The goal of the Boreal Ecosystem‐Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.
Abstract We study the interactions between plant evapotranspiration, controlled by photosynthesis (C3 and C4 grasses), and moist thermals responsible for the formation of shallow cumulus clouds (SCu). Our findings are based on a series of systematic numerical experiments at fine spatial and temporal scales using large eddy simulations explicitly coupled to a plant‐physiology model. The shading provided by SCu leads to strong spatial variability in photosynthesis and the surface energy balance. This in turn results in SCu characterized by less extreme and less skewed values of liquid water path. The larger water use efficiency of C4 grass leads to two opposite effects that influence boundary layer clouds: more vigorous and deeper thermals due to the larger buoyancy surface flux (positive effect) characterized by less moisture content (negative). We find that under these midlatitude and well‐watered soil conditions, SCu are characterized by a larger cloud cover and liquid water path over C4 grass fields.
Few scientists acquainted with the chemistry of biological systems at the molecular level can avoid being inspired. Evolution has produced chemical compounds exquisitely organized to accomplish the most complicated and delicate of tasks.Donald J. Cram, Nobel Prize Lecture, 1987
Summary This paper summarizes and analyses available data on the surface energy balance of Arctic tundra and boreal forest. The complex interactions between ecosystems and their surface energy balance are also examined, including climatically induced shifts in ecosystem type that might amplify or reduce the effects of potential climatic change. High latitudes are characterized by large annual changes in solar input. Albedo decreases strongly from winter, when the surface is snow‐covered, to summer, especially in nonforested regions such as Arctic tundra and boreal wetlands. Evapotranspiration ( Q E ) of high‐latitude ecosystems is less than from a freely evaporating surface and decreases late in the season, when soil moisture declines, indicating stomatal control over Q E , particularly in evergreen forests. Evergreen conifer forests have a canopy conductance half that of deciduous forests and consequently lower Q E and higher sensible heat flux ( Q H ). There is a broad overlap in energy partitioning between Arctic and boreal ecosystems, although Arctic ecosystems and light taiga generally have higher ground heat flux because there is less leaf and stem area to shade the ground surface, and the thermal gradient from the surface to permafrost is steeper. Permafrost creates a strong heat sink in summer that reduces surface temperature and therefore heat flux to the atmosphere. Loss of permafrost would therefore amplify climatic warming. If warming caused an increase in productivity and leaf area, or fire caused a shift from evergreen to deciduous forest, this would increase Q E and reduce Q H . Potential future shifts in vegetation would have varying climate feedbacks, with largest effects caused by shifts from boreal conifer to shrubland or deciduous forest (or vice versa) and from Arctic coastal to wet tundra. An increase of logging activity in the boreal forests appears to reduce Q E by roughly 50% with little change in Q H , while the ground heat flux is strongly enhanced.
Volcanic aerosols from the 1991 Mount Pinatubo eruption greatly increased diffuse radiation worldwide for the following 2 years. We estimated that this increase in diffuse radiation alone enhanced noontime photosynthesis of a deciduous forest by 23% in 1992 and 8% in 1993 under cloudless conditions. This finding indicates that the aerosol-induced increase in diffuse radiation by the volcano enhanced the terrestrial carbon sink and contributed to the temporary decline in the growth rate of atmospheric carbon dioxide after the eruption.
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Read moreSummary The boreal forest, one of the world's larger biomes, is distinct from other biomes because it experiences a short growing season and extremely cold winter temperatures. Despite its size and impact on the earth's climate system, measurements of mass and energy exchange have been rare until the past five years. This paper overviews results of recent and comprehensive field studies conducted in Canada, Siberia and Scandinavia on energy exchanges between boreal forests and the atmosphere. How the boreal biosphere and atmosphere interact to affect the interception of solar energy and how solar energy is used to evaporate water and heat the air and soil is examined in detail. Specifically, we analyse the magnitudes, temporal and spatial patterns and controls of solar energy, moisture and sensible heat fluxes across the land–atmosphere interface. We interpret and synthesize field data with the aid of a soil–vegetation–atmosphere transfer model, which considers the coupling of the energy and carbon fluxes and nutrient status. Low precipitation and low temperatures limit growth of many boreal forests. These factors restrict photosynthetic capacity and lower root hydraulic conductivity and stomatal conductance of the inhabitant forests. In such circumstances, these factors interact to form a canopy that has a low leaf area index and exerts a significant resistance to evaporation. Conifer forests, growing on upland soils, for example, evaporate at rates between 25 and 75% of equilibrium evaporation and lose less than 2.5 mm day −1 of water. The open nature of many boreal conifer forest stands causes a disproportionate amount of energy exchange to occur at the soil surface. The climatic and physiological factors that yield relatively low rates of evaporation over conifer stands also cause high rates of sensible heat exchange and the diurnal development of deep planetary boundary layers. In contrast, evaporation from broad‐leaved aspen stands and fen/wetlands approach equilibrium evaporation rates and lose up to 6 mm day −1 .
Read moreLDKT knowledge, perceived social support, and patient activation are associated with the socioeconomic position of people with kidney disease, and mediate approximately 50% of the association between the socioeconomic position and receipt of an LDKT. Interventions that target these factors may redress observed socioeconomic inequity.
Read moreThe Kyoto Protocol achieved a significant breakthrough by including terrestrial carbon sources and sinks into a legally binding emissions reduction framework. The effectiveness of the portocol can be improved by adopting a full carbon budget. Terrestrial carbon sinks are part of an active biological cycle and can offset fossil fuel emissions only temporarily, from decades to a century. They can thus buy time to address anthropogenic perturbation emissions.
Read moreA modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope ( 13 C/ 12 C and 18 O/ 16 O) ratios of updraft and downdraft air and thus the net fluxes of 13 C 16 O 2 and 12 C 18 O 16 O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO 2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO 2 mixing ratio ([CO 2 ]) fluctuations or those of another scalar are also used. Simulations using 10‐Hz data show that a wind‐based/scalar‐based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft [CO 2 ] differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down [CO 2 ] differences with HREA of 8–10 ppm are possible, compared with 3–5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.
Read moreAbstract Restored wetlands are a complex mosaic of open water and new and old emergent vegetation patches, where multiple environmental and biological drivers contribute to the measured heterogeneity in methane (CH 4 ) flux. In this analysis, we replicated the measurements of CH 4 flux using the eddy covariance technique at three tower locations within the same wetland site to parse the spatiotemporal variability in CH 4 flux contributed by large‐scale seasonal variations in climate and phenology and short‐term variations in flux footprint movement over a mosaic of vegetation and open water. Using a hierarchical statistical model accounting for site‐level environmental effects, tower‐level footprint and biological effects, and temporal autocorrelation, we partitioned the key drivers of the daily CH 4 flux variability among the three replicated towers. The daily mean air temperature and mean friction velocity, a measure of momentum transfer, explained a significant variability in CH 4 flux across the three towers, and the abundance and spatial aggregation of vegetation in the flux footprint along with the daily gross primary productivity explained much of the tower‐level variability. This statistical model captured 67% of the total variance in the daily integrated growing season CH 4 fluxes at this site, which bridged an order of magnitude from 80 to 480 mg C m −2 d −1 during the measurement period from 10 May 2012 to 24 October 2012.
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