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Leaf respiratory temperature responses and general leaf properties of Quercus rubra were measured throughout the 2003 growing season in a deciduous forest in the northeastern USA. Measurements were made in the upper and lower portions of the canopy at two sites with different soil water availability. Correlations among respiration and various leaf properties were examined. At a set temperature (10 and 20°C), area-based leaf respiration rates were higher in both the early and late growing season than in the mid-growing season (0•50 vs 0•33µmol CO2 m2 s-1at 10°C, on average). Upper-canopy leaves generally had higher respiration rates than lower-canopy leaves (0•53 vs 0•30 µmol CO2 m-2 s-1 at 10°C, on average). At the drier site a more significant seasonal pattern in respiration was observed, while at the more mesic site a stronger canopy-position effect was detected. E0, a model variable related to the overall energy of activation of respiration, varied only slightly (52±5 kJ mol-1K-1), and was not influenced by season, site or canopy position. Leaf properties (specific leaf area, nitrogen, soluble sugars) also varied with season, site and canopy position. Leaf N and reducing monose were positively correlated with leaf respiration rates. After isolating single factors (season, site, canopy position), reducing monose could partially explain the seasonality in respiration (32- 79%), and leaf N (Narea) was well correlated with the canopy-position effect. Our results suggest that the temporal and spatial heterogeneities of respiration need to be considered in ecosystem models, but significant simplifications may be made in Q. rubra by assuming a constant temperature coefficient (E0, 52•5 kJ mol-1in this study) or predicting the base respiration rate (R0) from well understood leaf properties.

During the 20th century the northeastern U.S.A. has undergone an annual temperature increase of 1 °C, the combined effect of winter warming and an increase in daily summer minimum temperatures. A significant cooling of spring through autumn in maximum air temperatures is also evident since 1950. Therefore, the primary objective of this study is to document these climate trends and variability over the last century. A secondary objective is to provide a preliminary analysis of how these changes may have impacted hydrologic and ecosystem processes. Specifically, with respect to ecosystem processes, we examine how the cooling of daytime maximum temperatures may have impacted plant respiration and biomass accumulation. The study site is the Black Rock Forest, an experimental forest located in Hudson Highlands of New York that has been maintained as a conservation area over the last 100 years. For the region centered about the forest, there exists a climate/weather record and an extensively maintained biomass record that extends continuously from the early part of the 20th century through present. With such an extensive physical and biological record to draw from, this forest provides a microcosm for studying how changes in 20th century local and regional climate may have impacted ecosystem processes such as species adaptation, biomass growth, and 20th century carbon sequestration. In a subsequent paper we will more extensively explore the relationship between this record of changing climate and eco-hydrological processes.

The objective of this study was to characterize the forest canopy in terms of species composition, tree size classes and spatial distribution of leaf area of a mature temperate rainforest to provide a basis for scaling up shoot-level processes of energy and mass exchange using a one-dimensional canopy model. Stem number per unit area was high (1660 stems ha −1 ) but species diversity was low ( n = 10). Two species ( Dacrydium cuppressinum and Weinmannia racemosa ) equally accounted for 70% of the stem numbers, with the remaining 30% distributed among the other eight tree species. However, D. cuppressinum alone accounted for 72% of basal area and 75% of crown volume. Across the site L e (“effective” leaf area index, uncorrected for foliage clumping and woody surface interceptance) varied between 2.2 and 5.6, with a mean value of 3.5. Half total woody surface area per unit ground area ( W ) was 1.1. The mean foliage clumping index within shoot elements ( γ E ) was 1.2, while that between shoot elements ( Ω E ) was 0.87, giving an overall foliage clumping factor ( Ω = Ω E / γ E ) of 0.73. After corrections for foliage clumping and woody surface interceptance, half total leaf area per unit ground area ( L hc = L e / Ω − W ) was 3.7. For individual shoots, γ E increased from 1.0 to 1.5 with height in the canopy, possibly reflecting shoot structural adaptation to local irradiance. Simulations with the canopy model over 1 year using mean clumping factors revealed that clumping reduced total canopy radiation interception by 5%. Shoot-level irradiance was reduced at the top of the canopy but increased in lower canopy layers resulting in a more even canopy irradiance distribution. This enhanced annual canopy photosynthesis by 8% over a canopy with randomly distributed foliage. When the simulations were repeated to include the observed variation of γ E with height, total canopy radiation absorption was 11% lower than for a random canopy and canopy photosynthesis was 12% greater than a random canopy. The combination of reduced canopy radiation absorption and increased photosynthesis due to foliage clumping resulted in a considerable enhancement of canopy light-use efficiency. These analyses reveal a significant advantage of clumped foliage over randomly distributed foliage in terms of carbon gain, even for a forest canopy with moderate leaf area index.

Summary • Measurements of CO2 efflux from stems and branches, sap velocity, and respiratory activity of excised wood cores were conducted in Dacrydium cupressinum trees that differed in diameter, age, and canopy emergence. The objective of this study was to determine if consistent linkages exist among respiratory production of CO2 within stems, xylem transport of CO2, and the rate of CO2 diffusing from stem surfaces. • Stem CO2 efflux was depressed during periods of sap flow compared with the efflux rate expected for a given stem temperature and was positively correlated with sapwood density. By contrast, no significant relationships were observed between CO2 efflux and the respiratory activity of wood tissues. • Between 86 and 91% of woody tissue respiration diffused to the atmosphere over a 24-h period. However, at certain times of the day, xylem transport and internal storage of CO2 may account for up to 13–38% and 12–18%, respectively, of woody tissue respiration. • These results demonstrate that differences in sap flow rates and xylem anatomy are critically important for explaining within- and between-tree variation in CO2 efflux from stems.

We assessed the relative limitations to photosynthesis imposed by stomatal and non-stomatal processes in Dacrydium cupressinum Lamb. (Podocarpaceae), which is the dominant species in a native, mixed conifer-broad-leaved rainforest in New Zealand. For comparison, we included three co-occurring broad-leaved tree species (Meterosideros umbellata Cav. (Myrtaceae), Weinmannia racemosa L.f. (Cunoniaceae) and Quintinia acutifolia Kirk (Escalloniaceae)) that differ in phylogeny and in leaf morphology from D. cupressinum. We found that low foliage phosphorus content on an area basis (P(a)) limited light-saturated photosynthesis on an area basis (A(sat)) in Q. acutifolia. Depth in the canopy did not generally affect A(sat) or the relative limitations to A(sat) because of stomatal and non-stomatal constraints, despite reductions in the ratio of foliage mass to area, foliar nitrogen on an area basis (N(a)) and P(a) with depth in the canopy. In the canopy-dominant conifer D. cupressinum, A(sat) was low, consistent with low values of the maximum rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V(cmax)). In comparison, the A(sat) response of the three broad-leaved tree species was quite variable. Although A(sat) was high in the canopy-dominant M. umbellata, it was low in the sub-canopy trees W. racemosa and Q. acutifolia. Relative stomatal limitation to photosynthesis was more pronounced in W. racemosa (40%) than in the other three species (28-33%). Despite differences in degree, non-stomatal limitation to A(sat) predominated in all tree species.

