Positive Carbon Gain Research Articles

Seasonal maintenance of leaf level carbon balance facilitated by thermal acclimation of leaf respiration but not photosynthesis in three angiosperm species

Thermal acclimation of leaf gas exchange could facilitate the maintenance of positive carbon gain under thermally contrasting environments, yet our knowledge regarding the physiological mechanisms underpinning thermal acclimation remains incomplete. We hypothesize that leaf level carbon balance under warmer growth temperature (T growth ) could be sustained through thermal acclimation in leaf net photosynthesis (A n ) and dark respiration (R d ) in three angiosperm species differing in growth form. To test this hypothesis, the temperature response of A n and R d were measured in two periods characterized by different ambient monthly average air temperatures, 14.8 °C (April) and 26.3 °C (July). Furthermore, the thermal response of the biochemical components of photosynthesis, including the apparent rates of maximum Rubisco carboxylation (V cmax ) and electron transport (J max ), as well as leaf nitrogen concentration, were examined to reveal the biochemical control over thermal acclimation of leaf gas exchange. Partial thermal acclimation was found for leaf A n in two of the three species. In addition, leaf V cmax and J max measured at reference temperature (25 °C) tended to be lower in July compared with April, while parameters defining thermal sensitivity, including the activation energy (E a ) and entropy (ΔS V ), were largely invariable in response to increased T growth . By contrast, leaf R d exhibited strong thermal acclimation potential. At warmer temperatures, R d measured at 25 °C (R d25 ) was lower than at cooler growth temperatures, leading to higher leaf level carbon balance represented by the ratio of A n measured at 25 °C (A n25 ) to R d25 . Adjustments in leaf gas exchange were partially governed by the variation in leaf nitrogen concentration, and potentially nitrogen partitioning. Collectively, these findings demonstrate strong acclimation potential in leaf R d relative to A n , and highlight the role of nitrogen in regulating leaf level carbon gain. Our data contribute to the understanding of carbon acquisition strategy in these species, and provide empirical evidence for modeling the carbon flux using ecosystem models. • Thermal acclimation of leaf gas exchange facilitates positive carbon gain under thermally unstable environments. • Thermal response of photosynthesis and respiration were assessed under contrasting growth temperature for three species. • Respiration showed stronger acclimation potential than photosynthesis for all species. • Thermal acclimation was dominated by decreased basal rates, which was partially associated with variation in leaf nitrogen. • Response of biochemical capacity to temperature warrants further study to improve the accuracy of ecosystem models.

Sex Differences in Desiccation Tolerance Varies by Colony in the Mesic Liverwort Plagiochila porelloides.

Water scarcity, a common stress factor, negatively impacts plant performance. Strategies to cope with it, such as desiccation tolerance, are becoming increasingly important to investigate. However, phenomena, such as intraspecific variation in stress responses have not received much attention. Knowledge of this variability and the environmental drivers can be leveraged to further investigate the mechanisms of desiccation tolerance. Here we tested for variation in desiccation tolerance in Plagiochila porelloides among colonies and sexes within the same riparian zone. Field-collected dehardened plants were subjected to a desiccation event, under controlled conditions and then rehydrated. Plant water status, photosynthetic rates, net carbon gain, and efficiency of photosystem II (PSII) were assayed to evaluate tissue desiccation, basic metabolic processes and plant recovery. To establish a linkage between plant response and environmental factors, field light conditions were measured. We detected intraspecific variation, where a more exposed colony (high percentage of open sky, large temporal range of light quantity, and high red/far-red ratio) showed sex differences in desiccation tolerance and recovery. Overall, PSII recovery occurred by 72 h after rehydration, with a positive carbon gain occurring by day 30. This within species variation suggests plastic or genetic effects, and likely association with light conditions.

