Canopy position affects the temperature response of leaf respiration in Populus deltoides

Temperature dependence of carbon metabolism in the leaves in sun and shade in a subtropical forest

Rising temperatures pose a threat to the stability of climate regulation by carbon metabolism in subtropical forests. Although the effects of temperature on leaf carbon metabolism traits in sun-exposed leaves are well understood, there is limited knowledge about its impacts on shade leaves and the implications for ecosystem-climate feedbacks. In this study, we measured temperature response curves of photosynthesis and respiration for 62 woody species in summer (including both evergreen and deciduous species) and 20 evergreen species in winter. The aim was to uncover the temperature dependence of carbon metabolism in both sun and shade leaves in subtropical forests. Our findings reveal that shade had no significant effects on the mean optimum photosynthetic temperatures (TOpt) or temperature range (T90). However, there were decreases observed in mean stomatal conductance, mean area-based photosynthetic rates at TOpt and 25 °C, as well as mean area-based dark respiration rates at 25 °C in both evergreen and deciduous species. Moreover, the respiration-temperature sensitivity (Q10) of sun leaves was higher than that of shade leaves in winter, with the reverse being true in summer. Leaf economics spectrum traits, such as leaf mass per area, and leaf concentration of nitrogen and phosphorus across species, proved to be good predictors of TOpt, T90, mass-based photosynthetic rate at TOpt, and mass-based photosynthetic and respiration rate at 25 °C. However, Q10 was poorly predicted by these leaf economics spectrum traits except for shade leaves in winter. Our results suggest that model estimates of carbon metabolism in multilayered subtropical forest canopies do not necessitate independent parameterization of T90 and TOpt temperature responses in sun and shade leaves. Nevertheless, a deeper understanding and quantification of canopy variations in Q10 responses to temperature are necessary to confirm the generality of temperature-carbon metabolism trait responses and enhance ecosystem model estimates of carbon dynamics under future climate warming.

It's only natural: Plant respiration in unmanaged systems

Respiration plays a key role in the terrestrial carbon cycle and is a fundamental metabolic process in all plant tissues and cells. We review respiration from the perspective of plants that grow in their natural habitat and how it is influenced by wide-ranging elements at different scales, from metabolic substrate availability to shifts in climate. Decades of field-based measurements have honed our understanding of the biological and environmental controls on leaf, root, stem, and whole-organism respiration. Despite this effort, there remain gaps in our knowledge within and across species and ecosystems, especially in more challenging-to-measure tissues like roots. Recent databases of respiration rates and associated leaf traits from species representing diverse biomes, plant functional types, and regional climates have allowed for a wider-lens view at modeling this important CO2 flux. We also re-analyze published data sets to show that maximum leaf respiration rates (Rmax) in species from around the globe are related both to leaf economic traits and environmental variables (precipitation and air temperature), but that root respiration does not follow the same latitudinal trends previously published for leaf data. We encourage the ecophysiological community to continue to expand their study of plant respiration in tissues that are difficult to measure and at the whole plant and ecosystem levels to address outstanding questions in the field.

Unraveling the mystery of canopy dieback caused by citrus disease Huanglongbing and its link to hypoxia stress

Devastating citrus disease Huanglongbing (HLB) is without existing cures. Herein, we present results demonstrating the possible mechanisms (hypoxia stress) behind HLB-triggered shoot dieback by comparing the transcriptomes, hormone profiles, and key enzyme activities in buds of severely and mildly symptomatic 'Hamlin' sweet orange (Citrus sinensis). Within six months (October - May) in field conditions, severe trees had 23% bud dieback, greater than mild trees (11%), with a concomitant reduction in canopy density. In February, differentially expressed genes (DEGs) associated with responses to osmotic stress, low oxygen levels, and cell death were upregulated, with those for photosynthesis and cell cycle downregulated in severe versus mild trees. For severe trees, not only were the key markers for hypoxia, including anaerobic fermentation, reactive oxygen species (ROS) production, and lipid oxidation, transcriptionally upregulated, but also alcohol dehydrogenase activity was significantly greater compared to mild trees, indicating a link between bud dieback and hypoxia. Tricarboxylic acid cycle revival, given the upregulation of glutamate dehydrogenase and alanine aminotransferase DEGs, suggests that ROS may also be generated during hypoxia-reoxygenation. Greater (hormonal) ratios of abscisic acid to cytokinins and jasmonates and upregulated DEGs encoding NADPH oxidases in severe versus mild trees indicate additional ROS production under limited oxygen availability due to stomata closure. Altogether, our results provided evidence that as HLB progresses, excessive ROS produced in response to hypoxia and during hypoxia-reoxygenation likely intensify the oxidative stress in buds leading to cell death, contributing to marked bud and shoot dieback and decline of the severely symptomatic sweet orange trees.

Reduced diurnal temperature range mitigates drought impacts on larch tree growth in North China

Forests are facing climate changes such as warmer temperatures, accelerated snowmelt, increased drought, as well as changing diurnal temperature ranges (DTR) and cloud cover regimes. How tree growth is influenced by the changes in daily to monthly temperatures and its associations with droughts has been extensively investigated, however, few studies have focused on how changes in sub-daily temperatures i.e., DTR, influence tree growth during drought events. Here, we used a network of Larix principis-rupprechtii tree-ring data from 1989 to 2018, covering most of the distribution of planted larch across North China, to investigate how DTR, cloud cover and their interactions influence the relationship between drought stress and tree growth. DTR showed a negative correlation with larch growth in 95 % of sites (r_mean = -0.30, significant in 42 % of sites). Cloud cover was positively correlated with growth in 87 % of sites (r_mean = 0.13, significant in 5 % of sites). Enhanced tree growth was found at lower DTR in the absence of severe drought. Our findings highlight that in the absence of severe droughts, reduced DTR benefits tree growth, while increased cloud cover tended to benefit tree growth only during severe drought periods. Given how DTR influences drought impacts on tree growth, net tree growth was found to be larger in regions with smaller DTR.

Variation in White spruce needle respiration at the species range limits: A potential impediment to Northern expansion

White spruce (Picea glauca) spans a massive range, yet the variability in respiratory physiology and related implications for tree carbon balance at the extremes of this distribution remain as enigmas. Working at both the most northern and southern extents of the distribution range more than 5000 km apart, we measured the short-term temperature response of dark respiration (R/T) at upper and lower canopy positions. R/T curves were fit to both polynomial and thermodynamic models so that model parameters could be compared among locations, canopy positions, and with previously published data. Respiration measured at 25°C (R₂₅ ) was 68% lower at the southern location than at the northern location, resulting in a significantly lower intercept in R/T response in temperate trees. Only at the southern location did upper canopy leaves have a steeper temperature response than lower canopy leaves, likely reflecting canopy gradients in light. At the northern range limit respiration is nearly twice that of the average R₂₅ reported in a global leaf respiration database. We predict that without significant thermal acclimation, respiration will increase with projected end-of-the-century warming and will likely constrain the future range limits of this important boreal species.

