Showing papers by "Ernst Detlef Schulze published in 2009"

Importance of methane and nitrous oxide for Europe's terrestrial greenhouse-gas balance

Abstract: Climate change negotiations aim to reduce net greenhouse-gas emissions by encouraging direct reductions of emissions and crediting countries for their terrestrial greenhouse-gas sinks. Ecosystem carbon dioxide uptake has offset nearly 10% of Europe's fossil fuel emissions, but not all of this may be creditable under the rules of the Kyoto Protocol. Although this treaty recognizes the importance of methane and nitrous oxide emissions, scientific research has largely focused on carbon dioxide. Here we review recent estimates of European carbon dioxide, methane and nitrous oxide fluxes between 2000 and 2005, using both top-down estimates based on atmospheric observations and bottom-up estimates derived from ground-based measurements. Both methods yield similar fluxes of greenhouse gases, suggesting that methane emissions from feedstock and nitrous oxide emissions from arable agriculture are fully compensated for by the carbon dioxide sink provided by forests and grasslands. As a result, the balance for all greenhouse gases across Europe's terrestrial biosphere is near neutral, despite carbon sequestration in forests and grasslands. The trend towards more intensive agriculture and logging is likely to make Europe's land surface a significant source of greenhouse gases. The development of land management policies which aim to reduce greenhouse-gas emissions should be a priority.

Importance of methane and nitrous oxide for Europe's terrestrial greenhouse-gas balance

Abstract: Climate change negotiations aim to reduce net greenhouse-gas emissions by encouraging direct reductions of emissions and crediting countries for their terrestrial greenhouse-gas sinks. Ecosystem carbon dioxide uptake has offset nearly 10% of Europe's fossil fuel emissions, but not all of this may be creditable under the rules of the Kyoto Protocol. Although this treaty recognizes the importance of methane and nitrous oxide emissions, scientific research has largely focused on carbon dioxide. Here we review recent estimates of European carbon dioxide, methane and nitrous oxide fluxes between 2000 and 2005, using both top-down estimates based on atmospheric observations and bottom-up estimates derived from ground-based measurements. Both methods yield similar fluxes of greenhouse gases, suggesting that methane emissions from feedstock and nitrous oxide emissions from arable agriculture are fully compensated for by the carbon dioxide sink provided by forests and grasslands. As a result, the balance for all greenhouse gases across Europe's terrestrial biosphere is near neutral, despite carbon sequestration in forests and grasslands. The trend towards more intensive agriculture and logging is likely to make Europe's land surface a significant source of greenhouse gases. The development of land management policies which aim to reduce greenhouse-gas emissions should be a priority.

The European carbon balance. Part 4: integration of carbon and other trace-gas fluxes

Abstract: Overviewing the European carbon (C), greenhouse gas (GHG), and non-GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr−1, and fossil fuel imports are 1.6 Pg yr−1. GPP is about 1.25% of solar radiation, containing about 360 × 1018 J energy – five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1‰ of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100-year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N2O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO-N, 70% of total N2O-N, and 95% of total NH3-N emissions of Europe. European soils are a net C sink (114 Tg yr−1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO2 C-equivalent yr−1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non-GHGs (NH3, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr−1. Nevertheless, for the European trace-gas balance, land-use intensity is more important than land-use change. This study shows that emissions of GHGs and non-GHGs significantly distort the C cycle and eliminate apparent C sinks.

Aboveground overyielding in grassland mixtures is associated with reduced biomass partitioning to belowground organs

Abstract: We investigated effects of plant species richness in experimental grassland plots on annual above- and belowground biomass production estimated from repeated harvests and ingrowth cores, respectively. Aboveground and total biomass production increased with increasing plant species richness while belowground production remained constant. Root to shoot biomass production ratios (R/S) in mixtures were lower than expected from monoculture performance of the species present in the mixtures, showing that interactions among species led to reduced biomass partitioning to belowground organs. This change in partitioning to belowground organs was not confined to mixtures with legumes, but also measured in mixtures without legumes, and correlated with aboveground overyielding in mixtures. It is suggested that species-rich communities invest less in belowground biomass than do monocultures to extract soil resources, thus leading to increased investment into aboveground organs and overyielding.

