| 1. |
- Aubinet, M., et al.
(författare)
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Direct advection measurements do not help to solve the night-time CO2 closure problem: Evidence from three different forests
- 2010
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 150:5, s. 655-664
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Tidskriftsartikel (refereegranskat)abstract
- The ADVEX project involved conducting extensive advection measurements at three sites, each with a different topography. One goal of the project was to measure the [CO2] balance under night-time conditions, in an attempt to improve NEE estimates. Four towers were arranged in a square around a main tower, with the sides of the square about 100 m long. Equipped with 16 sonic anemometers and [CO2] sampling points, the towers were installed to measure vertical and horizontal advection of [CO2]. Vertical turbulent fluxes were measured by an eddy covariance system at the top of the main tower. The results showed that horizontal advection varied greatly from site to site and from one wind sector to another, the highest values being reached when there were large friction velocities and fairly unstable conditions. There was less variation in vertical advection, the highest values being reached when there were low friction velocities and stable conditions. The night-time NEE estimates deduced from the mass balance were found to be incompatible with biologically driven fluxes because (i) they varied strongly from one wind sector to another and this variation could not be explained in terms of a response of the biologic flux to climate, (ii) their order of magnitude was not realistic and (iii) they still showed a trend vs. friction velocity. From a critical analysis of the measurement and data treatment we concluded that the causes of the problem are related to the representativeness of the measurement (control volume size, sampling resolution) or the hypotheses underlying the derivation of the [CO2] mass balance (ignoring the horizontal turbulent flux divergence). This suggests that the improvement of eddy flux measurements by developing an advection completed [CO2] mass balance at night would be practically difficult. (C) 2010 Elsevier B.V. All rights reserved.
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| 2. |
- Drobyshev, Igor, et al.
(författare)
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Forest fire activity in Sweden: Climatic controls and geographical patterns in 20th century
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 154-155, s. 174-186
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Tidskriftsartikel (refereegranskat)abstract
- We used Swedish county-scale forest fire statistics to quantify modern fire activity, identify its main temporal and geographical patterns, and evaluate statistical performance of six fire-related weather indices as proxy of fire activity in Sweden over 1942–1975 and 1996–2008, the periods with available countyscale fire statistics. The analyzed indices were monthly precipitation, SPI, MDC, PDSI, calibrated PDSI, and DI, a Drought Index calculated as a ratio between actual and equilibrium evapotranspiration. The modern fire cycle (FC) in the northern part of Sweden varies between 2 × 103 and 3 × 104 years, whereas in southern Sweden the FC is somewhat shorter (103–2 × 104 years). No temporal trend in average FC was evident at the country scale between the two periods. Significant and negative values of a Mantel test, obtained on county data for both periods (r = −0.494, p = 0.001 for 1942–1975 and r = −0.281 and p = 0.015 for 1996–2008) indicated the presence of a geographical pattern in annual forest fire activity. Over 1942–1975, PCA revealed that the central and northern counties formed one group with synchronized fire activity, and the southern and south–western counties formed another group. This pattern became less evident during the more recent period (1996–2008). Over 1996–2008, the analysis showed little synchronicity in annual fire activity across different parts of the country. The geographical position of a county had a clear effect on seasonal pattern of forest fires. In southern Sweden, the peak in the number of fires and the burnt area was in April–May, during a relatively short dry period immediately following the snowmelt. In northern Sweden, fires in the second half of fire season dominated the total annual area burnt. Analyzed indices differed considerably in their predictive power in respect to counties’ records of annual area burnt. Calibrated PDSI was a superior proxy of fire activity for the southern region (R2 = 60.8% in regression against total annual area burnt for respective provinces), and DIlate (Drought Index for the first half of the growing season) was superior for the northern counties (R2 = 73.3%). Predictive power of the indices was much higher for the recent period (1996–2002), with R2 values staying within 81.2 and 97.8%. Even if modern levels of forest fire activity in Sweden are very low from historical perspective, there is a strong spatiotemporal association between fire activity and climatic variability at regional scales, which provides a basis for modeling of the future fire hazard.
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| 3. |
- Drobyshev, Igor, et al.
