ALBERT

Notes: Abstract A portable system for CO2 gas exchange measurements is described that allows determination of net photosynthesis and transpiration rates as well as leaf conductance of salt marsh vascular plants, and photosynthesis rates of macrophytic algae and epibenthic algae of sediment cores during low tide periods of exposure. Carbon fixation processes of these several different types of organisms can be studied on the same day. Measurements may be carried out at an estuarine field site using controlled conditions of light, temperature, and air CO2 partial pressure. Algal samples are enclosed in the cuvette for only a matter of minutes and do not dry significantly during measurement. The rapidity with which gas exchange rates of samples may be assessed will allow routine processing of many sediment cores. Thus, the distribution of producer populations can be studied with greater resolution than previously possible.

Notes: Abstract  Databases describing branch gas exchange of Picea abies L. at two montane forest sites, Lägeren, Switzerland (National Forschungsprojekt 14 of the Schweizerische Nationalfonds) and Oberwarmensteinach, Germany (Bayerische Forschungsgruppe Forsttoxikologie), were analyzed in conjunction with a physiologically based model. Parameter estimates for describing carboxylase kinetics, electron transport, and stomatal function were derived, utilizing information from both single factor dependencies and diurnal time course measurements of gas exchange. Data subsets were used for testing the model at the branch level. Most of the observed variation in gas exchange characteristics can be explained with the model, while a number of systematic errors remain unexplained. Factors seen as contributing to the unexplained residual variation and not included in the model are light acclimation, degree of damage in adjustment to pollutant deposition, needle age, and cold stress effects. Nevertheless, a set of parameter values has been obtained for general application with spruce, e. g., for use in calculating canopy flux rates and to aid in planning of focused leaf and canopy level experiments. The value of the model for estimating fluxes between the forest and the atmosphere must be evaluated together with measurements at the stand level.

Notes: Abstract  The process-based simulation model STANDFLUX describes canopy water vapor and carbon dioxide exchange based on rates calculated for individual trees and as affected by local gradients in photon flux density (PFD), atmospheric humidity, atmospheric carbon dioxide concentration, and air temperature. Direct, diffuse, and reflected PFD incident on foliage elements within compartments of individual trees (defined by vertical layers and a series of concentric cylinders centered on the trunk) is calculated for a 3-dimensional matrix of points. Foliage element gas exchange rates are based on estimates of carboxylation, RuBP regeneration, and respiratory capacities as well as the correlated behavior found between stomatal conductance and assimilation rate. Because of the difficulties associated with effective sampling and description of spatial variation in structure and leaf level gas exchange parameters for trees comprising the forest canopy, the significance for canopy water and carbon dioxide exchange of varied representations of tree foliage distribution and of physiology is examined. The additional interactive effects encountered due to changes in tree density and, thus, spatial aggregation or disaggregation of foliage is also studied. The analysis is conducted within the context of observed structural and physiological variation encountered in Norway spruce (Picea abies) stands in the Fichtelgebirge region of central Germany. Potentials for simplifying the three-dimensional canopy gas exchange model without sizable influence on canopy flux rates were small. A relatively large number of sample points within the tree crowns is necessary to obtain consistent calculations of flux rates because of the non-linear relationship between PFD and net photosynthesis. Transpiration and net photosynthesis for stands with a low leaf area index (LAI) may be obtained from single tree estimates for each tree class weighted by class frequency, while 30 or more trees per class in differing relation to neighboring trees may be necessary to calculate reliable estimates of net photosynthesis in canopies with high LAI. The complexity in structure assumed for modeled trees was important, especially when overall canopy foliage area was either high or low due to spatial heterogeneity in clumping, e. g., potential canopy overlaps or side-lighting. Effects were greater for calculated net photosynthesis than for transpiration, reflecting higher sensitivity of net photosynthesis to differences in light distribution within individual trees. Accuracy in estimates of physiological parameters is equally important, and these characteristics have profound effects on estimated canopy gas exchange rates. While one-dimensional representations of canopy structure or approximations of tree physiological characteristics from other canopies or species may often be necessary in assessing vegetation/atmosphere exchanges, especially in the study of water balance of landscapes or regions, STANDFLUX provides a tool that can aid in evaluating the limitations of these simpler approaches.