High Leaf Respiration Rates May Limit the Success of White Spruce Saplings Growing in the Kampfzone at the Arctic Treeline

Arctic Treeline is the transition from the boreal forest to the treeless tundra and may be determined by growing season temperatures. The physiological mechanisms involved in determining the relationship between the physical and biological environment and the location of treeline are not fully understood. In Northern Alaska, we studied the relationship between temperature and leaf respiration in 36 white spruce (Picea glauca) trees, sampling both the upper and lower canopy, to test two research hypotheses. The first hypothesis is that upper canopy leaves, which are more directly coupled to the atmosphere, will experience more challenging environmental conditions and thus have higher respiration rates to facilitate metabolic function. The second hypothesis is that saplings [stems that are 5-10cm DBH (diameter at breast height)] will have higher respiration rates than trees (stems ≥10cm DBH) since saplings represent the transition from seedlings growing in the more favorable aerodynamic boundary layer, to trees which are fully coupled to the atmosphere but of sufficient size to persist. Respiration did not change with canopy position, however respiration at 25°C was 42% higher in saplings compared to trees (3.43±0.19 vs. 2.41±0.14μmolm^-2 s^-1). Furthermore, there were significant differences in the temperature response of respiration, and seedlings reached their maximum respiration rates at 59°C, more than two degrees higher than trees. Our results demonstrate that the respiratory characteristics of white spruce saplings at treeline impose a significant carbon cost that may contribute to their lack of perseverance beyond treeline. In the absence of thermal acclimation, the rate of leaf respiration could increase by 57% by the end of the century, posing further challenges to the ecology of this massive ecotone.

Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory acclimation

Tropical forest canopies cycle vast amounts of carbon, yet we still have a limited understanding of how these critical ecosystems will respond to climate warming. We implemented in situ leaf-level + 3°C experimental warming from the understory to the upper canopy of two Puerto Rican tropical tree species, Guarea guidonia and Ocotea sintenisii. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, chlorophyll fluorescence, stomatal conductance, leaf traits and foliar respiration. Warming did not alter net photosynthetic temperature response for either species; however, the optimum temperature of Ocotea understory leaf photosynthetic electron transport shifted upward. There was no Ocotea respiratory treatment effect, while Guarea respiratory temperature sensitivity (Q₁₀ ) was down-regulated in heated leaves. The optimum temperatures for photosynthesis (T_opt ) decreased 3-5°C from understory to the highest canopy position, perhaps due to upper canopy stomatal conductance limitations. Guarea upper canopy T_opt was similar to the mean daytime temperatures, while Ocotea canopy leaves often operated above T_opt . With minimal acclimation to warmer temperatures in the upper canopy, further warming could put these forests at risk of reduced CO₂ uptake, which could weaken the overall carbon sink strength of this tropical forest.

Complete or overcompensatory thermal acclimation of leaf dark respiration in African tropical trees

Tropical climates are getting warmer, with pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as leaf dark respiration (Rd ), to these new conditions. However, knowledge on this is currently limited due to data scarcity. We studied the impact of growth temperature on Rd and its dependency on net photosynthesis (An ), leaf nitrogen (N) and phosphorus (P) contents, and leaf mass per unit area (LMA) in 16 early-successional (ES) and late-successional (LS) tropical tree species in multispecies plantations along an elevation gradient (Rwanda TREE project). Moreover, we explored the effect of drought on Rd in one ES and one LS species. Leaf Rd at 20°C decreased at warmer sites, regardless if it was expressed per unit leaf area, mass, N or P. This acclimation resulted in an 8% and a 28% decrease in Rd at prevailing nighttime temperatures in trees at the intermediate and warmest sites, respectively. Moreover, drought reduced Rd , particularly in the ES species and at the coolest site. Thermal acclimation of Rd is complete or overcompensatory and independent of changes in leaf nutrients or LMA in African tropical trees.

Diel- and temperature-driven variation of leaf dark respiration rates and metabolite levels in rice

Leaf respiration in the dark (R_dark ) is often measured at a single time during the day, with hot-acclimation lowering R_dark at a common measuring temperature. However, it is unclear whether the diel cycle influences the extent of thermal acclimation of R_dark , or how temperature and time of day interact to influence respiratory metabolites. To examine these issues, we grew rice under 25°C : 20°C, 30°C : 25°C and 40°C : 35°C day : night cycles, measuring R_dark and changes in metabolites at five time points spanning a single 24-h period. R_dark differed among the treatments and with time of day. However, there was no significant interaction between time and growth temperature, indicating that the diel cycle does not alter thermal acclimation of R_dark . Amino acids were highly responsive to the diel cycle and growth temperature, and many were negatively correlated with carbohydrates and with organic acids of the tricarboxylic acid (TCA) cycle. Organic TCA intermediates were significantly altered by the diel cycle irrespective of growth temperature, which we attributed to light-dependent regulatory control of TCA enzyme activities. Collectively, our study shows that environmental disruption of the balance between respiratory substrate supply and demand is corrected for by shifts in TCA-dependent metabolites.

Similar temperature dependence of photosynthetic parameters in sun and shade leaves of three tropical tree species

Photosynthetic carbon uptake by tropical forests is of critical importance in regulating the earth's climate, but rising temperatures threaten this stabilizing influence of tropical forests. Most research on how temperature affects photosynthesis focuses on fully sun-exposed leaves, and little is known about shade leaves, even though shade leaves greatly outnumber sun leaves in lowland tropical forests. We measured temperature responses of light-saturated photosynthesis, stomatal conductance, and the biochemical parameters VCMax (maximum rate of RuBP carboxylation) and JMax (maximum rate of RuBP regeneration, or electron transport) on sun and shade leaves of mature tropical trees of three species in Panama. As expected, biochemical capacities and stomatal conductance were much lower in shade than in sun leaves, leading to lower net photosynthesis rates. However, the key temperature response traits of these parameters-the optimum temperature (TOpt) and the activation energy-did not differ systematically between sun and shade leaves. Consistency in the JMax to VCMax ratio further suggested that shade leaves are not acclimated to lower temperatures. For both sun and shade leaves, stomatal conductance had the lowest temperature optimum (~25 °C), followed by net photosynthesis (~30 °C), JMax (~34 °C) and VCMax (~38 °C). Stomatal conductance of sun leaves decreased more strongly with increasing vapor pressure deficit than that of shade leaves. Consistent with this, modeled stomatal limitation of photosynthesis increased with increasing temperature in sun but not shade leaves. Collectively, these results suggest that modeling photosynthetic carbon uptake in multi-layered canopies does not require independent parameterization of the temperature responses of the biochemical controls over photosynthesis of sun and shade leaves. Nonetheless, to improve the representation of the shade fraction of carbon uptake dynamics in tropical forests, better understanding of stomatal sensitivity of shade leaves to temperature and vapor pressure deficit will be required.

Temperature response of respiration and respiratory quotients of 16 co-occurring temperate tree species

The forests of the northeastern US are globally, one of the fastest growing terrestrial carbon sinks due to historical declines in large-scale agriculture, timber harvesting and fire disturbance. However, shifting range distributions of tree species with warming air temperatures are altering forest community composition and carbon dynamics. Here, we focus on respiration, a physiological process that is strongly temperature and species dependent. We specifically examined the response of respiration (R; CO2 release) to temperature in 10 broadleaved and six conifer species, as well as the respiratory quotient (RQ; ratio of CO2 released to O2 consumed) of nine broadleaved species that co-occur in the Hudson Highlands Region of New York, USA. The relationships between these physiological measurements and associated leaf traits were also explored. The rates of respiration at 20 °C were 71% higher in northern-ranged broadleaved species when compared with both central- and southern-ranged species. In contrast, the rates of respiration at 20 °C in northern-ranged conifers were 12% lower than in central-ranged conifers. The RQ of broadleaved species increased by 14% as temperatures increased from 15 °C to 35 °C. When RQ values were pooled across temperature, northern-ranged broadleaved species had 12% and 9% lower RQ values than central, and southern-ranged species, respectively, suggesting a reliance on alternative (non-carbohydrate) substrates to fulfill respiratory demands. A Pearson correlation analysis of leaf traits and respiration revealed strong correlations between leaf nitrogen, leaf mass area and R for both broadleaved and conifer species. Our results elucidate leaf trait relationships with tree physiology and reveal the various form and function strategies for species from differing range distributions. Compounded with predicted range distribution shifts and species replacement, this may reduce the carbon storage potential of northeast forests.