(författare)
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Reconstruction of a 253-year long mast record of European beech reveals its association with large scale temperature variability and no long-term trend in mast frequencies
- 2014
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 192, s. 9-17
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Tidskriftsartikel (refereegranskat)abstract
- Synchronous production of large seed crops, or mast years (MYs), is a common feature of many Fagus species, which is closely linked to the dynamics of forest ecosystems, including regeneration of canopy trees and changes in animal population densities. To better understand its climatic controls and check for the presence of long-term temporal trends in MY frequencies, we reconstructed MY record of the European beech (Fagus sylvatica L.) for the southern Swedish province of Halland over 1753-2006. We used superimposed epoch analysis (SEA) to relate MY (a) to summer temperature fields over the European subcontinent and (b) to the patterns of 500 mb geopotential heights over the 35-75 degrees N. For the MY reconstruction, we used newly developed regional beech ring-width chronology (1753-2006), an available summer temperature reconstruction, and a discontinuous historical MY record. A Monte Carlo experiment allowed identification of the thresholds in both growth and summer temperature anomalies, indicative of historical MYs, which were verified by dividing data into temporally independent calibration and verification sub-periods.MYs were strongly associated with both the 500 mb height anomalies and average summer temperatures during two years preceding a MY: a mast year (t) followed a cold summer two years (t-2) prior to the mast year and a warm summer one year prior (t-1) to the mast year. During t-2 years, the geographical pattern of 500 mb height anomalies exhibited a strong height depression in the region centered in the Northern Sea and extending toward eastern North America and statistically significant (p<0.05) temperature anomalies covering predominantly southern Scandinavia (area below 60 N) and British Isles. A year immediately preceding a mast year (t-1) was characterized by a strong regional high pressure anomaly centered in southern Scandinavia with significant temperature anomalies extended mostly over southern Scandinavia and Germany.The long-term mean MY return interval was 6.3 years, with 50 and 90% probabilities of MY occurrence corresponding to 6 and 15 years, respectively. Periods with intervals significantly shorter than the long-term mean were observed around 1820-1860 and 1990-2006 (means 3.9 and 3.2 years, respectively). However, the difference in return intervals between two sub-periods themselves was not significant.Geographically large and temporally rapid changes in atmospheric circulation among years, responsible for summer temperature conditions in the Northern Europe, are likely primary environmental drivers of masting phenomenon. However, decadal and centurial variability in MY intervals is difficult to relate directly to temperature variability, suggesting the presence of conditions "canceling" would-be MYs. Long-term MY reconstruction demonstrates high variability of reproductive behavior in European beech and indicates that a period with shorter MY intervals at the end of 20th may be not unique in a multi-century perspective. (C) 2014 Elsevier B.V. All rights reserved.
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| 4. |
- Feigenwinter, Christian, et al.
(författare)
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Spatiotemporal evolution of CO2 concentration, temperature, and wind field during stable nights at the Norunda forest site
- 2010
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 150:5, s. 692-701
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Tidskriftsartikel (refereegranskat)abstract
- Unusually high CO2 concentrations were frequently observed during stable nights in late summer 2006 at the CarboEurope-Integrated Project (CEIP) forest site in Norunda, Sweden. Mean CO2 concentrations in the layer below the height of the eddy-covariance measurement system at 30 m reached up to 500 mu mol mol(-1) and large vertical and horizontal gradients occurred, leading to very large advective fluxes with a high variability in size and direction. CO2 accumulation was found to build up in the second part of the night, when the stratification in the canopy sub-layer turned from stable to neutral. Largest vertical gradients of temperature and CO2 were shifted from close to the ground early in the night to the crown space of the forest late at night, decoupling the canopy sub-layer from the surface roughness layer. At the top of the canopy at 25 m CO2 concentrations up to 480 mu mol mol(-1) were observed at all four tower locations of the 3D cube setup and concentrations were still high (>400 mu mol mol(-1)) at the 100 m level of the Central tower. The vertical profiles of horizontal advective fluxes during the nights under investigation were similar and showed largest negative horizontal advection (equivalent to an additional CO2-sink) to occur in the crown space of the forest, and not, as usually expected, close to the ground. The magnitude of these fluxes was sometimes larger than 50 mu mol m(-2) s(-1)and they were caused by the large horizontal CO2 concentration gradients with maximum values of up to 1 mu mol mol(-1) m(-1). As a result of these high within canopy CO2 concentrations, the vertical advection also became large with frequent changes of direction according to the sign of the mean vertical wind component, which showed very small values scattering around zero. Inaccuracy of the sonic anemometer at such low wind velocities is the reason for uncertainty in vertical advection, whereas for horizontal advection, instrument errors were small compared to the fluxes. The advective fluxes during these nights were unusually high and it is not clear what they represent in relation to the biotic fluxes. Advection is most likely a scale overlapping process. With a control volume of about 100 m x 100 m x 30 m and the applied spatial resolution of the sensors, we obviously miss relevant information from processes in the mesoscale as well as in the turbulent scale. (C) 2009 Elsevier B.V. All rights reserved.
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| 5. |
- Grewling, L., et al.
