Rate of Belowground Carbon Allocation Differs with Successional Habit of Two Afromontane Trees

Background

Anthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO₂ efflux is crucial for addressing the carbon footprint of creeping degradation.

Methodology

We compared the carbon allocation pattern of the late successional gymnosperm Podocarpus falcatus (Thunb.) Mirb. and the early successional (gap filling) angiosperm Croton macrostachyus Hochst. es Del. in an Ethiopian Afromontane forest by whole tree ¹³CO₂ pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO₂. Further, we quantified the overall losses of assimilated ¹³C with soil CO₂ efflux.

Principal Findings

¹³C in leaves of C. macrostachyus declined more rapidly with a larger size of a fast pool (64% vs. 50% of the assimilated carbon), having a shorter mean residence time (14 h vs. 55 h) as in leaves of P. falcatus. Phloem sap velocity was about 4 times higher for C. macrostachyus. Likewise, the label appeared earlier in the arbuscular mycorrhiza of C. macrostachyus and in the soil CO₂ efflux as in case of P. falcatus (24 h vs. 72 h). Within one year soil CO₂ efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one.

Conclusions

Our results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e.g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.     

Foliage dynamics, leaf traits, and growth of coexisting evergreen and deciduous trees in a tropical montane forest in Ethiopia

Foliage dynamics of three functional tree types representing major components of the tropical montane evergreen forest in southern part of Central Ethiopia were compared. The species were Podocarpus falcatus (evergreen gymnosperm), Prunus africana (evergreen broadleaf), and Croton macrostachyus (facultative deciduous). The hypothesis examined is that in such tropical trees, endogenous control of foliage dynamics by the leaf life-spans (LLS) is largely dominant over external signals. Crown foliage turnover, leafiness of twigs, LLS, photosynthetic performance, respiration rate, specific leaf area, and relative growth rates of the stems were investigated. Foliage dynamics and leafiness of the twigs were monitored over 2?years while leaf traits were followed over 3?months. The degree of inter and intra-individual synchronization of foliage phenophases was examined to get an estimate of the contributions of endogenous and external signals to the dynamics of the foliages. Autoregression analysis indicated significant influence of the moisture regime on leaf sprouting of Croton and Podocarpus. During pronounced dry periods, new leaves were not developed. Analysis of phenological data using circular statistics revealed that in spite of strong inter-individual synchronization of leaf flush and fall (Podocarpus and Croton), the dynamics of individual parts of the crowns were less synchronized. LLS was independent of climate factors and it had substantial contribution to the control of foliage turnover. Moreover, examination of ecophysiological traits of developing leaves of the studied functional types showed differing patterns with LLS corroborating the ecophysiological characteristics. Although overlaid by fungal infestation, both the foliage and ecophysiological properties of Prunus resemble that of Podocarpus but the former exhibited a shorter LLS and slightly higher metabolic rates. Nevertheless, all species reacted positively to high moisture with respect to stem growth. In spite of largely differing weather conditions of the 2?years, direct competitive advantage of one of the species over the others could not be detected.     

Regeneration of seven indigenous tree species in a dry Afromontane forest,southern Ethiopia