Hard and tough: the coordination between leaf mechanical resistance and drought tolerance

Leaves of terrestrial plants vary tremendously in size, shape, and stiffness. Hard leaves with high mass per unit area (LMA) occur more frequently in water- and nutrient-limited habitats, where stress-tolerance is key to survival. Coordination between leaf mechanical properties and drought tolerance can arise because some (but not all) of the anatomical modifications leading to high LMA are mechanistically correlated to physiological traits conferring tolerance to dehydration. Thick cell walls are frequently coupled with low turgor loss point values, and also prevent cell shrinkage and collapse after turgor loss has occurred. This also protects the leaf against shrinking-induced loss of water transport capacity in the extra-vascular pathway. In turn, vascular water transport in the leaf veins is at risk of embolism-induced disruption under drought, but higher length of major veins per unit area can provide alternative pathways for water delivery to the mesophyll, while contributing to increased leaf stiffness. Water shortage in dry and warm habitats implies the risk of reduced photosynthetic rates, apparently favouring species that produce hard and drought-tolerant leaves, to warrant long leaf life span and to maintain gas exchange and positive carbon gain when conditions become harsh.

Seasonal coordination of leaf hydraulics and gas exchange in a wintergreen fern.

Wintergreen fern Polystichum acrostichoides has fronds that are photosynthetically active year-round, despite diurnal and seasonal changes in soil moisture, air temperature and light availability. This species can fix much of its annual carbon during periods when the deciduous canopy is open. Yet, remaining photosynthetically active year-round requires the maintenance of photosynthetic and hydraulic systems that are vulnerable to freeze–thaw cycles. We aimed to determine the anatomical and physiological strategies P. acrostichoides uses to maintain positive carbon gain, and the coordination between the hydraulic and photosynthetic systems. We found that the first night below 0 °C led to 25 % loss of conductivity (PLC) in stipes, suggesting that winter-induced embolism occurred. Maximum photosynthetic rate and chlorophyll fluorescence declined during winter but recovered by spring, despite PLC remaining high; the remaining hydraulic capacity was sufficient to supply the leaves with water. The onset of colder temperatures coincided with the development of a necrotic hinge zone at the stipe base, allowing fronds to overwinter lying prostrate and maintain a favourable leaf temperature. Our conductivity data show that the hinge zone did not affect leaf hydraulics because of the flexibility of the vasculature. Collectively, these strategies help P. acrostichoides to survive in northeastern forests.

Effects of different light intensities, CO2 concentrations, temperatures and drought stress on photosynthetic activity in two paleoendemic resurrection plant species Ramonda serbica and R. nathaliae

Ramonda serbica and R. nathaliae are rare resurrection flowering plants of the northern hemisphere. Although their habitats on the north-exposed slopes of canyons and gorges are similar, those inhabited by R. nathaliae are more varied in terms of humidity, temperature and type of bedrock. These fine differences in ecological preference lead us to ask whether diploid R. nathaliae is ecologically more plastic than hexaploid R. serbica. Photosynthetic performance and fluorescence parameters are central in evaluating a plant's capacity to withstand extreme environmental conditions. We have compared the effects of various light intensities, CO2 concentrations and temperatures on the photosynthetic response of well hydrated plants of the two species. We also tracked changes in chlorophyll a fluorescence, photochemical efficiency and non-photochemical quenching during plant desiccation. The results have shown that both species have a low compensation point and low dark respiration that are indispensable for their survival at shaded and humid sites; only there are they able to maintain an appropriate equilibrium between positive net carbon gain and optimal water status. Under conditions of saturating light and CO2, net CO2 uptake was always higher in R. nathaliae that showed lower sensitivity to temperatures between 25°C and 40°C. Moreover, the leaves of R. nathaliae are amphistomatic, with smaller but more numerous stomata than those in the hypostomatic leaves of R. serbica. These xeromorphic characters of R. nathaliae suggest the better regulation of gas exchange and efficiency of water use, under the conditions of everyday short periods of direct sunlight and higher temperatures, making it possible for this species to inhabit more open, warm and dry habitats than R. serbica. Pronounced drought stress induced, in both species, strong decline in fluorescence parameters, photochemical quenching and chain electron transport rate. At the same time the exceptionally high non-photochemical quenching provided the main and highly effective mechanism of energy dissipation and protection against over-excitation. Although being homoiochlorophyllous, chloroplasts in desiccated plants are well protected from photodamage in two ways. First, the leaves are curled inward with the palisade tissue oriented to the inner side of the leaf. Second, the lower epidermis, that hence becomes exposed to sunlight, is densely covered with dark non-glandular hairs and is rich in carotenoids, anthocyanins and phenolics. All this protects desiccated plant tissues against excessive light induced damage.

Forest Invasion by the African Tulip Tree (Spathodea campanulata) in the Hawaiian Islands: Are Seedlings Shade-Tolerant?