(författare)
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Variation in Artemisia pollen seasons in Central and Eastern Europe
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 160, s. 48-59
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Tidskriftsartikel (refereegranskat)abstract
- This paper aims to address some gaps in current knowledge by studying temporal and spatial variations in Artemisia pollen counts (2000-2009) at 13 sites located in different biogeographical areas of Central and Eastern Europe. Analysis showed that start dates of Artemisia pollen seasons are greatly dependent on temperature during June and July, with hot summer temperatures having a tendency to delay summer flowering. However, this relationship is not linear and the rate at which seasons become later increases when mean minimum June-July temperatures reach a threshold of about 13 degrees C. No explanation for variations in pollen season intensity could be found. The geographical location or amount of urbanisation did not influence, either positively or negatively, the seasonal pollen index. Second peaks in Artemisia pollen seasons can be described as the pollen seasons of late flowering Artemisia species, and mainly occurred in the geographical area south of the Carpathian Mountains. These second peaks can significantly influence the seasonal pollen index, contributing over 50% to the season's total Artemisia pollen recorded at one site. (C) 2012 Elsevier B.V. All rights reserved.
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| 6. |
- Groenendijk, M., et al.
(författare)
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Assessing parameter variability in a photosynthesis model within and between plant functional types using global Fluxnet eddy covariance data
- 2011
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 151:1, s. 22-38
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Tidskriftsartikel (refereegranskat)abstract
- The vegetation component in climate models has advanced since the late 1960s from a uniform prescription of surface parameters to plant functional types (PFTs) PFTs are used in global land-surface models to provide parameter values for every model grid cell With a simple photosynthesis model we derive parameters for all site years within the Fluxnet eddy covariance data set We compare the model parameters within and between PFTs and statistically group the sites Fluxnet data is used to validate the photosynthesis model parameter variation within a PFT classification Our major result is that model parameters appear more variable than assumed in PFTs Simulated fluxes are of higher quality when model parameters of individual sites or site years are used A simplification with less variation in model parameters results in poorer simulations This indicates that a PFT classification Introduces uncertainty in the variation of the photosynthesis and transpiration fluxes Statistically derived groups of sites with comparable model parameters do not share common vegetation types or climates A simple PFT classification does not reflect the real photosynthesis and transpiration variation Although site year parameters give the best predictions the parameters are generally too specific to be used in a global study The site year parameters can be further used to explore the possibilities of alternative classification schemes (C) 2010 Elsevier B V All rights reserved
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| 7. |
- Hentschel, Rainer, et al.
(författare)
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Simulation of stand transpiration based on a xylem water flow model for individual trees
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 182, s. 31-42
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Tidskriftsartikel (refereegranskat)abstract
- Quantifying the water exchange between a forest stand and the atmosphere is of major interest for the prediction of future growth conditions and the planning of silvicultural treatments. In the present study, we address (i) the uncertainties of sap flow estimations at the tree level and (ii) the performance of the simulation of stand transpiration. Terrestrial laser scan images (TLS) of a mature beech stand (Fagus sylvatica L) in Southwestern Germany serve as input data for a representation of the aboveground tree architecture of the study stand. In the single-tree xylem water flow model (XWF) used here, 98 beech trees are represented by 3D graphs of connected cylinders with explicit orientation and size. Beech-specific hydraulic parameters and physical properties of individual trees determine the physiological response of the tree model to environmental conditions. The XWF simulations are performed without further calibration to sap flow measurements. The simulations reliably match up with sap flow estimates derived from sap flow density measurements. The density measurements strongly depend on individual sapwood area estimates and the characterization of radial sap flow density gradients with xylem depth. Although the observed pure beech stand is even-aged, we observe a high variability in sap flow rates among the individual trees. Simulations of the individual sap flow rates show a corresponding variability due to the distribution of the crown projection area in the canopy and the different proportions of sapwood area. Stand transpiration is obtained by taking the sum of 98 single-tree simulations and the corresponding sap flow estimations, which are then compared with the stand-level root water uptake model (RWU model) simulation. Using the RWU model results in a 35% higher simulation of seasonal stand transpiration relative to the XWF model. These findings demonstrate the importance of individual tree dimensions and stand heterogeneity assessments in estimating stand water use. As a consequence of species-specific model parameterization and precise TLS-based stand characterization, the XWF model is applicable to various sites and tree species and is a promising tool for predicting the possible water supply limitations of pure and mixed forest stands. (C) 2013 Elsevier B.V. All rights reserved.
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| 8. |
- Jönsson, Anna Maria, et al.
(författare)
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Guess the impact of Ips typographus – An ecosystem modelling approach for simulating bark beetle outbreaks
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 166-167, s. 188-200
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Tidskriftsartikel (refereegranskat)abstract
- Spruce bark beetle outbreaks are common in Norway spruce forests following windstorm damage, due to ample availability of brood material. The realization of an outbreak depends on factors regulating the Ips typographus population dynamics, such as weather conditions and salvage cutting. In this study, we take an ecosystem modelling approach to analyse the influence of multiple environmental factors on the risk for I. typographus outbreaks. Model calculations of I. typographus phenology and population dynamics as a function of weather and brood tree availability were developed and implemented in the LPJ-GUESS ecosystem modelling framework. The model simulations were driven by gridded climate data covering Sweden with a spatial resolution of 0.5° and a daily temporal resolution. Records on storm damage and I. typographus outbreak periods in Sweden for the period of 1960–2009 were used for model evaluation, and a sensitivity analysis was performed to examine the model behaviour. The model simulations replicated the observed pattern in outbreak frequency, being more common in southern and central Sweden than in northern Sweden. A warmer climate allowing for more than one generation per year can increase the risk for attacks on living trees. The effect of countermeasures, aiming at either reduce the availability of brood material or the I. typographus population size, is dependent on a non-linear relation between I. typographus attack density and reproductive success. The sensitivity analysis indicated a major reduction in the risk of attacks on living trees by timely salvage cutting and cutting of infested trees. Knowledge uncertainties associated with attacks on standing trees, i.e. factors influencing tree defence capacity and I. typographus reproductive success, should be further addressed.
