Olaf Kolle

Large carbon uptake by an unmanaged 250-year-old deciduous forest in Central Germany

Unmanaged forests at a late stage of successional development are considered to be insignificant as carbon sinks, since in theory, assimilation is thought to be balanced by respiration. However, little experimental evidence for this hypothesis exists so far for forests at the ecosystem level. Therefore, we performed continuous eddy covariance measurements of carbon dioxide over an unmanaged beech forest in the Hainich National Park in Central Germany as part of the EU project CARBOEUROFLUX. This forest shows typical characteristics of an ‘advanced’ forest with large dead wood pools, a diverse stand structure and a wide tree age class distribution, up to 250 years. This forest was a large carbon sink over 2 years, with 494 g C m −2 per year in 2000 and 49 0gCm −2 per year in 2001. Daytime summer fluxes were strongly controlled by photosynthetic photon flux density ( R 2 = 0.7–0.9), with minor effects of the ratio of diffuse to total downward radiation or the vapor pressure deficit. Nighttime CO 2 fluxes were mainly controlled by soil temperature ( R 2 = 0.8) and soil moisture. In addition, high nighttime CO2 fluxes (4–6 mol m −2 s −1 ) were found directly before and during bud break in spring as well as just after leaf fall of both years (2000 and 2001), reflecting stand physiology corresponding to phenological changes, independent of soil temperature. Additional wind profile measurements at five heights within the canopy revealed a decoupling of above and below canopy air flow under conditions of low friction velocity ( u ∗ < 0. 4ms −1 ), probably indicating down slope drainage. In conclusion, unmanaged forests at a comparatively late stage of successional development can still act as significant carbon sinks with large implications for forest management practice and negotiations about biological sinks within the Kyoto Protocol.

Winter wheat carbon exchange in Thuringia, Germany

Eddy covariance measurements and estimates of biomass net primary production (NPP) in combination with soil carbon turnover modelled by the Roth-C model were used to assess the ecosystem carbon balance of an agricultural ecosystem in Thuringia, Germany, growing winter wheat in 2001. The eddy CO2 flux measurements indicate an annual net ecosystem exchange (NEE) uptake in the range from −185 to −245 g C m −2 per year. Main data analysis uncertainty in the annual NEE arises from night-time u ∗ screening, other effects (e.g. coordinate rotation scheme) have only a small influence on the annual NEE estimate. In agricultural ecosystems the fate of the carbon removed during harvest plays a role in the net biome production (NBP) of the ecosystem, where NBP is given by net ecosystem production (NEP =− NEE) minus non-respiratory losses of the ecosystem (e.g. harvest). Taking account of the carbon removed by the wheat harvest (290 g C m −2 ), the agricultural field becomes a source of carbon with a NBP in the order of −45 to −105 g C m −2 per year. Annual carbon balance modelled with the Roth-C model also indicated that the ecosystem was a source for carbon (NBP −25 to −55 g C m −2 per year). Based on the modelling most of carbon respired resulted from changes in the litter and fast soil organic matter pool. Also, the crop and management history, particularly the C input to soil in the previous year, significantly affect next year’s CO 2 exchange.

Thirst beats hunger - declining hydration during drought prevents carbon starvation in Norway spruce saplings.

Drought-induced tree mortality results from an interaction of several mechanisms. Plant water and carbon relations are interdependent and assessments of their individual contributions are difficult. Because drought always affects both plant hydration and carbon assimilation, it is challenging to disentangle their concomitant effects on carbon balance and carbon translocation. Here, we report results of a manipulation experiment specifically designed to separate drought effects on carbon and water relations from those on carbon translocation. In a glasshouse experiment, we manipulated the carbon balance of Norway spruce saplings exposed to either drought or carbon starvation (CO2 withdrawal), or both treatments, and compared the dynamics of carbon exchange, allocation and storage in different tissues. Drought killed trees much faster than did carbon starvation. Storage C pools were not depleted at death for droughted trees as they were for starved, well-watered trees. Hence drought has a significant detrimental effect on a plants ability to utilize stored carbon. Unless they can be transported to where they are needed, sufficient carbon reserves alone will not assure survival of a drought except under specific conditions, such as moderate drought, or in species that maintain plant water relations required for carbon re-mobilization.

