Use of a greenhouse as an open chamber for canopy gas exchange measurements: Methodology and validation

Measurements of whole-canopy gas exchange - of CO₂ and H₂O - are important for agricultural and ecological reasons. The objective of this study was to investigate the use of a full-size greenhouse as an open-chamber system for measuring canopy-scale gas exchange. Measurements were validated by comparison with gas exchange scaled up from leaf- and plant-level measurements. Leaf-level measurements used a conventional hand-held cuvette gas exchange system at many points in the greenhouse. The experiments were done in a greenhouse with an area of (15 × 24) m² in which a pepper crop was grown. Within the canopy photosynthetic activity and transpiration changed with height, as expected. In addition, it was shown both theoretically and experimentally that in the absence of air mixing within the chamber, gradients of CO₂ and H₂O developed along the airflow direction. Theoretical estimates of the gradients were in good agreement with measured values. In spite of the gradients, canopy photosynthesis and transpiration could be estimated relatively accurately. For instance, the values of canopy photosynthesis and transpiration, during the course of the day, as measured with the open-chamber approach, were in good agreement with mean values obtained from measurements on individual leaves. However, transpiration values obtained both from open-chamber measurements and from individual leaves were generally a little lower than those obtained with lysimeters.     

Spatial modelling of the fraction of photosynthetically active radiation absorbed by a boreal mixedwood forest using a lidar–hyperspectral approach

The spatial variability of the fraction of photosynthetically active radiation absorbed by the canopy (fPAR) was characterized for a heterogeneous boreal mixedwood forest site located in northern Ontario, Canada, based on relationships found between fPAR and light detection and ranging (lidar) data over different canopy architectures. Estimates of fPAR were derived from radiation measurements made above the canopy at a flux tower and below-canopy radiation was measured across a range of species compositions and canopy architectures. Airborne lidar data were used to characterize spatial variability of canopy structure around the flux tower and a map of mean canopy chlorophyll concentration was derived from airborne hyperspectral imagery. Different volumes of lidar points for the locations directly above each photosynthetically active radiation (PAR) sensor were examined to determine if there is an optimal method of relating lidar returns to estimated fPAR values.The strongest correlations between mean lidar height and fPAR occurred when using points that fell within a theoretical cone which originated at the PAR sensor having a solid angle α = 55°. For diffuse conditions, the correlation (r) between mean lidar height versus fPAR × chlorophyll was stronger than between mean lidar height versus fPAR by 8% for mean daily fPAR and from 10 to 20% for diurnal fPAR, depending on solar zenith angle. For direct light conditions, the relationship was improved by 12% for mean daily fPAR and 12–41% for diurnal relationships.Linear regression models of mean daily fPAR × chlorophyll versus mean lidar height were used in conjunction with gridded lidar data and the canopy chlorophyll map to generate maps of mean daily fPAR for direct and diffuse sunlight conditions. Site average fPAR calculated from these maps was 0.79 for direct light conditions and 0.78 for diffuse conditions. When compared to point estimates of mean daily fPAR calculated on the tower, the average fPAR was significantly lower than the point estimate. Subtracting the direct sunlight fPAR map from the diffuse sunlight fPAR map revealed a distinct spatial pattern showing that areas with open canopies and relatively low chlorophyll (e.g., black spruce patches) have a higher fPAR under direct sunlight conditions, while closed canopies with higher chlorophyll (e.g., deciduous species) absorb more PAR under diffuse conditions. These findings have implications for scaling from point measurements at flux towers to larger resolution satellite imagery and addressing local scale heterogeneity in flux tower footprints.     

