Nitrogen concentration and physical properties are key drivers of woody tissue respiration.

BACKGROUND AND AIMS: Despite the critical role of woody tissues in determining net carbon exchange of terrestrial ecosystems, relatively little is known regarding the drivers of sapwood and bark respiration. METHODS: Using one of the most comprehensive wood respiration datasets to date (82 species from Australian rainforest, savanna and temperate forest), we quantified relationships between tissue respiration rates (Rd) measured in vitro (i.e. 'respiration potential') and physical properties of bark and sapwood, and nitrogen concentration (Nmass) of leaves, sapwood and bark. KEY RESULTS: Across all sites, tissue density and thickness explained similar, and in some cases more, variation in bark and sapwood Rd than did Nmass. Higher density bark and sapwood tissues had lower Rd for a given Nmass than lower density tissues. Rd-Nmass slopes were less steep in thicker compared with thinner-barked species and less steep in sapwood than in bark. Including the interactive effects of Nmass, density and thickness significantly increased the explanatory power for bark and sapwood respiration in branches. Among these models, Nmass contributed more to explanatory power in trunks than in branches, and in sapwood than in bark. Our findings were largely consistent across sites, which varied in their climate, soils and dominant vegetation type, suggesting generality in the observed trait relationships. Compared with a global compilation of leaf, stem and root data, Australian species showed generally lower Rd and Nmass, and less steep Rd-Nmass relationships. CONCLUSIONS: To the best of our knowledge, this is the first study to report control of respiration-nitrogen relationships by physical properties of tissues, and one of few to report respiration-nitrogen relationships in bark and sapwood. Together, our findings indicate a potential path towards improving current estimates of autotrophic respiration by integrating variation across distinct plant tissues.

Differential controls by climate and physiology over the emission rates of biogenic volatile organic compounds from mature trees in a semi-arid pine forest.

Drought has the potential to influence the emission of biogenic volatile organic compounds (BVOCs) from forests and thus affect the oxidative capacity of the atmosphere. Our understanding of these influences is limited, in part, by a lack of field observations on mature trees and the small number of BVOCs monitored. We studied 50- to 60-year-old Pinus ponderosa trees in a semi-arid forest that experience early summer drought followed by late-summer monsoon rains, and observed emissions for five BVOCs-monoterpenes, methylbutenol, methanol, acetaldehyde and acetone. We also constructed a throughfall-interception experiment to create "wetter" and "drier" plots. Generally, trees in drier plots exhibited reduced sap flow, photosynthesis, and stomatal conductances, while BVOC emission rates were unaffected by the artificial drought treatments. During the natural, early summer drought, a physiological threshold appeared to be crossed when photosynthesis ≅2 µmol m(-2) s(-1) and conductance ≅0.02 mol m(-2) s(-1). Below this threshold, BVOC emissions are correlated with leaf physiology (photosynthesis and conductance) while BVOC emissions are not correlated with other physicochemical factors (e.g., compound volatility and tissue BVOC concentration) that have been shown in past studies to influence emissions. The proportional loss of C to BVOC emission was highest during the drought primarily due to reduced CO2 assimilation. It appears that seasonal drought changes the relations among BVOC emissions, photosynthesis and conductance. When drought is relaxed, BVOC emission rates are explained mostly by seasonal temperature, but when seasonal drought is maximal, photosynthesis and conductance-the physiological processes which best explain BVOC emission rates-decline, possibly indicating a more direct role of physiology in controlling BVOC emission.

Impact of Biofuel Poplar Cultivation on Ground-Level Ozone and Premature Human Mortality Depends on Cultivar Selection and Planting Location.

Isoprene and other volatile organic compounds emitted from vegetation play a key role in governing the formation of ground-level ozone. Emission rates of such compounds depend critically on the plant species. The cultivation of biofuel feedstocks will contribute to future land use change, altering the distribution of plant species and hence the magnitude and distribution of emissions. Here we use relationships between biomass yield and isoprene emissions derived from experimental data for 29 commercially available poplar hybrids to assess the impact that the large-scale cultivation of poplar for use as a biofuel feedstock will have on air quality, specifically ground-level ozone concentrations, in Europe. We show that the increases in ground-level ozone across Europe will increase the number of premature deaths attributable to ozone pollution each year by up to 6%. Substantial crop losses (up to ∼9 Mt y(-1) of wheat and maize) are also projected. We further demonstrate that these impacts are strongly dependent on the location of the poplar plantations, due to the prevailing meteorology, the population density, and the dominant crop type of the region. Our findings indicate the need for a concerted and centralized decision-making process that considers all aspects of future land use change in Europe, and not just the effect on greenhouse gas emissions.

Variation among different genotypes of hybrid poplar with regard to leaf volatile organic compound emissions.

