R. Ceulemans

Energy balance closure at FLUXNET sites

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha -1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Emerging model systems in plant biology: poplar (Populus) as a model forest tree.

Forest trees have tremendous economic and ecological value, as well as unique biological properties of basic scientific interest. The inherent difficulties of experimenting on very large long-lived organisms motivates the development of a model system for forest trees. Populus (poplars, cottonwoods, aspens) has several advantages as a model system, including rapid growth, prolific sexual reproduction, ease of cloning, small genome, facile transgenesis, and tight coupling between physiological traits and biomass productivity. A combination of genetics and physiology is being used to understand the detailed mechanisms of forest tree growth and development.

Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2

Forest ecosystems are important sinks for rising concentrations of atmospheric CO2. In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental forests were grown under atmospheric concentrations of CO2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO2. Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO2.

Net ecosystem CO2 exchange of mixed forest in Belgium over 5 years

In this paper, we present and discuss the annual net ecosystem exchange (NEE) results from 5 years (1997–2001) of continuous eddy covariance measurements of CO2 flux above a mixed temperate forest. The forest was a 70-year-old coniferous (Scots pine)—deciduous mixture, with slow growth rate and a leaf area index (LAI) of about 3, and was part of the European CARBOEUROFLUX research network. Effects of the data pre-treatment and the gap filling method on annual NEE estimates were analyzed. The u∗-correction increased the annual NEE by + 61 gCm −2 per year on average. The maximum difference in annual NEE estimates from different gap filling methods amounted up to 13 0gCm −2 per year in a year with a large gap in the CO2 flux series. The estimated average annual NEE over the 5 years was + 110 gCm −2 per year (ranging from − 9 to 255 gCm −2 per year) when using the most defensible gap filling strategy. We also analyzed the inter-annual variability of carbon balance, which was found to be mainly dependent on the length of the growing season and on the annual temperature. The observation that this forest acted as a CO2 source contrasts with previous results from most other temperate forests.

Energy and greenhouse gas balance of bioenergy production from poplar and willow: a review

Short‐rotation woody crops (SRWC) such as poplar and willow are an important source of renewable energy. They can be converted into electricity and/or heat using conventional or modern biomass technologies. In recent years many studies have examined the energy and greenhouse gas (GHG) balance of bioenergy production from poplar and willow using various approaches. The outcomes of these studies have, however, generated controversy among scientists, policy makers, and the society. This paper reviews 26 studies on energy and GHG balance of bioenergy production from poplar and willow published between 1990 and 2009. The data published in the reviewed literature gave energy ratios (ER) between 13 and 79 for the cradle‐to‐farm gate and between 3 and 16 for cradle‐to‐plant assessments, whereas the intensity of GHG emissions ranged from 0.6 to 10.6 g CO2 Eq MJbiomass−1 and 39 to 132 g CO2 Eq kWh−1. These values vary substantially among the reviewed studies depending on the system boundaries and methodological assumptions. The lack of transparency hampers meaningful comparisons among studies. Although specific numerical results differ, our review revealed a general consensus on two points: SRWC yielded 14.1–85.9 times more energy than coal (ERcoal∼0.9) per unit of fossil energy input, and GHG emissions were 9–161 times lower than those of coal (GHGcoal∼96.8). To help to reduce the substantial variability in results, this review suggests a standardization of the assumptions about methodological issues. Likewise, the development of a widely accepted framework toward a reliable analysis of energy in bioenergy production systems is most needed.

