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Arctic Ocean surface circulation change should not be viewed as the strength of the anticyclonic Beaufort Gyre. While the Beaufort Gyre is a dominant feature of average Arctic Ocean surface circulation, empirical orthogonal function analysis of dynamic height (1950–89) and satellite altimetry–derived dynamic ocean topography (2004–19) show the primary pattern of variability in its cyclonic mode is dominated by a depression of the sea surface and cyclonic surface circulation on the Russian side of the Arctic Ocean. Changes in surface circulation after Arctic Oscillation (AO) maxima in 1989 and 2007–08 and after an AO minimum in 2010 indicate the cyclonic mode is forced by the AO with a lag of about 1 year. Associated with a one standard deviation increase in the average AO starting in the early 1990s, Arctic Ocean surface circulation underwent a cyclonic shift evidenced by increased spatial-average vorticity. Under increased AO, the cyclonic mode complex also includes increased export of sea ice and near-surface freshwater, a changed path of Eurasian runoff, a freshened Beaufort Sea, and weakened cold halocline layer that insulates sea ice from Atlantic water heat, an impact compounded by increased Atlantic Water inflow and cyclonic circulation at depth. The cyclonic mode's connection with the AO is important because the AO is a major global scale climate index predicted to increase with global warming. Given the present bias in concentration of in situ measurements in the Beaufort Gyre and Transpolar Drift, a coordinated effort should be made to better observe the cyclonic mode. [ABSTRACT FROM AUTHOR]

Sustained observations of environmental conditions in the North Pole region are critical to understanding the changing Arctic Ocean. The Transpolar Drift conduit of sea ice and freshened upper-ocean waters across the Arctic Ocean passes over the North Pole region on its way to the North Atlantic through Fram and Nares Straits. The exported ice and freshened water stratifies the sub-Arctic seas and limits the vertical convection that ventilates the world ocean. Key variables such as ice thickness, bottom pressure, and hydrography in the North Pole region are thus sensitive indicators of changes over the whole Arctic Basin and how these affect the global ocean. Drifting buoys installed in the North Pole region by Great Britain, Canada, France, Germany, Japan, and the U.S. address what would otherwise be a dearth of ocean, ice, and atmosphere observations in the central Arctic. A suite of satellite remote sensing tools such as ICESat/ICESat-2 from the U.S., GRACE from the U.S. and Germany, and CryoSat2 from the European Union extend the conclusions from central Arctic Ocean in situ observations to other regions. Detecting and understanding climate change requires observations over decadal and longer scales. We propose an international program as the key to sustaining these observations in the North Pole region. Such an international program would help immeasurably by 1) facilitating financial sharing of the burden of long-term measurements among several nations, (2) reducing logistics costs through economies of scale, and 3) providing a buffer against national funding, logistics, and geopolitical difficulties. [ABSTRACT FROM AUTHOR]

Abstract: Model and observational evidence has shown that ocean current speeds in the Beaufort Gyre have increased and recently stabilized. Because these currents rival ice drift speeds, we examine the potential for the Beaufort Gyre's shift from a system in which the wind drives the ice and the ice drives a passive ocean to one in which the ocean often, in the absence of high winds, drives the ice. The resultant stress exerted on the ocean by the ice and the resultant Ekman pumping are reversed, without any change in average wind stress curl. Through these curl reversals, the ice‐ocean stress provides a key feedback in Beaufort Gyre stabilization. This manuscript constitutes one of the first observational studies of ice‐ocean stress inclusive of geostrophic ocean currents, by making use of recently available remote sensing data. [ABSTRACT FROM AUTHOR]

To understand the factors causing the interannual variations in the summer retreat of the Beaufort Sea ice edge, Seasonal Ice Zone Reconnaissance Surveys (SIZRS) aboard U.S. Coast Guard Arctic Domain Awareness flights were made monthly from June to October in 2012, 2013, and 2014. The seasonal ice zone (SIZ) is where sea ice melts and reforms annually and encompasses the nominally narrower marginal ice zone (MIZ) where a mix of open-ocean and ice pack processes prevail. Thus, SIZRS provides a regional context for the smaller-scale MIZ processes. Observations with aircraft expendable conductivity-temperature-depth probes reveal a salinity pattern associated with large-scale gyre circulation and the seasonal formation of a shallow (~20 m) fresh layer moving with the ice edge position. Repeat occupations of the SIZRS lines from 72° to 76°N on 140° and 150°W allow a comparison of observed hydrography to atmospheric indices. Using this relationship, the basinwide salinity signals are separated from the fresh layer associated with the ice edge. While this layer extends northward under the ice edge as the melt season progresses, low salinities and warm temperatures appear south of the edge. Within this fresh layer, average salinity is correlated with distance from the ice edge. The salinity observations suggest that the upper-ocean freshening over the summer is dominated by local sea ice melt and vertical mixing. A Price-Weller-Pinkel model analysis reveals that observed changes in heat content and density structure are also consistent with a 1D mixing process. [ABSTRACT FROM AUTHOR]

Measurements of ocean bottom pressure (OBP) anomalies from the satellite mission Gravity Recovery and Climate Experiment (GRACE), complemented by information from two ocean models, are used to investigate the variations and distribution of the Arctic Ocean mass from 2002 through 2011. The forcing and dynamics associated with the observed OBP changes are explored. Major findings are the identification of three primary temporal-spatial modes of OBP variability at monthly-to-interannual time scales with the following characteristics. Mode 1 (50% of the variance) is a wintertime basin-coherent Arctic mass change forced by southerly winds through Fram Strait, and to a lesser extent through Bering Strait. These winds generate northward geostrophic current anomalies that increase the mass in the Arctic Ocean. Mode 2 (20%) reveals a mass change along the Siberian shelves, driven by surface Ekman transport and associated with the Arctic Oscillation. Mode 3 (10%) reveals a mass dipole, with mass decreasing in the Chukchi, East Siberian, and Laptev Seas, and mass increasing in the Barents and Kara Seas. During the summer, the mass decrease on the East Siberian shelves is due to the basin-scale anticyclonic atmospheric circulation that removes mass from the shelves via Ekman transport. During the winter, the forcing mechanisms include a large-scale cyclonic atmospheric circulation in the eastern-central Arctic that produces mass divergence into the Canada Basin and the Barents Sea. In addition, strengthening of the Beaufort high tends to remove mass from the East Siberian and Chukchi Seas. Supporting previous modeling results, the month-to-month variability in OBP associated with each mode is predominantly of barotropic character. [ABSTRACT FROM AUTHOR]

Freshening in the Canada basin of the Arctic Ocean began in the 1990s and continued to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomalyof the 1970s, raising the spectre of slowing global ocean circulation. Freshening has been attributed to increased sea ice melting and contributions from runoff, but a leading explanation has been a strengthening of the Beaufort High-a characteristic peak in sea level atmospheric pressure-which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation. [ABSTRACT FROM AUTHOR]