This study explores the influence of temperature on the growth of six northern range margin (NRM) tree species in the Hudson River Valley (HRV). The HRV has excellent geographic and floristic qualities to study the influence of climate change on forested ecosystems. Indices of radial growth for three populations per species are developed and correlated against average minimum and maximum monthly temperatures from 1897 to 1994. Only positive correlations to temperature are considered for this analysis. Principal component analysis (PCA) is performed on chronologies over the entire HRV and at four subregions. PCA reveals a strong common signal among populations at subregional and regional scales. January temperatures most limit growth at the ecosystem level, supporting the hypothesis that winter temperatures may control vegetational ecotones. Surprisingly, growth of the oak–hickory ecosystem is most limited by January temperatures only in the southern half of the study region. Chestnut and white oak are the primary species driving the geographic pattern. As winter xylem embolism is a constant factor for ringporous species, snow cover and its interaction on fine root mortality may be the leading factors of the pattern of temperature sensitivity. Species-specific differences in temperature sensitivity are apparent. Atlantic white-cedar (AWC) and pitch pine are more sensitive to the entire winter season (December–March) while oak and hickory are most sensitive to January temperatures. AWC is most sensitive species to temperature. Chestnut and white oak in the HRV are more sensitive to winter temperature than red oak. Pignut hickory has the most unique response with significant relations to late growing season temperatures. Interestingly, AWC and pitch pine are sensitive to winter temperatures at their NRM while oak and hickory are not. Our results suggest that temperature limitations of growth may be species and phylogenetically specific. They also indicate that the influence of temperature on radial growth at species and ecosystem levels may operate differently at varying geographic scales. If these results apply broadly to other temperate regions, winter temperatures may play an important role in the terrestrial carbon cycle. r 2004 Elsevier GmbH. All rights reserved.

Summary • Here, we compared the carbon isotope ratios of leaf respiratory CO2 (δ13CR) and leaf organic components (soluble sugar, water soluble fraction, starch, protein and bulk organic matter) in five C3 plants grown in a glasshouse and inside Biosphere 2. One species, Populus deltoides, was grown under three different CO2 concentrations. • The Keeling plot approach was applied to the leaf scale to measure leaf δ13CR and these results were compared with the δ13C of leaf organic components. • In all cases, leaf respiratory CO2 was more 13C-enriched than leaf organic components. The amount of 13C enrichment displayed a significant species-specific pattern, but the effect of CO2 treatment was not significant on P. deltoides. • In C3 plant leaves, 13C-enriched respiratory CO2 appears widespread. Among currently hypothesized mechanisms contributing to this phenomenon, non-statistical carbon isotope distribution within the sugar substrates seems most likely. However, caution should be taken when attempting to predict the δ13C of leaf respiratory CO2 at the ecosystem scale by upscaling the relationship between leaf δ13CR and δ13C of leaf organic components.

Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night-time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7lmolm � 2 s � 1 ) were approximately four times higher than those in the lower leaves (4.3lmolm � 2 s � 1 ) and maximum photosynthesis rates in the upper shaded leaves (8.0lmolm � 2 s � 1 ) were about half those in the upper leaves. There was a strong linear relationship between total night-time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 201C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, Vcmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, Jmax (P40.05), and light use efficiency, « (Po0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model

Summary • The leaf-level response of respiration in the light ( R L ) is a vital component of a plant’s energy and carbon balance. • Xanthium strumarium (common cocklebur) plants were grown in various combinations of CO 2 , nitrogen and temperature, and R L was measured using the Kok effect method. • R L was significantly lower than respiration in the dark ( R D ), with the lowest percentage inhibition in the elevated CO 2 , high-N treatment. In general R L increased in response to increased CO 2 concentration and N availability across all temperature treatments. However, there was a significant interactive effect of growth CO 2 concentration and measurement temperature on R L which indicated that R L responded more positively to temperature changes in elevated CO 2 conditions. Additionally, across all CO 2 and N treatments the percentage of respired C with respect to assimilated C increased as temperature increased. • Collectively, these results improve our understanding of the magnitude and sensitivity to foreseen environmental changes of mitochondrial respiration during light hours.

Within-leaf variations in cell size, mitochondrial numbers and dark respiration rates were compared in the most recently expanded tip, the mid-section and base of needles of Pinus radiata D. Don trees grown for 4 years in open-top chambers at ambient (36 Pa) or elevated (65 Pa) carbon dioxide partial pressure (p(CO2)a). Mitochondrial numbers and respiratory activity varied along the length of the needle, with the highest number of mitochondria per unit cytoplasm and the highest rate of respiration per unit leaf area at the base of the needle. Regardless of the location of the cells (tip, middle or basal sections), needles collected from trees grown in elevated p(CO2)a had nearly twice the number of mitochondria per unit cytoplasm as those grown in ambient p(CO2)a. This stimulation of mitochondrial density by growth at elevated p(CO2)a was greater at the tip of the needle (2.7 times more mitochondria than in needles grown in ambient CO2) than at the base of the needle (1.7 times). The mean size of individual mitochondria was unaffected either by growth at elevated p(CO2)a or by position along the needle. Tree growth at elevated p(CO2)a had a variable effect on respiration per unit leaf area, significantly increasing respiration in the tip of the needles (+25%) and decreasing respiration at the mid-section and base of the needles (-14% and -25%, respectively). Although a simple relationship between respiration per unit leaf area and mitochondrial number per unit cytoplasm was found within each CO2 treatment, the variable effect of growth at elevated p(CO2)a on respiration along the length of the needles indicates that a more complex relationship must determine the association between structure and function in these needles.

Rising atmospheric CO2 concentration has been shown to affect photosynthesis and dark respiration of plants. However, it is unclear how CO2 enrichment will simultaneously affect rates of photosynthesis and respiration and the abundance of organelles in which they take place. In a study with Nicotiana sylvestris, we examined the relationships between chloroplast number and photosynthesis and between mitochondrion number and dark respiration of plants grown at 365 or 730 μmol mol−1 CO2 in environmentally controlled chambers. Elevated CO2 significantly increased the width of chloroplasts due to the large size of starch grains present in CO2-enriched leaves. Higher CO2 concentration, however, had no effect on the length of chloroplasts. The dimension of mitochondria and cross-section area of leaf cells were also unaffected by CO2 concentration. Elevated CO2 increased chloroplast number per unit cell area by 71% and net photosynthesis (A) per unit leaf area by a similar magnitude (67%). Mitochondrion number per unit cell area increased by 130%, whereas dark respiration (Rd) per unit leaf area was only 36% higher during daytime and 48% higher during nighttime periods at elevated CO2. Daytime and nighttime Rd decreased over time in both CO2 treatments, but A remained relatively unchanged throughout the growth period. As a result, photosynthetic efficiency increased over time. Our results demonstrate the agreement between chloroplast number and photosynthetic responses to CO2 enrichment and highlight the contrast between the large increase in mitochondrion numbers and the moderate increase in respiration at elevated CO2 in N. sylvestris.

Summary • We measured night-time respiration and daytime photosynthesis of leaves in canopies of 4 m tall cottonwood (Populus deltoides) trees to investigate the link between leaf respiration and photosynthetic capacity. • Trees were grown at three CO2 partial pressures [p(CO2)a] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25°C), but constant daytime temperatures (30–32°C), in a short-term whole-ecosystem environmental manipulation. • Rates of night-time leaf dark respiration (Rd) increased significantly at all growth CO2 partial pressures when nocturnal temperatures were increased from 15 to 25°C. Predawn leaf nonstructural carbohydrate (soluble sugars and starch) content was significantly lower at the higher night temperatures. Photosynthetic capacity (Amax) during the day increased significantly between 15 and 25°C at 42 and 80 Pa, but not at 120 Pa. • These findings indicate that the previously determined relationships between elevated night-time temperature, dark respiration and increased photosynthetic capacity may also hold at elevated p(CO2)a. This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.