Spatial and Seasonal Variations of Standardized Photosynthetic Parameters under Different Environmental Conditions for Young Planted Larix olgensis Henry Trees

Understanding the spatial and seasonal variations in leaf physiology is critical for accurately modeling the carbon uptake, physiological processes and growth of entire canopies and stands. For a 17-year-old Larix olgensis Henry plantation, vertical whorl-by-whorl sampling and analyses of seasonally repeated measurements of major photosynthetic parameters were conducted, and the correlations between photosynthetic parameters and environmental conditions, leaf morphological traits and spatial position within the crown were analyzed. According to the correlations, the photosynthetic parameters were standardized based on the environmental conditions to avoid the influence of the changing environment on the patterns of spatial and seasonal variations of photosynthetic parameters. The results showed that the standardized light-saturated net photosynthetic rate (SPmax), standardized dark respiration (SRd) and standardized stomatal conductance under saturated light (Sgs-sat) were all negatively related to the relative depth into the crown (RDINC) throughout the growing season. However, their vertical patterns were different during the development of the phenological phase. In addition, different gradients of environmental conditions also influenced the values and the range of the vertical variation in photosynthesis. High temperature and low humidity usually resulted in smaller values and weaker vertical variations of SPmax and Sgs-sat, but larger values and more obvious vertical variations in SRd. SPmax and Sgs-sat usually exhibited a parabolic seasonal pattern in different vertical positions within the crown; however, SRd generally followed a concave pattern. These seasonal patterns were all weaker with increasing RDINC. Different environments also exhibited a significant influence on the seasonal patterns of photosynthesis. We suggested that standardization is necessary before analyzing spatial and seasonal variations. A single environmental condition could not represent the spatial and seasonal patterns under all gradients of the environment. Spatial and seasonal variations should be simultaneously analyzed because they are related to each other.

Light inhibition of foliar respiration in response to soil water availability and seasonal changes in temperature in Mediterranean holm oak (Quercus ilex) forest

In the present study we investigated variations in leaf respiration in darkness (RD) and light (RL), and associated traits in response to season, and along a gradient of soil moisture, in Mediterranean woodland dominated by holm oak (Quercus ilex L.) in central and north-eastern Spain respectively. On seven occasions during the year in the central Spain site, and along the soil moisture gradient in north-eastern Spain, we measured rates of leaf RD, RL (using the Kok method), light-saturated photosynthesis (A) and related light response characteristics, leaf mass per unit area (MA) and leaf nitrogen (N) content. At the central Spain site, significant seasonal changes in soil water content and ambient temperature (T) were associated with changes in MA, foliar N, A and stomatal conductance. RD measured at the prevailing daily T and in instantaneous R-T responses, displayed signs of partial acclimation and was not significantly affected by time of year. RL was always less than, and strongly related to, RD, and RL/RD did not vary significantly or systematically with seasonal changes in T or soil water content. Averaged over the year, RL/RD was 0.66±0.05s.e. (n=14) at the central Spain site. At the north-eastern Spain site, the soil moisture gradient was characterised by increasing MA and RD, and reduced foliar N, A, and stomatal conductance as soil water availability decreased. Light inhibition of R occurred across all sites (mean RL/RD=0.69±0.01s.e. (n=18)), resulting in ratios of RL/A being lower than for RD/A. Importantly, the degree of light inhibition was largely insensitive to changes in soil water content. Our findings provide evidence for a relatively constrained degree of light inhibition of R (RL/RD ~ 0.7, or inhibition of ~30%) across gradients of water availability, although the combined impacts of seasonal changes in both T and soil water content increase the range of values expressed. The findings thus have implications in terms of the assumptions made by predictive models that seek to account for light inhibition of R, and for our understanding of how environmental gradients impact on leaf trait relationships in Mediterranean plant communities.

Vertical and seasonal variations in temperature responses of leaf respiration in a Chamaecyparis obtusa canopy

Leaf respiration (R) is a major component of carbon balance in forest ecosystems. Clarifying the variability of leaf R within a canopy is essential for predicting the impact of global warming on forest productivity and the potential future function of the forest ecosystem as a carbon sink. We examined vertical and seasonal variations in short-term temperature responses of leaf R as well as environmental factors (light and mean air temperature) and physiological factors [leaf nitrogen (N), leaf mass per area (LMA), and shoot growth] in the canopy of a 10-year-old stand of hinoki cypress [Chamaecyparis obtusa (Sieb. et Zucc.) Endl.] in Kyushu, Japan. Leaf respiration rate adjusted to 20 °C (R20) exhibited evident vertical gradients in each season and was correlated with light, LMA and leaf N. In contrast, the temperature sensitivity of leaf R (Q10) did not vary vertically throughout the seasons. Seasonally, Q10 was higher in winter than in summer and was strongly negatively correlated to mean air temperature. A negative correlation of R20 with mean air temperature was also observed for each of the three canopy layers. These results clearly indicate that leaf R was able to adjust to seasonal changes in ambient temperature under field conditions and down-regulate during warmer periods. We also found that the degree of thermal acclimation did not vary with canopy position. Overall, our results suggest that vertical and seasonal variations in temperature responses of leaf R within a hinoki cypress canopy could be predicted by relatively simple parameters (light and temperature). There was an exception of extremely high R20 values in April that may have been due to the onset of shoot growth in spring. Understanding thermal acclimation and variations in leaf R within forest canopies will improve global terrestrial carbon cycle models.

Consistent leaf respiratory response to experimental warming of three North American deciduous trees: a comparison across seasons, years, habitats and sites

Most vascular plants acclimate respiration to changes in ambient temperature, but explicit tests of these responses in field settings are rare, and how acclimation responses vary in space and time is relatively unstudied, hindering our ability to predict respiratory release of carbon under future climatic conditions. We measured temperature response curves of leaf respiration for three deciduous tree species from 2009 to 2012 in a field warming experiment (+3.4 °C above ambient) in both open and understory conditions at two sites in the southern boreal forest in Minnesota, USA. We analyzed the effects of warming on leaf respiration, and how the effects varied among species, times of season (early, middle and late parts of the growing season), sites, habitats (understory, open) and years. We hypothesized that the respiration exponent (Q10) of the short-term temperature response curve and the degree of acclimation would be smaller under conditions where plants were more likely to be substrate limited, such as in the understory or the margins of the growing season. However, in contrast to these predictions, stable Q10 and strong respiratory acclimation were consistently observed. For each species, the Q10 did not vary with experimental warming, nor was its response to warming influenced by time of season, year, site or habitat. Strong leaf respiratory acclimation to warming occurred in each species and was consistent across most sources of variation. Most of the leaf traits studied were not affected by warming, while the Q10-leaf nitrogen and R25-soluble carbohydrate relationships were observed, and shifted with warming, implying that acclimation may be associated with the adjustment in respiratory capacity and its relation to leaf nitrogen and soluble carbohydrate content. Consistent Q10 and acclimation across habitats, sites, times of season and years suggest that modeling of temperature acclimation may be possible with relatively simple functions.

