Meelis Mölder

A new mass conservation approach to the study of CO2 advection in an alpine forest

A new method is proposed for the computation of CO2 Net Ecosystem Exchange (NEE) and its components in a forest ecosystem. Advective flux is estimated by taking into account the air mass conservation principle. For this purpose, wind and dry air density values on the surface of the control volume are first corrected and then the advective flux is estimated on the surface of the control volume. Turbulent flux is also computed along the surface of the control volume while storage flux is computed inside the volume. Additional characteristics of this method are that incompressibility of the mean flow is not assumed a priori, and that vertical and horizontal advective fluxes are not treated separately, but their sum is estimated directly. The methodology is applied to experimental data collected with a three-dimensional scheme at the alpine site of Renon during the Advex project (July 2005). The advection flux was found to be prevailing positive at night and negative during the day, as was found in previous studies on advection for the same site, but showed a lower scatter in half-hour calculated values. We tested the effect of its summation on turbulent and storage fluxes to produce half-hourly values of NEE. Nighttime NEE values were used in functional relations with soil temperature, daytime values with PPFD. The effect of addition of the advection component was an increase in the values of parameters indicating ecosystem respiration, quantum yield, and photosynthetic capacity. The coefficient of correlation between NEE and environmental drivers increased. (Less)

Vertical variability and effect of stability on turbulence characteristics down to the floor of a pine forest

Among the fundamental problems in canopy turbulence, particularly near the forest floor, remain the local diabatic effects and linkages between turbulent length scales and the canopy morphology. To progress on these problems, mean and higher order turbulence statistics are collected in a uniform pine forest across a wide range of atmospheric stability conditions using five 3-D anemometers in the subcanopy. The main novelties from this experiment are: (1) the agreement between second-order closure model results and measurements suggest that diabatic states in the layer above the canopy explain much of the modulations of the key velocity statistics inside the canopy except in the immediate vicinity of the trunk space and for very stable conditions. (2) The dimensionless turbulent kinetic energy in the trunk space is large due to a large longitudinal velocity variance but it is inactive and contributes little to momentum fluxes. (3) Near the floor layer, a logarithmic mean velocity profile is formed and vertical eddies are strongly suppressed modifying all power spectra. (4) A spectral peak in the vertical velocity near the ground commensurate with the trunk diameter emerged at a moderate element Reynolds number consistent with Strouhal instabilities describing wake production.

Seasonal variation of carbon fluxes in a sparse savanna in semi arid Sudan

BackgroundLarge spatial, seasonal and annual variability of major drivers of the carbon cycle (precipitation, temperature, fire regime and nutrient availability) are common in the Sahel region. This causes large variability in net ecosystem exchange and in vegetation productivity, the subsistence basis for a major part of the rural population in Sahel. This study compares the 2005 dry and wet season fluxes of CO2 for a grass land/sparse savanna site in semi arid Sudan and relates these fluxes to water availability and incoming photosynthetic photon flux density (PPFD). Data from this site could complement the current sparse observation network in Africa, a continent where climatic change could significantly impact the future and which constitute a weak link in our understanding of the global carbon cycle.ResultsThe dry season (represented by Julian day 35–46, February 2005) was characterized by low soil moisture availability, low evapotranspiration and a high vapor pressure deficit. The mean daily NEE (net ecosystem exchange, Eq. 1) was -14.7 mmol d-1 for the 12 day period (negative numbers denote sinks, i.e. flux from the atmosphere to the biosphere). The water use efficiency (WUE) was 1.6 mmol CO2 mol H2O-1 and the light use efficiency (LUE) was 0.95 mmol CO2 mol PPFD-1. Photosynthesis is a weak, but linear function of PPFD. The wet season (represented by Julian day 266–273, September 2005) was, compared to the dry season, characterized by slightly higher soil moisture availability, higher evapotranspiration and a slightly lower vapor pressure deficit. The mean daily NEE was -152 mmol d-1 for the 8 day period. The WUE was lower, 0.97 mmol CO2 mol H2O-1 and the LUE was higher, 7.2 μ mol CO2 mmol PPFD-1 during the wet season compared to the dry season. During the wet season photosynthesis increases with PPFD to about 1600 μ mol m-2s-1 and then levels off.ConclusionBased on data collected during two short periods, the studied ecosystem was a sink of carbon both during the dry and wet season 2005. The small sink during the dry season is surprising and similar dry season sinks have not to our knowledge been reported from other similar savanna ecosystems and could have potential management implications for agroforestry. A strong response of NEE versus small changes in plant available soil water content was found. Collection and analysis of flux data for several consecutive years including variations in precipitation, available soil moisture and labile soil carbon are needed for understanding the year to year variation of the carbon budget of this grass land/sparse savanna site in semi arid Sudan.