Since its introduction to North America, Lythrum salicaria (L.) (purple loosestrife) has become invasive in marshy and riparian habitats. We compared gas-exchange responses to external CO2 partial pressure and light, as well as related leaf structural and biochemical characteristics, of L. salicaria with those of co-occurring native Asclepias syriaca (common milkweed) and Solidago graminifolia (lance-leaved goldenrod) along a pond bank in the Black Rock Forest, Cornwall, New York, USA to examine if the invasive success of L. salicaria may be influenced by robust leaf gas-exchange characteristics, including relatively high rates of photosynthesis and low rates of respiration, compared with those of less successful co-occurring native plant species. Neither the mean rate of net photosynthesis measured at ambient CO2 and saturating photon flux density (A) nor the mean dark respiration rate (RD) differed significantly between L. salicaria and either of the native species, while both the mean maximum rate of photosynthesis at saturating CO2 concentration and photon flux density (A max) and the mean rate of respiration measured in light (RL) were significantly higher in L. salicaria than A. syriaca, but no different between L. salicaria and S. graminifolia. Likewise, photosynthetic nitrogen-use efficiency was greater in L. salicaria than A. syriaca only, while photosynthetic water-use efficiency was significantly less in both L. salicaria and S. graminifolia than in A. syriaca. Despite limited interspecific differences in leaf photosynthesis, respiration, and resource-use efficiency, particularly between L. salicaria and S. graminifolia, we found that L. salicaria assimilated 208% more carbon per unit of energy invested in leaf biomass than either of the co-occurring native species, suggesting that increased photosynthetic energy-use efficiency may influence its observed invasive success.

To examine how global change could influence species invasions, we compared the responses of energetic processes and growth of invasive and native grass species to atmospheric CO2 enrichment in an intact Mojave Desert ecosystem. Combined with its modest influence on photosynthetic activity, elevated atmospheric CO2 was associated with a significant reduction in the energetic cost of aboveground biomass construction in invasive Bromus madritensis spp. rubens (red brome) without a concurrent cost reduction in native Vulpia octoflora (six-weeks fescue). Consequently, the invasive grass species grew faster, grew bigger, and produced more seeds with atmospheric CO2 enrichment than the native grass species. As a physiological mechanism of invasive species success driven by CO2 enrichment, such alterations in biomass construction costs combined with increased photosynthetic activity could trigger a shift in the species composition of this ecosystem, and potentially that of other invaded ecosystems, toward increa...

Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp-broadleaved forest in New Zealand, dominated by 100–400-year-old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth

Because of scaling problems point measurements of soil CO2 efflux on a small volume of soil may not necessarily reflect an overall community response. The aim of this study was to test this hypothesis in the Biosphere 2 facility and achieve the following broad goals: (1) investigate soil net CO2 exchange‐temperature relationship at the community level; (2) compare soil net CO2 exchange at the community level to the traditional sample point estimates of CO2 efflux scaled up to the community level; (3) evaluate the usefulness of a facility such as Biosphere 2 for conducting community level experiments for studying response to a climatic perturbation under controlled environmental conditions. A 550 m 3 volume of soil with 282, 15 cm tree stumps was enclosed at the Biosphere 2 Center and warmed from 10 to 25 8C over a period of 34 days. Net CO2 exchange from this community was measured at various points on the soil surface with 78.5 cm 2 chambers and for the whole community using each of the three bays at Biosphere 2 Center as a closed system. Soil CO2 efflux rates obtained by point measurements showed tremendous variability from location to location. At the community level and with point measurements, net CO2 exchange increased exponentially with increasing soil temperatures. Q10 values from both the point and community level measurements ranged from 1.7 to 2.5. Scaling of point measurements by soil surface area and time overestimated community rates by 36% revealing some of the limitations of point measurements. This experiment demonstrates how Biosphere 2 facility could be used to study behavior of individual components and measure responses at the community level and test our capacity to scale point in time and space measures of community processes to the community level. # 2002 Elsevier Science B.V. All rights reserved.

This study explores the relationship between the normalized difference vegetation index (NDVI), aboveground plant biomass, and ecosystem C fluxes including gross ecosystem production (GEP), ecosystem respiration (ER) and net ecosystem production. We measured NDVI across long-term experimental treatments in wet sedge tundra at the Toolik Lake LTER site, in northern Alaska. Over 13 years, N and P were applied in factorial experiments (N, P and N + P), air temperature was increased using greenhouses with and without N + P fertilizer, and light intensity (photosynthetically active photon flux density) was reduced by 50% using shade cloth. Within each treatment plot, NDVI, aboveground biomass and whole-system CO(2) flux measurements were made at the same sampling points during the peak-growing season of 2001. We found that across all treatments, NDVI is correlated with aboveground biomass ( r(2)=0.84), GEP ( r(2)=0.75) and ER ( r(2)=0.71), providing a basis for linking remotely sensed NDVI to aboveground biomass and ecosystem carbon flux.

Summary 1. The responses of night-time dark respiration ( R d ) to temperature and leaf characteristics were measured through the canopies of tree species from two distinct forests ‐ an oak-dominated deciduous forest in north-eastern USA, and a conifer-dominated temperate rainforest in New Zealand. These were chosen to examine the extent to which canopy level changes in dark respiration can be applied across forest biomes, and the appropriateness of scaling rules to calculations of whole-canopy carbon efflux. 2. The response of respiration to temperature differed significantly between species and with height in the canopy. This involved changes in both R d at a reference temperature, and the extent to which R d increased with temperature (described by the energy of activation, E o , or the change in R d over a 10 ° C range, Q 10 ). E o ranged from 25 (lowercanopy leaves) to 53·8 kJ mol - 1 K - 1 (upper-canopy leaves) in the deciduous forest, and from 24‐37 kJ mol - 1 K - 1 in the temperate rainforest site. 3. Relationships between respiratory and leaf characteristics indicated that the instantaneous rate of respiration covaries with soluble sugar concentration and leaf nitrogen, but the temperature response of respiration ( E o or Q 10 ) appears to be driven by leaf N. 4. Scaling leaf respiratory carbon loss to the whole-canopy level indicated that simplifying assumptions regarding the variation in respiration and its temperature response with canopy height tend to underestimate carbon loss if the assumptions are based on lower-canopy leaf physiology, but overestimate carbon loss if the assumptions are based on upper-canopy physiology. Thus, canopy-level differences in leaf respiratory characteristics should be considered in modelling efforts attempting to estimate wholecanopy respiration.