Substantial lateral CO2 diffusion rates into leaf areas where stomata were blocked by grease patches were quantified by gas exchange and chlorophyll a fluorescence imaging in different species across the full range of photosynthetic photon flux densities (PPFD). The lateral CO2 flux rate over short distances was substantial and very similar in five dicotyledonous species with different vascular anatomies (two species with bundle sheath extensions, sunflower [Helian thus annuus] and dwarf bean [Phaseolus vulgaris]; and three species without bundle sheath extensions, faba bean [ Viciafaba], petunia [Petunia hybrida], and tobacco [Nicotiana tabacum]). Only in the monocot maize (Zea mays) was there little or no evident lateral CO2 flux. Lateral diffusion rates were low when PPFD <300 μmol m-2 s-1 but approached saturation in moderate PPFD (300 μmol m-2 s-1) when lateral CO2 diffusion represented 15% to 24% of the normal CO2 assimilation rate. Smaller patches and higher ambient CO2 concentration increased lateral CO2 diffusion rates. Calculations with a two-dimensional diffusion model supported these observations that lateral CO2 diffusion over short distances inside dicotyledonous leaves can be important to photosynthesis. The results emphasize that sup- ply of CO2 from nearby stomata usually dominates assimilation, but that lateral supply over distances up to approximately 1 mm can be important if stomata are blocked, particularly when assimilation rate is low. [ABSTRACT FROM AUTHOR]

Photosynthesis depends on the diffusion of gaseous CO2 inside the leaf spaces from the stomatal entry point to the mesophyll cell walls. Although most research considers only the vertical diffusion from stomata on upper and/or leaf lower surfaces, some of the gas will diffuse in the lateral (paradermal) direction. The importance of lateral CO2 diffusion is reviewed, and the anatomical characteristics of leaves, including the variation of air space volume between species and conditions are discussed. The contribution of the air space conductance to the limitation of photosynthesis by the overall CO2 diffusion pathway is usually ignored. However, the need to consider three-dimensional diffusion at the small scale of a few stomata is emphasized because stomata are discrete, and separated by 20–300 µm. At the large scale of 100s of micrometres, there may be barriers to CO2 caused by the vascular tissue, particularly if there are bundle sheath extensions. The possible extent and controls on CO2 lateral and vertical diffusion in different species and conditions are illustrated using chlorophyll a fluorescence imaging techniques. It is clear that there is a range of effective lateral permeabilities depending on the particular vascular patterns and cell arrangements, and that species cannot be simply divided into homobaric and heterobaric anatomies. Lateral diffusion in more permeable leaves can be sufficient to affect measurements of leaf gas exchange, particularly when fluxes are low, although its contribution to leaf photosynthesis in natural conditions needs clarification. [ABSTRACT FROM AUTHOR]

Lateral diffusion of CO2 was investigated in photosynthesizing leaves with different anatomy by gas exchange and chlorophyll a fluorescence imaging using grease to block stomata. When one-half of the leaf surface of the heterobaric species Helianthus annuus was covered by 4-mm-diameter patches of grease, the response of net CO2 assimilation rate (A) to intercellular CO2 concentration (Ci) indicated that higher ambient CO2 concentrations (Ca) caused only limited lateral diffusion into the greased areas. When single 4-mm patches were applied to leaves of heterobaric Phaseolus vulgaris and homobaric Commelina communis, chlorophyll a fluorescence images showed dramatic declines in the quantum efficiency of photosystem II electron transport (measured as Fq′ Fm′) across the patch, demonstrating that lateral CO2 diffusion could not support A. The Fq′/Fm′ values were used to compute images of Ci across patches, and their dependence on Ca was assessed. At high Ca, the patch effect was less in C. communis than P. vulgaris. A finite-volume porous-medium model for assimilation rate and lateral CO2 diffusion was developed to analyze the patch images. The model estimated that the effective lateral CO2 diffusion coefficients inside C. communis and P. vulgaris leaves were 22% and 12% of that for free air, respectively. We conclude that, in the light, lateral CO2 diffusion cannot support appreciable photosynthesis over distances of more than approximately 0.3 mm in normal leaves, irrespective of the presence or absence of bundle sheath extensions, because of the CO2 assimilation by cells along the diffusion pathway. [ABSTRACT FROM AUTHOR]

Provides information on a study which showed that vertical turbulent fluxes in the upper ocean can be measured directly with an autonomous underwater vehicle (AUV). Methodology; Illustration of the utility of the Kalman smoothing method; Description of the Antarctic Zone Flux Experiment; Surface heat balance of the Arctic Ocean.

ABSTRACT Native scrub-oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO2] (approx. 350 μmol mol-1) and to high [CO2] (increased by 350 μmol mol-1). Stomatal and photosynthetic acclimation to high [CO2] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (gs) was approximately 40% lower in the high- compared to low-[CO2]-grown plants when measured at their respective growth concentrations. Reciprocal measurements of gs in both high- and low-[CO2]-grown plants showed that there was negative acclimation in the high-[CO2]-grown plants (9–16% reduction in gs when measured at 700 μmol mol-1), but these were small compared to those for net CO2 assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO2] (ci) which showed a reduction in stomatal sensitivity at low ci in the high-[CO2]-grown plants. Stomatal density showed no change in response to growth in high growth [CO2]. Long-term stomatal and photosynthetic acclimation to growth in high [CO2] did not markedly change the 2·5- to 3-fold increase in gas-exchange-derived water use efficiency caused by high [CO2]. [ABSTRACT FROM AUTHOR]

Focuses on the measurement of the upper Arctic Ocean during a cruise by the U.S.S. Pargo submarine in 1993. Details on the use of the hydrographic measurements collected; Information on the submarine; What findings of studies conducted during the cruise indicate; Reference to the journey of the submarine.

The observed responses of stomata to carbon dioxide are reviewed, and the interaction of other known factors on the sensitivity to CO[sub2] are summarized. The role of stomatal response to CO[sub2] is discussed, and it is argued that while the effect of the CO[sub2] response in normal daily stomatal behaviour is presently poorly understood the stomatal response to CO[sub2] will have major impact in improving water use efficiency in future CO[sub2] atmospheres. However, the attenuation of this increase is emphasized so that increases at the crop level will probably be much smaller than those observed at the single leaf assimilation level. [ABSTRACT FROM AUTHOR]

The article comments on a study of lateral diffusion of carbon dioxide conducted by R. Pieruschka et al and published in a 2006 issue of "New Phytologist." It outlines the major arguments of the study and presents a discussion on leaf anatomy and patchy stomata behavior of leaves. It considers the importance of lateral carbon dioxide diffusion.