Evaluation of a radiosity based light model for greenhouse cucumber canopies

Light distribution is a key factor of developmental and growth processes, and strongly depends on the foliage distribution which is affected, e.g., by the arrangement of the plants in the canopy. The precise simulation of the light distribution on organ level is an essential component for dynamical plant models which incorporate structural and physiological adaptions of plants to their environment. Combinations of static 3D plant models with 3D light models are used for analyzing the complex light distribution on leaf level in silico, but detailed measurements for evaluation of simulation results are almost non-existent. This study addressed the evaluation of a model on a high level of detail using individual leaf based light measurements in canopies of cucumber (Cucumis sativus L.). We combined a static 3D plant model derived from digitized plants on an individual organ scale with a mock-up of the surrounding canopy and a 3D radiosity based light distribution model. Variations of plant density and spacing were analyzed to cover a range of canopy architectures. An exclusion of components of the light environment by applying a shading encasement followed by a successive uncovering allowed investigating the scene under increasing levels of complexity. The combined 3D plant-light distribution approach allowed determining the interaction of the light directions and the canopy architecture as well as differences in the accuracy of the simulations. Depending on canopy architecture and shading treatment, the light distributions covered a range from exponentially shaped vertical gradients in encased treatments to nearly flat light profiles in nonencased conditions. In conclusion, simulations of leaf level PAR based on combinations of detailed 3D surfaced-based plant and light distribution models are suitable to derive light-induced physiological responses on organ level.     

FACE facility CO2 concentration control and CO2 use in 1990 and 1991

CO₂ treatment level control and CO₂ use are reported for free-air carbon dioxide enrichment (FACE) facility operations at the University of Arizona's Maricopa Agricultural Center in 1990 and 1991. These are required for evaluation of the validity of biological experiments conducted in four replicates of paired experimental and control plots in a large cotton field and the cost-effectiveness of the plant fumigation facility. Gas concentration was controlled to 550 γmol mol^-1 at the center of each experimental plot, just above the canopy. In both years, season-long (April–September) average CO₂ levels during treatment hours (05:00–19:00 h Mountain Standard Time) were 550 γmol mol⁻¹ measured at treatment plot centers when the facility was operating. Including downtime, the season average was 548 γmol mol⁻¹ in 1991. In 1990, the season averages for the four elevated CO₂ treatments varied from 522 to 544 γmol mol⁻¹, owing to extended periods of downtime after lightning damage. Ambient CO₂ concentration during treatment was 370 γmol mol⁻¹. Instantaneous measurements of CO₂ concentration were within 10% of the target concentration of 550 γmol mol⁻¹ more than 65% of the time when the facility was operating, and 1 min averages were within 10% of the target concentration for 90% of the time. The long-term average of CO₂ concentration measured over the 20 m diameter experimental area of one array at the height of the canopy was in the range 550–580 γmol mol⁻¹ during July 1991, with the higher values near the edges. In 1991, CO₂ demand averaged 1250 kg per array per 14 h treatment day, or 4 kg m⁻² of fumigated plant canopy. The FACE facility provided good temporal and spatial control of CO₂ concentration and was a cost-effective method for large-scale field evaluations of the biological effects of CO₂.     

Two-dimensional simulation of airflow and carbon dioxide transport over a forested mountain: Part II. Carbon dioxide budget analysis and advection effects

We apply a high-resolution atmospheric model to assess the influence of mesoscale advection of CO₂ on the estimation of net ecosystem exchange (NEE) using eddy-covariance CO₂ flux measurements at a Fluxnet-Canada forest site located on sloping terrain on Vancouver Island, Canada. The numerical simulation is performed for fair-weather conditions over an idealized two-dimensional mountain bounded by water. The model is enhanced to include a CO₂ budget with a treatment of canopy photosynthesis and soil respiration.The simulation captures the transport of CO₂ by nocturnal drainage flows and weak land breezes. The resulting vertical profiles and time evolution of CO₂ concentration show a significant variation near the ground, associated with stability changes in the atmospheric boundary layer. The simulated vertical CO₂ gradients are found to be large around sunset and sunrise. The decrease of CO₂ concentration over land after midnight and the CO₂ accumulation over the neighboring water surface indicate CO₂ advection.A CO₂ budget analysis of the numerical-model output shows that the mean horizontal and vertical advection have significant fluctuations and opposite signs during daytime, with the net result that they largely counteract each other. At night, mean advection results in the underestimation by 20% of the nocturnal respiration. The estimated NEE at night is dominated by sub-grid-scale vertical flux in this simulation. Further evaluation using 3D simulations with higher resolution is needed to see if our results hold where vertical fluxes are much better resolved.     

