Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

Climate change may have an impact on the productivity of conifer trees by influencing the morphology (size and surface characteristics) and function (capacity for gas exchange) of conifer needles. In order to test the responses of needles to climatic variables, Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), saplings were grown in sunlit controlled environment chambers at ambient or elevated (+200 parts per million above ambient) CO2 and at ambient or elevated temperature (+4 degrees C above ambient). Needle characteristics, including length, width, area, stomatal density (stomata per mm2), percentage of stomatal occlusion, and the morphology of epicuticular wax, were evaluated. Needle function was evaluated as stomatal conductance to water vapor and transpiration. Needle length increased significantly with elevated temperature but not with elevated CO2. Neither elevated CO2 nor elevated temperature affected stomatal density or stomatal number in these hypostomatous needles. Epicuticular wax was less finely granular at elevated than at ambient temperature and was similar in appearance at elevated and ambient CO2. Stomatal conductance and transpiration increased with elevated temperature and associated increased vapor pressure deficit; however, neither conductance nor transpiration was affected by elevated CO2. These results indicate that simulated climate change influences Douglas fir needle structure and function.

Hyperspectral plant signatures can be used as a short-term, as well as long-term (100-year timescale) monitoring technique to verify that CO 2 sequestration fields have not been compromised. An influx of CO 2 gas into the soil can stress vegetation, which causes changes in the visible to near-infrared reflectance spectral signature of the vegetation. For 29 days, beginning on July 9, 2008, pure carbon dioxide gas was released through a 100-m long horizontal injection well, at a flow rate of 300 kg day -1 . Spectral signatures were recorded almost daily from an unmown patch of plants over the injection with a ''FieldSpec Pro'' spectrometer by Analytical Spectral Devices, Inc. Measurements were taken both inside and outside of the CO 2 leak zone to normalize observations for other environmental factors affecting the plants. Four to five days after the injection began, stress was observed in the spectral signatures of plants within 1 m of the well. After approximately 10 days, moderate to high amounts of stress were measured out to 2.5 m from the well. This spatial distribution corresponded to areas of high CO 2 flux from the injection. Airborne hyperspectral imagery, acquired by Resonon, Inc. of Bozeman, MT using their hyperspectral camera, also showed the same pattern of plant stress. Spectral signatures of the plants were also compared to the CO 2 concentrations in the soil, which indicated that the lower limit of soil CO 2 needed to stress vegetation is between 4 and 8% by volume.

Morphological differences between old-growth trees and saplings of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) may extend to differences in needle anatomy. We used microscopy with image analysis to compare and quantify anatomical parameters in cross sections of previous-year needles of old-growth Douglas-fir trees and saplings at the Wind River Canopy Crane site in Washington and at three sites in the Cascade Mountains of Oregon. We also compared needle anatomy across a chronosequence of 10-, 20-, 40- and 450-year-old Douglas-fir trees from the Wind River site. Anatomy differed significantly between needles of old-growth trees and saplings at all sites, suggesting a developmental change in needle anatomy with increasing tree age. Compared with needles of old-growth trees, needles of saplings were longer and had proportionately smaller vascular cylinders, larger resin canals and few hypodermal cells. Astrosclereids, which sequester lignin in their secondary cell walls and occupy space otherwise filled by photosynthetic cells, were scarce in needles of saplings but abundant in needles of old-growth trees. Needles of old-growth trees had an average of 11% less photosynthetic mesophyll area than needles of saplings. The percentage of non-photosynthetic area in needles increased significantly with increasing tree age from the chronosequence of 10-, 20-, 40- and 450-year-old trees at the Wind River site. This reduction in photosynthetic area may contribute to decreased growth rates in old trees.

Increased atmospheric CO 2 and global warming may affect overall tree growth, but impacts of these combined stresses are largely unknown in terms of multiple growing season impacts on specific flushes. Thus, the effects of ambient or elevated CO 2 (approximately 200 µmol·mol -1 above ambient) and ambient or elevated temperature (approximately 4°C above ambient) were evaluated for both main and second (lammas) flushes of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Established seedlings were grown for three full growing seasons in outdoor, sunlit chambers, which maintained diel and seasonal variation in climate. A reconstructed forest soil was used with a seasonal wet and dry cycle and without added fertilizer. Compared with ambient CO 2 , elevated CO 2 had no impact on overall phenology and growth of terminal shoots, needles, or buds. In contrast, compared with ambient temperature, elevated temperature resulted in higher shoot and needle growth rates early in the season; reduced final terminal shoot length; and either reduced, increased, or unchanged final needle length, depending on season. Initiation of the lammas flush was delayed and (or) decreased at elevated temperature. Leading terminal bud break and growth occurred earlier; however, resting bud length was reduced, and bud width tended to increase with elevated temperature. Thus, at least during early seedling growth, elevated temperatures may reduce both main-and lammas-flush growth, thereby altering tree productivity, whereas elevated CO 2 may have little effect on main or lammas growth at either the current or elevated temperature.Résumé : L'augmentation du CO 2 atmosphérique et le réchauffement global peuvent affecter la croissance des arbres en général, mais on connait très mal les impacts de ces stress combinés en termes d'impacts au cours de plusieurs saisons sur les pousses spécifiques. Les auteurs ont évalué les effets de CO 2 ambiant et élevé (± 200 µmol mol -1 plus d'ambiant) et de température ambiante et élevée (± 4°C plus d'ambiante) sur les rameaux principaux et sur les rameaux de seconde venue (lammas) chez le douglas taxifolié (Pseudotsuga menziesii (Mirb.) Franco). Ils ont cultivé des plantules établies, pendant trois saisons de croissance complètes, dans des chambres situées à l'extérieur, naturellement illuminées et qui maintiennent la variation quotidienne et saisonnière du climat. Ils ont utilisé un sol forestier reconstitué et un cycle saisonnier d'humidité et de sécheresse, sans addition de fertilisant. Comparativement au CO 2 ambiant, l'augmentation du CO 2 n'a pas d'effet sur la phénologie générale et la croissance de la pousse terminale, des aiguilles et des bourgeons. Au contraire, comparativement à la température ambiante, la température élevée conduit à des taux accrus de croissance de la tige et des aiguilles tôt dans la saison, à une réduction de la longueur totale de la pousse terminale et réduit, augmente ou laisse inchangée la longueur finale des aiguilles, selon la saison. Le déclenchement des pousses de seconde ve...