As leaves are typically very thin, most plant physiologists think of this diffusion as a vertical one-dimensional flux of CO SB 2 sb , across one or both epidermes into the mesophyll, imagining a typical textbook cross-section of a leaf (see Fig. Keywords: anatomy; heterobaric; heterogeneity; homobaric; lateral diffusion; stomata EN anatomy heterobaric heterogeneity homobaric lateral diffusion stomata 641 643 3 04/16/21 20060215 NES 060215 Photosynthesis depends on the diffusion of CO SB 2 sb from the air surrounding a leaf into the leaf and into the chloroplasts. Redrawn from Weyers & Lawson (1997). gl Leaf anatomy Lateral CO SB 2 sb diffusion in leaves depends on the anatomy of the leaf, and two extreme types are usually considered (Fig. Lateral CO SB 2 sb diffusion is considered to be totally restricted in "heterobaric" leaves, in which the bundle sheath surrounding the vein extends to the upper and lower epidermis, compartmentalising the leaf into small areoles and restricting lateral movement of CO SB 2 sb (Fig. [Extracted from the article]

Focuses on the effect of increasing atmospheric carbon dioxide concentration on stomal conductance in plants. Importance of stomal response to environment in plant growth and water status; Application of meta-analysis; Role of stomata in photosynthesis.

Provides information on autonomous vehicle and hydrographic buoy measurement programs during the Antarctic Zone Flux Experiment in 1994. Use of an autonomous conductivity temperature vehicle (ACTV) and a polar ocean profile buoy in the project; Objective of the ACTV operation regarding the deep mixed layer and high heat flux in the Weddell Sea; Significance of the project in obtaining data concerning the horizontal structure of boundary layer turbulence and lead convection in the area.

Afforestation and reforestation to meet 'Net Zero' emissions targets are considered a necessary policy by many countries. Their potential benefits are usually assessed through forest carbon and growth models. The implementation of vegetation demography gives scope to represent forest management and other size-dependent processes within land surface models (LSMs). In this paper, we evaluate the impact of including management within an LSM that represents demography, using both in-situ and reanalysis climate drivers at a mature, upland Sitka spruce plantation in Northumberland, UK. We compare historical simulations with fixed and variable CO2 concentrations, and with and without tree thinning implemented. Simulations are evaluated against the observed vegetation structure and carbon fluxes. Including thinning and the impact of increasing CO2 concentration ('CO2 fertilisation') gave more realistic estimates of stand-structure and physical characteristics. Historical CO2 fertilisation had a noticeable effect on the Gross Primary Productivity seasonal–diurnal cycle and contributed to approximately 7% higher stand biomass by 2018. The net effect of both processes resulted in a decrease of tree density and biomass, but an increase in tree height and leaf area index. [ABSTRACT FROM AUTHOR]

Understanding and predicting Arctic change and its impacts on global climate requires broad, sustained observations of the atmosphere-ice-ocean system, yet technological and logistical challenges severely restrict the temporal and spatial scope of observing efforts. Satellite remote sensing provides unprecedented, pan-Arctic measurements of the surface, but complementary in situ observations are required to complete the picture. Over the past few decades, a diverse range of autonomous platforms have been developed to make broad, sustained observations of the ice-free ocean, often with near-real-time data delivery. Though these technologies are well suited to the difficult environmental conditions and remote logistics that complicate Arctic observing, they face a suite of additional challenges, such as limited access to satellite services that make geolocation and communication possible. This paper reviews new platform and sensor developments, adaptations of mature technologies, and approaches for their use, placed within the framework of Arctic Ocean observing needs. [ABSTRACT FROM AUTHOR]

A preface to the "Journal of Experimental Botany" is presented.

Numerous recent observations indicate that the Arctic is undergoing a significant change. In the last decade, the hydrography of the Arctic Ocean has shifted, and the atmospheric circulation has undergone achange from the lower stratosphere to the surface. Typically the eastern Arctic Ocean, on the European side of the Lomonosov Ridge, is dominated by water of Atlantic origin. A cold halocline of varying thickness overlies the warmer Atlantic water and isolates it from the seaice and surface mixed layer. The western Arctic Ocean, on the North American side of the Lomonosov Ridge, is characterized by an added layer of water from the Pacific immediately below the surface mixed layer. Data collected during several cruises from 1991 to 1995 indicate that in the 1990s the boundary between these eastern and western halocline types shifted from a position roughly parallel to the LomonosovRidge to near alignment with the Alpha and Mendeleyev Ridges. The Atlantic Water temperature has also increased, and the cold haloclirle has become thinner. The change has resulted in increased surface salinity in the Makarov Basin. Recent results suggest that the change also includes decreased surface salinity and greater summer ice melt in the Beaufort Sea. Atmospheric pressure fietlds and ice drift data show that the whole patterns of atmospheric pressure and ice drift for the early 1990s were shifted counterclockwise 40 deg. -60 deg. from earlier patterns. The shift in atmospheric circulation seems related to the Arctic Oscillation in the Northern Hemisphere atmospheric pressure pattern. The changes in the ocean circulation, ice drift, air temperatures, and permafrost can be explained as responses to the ArcticOscillation, as can changes inair temperatures over the Russian Arctic. [ABSTRACT FROM AUTHOR]

Examines stomatal responses to increased carbon dioxide concentration. Reduction of stomatal aperture; Differences in the short-term and long-term effects; Impact of the stomatal acclimation effect on conductance; Differences in the responses between stomata on adaxial and abaxial leaf surfaces; Changes in stomatal frequencies.