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| 9. |
- Linder, Sune
(författare)
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Whole-tree chambers for elevated atmospheric CO2 experimentation and tree scale flux measurements in south-eastern Australia: The Hawkesbury Forest Experiment
- 2010
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 150, s. 941-951
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Tidskriftsartikel (refereegranskat)abstract
- Resolving ecophysiological processes in elevated atmospheric CO2 (C-a) at scales larger than single leaves poses significant challenges. Here, we describe a field-based experimental system designed to grow trees up to 9m tall in elevated C-a with the capacity to control air temperature and simultaneously measure whole-tree gas exchange. In western Sydney, Australia, we established the Hawkesbury Forest Experiment (HFE) where we built whole-tree chambers (WTC) to measure whole-tree CO2 and water fluxes of an evergreen broadleaf tree, Eucalyptus saligna. A single E. saligna tree was grown from seedling to small tree within each of 12 WTCs; six WTCs were maintained at ambient C-a and six WTCs were maintained at elevated C-a, targeted at ambient C-a +240 mu mol mol(-1). All 12 WTCs were controlled to track ambient outside air temperature (T-air) and air water vapour deficit (D-air). During the experimental period, T-air, D-air and C-a in the WTCs were within 0.5 degrees C, 0.3 kPa, and 15 mu mol mol(-1) of the set-points for 90% of the time, respectively. Diurnal responses of whole-tree CO2 and water vapour fluxes are analysed, demonstrating the ability of the tree chamber system to measure rapid environmental responses of these fluxes of entire trees. The light response of CO2 uptake for entire trees showed a clear diurnal hysteresis, attributed to stomatal closure at high Dair. Tree scale CO2 fluxes confirm the hypothesised deleterious effect of chilling night-time temperatures on whole-tree carbon gain in this subtropical Eucalyptus. The whole-tree chamber flux data add an invaluable scale to measurements in both ambient and elevated C-a and allow us to elucidate the mechanisms driving tree productivity responses to elevated C-a in interaction with water availability and temperature. (C) 2010 Elsevier B.V. All rights reserved.
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| 10. |
- Linderson, Maj-Lena, et al.
(författare)
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Up-scaling of water use efficiency from leaf to canopy as based on leaf gas exchange relationships and the modeled in-canopy light distribution
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 152, s. 201-211
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Tidskriftsartikel (refereegranskat)abstract
- Abstract in UndeterminedThe aim of this study was to evaluate the extent to which water use efficiency (WUE) at leaf scale can be used to assess WUE at canopy scale, leaf WUE being assumed to be a constant function of vapor pressure deficit and to thus not be dependent upon other environmental factors or varying leaf properties. Leaf WUE and its variability and dependencies were assessed using leaf gas-exchange measurements obtained during two growing seasons, 1999 and 2000, at the Soroe beech forest study site on Zealand in Denmark. It was found that the VPD-normalized leaf VVUE, WUEnormleaf, although dependent on incoming PAR below 500 p,mol m(-2) s(-1) is independent, both of the canopy levels and of variations in the environmental parameters. The average WUEnormleaf for PAR above 500 mu mol m(-2) s(-1) was found to be 5.5 mu mol CO2 (mmol H2O)(-1) hPa and, for the full range, 2.3 mu mol CO2 (mmol H2O)(-1) hPa. These results showed that WUE can be up-scaled from leaf to canopy on the basis of WUEnormleaf and the PAR distribution within the canopy. The up-scaling conducted was based on this WUEnormleaf - PAR relationship, the light distribution being assessed using the MAESTRA model, parameterized in accordance with measurements obtained for the Soroe forest. The up-scaled WUE was then compared with WUE as estimated from turbulent flux data measured above the forest with the eddy-covariance technique. The modeled daily canopy WUE obtained for daytime fluxes (6:00 AM-6:00 PM) was found to be in agreement with corresponding canopy WUE estimates based on the turbulent fluxes observed and to be dependent on VDP and light intensity alone, its thus being independent of other environmental factors. Accordingly, canopy WUE can be estimated on the basis of the up-scaled WUE relationships, provided incident PAR and VPD within the canopy are known.