Production physiology and growth potential of poplars under short-rotation forestry culture

The possibility of the genus Populus in reaching impressive growth performance and high productivity levels is partly based on our improved insight in its production physiology at different levels, and on the fact that genetic variability has been optimally tapped and combined with the cultural management regime of short-rotation forestry. The present contribution is primarily a literature review of studies on the production physiology and growth potential of poplar, and on a number of past, and on-going studies concerning short-rotation poplar plantations. The selected studies from which results are being presented incorporate both coppiced and non-coppiced regimes, and field trials on former agricultural land as well as on marginal wasteland or landfill sites. A number of production physiological characteristics are being highlighted for various clones, hybrids and genotypes. The main focus is on the genotypic or clonal differences in the production performance and growth physiology in short-rotation, high-density systems. The point is being made that the optimal combination of superior, selected genotypes with the concept of intensive short-rotation forestry practices is the key to improved yield and productivity levels. Studies on the production physiology of poplar are relevant and crucial to back-up genetic selection programs and to maximize yield levels of short-rotation forestry culture. Poplar is the ideal prototype for such studies, but further research at the plantation or stand scale is still required.

Next generation of elevated [CO2] experiments with crops: A critical investment for feeding the future world

A rising global population and demand for protein-rich diets are increasing pressure to maximize agricultural productivity. Rising atmospheric [CO(2)] is altering global temperature and precipitation patterns, which challenges agricultural productivity. While rising [CO(2)] provides a unique opportunity to increase the productivity of C(3) crops, average yield stimulation observed to date is well below potential gains. Thus, there is room for improving productivity. However, only a fraction of available germplasm of crops has been tested for CO(2) responsiveness. Yield is a complex phenotypic trait determined by the interactions of a genotype with the environment. Selection of promising genotypes and characterization of response mechanisms will only be effective if crop improvement and systems biology approaches are closely linked to production environments, that is, on the farm within major growing regions. Free air CO(2) enrichment (FACE) experiments can provide the platform upon which to conduct genetic screening and elucidate the inheritance and mechanisms that underlie genotypic differences in productivity under elevated [CO(2)]. We propose a new generation of large-scale, low-cost per unit area FACE experiments to identify the most CO(2)-responsive genotypes and provide starting lines for future breeding programmes. This is necessary if we are to realize the potential for yield gains in the future.

Enhanced ozone strongly reduces carbon sink strength of adult beech (Fagus sylvatica) – Resume from the free-air fumigation study at Kranzberg Forest

Ground-level ozone (O(3)) has gained awareness as an agent of climate change. In this respect, key results are comprehended from a unique 8-year free-air O(3)-fumigation experiment, conducted on adult beech (Fagus sylvatica) at Kranzberg Forest (Germany). A novel canopy O(3) exposure methodology was employed that allowed whole-tree assessment in situ under twice-ambient O(3) levels. Elevated O(3) significantly weakened the C sink strength of the tree-soil system as evidenced by lowered photosynthesis and 44% reduction in whole-stem growth, but increased soil respiration. Associated effects in leaves and roots at the gene, cell and organ level varied from year to year, with drought being a crucial determinant of O(3) responsiveness. Regarding adult individuals of a late-successional tree species, empirical proof is provided first time in relation to recent modelling predictions that enhanced ground-level O(3) can substantially mitigate the C sequestration of forests in view of climate change.

A comparison among eucalypt, poplar and willow characteristics with particular reference to a coppice, growth-modelling approach.

Abstract A comparative overview is made of a suite of characteristics related to physiological processes of development, growth and productivity of three functionally related genera, namely eucalypt ( Eucalyptus ), poplar ( Populus ) and willow ( Salix ). The approach provides basic information for top-down as well as bottom-up models, with particular reference to coppice growth and productivity. Eucalypt, poplar and willow are similar in many aspects but differ in others. All three genera have similar indeterminate shoot growth, leaf life span and biomass development patterns. Information on the development of leaf area index is central to current models of biomass production. However, there are very few data in the literature on the time-course of leaf area development for short-rotation crops, particularly after coppicing. There is considerable evidence that coppicing schedules affect the spatial and temporal development of canopy structure. More information is needed on how canopy development in coppice regrowth differs from that in plantations of seedlings or first-rotation cuttings, if comparative assessment of the three genera in a longer term silvicultural context is to be meaningful. Key components are leaf size and photosynthetic rates during sprouting, in relation to carbohydrate status of the stool. The longer term responses must take into account the interaction between coppicing schedules and the susceptibility of new shoots to herbivory and disease.