Advection and resulting CO2 exchange uncertainty in a tall forest in central Germany.

Potential losses by advection were estimated at Hainich Forest, Thuringia, Germany, where the tower is located at a gentle slope. Three approaches were used: (1) comparing nighttime eddy covariance fluxes to an independent value of total ecosystem respiration by bottom-up modeling of the underlying processes, (2) direct measurements of a horizontal CO2 gradient and horizontal wind speed at 2 m height in order to calculate horizontal advection, and (3) direct measurements of a vertical CO2 gradient and a three-dimensional wind profile in order to calculate vertical advection. In the first approach, nighttime eddy covariance measurements were compared to independent values of total ecosystem respiration by means of bottom-up modeling of the underlying biological processes. Turbulent fluxes and storage term were normalized to the fluxes calculated by the bottom-up model. Below a u(*) threshold of 0.6 m/s the normalized turbulent fluxes decreased with decreasing u(*), but the flux to the storage increased only up to values less than 20% of the modeled flux at low turbulence. Horizontal advection was measured by a horizontal CO2 gradient over a distance of 130 m combined with horizontal wind speed measurements. Horizontal advection occurred at most of the evenings independently of friction velocity above the canopy. Nevertheless, horizontal advection was higher when u(*) was low. The peaks of horizontal advection correlated with changes in temperature. A full mass balance including turbulent fluxes, storage, and horizontal and vertical advection resulted in an increase of spikes and scatter but seemed to generally improve the results from the flux measurements. The comparison of flux data with independent bottom-up modeling results as well as the direct measurements resulted in strong indications that katabatic flows along the hill slope during evening and night reduces the measured apparent ecosystem respiration rate. In addition, anabatic flows may occur during the morning. We conclude that direct measurements of horizontal and vertical advection are highly necessary at sites located even on gentle hill slopes.

Data Acquisition and Flux Calculations

In this chapter, the basic theory and the procedures used to obtain turbulent fluxes of energy, mass, and momentum with the eddy covariance technique will be detailed. This includes a description of data acquisition, pretreatment of high-frequency data and flux calculation.

Methane airborne measurements and comparison to global models during BARCA

[1] Tropical regions, especially the Amazon region, account for large emissions of methane (CH_4). Here, we present CH_4 observations from two airborne campaigns conducted within the BARCA (Balanco Atmosferico Regional de Carbono na Amazonia) project in the Amazon basin in November 2008 (end of the dry season) and May 2009 (end of the wet season). We performed continuous measurements of CH_4 onboard an aircraft for the first time in the Amazon region, covering the whole Amazon basin with over 150 vertical profiles between altitudes of 500 m and 4000 m. The observations support the finding of previous ground-based, airborne, and satellite measurements that the Amazon basin is a large source of atmospheric CH_4. Isotope analysis verified that the majority of emissions can be attributed to CH_4 emissions from wetlands, while urban CH_4 emissions could be also traced back to biogenic origin. A comparison of five TM5 based global CH_4 inversions with the observations clearly indicates that the inversions using SCIAMACHY observations represent the BARCA observations best. The calculated CH_4 flux estimate obtained from the mismatch between observations and TM5-modeled CH_4 fields ranges from 36 to 43 mg m^(−2) d^(−1) for the Amazon lowland region.

Adjustment to the light environment in small-statured forbs as a strategy for complementary resource use in mixtures of grassland species