Native to West Africa, Spathodea campanulata (African tulip tree) is frequently viewed as a shade-intolerant invader. It commonly colonizes roadsides, human-disturbed forests and abandoned agricultural land in tropical islands, where it can then become dominant in secondary forests. Some authors have suggested that the seedlings may be shade-tolerant and able to establish in closed forest, but the shade tolerance of seedlings has never been evaluated. We identified tolerated light environments of S. campanulata seedlings in wet forests in Hawaii by measuring photosynthetically active radiation (PAR) around naturally occurring seedlings (< 30 cm height) in the field. We also measured photosynthetic responses of seedlings to light under field and lab conditions, and determined seedling growth rates in sun and shade. Seedlings were found in shaded conditions in the field, and they consistently had positive net carbon gain at 50 µmol photons m-2·s-1 PAR, with an estimated mean compensation point below 10 µmol photons m-2·s-1, indicating high shade tolerance. The most frequent midday light environments of S. campanulata seedlings in the field were in the range around 50 to 200 µmol photons m-2·s-1 PAR, i.e., 2.5% to 10% of full sunlight. Among seedlings found growing in shade, minimum saturating light (Ek), determined from chlorophyll fluorescence, averaged 260 µmol photons m-2·s-1, suggesting that seedling maximum photosynthesis can occur at less than 13% of full sun. Growth rates of young seedlings in shade and sun were comparable. Widespread wind dispersal of seeds, seedling tolerance of low light, and our observations of some S. campanulata saplings establishing in rainforest without recent disturbance suggest that S. campanulata will be a persistent component of Hawaiian lowland rainforests.

Vertical variation in leaf gas exchange parameters for a Southeast Asian tropical rainforest in Peninsular Malaysia

Vertical variation in leaf gas exchange characteristics of trees grown in a lowland dipterocarp forest in Peninsular Malaysia was investigated. Maximum net photosynthetic rate, stomatal conductance, and electron transport rate of leaves at the upper canopy, lower canopy, and forest floor were studied in situ with saturated condition photosynthetic photon flux density. The dark respiration rate of leaves at the various heights was also studied. Relationships among gas exchange characteristics, and also with nitrogen content per unit leaf area and leaf dry matter per area were clearly detected, forming general equations representing the vertical profile of several important parameters related to gas exchange. Numerical analysis revealed that the vertical distribution of gas exchange parameters was well determined showing both larger carbon gain for the whole canopy and at the same time positive carbon gain for the leaves of the lowest layer. For correct estimation of gas exchange at both leaf and canopy scales using multi-layer models, it is essential to consider the vertical distribution of gas exchange parameters with proper scaling coefficients.

Enhanced growth of Juniperus thurifera under a warmer climate is explained by a positive carbon gain under cold and drought

Juniperus thurifera L. is an endemic conifer of the western Mediterranean Basin where it is subjected to a severe climatic stress characterized by low winter temperatures and summer drought. Given the trend of increased warming-induced drought stress in this area and the climatic sensitivity of this species, we expect a negative impact of climate change on growth and ecophysiological performance of J. thurifera in the harsh environments where it dominates. To evaluate this, we measured long- and short-term radial growth using dendrochronology, photosynthesis and water-use efficiency in males, females and juveniles in three sites in Central Spain. Climate was monitored and completed with historical records. Mean annual temperature has increased +0.2 °C per decade in the study area, and the main warming trends corresponded to spring (+0.2 °C per decade) and summer (+0.3 °C per decade). Radial growth and maximum photosynthesis peaked in spring and autumn. Positive photosynthetic rates were maintained all year long, albeit at reduced rates in winter and summer. Radial growth was enhanced by wet conditions in the previous autumn and by warm springs and high precipitation in summer of the year of tree-ring formation. Cloud cover during the summer increased growth, while cloudy winters led to impaired carbon gain and reduced growth in the long term. We argue that maintenance of carbon gain under harsh conditions (low winter temperatures and dry summer months) and plastic xylogenesis underlie J. thurifera's ability to profit from changing climatic conditions such as earlier spring onset and erratic summer rainfall. Our results highlight that not only the magnitude but also the sign of the impact of climate change on growth and persistence of Mediterranean trees is species specific.