Current Carbon Balance of the Forested Area in Sweden and its Sensitivity to Global Change as Simulated by Biome-BGC

Detailed information from the Swedish National Forest Inventory was used to simulate the carbon balance for Sweden by the process-based model Biome-BGC. A few shortcomings of the model were identified and solutions to those are proposed and also used in the simulations. The model was calibrated against CO2 flux data from 3 forests in central Sweden and then applied to the whole country divided into 30 districts and 4 age classes. Gross primary production (GPP) ranged over districts and age classes from 0.20 to 1.71 kg C m−2 y−1 and net ecosystem production (NEP) ranged from −0.01 to 0.44. The 10- to 30-year age class was the strongest carbon sink because of its relatively low respiration rates. When the simulation results were scaled up to the whole country, GPP and NEP were 175 and 29 Mton C y−1, respectively, for the 22.7 Mha of forests in Sweden. A climate change scenario was simulated by assuming a 4°C increase in temperature and a doubling of the CO2 concentration; GPP and NEP then increased to 253 and 48 Mton C y−1, respectively. A sensitivity analysis showed that at present CO2 concentrations NEP would peak at an increase of 5°C for the mean annual temperature. At higher CO2 levels NEP showed a logarithmic increase.

Quantification of C uptake in subarctic birch forest after setback by an extreme insect outbreak

The carbon dynamics of northern natural ecosystems contribute significantly to the global carbon balance. Periodic disturbances to these dynamics include insect herbivory. Larvae of autumn and winter moths (Epirrita autumnata and Operophtera brumata) defoliate mountain birch (Betula pubescens) forests in northern Scandinavia cyclically every 9-10 years and occasionally (50-150 years) extreme population densities can threaten ecosystem stability. Here we report impacts on C balance following a 2004 outbreak where a widespread area of Lake Tornetrask catchment was severely defoliated. We show that in the growing season of 2004 the forest was a much smaller net sink of C than in a reference year, most likely due to lower gross photosynthesis. Ecosystem respiration in 2004 was smaller and less sensitive to air temperature at nighttime relative to 2006. The difference in growing season uptake between an insect affected and non-affected year over the 316 km(2) area is in the order of 29 x 10(3) tonnes C equal to a reduction of the sink strength by 89%. Citation: Heliasz, M., T. Johansson, A. Lindroth, M. Molder, M. Mastepanov, T. Friborg, T. V. Callaghan, and T. R. Christensen (2011), Quantification of C uptake in subarctic birch forest after setback by an extreme insect outbreak, Geophys. Res. Lett., 38, L01704, doi:10.1029/2010GL044733. (Less)

Contributions of climate, leaf area index and leaf physiology to variation in gross primary production of six coniferous forests across Europe: a model-based analysis

Gross primary production (GPP) is the primary source of all carbon fluxes in the ecosystem. Understanding variation in this flux is vital to understanding variation in the carbon sink of forest ecosystems, and this would serve as input to forest production models. Using GPP derived from eddy-covariance (EC) measurements, it is now possible to determine the most important factor to scale GPP across sites. We use long-term EC measurements for six coniferous forest stands in Europe, for a total of 25 site-years, located on a gradient between southern France and northern Finland. Eddy-derived GPP varied threefold across the six sites, peak ecosystem leaf area index (LAI) (all-sided) varied from 4 to 22 m(2) m(-2) and mean annual temperature varied from -1 to 13 degrees C. A process-based model operating at a half-hourly time-step was parameterized with available information for each site, and explained 71-96% in variation between daily totals of GPP within site-years and 62% of annual total GPP across site-years. Using the parameterized model, we performed two simulation experiments: weather datasets were interchanged between sites, so that the model was used to predict GPP at some site using data from either a different year or a different site. The resulting bias in GPP prediction was related to several aggregated weather variables and was found to be closely related to the change in the effective temperature sum or mean annual temperature. High R(2)s resulted even when using weather datasets from unrelated sites, providing a cautionary note on the interpretation of R(2) in model comparisons. A second experiment interchanged stand-structure information between sites, and the resulting bias was strongly related to the difference in LAI, or the difference in integrated absorbed light. Across the six sites, variation in mean annual temperature had more effect on simulated GPP than the variation in LAI, but both were important determinants of GPP. A sensitivity analysis of leaf physiology parameters showed that the quantum yield was the most influential parameter on annual GPP, followed by a parameter controlling the seasonality of photosynthesis and photosynthetic capacity. Overall, the results are promising for the development of a parsimonious model of GPP.