Dynamic spatial patterns of leaf traits affect total respiration on the crown scale

Temporal and spatial variations of leaf traits caused conflicting conclusions and great estimating errors of total carbon budget on crown scales. However, there is no effective method to quantitatively describe and study heterogeneous patterns of crowns yet. In this study, dynamic spatial patterns of typical ecological factors on crown scales were investigated during two sky conditions, and CEZs (crown ecological zones) method was developed for spatial crown zoning, within which leaf traits were statistically unchanged. The influencing factors on hourly and spatial variations of leaf dark respiration (Rd) were analysed, and total crown respiration (Rt) was estimated based on patterns of CEZs. The results showed that dynamic spatial patterns of air temperature and light intensity changed significantly by CEZs in special periods and positions, but not continuously. The contributions of influencing factors on variations of Rd changed with crown depth and sky conditions, and total contributions of leaf structural and chemical traits were higher during sunny days than ecological factors, but lower during cloudy days. The estimated errors of Rt may be obviously reduced with CEZs. These results provided some references for scaling from leaves to crown, and technical foundations for expanding lab-control experiments to open field ones.

Whole-plant respiration and its temperature sensitivity during progressive carbon starvation

Plant respiration plays a critical role in the C balance of plants. Respiration is highly temperature sensitive and small temperature-induced increases in whole-plant respiration could change the C balance of plants that operate close to their light-compensation points from positive to negative. Nonstructural carbohydrates are thought to play an important role in controlling respiration and its temperature sensitivity, but this role has not been studied at the whole-plant level. We measured respiration of whole Ardisia crenata Sims. seedlings and tested the hypothesis that darkness-induced C starvation would decrease the temperature sensitivity of whole-plant respiration. Compared with control plants, sugar and starch concentrations in darkened plants declined over time in all organs. Similarly, whole-plant respiration decreased. However, the temperature sensitivity of whole-plant respiration, expressed as the proportional increase in respiration per 10°C warming (Q10), increased with progressive C starvation. We hypothesise that growth respiration was suppressed in darkened plants and that whole-plant respiration represented maintenance respiration almost exclusively, which is more temperature sensitive. Alternatively, changes in the respiratory substrate during C starvation or increased involvement of alternative oxidase pathway respiration may explain the increase in Q10. Carbohydrates are important for respiration but it appears that even in C-starved A. crenata plants, carbohydrate availability does not limit respiration during short-term warming.

Mechanistic insights on the responses of plant and ecosystem gas exchange to global environmental change: lessons from Biosphere 2

Scaling up leaf processes to canopy/ecosystem level fluxes is critical for examining feedbacks between vegetation and climate. Collectively, studies from Biosphere 2 Laboratory have provided important insight of leaf-to-ecosystem investigations of multiple environmental parameters that were not before possible in enclosed or field studies. B2L has been a testing lab for the applicability of new technologies such as spectral approaches to detect spatial and temporal changes in photosynthesis within canopies, or for the development of cavity ring-down isotope applications for ecosystem evapotranspiration. Short and long term changes in atmospheric CO2, drought or temperature allowed for intensive investigation of the interactions between photosynthesis and leaf, soil and ecosystem respiration. Experiments conducted in the rainforest biome have provided some of the most comprehensive dataset to date on the effects of climate change variables on tropical ecosystems. Results from these studies have been later corroborated in natural rainforest ecosystems and have improved the predictive capabilities of models that now show increased resilience of tropics to climate change. Studies of temperature and CO2 effects on ecosystem respiration and its leaf and soil components have helped reconsider the use of simple first-order kinetics for characterizing respiration in models. The B2L also provided opportunities to quantify the rhizosphere priming effect, or establish the relationships between net primary productivity, atmospheric CO2 and isoprene emissions.

Canopy position affects the relationships between leaf respiration and associated traits in a tropical rainforest in Far North Queensland

We explored the impact of canopy position on leaf respiration (R) and associated traits in tree and shrub species growing in a lowland tropical rainforest in Far North Queensland, Australia. The range of traits quantified included: leaf R in darkness (RD) and in the light (RL; estimated using the Kok method); the temperature (T)-sensitivity of RD; light-saturated photosynthesis (Asat); leaf dry mass per unit area (LMA); and concentrations of leaf nitrogen (N), phosphorus (P), soluble sugars and starch. We found that LMA, and area-based N, P, sugars and starch concentrations were all higher in sun-exposed/upper canopy leaves, compared with their shaded/lower canopy and deep-shade/understory counterparts; similarly, area-based rates of RD, RL and Asat (at 28 °C) were all higher in the upper canopy leaves, indicating higher metabolic capacity in the upper canopy. The extent to which light inhibited R did not differ significantly between upper and lower canopy leaves, with the overall average inhibition being 32% across both canopy levels. Log-log RD-Asat relationships differed between upper and lower canopy leaves, with upper canopy leaves exhibiting higher rates of RD for a given Asat (both on an area and mass basis), as well as higher mass-based rates of RD for a given [N] and [P]. Over the 25-45 °C range, the T-sensitivity of RD was similar in upper and lower canopy leaves, with both canopy positions exhibiting Q10 values near 2.0 (i.e., doubling for every 10 °C rise in T) and Tmax values near 60 °C (i.e., T where RD reached maximal values). Thus, while rates of RD at 28 °C decreased with increasing depth in the canopy, the T-dependence of RD remained constant; these findings have important implications for vegetation-climate models that seek to predict carbon fluxes between tropical lowland rainforests and the atmosphere.

Predicting ecosystem carbon balance in a warming Arctic: the importance of long-term thermal acclimation potential and inhibitory effects of light on respiration

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.

Soil water availability influences the temperature response of photosynthesis and respiration in a grass and a woody shrub

Seedlings of the shrub kānuka (Kunzea ericoides var. ericoides (A. Rich) J. Thompson) and the pasture grass brown top (Agrostis capillarus L.) were grown in intact soil cores in climate-controlled cabinets to analyse the thermal response of leaf-level carbon exchange at four levels of volumetric soil water content (θ). The objective was to resolve the combined effects of relatively rapid and short-term changes in θ and temperature on the thermal responses of both photosynthesis and respiration in these two contrasting plant types. Results showed that θ had a greater effect on the short-term temperature response of photosynthesis than the temperature response of respiration. The optimum value of θ for net photosynthesis was around 30% for both plants. The photosynthetic capacity of kānuka and the grass declined significantly when θ fell below 20%. The temperature sensitivity of photosynthesis was low at low soil water content and increased at moderate to high soil water content in both plant types. Statistical analysis showed that the temperature sensitivity of photosynthetic parameters was similar for both plant types, but the sensitivity of respiratory parameters differed. Respiratory capacity increased with increasing soil water content in kānuka but declined significantly when θ fell below 15%. There was no significant influence of soil water content on respiratory capacity in the grass. Collectively, our results indicate that θ influenced the temperature sensitivity of photosynthesis and respiration, and altered the balance between foliar respiration and photosynthetic capacity in both plant types.