Latitude, solar elevation angles and gap‐regenerating rain forest pioneers

Summary 1. Tropical rain forests have more species‐rich tree assemblages than forests at higher latitudes, but is this because they comprise a wider array of niches or functional types? We address this by considering one tree functional type – light‐demanding canopy trees with fast foliage turnover and growth – that is common in the tropics and subtropics, but virtually absent from mid‐latitude rain forests. Although often referred to as ‘tall pioneers’ or ‘large pioneers’, they are by no means confined to early‐successional stages, also recruiting directly to the canopy in old‐growth stands by rapid growth beneath tree‐fall gaps. 2. We also explored the influence of latitude on tree‐fall gap light environments as a possible constraint on the geographic distribution of this functional type, using the YPLANT program to simulate light interception and potential carbon gain by seedlings of the Australian rain forest pioneer Polyscias murrayi beneath idealized gaps at tropical, subtropical and cool temperate sites (latitudes 17, 29 and 42°S, respectively). P. murrayi grows quickly to heights of 20–25 m, has high photosynthetic capacity and respiration rates, and a leaf life span of 6–9 months. 3. Simulated light interception and potential carbon gain were strongly influenced by latitude, and by the interaction of latitude with position within an idealized tree‐fall gap of 100 m2. Potential net daily carbon gain of P. murrayi was strongly positive beneath the gap centre at latitude 17, and beneath the poleward (i.e. southern) gap margin at latitude 29, but negative beneath both the gap centre and margin at latitude 42. Light interception and carbon gain were also influenced by geographic variation in sunshine hours, which were highest at latitude 29 and lowest at latitude 42. A larger gap of 300 m2 permitted positive net carbon gain at all latitudes, although rates were again predicted to be highest beneath the gap centre in the tropics. 4. Synthesis. YPLANT simulations supported the hypothesis that sun angles could prevent trees with high metabolic rates from invading old‐growth mid‐latitude rain forests, where light environments suitable for their establishment will be scarce. Geographic variation in forest light environments is therefore likely to influence the range of viable functional types at different latitudes.

Aspen shoots are carbon autonomous during bud break

Current thinking holds that carbon autonomy of branches in trees is unlikely, particularly during bud break, when the new developing shoots require significant influx of carbon resources from more distant sources. Results from recent studies indicate that the impact of bud break on overall tree reserves might be small. In two studies the independence of flushing shoots from stored carbon reserves and the photosynthesis in developing new leaves and shoots of Populus tremuloides were explored. New developing shoots quickly became a positive carbon source and only a few days into flush, the photosynthetic system of the newly developing shoots was efficient enough to achieve positive carbon gain even at low light levels. Only 14% of the stored shoot reserves, without any mobilization from more distant reserves, were used during bud break and early shoot expansion. Without any underlying stress, shoots of deciduous trees appear to be carbon autonomous during bud break when demand on stored carbon should be the highest. The development of an efficient photosynthetic system in new shoots is critical in the recovery of carbon reserves in aspen. It minimizes the cost of bud break to the overall stored carbon reserves by optimizing the assimilation of carbon in the newly developed leaves, while eliminating the cost for mobilizing carbon reserves from more distant sources. This carbon autonomy of shoots has important implications for the whole tree carbon balance particularly to the non-photosynthetic tissues which functions solely depending on carbon export from the newly developing leaves and shoots.

Seasonal changes in the physiological activity of regeneration under a natural light gradient in a Pinus pinea regular stand

Seasonal courses of chlorophyll a fluorescence, gas exchange parameters and water status, joined to environmental variables, were measured in four age classes of stone pine (Pinus pinea L.) natural regeneration under a natural light gradient in a stone pine regular stand throughout 2007 and 2008. One-year old seedlings were worse adapted to the extreme drought conditions recorded in summer (especially in 2008), showing lower predawn and midday Fv/Fm values, slightly lower photosynthetic rates at midday, lower PSII values, lower water potentials (pd andmin), and high mortality (90%). The oldest seedlings (class 4, 51-200 cm high), although they do not experience the lowest min in July 2008, were not able to maintain a positive carbon gain at midday, even showing the lowest photosynthetic rates. However, they rapidly recovered once the climatic conditions were softened in early autumn. During late autumn low Fv/Fm values were found especially in one-year old seedlings, indicating that maximal photochemical efficiency of PSII is sensitive to the autumn temperatures below cero recorded during both years. Thus, stone pine regeneration adapts its physiology to the seasonal course of temperature and water availability, showing a different photosynthetic performance according to age class. However, seedling physiology is not affected by the natural light gradient found in the studied stand, probably related to the low variability recorded in GSF values and the low density of the stand. We suggest that such low stand densities cannot assure one-year old seedlings’ survival, but are sufficient for older seedlings.