Regeneration of seven indigenous tree species having significant ecological and economic importance was investigated in the Munessa-Shashemene dry Afromontane forest (MSF), southern Ethiopia. Densities and distributions of seedlings, saplings and trees were assessed along gradients of altitude, light and disturbance using quadrat sizes of 10×5 m (for seedlings) and 20×20 m (saplings and trees) following line transects. The number of individuals, frequency and height of the study species were recorded in the quadrats at every 100 m drop in altitude. Seedling densities varied markedly among the species and altitudes. Mean densities (number of individuals ha⁻¹) of seedlings ranged from zero (Polyscias fulva) to 5334 (Prunus africana), and from three (Polyscias fulva) to 102 (Podocarpus falcatus) for trees and saplings. Canonical Correspondence Analysis revealed that seedlings of Celtis africana and Croton macrostachyus were highly favored by disturbance and, hence, were concentrated in canopy gaps within the forest. Based on their population structures, the study species could be categorized into three groups: (1) Species that showed the highest proportion of individuals in the lowest height class and with a gradual decrease towards the upper height classes, which suggests good regeneration; Podocarpus falcatus, C. africana, C. macrostachys and P. africana belonged to this group. (2) Species that showed higher proportions of individuals in the lowest height class and with missing individuals in the subsequent middle height classes, indicative of hampered regeneration; Syzygium guineense and Pouteria adolfi-friederici belonged to this group. (3) Species with no individuals in the lowest and middle height classes but represented by individuals in upper height classes; P. fulva belonged to this group. The species categorized in the last two groups exhibited hampered regeneration, and P. fulva is in the verge of local extermination. High seedling densities (e.g. C. africana and P. africana) and/or adaptive defense mechanisms to herbivory (e.g. P. falcatus and C. macrostachyus) were common attributes of species, which exhibited good regeneration. Regeneration problems were largely attributed to human disturbance, lack of suitable habitat for seed germination or problems associated to seed set (seed predation or abortion). Our study indicated that P. fulva, P. adolfi-friederici and S. guineense require the highest immediate attention for conservation in the MSF.     

Evidence for facultative deciduousness in Colophospermum mopane in semi‐arid African savannas

Leaf phenology dictates the time available for carbon assimilation, transpiration and nutrient uptake in plants. Understanding the environmental cues that control phenology is therefore vital for predicting climate‐related changes to plant and ecosystem function. In contrast to temperate systems, and to a lesser degree, tropical forest systems, the cues initiating leaf drop in tropical savannas are poorly studied. We investigated the cues for leaf fall in a tropical monodominant arid savanna species, Colophospermum mopane, using an irrigation experiment. We tracked soil moisture, solar radiation, air temperature, leaf water status, leaf health and leaf carbon balance through the dry season in both irrigated and control plants. Water was the primary cue driving leaf loss of C. mopane rather than temperature or light. Trees watered throughout the dry season retained their canopies. These leaves remained functional and continued photosynthesis throughout the dry season. Leaf carbon acquisition rates did not decline with leaf age but were affected by soil moisture availability and temperature. Leaf loss did not occur when leaf carbon gain was zero, or when a particular leaf carbon threshold was reached. Colophospermum mopane is facultatively deciduous as water availability determines leaf drop in this widespread arid savanna species. Obligate deciduosity is not the only successful strategy in climates with a long dry season.     

Source water, phenology and growth of two tropical dry forest tree species growing on shallow karst soils

Seasonally dry tropical forests are dominated by deciduous and evergreen tree species with a wide range of leaf phenology. We hypothesized that Piscidia piscipula is able to extend leaf senescence until later in the dry season due to deeper and more reliable water sources than Gymnopodium floribundum, which loses leaves earlier in the dry season. Physiological performance was assessed as timing of leaf production and loss, growth, leaf water potential, depth of water uptake determined by stable isotopes, and leaf stable isotopic composition of carbon (δ¹³C) and oxygen (δ¹⁸O). P. piscipula took water primarily from shallow sources, whereas G. floribundum took water from shallow and deep sources. The greatest variation in water sources occurred during the onset of the dry season, when G. floribundum was shedding old leaves and growing new leaves, but P. piscipula maintained its leaves from the previous wet season. P. piscipula showed greater relative growth rate, greater leaf expansion rates, and more negative predawn and midday water potentials than G. floribundum. P. piscipula also exhibited greater leaf organic δ¹³C and lower δ¹⁸O values, indicating that the decrease in photosynthetic carbon isotope discrimination was associated with greater stomatal conductance and greater photosynthesis. Our results indicate that the contrasting early and late dry season leaf loss phenology of these two species is not simply determined by rooting depth, but rather a more complicated suite of characteristics based on opportunistic use of dynamic water sources, maximizing carbon gain, and maintenance of water potential during the dry season.     