Our eclectic adventures in the slower eras of photosynthesis: from New England Down Under to biosphere 2 and beyond

This is a tale of a career in plant physiological ecology that enjoyed the freedom to address photosynthetic physiology and biochemistry in leaves of plants from diverse environments. It was supported by block funding (now sadly a thing of the past) for research at the Australian National University, by grants during appointments in the United States and in Germany, and by Columbia University. It became a "career experiment" in which long-term, high-trust support for curiosity-driven plant biology in Australia, and at times in the United States, led to surprisingly innovative results. Although the rich diversity of short-term competitive grant opportunities in the United States sustained ongoing research, it proved difficult to mobilize support for more risky long-term projects. A decade after the closure of the Biosphere 2 Laboratory, this article highlights the achievements of colleagues in experimental climate change research from 1998 to 2003.

Impacts of temperature on primary productivity and respiration in naturally structured macroalgal assemblages

Rising global temperatures caused by human-mediated change has already triggered significant responses in organismal physiology, distribution and ecosystem functioning. Although the effects of rising temperature on the physiology of individual organisms are well understood, the effect on community-wide processes has remained elusive. The fixation of carbon via primary productivity is an essential ecosystem function and any shifts in the balance of primary productivity and respiration could alter the carbon balance of ecosystems. Here we show through a series of tests that respiration of naturally structured algal assemblages in southern New Zealand greatly increases with rising temperature, with implications for net primary productivity (NPP). The NPP of in situ macroalgal assemblages was minimally affected by natural temperature variation, possibly through photo-acclimation or temperature acclimation responses, but respiration rates and compensating irradiance were negatively affected. However, laboratory experiments testing the impacts of rising temperature on several photosynthetic parameters showed a decline in NPP, increasing respiration rates and increasing compensating irradiance. The respiration Q10 of laboratory assemblages (the difference in metabolic rates over 10°C) averaged 2.9 compared to a Q10 of 2 often seen in other autotrophs. However, gross primary productivity (GPP) Q10 averaged 2, indicating that respiration was more severely affected by rising temperature. Furthermore, combined high irradiance and high temperature caused photoinhibition in the laboratory, and resulted in 50% lower NPP at high irradiance. Our study shows that communities may be more severely affected by rising global temperatures than would be expected by responses of individual species. In particular, enhanced respiration rates and rising compensation points have the potential to greatly affect the carbon balance of macroalgal assemblages through declines in sub-canopy NPP, the impacts of which may be exacerbated over longer time-scales and could result in declines in sub-canopy species richness and abundance.

Effect of environmental variables and stand structure on ecosystem respiration components in a Mediterranean beech forest

The temporal variability of ecosystem respiration (RECO) has been reported to have important effects on the temporal variability of net ecosystem exchange, the net amount of carbon exchanged between an ecosystem and the atmosphere. However, our understanding of ecosystem respiration is rather limited compared with photosynthesis or gross primary productivity, particularly in Mediterranean montane ecosystems. In order to investigate how environmental variables and forest structure (tree classes) affect different respiration components and RECO in a Mediterranean beech forest, we measured soil, stem and leaf CO2 efflux rates with dynamic chambers and RECO by the eddy-covariance technique over 1 year (2007-2008). Ecosystem respiration showed marked seasonal variation, with the highest rates in spring and autumn and the lowest in summer. We found that the soil respiration (SR) was mainly controlled by soil water content below a threshold value of 0.2 m(3) m(-3), above which the soil temperature explained temporal variation in SR. Stem CO2 effluxes were influenced by air temperature and difference between tree classes with higher rates measured in dominant trees than in co-dominant ones. Leaf respiration (LR) varied significantly between the two canopy layers considered. Non-structural carbohydrates were a very good predictor of LR variability. We used these measurements to scale up respiration components to ecosystem respiration for the whole canopy and obtained cumulative amounts of carbon losses over the year. Based on the up-scaled chamber measurements, the relative contributions of soil, stem and leaves to the total annual CO2 efflux were: 56, 8 and 36%, respectively. These results confirm that SR is the main contributor of ecosystem respiration and provided an insight on the driving factors of respiration in Mediterranean montane beech forests.

High-resolution temperature responses of leaf respiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function

We tested whether snow gum (Eucalyptus pauciflora) trees growing in thermally contrasting environments exhibit generalizable temperature (T) response functions of leaf respiration (R) and fluorescence (Fo). Measurements were made on pot-grown saplings and field-grown trees (growing between 1380 and 2110 m a.s.l.). Using a continuous, high-resolution protocol, we quantified T response curves of R and Fo--these data were used to identify an algorithm for modelling R-T curves at subcritical T's and establish variations in heat tolerance. For the latter, we quantified Tmax [T where R is maximal] and Tcrit [T where Fo rises rapidly]. Tmax ranged from 51 to 57 °C, varying with season (e.g. winter  summer). Tcrit ranged from 41 to 49 °C in summer and from 58 to 63 °C in winter. Thus, surprisingly, leaf energy metabolism was more heat-tolerant in trees experiencing ice-encasement in winter than warmer conditions in summer. A polynomial model fitted to log-transformed R data provided the best description of the T-sensitivity of R (between 10 and 45 °C); using these model fits, we found that the negative slope of the Q10 -T relationship was greater in winter than in summer. Collectively, our results (1) highlight high-T limits of energy metabolism in E. pauciflora and (2) provide a framework for improving representation of T-responses of leaf R in predictive models.

Foliar respiration and its temperature sensitivity in trees and lianas: in situ measurements in the upper canopy of a tropical forest

Leaf dark respiration (R) and its temperature sensitivity are essential for efforts to model carbon fluxes in tropical forests under current and future temperature regimes, but insufficient data exist to generalize patterns of R in species-rich tropical forests. Here, we tested the hypothesis that R and its temperature sensitivity (expressed as Q10, the proportional increase in R with a 10 °C rise in temperature) vary in relation to leaf functional traits, and among plant functional types (PFTs). We conducted in situ measurements of R of 461 leaves of 26 species of tree and liana in the upper canopy of a tropical forest in Panama. A construction crane allowed repeated non-destructive access to measure leaves kept in the dark since the previous night and equilibrated to the ambient temperature of 23-31 °C in the morning. R at 25 °C (R25) varied among species (mean 1.11 μmol m(-2) s(-1); range 0.72-1.79 μmol m(-2) s(-1)) but did not differ significantly among PFTs. R25 correlated positively with photosynthetic capacity, leaf mass per unit area, concentrations of nitrogen and phosphorus, and negatively with leaf lifespan. Q10 estimated for each species was on average higher than the 2.0 often assumed in coupled climate-vegetation models (mean 2.19; range 1.24-3.66). Early-successional tree species had higher Q10 values than other functional types, but interspecific variation in Q10 values was not correlated with other leaf traits. Similarity in respiration characteristics across PFTs, and relatively strong correlations of R with other leaf functional traits offer potential for trait-based vegetation modeling in species-rich tropical forests.

Acclimation of foliar respiration and photosynthesis in response to experimental warming in a temperate steppe in northern China

BACKGROUND

Thermal acclimation of foliar respiration and photosynthesis is critical for projection of changes in carbon exchange of terrestrial ecosystems under global warming.