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| 11. |
- Nicholas, Kimberly, et al.
(författare)
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Effect of vineyard-scale climate variability on Pinot noir phenolic composition
- 2011
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 151:12, s. 1556-1567
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of agricultural crops to climate change is a major area for climate impact studies. The relationship between climate and three key phenolic compounds in grape skins important to premium wine quality (anthocyanins, tannins, and total phenolics) has not been well-studied. Here we conducted a three-year field study to collect and analyze berry samples from Pinot noir vineyards in the Carneros and Sonoma Valley American Viticultural Areas of California's North Coast wine country, and correlate phenolic measurements with climate statistics derived from hourly temperature measures at each vineyard site. We used several statistical approaches to identify key phenologically-based periods influencing phenolic concentration at maturity, including classification and regression trees, factor screening, principal component analysis, and pairwise correlations. The results from these statistical models showed that cool conditions following harvest the year before maturity, warm temperatures from budburst to bloom, and cool temperatures from bloom to veraison (the onset of ripening) were positively correlated with concentrations of all three classes of phenolics, although not all trends were statistically significant. Anthocyanins were positively and significantly correlated with temperatures between 16 and 22 degrees C from veraison to harvest. Tannins were significantly increased by warm nights preceding budburst and warm days from budburst to bloom. We measured relatively high levels of light interception (35% of incident photosynthetically active radiation), and we found that increased light interception was significantly correlated with lower levels of all three classes of phenolic compounds in this study. For the Pinot noir sites in this study, warm temperatures from budburst to bloom appear to increase phenolic concentrations, which is likely beneficial for wine quality. However, warmer periods during the preceding fall and summer during ripening appear to offset these effects. Given projections for greater summer warming in California with climate change, the overall impact of climate change on winegrowing is likely to be negative. (C) 2011 Elsevier B.V. All rights reserved.
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| 12. |
- Oren, Ram
(författare)
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Sensitivity of stand transpiration to wind velocity in a mixed broadleaved deciduous forest
- 2014
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 187, s. 62-71
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Tidskriftsartikel (refereegranskat)abstract
- Wind velocity (U) within and above forest canopies can alter the coupling between the vapor-saturated sub-stomatal airspace and the drier atmosphere aloft, thereby influencing transpiration rates. In practice, however, the actual increase in transpiration with increasing U depends on the aerodynamic resistance (R-A) to vapor transfer compared to canopy resistance to water vapor flux out of leaves (R-C, dominated by stomatal resistance, R-stom), and the rate at which R-A decreases with increasing U. We investigated the effect of U on transpiration at the canopy scale using filtered meteorological data and sap flux measurements gathered from six diverse species of a mature broadleaved deciduous forest. Only under high light conditions, stand transpiration (E-C) increased slightly (6.5%) with increasing U ranging from 0.7 to similar to 4.7 ms(-1). Under other conditions, sap flux density (J(s)) and E-C responded weakly or did not change with U. R-A, estimated from Monin-Obukhov similarity theory, decreased with increasing U, but this decline was offset by increasing R-C, estimated from a rearranged Penman-Monteith equation, due to a concurrent increase in vapor pressure deficit (D). The increase of R-C with D over the observed range of U was consistent with increased R-stom by similar to 40% based on hydraulic theory. Except for very rare half-hourly values, the proportion of R-A to total resistance (R-T) remained <15% over the observed range of conditions. These results suggest that in similar forests and conditions, the direct effect of U reducing R-A and thus increasing transpiration is negligible. However, the observed U-D relationship and its effect on R-stom must be considered when modeling canopy photosynthesis. (C) 2013 Elsevier B.V. All rights reserved.
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| 13. |
- Oren, Ram
(författare)
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Trenching reduces soil heterotrophic activity in a loblolly pine (Pinus taeda) forest exposed to elevated atmospheric [CO2] and N fertilization
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 165, s. 43-52
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Tidskriftsartikel (refereegranskat)abstract
- Forests return large quantities of C to the atmosphere through soil respiration (R-soil), which is often conceptually separated into autotrophic C respired by living roots (R-root) and heterotrophic decomposition (R-het) of soil organic matter (SOM). Live roots provide C sources for microbial metabolism via exudation, allocation to fungal associates, sloughed-off cells, and secretions such as mucilage production, suggesting a coupling between the activity of roots and heterotrophs. We addressed the strength of root effects on the activity of microbes and exo-enzymes by removing live-root-C inputs to areas of soil with a trenching experiment. We examined the extent to which trenching affected metrics of soil heterotrophic activity (proteolytic enzyme activity, microbial respiration, potential net N mineralization and nitrification, and exo-enzyme activities) in a forest exposed to elevated atmospheric [CO2] and N fertilization, and used automated measurements of R-soil in trenched and un-trenched plots to estimate R-root and R-het components. Trenching decreased many metrics of heterotrophic activity and increased net N mineralization and nitrification, suggesting that the removal of root-C inputs reduced R-het by exacerbating microbial C limitation and stimulating waste-N excretion. This trenching effect was muted by N fertilization alone but not when N fertilization was combined with elevated CO2, consistent with known patterns of below-ground C allocation at this site. Live-root-C inputs to soils and heterotrophic activity are tightly coupled, so root severing techniques like trenching are not likely to achieve robust quantitative estimates of Rroot or R-het. (c) 2012 Elsevier B.V. All rights reserved.
