicate sites exhibited significant variation, suggesting that shade tolerance may vary with site conditions (presumably soil moisture in our study sites). The mortality models are consistent with previous qualitative categorizations into shade tolerance classes and parallel the dominance of different species in postdisturbance succession. Our results suggest that species differences in noncatastrophic mortality are critical to understanding and predicting forest dynamics.

Modelling the effects of atmospheric C0 2 on vegetation-atmosphere interactions

The effect of doubling atmospheric CO 2 concentration ( C a) on climate and vegetation is investigated using a combined climate-vegetation model. The vegetation model predicts the response of leaf area index, canopy transpiration ( E T) and whole-plant carbon balance to changes in climate, soil moisture, and atmospheric CO 2 forcing. This model has been embedded in the UK Meteorological Office Single Column Model (SCM), which provides the climate feedback to the vegetation. The vegetation model uses an optimisation approach to predict stomatal resistance, a biochemical model to predict photosynthesis and a simple carbon balance model to predict leaf area. Respiration is calculated as a function of leaf area and vegetation height. Clouds are assumed to be radiatively passive in the SCM to avoid unrealistic feedbacks. Simulations were performed with the fully interactive vegetation-climate model for an Amazon location with the present-day value of C a (1 × CO 2), and twice this value (2 × CO 2). In addition, two other types of simulation were performed at both CO 2 concentrations: one in which the vegetation component was forced only with 1 × CO 2, and one using a fixed surface resistance. The latter case is equivalent to simulations using most current general circulation models. In all the simulations, increased atmospheric CO 2 caused an increase in surface temperature owing to increased radiative forcing. With a fixed resistance, mean E T was increased by 5.6% and sensible heat flux was reduced by 3.8%. The fully interactive model had significant effects on the response of both climate and productivity to C a. Increased C a caused stomatal closure, which resulted in a reduction in mean E T of 25%. The effect of C a on E T was amplified by the positive feedback resulting from the effect of increased air humidity deficit on stomatal resistance. Gross primary productivity (GPP) was predicted to increase by 19.4% in the fully interactive model. With a fixed resistance, GPP increased by 34.3%. This difference in the predicted increase in GPP is due to the effect of stomata on the humidity of the planetary boundary layer. It is concluded that vegetation and climate responses to atmospheric CO 2 concentration cannot be considered in isolation: the effects of the feedbacks could be at least as important as any direct effects.