Light use efficiency of California redwood forest understory plants along a moisture gradient

We investigated photosynthesis of five plant species growing in the understory at three sites (1,170-, 1,600- and 2,100-mm annual moisture inputs), along the geographical range of coastal California redwood forest, to determine whether greater inputs of rain and fog at northern sites enhance photosynthetic utilization of fluctuating light. Measurements of understory light environment and gas exchange were carried out to determine steady state and dynamic photosynthetic responses to light. Leaf area index ranged from 4.84 at the 2,100-mm site to 5.98 at the 1,170-mm site. Maximum rates of net photosynthesis and stomatal conductance (g) did not vary appreciably within species across sites. Photosynthetic induction after a change from low to high light was significantly greater in plants growing in lower light conditions regardless of site. Photosynthetic induction also increased with the rate of g in diffuse light, prior to the increase to saturating light levels. Post-illumination CO₂ assimilation was the largest factor contributing to variation in C gain during simulated lightflecks. The duration of post-illumination photosynthetic activity, total CO₂ assimilation per light received, and light use efficiency during simulated lightflecks increased significantly with moisture inputs in four out of five species. Increasing leaf N concentration with increasing moisture inputs in three out of five species, coupled with changes in leaf N isotopic composition with the onset of the summer fog season suggest that natural N deposition increases with rain and fog inputs and contributes to greater utilization of fluctuating light availability in coastal California redwood forests.     

Simulation of leaf photosynthesis of C3 plants under fluctuating light and different temperatures

An induction-dependent empirical model was developed to simulate the C₃ leaf photosynthesis under fluctuating light and different temperatures. The model also takes into account the stomatal conductance when the light intensity just exceeds the compensation point after a prolonged period of darkness (initial stomatal conductance, $ g_{{{\text{S}}_{\text{ini}} }} $ ). The model was parameterized for both Chrysanthemum morifolium and Spinacia oleracea by artificially changing the induction states of the leaves in the climate chamber. The model was tested under natural conditions that were including frequent light flecks due to partial cloud cover and varying temperatures. The temporal course of observed photosynthesis rate and the carbon gain was compared to the simulation. The ability of the current model to predict the carbon assimilation rate was assessed using different statistical indexes. The model predictions were accurate but the model slightly underestimated the actual overall carbon gain. The accuracy of the simulation was largely dependent on the parameters that were calculated for the particular plant species, of which the simulation is intended for. In particular, the rate of change of induction and the initial stomatal conductance were found to be highly important and these were species-specific parameters for the predictions. The model is suitable for estimating instantaneous leaf CO₂ assimilation for different herbaceous plant species under dynamic environmental conditions. It can be simply calibrated for other crops, by estimating the individual parameters.     

Crassulacean acid metabolism (CAM) in an epiphytic ant-plant, Myrmecodia beccarii Hook.f. (Rubiaceae)

This study demonstrates unequivocally the presence of crassulacean acid metabolism (CAM) in a species of the Rubiaceae, the fourth largest angiosperm plant family. The tropical Australian endemic epiphytic ant-plant, Myrmecodia beccarii Hook.f., exhibits net CO₂ uptake in the dark and a concomitant accumulation of titratable acidity in plants in the field and in cultivation. Plants growing near Cardwell, in a north Queensland coastal seasonally dry forest of Melaleuca viridiflora Sol. ex Gaertn., accumulated ~50 % of their 24 h carbon gain in the dark during the warm wet season. During the transition from the wet season to the dry season, 24 h carbon gain was reduced whilst the proportion of carbon accumulated during the dark increased. By mid dry season many plants exhibited zero net carbon uptake over 24 h, but CO₂ uptake in the dark was observed in some plants following localised rainfall. In a shade-house experiment, droughted plants in which CO₂ uptake in the light was absent and dark CO₂ uptake was reduced, were able to return to relatively high rates of CO₂ uptake in the light and dark within 12 h of rewatering.     