Notes: Leaf gas-exchange and chemical composition were investigated in seedlings of Quercus suber L. grown for 21 months either at elevated (700 μmol mol–1) or normal (350 μmol mol–1) ambient atmospheric CO2 concentrations, [CO2], in a sandy nutrient-poor soil with either ‘high’ N (0.3 mol N m–3 in the irrigation solution) or with ‘low’ N (0.05 mol N m–3) and with a constant suboptimal concentration of the other macro- and micronutrients. Although elevated [CO2] yielded the greatest total plant biomass in ‘high’ nitrogen treatment, it resulted in lower leaf nutrient concentrations in all cases, independent of the nutrient addition regime, and in greater nonstructural carbohydrate concentrations. By contrast, nitrogen treatment did not affect foliar N concentrations, but resulted in lower phosphorus concentrations, suggesting that under lower N, P use-efficiency in foliar biomass production was lower. Phosphorus deficiency was evident in all treatments, as photosynthesis became CO2 insensitive at intercellular CO2 concentrations larger than ≈ 300 μmol mol–1, and net assimilation rates measured at an ambient [CO2] of 350 μmol mol–1 or at 700 μmol mol–1 were not significantly different. Moreover, there was a positive correlation of foliar P with maximum Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) carboxylase activity (Vcmax), which potentially limits photosynthesis at low [CO2], and the capacities of photosynthetic electron transport (Jmax) and phosphate utilization (Pmax), which are potentially limiting at high [CO2]. None of these potential limits was correlated with foliar nitrogen concentration, indicating that photosynthetic N use-efficiency was directly dependent on foliar P availability. Though the tendencies were towards lower capacities of potential limitations of photosynthesis in high [CO2] grown specimens, the effects were statistically insignificant, because of (i) large within-treatment variability related to foliar P, and (ii) small decreases in P/N ratio with increasing [CO2], resulting in balanced changes in other foliar compounds potentially limiting carbon acquisition. The results of the current study indicate that under P-deficiency, the down-regulation of excess biochemical capacities proceeds in a similar manner in leaves grown under normal and elevated [CO2], and also that foliar P/N ratios for optimum photosynthesis are likely to increase with increasing growth CO2 concentrations. Symbols: A, net assimilation rate (μmol m–2 s–1); Amax, light-saturated A (μmol m–2 s–1); α, initial quantum yield at saturating [CO2] and for an incident Q (mol mol–1); [CO2], atmospheric CO2 concentration (μmol mol–1); Ci, intercellular CO2 concentration (μmol mol–1); Ca, CO2 concentration in the gas-exchange cuvette (μmol mol–1); FB, fraction of leaf N in ‘photoenergetics’; FL, fraction of leaf N in light harvesting; FR, fraction of leaf N in Rubisco; Γ*, CO2 compensation concentration in the absence of Rd (μmol mol–1); Jmax*, capacity for photosynthetic electron transport; Jmc, capacity for photosynthetic electron transport per unit cytochrome f (mol e–[mol cyt f]–1 s–1); Kc, Michaelis-Menten constant for carboxylation (μmol mol–1); Ko, Michaelis-Menten constant for oxygenation (mmol mol–1); MA, leaf dry mass per area (g m–2); O, intercellular oxygen concentration (mmol mol–1); [Pi], concentration of inorganic phosphate (mM); Pmax*, capacity for phosphate utilization; Q, photosynthetically active quantum flux density (μmol m–2 s–1); Rd*, day respiration (CO2 evolution from nonphotorespiratory processes continuing in the light); Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; RUBP, ribulose-1,5-bisphosphate; Tl, leaf temperature (°C); UTPU*, rate of triose phosphate utilization; Vcmax*, maximum Rubisco carboxylase activity; Vcr, specific activity of Rubisco (μmol CO2[g Rubisco]–1 s–1]*given in either μmol m–2 s–1 or in μmol g–1 s–1 as described in the text.

Notes: Leaf conductance often decreases in response to elevated atmospheric CO2 concentration (Ca) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated Ca during an eight-month period two years after Ca manipulation began. Whole-chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated Ca, despite an increase in leaf area index (LAI). Elevated Ca also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated Ca in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated Ca.

Notes: Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (GSi) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that GSi decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, GSi decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Qo). Under conditions of D= 1 kPa and saturating Qo, GSi decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood-to-leaf area ratio (AS:AL) doubled. A simple hydraulic model explained the variation in GSi based on an inverse relationship with height, and a linear relationship with AS:AL. Thus, in F. sylvatica, adjustments in AS:AL partially compensate for the negative effect of increased flow-path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.