Conversion of cropland into grassland: Implications for soil organic‐carbon stocks in two soils with different texture

Abstract: Soil organic-carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (Ntot) were measured before and after density fractionation in two light fractions and a mineral-associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (14C) concentrations of mineral associated C were determined. Carbon stocks and mineral-associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root-biomass distribution, with 59% and 86% of the total root biomass at 0–5cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral-associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land-use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high 14C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land-use change effects on SOC are assessed.

Forest management and carbon sequestration in wood products

Abstract: Wood products are considered to contribute to the mitigation of carbon dioxide emissions. A critical gap in the life cycle of wood products is to transfer the raw timber from the forest to the processing wood industry and, thus, the primary wood products. Therefore, often rough estimates are used for this step to obtain total forestry carbon balances. The objectives of this study were (1) to examine the fate of timber harvested in Thuringian state forests (central Germany), representing a large, intensively managed forested region, and (2) to quantify carbon stocks and the lifetime of primary wood products made from this timber. The analyses were based on the amount and assortments of actually sold timber, and production parameters of the companies that bought and processed this timber. In addition, for coniferous stands of a selected Thuringian forest district, we calculated potential effects of management, as expressed by different thinning regimes on wood products and their lifetimes. Total annual timber sale of soft- and hardwoods from Thuringian state forests (195,000 ha) increased from about 136,893 t C (~0.7 t C ha−1 year−1) in 1996 to 280,194 t C (~1.4 t C ha−1 year−1) in 2005. About 47% of annual total timber harvest went into short-lived wood products with a mean residence time (MRT) < 25 years. Thirty-one per cent of the total harvest went into wood products with an MRT of 25–43 years, and only 22% was used as construction wood and glued wood, products with the longest MRT (50 years). The average MRT of carbon in harvested wood products was 20 years. Thinning from above throughout the rotation of spruce forests would lead to an average MRT in harvested wood products of about 23 years, thinning from below of about 18 years. A comparison of our calculations with estimates that resulted from the products module of the CO2FIX model (Nabuurs et al. 2001) demonstrates the influence of regional differences in forest management and wood processing industry on the lifetime of harvested wood products. To our knowledge, the present study provides for the first time real carbon inputs of a defined forest management unit to the wood product sector by linking data on raw timber production, timber sales and wood processing. With this new approach and using this data, it should be possible to substantially improve the net-carbon balance of the entire forestry sector.

Artificial drainage and associated carbon fluxes (CO2/CH4) in a tundra ecosystem

Abstract: Ecosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near Cherskii, Far-Eastern Federal District, Russia. Methane (CH4) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m−2 (i.e. an ecosystem C loss) and +8 g C m−2 after draining the study site. When taking CH4 as another important greenhouse gas into account and considering the global warming potential (GWP) of CH4 vs. CO2, the ecosystem had a positive GWP during all summers. However CH4 emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C-CO2-e m−2 before drainage in 2003 vs. 23 ± 26 g C-CO2-e m−2 after drainage in 2005).

Intra-annual variability of anatomical structure and δ13C values within tree rings of spruce and pine in alpine, temperate and boreal Europe

Abstract: Tree-ring width, wood density, anatomical structure and 13C/12C ratios expressed as δ13C-values of whole wood of Picea abies were investigated for trees growing in closed canopy forest stands. Samples were collected from the alpine Renon site in North Italy, the lowland Hainich site in Central Germany and the boreal Flakaliden site in North Sweden. In addition, Pinus cembra was studied at the alpine site and Pinus sylvestris at the boreal site. The density profiles of tree rings were measured using the DENDRO-2003 densitometer, δ13C was measured using high-resolution laser-ablation-combustion-gas chromatography-infra-red mass spectrometry and anatomical characteristics of tree rings (tracheid diameter, cell-wall thickness, cell-wall area and cell-lumen area) were measured using an image analyzer. Based on long-term statistics, climatic variables, such as temperature, precipitation, solar radiation and vapor pressure deficit, explained <20% of the variation in tree-ring width and wood density over consecutive years, while 29–58% of the variation in tree-ring width were explained by autocorrelation between tree rings. An intensive study of tree rings between 1999 and 2003 revealed that tree ring width and δ13C-values of whole wood were significantly correlated with length of the growing season, net radiation and vapor pressure deficit. The δ13C-values were not correlated with precipitation or temperature. A highly significant correlation was also found between δ13C of the early wood of one year and the late wood of the previous year, indicating a carry-over effect of the growing conditions of the previous season on current wood production. This latter effect may explain the high autocorrelation of long-term tree-ring statistics. The pattern, however, was complex, showing stepwise decreases as well as stepwise increases in the δ13C between late wood and early wood. The results are interpreted in the context of the biochemistry of wood formation and its linkage to storage products. It is clear that the relations between δ13C and tree-ring width and climate are multi-factorial in seasonal climates.