Photosynthetic Characteristics of the C4Invasive Exotic Grass Miscanthus sinensis Andersson Growing Along Gradients of Light Intensity in the Southeastern United States

Miscanthus sinensis, a C4 grass, is an emerging invasive species in open and edge habitats throughout the eastern United States. To assess its ability to invade forest understories, we examined photosynthetic responses of Miscanthus sinensis grown under natural light gradients (5% to 100% full sunlight). Although there was a slight reduction in reproductive output under lower light, Miscanthus sinensis was able to maintain high rates of photosynthesis and positive carbon gain along the entire light gradient. Positive carbon gain at low light was likely due to an increased quantum yield. Photosynthetic induction for all plants was rapid, while stomatal opening during induction was slower. Rates of induction loss were slower than induction gain, and were slower than another major C4 invasive exotic grass, Microstegium vimineum that is shade tolerant. During transient high light events, the combination of rapid induction, maintenance of induction state between sunflecks, and high photosynthetic rates, may allow for greater carbon gain than native understory plants. Miscanthus sinensis possesses some characteristics that allow it to persist in forest understories and to rapidly utilize increases in light availability, rendering it a potentially important invader of southeastern United States forest understories.

Epiphytes and hemiepiphytes have slower photosynthetic response to lightflecks than terrestrial plants: evidence from ferns and figs

Abstract:Photosynthetic responses of 12 species including six fern species (Neottopteris nidus, Microsorum punctatum, Pseudodrynaria coronans, Asplenium finlaysonianum, Paraleptochilus decurrens and Tectaria fauriei) and seedlings of six fig species (Ficus curtipes, F. gibbosa, F. altissima, F. auriculata, F. oligodon and F. hookeriana) in different life forms to lightfleck were investigated, to test whether epiphytes and hemiepiphytes display a slower response to lightfleck and fast induction loss after a lightfleck compared with their terrestrial counterparts, and whether ferns display a slower response to lightfleck and slower induction loss compared to figs. The measurements of functional traits and physiological parameters were determined in a screenhouse of 4% full sunlight. Epiphytic ferns and hemiepiphytic figs had thicker leaves compared with their terrestrial counterparts. Compared with figs, ferns had thicker fronds, larger stomata with a low density, and lower stomatal conductance and photosynthetic capacity; ferns had lower light compensation point and dark respiration rate, conferring a positive carbon gain under low diffuse light beneath the canopy. The induction time to reach 90% maximum net photosynthetic rate (T90) upon the exposure to a saturated light varied strongly among life forms. Epiphytic ferns had slower T90 than terrestrial ferns (19.9–26.3 vs 5.9–16.3 min, respectively), and hemiepiphytic figs had slower T90 than terrestrial figs (13.1–20.4 vs 5.2–7.8 min, respectively). Compared with figs, ferns showed a slower response to lightfleck. Across ferns and figs, the induction time was negatively correlated with initial stomatal conductance. No significant difference in induction loss was found between two life forms within ferns or figs, whereas ferns had a significantly slower induction loss compared with figs. These results showed that the inherent conservative water use strategy of the epiphytes and hemiepiphytes constrain their lightfleck utilization.

Water budget and the consequent duration of canopy carbon gain in a teak plantation in a dry tropical region: Analysis using a soil–plant–air continuum multilayer model