Notes: A process-based leaf gas exchange model for C3 plants was developed which specifically describes the effects observed along light gradients of shifting nitrogen investment in carboxylation and bioenergetics and modified leaf thickness due to altered stacking of photosynthetic units. The model was parametrized for the late-successional, shade-tolerant deciduous species Acer saccharum Marsh. The specific activity of ribulose-1,5-bisphosphate carboxylase (Rubisco) and the maximum photosynthetic electron transport rate per unit cytochrome f (cyt f) were used as indices that vary proportionally with nitrogen investment in the capacities for carboxylation and electron transport. Rubisco and cyt f per unit leaf area are related in the model to leaf dry mass per area (MA), leaf nitrogen content per unit leaf dry mass (Nm), and partitioning coefficients for leaf nitrogen in Rubisco (PR) and in bioenergetics (PB). These partitioning coefficients are estimated from characteristic response curves of photosynthesis along with information on lear structure and composition. While PR and PB determine the light-saturated value of photosynthesis, the fraction of leaf nitrogen in thylakoid light-harvesting components (PL) and the ratio of leaf chlorophyll to leaf nitrogen invested in light harvesting (CB), which is dependent on thylakoid stoichiometry, determine the initial photosynthetic light utilization efficiency in the model. Carbon loss due to mitochondrial respiration, which also changes along light gradients, was considered to vary in proportion with carboxylation capacity. Key model parameters - Nm, PR, PB, PLCB and stomatal sensitivity with respect to changes in net photosynthesis (Gr) – were examined as a function of MA, which is linearly related to irradiance during growth of the leaves. The results of the analysis applied to A. saccharum indicate that PB and PR increase, and Gf, PL and CB decrease with increasing MA. As a result of these effects of irradiaiice on nitrogen partitioning, the slope of the light-saturated net photosynthesis rate per unit leaf dry mass (Ammax) versus Nm relationship increased with increasing growth irradiance in mid-season. Furthermore, the nitrogen partitioning coefficients as well as the slopes of Ammax versus Nm were independent of season, except during development of the leaf photosynthetic apparatus. Simulations revealed that the acclimation to high light increased Ammax by 40% with respect to the low light regime. However, light-saturated photosynthesis per leaf area (Aamax) varied 3-fold between these habitats, suggesting that the acclimation to high light was dominated by adjustments in leaf anatomy (Aamax=AmmaxMA) rather than in foliar biochemistry. This differed from adaptation to low light, where the alterations in foliar biochemistry were predicted to be at least as important as anatomical modifications. Due to the light-related accumulation of photosynthetic mass per unit area, Aamax depended on MA and leaf nitrogen per unit area (Na). However, Na conceals the variation in both MA and Nm (Na=NmMA), and prevents clear separation of anatomical adjustments in foliage structure and biochemical modifications in foliar composition. Given the large seasonal and site nutrient availability-related variation in Nm, and the influences of growth irradiance on nitrogen partitioning, the relationship between Aamax and Na is universal neither in time nor in space and in natural canopies at mid-season is mostly driven by variability in MA. Thus, we conclude that analyses of the effects of nitrogen investments on potential carbon acquisition should use mass-based rather than area-based expressions.

Notes: We present a physiological model of isoprene (2-methyl-1,3-butadiene) emission which considers the cost for isoprene synthesis, and the production of reductive equivalents in reactions of photosynthetic electron transport for Liquidambar styraciflua L. and for North American and European deciduous temperate Quercus species. In the model, we differentiate between leaf morphology (leaf dry mass per area, MA, g m−2) altering the content of enzymes of isoprene synthesis pathway per unit leaf area, and biochemical potentials of average leaf cells determining their capacity for isoprene emission. Isoprene emission rate per unit leaf area (μmol m−2 s−1) is calculated as the product of MA, the fraction of total electron flow used for isoprene synthesis (ɛ, mol mol−1), the rate of photosynthetic electron transport (J) per unit leaf dry mass (Jm, μmol g−1 s−1), and the reciprocal of the electron cost of isoprene synthesis [mol isoprene (mol electrons−1)]. The initial estimate of electron cost of isoprene synthesis is calculated according to the 1-deoxy- D-xylulose-5-phosphate pathway recently discovered in the chloroplasts, and is further modified to account for extra electron requirements because of photorespiration. The rate of photosynthetic electron transport is calculated by a process-based leaf photosynthesis model. A satisfactory fit to the light-dependence of isoprene emission is obtained using the light response curve of J, and a single value of ɛ, that is dependent on the isoprene synthase activity in the leaves. Temperature dependence of isoprene emission is obtained by combining the temperature response curves of photosynthetic electron transport, the shape of which is related to long-term temperature during leaf growth and development, and the specific activity of isoprene synthase, which is considered as essentially constant for all plants. The results of simulations demonstrate that the variety of temperature responses of isoprene emission observed within and among the species in previous studies may be explained by different optimum temperatures of J and/or limited maximum fraction of electrons used for isoprene synthesis. The model provides good fits to diurnal courses of field measurements of isoprene emission, and is also able to describe the changes in isoprene emission under stress conditions, for example, the decline in isoprene emission in water-stressed leaves.