Performance characteristics of an area distributed free air carbon dioxide enrichment (FACE) system

Free air carbon dioxide enrichment (FACE) systems rely primarily on wind to distribute CO₂ across treatment plots. Most current FACE designs inject CO₂ at the perimeter of the plot, and low wind speeds can result in large horizontal gradients of CO₂ concentration. These gradients can be reduced by using blowers to inject CO₂ or a mixture of CO₂ and air into the plot, but this process can alter the microclimate enough to affect plant growth. This report describes a new FACE system in which CO₂ emitters are evenly distributed across a plot in a square array to provide an area-source of CO₂. A variable mixture of CO₂ and air is supplied continuously to each emitter at a constant low total volume flow. Tests in 13 m² plots of grassland and maize using set points of 1.4 times the ambient concentration showed good horizontal uniformity of CO₂ concentration except for a small radius around each emitter. Temporal variation in CO₂ concentration in the daytime at canopy height at the center of the plot was similar to that of other FACE designs, with 1 min average CO₂ concentrations within 10% of the target 86% of the time. At night, the CO₂ concentration achieved was more variable, but 1 min averages of CO₂ concentration were within 20% of the target 83% of the time. The frequency distribution of instantaneous measurements made with an open path CO₂ analyzer indicated a distribution strongly skewed toward low concentrations compared with the 1 min averages, which literature suggests may be a general characteristic of FACE systems. Canopy air temperatures on nights with low wind differed by less than 0.1 °C in plots with and without flow to the emitters, indicating minimal disturbance of the canopy microclimate. A square array FACE system could be scaled to any horizontal dimension without compromising the horizontal uniformity of CO₂ concentration, and could be useful in large-scale screening of germplasm for CO₂ responsiveness under field conditions.     

Soil carbon dioxide flux and rice photosynthesis

Measurement of CO₂ concentration in air at 25 em below and 100 em above the canopy of a good rice crop indicated that a severe CO₂ deficit occurred around the photosynthetic surface of crop when light intensity was high. Soil CO₂ flux as measured by the soda lime method in a closed system ranged from 3.9 to 5.7 g.m^-2. day^-1 under flooded conditions and from 6.0 to 8,6g.m^-2 .day^-1 under drained conditions. Cropped soil released more CO₂ than bare soil under both flooded and drained conditions. The estimated contribution of soil CO₂ to gross photosynthesis was 6%, for the flooded soil and 7% for the drained soil or a contribution of 9 and 12% to net dry matter production. These results together with other information indicate that atmospheric CO₂ is the most important source of CO₂, in crop photosynthesis, soil CO₂ released into atmosphere Is second most important, and soil CO₂ absorbed by plant roots is almost negligible.     