Impact of afforestation-associated management changes on the carbon balance of grassland.

Abstract: Afforestations can be considerable carbon (C) sources due to C losses from the soil after site preparation for tree planting and decreased primary production. In this study, the transition from grassland to afforestation was investigated using two eddy flux towers, which were operated in parallel for 3 years, one on a young afforestation and one on an adjacent grassland. Differences between the fluxes at the two sites were attributable to the management of the sites, without confounding influences of meteorological variability. Site preparation with deep ploughing of the planting rows destroyed 30% of the grassland vegetation at the afforestation site and reduced gross primary productivity by 41% in the first year. At the afforestation site 38 gm ―2 less C was sequestered compared with the nonafforested grassland during the first year. In the following years, the C sink at the afforestation site was higher than at the grassland indicating that soil C loss due to site preparation and land use change on the afforestation occurred only during the first year. Metrological conditions, especially summer drought, caused a high interannual variability of the C balance: both sites were small C sources in 2005 (67 g C m ―2 a ―1 at the grassland and 19g Cg ―1 a ―1 at the afforestation site) and small C sinks in 2004 and 2006 (―72.5 and ―16gCm ―2 a ―1 at the grassland and ―34 and ―61 g C g ―1 a ―1 at the afforestation). Sheep grazing and mowing affected the short-term dynamics of the C balance and sheep grazing accelerated the C turnover on the grassland site. The investigated afforestation site did not provide any short-term way of sequestering additional C even though soil C losses during the first 3 years were relatively small.

Toward a consistency cross-check of eddy covariance flux-based and biometric estimates of ecosystem carbon balance

Abstract: Quantification of an ecosystem's carbon balance and its components is pivotal for understanding both ecosystem functioning and global cycling. Several methods are being applied in parallel to estimate the different components of the CO2 balance. However, different methods are subject to different sources of error. Therefore, it is necessary that site level component estimates are cross-checked against each other before being reported. Here we present a two-step approach for testing the accuracy and consistency of eddy covariance–based gross primary production (GPP) and ecosystem respiration (Re) estimates with biometric measurements of net primary production (NPP), autotrophic (Ra) and heterotrophic (Rh) respiration. The test starts with closing the CO2 balance to account for reasonable errors in each of the component fluxes. Failure to do so within the constraints will classify the flux estimates on the site level as inconsistent. If the CO2 balance can be closed, the test continues by comparing the closed site level Ra/GPP with the Rh/GPP ratio. The consistency of these ratios is then judged against expert knowledge. Flux estimates of sites that pass both steps are considered consistent. An inconsistent ratio is not necessarily incorrect but provides a signal for careful data screening that may require further analysis to identify the possible biological reasons of the unexpected ratios. We reviewed the literature and found 16 sites, out of a total of 529 research forest sites, that met the data requirements for the consistency test. Thirteen of these sites passed both steps of the consistency cross-check. Subsequently, flux ratios (NPP/GPP, Rh/NPP, Rh/Re, and Re/GPP) were calculated for the consistent sites. Similar ratios were observed at sites which lacked information to check consistency, indicating that the flux data that are currently used for validating models and testing ecological hypotheses are largely consistent across a wide range of site productivities. Confidence in the output of flux networks could be further enhanced if the required fluxes are independently estimated at all sites for multiple years and harmonized methods are used.

Resources, recruitment limitation and invader species identity determine pattern of spontaneous invasion in experimental grasslands.