Salinity is the primary determinant of the Arctic Ocean's density structure. Freshwater accumulation and distribution in the Arctic Ocean have varied significantly in recent decades and certainly in the Beaufort Gyre (BG). In this study, we analyze salinity variations in the BG region between 2012 and 2017. We use in situ salinity observations from the Seasonal Ice Zone Reconnaissance Surveys (SIZRS), CTD casts from the Beaufort Gyre Exploration Project (BGP), and the EN4 data to validate and compare with satellite observations from Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), and Aquarius Optimally Interpolated Sea Surface Salinity (OISSS), and Arctic Ocean models: ECCO, MIZMAS, HYCOM, ORAS5, and GLORYS12. Overall, satellite observations are restricted to ice-free regions in the BG area, and models tend to overestimate sea surface salinity (SSS). Freshwater Content (FWC), an important component of the BG, is computed for EN4 and most models. ORAS5 provides the strongest positive SSS correlation coefficient (0.612) and lowest bias to in situ observations compared to the other products. ORAS5 subsurface salinity and FWC compare well with the EN4 data. Discrepancies between models and SIZRS data are highest in GLORYS12 and ECCO. These comparisons identify dissimilarities between salinity products and extend challenges to observations applicable to other areas of the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Abstract: Here we report the first optical, sensor-based profiles of nitrate from the central Makarov and Amundsen and southern Canada Basins of the Arctic Ocean. These profiles were obtained as part of the International Polar Year program during spring 2007 and 2008 field seasons of the North Pole Environmental Observatory (NPEO) and Beaufort Gyre Exploration Program (BGEP). These nitrate data were combined with in-situ, sensor-based profiles of dissolved oxygen to derive the first high-resolution vertical NO profiles to be reported for the Arctic Ocean. The focus of this paper is on the halocline layer that insulates sea ice from Atlantic water heat and is an important source of nutrients for marine ecosystems within and downstream of the Arctic. Previous reports based on bottle data have identified a distinct lower halocline layer associated with an NO minimum at about S=34.2 that was proposed to be formed initially in the Nansen Basin and then advected downstream. Greater resolution afforded by our data reveal an even more pronounced NO minimum within the upper, cold halocline of the Makarov Basin. Thus a distinct lower salinity source ventilated the Makarov and not the Amundsen Basin. In addition, a larger Eurasian River water influence overlies this halocline source in the Makarov. Observations in the southern Canada Basin corroborate previous studies confirming multiple lower halocline influences including diapycnal mixing between Pacific winter waters and Atlantic-derived lower halocline waters, ventilation via brine formation induced in persistent openings in the ice, and cold, O2-rich lower halocline waters originating in the Eurasian Basin. These findings demonstrate that continuous sensing of chemical properties promises to significantly advance understanding of the maintenance and circulation of the halocline. [ABSTRACT FROM AUTHOR]

Eucalyptus gunnii is a fast-growing, cold-tolerant tree species endemic to Tasmania that is suitable for growing as short-rotation coppice (SRC) plantations in the UK. Fast growing eucalypts such as E. gunnii could potentially deliver higher biomass yields with a superior calorific value for the domestic bioenergy market than other SRC plantation species such as willow or poplar. However, eucalypts are known emitters of biogenic volatile organic compounds (BVOC) like isoprene and monoterpenes. These compounds contribute to the formation of atmospheric pollutants such as ozone and secondary organic aerosols. An assessment of the sources of BVOCs during the lifecycle of a UK E. gunnii SRC plantation found the mean standardised emissions of isoprene and total monoterpenes from branches of juvenile foliage to be 7.50 μg C g dw −1 h−1 and 1.30 μg C g dw −1 h−1, respectively. The predominant monoterpene emitted was cis-β-ocimene. Isoprene emissions from the forest floor were extremely low but monoterpene emissions peaked at 50 μg C m−2 h−1. α-Pinene and d-limonene were the major components of the monoterpene emissions, with higher emissions correlated to the abundance of leaf litter. Both the magnitude and composition of monoterpene emissions from the forest floor varied during the SRC plantation life cycle, with the coppiced and regrowth stands of eucalyptus producing less emissions. The woodchip produced at harvesting emitted only trace levels of isoprene but substantial monoterpene emissions, up to 90 μg C m−2 h−1, predominately eucalyptol. Harvesting and resulting biomass chips may provide a short-lived concentrated source of BVOCs in winter at SRC plantations. Modelled annual emissions using MEGAN 2.1 (canopy emissions only) suggest that BVOC emissions from a UK E. gunnii SRC plantation are most abundant in summer, and that modelled annual isoprene and total monoterpenes emissions could be around 6.9 kg C ha−1 and 2.4 kg C ha−1 respectively, for a young plantation. Based on the very limited data, the per-hectare E. gunnii isoprene emissions are smaller than estimates for other SRC/SRF plantation species in the UK; the per-hectare monoterpene emissions are in the span of estimates for other plantation species. • First data for emissions of speciated BVOC from a UK eucalypt bioenergy plantation • Isoprene emission from juvenile foliage six times greater than monoterpene emission • cis-β-Ocimene and α-phellandrene major monoterpenes from juvenile foliage • Forest floor monoterpene emissions smaller than branches and vary with lifecycle • α-Pinene and d-limonene dominate forest floor emissions; eucalyptol from woodchip [ABSTRACT FROM AUTHOR]

Abstract: A new method of constructing light, flexible and more realistic replica leaves for continuous determination of leaf boundary layer conductance to heat transfer () was developed and tested in a mature oak (Quercus robur L.) and sycamore (Acer pseudoplatanus L.) tree canopy. The replicas were used to determine the difference between oak and sycamore leaf in exposed sites in the upper canopy, the relationship of with wind speed, the seasonal changes in in the canopy as leaf cover developed, and values for Ω, the decoupling coefficient. The replicas showed similar gradients in temperature at their margins to those in real leaves. When exposed, the of the larger sycamore leaves was 66% of that of oak leaves under the same conditions. Linear relationships were found with and wind speed across the measured range of 0.3–3.5ms−1, and flow in the replica boundary layers was laminar in all conditions. The leafless canopy produced a substantial sheltering effect, reducing by 12–28% in light winds. Sycamore replicas in the leafed canopy showed a 19–29% lower at a given external wind speed than when outside, but there was little difference between ‘sun’ and ‘shade’ position shoots, because of the density of the shoots, and the close branching pattern. In contrast, in oak at a given wind speed was 15–21% lower for ‘sun’ leaves than that for replicas outside the canopy, with a larger reduction (approximately 28%) in denser ‘shade’ sites. Although wind speed in the canopy was often low, leaves of both species were usually well coupled to the canopy airstream (Ω <0.3). Sun leaves were substantially less well coupled than shade leaves, despite the lower shelter effect, because of their higher stomatal conductance values. In the lightest winds (<0.5ms−1) and with high stomatal conductance, coupling may on many occasions be poor for sun leaves, particularly for the larger sycamore leaves. [Copyright &y& Elsevier]

High resolution chlorophyll a fluorescence imaging was used to compare the photosynthetic efficiency of PSII electron transport (estimated by Fq′/Fm′) in guard cell chloroplasts and the underlying mesophyll in intact leaves of six different species: Commelina communis, Vicia faba, Amaranthus caudatus, Polypodium vulgare, Nicotiana tabacum, and Tradescantia albifora. While photosynthetic efficiency varied between the species, the efficiencies of guard cells and mesophyll cells were always closely matched. As measurement light intensity was increased, guard cells from the lower leaf surfaces of C. communis and V. faba showed larger reductions in photosynthetic efficiency than those from the upper surfaces. In these two species, guard cell photosynthetic efficiency responded similarly to that of the mesophyll when either light intensity or CO2 concentration during either measurement or growth was changed. In all six species, reducing the O2 concentration from 21% to 2% reduced guard cell photosynthetic efficiency, even for the C4 species A. caudatus, although the mesophyll of the C4 species did not show any O2 modulation of photosynthetic efficiency. This suggests that Rubisco activity is significant in the guard cells of these six species. When C. communis plants were water‐stressed, the guard cell photosynthetic efficiency declined in parallel with that of the mesophyll. It was concluded that the photosynthetic efficiency in guard cells is determined by the same factors that determine it in the mesophyll. [ABSTRACT FROM PUBLISHER]