枣麦间作系统中冬小麦的冠层光分布特征及产量研究

针对近年来"林农生产争地争光"的问题,以株行距为3 m?4 m南北行向栽植的枣树||冬小麦间作系统为研究对象,大田条件下设置枣麦间作(JZ)和冬小麦单作对照(CK)两个处理,以2013—2014年生长季冬小麦光合生理参数和冠层光照强度为基础,以两棵枣树的定植点连成1条测定样线,在样线上以距枣树的东(E)、西(W)距离为基准,每50 cm设置1个测定点,设E50 cm、E100 cm、E150 cm、E200 cm(W200 cm)、W150 cm、W100 cm、W50 cm共7个测定位置,在不同调查时期测定各测定位置的冬小麦冠层光合有效辐射(PAR),并在冬小麦成熟期调查各测定位置上的产量。采用多项式回归和定区间积分等方法计算冬小麦分蘖期、拔节期、抽穗期、扬花期、灌浆期和成熟期冠层达到饱和PAR的时长与时空窗,探讨枣麦间作系统中枣树遮光对间作作物冬小麦冠层光照分布及产量的影响。结果显示,间作系统中冬小麦冠层光照强度及产量整体呈现出不同的时空分布特征,且相较于单作小麦系统均有一定程度的衰减。单作冬小麦冠层的饱和PAR时空窗比间作处理大56.1%,穗粒数、有效穗数、千粒重和产量分别比间作小麦高14.7%、15.9%、33.5%和53.0%。相对于单作对照,间作物冬小麦整个生育时期内在距枣树E50~E100 cm、E100~E150 cm、E150~E200 cm、W150~W200 cm、W100~W150 cm、W50~W100 cm处的冠层PAR时空窗损失严重,分别达92.5%、45.7%、7.0%、5.4%、10.9%、54.0%。冠层PAR时空窗损失导致冬小麦减产,在以上各处减产程度分别达46.2%、39.6%、26.3%、24.7%、32.4%和37.6%。故枣树遮阴程度的差异导致间作物冬小麦不同程度减产,且间作巷道内西侧光照质量整体优于东侧。这就要求在冬小麦扬花期后对枣树进行适当修剪,且适当增加巷道东侧枣树株距,以避免枣树新生枝徒长,提高冬小麦冠层光合有效辐射截获量,使间作系统获得更高产量。     

Aspen bark photosynthesis and its significance to remote sensing and carbon budget estimates in the boreal ecosystem

Aspen bark was investigated for photosynthetic function, pigment content, and spectral characteristics during the 1993–1994 Boreal Ecosystem-Atmosphere Study (BOREAS) summer field campaigns in the boreal zone of Saskatchewan, Canada. Parameters related to photosynthetic function were similar for bark and leaves: chlorophyll (Chl) concentration; fluorescence responses; and spectral reflectance. Similar increases along a vertical gradient from base to tree top were observed for incident photosynthetically active radiation (PAR), photosynthetic pigment content, photosynthetic capacity, and spectral reflectance variables. Since transmittance of aspen bark periderm was 20–30% in the blue, and 50–60% in the red Chl absorption bands, the PAR available to the photosynthetic cortical layer in the natural, canopy environment (<1000 μmol m^?2 s^?1) was sufficient to support positive net assimilation (<8–10 νmol CO₂ m^?2 s^?1) under ideal conditions (e.g., light, temperature, saturating CO₂), a rate approximately 30–50% that of leaves. However, the respiring tissues comprising the greater fraction of bark tissue bias the balance of CO₂ exchange in favour of respiration for the whole bark. Therefore, net photosynthesis under ambient conditions on the whole bark was, in general, negative. The total bark surface area was estimated to contain 17–40% of the whole tree Chl. The contribution of the bark surface area fraction of the full canopy (leaves plus bark) increased with age (<60 years), with a similar trend expected for bark in total tree (and stand) photosynthesis. A spectral reflectance variable, the red edge inflection point (REIP), was related to total bark Chl content (r²=0.74). A better predictive relationship (r²=0.82) for total bark Chl was observed using a spectral index calculated from the reflectance ratio of two narrow wavebands (R3/R2: R2 and R3 are between 0.715–0.726 μm and 0.734–0.747 μm, respectively), which may have greater utility in landscape remote sensing. The bark spectra for Chlcontaining bark should improve understanding of carbon balance in aspen forests, based on landscape-level radiative transfer simulations.     