BACKGROUND AND AIMS The biological mechanisms of niche complementarity allowing for a stable coexistence of a large number of species in a plant community are still poorly understood. This study investigated how small-statured forbs use environmental niches in light and CO(2) to explain their persistence in diverse temperate grasslands. METHODS Light and CO(2) profiles and the corresponding leaf characteristics of seven small-statured forbs were measured in monocultures and a multi-species mixture within a biodiversity experiment (Jena Experiment) to assess their adjustment to growth conditions in the canopy. KEY RESULTS Environmental conditions near the ground varied throughout the season with a substantial CO(2) enrichment (>70 µmol mol(-1) at 2 cm, >20 µmol mol(-1) at 10 cm above soil surface) and a decrease in light transmittance (to <5 % deep in the canopy) with large standing biomass (>500 g d. wt m(-2)) in the multi-species assemblage. Leaf morphology, biochemistry and physiology of small-statured forbs adjusted to low light in the mixture compared with the monocultures. However, the net carbon assimilation balance during the period of low light only compensated the costs of maintenance respiration, while CO(2) enrichment near the ground did not allow for additional carbon gain. Close correlations of leaf mass per area with changes in light availability suggested that small-statured forbs are capable of adjusting to exploit seasonal niches with better light supply for growth and to maintain the carbon metabolism for survival if light transmittance is substantially reduced in multi-species assemblages. CONCLUSIONS This study shows that adjustment to a highly dynamic light environment is most important for spatial and seasonal niche separation of small-statured forb species in regularly mown, species-rich grasslands. The utilization of short-period CO(2) enrichment developing in dense vegetation close to the ground hardly improves their carbon balance and contributes little to species segregation along environmental niche axes.

Seasonal and inter-annual photosynthetic response of representative C4 species to soil water content and leaf nitrogen concentration across a tropical seasonal floodplain

We examined the seasonal and inter-annual variation of leaf-level photosynthetic characteristics of three C-4 perennial species, Cyperus articulatus, Panicum repens and Imperata cylindrica, and their response to environmental variables, to determine comparative physiological responses of plants representing particular microhabitats within a seasonal tropical floodplain in the Okavango River Delta, Botswana. Five measurement campaigns were carried out over a period of 2 y which covered two early rainy seasons, two late rainy seasons and one dry season. For all three species, light-saturated net photosynthetic rates (A(sat)) and stomatal conductance (9,at) decreased with decreasing soil water content with a seasonal range for A(sat) of approximately 5-45 mu mol m(-2) s(-1), and for g(sat) of 0.03-0.35 mol m-2 s(-1). The species representing the wettest microhabitat (Cyperus) had the highest g(sat) at low leaf-to-air vapour pressure deficits (D-l), the highest ratio of intercellular to ambient CO2 concentration (C-i/C-a), as well as the highest degree of variation in C-l/C-a from season to season. We interpret this as being indicative of its adaptation to a moist growth environment allowing for non-conservative water use strategies as soil moisture is usually abundant. For all three species there was significant variation in photosynthetic fluxes from one year to another that was related to variation in leaf nitrogen and phosphorus. This study shows that when assessing the role of savanna stands in large-scale carbon balance models, the remarkable inter-annual variation in leaf photosynthesis reported in this study should be taken into account. (Less)

Eddy Covariance Assessment of CO2 Accumulation by Mature Pine Forest

In recent years, there has been a significant progress in methods of direct continuous monitoring of the carbon balance of ecosystems. This progress was mainly provided by the upgrade of the hardware and software for the eddy covariance technique. The eddy covariance method is based on determination of the high-frequency covariance between the vertical component of wind velocity and the concentration of carbon dioxide [5–7].

High-quality eddy-covariance CO2 budgets under cold climate conditions

This study aimed at quantifying potential negative effects of instrument heating to improve eddy covariance flux data quality in cold environments. Our overarching objective was to minimize heating-related bias in annual CO2 budgets from an Arctic permafrost system. We used continuous eddy-covariance measurements covering three full years within an Arctic permafrost ecosystem with parallel sonic anemometers operation with activated heating and without heating as well as parallel operation of open- and closed-path gas analyzers, the latter serving as a reference. Our results demonstrate that the sonic anemometer heating has a direct effect on temperature measurements while the turbulent wind field is not affected. As a consequence, fluxes of sensible heat are increased by an average 5 W m-2 with activated heating, while no direct effect on other scalar fluxes was observed. However, the biased measurements in sensible heat fluxes can have an indirect effect on the CO2 fluxes in case they are used as input for a density-flux WPL correction of an open-path gas analyzer. Evaluating the self-heating effect of the open-path gas analyzer by comparing CO2 flux measurements between open- and closed-path gas analyzers we found systematically higher CO2 uptake recorded with the open-path sensor, leading to a cumulative annual offset of 96 gC m-2, which was not only the result of the cold winter season but also due to substantial self-heating effects during summer. With an inclined sensor mounting, only a fraction of the self-heating correction for vertically mounted instruments is required.