Respiratory characteristics of wheat (Trticum aestivum L. cvs Gabo and WW15), mung bean (Vigna radiata L. Wilczek cv. Celera) and sunflower (Helianthus annuus L. cv. Sunfola) were studied in plants grown under a normal CO2 concentration and in air containing an additional 340 (or 250) μl l-1 CO2. Such an increase in global atmospheric CO2 concentration has been forecast for about the middle of the next century. The aim was to measure the effect of high CO2 on respiration and its components. Polarographic and, with wheat, CO2 exchange techniques were used. The capacity of the alternative pathway of respiration in roots was determined polarographically in the presence of 0.1 mM KCN. The actual rate of alternative pathway respiration was assessed by reduction in oxygen consumption caused by 10 mM sail- cylhydroxamic acid. Each species responded differently. In wheat, growth in high atmospheric CO2 was associated with up to 45% reduction in respiration by both roots and whole plants. Use of respiratory inhibitors in polarographic measurements on wheat roots implicated reduction in the degree of engagement of the alternative pathway as a major contributor to this reduced respiratory activity of high-CO2 plants. No change was found in the total sugar content per unit wheat root dry weight as a result of high CO2. In none of the species was there an increase in the absolute, or relative, contribution by the alternative pathway to total respiration of the root systems. Thus the improved photosynthetic assimilate supply of plants grown in high CO2 did not lead to increased diversion of carbon through the non-phosphorylating alternative path- way of respiration in the root. On the contrary, in wheat grown in high CO2 the reduced loss of carbon through that route must have contributed to their larger dry... [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 (ca) has been shown to increase forest carbon uptake. Yet, whether the ca-fertilization effect on forests is modulated by changes in sulphur (Sdep) and nitrogen (Ndep) deposition and how Ndep affects ecosystem N availability remains unclear. We explored spatial and temporal (over 30-years) changes in tree-ring δ13C-derived intrinsic water-use efficiency (iWUE), δ18O and δ15N for four species in twelve forests across climate and atmospheric deposition gradients in Britain. The increase in iWUE was not uniform across sites and species-specific underlying physiological mechanisms reflected the interactions between climate and atmospheric drivers (oak and Scots pine), but also an age effect (Sitka spruce). Most species showed no significant trends for tree-ring δ15N, suggesting no changes in N availability. Increase in iWUE was mostly associated with increase in temperature and decrease in moisture conditions across the South–North gradient and over 30-years. However, when excluding Sitka spruce (to account for age or stand development effects), variations in iWUE were significantly associated with changes in ca and Sdep. Our data suggest that overall climate had the prevailing effect on changes in iWUE across the investigated sites. Whereas, detection of Ndep, Sdep and ca signals was partially confounded by structural changes during stand development. [ABSTRACT FROM AUTHOR]

Accurate model representation of land- atmosphere carbon fluxes is essential for climate projections. However, the exact responses of carbon cycle processes to climatic drivers often remain uncertain. Presently, knowledge derived from experiments, complemented by a steadily evolving body of mechanistic theory, provides the main basis for developing such models. The strongly increasing availability of measurements may facilitate new ways of identifying suitable model structures using machine learning. Here, we explore the potential of gene expression programming (GEP) to derive relevant model formulations based solely on the signals present in data by automatically applying various mathematical transformations to potential predictors and repeatedly evolving the resulting model structures. In contrast to most other machine learning regression techniques, the GEP approach generates "readable" models that allow for prediction and possibly for interpretation. Our study is based on two cases: artificially generated data and real observations. Simulations based on artificial data show that GEP is successful in identifying prescribed functions, with the prediction capacity of the models comparable to four state-of-the-art machine learning methods (random forests, support vector machines, artificial neural networks, and kernel ridge regressions). Based on real observations we explore the responses of the different components of terrestrial respiration at an oak forest in south-eastern England. We find that the GEP-retrieved models are often better in prediction than some established respiration models. Based on their structures, we find previously unconsidered exponential dependencies of respiration on seasonal ecosystem carbon assimilation and water dynamics. We noticed that the GEP models are only partly portable across respiration components, the identification of a "general" terrestrial respiration model possibly prevented by equifinality issues. Overall, GEP is a promising tool for uncovering new model structures for terrestrial ecology in the data-rich era, complementing more traditional modelling approaches. [ABSTRACT FROM AUTHOR]

Arctic sea-ice loss is a leading indicator of climate change and can be attributed, in large part, to atmospheric forcing. Here, we show that recent ice reductions, weakening of the halocline, and shoaling of the intermediate-depth Atlantic Water layer in the eastern Eurasian Basin have increased winter ventilation in the ocean interior, making this region structurally similar to that of the western Eurasian Basin. The associated enhanced release of oceanic heat has reduced winter sea-ice formation at a rate now comparable to losses from atmospheric thermodynamic forcing, thus explaining the recent reduction insea-ice cover in the eastern Eurasian Basin. This encroaching “atlantification" of the Eurasian Basin represents an essential step toward a new Arctic climate state, with a substantially greater role for Atlantic inflows. [ABSTRACT FROM AUTHOR]

The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 10s of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced ~ 70 m diameter footprints on the surface of the Earth at ~ 150 m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3 km cross-track with a pair spacing of 90 m. Each of the beams will have a nominal 17 m diameter footprint with an along-track sampling interval of 0.7 m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross-track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along-track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532 nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15 + year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness. [ABSTRACT FROM AUTHOR]

Melting of the Greenland Ice Sheet represents a major uncertainty in projecting future rates of global sea level rise. Much of this uncertainty is related to a lack of knowledge about subsurface ocean hydrographic properties, particularly heat content, how these properties are modified across the continental shelf, and about the extent to which the ocean interacts with glaciers. Early results from NASA's five-year Oceans Melting Greenland (OMG) mission, based on extensive hydrographic and bathymetric surveys, suggest that many glaciers terminate in deep water and are hence vulnerable to increased melting due to ocean-ice interaction. OMG will track ocean conditions and ice loss at glaciers around Greenland through the year 2020, providing critical information about ocean-driven Greenland ice mass loss in a warming climate. [ABSTRACT FROM AUTHOR]