Two-dimensional solar radiation interception model for hedgerow fruit trees

A two-dimensional, hourly or daily time step model was developed, which takes canopy characteristics and row orientation into account to simulate solar radiation interception in hedgerow orchards. In order to determine the spatial and temporal distribution of soil irradiance across the tree row, the canopy path length through which the radiation must travel to reach a certain point on the soil surface is calculated. The model assumes leaves to be uniformly distributed within an ellipsoid, and radiation penetrating the canopy is attenuated according to Beer’s law. Beam or direct radiation and diffuse radiation for the PAR (photosynthetically active radiation) and NIR (near-infrared radiation) wavebands are calculated separately, as they interact differently with the canopy. The attenuation of beam radiation by the canopy is strongly dependent on canopy dimensions and architecture, zenith and azimuth angle, as well as row orientation. Radiation can penetrate neighbouring rows, so two rows on either side of the simulated row are considered. Validation of the model was carried out for a wide range of conditions (crops, row orientation, canopy density, tree size and shape). Field measurements included solar radiation, soil irradiance at different distances from the tree row with tube solarimeters, leaf area density, as well as canopy size and row orientation. Model predictions of soil irradiance were excellent in orchards with symmetrical and elliptical canopies having a uniform leaf distribution. In orchards where the canopy was non-symmetric and/or had non-uniform leaf distribution, errors in predictions of solar radiation transmittance occurred. As a result of these discrepancies, the overall MAE was 40% of the average measured value of radiant transmittance over the whole day.     

The effect of turbulence on the light environment of alfalfa

The properties of sunflecks in alfalfa canopies and their relationship to turbulence were quantified from measurements of light fluctuations and winds. Winds were measured above the canopy at 10 Hz, while light was measured at rates as high as 27 Hz inside the canopy. Power spectral analyses were used to determine the importance of various time scales of light fluctuations. Under all wind conditions a prominent spectral peak ranging from 1 to 2 Hz was observed for photosynthetically active radiation (PAR) in the canopy at LAI of 1.7. As mean winds increased from 1 to 7 ms⁻¹, the peak broadened and large increases were observed in the contribution of higher frequencies up to 10 Hz. The slope of spectral densities of PAR at the higher frequencies exhibited a linear relationship with the friction velocity or intensity of turbulence above the canopy. This implies that certain properties of sunflecks may be evaluated from measured properties of turbulence.The most prominent peaks in the PAR power spectra are likely due to stalk waving, which is triggered by gusts or turbulent structures. The strong damping effects of plants on the resonant interactions, as well as leaf flutter, account for the broad, flat peak under windy conditions. The frequency of leaf flutter is considered to be the main factor governing the slope of the linear portion of the PAR spectrum.     

Measuring forest floor CO2 fluxes in a Douglas-fir forest

CO₂ exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO₂ fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m⁻² s⁻¹. Generally, fluxes ranged between 2 and 10 μmol C m⁻² s⁻¹ during the measurement period. CO₂ flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q₁₀ values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO₂ flux by about 25%. Soil moisture had very little effect on forest floor CO₂ flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m⁻² per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO₂ flux at the site and uncertainty in the eddy covariance respiration measurements.     

Decoupling of air flow above and in plant canopies and gravity waves affect micrometeorological estimates of net scalar exchange

Turbulence within open canopies is shown to undergo a dramatic change in character during the transition from convective to stable conditions. In convective conditions the flow within the canopy is coupled through turbulent exchange to the flow aloft. As the transition proceeds, the within- and above-canopy flows decouple and vertically coherent waves form within the canopy. The intensity of above-canopy turbulence is not a good indicator of flow decoupling. Within-canopy waves can lead to large random error in the measurement of the change of storage and the advection terms in the mass balance equation. More importantly, errors associated with sampling over incomplete wave cycles will inevitably be combined with true advective flux divergences at non-ideal sites. Quantitative estimates of likely errors on storage of heat and CO₂ within the canopy are presented.     