In 1953, Sir Hans Krebs won the Nobel Prize for his classic work elucidating the metabolic steps by which citrate is metabolized within the mitochondria and drives energy recovery from stored carbohydrates. Through an elegant series of reactions fed from a common substrate, the tricarboxylic acid cycle (TCA) creates carbon compounds of various sizes and configurations to support a multitude of metabolic reactions (Krebs 1936). Contemporaneously, physiologists like Bessel Kok, concentrating on photosynthetic metabolism, suggested an in vivo linkage between photosynthesis and respiration that was sensitive to environmental conditions, particularly light (Kok 1948). Many talented scientists from the fields of molecular biology, biochemistry, physiology, and ecology have continued to examine this linkage and the effect of light on it (Hurry et al. 2005). Significant progress has integrated and advanced our knowledge of the biochemical and physiological controls of respiratory metabolism, and now it is well established that even low levels of light can not only decrease respiratory CO2 release but can also cause the long-studied clockwise Krebs cycle to be decidedly non-cyclical (Fig. 1, ‘light’). On pages 2208–2220 of the December issue, Guillaume Tcherkez et al. (2012) use emerging technologies, developing theories and clever experimental manipulations to further advance our knowledge of the structure, function and control of respiratory metabolism in the light.The result is a clearer understanding of the tricarboxylic acid pathway (TCAP – not cycle!) and specifically the dynamic effects of atmospheric CO2 and O2 on respiratory metabolism. Changes in the atmospheric CO2 and/or O2 partial pressure are likely to influence respiratory metabolism during illumination in a variety of ways, though underlying functional controls of these effects are not yet clear. Elevated CO2 conditions can stimulate carboxylation and can decrease oxygenation of ribulose-1,5-bisphosphate (RuBP) in photosynthesis, altering the energy balance and efficiency of carbon fixation and the rate of triose-phosphate production (Sage, Sharkey & Seemann 1990). Kok’s observation of lower mitochondrial CO2 efflux in the light has often been interpreted as a decreased demand for respiratory products when photosynthesis could more directly supply ATP, reductants and reduced sugars. However, increased rates of carbon fixation in elevated CO2 environments have been shown to vary in their influence on the degree of light inhibition of respiration, with studies reporting decreases (Wang et al. 2001; Shapiro et al. 2004) or little effect (Sage et al. 1990; Ayub et al. 2011; Crous et al. 2012). While acknowledging previous interpretations of mechanistic controls on the light inhibition of respiration, Tcherkez et al. view the complexity of plant metabolism in the light under different gaseous environments as a ‘persisting conundrum’ and tackle this issue using an arsenal of techniques. Key to their work is the development of tools to quantitatively follow the flow of individual carbon atoms among the various pools of the TCAP. Such detailed tracing allows for modelling of the probability and kinetics of individual reactions and provides qualitative links to isotopic fluxomics (Tcherkez et al. 2009). The last decade has seen a strong theoretical advancement in our understanding of mitochondrial metabolism in illuminated photosynthetic cells (Hurry et al. 2005; NunesNesi, Sweetlove & Fernie 2007; Leakey et al. 2009). As light impacts the redox state of cells and organelles, the regulation of various enzyme systems leads the TCAP ‘cycle’ to become decidedly less cyclical, opening the sequence of reactions to provide parallel but linked metabolic pathways (Fig. 1). In the dark, Krebs’ classical clockwise view is maintained, and citrate metabolism fed from glycolysis creates carbon substrates, releases CO2 as various intermediates are oxidized and supplies reductant to drive the formation of ATP by the electron transport chain/oxidative phosphorylation. By contrast, in illuminated photosynthetic cells (right panel), TCAP activity is fed directly from stored citrate, bypassing the incorporation of acetyl-coenzyme A, and is used primarily to drive the formation of glutamine/ glutamate rather than participating in the full cycle. Moreover, triose phosphates from the Calvin cycle can feed the formation of phosphoenolpyruvate, which itself can be carboyxlated, and the resulting oxaloacetate can be then transformed into malate or fumarate in the ‘left-hand’ side of the opened cycle. Using a stable isotope pulse-chase experiment, Tcherkez et al. show that the rate of respiration in the light is unaffected by short-term (hours) manipulations of [CO2] and further conclude that CO2 evolution from the TCAP accounts for only 20% of the total decarboxylations. Correspondence: K. L. Griffin. E-mail: griff@ldeo.columbia.edu Plant, Cell and Environment (2012) doi: 10.1111/pce.12039 bs_bs_banner

In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night-time respiration and daytime photosynthesis of leaves in canopies of 4-m-tall cottonwood ( Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 ∞ C) and either (i) a constant nocturnal temperature of 20 ∞ C or (ii) increasing nocturnal temperatures (15, 20 or 25 ∞ ∞ ∞ C). In the first (day warming only) experiment, rates of night-time leaf dark respiration ( R dark ) remained constant and leaves displayed a modest increase (11%) in light-saturated photosynthetic capacity ( A max ) during the day (1000‐1300 h) over the 6 ∞ C range. In the second (dual night and day warming) experiment, R dark increased by 77% when nocturnal temperatures were

We present observations and simulations examining plant–water relations in a forested catchment characterized by strong topographic control over surface hydrology and stand structure. The system is dominated by xeric and mesic ecotypes of Quercus rubra L. Soil depths are typically very thin and the severity of seasonal water stress in these trees is determined largely by position along the local topographic gradient. Water balance related ecotypic differences in Q. rubra were measured during the 2000 growing season at an upland and lowland site. Significant site differences in stomatal conductance, sap flux density, and leaf to sapwood area ratios were observed. However, mid-day leaf water potentials and the leaf area-specific canopy hydraulic conductance were not significantly different despite a 20% increase in soil moisture content, an average 10 m increase in tree height, and a higher leaf area index at the lowland site. These results suggest a close coordination of tree morphology, stand structure, and the hydraulic conductance of the combined soil–root–leaf pathway that governs leaf-level water vapor exchange rates similarly across the topographic gradient. To place these stand-level observations in the context of catchment-scale water balance we linked the SPA canopy model with a 1-D soil column model and TOPMODEL hydrologic formulations. The SPA model was used to represent the canopy because of its specific inclusion of hydraulic constraints on transpiration and leaf water potential. The combined model is spatially explicit in the distribution of ecotype morphology, and calculates transpiration rates for TOPMODEL-derived saturated and unsaturated area fractions within the watershed separately. Model predictions of stomatal conductance, upland latent energy flux, stream discharge, and soil moisture content compared favorably with observations. Sensitivity analyses of canopy model parameters indicate stream discharge in this system is most sensitive to changes in the maximum leaf area index, the minimum leaf water potential, and belowground resistance. Discharge was least sensitive to changes in stem hydraulic conductivity and capacitance. Model results and observations are discussed with respect to adaptations to water stress, hydraulic controls on canopy water use, and ecosystem water use.

Plant respiration is an important physiological process in the global carbon cycle serving as a major carbon flux from the biosphere to the atmosphere. Respiration is sensitive to temperature providing a link between environmental variability, climate change and the global carbon cycle. We measured leaf respiration in Populus deltoides after manipulating the air temperature surrounding part of a single leaf, and compared this to the temperature response of the same leaves after manipulating the temperature of the stand. The short-term temperature response of respiration (Q10– change in the respiration rate with a 10 °C increase in leaf temperature) was 1.7 when the leaf temperature was manipulated, but 2.1 when the stand-level temperature was changed. As a result, total night-time carbon release during the five-day experiment was 21% lower when using the Q10 estimates from the tradition leaf manipulation compared to the stand-level manipulation. We conclude that the temperature response of leaf respiration is related to whole plant carbon and energy demands, and that appropriate experimental procedures are required in examining respiratory CO2 release under variable temperature conditions.