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| 14. |
- Palmroth, Sari, et al.
(författare)
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Spatial and temporal variability of soil CO2 efflux in three proximate temperate forest ecosystems
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 171-172, s. 256-269
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Tidskriftsartikel (refereegranskat)abstract
- The magnitude of CO2 flux from soil (F-soil) varies with primary productivity and environmental drivers of respiration, soil temperature (T-soil) and moisture, all of which vary temporally and spatially. To quantify the sources of Fson variability, we first compared F-soil of three proximate forests within 30 km of one another, ranging in age, composition, soil, and environment and, thus, productivity. We collected data with automated soil respiration chambers during a 10-year period in a mid-rotation Pinus taeda plantation (PP), for three-years in a mature P. taeda stand (OP), and for five-years in a mature, mixed-species hardwood (HW) stand; PP and HW were on clay-loam soil and OP on a sandy soil. Among stands, Fsoil sensitivity to T-soil was lowest in OP and highest in PP, reflected in mean annual F-soil (+/- standard deviation) of 1033 +/- 226 (OP), 1206 +/- 99(11W), and 1383 +/- 152 (PP) g Cm-2; both F-soil sensitivity to T5011 and annual F-soil increased with leaf litterfall. For the second portion of our study, we established an additional three plots at PP for a six-year period to examine within-stand variability. Within PP, sensitivity of F-soil to T-soil was similar, yet higher leaf area was correlated with a combination of lower soil temperature and belowground carbon flux, resulting in lower F-soil. Temporally, diurnal to seasonal F-soil followed T-soil whereas annual values were driven by soil moisture. Spatially, among the three stands F-soil increased with leaf production, whereas within a stand (PP) F-soil decreased with increasing leaf production. (C) 2012 Elsevier B.V. All rights reserved.
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| 15. |
- Rosenqvist, Lars
(författare)
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Water balance in afforestation chronosequences of common oak and Norway spruce on former arable land in Denmark and southern Sweden
- 2010
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 150, s. 196-207
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation, throughfall and soil moisture were measured, and interception, transpiration and water recharge were estimated in four afforestation chronosequences on former arable land at two Danish locations (Vestskoven and Gejlvang) and at one southern Swedish location (Tonnersjoheden). Afforestation was performed using Norway spruce (Picea abies (Karst.) L) and common oak (Quercus robur L.) at Vestskoven and only Norway spruce at Gejlvang and Tonnersjoheden. Four to five stands of different ages (5-92 years) were studied in each of these chronosequences. Hydrological fluxes were calculated using the soil hydrological model SWAP. Throughfall flux and soil water content were used for calibration of the model. The simulated water recharge decreased with increased stand age within 30-40 years of afforestation. This was mainly due to increased interception evaporation with age. The annual water recharge was higher below oak stands (149-192 mm yr(-1)) than below spruce stands (107-191 mm yr(-1)) of similar age. The relative water recharge was also considerably higher from the sandy glaciofluvial soils at Gejlvang and Tonnersjoheden than from the sandy loamy till soils at Vestskoven. (C) 2009 Elsevier B.V. All rights reserved.
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| 16. |
- Sikoparija, B., et al.
(författare)
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A mechanism for long distance transport of Ambrosia pollen from the Pannonian Plain
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 180, s. 112-117
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Tidskriftsartikel (refereegranskat)abstract
- The pollen grains of ragweed are important aeroallergens that have the potential to be transported long distances through the air. The arrival of ragweed pollen in Nordic countries from the Pannonian Plain can occur when certain conditions are met, which this study aims to describe for the first time. Atmospheric ragweed pollen concentrations were collected at 16 pollen-monitoring sites. Other factors included in the analysis were the overall synoptic weather situation, surface wind speeds, wind direction and temperatures as well as examining regional scale orography and satellite observations. Hot and dry weather in source areas on the Pannonian Plain aid the release of ragweed pollen during the flowering season and result in the deep Planetary Boundary Layers needed to lift the pollen over the Carpathian Mountains to the north. Suitable synoptic conditions are also required for the pollen bearing air masses to move northward. These same conditions produce the jet-effect Kosava and orographic foehn winds that aid the release and dispersal of ragweed pollen and contribute towards its movement into Poland and beyond.
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| 17. |
- Stoy, Paul C., et al.