Notes: The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (Jmax;A), and maximum carboxylase activity of Rubisco (Vcmax;A) is proportional to average integrated daily quantum flux density (Qint) in a mixed deciduous forest dominated by the shade-intolerant species Populus tremula L., and the shade-tolerant species Tilia cordata Mill. We distinguished between the age-dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (MA), foliar nitrogen content per unit dry mass (NM), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (FB), Rubisco (FR) and in light-harvesting chlorophyll-protein complexes (Vcmax;A ∝ MANMFR; Jmax;A ∝ MANMFB). In both species, increases in Jmax;A and Vcmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in MA. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in MA. There was only small within-canopy variation in the initial rate of biomass accumulation per unit Qint (slope of MA versus leaf age relationship per unit Qint), suggesting that canopy differences in the rate of development of Jmax;A and Vcmax;A are directly proportional to Qint. Nevertheless, MA, nitrogen, Jmax;A and Vcmax;A of mature leaves were not proportional to Qint because of a finite MA in leaves immediately after bud-burst (light-independent component of MA). MA, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day-to-day fluctuations in Qint. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non-senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light-dependent variation in the rate of developmental changes in MA determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.

Notes: To test the hypothesis that in temperate deciduous trees acclimation to potentially damaging high irradiances occurs via long-term adjustments in foliar photosynthetic capacity, and short-term changes in xanthophyll cycle pool size in response to weather fluctuations, nitrogen concentration and pigment composition were examined along a canopy light gradient in three species –Betula pendula, Populus tremula and Tilia cordata (from most shade intolerant to tolerant), and foliage photosynthetic potentials in P. tremula and T. cordata. Integrated quantum flux density (Qi) incident on leaves was estimated with a method combining hemispherical photography and light measurements with quantum sensors made over the growing season. Long- and short-term light indices – average total seasonal daily integrated quantum flux density (Ts, mol m–2 d–1) and that of the 3 d preceding foliage sampling (T3d) – were calculated for each sampled leaf. In addition to total integrated quantum flux density, the part of Qi attributable to direct flux was also computed. Strong linear relationships between the capacity for photosynthetic electron transport per area (Jamax), estimated from in situ measurements of effective quantum yield of photosystem II (PS II), and Qi averaged over the season and over the preceding 3 d were found for all studied species. However, the major determinant of Jamax, the product of electron transport capacity per leaf dry mass (Jmmax) and leaf dry mass per area (MA), was MA rather than Jmmax, which was relatively constant along the light gradient. There was evidence that Jamax is more tightly related to Ts, which characterizes the light climate during foliar development, than to short-term integrated light, possibly because there is little flexibility in adjustments in MA after the completion of foliar growth. Leaf chlorophyll concentrations and the investment of leaf nitrogen in chlorophyll (Chl/N) were negatively related to Qi– an investment pattern which improves light harvesting in low light. Xanthophyll cycle pool size (VAZ, violaxanthin + antheraxanthin + zeaxanthin) either expressed per unit chlorophyll (VAZ/Chl) or as a fraction of total carotenoids (VAZ/Car) increased with increasing Qi in all species. However, contrary to Jamax, it tended to correlate more strongly with short-term than with long-term average integrated light. There were few interspecific differences in Jamax, Chl/N, VAZ/Chl and VAZ/Car when the variability in light level incident to the leaves was accounted for, indicating that the foliage of both shade-intolerant and -tolerant temperate tree species possesses considerable phenotypic flexibility. Collectively these results support the view that rapid adjustment of the xanthophyll cycle pool size provides an important means for acclimation to light fluctuations in a time scale of days, during which the potential for photosynthetic quenching of excitation energy is not likely to change appreciably.