Plant responses to elevated CO 2 concentrations ([CO 2 ]) may be regulated by both accelerated ontogeny and allocational changes as plants grow. However, isolating ontogenyrelated effects from age-related effects are difficult because these factors are often confounded. In this study, the roles of age and ontogeny in photosynthetic responses to elevated [CO 2 ] were examined on Xanthium strumarium L. grown at ambient (365 µ mol mol − − − 1 ) and elevated (730 µ mol mol − − − 1 ) [CO 2 ]. To examine age-related effects, six cohorts were planted at 5-day intervals. To examine ontogenyrelated effects, all plants were induced to flower at the same time; ontogeny in Xanthium is relatively unaffected by growth in elevated [CO 2 ]. Growth in elevated [CO 2 ] increased net photosynthetic rates by approximately 30% throughout vegetative growth (i.e. active carbohydrate sinks), approximately 10% during flowering (i.e. minimal sink activity), and approximately 20% during fruit production (i.e. active sinks). At the harvest, the ratio of source to sink tissue significantly decreased with increasing plant age and was correlated with leaf soluble sugar concentration. Leaf soluble sugar concentration was negatively correlated with the relative photosynthetic response to elevated [CO 2 ]. These results suggest that age and ontogeny independently affect photosynthetic responses to elevated [CO 2 ] and the effects are mediated by reversible changes in source : sink balance.

Lythrum salicaria (purple loosestrife) is a nonindigenous invasive species characterized by prolific growth and abundance in marshy and riparian habitats across North America. Given its invasive success, we hypothesized this species may require less energy and/or use energy more efficiently for biomass construction than co-occurring noninvasive plant species. We measured leaf construction cost (CC), leaf mass per unit area (LMA), and leaf organic nitrogen and carbon content of L. salicaria and the five most abundant co-occurring species, Parthenocissus quinquefolia, Erigeron philadelphicus, Asclepias syriaca, Spiraea latifolia, and Solidago graminifolia, along dammed ponds in the Black Rock Forest, Cornwall, New York, USA. Lythrum salicaria, which was highly abundant (2.52 individuals/m(2)), exhibited significantly lower area-based leaf CC (44.47 ± 4.24 g glucose/m(2) leaf) than relatively less abundant species, suggesting energetics may influence its invasive success. Conversely, least abundant Solidago graminifolia (0.67 individuals/m(2)) exhibited the significantly highest leaf CC per unit leaf area (141.87 ± 39.21 g glucose/m(2) leaf). Overall, a negative correlation between species abundance and area-based leaf CC (r(2) = 0.73) indicated low energy requirements and/or high energy efficiency may influence relative abundance in the plant species studied. As it correlates with species abundance in this study, CC may be a useful tool for evaluating invasive potential.

Summary 1 Mass-based and area-based rates of respiration, leaf nitrogen content, leaf total protein content, non-structural carbohydrates and leaf mass per unit area (LMA) all decreased with depth in the canopy of Nothofagus fusca (Hook. F.) Oerst. (Red beech) trees grown for 5 years at ambient (36 Pa) or elevated (66 Pa) CO2 partial pressures. 2 Elevated CO2 partial pressure had a strong effect on dark respiration, decreasing both mass-based and area-based rates at all canopy positions, but had little or no effect on leaf physical and biochemical properties. 3 Leaf sugars, starch, protein, N and LMA were all correlated with respiration rate, and are therefore strong predictors of area-based dark respiration rates. The y axis intercept of regressions of respiration rate on mean leaf N, protein, starch and LMA was lower for plants grown at elevated compared to ambient CO2 partial pressures because of the differential effect of growth at elevated CO2 partial pressure on leaf gas-exchange, chemical and physical characteristics. 4 The lower respiration rates for leaves from trees grown at elevated CO2 partial pressure resulted in a significant increase in the ratio of light-saturated net photosynthesis to respiration, increasing the potential carbon-use efficiency of these leaves.

Leaf dark respiration ( R ) is an important component of plant carbon balance, but the effects of rising atmospheric CO 2 on leaf R during illumination are largely unknown. We studied the effects of elevated CO 2 on leaf R in light ( R L ) and in darkness ( R D ) in Xanthium strumarium at different developmental stages. Leaf R L was estimated by using the Kok method, whereas leaf R D was measured as the rate of CO 2 efflux at zero light. Leaf R L and R D were significantly higher at elevated than at ambient CO 2 throughout the growing period. Elevated CO 2 increased the ratio of leaf R L to net photosynthesis at saturated light ( A max ) when plants were young and also after flowering, but the ratio of leaf R D to A max was unaffected by CO 2 levels. Leaf R N was significantly higher at the beginning but significantly lower at the end of the growing period in elevated CO 2 -grown plants. The ratio of leaf R L to R D was used to estimate the effect of light on leaf R during the day. We found that light inhibited leaf R at both CO 2 concentrations but to a lesser degree for elevated (17–24%) than for ambient (29–35%) CO 2 -grown plants, presumably because elevated CO 2 -grown plants had a higher demand for energy and carbon skeletons than ambient CO 2 -grown plants in light. Our results suggest that using the CO 2 efflux rate, determined by shading leaves during the day, as a measure for leaf R is likely to underestimate carbon loss from elevated CO 2 -grown plants.

With increasing interest in the effects of elevated atmospheric CO 2 on plant growth and the global carbon balance, there is a need for greater understanding of how plants respond to variations in atmospheric partial pressure of CO 2 . Our research shows that elevated CO 2 produces significant fine structural changes in major cellular organelles that appear to be an important component of the metabolic responses of plants to this global change. Nine species (representing seven plant families) in several experimental facilities with different CO 2 -dosing technologies were examined. Growth in elevated CO 2 increased numbers of mitochondria per unit cell area by 1.3–2.4 times the number in control plants grown in lower CO 2 and produced a statistically significant increase in the amount of chloroplast stroma (nonappressed) thylakoid membranes compared with those in lower CO 2 treatments. There was no observable change in size of the mitochondria. However, in contrast to the CO 2 effect on mitochondrial number, elevated CO 2 promoted a decrease in the rate of mass-based dark respiration. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO 2 concentrations.

The effects of CO 2 enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) were studied in current year and 1-year-old needles of the same branch of field-grown Pinus radiata D. Don trees. All measurements were made in the fourth year of growth in large, open-top chambers continuously maintained at ambient (36 Pa) or elevated (65 Pa) CO 2 partial pressures. Photosynthetic rates of the 1-year-old needles made at the growth CO 2 partial pressure averaged 10.5± 0-5 μmol m -2 s -1 in the 36 Pa grown trees and 11.8± 0-4 pmol m -2 s -1 in the 65 Pa grown trees, and were not significantly different from each other. The photosynthetic capacity of 1-year-old needles was reduced by 25% from 23-0 ± 1-8 pmol m -2 s -1 in the 36 Pa CO 2 grown trees to 17.3 ± 0.7 μmol m -2 s -1 in the 65 Pa grown trees. Growth in elevated CO 2 also resulted in a 25% reduction in V cmax (maximum carboxylation rate), a 23% reduction in J max (RuBP regeneration capacity mediated by maximum electron transport rate) and a 30% reduction in Rubisco activity and content. Total non-structural carbohydrates (TNC) as a fraction of total dry mass increased from 12.8 ± 0-4% in 1-year-old needles from the 36 Pa grown trees to 14.2±0.7% in 1-year-old needles from the 65 Pa grown trees and leaf nitrogen content decreased from 1-30 ± 0-02 to 1.09 ± 0.10 g m -2 . The current-year needles were not of sufficient size for gas exchange measurements, but none of the biochemical parameters measured (Rubisco, leaf chlorophyll, TNC and N), were effected by growth in elevated CO 2 . These results demonstrate that photosynthetic acclimation, which was not found in the first 2 years of this experiment, can develop over time in field-grown trees and may be regulated by source-sink balance, sugar feedback mechanisms and nitrogen allocation.