(författare)
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A data-driven analysis of energy balance closure across FLUXNET research sites: The role of landscape scale heterogeneity
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 1873-2240 .- 0168-1923. ; 171, s. 137-152
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Tidskriftsartikel (refereegranskat)abstract
- The energy balance at most surface-atmosphere flux research sites remains unclosed. The mechanisms underlying the discrepancy between measured energy inputs and outputs across the global FLUXNET tower network are still under debate. Recent reviews have identified exchange processes and turbulent motions at large spatial and temporal scales in heterogeneous landscapes as the primary cause of the lack of energy balance closure at some intensively-researched sites, while unmeasured storage terms cannot be ruled out as a dominant contributor to the lack of energy balance closure at many other sites. We analyzed energy balance closure across 173 ecosystems in the FLUXNET database and explored the relationship between energy balance closure and landscape heterogeneity using MODIS products and GLOBEstat elevation data. Energy balance closure per research site (C-EBS)averaged 0.84 +/- 0.20, with best average closures in evergreen broadleaf forests and savannas (0.91-0.94) and worst average closures in crops, deciduous broadleaf forests, mixed forests and wetlands (0.70-0.78). Half-hourly or hourly energy balance closure on a percent basis increased with friction velocity (u.) and was highest on average under near-neutral atmospheric conditions. C-EBS was significantly related to mean precipitation, gross primary productivity and landscape-level enhanced vegetation index (EVI) from MODIS, and the variability in elevation, MODIS plant functional type, and MODIS EVI. A linear model including landscape-level variability in both EVI and elevation, mean precipitation, and an interaction term between EVI variability and precipitation had the lowest Akaike's information criterion value. C-EBS in landscapes with uniform plant functional type approached 0.9 and C-EBS in landscapes with uniform EVI approached 1. These results suggest that landscape-level heterogeneity in vegetation and topography cannot be ignored as a contributor to incomplete energy balance closure at the flux network level, although net radiation measurements, biological energy assimilation, unmeasured storage terms, and the importance of good practice including site selection when making flux measurements should not be discounted. Our results suggest that future research should focus on the quantitative mechanistic relationships between energy balance closure and landscape-scale heterogeneity, and the consequences of mesoscale circulations for surface-atmosphere exchange measurements. (C) 2012 Elsevier B.V. All rights reserved.
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| 18. |
- Tyagi, Bhishma, et al.
(författare)
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Coherent structures contribution to fluxes of momentum and heat during stable conditions for pre monsoon thunderstorm season
- 2014
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 186, s. 43-47
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Tidskriftsartikel (refereegranskat)abstract
- The contributions of Coherent Structures (CS) to fluxes of momentum and heat have been analyzed for night time stable conditions during pre-monsoon thunderstorm periods over two tropical stations: Kharagpur and Ranchi. Fast response data recorded using sonic anemometers have been used for 2007, 2009 and 2010 for Kharagpur and 2008-2010 for Ranchi. The signals were decomposed in several time scales and then used for zero-crossing method using discrete wavelet transform. Significant contributions by CS have been found for both momentum and heat flux at both the sites and these contributions during night time stable conditions differs for days with a thunderstorm activity from a clear day with no activity, even though at both sites thunderstorm events are occurring during daytime unstable conditions.
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| 19. |
- van Oijen, M., et al.
(författare)
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A Bayesian framework for model calibration, comparison and analysis : Application to four models for the biogeochemistry of a Norway spruce forest
- 2011
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 151:12, s. 1609-1621
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Tidskriftsartikel (refereegranskat)abstract
- Four different parameter-rich process-based models of forest biogeochemistry were analysed in a Bayesian framework consisting of three operations: (1) Model calibration, (2) Model comparison, (3) Analysis of model-data mismatch. Data were available for four output variables common to the models: soil water content and emissions of N(2)O, NO and CO(2). All datasets consisted of time series of daily measurements. Monthly averages and quantiles of the annual frequency distributions of daily emission rates were calculated for comparison with equivalent model outputs. This use of the data at model-appropriate temporal scale, together with the choice of heavy-tailed likelihood functions that accounted for data uncertainty through random and systematic errors, helped prevent asymptotic collapse of the parameter distributions in the calibration. Model behaviour and how it was affected by calibration was analysed by quantifying the normalised RMSE and r(2) for the different output variables, and by decomposition of the MSE into contributions from bias, phase shift and variance error. The simplest model, BASFOR, seemed to underestimate the temporal variance of nitrogenous emissions even after calibration. The model of intermediate complexity. DAYCENT, simulated the time series well but with large phase shift. COUP and MoBiLE-DNDC were able to remove most bias through calibration. The Bayesian framework was shown to be effective in improving the parameterisation of the models, quantifying the uncertainties in parameters and outputs, and evaluating the different models. The analysis showed that there remain patterns in the data - in particular infrequent events of very high nitrogenous emission rate - that are unexplained by any of the selected forest models and that this is unlikely to be due to incorrect model parameterisation.