Abstract: Summary 1 A number of experimental studies have supported the hypothesis that diversity increases invasion resistance, but several mechanisms were proposed to explain this relationship We studied spontaneous invasion in experimental grasslands varying in species richness (1–16 (60)) and plant functional group richness and identity (1–4; grasses, legumes, tall herbs small herbs) during the first 3 years after establishment on large-area plots of 20 × 20 m size 2 Invader species number, biomass and density decreased with increasing community species richness The identity of successful invaders changed through time as the relative importance of external invaders (= species not belonging to the experimental species pool) decreased, while internal invaders (= species belonging to the experimental species pool) became more important The presence of legumes increased, while the presence of grasses in the plant communities decreased density, biomass and species number of external invaders and biomass of internal invaders in the second and third year after establishment 3 Analyses of viable seeds in the topsoil, a higher invasion in communities with more potential invaders in adjacent plots and an edge effect with a higher invader density and species number at the margin of the plots suggested recruitment limitation of internal invader species in contrast to external invader species that were more strongly limited by community characteristics 4 Resource availability, in particular percentage cover of resident species, root length density and soil nitrate, and propagule availability had significant effects on invasibility However, statistical analyses revealed that these variables did not completely control for effects of community species richness and the presence of grasses and legumes respectively on invasion resistance 5 Separate analyses of frequent invader species showed that the invasion success of individual species varied in response to biodiversity and resource niches 6 Synthesis Our results confirm that a more complete use of available resources in plant communities of increasing species richness generally decreases invasibility However, the overall positive effects of biodiversity on invasion resistance are dependent on species identity of the invader species Thus, mechanisms at species level are important to understand community invasion resistance

Community assembly and biomass production in regularly and never weeded experimental grasslands.

Abstract: We studied the natural colonisation of new species in experimental grasslands varying in plant species richness (from 1 to 60) and plant functional group richness (from 1 to 4) in either regularly or never weeded subplots during the first 3 years after establishment Sown species established successfully, with no differences in species richness or their relative abundances between the regularly and never weeded subplots during the study period Aboveground biomass of sown species increased with increasing sown species richness in both treatments While a positive relationship between sown species richness and total aboveground biomass (including colonising species) existed in the 2nd year after sowing in the regularly and never weeded subplots, this positive relationship decayed in the 3rd year in the never weeded subplots because of a higher biomass of colonising species in species-poor mixtures Total aboveground biomass varied independently of total species richness 3 years after sowing in both treatments Jaccard similarity of coloniser species composition between regularly and never weeded subplots decreased from the 2nd to the 3rd year, indicating a divergence in coloniser species composition Coloniser immigration and turnover rates were higher in regularly weeded subplots, confirming that weeding counteracts species saturation and increases the chance that new colonisers would establish Although our study shows that low diversity plant communities are unstable and converge to higher levels of biodiversity, the effects of initially sown species on community composition persisted 3 years after sowing even when allowing for succession, suggesting that colonising species mainly filled empty niche space

Using drill resistance to quantify the density in coarse woody debris of Norway spruce

Abstract: To evaluate the mass of coarse woody debris (CWD), it is necessary to quantify its density. Drill resis- tance measurements are introduced as a approach to esti- mate the density of CWD in different stages of decay. Dead logs of Norway spruce (Picea abies (L.) Karst.) from a Central European mountainous site were used as a test system to compare the new method with conventional predictors of wood density such as fast quantitative field estimates (e.g., knife probe) and classification of decay classes based on a set of qualitative traits and quantitative estimates. The model containing only drill resistance as a predictor explained 65% of the variation in wood density and was markedly better than models containing one or more of several conventional predictors. However, we show that the relationship between drill resistance and gravimetric wood density relationship is sensitive to the decay status. Therefore, the best model combines drill resistance and decay class (adj. R 2 = 0.732). An additional experiment showed that drill resistance is also sensitive to the moisture state (fresh vs. oven-dry) of the sample. The major potential of the method lies in its non-destructive nature which allows repeated sampling in long-term eco- system studies or in protected areas where destructive sampling is prohibited. The limitations of the method are discussed and recommendations for applications are given.

Non-random recruitment of invader species in experimental grasslands.