Efforts to implement variational data assimilation routines with functional ecology models and land surface models have been limited, with sequential and Markov chain Monte Carlo data assimilation methods being prevalent. When data assimilation has been used with models of carbon balance, prior or “background” errors (in the initial state and parameter values) and observation errors have largely been treated as independent and uncorrelated. Correlations between background errors have long been known to be a key aspect of data assimilation in numerical weather prediction. More recently, it has been shown that accounting for correlated observation errors in the assimilation algorithm can considerably improve data assimilation results and forecasts. In this paper we implement a Four-dimensional Variational data assimilation (4D-Var) scheme with a simple model of forest carbon balance, for joint parameter and state estimation and assimilate daily observations of Net Ecosystem CO 2 Exchange (NEE) taken at the Alice Holt forest CO 2 flux site in Hampshire, UK. We then investigate the effect of specifying correlations between parameter and state variables in background error statistics and the effect of specifying correlations in time between observation errors. The idea of including these correlations in time is new and has not been previously explored in carbon balance model data assimilation. In data assimilation, background and observation error statistics are often described by the background error covariance matrix and the observation error covariance matrix. We outline novel methods for creating correlated versions of these matrices, using a set of previously postulated dynamical constraints to include correlations in the background error statistics and a Gaussian correlation function to include time correlations in the observation error statistics. The methods used in this paper will allow the inclusion of time correlations between many different observation types in the assimilation algorithm, meaning that previously neglected information can be accounted for. In our experiments we assimilate a single year of NEE observations and then run a forecast for the next 14 years. We compare the results using our new correlated background and observation error covariance matrices and those using diagonal covariance matrices. We find that using the new correlated matrices reduces the root mean square error in the 14 year forecast of daily NEE by 44% decreasing from 4.22 g C m −2 day −1 to 2.38 g C m −2 day −1 . [ABSTRACT FROM AUTHOR]

Accurate modelling of land-atmosphere carbon fluxes is essential for future climate projections. However, the exact responses of carbon cycle processes to climatic drivers often remain uncertain. Presently, knowledge derived from experiments complemented with a steadily evolving body of mechanistic theory provides the main basis for developing the respective models. The strongly increasing availability of measurements may complicate the traditional hypothesis driven path to developing mechanistic models, but it may facilitate new ways of identifying suitable model structures using machine learning as well. Here we explore the potential to derive model formulations automatically from data based on gene expression programming (GEP). GEP automatically (re)combines various mathematical operators to model formulations that are further evolved, eventually identifying the most suitable structures. In contrast to most other machine learning regression techniques, the GEP approach generates models that allow for prediction and possibly for interpretation. Our study is based on two cases: artificially generated data and real observations. Simulations based on artificial data show that GEP is successful in identifying prescribed functions with the prediction capacity of the models comparable to four state-of-the-art machine learning methods (Random Forests, Support Vector Machines, Artificial Neural Networks, and Kernel Ridge Regressions). The case of real observations explores different components of terrestrial respiration at an oak forest in south-east England. We find that GEP retrieved models are often better in prediction than established respiration models. Furthermore, the structure of the GEP models offers new insights to driver selection and interactions. We find previously unconsidered exponential dependencies of respiration on seasonal ecosystem carbon assimilation and water dynamics. However, we also noticed that the GEP models are only partly portable across respiration components; equifinality issues possibly preventing the identification of a "general" terrestrial respiration model. Overall, GEP is a promising tool to uncover new model structures for terrestrial ecology in the data rich era, complementing the traditional approach of model building. [ABSTRACT FROM AUTHOR]

Plant phenology is a key indicator of the terrestrial biosphere's response to climate change, as well as a driver of global climate through changes in the carbon, energy and water cycles. Remote sensing observations of seasonal canopy greenness dynamics represent a valuable means to study land surface phenology (LSP) at scales relevant for comparison with regional climate information as well as ecosystem-level CO 2 fluxes. We explore relationships among key LSP dates at the start and end of the season captured by three remote sensing products (i.e., NDVI: Normalized Difference Vegetation Index; PI: Phenology Index; MODIS Land Cover Dynamics Product based on the Enhanced Vegetation Index, EVI) over 19 deciduous broadleaf and mixed forest sites in the northern hemisphere for 2000–2012, and compare these estimates to estimates of start and end of photosynthesis phenology extracted from gross primary productivity (GPP) from CO 2 flux measurements. To derive phenological transition dates, we use analytical solutions of various derivatives from the fitted logistic curves. LSP dates estimated by the three remote sensing products were not equivalent and differed in their sign and magnitude of lags with photosynthesis phenology dates. NDVI-derived phenology was characterized by shorter growing seasons, while EVI prolonged it by about two weeks compared to the photosynthesis phenology season length. PI start and end of season dates more closely matched the start ( r 2 = 0.84, RMSE = 7.61) and end ( r 2 = 0.61, RMSE = 8.57) of photosynthesis phenology as estimated by GPP time series. PI was also found agreeing best with LSP estimates from highly spatially resolved ground digital camera observations, available for about half of the investigated FLUXNET sites. Although there were strong relationships between remotely sensed LSP and photosynthesis phenology, the relationships were not consistent across deciduous forest ecosystems implying that the vegetative and photosynthetic timing do not always follow each other in the same direction. [ABSTRACT FROM AUTHOR]

The paper presents a model system, which consists of a partial equilibrium model and process-based terrestrial biogeochemistry models, to determine the optimal distributions of both Miscanthus ( Miscanthus × giganteus) and short rotation coppice willow ( SRC) ( Salix. viminalis L. x S. viminalis var Joruun) in Great Britain ( GB), as well as their potential contribution to meet heat and electricity demand in GB. Results show that the potential contribution of Miscanthus and SRC to heat and electricity demand is significant. Without considering farm-scale economic constraints, Miscanthus and SRC could generate, in an economically competitive way compared with other energy generation costs, 224 800 GWh yr−1 heat and 112 500 GWh yr−1 electricity, with 8 Mha of available land under Miscanthus and SRC, accounting for 66% of total heat demand and 62% of total electricity demand respectively. Given the pattern of heat and electricity demand, and the relative yields of Miscanthus and SRC in different parts of GB, Miscanthus is mainly favoured in the Midlands and areas in the South of GB, whereas SRC is favoured in Scotland, the Midlands and areas in the South of GB. [ABSTRACT FROM AUTHOR]