Theoretical studies suggest that partitioning leaf photosynthetic responses to CO 2 partial pressures into two components, sensitivity and acclimation, facilitates both scaling-up photosynthetic responses and predicting global terrestrial carbon influx. Here, we experimentally examine these two components by growing soybean ( Glycine max ) in two CO 2 partial pressures, 35 and 70 Pa, and making a suite of ecophysiological measurements on expanding and fully expanded leaves. These CO 2 treatments resulted in a variety of acclimation responses, including changes in net photosynthetic rate and capacity, stomatal conductance, transpiration, and respiration. These responses were strongly dependent on leaf age. Despite the wide variety of acclimation responses, the experimentally derived photosynthetic sensitivity did not vary with CO 2 treatments or leaf age. In addition, the photosynthetic sensitivity to ambient CO 2 partial pressure was consistent with the sensitivity to intercellular CO 2 partial pressure, indicating little effect of stomatal conductance on photosynthetic sensitivity. This study supports the theoretical conclusion that photosynthetic sensitivity is independent of growth environment and leaf age, as well as photosynthetic acclimation, even though the latter varies with both environmental and developmental factors. Accordingly, photosynthetic sensitivity may be directly extrapolated from leaf to globe to predict the increment in terrestrial carbon influx stimulated by the yearly increase in atmospheric CO 2 , whereas the acclimation component must be used to adjust the overall global estimate.

Previous modelling exercises and conceptual arguments have predicted that a reduction in biochemical capacity for photosynthesis (Aarea) at elevated CO2 may be compensated by an increase in mesophyll tissue growth if the total amount of photosynthetic machinery per unit leaf area is maintained (i.e. morphological upregulation). The model prediction was based on modelling photosynthesis as a function of leaf N per unit leaf area (Narea), where Narea = Nmass×LMA. Here, Nmass is percentage leaf N and is used to estimate biochemical capacity and LMA is leaf mass per unit leaf area and is an index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multiple correlations that are known, or that are likely to exist between CO2 concentration, Narea, Nmass, LMA and Aarea. Although this is impractical experimentally, we can control for these correlations statistically using systems of linear multiple-regression equations. We developed a linear model to partition the response of Aarea to elevated CO2 into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO2 limitation of photosynthesis. The model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancement at elevated CO2 due to morphological upregulation was negligible for most species. The response of Aarea in these species was dominated by the reduction in CO2 limitation occurring at higher CO2 concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO2 due to an increase in LMA that was independent of any changes in Narea. This morphologically based inhibition of Aarea combined additively with a reduction in biochemical capacity to significantly offset the direct enhancement of Aarea caused by reduced CO2 limitation in two species. This offset was 100% for Acer rubrum, resulting in no net effect of elevated CO2 on Aarea for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological responses to elevated CO2 can have important effects on photosynthesis.

Estimation of leaf photosynthetic rate (A) from leaf nitrogen content (N) is both conceptually and numerically important in models of plant, ecosystem and biosphere responses to global change. The relationship between A and N has been studied extensively at ambient CO{sub 2} but much less at elevated CO{sub 2}. This study was designed to (1) assess whether the A-N relationship was more similar for species within than between community and vegetation types, and (2) examine how growth at elevated CO{sub 2} affects the A-N relationship. Data were obtained for 39 C{sub 3} species grown at ambient CO{sub 2} and 10 C{sub 3} species grown at ambient and elevated CO{sub 2}. A regression model was applied to each species as well as to species pooled within different community and vegetation types. Cluster analysis of the regression coefficients indicated that species measured at ambient CO{sub 2} did not separate into distinct groups matching community or vegetation type. Instead, most community and vegetation types shared the same general parameter space for regression coefficients. Growth at elevated CO{sub 2} increased photosynthetic nitrogen use efficiency for pines and deciduous trees. When species were pooled by vegetation type, the A-N relationship for deciduous trees expressed onmore » a leaf-mass bask was not altered by elevated CO{sub 2}, while the intercept increased for pines. When regression coefficients were averaged to give mean responses for different vegetation types, elevated CO{sub 2} increased the intercept and the slope for deciduous trees but increased only the intercept for pines. There were no statistical differences between the pines and deciduous trees for the effect of CO{sub 2}. Generalizations about the effect of elevated CO{sub 2} on the A-N relationship, and differences between pines and deciduous trees will be enhanced as more data become available.« less

The effects of CO2 enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1-year-old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open-top chambers at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO2 concentration [(pCO2)a] were lower in 1-year-old needles of trees grown at elevated CO2 concentrations than in those of trees grown at ambient (pCO2)a. At elevated CO2 concentrations Vcmax (maximum carboxylation rate) was reduced by 13% and Jmax (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth (pCO2)a of 4·68 ± 0·41 μmol m–2 s–1 and 6·15 ± 0·46 μmol m–2 s–1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO2)a were 6·2 ± 0·72 μmol m–2 s–1 at 36 Pa and 10·15 ± 0·64 μmol m–2 s–1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1-year-old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1-year-old needles were not significantly different in the CO2 treatments. There was no evidence in biochemical parameters for down-regulation at elevated (pCO2)a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO2 concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO2 concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age-dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO2 concentration.

The purpose of this paper is to summarize the results of a series of greenhouse and open- top chamber studies on the effects of N and elevated atmospheric CO 2 on ponderosa and loblolly pine (Pinus ponderosa Laws. and P. taeda L.) to evaluate common patterns of response. Growth response to elevated CO 2 ranged from zero to more than 1000%, depending largely upon N status. In both species, growth response to CO 2 was greater under moderate N deficiency than under extreme N deficiency or N sufficiency/excess. Elevated CO 2 generally caused lowered tissue N concentrations in many (but not all) cases, which in turn resulted in smaller increases in N uptake than in biomass. Growth response to N ranged from -50 (in ponderosa pine) to more than 1000%, depending upon the N status of the control medium. Growth response to N was enhanced by elevated CO 2 when N was in the extreme deficiency range but not when N was in the moderate deficiency range. In two separate studies, ponderosa pine responded negatively to high N inputs. and in each case this response was mitigated by elevated CO 2 . Collectively, these results show that (i) N deficiency is a continuum rather than a step function, (ii) responses to elevated CO 2 vary across this continuum of N deficiency. and (iii) elevated CO 2 greatly enhances growth response to N additions when N is initially-in the extremely deficient range.

Nonlinear responses of photosynthesis to the CO2 concentration at which plants were grown (Cg) have been often reported in the literatu~e. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of Cg (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at Cg was highest in plants from the 700 ppm CO2 treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective Cg. Measurements of CO2-induced changes in mass-based leaf nitrogen concentration (nnlf an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to Cg is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO2-induced changes in nm and h. These compiled data indicate that h generally increases and nm usually decreases with increasing Cg, but that the trajectories and magnitudes of the changes in h and nm vary with species and growth environments. Integration of these variables (nm and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to Cg. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to Cg reported in the literature.

Root growth and physiological uptake capacity for NH 4 + and NO 3 − were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO2 levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fraction of biomass allocated to active fine roots as well as total N (NH 4 + + NO 3 − ) absorption per unit root dry mass were unaffected by CO2. On a whole-plant basis, elevated CO2 led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO2 significantly increased plant N acquisition, the relative increase, in biomass far exceeded the gain in N, i.e. a 60% increase in total dry weight was accompanied by only a 30% increase in N gain in response to high CO2. We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO2 is largely caused by inability of the root systems to sufficiently compensate for increased N demand. Elevated CO2 significantly altered root uptake capacity of the different N forms, i.e., high CO2 significantly increased NO 3 − absorption rates, but decreased NH 4 + absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO2 increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO2-induced changes in plant preference for inorganic N forms is not simply regulated by root energy status. If changes in plant preference for inorganic N forms represent typical responses to elevated CO2, the results could have important implications for N dynamics in managed and natural plant communities.