A soil–plant–air continuum multilayer model was used to numerically simulate canopy net assimilation ( A n), evapotranspiration (ET), and soil moisture in a deciduous teak plantation in a dry tropical climate of northern Thailand to examine the influence of soil drought on A n. The timings of leaf flush and the end of the canopy duration period (CDP) were also investigated from the perspective of the temporal positive carbon gain. Two numerical experiments with different seasonal patterns of leaf area index (LAI) were carried out using above-canopy hydrometeorological data as input data. The first experiment involved seasonally varying LAI estimated based on time-series of radiative transmittance through the canopy, and the second experiment applied an annually constant LAI. The first simulation captured the measured seasonal changes in soil surface moisture; the simulated transpiration agreed with seasonal changes in heat pulse velocity, corresponding to the water use of individual trees, and the simulated A n became slightly negative. However, in the second simulation, A n became negative in the dry season because the decline in stomatal conductance due to severe soil drought limited the assimilation, and the simultaneous increase in leaf temperature increased dark respiration. Thus, these experiments revealed that the leaflessness in the dry season is reasonable for carbon gain and emphasized the unfavorable soil water status for carbon gain in the dry season. Examining the duration of positive A n (DPA) in the second simulation showed that the start of the longest DPA (LDPA) in a year approached the timing of leaf flush in the teak plantation after the spring equinox. On the other hand, the end appeared earlier than that of all CDPs. This result is consistent with the sap flow stopping earlier than the complete leaf fall, implying that the carbon assimilation period ends before the completion of defoliation. The model sensitivity analysis in the second simulation suggests that a smaller LAI and slower maximum rate of carboxylation likely extend the LDPA because soil water from the surface to rooting depth is maintained longer at levels adequate for carbon gain by decreased canopy transpiration. The experiments also suggest that lower soil hydraulic conductivity and deeper rooting depth can postpone the end of the LDPA by increasing soil water retention and the soil water capacity, respectively.

Importance of mesophyll diffusion conductance in estimation of plant photosynthesis in the field

Mesophyll diffusion conductance to CO(2) (g(m)) is an important leaf characteristic determining the drawdown of CO(2) from substomatal cavities (C(i)) to chloroplasts (C(C)). Finite g(m) results in modifications in the shape of the net assimilation (A) versus C(i) response curves, with the final outcome of reduced maximal carboxylase activity of Rubisco (V(cmax)), and a greater ratio of the capacity for photosynthetic electron transport to V(cmax) (J(max)/V(cmax)) and alterations in mitochondrial respiration rate (R(d)) when estimated from A/C(i) responses without considering g(m). The influence of different Farquhar et al. model parameterizations on daily photosynthesis under non-stressed (C(i) kept constant throughout the day) and stressed conditions (mid-day reduction in C(i)) was compared. The model was parameterized on the basis of A/C(C) curves and A/C(i) curves using both the conventional fitting procedure (V(cmax) and R(d) fitted separately to the linear part of the response curve and J(max) to the saturating part) and a procedure that fitted all parameters simultaneously. The analyses demonstrated that A/C(i) parameterizations overestimated daily assimilation by 6-8% for high g(m) values, while they underestimated if by up to 70% for low g(m) values. Qualitative differences between the A/C(i) and A/C(C) parameterizations were observed under stressed conditions, when underestimated V(cmax) and overestimated R(d) of A/C(i) parameterizations resulted in excessive mid-day depression of photosynthesis. Comparison with measured diurnal assimilation rates in the Mediterranean sclerophyll species Quercus ilex under drought further supported this bias of A/C(i) parameterizations. While A/C(i) parameterization predicted negative carbon balance at mid-day, actual measurements and simulations with the A/C(C) approach yielded positive carbon gain under these conditions. In addition, overall variation captured by the best A/C(i) parameterization was poor compared with the A/C(C) approach. This analysis strongly suggests that for correct parameterization of daily time-courses of photosynthesis under realistic field conditions, g(m) must be included in photosynthesis models.

Influence of canopy position, needle age and season on the foliar gas exchange of Pinus canariensis

We investigated the seasonal variation in the gas exchange of current and 1-year-old needles in the upper sun and lower shade crown of adult Pinus canariensis trees. In general, current year needles displayed lower gas exchange rates than the 1-year-old needles. In both needle age classes, gas exchange was significantly lower in the shade than in the sun crown. However irrespective of crown position and needle age, maximum daily net photosynthesis, transpiration, and stomatal conductance for water vapour were generally higher during the wet and cold winter as compared to the dry and hot summer. These higher gas exchange values obtained during the cold and wet season can mainly be explained by higher soil-water availability and lower evaporative demand as compared to the warm and dry seaon. In addition, we also observed a displacement in the temperature optimum of net photosynthesis towards lower temperatures during the cold and wet season as compared to the warm and dry season. The observed gas exchange characteristics indicate a conservative water saving strategy and thus allowing P. canariensis needles to maintain a positive carbon gain even at periods of high evaporative demand and low soil-water availability.

