Bruce Osborne

Land-use change to bioenergy production in Europe: implications for the greenhouse gas balance and soil carbon

Bioenergy from crops is expected to make a considerable contribution to climate change mitigation. However, bioenergy is not necessarily carbon neutral because emissions of CO2, N2O and CH4 during crop production may reduce or completely counterbalance CO2 savings of the substituted fossil fuels. These greenhouse gases (GHGs) need to be included into the carbon footprint calculation of different bioenergy crops under a range of soil conditions and management practices. This review compiles existing knowledge on agronomic and environmental constraints and GHG balances of the major European bioenergy crops, although it focuses on dedicated perennial crops such as Miscanthus and short rotation coppice species. Such second‐generation crops account for only 3% of the current European bioenergy production, but field data suggest they emit 40% to >99% less N2O than conventional annual crops. This is a result of lower fertilizer requirements as well as a higher N‐use efficiency, due to effective N‐recycling. Perennial energy crops have the potential to sequester additional carbon in soil biomass if established on former cropland (0.44 Mg soil C ha−1 yr−1 for poplar and willow and 0.66 Mg soil C ha−1 yr−1 for Miscanthus). However, there was no positive or even negative effects on the C balance if energy crops are established on former grassland. Increased bioenergy production may also result in direct and indirect land‐use changes with potential high C losses when native vegetation is converted to annual crops. Although dedicated perennial energy crops have a high potential to improve the GHG balance of bioenergy production, several agronomic and economic constraints still have to be overcome.

Resource competition in plant invasions: emerging patterns and research needs

Invasions by alien plants provide a unique opportunity to examine competitive interactions among plants. While resource competition has long been regarded as a major mechanism responsible for successful invasions, given a well-known capacity for many invaders to become dominant and reduce plant diversity in the invaded communities, few studies have measured resource competition directly or have assessed its importance relative to that of other mechanisms, at different stages of an invasion process. Here, we review evidence comparing the competitive ability of invasive species vs. that of co-occurring native plants, along a range of environmental gradients, showing that many invasive species have a superior competitive ability over native species, although invasive congeners are not necessarily competitively superior over native congeners, nor are alien dominants are better competitors than native dominants. We discuss how the outcomes of competition depend on a number of factors, such as the heterogeneous distribution of resources, the stage of the invasion process, as well as phenotypic plasticity and evolutionary adaptation, which may result in increased or decreased competitive ability in both invasive and native species. Competitive advantages of invasive species over natives are often transient and only important at the early stages of an invasion process. It remains unclear how important resource competition is relative to other mechanisms (competition avoidance via phenological differences, niche differentiation in space associated with phylogenetic distance, recruitment and dispersal limitation, indirect competition, and allelopathy). Finally, we identify the conceptual and methodological issues characterizing competition studies in plant invasions, and we discuss future research needs, including examination of resource competition dynamics and the impact of global environmental change on competitive interactions between invasive and native species.

Photosynthesis and nutrient‐use efficiency of barley in response to low arbuscular mycorrhizal colonization and addition of phosphorus

The effects of arbuscular mycorrhizal infection by Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe on growth and photosynthesis of barley (Hordeum vulgare L. cv. Manitou) were investigated in sand culture at five levels of calcium phosphate (50, 100, 200, 400 and 800 mg P kg(-1) ). Mycorrhizal infection was low and varied with P supply, declining from 3.3% at 50 mg P kg(-1) to 1.5% at the highest P concentration. In general, there were small differences in biomass between mycorrhizal (+AM) and non-mycorrhizal (-AM) barley but a significant reduction in dry mass of senesced leaves occurred in the +AM plants. Leaf P concentrations increased with P application, but did not differ between + AM and -AM plants. Although there were no differences in dry mass between + AM and -AM plants at 50 mg P kg(-1) , it was at this lowest P supply that +AM plants had higher rates of photosynthesis and greater P-use and N-use efficiencies. The mycorrhizal enhancement of maximum photosynthetic rate at the lowest P level was associated with a higher stomatal conductance, but was not related to increased leaf P or to changes in photon yield or the ratio of variable (FV) to maximum (FM) chlorophyll fluorescence.