Dedicated non-food bioenergy crops like poplar are needed as sustainable, lowinput feedstocks for renewable energy in a future drier climate, where they can be grown on marginal soils. Such plants should have a low water, carbon, and chemical footprint. Capturing natural variation in traits associated with water use efficiency (WUE) is the first step to developing trees that require less water and may be adapted to drier environments. We have assessed stomatal conductance (gs) and leaf carbon isotope composition (δ13C, an indirect indicator of leaf WUE) in two Populus species, P. deltoides and P. trichocarpa and their F2 progeny, grown in the United Kingdom and in Italy. Populus deltoides leaves showed lower δ13C than P. trichocarpa, suggesting a higher WUE in P. trichocarpa, although without drought preconditioning, gs of P. trichocarpa was less responsive to dehydration and abscisic acid treatment than P. deltoides, suggesting that leaf anatomy may also contribute to δ13C in Populus. Quantitative trait loci (QTL) were identified for δ13C on eight linkage groups (LG) and two QTL for gs. From these. QTL and differential gene expression in response to drought from microarray data, we focused on three hotspots and identified 23 novel candidate genes on LG VI, X, and XVI. We have begun to unravel the genetic basis of WUE in bioenergy Populus revealing important underpinning data for breeding and improvement in poplar genotypes for a future drier climate. [ABSTRACT FROM AUTHOR]

Heat-stressed crops suffer dehydration, depressed growth, and a consequent decline in water productivity, which is the yield of harvestable product as a function of lifetime water consumption and is a trait associated with plant growth and development. Heat shock transcription factor (HSF) genes have been implicated not only in thermotolerance but also in plant growth and development, and therefore could influence water productivity. Here it is demonstrated that Arabidopsis thaliana plants with increased HSFA1b expression showed increased water productivity and harvest index under water-replete and water-limiting conditions. In non-stressed HSFA1b-overexpressing (HSFA1bOx) plants, 509 genes showed altered expression, and these genes were not over-represented for development-associated genes but were for response to biotic stress. This confirmed an additional role for HSFA1b in maintaining basal disease resistance, which was stress hormone independent but involved H2O2 signalling. Fifty-five of the 509 genes harbour a variant of the heat shock element (HSE) in their promoters, here named HSE1b. Chromatin immunoprecipitation-PCR confirmed binding of HSFA1b to HSE1b in vivo, including in seven transcription factor genes. One of these is MULTIPROTEIN BRIDGING FACTOR1c (MBF1c). Plants overexpressing MBF1c showed enhanced basal resistance but not water productivity, thus partially phenocopying HSFA1bOx plants. A comparison of genes responsive to HSFA1b and MBF1c overexpression revealed a common group, none of which harbours a HSE1b motif. From this example, it is suggested that HSFA1b directly regulates 55 HSE1b-containing genes, which control the remaining 454 genes, collectively accounting for the stress defence and developmental phenotypes of HSFA1bOx. [ABSTRACT FROM AUTHOR]

Carbon dioxide flux measurements using the eddy covariance (EC) methodology have helped researchers to develop models of ecosystem carbon balance. However, making reliable predictions of carbon fluxes is not straightforward due to phenological changes and possible abiotic/biotic stresses that profoundly influence tree functioning., To assess the influence of canopy phenological state on CO2 flux, we installed two different digital camera systems at different viewing angles (an outdoor webcam with a near-horizontal view and a commercial 'fisheye' digital camera with a downward view) on a flux measurement tower in southern England and tracked the visual change of the canopy in this oak-dominated ( Quercus robur L.) forest over two growing seasons., Changes in the setting of the camera's white balance substantially affected the quality of the webcam images. However, the timing of the onset of greening and senescence was, nevertheless, detectable for the individual trees as well as the overall canopy for both years. The greening-up date assessed from the downward images from a hemispherical lens was ∼5 days earlier than from the horizontal-view images, because of ground vegetation development (not visible in the horizontal view)., The effects of a late air frost in 2010 were evident in the canopy greenness, and these led to reductions in daily gross primary productivity ( GPP). The cameras recorded differences between individual tree crowns, showing their different responses to the late frost., A major new finding from this work is the strong relationship between GPP and Hue, which was stronger than the relationship between GPP and NDVI. [ABSTRACT FROM AUTHOR]

Woody biomass produced from short rotation coppice ( SRC) poplar ( Populus spp.) and willow ( Salix spp.) is a bioenergy feedstock that can be grown widely across temperate landscapes and its use is likely to increase in future. Process-based models are therefore required to predict current and future yield potential that are spatially resolved and can consider new genotypes and climates that will influence future yield. The development of a process-based model for SRC poplar and willow, ForestGrowth- SRC, is described and the ability of the model to predict SRC yield and water use efficiency ( WUE) was evaluated. ForestGrowth- SRC was parameterized from a process-based model, ForestGrowth for high forest. The new model predicted annual above ground yield well for poplar ( r2 = 0.91, RMSE = 1.46 ODT ha−1 yr−1) and willow ( r2 = 0.85, RMSE = 1.53 ODT ha−1 yr−1), when compared with measured data from seven sites in contrasting climatic zones across the United Kingdom. Average modelled yields for poplar and willow were 10.3 and 9.0 ODT ha−1 yr−1, respectively, and interestingly, the model predicted a higher WUE for poplar than for willow: 9.5 and 5.5 g kg−1 respectively. Using regional mapped climate and soil inputs, modelled and measured yields for willow compared well ( r2 = 0.58, RMSE = 1.27 ODT ha−1 yr−1), providing the first UK map of SRC yield, from a process-based model. We suggest that the model can be used for predicting current and future SRC yields at a regional scale, highlighting important species and genotype choices with respect to water use efficiency and yield potential. [ABSTRACT FROM AUTHOR]

Transgenic antisense tobacco plants with a range of reductions in sedoheptulose-1,7-bisphosphatase (SBPase) activity were used to investigate the role of photosynthesis in stomatal opening responses. High resolution chlorophyll a fluorescence imaging showed that the quantum efficiency of photosystem II electron transport (Fq′/Fm′) was decreased similarly in both guard and mesophyll cells of the SBPase antisense plants compared to the wild-type plants. This demonstrated for the first time that photosynthetic operating efficiency in the guard cells responds to changes in the regeneration capacity of the Calvin cycle. The rate of stomatal opening in response to a 30 min, 10-fold step increase in red photon flux density in the leaves from the SBPase antisense plants was significantly greater than wild-type plants. Final stomatal conductance under red and mixed blue/red irradiance was greater in the antisense plants than in the wild-type control plants despite lower CO2 assimilation rates and higher internal CO2 concentrations. Increasing CO2 concentration resulted in a similar stomatal closing response in wild-type and antisense plants when measured in red light. However, in the antisense plants with small reductions in SBPase activity greater stomatal conductances were observed at all Ci levels. Together, these data suggest that the primary light-induced opening or CO2-dependent closing response of stomata is not dependent upon guard or mesophyll cell photosynthetic capacity, but that photosynthetic electron transport, or its end-products, regulate the control of stomatal responses to light and CO2. [ABSTRACT FROM PUBLISHER]