Cambial growth dynamics and climatic control of different tree life forms in tropical mountain forest in Ethiopia

Munessa Forest is a mountain forest in south-eastern Ethiopia experiencing seasonal rainfall variation. We investigated seasonal cambial activity and dormancy from increment rates of four different tree species belonging to varying life forms, namely, evergreen native conifer (Podocarpus falcatus), evergreen introduced conifer (Pinus patula), evergreen broadleaved tree (Prunus africana) and deciduous broadleaved tree (Celtis africana). Measurements of stem radius fluctuations were registered with the help of high-resolution electronic dendrometers. Daily amplitudes of stem diameter variations and daily and monthly net growth rates were determined and related to climatic variables measured at local climate stations. Thin sections of wood collected with a microcorer every 3–6 weeks allowed a visual control of newly formed wood cells during consecutive time intervals. Lack of water availability during the long dry season induced cambial dormancy of 5–7 months depending on life forms. After the onset of the short rainy season, stem swelling started quite synchronously with a variation of only single days in all studied species. Evergreen tree species were able to initiate wood formation during the short rainy season, whereas growth in the deciduous broadleaved species started in the long rainy season. The acquired data provide a basis for delineating the species-specific growth boundaries and the duration of the cambial growing season.     

Modelling photosynthesis in fluctuating light with inclusion of stomatal conductance, biochemical activation and pools of key photosynthetic intermediates

Photosynthetic carbon gain in rapidly fluctuating light is controlled by stomatal conductance, activation of ribulose-1,5-bisphosphate carboxylase-oxygenase, a fast induction step in the regeneration of ribulose-1,5-bisphosphate, and the build-up of pools of photosynthetic intermediates that allow post-illumination CO₂ fixation. Experimental work over recent years has identified and characterised these factors. A physiologically-based dynamic model is described here that incorporates these factors and allows the simulation of carbon gain in response to any arbitrary sequence of light levels. The model output is found to conform well to previously reported plant responses of Alocasia macrorrhiza (L.) G. Don. observed under widely differing conditions. The model shows (i) responses of net assimilation rate and stomatal conductance to constant light levels and different CO₂ concentrations that are consistent with experimental observations and predictions of a steady-state model; (ii) carbon gain to continue after the end of lightflecks, especially in uninduced leaves; (iii) carbon gain to be only marginally reduced during low-light periods of up to 2 s; (iv) a fast-inducing component in the regeneration of ribulose-1,5-bisphosphate to be limiting for up to 60 s after an increase in light in uninduced leaves: the duration of this limitation lengthens with increasing CO₂ concentration and is absent at low CO₂ concentration; (v) oxygen evolution to exceed CO₂ fixation during the first few seconds of a lightfleck, but CO₂ fixation to continue after the end of the lightfleck whereas oxygen evolution decreases to low-light rates immediately. The model is thus able to reproduce published responses of leaves to a variety of perturbations. This provides good evidence that the present formulation of the model includes the essential rate-determining factors of photosynthesis under fluctuating light conditions. Received: 27 January 1997 / Accepted: 15 April 1997     

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

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

Physiological and morphological short-term responses to light and temperature in two Nothofagus species of Patagonia, South America