Plantations of hybrid poplar are used in temperate regions to produce woody biomass for forestry-related industries and are likely to become more prevalent if they are used as a source of cellulose for second-generation biofuels. Species in the genus Populus are known to emit great quantities of the volatile organic compounds (VOCs) isoprene and methanol, and lesser quantities of terpene VOCs, giving poplar plantations the potential to significantly influence regional atmospheric chemistry. The goals of this study were to quantify the differences in isoprene, methanol, and monoterpene emissions from 30 hybrid poplar genotypes, determine how well VOC emissions could be explained by growth, photosynthesis, and stomatal conductance, determine whether the parental crosses that created a genotype could be used to predict its emissions, and determine whether VOC emissions from different genotypes exhibit different responses to elevated CO2. We found that 40-50% of the variation in isoprene emissions across genotypes could be explained by a combination of instantaneous photosynthesis rate and seasonal aboveground growth and 30-35% of methanol emissions could be explained by stomatal conductance. We observed a threefold range in isoprene emissions across all 30 genotypes. Both genotype and parental cross were significant predictors of isoprene and monoterpene emissions. Genotypes from P. tricocarpa X P. deltoides (T x D) crosses generally had higher isoprene emissions and lower monoterpene emissions than those from P. deltoides x P. nigra (D x N) crosses. While isoprene and monoterpene emissions generally decreased under elevated CO2 and methanol emissions generally increased, the responses varied among genotypes. Our findings suggest that genotypes with greater productivity tend to have higher isoprene emissions. Additionally, the genotypes with the lowest isoprene emissions under current CO2 are not necessarily the ones with the lowest emissions under elevated CO2.

Contribution of various carbon sources toward isoprene biosynthesis in poplar leaves mediated by altered atmospheric CO2 concentrations.

Biogenically released isoprene plays important roles in both tropospheric photochemistry and plant metabolism. We performed a (13)CO(2)-labeling study using proton-transfer-reaction mass spectrometry (PTR-MS) to examine the kinetics of recently assimilated photosynthate into isoprene emitted from poplar (Populus × canescens) trees grown and measured at different atmospheric CO(2) concentrations. This is the first study to explicitly consider the effects of altered atmospheric CO(2) concentration on carbon partitioning to isoprene biosynthesis. We studied changes in the proportion of labeled carbon as a function of time in two mass fragments, M41(+), which represents, in part, substrate derived from pyruvate, and M69(+), which represents the whole unlabeled isoprene molecule. We observed a trend of slower (13)C incorporation into isoprene carbon derived from pyruvate, consistent with the previously hypothesized origin of chloroplastic pyruvate from cytosolic phosphenolpyruvate (PEP). Trees grown under sub-ambient CO(2) (190 ppmv) had rates of isoprene emission and rates of labeling of M41(+) and M69(+) that were nearly twice those observed in trees grown under elevated CO(2) (590 ppmv). However, they also demonstrated the lowest proportion of completely labeled isoprene molecules. These results suggest that under reduced atmospheric CO(2) availability, more carbon from stored/older carbon sources is involved in isoprene biosynthesis, and this carbon most likely enters the isoprene biosynthesis pathway through the pyruvate substrate. We offer direct evidence that extra-chloroplastic rather than chloroplastic carbon sources are mobilized to increase the availability of pyruvate required to up-regulate the isoprene biosynthesis pathway when trees are grown under sub-ambient CO(2).

Responses of sugar maple and hemlock seedlings to elevated carbon dioxide under altered above- and belowground nitrogen sources.

Various human-induced changes to the atmosphere have caused carbon dioxide (CO2), nitrogen dioxide (NO2) and nitrate deposition (NO3⁻) to increase in many regions of the world. The goal of this study was to examine the simultaneous influence of these three factors on tree seedlings. We used open-top chambers to fumigate sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) with ambient or elevated CO2 and NO2 (elevated concentrations were 760 ppm and 40 ppb, respectively). In addition, we applied an artificial wet deposition of 30 kg ha⁻¹ year⁻¹ NO3⁻ to half of the open-top chambers. After two growing seasons, hemlocks showed a stimulation of growth under elevated CO2, but the addition of elevated NO2 or NO3⁻ eliminated this effect. In contrast, sugar maple seedlings showed no growth enhancement under elevated CO2 alone and decreased growth in the presence of NO2 or NO3⁻, and the combined treatments of elevated CO2 with increased NO2 or NO3⁻ were similar to control plants. Elevated CO2 induced changes in the leaf characteristics of both species, including decreased specific leaf area, decreased %N and increased C:N. The effects of elevated CO2, NO2 and NO3⁻ on growth were not additive and treatments that singly had no effect often modified the effects of other treatments. The growth of both maple and hemlock seedlings under the full combination of treatments (CO2 + NO2 + NO3⁻) was similar to that of seedlings grown under control conditions, suggesting that models predicting increased seedling growth under future atmospheric conditions may be overestimating the growth and carbon storage potential of young trees.

Predicting leaf-level fluxes of O3 and NO2: the relative roles of diffusion and biochemical processes.

Pollutants like O(3) and NO(2) enter leaves through the stomata and cause damage during reactions with components of biological cell membranes. The steady-state flux rates of these gases into the leaf are determined by a series of physical and biochemical resistances including stomatal aperture, reactions occurring within the cell wall and the ability of the leaf to remove the products of apoplastic reactions. In the present study, multiple regression models incorporating stomatal conductance, apoplastic and symplastic ascorbate concentrations, and nitrate reductase (NR) activities were generated to explain the observed variations in leaf-level flux rates of O(3) and NO(2). These measurements were made on the plant Catharanthus roseus (Madagascar periwinkle). The best-fit model explaining NO(2) flux included stomatal conductance, apoplastic ascorbate and NR activity. This model explained 89% of the variation in observed leaf fluxes and suggested physical resistances, reaction between NO(2) and apoplastic ascorbate, and the removal rate of nitrate (generated by reactions of NO(2) and water) from the apoplast all play controlling roles in NO(2) flux to leaves. O(3) flux was best explained by stomatal conductance and symplastic ascorbate explaining 66% of the total variation in leaf flux. Both models demonstrate the importance of measuring processes other than stomatal conductance to explain steady-state leaf-level fluxes of pollutant gases.