• Clear-felling an upland Sitka spruce resulted in a strong annual CO 2 emission. • The magnitude of the CO 2 source was similar to the sink of a mature stand. • Two years after replanting the clear-fell was a weak source. • Drought in 2018 reduced net CO 2 uptake of the mature stand by 30%. • Drought turned the clear-fell into a CO 2 source due to increase in respiration. Sitka spruce is the major conifer species in British upland forests and is predominantly managed as even-aged, single-species plantations with rotation lengths of less than 50 years using a "patch clear-felling" system. Evidence on the impact of clear-felling on the carbon, water and energy balances of plantation forestry is sparse and extreme weather events, such as the 2018 drought that occurred across Europe, are expected to have considerable impact. We used energy balance and gas flux data from a typical mature stand and a clear-felled site on peaty-gley soil at Harwood Forest in Northumberland, UK, over 4 years to quantify the impact of clear-felling and investigate how the drought of 2018 affected these fluxes. Felling resulted in an annual CO 2 source (7.05 tC ha − 1 yr − 1 ) of similar magnitude in absolute terms to the mature stand sink (−7.09 tC ha − 1 yr − 1 ). The source was reduced with the growth of ground vegetation, and after tree replanting two years post-felling, the site was a weak source. Felling increased reflected radiation by 92%, reduced evapotranspiration by 28% and therefore increased site wetness by 8%, while replanting and the spread of ground vegetation increased evapotranspiration by 16% on average. The warm, dry spring and summer in 2018 increased evapotranspiration and reduced net carbon uptake of the mature stand by 30% compared to the mean of the previous three years, because of a small decrease in photosynthetic uptake and a small increase in ecosystem respiration. The drought conditions also increased ecosystem losses of CO 2 at the clear-felled site by 28%. The clear-fell site would have turned into a small CO 2 sink in the second year after replanting, if there had not been a drought. [ABSTRACT FROM AUTHOR]

Abstract: Temperature, salinity, and chemical measurements, including the nutrients silicic acid (Si), nitrate (NO3), nitrite (NO2), ammonium (NH4), and phosphate (PO4 or P), the oxygen isotopic composition of seawater (δ 18O), and barium (Ba) concentrations were obtained from the central Arctic Ocean along transects radiating from the North Pole in early spring, 2000–2006. Stations that were reoccupied over this time period were grouped into five regions: from Ellesmere Island, (1) north along 70°W and (2) northwest along 90°W; near the North Pole, (3) on the Amundsen Basin flank and (4) directly over the Lomonosov Ridge; (5) through the Makarov Basin along 170–180°W. These regions had been shown by others to have undergone marked changes in water-mass assemblies in the early 1990s, but our time series tracer hydrographic data indicate a partial return of Pacific origin water within the mixed layer and the upper halocline layers beginning in 2003–2004. Back-trajectories derived from satellite-tracked ice buoys for these stations indicate that the upper levels of Pacific water in the central Arctic in 2004–2006 transited westward from the Bering Strait along the Siberian continental slope into the East Siberian Sea before entering the Transpolar Drift Stream (TPD). By 2004, the TPD shifted back from an alignment over the Alpha-Mendeleev Ridge toward the Lomonosov Ridge, as was characteristic prior to the early 1990s. At most stations occupied in 2006, a decrease in the Pacific influence was observed, both in the mixed layer and in the upper halocline, which suggests the Canadian branch of the TPD was shifting back toward North America. Clearly the system is more variable than has been previously appreciated. [Copyright &y& Elsevier]

Abstract: Dissolved oxygen (O2) profiling by new generation sensors was conducted in the Arctic Ocean via aircraft during May 2003 as part of the North Pole Environmental Observatory (NPEO) and Freshwater Switchyard (SWYD) projects. At stations extending from the North Pole to the shelf off Ellesmere Island, such profiles display what appear to be various O2 maxima (with concentrations 70% of saturation or less) over depths of 70–110m in the halocline, corresponding to salinity and temperature ranges of 33.3–33.9 and −1.7 to −1.5°C. The features appear to be widely distributed: Similar features based on bottle data were recently reported for a subset of the 1997–1998 SHEBA stations in the southern Canada Basin and in recent Beaufort Sea sensor profiles. Oxygen sensor data from August 2002 Chukchi Borderlands (CBL) and 1994 Arctic Ocean Section (AOS) projects suggest that such features arise from interleaving of shelf-derived, O2-depleted waters. This generates apparent oxygen maxima in Arctic Basin profiles that would otherwise trend more smoothly from near-saturation at the surface to lower concentrations at depth. For example, in the Eurasian Basin, relatively low O2 concentrations are observed at salinities of about 34.2 and 34.7. The less saline variant is identified as part of the lower halocline, a layer originally identified by a Eurasian Basin minimum in “NO,” which, in the Canadian Basin, is reinforced by additional inputs. The more saline and thus denser variant appears to arise from transformations of Atlantic source waters over the Barents and/or Kara shelves. Additional low-oxygen waters are generated in the vicinity of the Chukchi Borderlands, from Pacific shelf water outflows that interleave with Eurasian waters that flow over the Lomonosov Ridge into the Makarov Basin and then into the Canada Basin. One such input is associated with the well-known silicate maximum that historically has been associated with a salinity of ≈33.1. Above that (), there is a layer moderately elevated in temperature (summer Bering Sea water) that we show is also O2-depleted. We propose that these low O2 waters influence the NPEO and SWYD profiles to varying extents in a manner reflective of the large-scale circulation. The patterns of halocline circulation we infer from the intrusive features defy a simple boundary-following cyclonic flow. These results demonstrate the value of the improved resolution made feasible with continuous O2 profiling. In the drive to better understand variability and change in the Arctic Ocean, deployment of appropriately calibrated CTD-O2 packages offers the promise of important new insights into circulation and ecosystem function. [Copyright &y& Elsevier]

Although certain regions of the Arctic have experienced periods of decadal warming during the last 100 years, recent decades show an ongoing suite of Arctic-wide, interrelated atmospheric, oceanic, terrestrial, and human dimension changes. A major workshop conclusion stresses the necessity for coordination between arctic disciplines to form a continuing and enhanced dataset of arctic change. A major workshop recommendation is to increase the use of past and evolving datasets for understanding arctic change through, for example, an arctic change protocol, an aaaarctic system reanalysis, timely intercalibration of satellite products, and multidisciplinary, multiregional analyses. Understanding of the Arctic can improve change detection due to the roles of vegetation type, sea ice, and other feedbacks in providing a multiyear memory for the climate system. [ABSTRACT FROM AUTHOR]

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collectocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice-albedo feedback and cloud-radiation feedback. This information is being used to improve formulations of arctic ice-ocean-atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice packand allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goals, experimental design, instrumentation, and the resulting datasets. Examples of various data products available from the SHEBA project are presented. [ABSTRACT FROM AUTHOR]