We measured CO2 efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO2 on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mt M NH 4 + , and maintained in an atmosphere of either 35 or 70 Pa CO2. Carbon dioxide efflux (μmol CO2 plant−1 s−1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO2, primarily due to large increases in root mass. Specific CO2 efflux (μmol CO2 g root−1 s−1) responded to CO2 only under conditions of adequate soil nitrogen availability (3.5 mt M). Under these conditions, CO2 efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 μmol g−1 s−1 with elevated CO2 while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 μmol g−1 s−1. Although below ground CO2 efflux from seedlings grown in either sub-optimal (1 mt M) or supra-optimal (7 mt M) nitrogen availability did not respond to CO2, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO2 efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systems are responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and may lead to significantly different effects on whole plant carbon balance of these two forest tree species.

Our study sought to understand how past, low atmospheric CO2 concentrations ([CO2]) impact respiration (R) of soybean (Glycine max), when compared to plants grown under current and future [CO2]s. Experiments were conducted using plants grown under 290, 400 and 700 ppm [CO2]. Leaf R was measured in both darkness (RD) and in the light (RL; using the Kok method), with short-term changes in measurement [CO2] and [O2] being used to explore the relationship between light inhibition of leaf R and photorespiration. Root R, photosynthesis (A), leaf [N] and biomass allocation traits were also quantified. In contrast to the inhibitory effect of low growth [CO2] on A, growth [CO2] had no significant effect on leaf RD or root R. Irrespective of growth [CO2], RL was always lower than RD, with light inhibiting leaf R by 17-47%. Importantly, the degree of light inhibition of leaf R was lowest in plants grown under low [CO2], with variations in RL being positively correlated with RD and photorespiration. Irrespective of whether leaf R was measured in the light or dark, a greater proportion of the carbon fixed by leaf photosynthesis was released by leaf R in plants grown under low [CO2] than under current/future [CO2]'s. Collectively, our results highlight the differential responses of A and R to growth of plants under low to elevated atmospheric [CO2].

The carbon balance of Arctic ecosystems is particularly sensitive to global environmental change. Leaf respiration (R), a temperature-dependent key process in determining the carbon balance, is not well-understood in Arctic plants. The potential for plants to acclimate to warmer conditions could strongly impact future global carbon balance. Two key unanswered questions are (1) whether short-term temperature responses can predict long-term respiratory responses to growth in elevated temperatures and (2) to what extent the constant daylight conditions of the Arctic growing season inhibit leaf respiration. In two dominant Arctic species Eriophorum vaginatum (tussock grass) and Betula nana (woody shrub), we assessed the extent of respiratory inhibition in the light (RL/RD), respiratory response to short-term temperature change, and respiratory acclimation to long-term warming treatments. We found that R of both species is strongly inhibited by light (averaging 35% across all measurement temperatures). In E. vaginatum both RL and RD acclimated to the long-term warming treatment, reducing the magnitude of respiratory response relative to the short-term response to temperature increase. In B. nana, both RL and RD responded to short-term temperature increase but showed no acclimation to the long-term warming. The ability to predict plant respiratory response to global warming with short-term temperature responses will depend on species-specific acclimation potential and the differential response of RL and RD to temperature. With projected woody shrub encroachment in Arctic tundra and continued warming, changing species dominance between these two functional groups, may impact ecosystem respiratory response and carbon balance.

We examined changes in root growth and (15)NH(4) (+) uptake capacity of loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Douglas. Ex Laws.) seedlings that were grown in pots in a phytotron at CO(2) partial pressures of 35 or 70 Pa with NH(4) (+) as the sole N source. Kinetics of (15)N-labeled NH(4) (+) uptake were determined in excised roots, whereas total NH(4) (+) uptake and uptake rates were determined in intact root systems following a 48-h labeling of intact seedlings with (15)N. In both species, the elevated CO(2) treatment caused a significant downregulation of (15)NH(4) (+) uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root (15)NH(4) (+) uptake (V(max)). Rates of (15)NH(4) (+) uptake in intact roots were, however, unaffected by CO(2) treatment and were on average 4- to 10-fold less than the V(max) in excised roots, suggesting that (15)NH(4) (+) absorption from the soil was not limited by the kinetics of root (15)NH(4) (+) uptake. Despite the lack of a CO(2) effect on intact root absorption rates, (15)NH(4) (+) uptake on a per plant basis was enhanced at high CO(2) concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO(2) concentration increased total (15)NH(4) (+) uptake and the fraction of total biomass allocated to fine roots (< 2 mm in diameter) to a similar relative extent. We suggest that the increased uptake on a per plant basis in response to CO(2) enrichment is largely the result of a compensatory increase in root absorbing surfaces.

Human activity in the last 200 years has led to a marked increase in the level of C02 in the atmosphere. Plants sense increases in C02 levels and initially respond with an increase in photosynthetic rate, which may then slow as the plant adapts. This increase in photosynthetic rate may account in part for the ‘disappearance’ of an estimated 1.8 gigatons of carbon per year.

Direct and indirect effects of warming are increasingly modifying the carbon-rich vegetation and soils of the Arctic tundra, with important implications for the terrestrial carbon cycle. Understanding the biological and environmental influences on the processes that regulate foliar carbon cycling in tundra species is essential for predicting the future terrestrial carbon balance in this region. To determine the effect of climate change impacts on gas exchange in tundra, we quantified foliar photosynthesis (A net), respiration in the dark and light (R D and R L, determined using the Kok method), photorespiration (PR), carbon gain efficiency (CGE, the ratio of photosynthetic CO2 uptake to total CO2 exchange of photosynthesis, PR, and respiration), and leaf traits of three dominant species - Betula nana, a woody shrub; Eriophorum vaginatum, a graminoid; and Rubus chamaemorus, a forb - grown under long-term warming and fertilization treatments since 1989 at Toolik Lake, Alaska. Under warming, B. nana exhibited the highest rates of A net and strongest light inhibition of respiration, increasing CGE nearly 50% compared with leaves grown in ambient conditions, which corresponded to a 52% increase in relative abundance. Gas exchange did not shift under fertilization in B. nana despite increases in leaf N and P and near-complete dominance at the community scale, suggesting a morphological rather than physiological response. Rubus chamaemorus, exhibited minimal shifts in foliar gas exchange, and responded similarly to B. nana under treatment conditions. By contrast, E. vaginatum, did not significantly alter its gas exchange physiology under treatments and exhibited dramatic decreases in relative cover (warming: -19.7%; fertilization: -79.7%; warming with fertilization: -91.1%). Our findings suggest a foliar physiological advantage in the woody shrub B. nana that is further mediated by warming and increased soil nutrient availability, which may facilitate shrub expansion and in turn alter the terrestrial carbon cycle in future tundra environments.