Carbon Dioxide in Arable Soil Profiles: A Comparison of Automated and Manual Measuring Systems

Carbon dioxide (CO₂) concentrations in arable soil profiles are influenced by autotrophic and heterotrophic respiration as well as soil physical properties that regulate gas transport. Although different methods have been used to assess dynamics of soil CO₂ concentrations, our understanding of the comparability of results obtained using different methods is limited. We therefore aimed to compare the dynamics in soil CO₂ concentrations obtained from an automated system (GMP343 sensors) to those from a manually operated measurement system (i.e., soil gas sampled using stainless steel needles and rods). In a winter wheat field in Denmark, soil CO₂ concentrations were measured from 29 November 2011 to 14 June 2012 at upslope and footslope positions of a short catena (25 m). Carbon dioxide was measured at 20- and 40-cm soil depths (i.e., within and below the nominal plow layer) using the two measurement systems. Within the measurement range for the GMP343 sensors (0–20,000 ppm), mean results from the two systems were similar within the plow layer at the upslope (P = 0.060) and footslope (P = 0.139) position, and also below the plow layer at the upslope position (P = 0.795). However, results from the two systems deviated for the soil from the footslope position below the plow layer (P = 0.001). These results were partly attributed to larger variation in soil parameters below than within the nominal plow layer. The data suggested that generally the application of either system may be adequate; however, differences may occur in response to soil spatial variability. A better coverage of spatial variability is more easily addressed using manually operated systems, whereas temporal variability can be covered using the automated system. Depending on the aim of the study, the two systems may be used in combination to enhance both spatial and temporal data coverage.     

N2O flux from plant-soil systems in polar deserts switch between sources and sinks under different light conditions

Production and consumption of greenhouse gases such as CO₂, CH₄ and N₂O are key factors driving climate change. While CO₂ sinks are commonly reported and the mechanisms relatively well understood, N₂O sinks have often been overlooked and the driving factors for these sinks are poorly understood. We examined CO₂, CH₄ and N₂O flux in three High Arctic polar deserts under both light (measured in transparent chambers) and dark (measured in opaque chambers) conditions. We further examined if differences in soil moisture, evapotranspiration, Photosynthetically Active Radiation (PAR), and/or plant communities were driving gas fluxes measured in transparent and opaque chambers at each of our sites. Nitrous oxide sinks were found at all of our sites suggesting that N₂O uptake can occur under extreme polar desert conditions, with relatively low soil moisture, soil temperature and limited soil N. Fluxes of CO₂ and N₂O switched from sources under dark conditions to sinks under light conditions, while CH₄ fluxes at our sites were not affected by light conditions. Neither evapotranspiration nor PAR were significantly correlated with CO₂ or N₂O flux, however, soil moisture was significantly correlated with both gas fluxes. The relationship between soil moisture and N₂O flux was different under light and dark conditions, suggesting that there are other factors, in addition to moisture, driving N₂O sinks. We found significant differences in N₂O and CO₂ flux between plant communities under both light and dark conditions and observed individual communities that shifted between sources and sinks depending on light conditions. Failure of many studies to include plant-mediated N₂O flux, as well as, N₂O soil sinks may account for the currently unbalanced global N₂O budget.     

Foliar exchange of mercury vapor: Evidence for a compensation point

Historical studies for crop and weed species documented elemental Hg vapor (Hg°) deposition to foliage, but they used Hg° concentrations that were orders of magnitude higher than levels now known to occur under background conditions, possibly creating artificially high gradients between the atmosphere and landscape surfaces. Measurements of Hg° exchange with white oak (Quercus alba L.), red maple (Acer rubrum L.), Norway spruce (Picea abies L.), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO₂, H₂O and Hg° exchange under controlled environmental conditions. When Hg° concentrations were held at 0.5 to 1.5 ng m^?3, red maple (Acer rubrum L.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.) foliage exhibited mean Hg° emissions of 5.5, 1.7, 2.7, and 5.3 ng m^?2 h^?1, respectively. At Hg° concentrations between 9 and 20 ng m^?3 little net exchange of Hg° was observed. However at concentrations between 50 and 70 ng m^?3 the Hg° was deposited to foliage at rates between 22 and 38 ng m^?2 h^?1. These data suggest that dry foliar surfaces in terrestrial forest landscapes may be a dynamic exchange surface that can function as a source or sink dependent on the magnitude of current Hg° concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg° deposition to seedling foliage in the 10–25 ng m^?3 range.     