METHODOLOGY/PRINCIPAL FINDINGS

A field manipulative experiment was conducted to elevate foliar temperature (Tleaf) by 2.07°C in a temperate steppe in northern China. Rd/Tleaf curves (responses of dark respiration to Tleaf), An/Tleaf curves (responses of light-saturated net CO2 assimilation rates to Tleaf), responses of biochemical limitations and diffusion limitations in gross CO2 assimilation rates (Ag) to Tleaf, and foliar nitrogen (N) concentration in Stipa krylovii Roshev. were measured in 2010 (a dry year) and 2011 (a wet year). Significant thermal acclimation of Rd to 6-year experimental warming was found. However, An had a limited ability to acclimate to a warmer climate regime. Thermal acclimation of Rd was associated with not only the direct effects of warming, but also the changes in foliar N concentration induced by warming.

CONCLUSIONS/SIGNIFICANCE

Warming decreased the temperature sensitivity (Q10) of the response of Rd/Ag ratio to Tleaf. Our findings may have important implications for improving ecosystem models in simulating carbon cycles and advancing understanding on the interactions between climate change and ecosystem functions.

Needle-age related variability in nitrogen, mobile carbohydrates, and δ13C within Pinus koraiensis tree crowns

For both ecologists and physiologists, foliar physioecology as a function of spatially and temporally variable environmental factors such as sunlight exposure within a tree crown is important for understanding whole tree physiology and for predicting ecosystem carbon balance and productivity. Hence, we studied concentrations of nitrogen (N), non-structural carbohydrates (NSC = soluble sugars + starch), and δ(13)C in different-aged needles within Pinus koraiensis tree crowns, to understand the needle age- and crown position-related physiology, in order to test the hypothesis that concentrations of N, NSC, and δ(13)C are needle-age and crown position dependent (more light, more photosynthesis affecting N, NSC, and δ(13)C), and to develop an accurate sampling strategy. The present study indicated that the 1-yr-old needles had significantly higher concentration levels of mobile carbohydrates (both on a mass and an area basis) and N(area) (on an area basis), as well as NSC-N ratios, but significantly lower levels of N(mass) (on a mass basis) concentration and specific leaf area (SLA), compared to the current-year needles. Azimuthal (south-facing vs. north-facing crown side) effects were found to be significant on starch [both on a mass (ST(mass)) and an area basis (ST(area))], δ(13)C values, and N(area), with higher levels in needles on the S-facing crown side than the N-facing crown side. Needle N(mass) concentrations significantly decreased but needle ST(mass), ST(area), and δ(13)C values significantly increased with increasing vertical crown levels. Our results suggest that the sun-exposed crown position related to photosynthetic activity and water availability affects starch accumulation and carbon isotope discrimination. Needle age associated with physiological activity plays an important role in determining carbon and nitrogen physiology. The present study indicates that across-scale sampling needs to carefully select tissue samples with equal age from a comparable crown position.

Leaf photosynthesis and respiration of three bioenergy crops in relation to temperature and leaf nitrogen: how conserved are biochemical model parameters among crop species?

Given the need for parallel increases in food and energy production from crops in the context of global change, crop simulation models and data sets to feed these models with photosynthesis and respiration parameters are increasingly important. This study provides information on photosynthesis and respiration for three energy crops (sunflower, kenaf, and cynara), reviews relevant information for five other crops (wheat, barley, cotton, tobacco, and grape), and assesses how conserved photosynthesis parameters are among crops. Using large data sets and optimization techniques, the C(3) leaf photosynthesis model of Farquhar, von Caemmerer, and Berry (FvCB) and an empirical night respiration model for tested energy crops accounting for effects of temperature and leaf nitrogen were parameterized. Instead of the common approach of using information on net photosynthesis response to CO(2) at the stomatal cavity (A(n)-C(i)), the model was parameterized by analysing the photosynthesis response to incident light intensity (A(n)-I(inc)). Convincing evidence is provided that the maximum Rubisco carboxylation rate or the maximum electron transport rate was very similar whether derived from A(n)-C(i) or from A(n)-I(inc) data sets. Parameters characterizing Rubisco limitation, electron transport limitation, the degree to which light inhibits leaf respiration, night respiration, and the minimum leaf nitrogen required for photosynthesis were then determined. Model predictions were validated against independent sets. Only a few FvCB parameters were conserved among crop species, thus species-specific FvCB model parameters are needed for crop modelling. Therefore, information from readily available but underexplored A(n)-I(inc) data should be re-analysed, thereby expanding the potential of combining classical photosynthetic data and the biochemical model.

Seasonal patterns of foliage respiration in dominant and suppressed Eucalyptus globulus canopies

We examined spatial and temporal dynamics of foliage respiration in canopies of dominant and suppressed Eucalyptus globulus trees to better understand processes regulating foliage respiration in a young fast-growing stand. Temperature response functions and seasonal measures of respiration (measured at a reference temperature of 15 °C, R₁₅) were studied for approximately 1 year to (i) examine controls on respiration as a function of canopy position, foliar nitrogen and non-structural carbohydrate concentrations and (ii) assess the capacity for thermal acclimation within E. globulus canopies. The short-term temperature response of respiration varied both with canopy position and seasonally. Area-based R(15) measurements declined with increasing canopy depth and were strongly related to foliar N concentrations, especially in upper-canopy positions. R₁₅ was negatively correlated with the average temperature of the preceding 14 days, a pattern consistent with thermal acclimation. In suppressed canopies, R₁₅ was higher than that at similar canopy heights in dominant trees. Similarly, foliar concentrations of non-structural carbohydrates were also relatively higher in suppressed canopies than dominant canopies, providing support for a substrate-based model of leaf respiration. Our data highlight the dynamic nature of foliar respiration within E. globulus canopies, which contrasts with the generally simplistic representation of respiration within most process-based models.

Thermal acclimation of photosynthesis: a comparison of boreal and temperate tree species along a latitudinal transect

Common gardens were established along a approximately 900 km latitudinal transect to examine factors limiting geographical distributions of boreal and temperate tree species in eastern North America. Boreal representatives were trembling aspen (Populus tremuloides Michx.) and paper birch (Betula papyrifera Marsh.), while temperate species were eastern cottonwood (Populus deltoides Bartr ex. Marsh var. deltoides) and sweetgum (Liquidambar styraciflua L.). The species were compared with respect to adjustments of leaf photosynthetic metabolism along the transect, with emphasis on temperature sensitivities of the maximum rate of ribulose bisphosphate (RuBP) carboxylation (E(V)) and regeneration (E(J)). During leaf development, the average air temperature (T(growth)) differed between the coolest and warmest gardens by 12 degrees C. Evidence of photosynthetic thermal acclimation (metabolic shifts compensating for differences in T(growth)) was generally lacking in all species. Namely, neither E(V) nor E(J) was positively related to T(growth). Correspondingly, the optimum temperature (T(opt)) of ambient photosynthesis (A(sat)) did not vary significantly with T(growth). Modest variation in T(opt) was explained by the combination of E(V) plus the slope and curvature of the parabolic temperature response of mesophyll conductance (g(m)). All in all, species differed little in photosynthetic responses to climate. Furthermore, the adaptive importance of photosynthetic thermal acclimation was overshadowed by g(m)'s influence on A(sat)'s temperature response.