Early spring leaf out enhances growth and survival of saplings in a temperate deciduous forest

Saplings of many canopy tree species in winter deciduous forests receive the major portion of their light budget for their growing season prior to canopy closure in the spring. This period of high light may be critical for achieving a positive carbon (C) gain, thus contributing strongly to their growth and survival. This study of saplings of Aesculus glabra and Acer saccharum in Trelease Woods, Illinois, USA, tested this hypothesis experimentally by placing tents of shade cloth over saplings during their spring period of high light prior to canopy closure in three consecutive years. Leaf senescence began 16 days (year 0) and 60 days (year 1) earlier for shaded A. glabra saplings than control saplings. No change in senescence occurred for A. saccharum. The annual absolute growth in stem diameter of both species was negligible or negative for shaded saplings, but positive for control saplings. Only 7% of the shaded A. glabra saplings were alive after 2 years, while all control saplings survived for 3 years; only 20% of the shaded A. saccharum saplings survived for 3 years, while 73% of control saplings were alive after the same period. Early spring leaf out is a critical mechanism that allows the long-term persistence of saplings of these species in this winter deciduous forest. Studies and models of C gain, growth, and survival of saplings in deciduous forests may need to take into account their spring phenology because saplings of many species are actually "sun" individuals in the spring prior to their longer period in the summer shade.

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.

Cushion size, surface roughness, and the control of water balance and carbon flux in the cushion mossLeucobryum glaucum(Leucobryaceae)

We explored the size dependence of water balance and carbon flux in the cushion moss Leucobryum glaucum (Leucobryaceae). Conductance to water vapor (g(a)) was modeled empirically using 4-24 cm diameter cushions (N = 14) evaluated across wind speeds from 0.7 to 4.3 m/s in a wind tunnel. Model parameters included wind speed (u), kinematic viscosity (v), cushion diameter (L(d)), and surface roughness (L(r)). The model g(a) = -9.62(u/v)(1.21) · L(d)(-0.35) · L(r-in)(-1.85) (where L(r-in) represents a dimensionless form of L(r); R(2) = 0.88) indicates negative relationships between g(a) and both L(d) and L(r). These predictions were evaluated during a 5-d field experiment where water loss and net carbon exchange (estimated by ΔF/F(m)') were monitored. In the field (N = 18, 4-34 cm diameter cushions), L(r), but not L(d), controlled rates of evaporation due to additional turbulence that reduced size dependence of cushions along the forest floor. However, the duration of positive net carbon gain varied from 1.4 to 4.4 d and was significantly longer in larger diameter cushions. Thus, under field conditions, size-dependent changes in surface-area-to-volume relationships influence the duration of net carbon gain more than differences in water flux and lead to a strong size dependence of water balance and carbon flux.

Interactive effects of drought and shade on three arid zone chenopod shrubs with contrasting distributions in relation to tree canopies

Summary Plants that grow beneath trees in arid systems may frequently experience both water and light limitation, although protection from high radiation loads during drought may compensate for a loss of productivity due to reduced light availability when water is plentiful. We examined the effects of shading, during an imposed water deficit, on the carbon gain, stomatal conductance (gs) and shoot water potential (Ψs) of seedlings of three shrubs: Atriplex vesicaria (Heward ex Benth.), a C4 species, and Enchylaena tomentosa (R. Br.) and Rhagodia spinescens (R. Br.), which are restricted to shaded sites beneath trees. Under conditions of limiting water, photosynthetic rates measured at saturating light (Amax) were negative in high‐light grown Enchylaena plants but remained positive in shade‐grown plants. When water was not limiting, Amax was reduced in shade‐grown Atriplex but shade did not affect carbon gain in the other two species. AtriplexΨs was higher in shaded than in unshaded plants, but in unshaded plants positive carbon gain was maintained at Ψs below −10 MPa. Stomatal conductance and Amax decreased more slowly with increasing water deficit in shaded conditions in all species. Atriplex was tolerant of a broader range of light and soil moisture conditions than Enchylaena, with Rhagodia intermediate between these two species. The interactive effect between drought and shade and the ecophysiological tolerances of these three species have consequences for their field distributions.