Branch transpiration of pine and spruce scaled to tree and canopy using needle biomass distributions

Abstractu2002Branch water exchange and total tree water uptake were measured in a mixed Norway spruce and Scots pine stand in central Sweden during the 1995 and 1996 growing seasons. Branch transpiration was scaled to canopy level on the basis of a branch conductance model, using vertical needle-area distributions obtained by destructive sampling. Comparison with total tree water uptake scaled to canopy level showed agreement within 10%, for periods when the canopy was not affected by climatically induced stress. Comparison of scaled fluxes on individual trees showed that measurements of transpiration at branch level provide information on the direct response of transpiration to variations in weather, and furthermore that the time-lag between transpiration and tree water uptake was as much as 3 h. The vertical needle-area distribution of Scots pine was similar to that found by other authors. Needle-area distribution on Norway spruce, which has not been described before, showed that it has its largest needle area at the top of the crown. Specific needle area varied considerably both within trees and between trees. For spruce, mean specific needle area (±SD) varied from 2.4±0.5 mm2 mg–1 at the top of the crown to 7.1±1.9 mm2 mg–1 at the base. Corresponding figures for Scots pine were 3.4±2.0 and 9.1±2.1.

Excess resistance of bog surfaces in central Sweden

Typical bogs in boreal forest zone can be characterized by hummock and hollow micro-topography and sparsely vegetated surfaces, thus, are quite distinct from other types of wetlands. Micrometeorological measurements were carried out in central Sweden at two bogs in different summer seasons. The data analysis aimed at obtaining good estimates for the roughness length z(ou) and the kB(-1) factor. The roughness length for wind speed (z(ou)) was estimated to be 2 cm. A good surface temperature estimate is crucial for kB-1 calculations. The surface temperature of such a heterogeneous surface showed high spatial variability, the differences between its minimum and maximum values reaching 10 K during daytime. However, an infrared thermometer averaging over a circle of a few meters still gave an acceptable result. The kB-1 factor was determined through the roughness sublayer Stanton number and the corresponding drag coefficient and was dependent on the roughness Reynolds number (Re-o). It took the form: kB(-1) = 1.58Re(o)(0.25) - 3.4. Within considerable scatter of data, a constant kB(-1) = 3.2 was also acceptable. This result lies in between the bluff-rough and vegetated surface cases. (Less)

Modelling of growing season methane fluxes in a high-Arctic wet tundra ecosystem 1997-2010 using in situ and high-resolution satellite data

Methane (CH4) fluxes 1997–2010 were studied by combining remotely sensed normalised difference water index (NDWI) with in situ CH4 fluxes from Rylekærene, a high-Arctic wet tundra ecosystem in the Zackenberg valley, north-eastern Greenland. In situ CH4 fluxes were measured using the closed-chamber technique. Regression models between in situ CH4 fluxes and environmental variables [soil temperature (Tsoil), water table depth (WtD) and active layer (AL) thickness] were established for different temporal and spatial scales. The relationship between in situ WtD and remotely sensed NDWI was also studied. The regression models were combined and evaluated against in situ CH4 fluxes. The models including NDWI as the input data performed on average slightly better [root mean square error (RMSE) =1.56] than the models without NDWI (RMSE=1.67), and they were better in reproducing CH4 flux variability. The CH4 flux model that performed the best included exponential relationships against temporal variation in T soil and AL, an exponential relationship against spatial variation in WtD and a linear relationship between WtD and remotely sensed NDWI (RMSE=1.50). There were no trends in modelled CH4 flux budgets between 1997 and 2010. Hence, during this period there were no trends in the soil temperature at 10 cm depth and NDWI.

Atmospheric methane removal by boreal plants

Several studies have proposed aerobic methane (CH4) emissions by plants. If confirmed, these findings would further increase the imbalance in the global CH4 budget which today underestimates CH4 sinks. Oxidation by OH-radicals in the troposphere is the major identified sink followed by smaller contribution from stratospheric loss and oxidation by methano- and methylotrophic bacteria in soils. This study directly investigated CH4 exchange by plants in their natural environment. At a forest site in central Sweden, in situ branch chamber measurements were used to study plant ambient CH4 exchange by spruce (Picea abies), birch (Betula pubescens), rowan (Sorbus aucuparia) and pine (Pinus sylvestris). The results show a net uptake of CH4 by all the studied plants, which might be of importance for the methane budget. (Less)