The study of plant responses to environmental stress factors is essential for management of plant systems and for anticipating their response to climate change. The main goal of this study was to determine morphological and physiological responses of Nothofagus obliqua and N. nervosa seedlings to light and temperature, two of the main stress factors acting in their current natural distribution in NW Patagonia. Responses to light were evaluated analyzing growth and survival, as well as morphological and physiological traits related to them, in seedlings subjected to three contrasting light conditions (full-sun conditions, 50% of sunlight and 20% of sunlight) during one growth season. Temperature photosynthetic responses were evaluated in seedlings subjected to temperature treatments between ?5 and 40°C for 2 and 4 h. Growth rate and biomass partition were similar between light treatments in both species. High apical meristem damage and decreased photosynthetic capacity of preformed leaves were observed under full-sun conditions, suggesting that high light levels have a deleterious effect on plant yield. Both species produced neoformed leaves during the growing season with better photosynthetic capacity than preformed leaves under full sun conditions, contributing to plant acclimation. Almost no plasticity was observed in morphological traits in response to shade. Both species differed in optimum temperature for photosynthesis, with a wider temperature range at which high photosynthesis is maintained in N. obliqua. In both species the higher values of net photosynthetic rate were found at higher temperatures than the mean annual temperature of its current natural distribution range. Under no water-stress conditions, future higher temperatures could increase carbon fixation of these species, with a little advantage of N. obliqua if temperature variance is high. Synergy effect of various environmental stress factors, particularly considering cultivation of these species outside their current natural distribution sites require further studies.     

Simulating carbon dioxide exchange rates of deciduous tree species: evidence for a general pattern in biochemical changes and water stress response

Background and Aims

Deciduous trees have a seasonal carbon dioxide exchange pattern that is attributed to changes in leaf biochemical properties. However, it is not known if the pattern in leaf biochemical properties – maximum Rubisco carboxylation (V_cmax) and electron transport (J_max) – differ between species. This study explored whether a general pattern of changes in V_cmax, J_max, and a standardized soil moisture response accounted for carbon dioxide exchange of deciduous trees throughout the growing season.

Methods

The model MAESTRA was used to examine V_cmax and J_max of leaves of five deciduous trees, Acer rubrum ‘Summer Red’, Betula nigra, Quercus nuttallii, Quercus phellos and Paulownia elongata, and their response to soil moisture. MAESTRA was parameterized using data from in situ measurements on organs. Linking the changes in biochemical properties of leaves to the whole tree, MAESTRA integrated the general pattern in V_cmax and J_max from gas exchange parameters of leaves with a standardized soil moisture response to describe carbon dioxide exchange throughout the growing season. The model estimates were tested against measurements made on the five species under both irrigated and water-stressed conditions.

Key Results

Measurements and modelling demonstrate that the seasonal pattern of biochemical activity in leaves and soil moisture response can be parameterized with straightforward general relationships. Over the course of the season, differences in carbon exchange between measured and modelled values were within 6–12 % under well-watered conditions and 2–25 % under water stress conditions. Hence, a generalized seasonal pattern in the leaf-level physiological change of V_cmax and J_max, and a standardized response to soil moisture was sufficient to parameterize carbon dioxide exchange for large-scale evaluations.

Conclusions

Simplification in parameterization of the seasonal pattern of leaf biochemical activity and soil moisture response of deciduous forest species is demonstrated. This allows reliable modelling of carbon exchange for deciduous trees, thus circumventing the need for extensive gas exchange experiments on different species.Key words: Carbon budget, deciduous trees, modelling, MAESTRA, soil moisture, species response, transpiration, Acer rubrum, Betula nigra, Quercus nuttallii, Q. phellos, Paulownia elongata     

The effects of the spatial pattern of defoliation on regrowth of a tussock grass

Summary The spatial pattern of foliage removal from a tussock grass can influence regrowth through effects on daily carbon gain (CER_d). This field study examined the extent to which tussock photosynthetic responses to different defoliation patterns were associated with changes in whole-canopy attributes (e.g., foliage age structure, canopy light microclimate). During the spring growing season, 60% of the green foliage area was removed from individual Agropyron desertorum tussocks with scissors in different spatial patterns. These patterns represented extremes of defoliation patterns that might be inflicted by natural herbivores. Tussock photosynthesis (per unit foliage area) at high light (2000 mol photons m^–2 s^–1 between 400 and 700 nm; P₂₀₀₀) increased following clipping with all defoliation patterns. The increases in P₂₀₀₀ were greater when leaves were removed from low in the tussock (older leaves) than if leaves high in the canopy (younger leaves) were removed. These relative changes of P₂₀₀₀ among clipping patterns paralleled the responses of CER_d and regrowth from an earlier study. Furthermore, the changes in P₂₀₀₀ corresponded with increases in the proportion of foliage within the tussocks that was directly illuminated at midday. The greater photosynthesis of tussocks after lower-leaf removal was directly related to a higher proportion of younger foliage and a smaller fraction of foliage shaded within the tussock. In a dense canopy, such as these grass tussocks, the influence of defoliation on whole-canopy attributes may be of primary importance to whole-plant photosynthetic responses.     