Acclimation of respiratory temperature responses in northern and southern populations of Pinus banksiana

Temperature acclimation of respiration may contribute to climatic adaptation and thus differ among populations from contrasting climates. Short-term temperature responses of foliar dark respiration were measured in 33-yr-old trees of jack pine (Pinus banksiana) in eight populations of wide-ranging origin (44-55 degrees N) grown in a common garden at 46.7 degrees N. It was tested whether seasonal adjustments in respiration and population differences in this regard resulted from changes in base respiration rate at 5 degrees C (R(5)) or Q(10) (temperature sensitivity) and covaried with nitrogen and soluble sugars. In all populations, acclimation was manifest primarily through shifts in R(5) rather than altered Q(10). R(5) was higher in cooler periods in late autumn and winter and lower in spring and summer, inversely tracking variation in ambient air temperature. Overall, R(5) covaried with sugars and not with nitrogen. Although acclimation was comparable among all populations, the observed seasonal ranges in R(5) and Q(10) were greater in populations originating from warmer than from colder sites. Population differences in respiratory traits appeared associated with autumnal cold hardening. Common patterns of respiratory temperature acclimation among biogeographically diverse populations provide a basis for predicting respiratory carbon fluxes in a wide-ranging species.

Temperature responses are a window to the physiology of dark respiration: differences between CO2 release and O2 reduction shed light on energy conservation

We showed that temperature responses of dark respiration for foliage of Pinus radiata could be approximated by Arrhenius kinetics, whereby E(0) determines shape of the exponential response and denotes overall activation energy of respiratory metabolism. Reproducible and predictable deviation from strict Arrhenius kinetics depended on foliage age, and differed between R(CO2) and R(O2). Inhibition of oxygen reduction (R(O2)) by cyanide (inhibiting COX) or SHAM (inhibiting AOX) resulted in reproducible changes of the temperature sensitivity for R(O2), but did not affect R(CO2). Enthalpic growth--preservation of electrons in anabolic products--could be approximated with knowledge of four variables: activation energies (E(0)) for both R(CO2) and R(O2), and basal rates of respiration at a low reference temperature (R(REF)). Rates of enthalpic growth by P. radiata needles were large in spring due to differences between R(REF) of oxidative decarboxylation and that of oxygen reduction, while overall activation energies for the two processes were similar. Later during needle development, enthalpic growth was dependent on differences between E(0) for R(CO2) as compared with R(O2), and increased E(0)(R(O2)) indicated greater contributions of cytochrome oxidase to accompany the switch from carbohydrate sink to source. Temperature-dependent increments in stored energy can be calculated as the difference between R(CO2)DeltaH(CO2) and R(O2)DeltaH(O2).

Thermal acclimation of leaf respiration but not photosynthesis in Populus deltoides x nigra

Dark respiration and photosynthesis were measured in leaves of poplar Populus deltoides x nigra ('Veronese') saplings to investigate the extent of respiratory and photosynthetic acclimation in pre-existing and newly emerged leaves to abrupt changes in air temperature. The saplings were grown at three temperature regimes and at high and low nitrogen availabilities. Rates of photosynthesis and dark respiration (R(d)) were measured at the initial temperature and the saplings were then transferred to a different temperature regime, where the plants remained for a second and third round of measurements on pre-existing and newly emerged leaves. Acclimation of photosynthesis was limited following transfer to warmer or cooler growing conditions. There was strong evidence of cold and warm acclimation of R(d) to growth temperature, but this was limited in pre-existing leaves. Full acclimation of R(d )was restricted to newly emerged leaves grown at the new growth temperature. These findings indicate that the extent of thermal acclimation differs significantly between photosynthesis and respiration. Importantly, pre-existing leaves in poplar were capable of some respiratory acclimation, but full acclimation was observed only in newly emerged leaves. The R(d)/A(max) ratio declined at higher growth temperatures, and nitrogen status of leaves had little impact on the degree of acclimation.

Seasonal variation of temperature response of respiration in invasive Berberis thunbergii (Japanese barberry) and two co-occurring native understory shrubs in a northeastern US deciduous forest

In the understory of a closed forest, plant growth is limited by light availability, and early leafing is proposed to be an important mechanism of plant invasion by providing a spring C "subsidy" when high light is available. However, studies on respiration, another important process determining plant net C gain, are rare in understory invasive plants. In this study, leaf properties and the temperature response of leaf respiration were compared between invasive Berberis thunbergii, an early leafing understory shrub, and two native shrubs, Kalmia latifolia, a broadleaf evergreen and Vaccinium corymbosum, a late-leafing deciduous species, in an oak-dominated deciduous forest. The seasonal trend of the basal respiration rates (R(0)) and the temperature response coefficient (E(0)), were different among the three shrubs and species-specific negative correlations were observed between R(0) and E(0). All three shrubs showed significant correlation between respiration rate on an area basis (20 degrees C) and leaf N on an area basis. The relationship was attributed to the variation of both leaf N on a mass basis and leaf mass per area (LMA) in B. thunbergii, but to LMA only in K. latifolia and V. corymbosum. After modeling leaf respiration throughout 2004, B. thunbergii displayed much higher annual leaf respiration (mass based) than the two native shrubs, indicating a higher cost per unit of biomass investment. Thus, respiratory properties alone were not likely to lead to C balance advantage of B. thunbergii. Future studies on whole plant C budgets and leaf construction cost are needed to address the C balance advantage in early leafing understory shrubs like B. thunbergii.

Does growth irradiance affect temperature dependence and thermal acclimation of leaf respiration? Insights from a Mediterranean tree with long-lived leaves

Understanding the response of leaf respiration (R) to changes in irradiance and temperature is a prerequisite for predicting the impacts of climate change on plant function and future atmospheric CO2 concentrations. Little is known, however, about the interactive effects of irradiance and temperature on leaf R. We investigated whether growth irradiance affects the temperature response of leaf R in darkness (Rdark) and in light (Rlight) in seedlings of a broad-leaved evergreen species, Quercus ilex. Two hypotheses concerning Rdark were tested: (1) the Q10 (i.e. the proportional increase in R per 10 degrees C rise in temperature) of leaf Rdark is lower in shaded plants than in high-light-grown plants, and (2) shade-grown plants exhibit a lower degree of thermal acclimation of Rdark than plants exposed to higher growth irradiance. We also assessed whether light inhibition of Rlight differs between leaves exposed to contrasting temperatures and growth irradiances, and whether the degree of thermal acclimation of Rlight is dependent on growth irradiance. We showed that while growth irradiance did impact on photosynthesis, it had no effect on the Q10 of leaf Rdark. Growth irradiance had little impact on thermal acclimation when fully expanded, pre-existing leaves were exposed to contrasting temperatures for several weeks. When Rlight was measured at a common irradiance, Rlight/Rdark ratios were higher in shaded plants due to homeostasis of Rlight between growth irradiance treatments and to the lower Rdark in shaded leaves. We also showed that Rlight does not acclimate to the same degree as Rdark, and that Rlight/Rdark decreases with increasing measuring and growth temperatures, irrespective of the growth irradiance. Collectively, we raised the possibility that predictive carbon cycle models can assume that growth irradiance and photosynthesis do not affect the temperature sensitivity of leaf Rdark of long-lived evergreen leaves, thus simplifying incorporation of leaf R into such models.