Spatial variability of photosynthetically active radiation in European beech and Norway spruce

The vertical and horizontal variability of solar radiation within a mature European beech (Fagus sylvatica L.)-Norway spruce (Picea abies [L.] Karst) mixed stand in Southern Germany is investigated. A large dataset with more than one million spectral measurements of photon fluence rates at six vertical levels within the stand is analyzed with respect to tree species, meteorological sky conditions, and the influence of solar elevation angle on canopy penetration. Irradiance probability density functions of the photosynthetically active waveband are used to describe the three-dimensional radiation field. For a quantification of umbra, penumbra, and sunfleck frequencies, in-canopy fractions of photon fluence rates within the photosynthetically active waveband are investigated. Different phenological stages of beech and their effects on the in-canopy light climate are compared. The results show that during overcast conditions (OVC) fractions of photosynthetically active radiation (PAR) are higher at all canopy levels than during clear sky (CS) conditions due to their exclusively diffuse character. The lowest median PAR level of less than 1% of above-canopy PAR can be observed in the shade crown of beech and at ground level. More PAR can penetrate the canopy at a higher solar elevation under CS conditions. This effect is more pronounced for spruce than for beech due to the conical crown shape of the conifers that allows photons from higher angles to enter the gaps inbetween trees in contrast to the more homogeneously closed beech canopy. Solar elevation is not an important factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing species or different sky conditions. The frequency of sunflecks differs more by species and by the vertical position within the canopy than by sky condition.     

Hot spots,hot moments,and spatio-temporal controls on soil CO2 efflux in a water-limited ecosystem

Soil CO₂ efflux is the primary source of CO₂ emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO₂ efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO₂ efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO₂ efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO₂ efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO₂ efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO₂ efflux, and during the wet season the emergence of a hot spot of soil CO₂ efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO₂ efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO₂ efflux in water-limited ecosystems.     

Modeling soil respiration based on carbon, nitrogen, and root mass across diverse Great Lake forests

The variability in the net ecosystem exchange of carbon (NEE) is a major source of uncertainty in quantifying global carbon budget and atmospheric CO₂. Soil respiration, which is a large component of NEE, could be strongly influential to NEE variability. Vegetation type, landscape position, and site history can influence soil properties and therefore drive the microbial and root production of soil CO₂. This study measured soil respiration and soil chemical, biological and physical properties on various types of temperate forest stands in Northern Wisconsin (USA), which included ash elm, aspen, northern hardwood, red pine forest types, clear-cuts, and wetland edges. Soil respiration at each of the 19 locations was measured six times during 1 year from early June to mid-November. These data were combined with two additional data sets from the same landscape that represent two smaller spatial scales. Large spatial variation of soil respiration occurred within and among each forest type, which appeared to be from differences in soil moisture, root mass and the ratio of soil carbon to soil nitrogen (C:N). A soil climate driven model was developed that contained quadratic functions for root mass and the ratio of soil carbon to soil nitrogen. The data from the large range of forest types and site conditions indicated that the range of root mass and C:N on the landscape was also large, and that trends between C:N, root mass, and soil respiration were not linear as previously reported, but rather curvilinear. It should be noted this function appeared to level off and decline at C:N larger than 25, approximately the value where microbial nitrogen immobilization limits free soil nitrogen. Weak but significant relationships between soil water and soil C:N, and between soil C:N and root mass were observed indicating an interrelatedness of (1) topographically induced hydrologic patterns and soil chemistry, and (2) soil chemistry and root production. Future models of soil respiration should address multiple spatial and temporal factors as well as their co-dependence.