The Only African Wild Tobacco,Nicotiana africana: Alkaloid Content and the Effect of Herbivory

Herbivory in some Nicotiana species is known to induce alkaloid production. This study examined herbivore-induced defenses in the nornicotine-rich African tobacco N. africana, the only Nicotiana species indigenous to Africa. We tested the predictions that: 1) N. africana will have high constitutive levels of leaf, flower and nectar alkaloids; 2) leaf herbivory by the African bollworm Helicoverpa armigera will induce increased alkaloid levels in leaves, flowers and nectar; and 3) increased alkaloid concentrations in herbivore-damaged plants will negatively affect larval growth. We grew N. africana in large pots in a greenhouse and exposed flowering plants to densities of one, three and six fourth-instar larvae of H. armigera, for four days. Leaves, flowers and nectar were analyzed for nicotine, nornicotine and anabasine. The principal leaf alkaloid was nornicotine (mean: 28 µg/g dry mass) followed by anabasine (4.9 µg/g) and nicotine (0.6 µg/g). Nornicotine was found in low quantities in the flowers, but no nicotine or anabasine were recorded. The nectar contained none of the alkaloids measured. Larval growth was reduced when leaves of flowering plants were exposed to six larvae. As predicted by the optimal defense theory, herbivory had a localized effect and caused an increase in nornicotine concentrations in both undamaged top leaves of herbivore damaged plants and herbivore damaged leaves exposed to one and three larvae. The nicotine concentration increased in damaged compared to undamaged middle leaves. The nornicotine concentration was lower in damaged leaves of plants exposed to six compared to three larvae, suggesting that N. africana rather invests in new growth as opposed to protecting older leaves under severe attack. The results indicate that the nornicotine-rich N. africana will be unattractive to herbivores and more so when damaged, but that potential pollinators will be unaffected because the nectar remains alkaloid-free even after herbivory.     

Leaf dynamics and profitability in wild strawberries

Summary Leaf dynamics and carbon gain were evaluated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, N.Y., U.S.A., were studied for two or three years each. A computer-based model of plant growth and CO₂ exchange combined field studies of leaf biomass dynamics with previously-determined gas exchange rates to estimate carbon balances of leaves and whole plants in different environments.Leaves were produced throughout the growing season, although there was usually a decline in rate of leaf-production in mid-summer. Leaves produced in late spring had the largest area and longest lifespan (except for overwintering leaves produced in the fall). Specific Leaf Weight (SLW) varied little with time of leaf production, but differed greatly among populations; SLW increased with amount of light received in each habitat. The population in the most open habitat had the least seasonal variation in all leaf characters. F. vesca produced lighter, longer-lived leaves than F. virginiana.Simulations showed that age had the largest effect on leaf carbon gain in high-light environments; water stress and temperature had lesser effects. Leaf carbon gain in lowlight environments was relatively unaffected by age and environmental factors other than light. Leaves in high-light environments had the greatest lifetime profit and the greatest ratio of profit to cost. Increasing lifespan by 1/3 increased profit by 80% in low-light leaves and 50% in high-light leaves. Increasing the number of days during which the leaf had the potential to exhibit high photosynthetic rate in response to high light led to little change in profit of low-light leaves while increasing profit of high-light leaves by 49%.     