Leaf- and stand-level responses of a forested mesocosm to independent manipulations of temperature and vapor pressure deficit

Alterations in temperature (T) and vapor pressure deficit (VPD) strongly influence gas exchange, but because VPD is highly influenced by T, the effects of these two factors are difficult to separate. Here, the concomitant effects of T and VPD on CO(2) uptake, stomatal conductance, and transpiration at leaf- and canopy-levels were examined for a stand of trees (Populus deltoides) enclosed within large mesocosms. T and VPD were independently altered to yield a factorial combination of treatments of low (24 degrees C) or high (30 degrees C) T and low (0.75) or high (1.75 kPa) VPD. Traditional leaf-level gas exchange measurements were compared with whole-canopy exchange to verify typical scaling methods. Elevated T resulted in an average 40% and 14% increase in midday leaf-level and canopy-level net CO(2) uptake, respectively. Other physiological responses to elevated T and VPD were similar at both scales, but the magnitude of change was usually less pronounced at the canopy-level. Surprisingly, only minimal interactions between T and VPD were found to influence responses of CO(2) uptake and stomatal conductance at either level.

The influence of temperature on within-canopy acclimation and variation in leaf photosynthesis: spatial acclimation to microclimate gradients among climatically divergent Acer rubrum L. genotypes

Leaf gas exchange and temperature response were measured to assess temperature acclimation within a tree canopy in climatically contrasting genotypes of Acer rubrum L. Over the course of two 50 d continuous periods, growth temperature was controlled within tree crowns and the steady-state rate of leaf gas exchange was measured. Data were then modelled to calculate the influence of genotype variation and vertical distribution of physiological activity on carbon uptake. The maximal rate of Rubisco carboxylation (V(cmax)), the maximum rate of electron transport (J(max)), leaf dark respiration rate (R(d)), maximum photosynthesis (A(max)), and the CO(2) compensation point (Gamma) increased with temperature during both (i) a constant long-term (50 d) daytime temperature or (ii) ambient daytime temperature with short-term temperature control (25-38 degrees C). In addition, within-crown variation in the temperature response of photosynthesis and R(d) was influenced by acclimation to local microclimate temperature gradients. Results indicated that carbon uptake estimates could be overestimated by 22-25% if the vertical distribution of temperature gradients is disregarded. Temperature is a major factor driving photosynthetic acclimation and within-crown gas exchange variation. Thus, this study established the importance of including spatial acclimation to temperature- and provenance-, ecotype-, and/or genotype-specific parameter sets into carbon uptake models.

The hot and the cold: unravelling the variable response of plant respiration to temperature

When predicting the effects of climate change, global carbon circulation models that include a positive feedback effect of climate warming on the carbon cycle often assume that (1) plant respiration increases exponentially with temperature (with a constant Q₁₀) and (2) that there is no acclimation of respiration to long-term changes in temperature. In this review, we show that these two assumptions are incorrect. While Q₁₀ does not respond systematically to elevated atmospheric CO₂ concentrations, other factors such as temperature, light, and water availability all have the potential to influence the temperature sensitivity of respiratory CO₂ efflux. Roots and leaves can also differ in their Q₁₀ values, as can upper and lower canopy leaves. The consequences of such variable Q₁₀ values need to be fully explored in carbon modelling. Here, we consider the extent of variability in the degree of thermal acclimation of respiration, and discuss in detail the biochemical mechanisms underpinning this variability; the response of respiration to long-term changes in temperature is highly dependent on the effect of temperature on plant development, and on interactive effects of temperature and other abiotic factors (e.g. irradiance, drought and nutrient availability). Rather than acclimating to the daily mean temperature, recent studies suggest that other components of the daily temperature regime can be important (e.g. daily minimum and / or night temperature). In some cases, acclimation may simply reflect a passive response to changes in respiratory substrate availability, whereas in others acclimation may be critical in helping plants grow and survive at contrasting temperatures. We also consider the impact of acclimation on the balance between respiration and photosynthesis; although environmental factors such as water availability can alter the balance between these two processes, the available data suggests that temperature-mediated differences in dark leaf respiration are closely linked to concomitant differences in leaf photosynthesis. We conclude by highlighting the need for a greater process-based understanding of thermal acclimation of respiration if we are to successfully predict future ecosystem CO₂ fluxes and potential feedbacks on atmospheric CO₂ concentrations.

Respiration characteristics in temperate rainforest tree species differ along a long-term soil-development chronosequence

We measured the response of dark respiration (R(d)) to temperature and foliage characteristics in the upper canopies of tree species in temperate rainforest communities in New Zealand along a soil chronosequence (six sites from 6 years to 120,000 years). The chronosequence provided a vegetation gradient characterised by significant changes in soil nutrition. This enabled us to examine the extent to which changes in dark respiration can be applied across forest biomes and the utility of scaling rules in whole-canopy carbon modelling. The response of respiration to temperature in the dominant tree species differed significantly between sites along the sequence. This involved changes in both R(d) at a reference temperature (R(10)) and the extent to which R(d) increased with temperature (described by E(o), a parameter related to the energy of activation, or the change in R(d) over a 10 degrees C range, Q(10)). Site averaged E(o) ranged from 44.4 kJ mol(-1) K(-1) at the 60-year-old site to 26.0 kJ mol(-1) K(-1) at the oldest, most nutrient poor, site. Relationships between respiratory and foliage characteristics indicated that both the temperature response of respiration (E(o) or Q(10)) and the instantaneous rate of respiration increased with both foliar nitrogen and phosphorus content. The ratio of photosynthetic capacity (Whitehead et al. in Oecologia 2005) to respiration (A(max)/R(d)) attained values in excess of 15 for species in the 6- to 120-year-old sites, but thereafter decreased significantly to around five at the 120,000-year-old site. This indicates that shoot carbon acquisition is regulated by nutrient limitations in the retrogressing ecosystems on the oldest sites. Our findings indicate that respiration and its temperature response will vary according to soil age and, therefore, to soil nutrient availability and the stage of forest development. Thus, variability in respiratory characteristics for canopies should be considered when using models to integrate respiration at large spatial scales.

Nocturnal warming increases photosynthesis at elevated CO2 partial pressure in Populus deltoides

•  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 (R_d ) 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 (A_max ) 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.

Photosynthetic consequences of Marssonina leaf spot differ between two poplar hybrids

•  In foliage of two hybrid poplars, clone DN-34 (Populus deltoides × P. nigra) and clone NM-6 (P. nigra × P. maximowiczii), we examined relationships between photosynthesis and severity of leaf spot induced by Marssonina brunnea f. sp. brunnea, a common disease of many tree species in the Populus genus with the potential to affect growth. •  Gas exchange was measured on asymptomatic and diseased foliage in monoculture stands of each clone. The equation Y = (1 - x)^β was used to characterize the relationship between relative photosynthesis (Y) and percent leaf spot (x), where β represents the ratio between functional impairment and measured lesion area. •  Leaf photosynthesis was strongly and negatively correlated with leaf spot severity in both hybrids, and β-values indicated that photosynthetic impairment extended beyond visibly damaged leaf tissue. However, large differences in β between hybrids indicated differential photosynthetic consequences for a given leaf spot severity. For each hybrid, values of β were positively related to photosynthetic photon flux density incident upon the leaf during gas exchange measurement. •  Declines in leaf photosynthesis appeared to result from a disruption of the photosynthetic apparatus by the invading pathogen. However, specific causes for the differential photosynthetic responses of the two hybrids to disease remained elusive.