Abstract: To assess potential effects of seed limitation, characteristics of invader species and characteristics of established plant communities on recruitment success, we conducted a split-plot experiment factorially combining three weeding treatments corresponding to increasing successional age (regular weeding < cessation of weeding after three years < never weeded since sowing) with two seed limitation treatments (control vs sowing a seed mixture of all experimental species = internal invaders) in experimental grasslands varying in species richness (1, 2, 4, 8, 16) and in functional group richness and composition (1, 2, 3, 4 functional groups: presence/absence of legumes x grasses x tall herbs x small herbs). Seed addition increased internal invader seedling densities and the probability of successful colonization per species. Legumes, tall herbs and species with large and long-lived seeds or a requirement for specific vectors for pollination and dispersal benefited from seed addition most. The number of successfully established internal invader species was highest in plots with low initial species richness and in the regularly weeded treatments and lowest in plots with high initial species richness and in the never weeded treatments, indicating decreased recruitment with increased successional age. Resident plant communities with legumes had mostly negative (legume and small- and tall-herb internal invaders, external invaders) or neutral (grass invaders) effects on seedling density and colonization probability of invader species whereas resident communities with grasses had positive effects on the colonization probability of invader species except grasses themselves. These results show that seed limitation, invader characteristics, and community characteristics all can affect recruitment success in predictable ways, suggesting non-random community assembly during succession starting from different initial species pools.

Das Potenzial von Wäldern und Mooren für den Klimaschutz in Deutschland und auf globaler Ebene

Abstract: Walder und Moore beherbergen weltweit ebenso wie auch in Deutschland von allen Okosystemen die grosten Kohlenstoffvorrate pro Hektar. Beide Okosystemtypen sind besonders gefahrdet durch die zunehmende menschliche Nutzung und in vielen Regionen auch durch den Klimawandel. Ihr Schutz ist eine besonders effiziente Klimaschutzmasnahme mit direkten Synergien zum Naturschutz. Effiziente Schutzmasnahmen mussen ursachenorientiert sein. Eine Entlastung vom Nutzungsdruck - unter anderem durch eine effektivere Nutzung von Ressourcen sowie angemessene Kompensationszahlungen - ist daher notig. Beides ist derzeit nicht flachendeckend in Sicht. In den Waldern Deutschlands sollte angesichts des Klimawandels ein signifikanter Waldumbau zu artenreichen Mischwaldern erfolgen, um auch zukunftig stabile Bestande und deren Senkenfunktion zu sichern. Eine klimafreundlich durchgefuhrte Renaturierung der dranierten Moore Deutschlands konnte theoretisch bis zu 35 mio. t Kohlendioxid-Aquivalente pro Jahr einsparen. Naturschutz und Landnutzung mussen sich arrangieren, wenn Klimaschutz effizient geleistet und Verlagerungseffekte vermieden werden sollen.

Temperate and Boreal Old-Growth Forests: How do Their Growth Dynamics and Biodiversity Differ from Young Stands and Managed Forests?

Abstract: This chapter investigates biomass, net primary productivity (NPP), and net ecosystem productivity (NEP) of boreal and temperate forest ecosystems in relation to stand density and age. Forests may accumulate woody biomass at constant rate for centuries and there is little evidence of an age-related decline in productivity. Self thinning and management may lead to a loss of tree-individuals to the extent that the available ground surface is no longer covered, thus leading to a decline in productivity per unit ground area. Carbon-accumulation of old forests is similar to that of young forests at the same yield class and of the same species. However, due to the accumulated mass per area and the increased spread of fungal heartwood rot, old forests become unstable and collapse due to external forces, mainly wind. Since carbon accumulation and collapse are highly asymmetric in time, old stands are rarer than young stands. Forest structure and management, rather than stand age, determine NPP. There is no clear distinction in productivity between primary and managed forest, except that managed forests are generally harvested at an age below 100 years. Although unmanaged forests sustain natural processes, biodiversity – expressed as species richness – is not necessarily higher in unmanaged compared to managed forests.

Biosphere–Atmosphere Exchange of Old-Growth Forests: Processes and Pattern

Abstract: Old growth forests are a substantial part of the global forested area, but differ in age, structure and composition from younger or managed forests. In this chapter we review how these characteristics lead to differences in the biosphere–atmosphere exchange of carbon, water, and energy of old-growth forests compared to younger and managed forests. A global compilation of net ecosystem exchange data from eddy covariance measurements reveals that – in contrast to classical theory – most of the old-growth forests being studied remain net carbon sinks, and suggests that, rather than age, other factors such as tree structure and resource competition limits growth in forests. Water fluxes exhibit an ambiguous response to age with some old forests showing reduced water fluxes due to hydraulic limitation, while others show higher water fluxes due to better groundwater access and lower surface reflectance (albedo) in old and tall forests.