Carbon uptake and respiration in above-ground parts of a Larix decidua × leptolepis tree

Summary Shade needles of hybrid larch (Larix decidua × leptolepis) had the same rates of photosynthesis as sun needles per dry weight and nitrogen, and a similar leaf conductance under conditions of light saturation at ambient CO₂ (A_max). However, on an area basis, A_max and specific leaf weight were lower in shade than in sun needles. Stomata of sun needles limited CO₂ uptake at light saturation by about 20%, but under natural conditions of light in the shade crown, shade needles operated in a range of saturating internal CO₂ without stomatal limitation of CO₂ uptake. In both needle types, stomata responded similarly to changes in light, but shade needles were more sensitive to changes in vapor pressure deficit than sun needles. Despite a high photosynthetic capacity, the ambient light conditions reduced the mean daily (in summer) and annual carbon gain of shade needles to less than 50% of that in sun needles. In sun needles, the transpiration per carbon gain was about 220 mol mol^–1 on an annual basis. The carbon budget of branches was determined from the photosynthetic rate, the needle biomass and respiration, the latter of which was (per growth and on a carbon basis) 1.6 mol mol^–1 year^–1 in branch and stem wood. In shade branches carbon gains exceeded carbon costs (growth + respiration) by only a factor of 1.6 compared with 3.5 in sun branches. The carbon balance of sun branches was 5 times higher per needle biomass of a branch or 9 times higher on a branch length basis than shade branches. The shade foliage (including the shaded near-stem sun foliage) only contributed approximately 23% to the total annual carbon gain of the tree.     

A review of light interception in plant stands from leaf to canopy in different plant functional types and in species with varying shade tolerance

Changes in the efficiency of light interception and in the costs for light harvesting along the light gradients from the top of the plant canopy to the bottom are the major means by which efficient light harvesting is achieved in ecosystems. In the current review analysis, leaf, shoot and canopy level determinants of plant light harvesting, the light-driven plasticity in key traits altering light harvesting, and variations among different plant functional types and between species of different shade tolerance are analyzed. In addition, plant age- and size-dependent alterations in light harvesting efficiency are also examined. At the leaf level, the variations in light harvesting are driven by alterations in leaf chlorophyll content modifies the fraction of incident light harvested by given leaf area, and in leaf dry mass per unit area (M _A) that determines the amount of leaf area formed with certain fraction of plant biomass in the leaves. In needle-leaved species with complex foliage cross-section, the degree of foliage surface exposure also depends on the leaf total-to-projected surface area ratio. At the shoot scale, foliage inclination angle distribution and foliage spatial aggregation are the major determinants of light harvesting, while at the canopy scale, branching frequency, foliage distribution and biomass allocation to leaves (F _L) modify light harvesting significantly. F _L decreases with increasing plant size from herbs to shrubs to trees due to progressively larger support costs in plant functional types with greater stature. Among trees, F _L and stand leaf area index scale positively with foliage longevity. Plant traits altering light harvesting have a large potential to adjust to light availability. Chlorophyll per mass increases, while M _A, foliage inclination from the horizontal and degree of spatial aggregation decrease with decreasing light availability. In addition, branching frequency decreases and canopies become flatter in lower light. All these plastic modifications greatly enhance light harvesting in low light. Species with greater shade tolerance typically form a more extensive canopy by having lower M _A in deciduous species and enhanced leaf longevity in evergreens. In addition, young plants of shade tolerators commonly have less strongly aggregated foliage and flatter canopies, while in adult plants partly exposed to high light, higher shade tolerance of foliage allows the shade tolerators to maintain more leaf layers, resulting in extended crowns. Within a given plant functional type, increases in plant age and size result in increases in M _A, reductions in F _L and increases in foliage aggregation, thereby reducing plant leaf area index and the efficiency of light harvesting. Such dynamic modifications in plant light harvesting play a key role in stand development and productivity. Overall, the current review analysis demonstrates that a suite of chemical and architectural traits at various scales and their plasticity drive plant light harvesting efficiency. Enhanced light harvesting can be achieved by various combinations of traits, and these suites of traits vary during plant ontogeny.     

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

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

     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).