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| 20. |
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| 21. |
- Vico, Giulia, et al.
(författare)
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Snowed in for survival : quantifying the risk of winter damage to overwintering field crops in northern temperate latitudes
- 2014
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 197, s. 65-75
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Tidskriftsartikel (refereegranskat)abstract
- Autumn-sown field crops have important agronomic advantages (e.g., reduction of soil erosion and nutrient leaching, maximizing the use of spring moisture) and have the potential to be highly productive even though adverse winter conditions can negatively affect crop viability and yield. In the face of the unpredictable weather patterns and the expected shifts in climate in the near future, there is an imperative to develop methods to quantify both the risk of winter damage and how it is affected by altered climatic conditions and crop variety. We propose a set of indices to characterize synthetically the risk of crop damage stemming from cold spells, extended periods at low temperature, frequent occurrence of freeze-thaw cycles, and prolonged snow cover. An existing model of crop hardening and dehardening is further developed to account in full for the variability of lethal threshold temperature among individual plants. This model is coupled to a simple yet realistic description of crop-sensed temperature, so that required inputs are limited to crop-specific responses to low temperature and standard meteorogical data (average daily temperature and snow depth). This framework is applied to winter wheat under the current climatic conditions for central and southern Sweden. The roles of variety-specific hardening ability, temperature, and snow are assessed separately, thus obtaining indications of the potential impacts of variety selection and future predicted changes in temperature and snow cover in the region. Variety-specific hardening ability and response to exposure to low temperature may drastically alter the extent of winter damage. The most prevalent damaging mechanism depends on the climatic regime, with crops in colder areas benefiting from extended snow cover. A tradeoff between temperature (and hence latitude) and snow emerges, with locations at intermediate latitudes subjected to the highest risk of crop damage from exposure to low temperature and frequent freeze-thaw cycles. The same locations are also characterized by the highest inter-annual variability in the extent of winter damage - a fact that has potential implications for yield reliability.
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| 22. |
- Wu, Sihong, et al.
(författare)
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The role of air and soil temperature in the seasonality of photosynthesis and transpiration in a boreal Scots pine ecosystem
- 2012
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923 .- 1873-2240. ; 156, s. 85-103
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis and transpiration in boreal forests are restricted by air temperature (T a) and soil temperature (T s), especially in spring after the dormant period, but the extent to which the recovery process is regulated these factors is still uncertain. To examine the role of air temperature and soil temperature, years with three types of typical patterns of temperature rises were identified from 13 years of continuous flux measurements for a Scots pine ecosystem in Hyytiälä, southern Finland. By combining a process-based ecosystem model (CoupModel) with an uncertainty estimation procedure (GLUE), the role of regulating factors was explored and 45 of 51 parameters were found to have reduced uncertainty after calibration. Air temperature was the major limiting factor for photosynthesis in early spring, autumn and winter, but soil temperature was a rather important limiting factor in late spring. Especially during warm spring with a large delay of soil temperature rise both water uptake and photosynthesis was strongly reduced due to low soil temperature. Soil moisture and nitrogen showed indications of being more important for regulating photosynthesis in the summer period. It proved possible to replace the soil temperature acclimation function on photosynthesis and transpiration with a corresponding air temperature function only during warm years with a small delay between T a and T s. Fluxes of photosynthesis and transpiration showed a sensitivity to the carbon footprint representation, as expected from the high spatial variability in soil temperature during the spring of a warm year with a large delay between T a and T s.
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| 23. |
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| 24. |
- Prank, Marje, et al.
(författare)
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An operational model for forecasting ragweed pollen release and dispersion in Europe.
- 2013
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 182-183, s. 43-53
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Tidskriftsartikel (refereegranskat)abstract
- The paper considers the possibilities of modelling the release and dispersion of the pollen of common ragweed (Ambrosia artemisiifolia L.), a highly allergenic invasive weed, which is spreading through southern and central Europe. In order to provide timely warnings for the allergy sufferers, a model was developed for forecasting ragweed pollen concentrations in the air. The development was based on the system for integrated modelling of atmospheric composition (SILAM) and concentrated on spatio-temporal modelling of ragweed flowering season and pollen release, which constitutes the emission term. Evaluation of the new model against multi-annual ragweed pollen observations demonstrated that the model reproduces well the main ragweed pollen season in the areas with major plant presence, such as the Pannonian Plain, the Lyon area in France, the Milan region in Italy, Ukraine and southern Russia. The predicted start of the season is mostly within 3 days of the observed for the majority of stations in these areas. The temporal correlation between modelled and observed concentrations exceeds 0.6 for the bulk of the stations. Model application to the seasons of 2005–2011 indicated the regions with high ragweed pollen concentrations, in particular the areas where allergenic thresholds are exceeded. It is demonstrated that, due to long-range transport of pollen, high-concentration areas are substantially more extensive than the heavily infested territories.
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