summary We grew loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings in a factorial experiment with two CO2 partial pressures (35 and 70 Pa) and two nitrogen treatments (1-0 and 3–5 mM NH4+) for one growing season in a ‘common garden’ experiment designed to examine the extent that dry matter and nitrogen accumulation and partitioning are environmentally controlled. Ponderosa pine seedlings grown in 35 Pa CO2 and 3.5 mM NH4+ showed symptoms of nitrogen toxicity, characterized by greatly reduced growth, and moderately reduced total plant N. With the exception of this treatment combination, there were no significant differences between species in total plant dry matter or total plant nitrogen, suggesting that responses of growth to environmental conditions were stronger than heritable responses. There were however large differences in dry matter and N partitioning between the two species. Increases in leaf mass were largest in loblolly pine, whilst ponderosa pine tended to have higher root: shoot (R:S) ratios. R:S ratio of loblolly increased in response to C availability and decreased in response to N availability, whilst R: S ratio of ponderosa pine was much less responsive to resource availability. Total plant N varied with N supply, and N partitioning was related to plant growth and carbon partitioning. Carbon and N were interactive, such that an increase in the accumulation of either resource was always accompanied by an increase in the other. Over several seasons the different patterns of resource acquisition and biomass allocation that we observed in a uniform environment could potentially result in different growth rates at most resource levels. In the first season, contrary to our expectations, heritable differences in growth rate did not appear.

Abutilon theophrasti (C3) and Amaranthus retroflexus (C4), were grown from seed at four partial pressures of CO2: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO2. Net photosynthesis at growth CO2 partial pressures increased with increasing CO2 for C3 plants, but not C4 plants. Net photosynthesis of Abutilon at 15 Pa CO2 was 70% less than that of plants grown at 35 Pa CO2, due to greater stomatal and biochemical limitations at 15 Pa CO2. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO2 was nearly 3 times greater than at 35 Pa CO2. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vcmax), electron transport mediated RuBP regeneration capacity (J max), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO2 (A-C i) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO2. Abutilon grown at 15 Pa CO2 had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO2. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J max and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO2. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO2 compared with 35 Pa CO2, suggesting that the rise in CO2 since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO2 treatments. Differences in photosynthetic response to growth over a wide range of CO2 partial pressures suggest changes in the relative performance of C3 and C4 annuals as atmospheric CO2 has fluctuated over geologic time.

Laboratory studies indicate that plant respiratory efficiency may decrease in response to low nutrient availability due to increased partitioning of electrons to the energy-wasteful alternative oxidase (AOX); however, field confirmation of this hypothesis is lacking. We therefore investigated plant respiratory changes associated with succession and retrogression in soils aged from 10 to 120 000 years along the Franz Josef soil chronosequence, New Zealand. Respiration rates and electron partitioning were determined based on oxygen isotopic fractionation. Leaf structural traits, foliar nutrient status, carbohydrates and species composition were measured as explanatory variables. Although soil nutrient levels and species composition varied by site along the chronosequence, foliar respiration across all sites and species corresponded strongly with leaf nitrogen concentration (r(2) = 0.8). In contrast, electron partitioning declined with increasing nitrogen/phosphorus (r(2) = 0.23) and AOX activity correlated with phosphorus (r(2) = 0.64). Independently, total respiration was further associated with foliar Cu, possibly linked to its effect on AOX. Independent control of AOX and cytochrome pathway activities is also discussed. These responses of plant terminal respiratory oxidases - and therefore respiratory carbon efficiency - to multiple nutrient deficiencies demonstrate that modulation of respiratory metabolism may play an important role in plant responses to nutrient gradients.

Laboratory studies indicate that, in response to environmental conditions, plants modulate respiratory electron partitioning between the 'energy-wasteful' alternative pathway (AP) and the 'energy-conserving' cytochrome pathway (CP). Field data, however, are scarce. Here we investigate how 20-yr field manipulations simulating global change affected electron partitioning in Alaskan Arctic tundra species. We sampled leaves from three dominant tundra species - Betula nana, Eriophorum vaginatum and Rubus chamaemorus - that had been strongly affected by manipulations of soil nutrients, light availability, and warming. We measured foliar dark respiration, in-vivo electron partitioning and alternative oxidase/cytochrome c oxidase concentrations in addition to leaf traits and mitochondrial ultrastructure. Changes in leaf traits and ultrastructure were similar across species. Respiration at 20°C (R(20)) was reduced 15% in all three species grown at elevated temperature, suggesting thermal acclimation of respiration. In Betula, the species with the largest growth response to added nutrients, CP activity increased from 9.4 ± 0.8 to 16.6 ± 1.6 nmol O(2) g(-1) DM s(-1) whereas AP activity was unchanged. The ability of Betula to selectively increase CP activity in response to the environment may contribute to its overall ecological success by increasing respiratory energy efficiency, and thus retaining more carbon for growth.

[1] Variations in the Dole Effect, which have been used to infer past changes in biospheric productivity, are strongly affected by isotopic discrimination in soil respiration. Respiration through the alternative oxidase (AOX) pathway is associated with a higher discrimination than the one associated with the “normal” dark respiration pathway (the cytochrome pathway, COX). However, observations of O2 discrimination and AOX activity in undisturbed natural environments are scarce. In the current study we measured the O2 concentration and stable isotopes in the root zone of tundra, boreal forest and alpine forest soils. To estimate the discrimination from this data, we have performed O2diffusion experiments in gamma-sterilized soil columns, with varying soil clay content. The discrimination found in the diffusion experiments was independent of clay content, and the value found, 14 ± 2‰, is the same as the one for binary diffusion of O2 in N2, indicating no interaction between the O2 and clay particles. Based on the field and laboratory results, the respiratory discrimination in the soils studied is 15–31‰, with the higher values associated with colder soils. The high discrimination found for cold (

Urbanization is accelerating across the globe, elevating the importance of studying urban ecology. Urban environments exhibit several factors affecting plant growth and function, including high temperatures (particularly at night), CO(2) concentrations and atmospheric nitrogen deposition. We investigated the effects of urban environments on growth in Quercus rubra L. seedlings. We grew seedlings from acorns for one season at four sites along an urban-rural transect from Central Park in New York City to the Catskill Mountains in upstate New York (difference in average maximum temperatures of 2.4 °C; difference in minimum temperatures of 4.6 °C). In addition, we grew Q. rubra seedlings in growth cabinets (GCs) mimicking the seasonal differential between the city and rural sites (based on a 5-year average). In the field experiment, we found an eightfold increase in biomass in urban-grown seedlings relative to those grown at rural sites. This difference was primarily related to changes in growth allocation. Urban-grown seedlings and seedlings grown at urban temperatures in the GCs exhibited a lower root: shoot ratio (urban ~0.8, rural/remote ~1.5), reducing below-ground carbon costs associated with construction and maintenance. These urban seedlings instead allocated more growth to leaves than did rural-grown seedlings, resulting in 10-fold greater photosynthetic area but no difference in photosynthetic capacity of foliage per unit area. Seedlings grown at urban temperatures in both the field and GC experiments had higher leaf nitrogen concentrations per unit area than those grown at cooler temperatures (increases of 23% in field, 32% in GC). Lastly, we measured threefold greater (13)C enrichment of respired CO(2) (relative to substrate) in urban-grown leaves than at other sites, which may suggest greater allocation of respiratory function to growth over maintenance. It also shows that lack of differences in total R flux in response to environmental conditions may mask dramatic shifts in respiratory functioning. Overall, our findings indicating greater seedling growth and establishment at a critical regeneration phase of forest development may have important implications for the ecology of urban forests as well as the predicted growth of the terrestrial biosphere in temperate regions in response to climate change.