Light absorption by a marine diatom: experimental observations and theoretical calculations of the package effect in a small Thalassiosira species

The relationship between in vivo light absorption efficiency of whole cells and in vitro absorption efficiency of algal pigments has been examined experimentally in the marine diatom Thalassiosira sp. In vitro absorption spectra were obtained for cells disrupted by either ultrasonic treatment or high-pressure shearing stress in a low-temperature (-40°C) pressure cell. A dimensionless measure of the magnitude of the package effect (Qa*), calculated from the ratio of whole-cell to disrupted-cell absorption, ranged from about 0.5 at the blue absorption peak of chlorophyll a (λ=435 nm) to 0.7 at the red chlorophyll a peak (λ=670 nm) to 1.0 at the absorption minimum (λ=600 nm). Cell diameter was found to be an inappropriate measure of size for assessing the magnitude of the package effect. Instead, the effective optical diameter for calculation of intracellular self-shading was found to be less than the cell diameter. This observation is consistent with the fact that most algal pigments are contained within chloroplasts, and that chloroplast volume is necessarily smaller than cell volume.

Gunnera tinctoria: An Unusual Nitrogen-fixing InvaderThis water-loving species may offer insights into the development of terrestrial plants

toria indicate that the plant is highly dependent on the cyanobacterium Nostoc punctiforme to meet its nitrogen needs. Puzzling aspects of the symbiosis and nitrogen fixation are being explored. This symbiotic relationship may have important evolutionary significance in the colonization of land by plants, given that Gunnera are among the oldest of angiosperm genera and the only angiosperm genus known to form a symbiotic association with a nitrogen-fixing cyanobacterium.

Rapid predictions of cold tolerance in Douglas-fir seedlings using chlorophyll fluorescence after freezing

Chlorophyll fluorescence measurements were performed on the foliage of 3-year-old (11/2+11/2) nursery-grown Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings after exposure to controlled freezing temperatures, in the laboratory, to assess low temperature tolerance. The seedlings were propagated in an Irish nursery and lifted at monthly intervals overwinter 1999 and 1999–2000. Excised shoots from first-order laterals were frozen, in the dark. After freezing, needles were immediately assessed using chlorophyll fluorescence. The excised shoots were then maintained under controlled conditions for 14 days and visually assessed for needle damage. The chlorophyll fluorescence parameter, Fv/Fm, accurately predicted cold hardiness and was linearly related to visual needle damage and short-term survival. An equation was constructed using Fv/Fm data for determining the LT50, that is, the freeze temperature causing 50% seedling damage. The predictions of F.LT50 (fluorescence-based empirical determination of LT50) have been tested over two seasons (i.e., against a second independent data set) with variability between 0 and 1.8 °C of visual estimates, though predictions were often ≤ 1.1 °C of the visual assessment. This approach provided a simple, rapid and accurate prediction of cold tolerance, under climatic conditions where in situ measurements are unreliable. The method can be used to predict if Douglas-fir seedlings have developed sufficient tolerance for lifting to the cold-store, or for planting.

Light absorption, photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 μmol m-2 s-1. Growth was lightsaturated at PFDs in excess of 100 μmol m-2 s-1, with a mean light-saturated growth rate at 23 °C of 1.5×10-5s-1 (1.2 d-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 μmol m-2s-1. Dark-respiration rates were independent of PFD, averaging 1.7×10-6 mol O2 mol-1 C s-1 (0.14 mol O2 mol-1 C d-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O2 mol-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10-3 m2 mg-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg-1 chl a between PFDs of 14 and 200 μmol m-2 s-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.

Simulating the impacts of land use in Northwest Europe on Net Ecosystem Exchange (NEE): The role of arable ecosystems, grasslands and forest plantations in climate change mitigation

In this study, we compared measured and simulated Net Ecosystem Exchange (NEE) values from three wide spread ecosystems in the southeast of Ireland (forest, arable and grassland), and investigated the suitability of the DNDC (the DeNitrification-DeComposition) model to estimate present and future NEE. Although, the field-DNDC version overestimated NEE at temperatures >5 °C, forest-DNDC under-estimated NEE at temperatures >5 °C. The results suggest that the field/forest DNDC models can successfully estimate changes in seasonal and annual NEE from these ecosystems. Differences in NEE were found to be primarily land cover specific. The annual NEE was similar for the grassland and arable sites, but due to the contribution of exported carbon, the soil carbon increased at the grassland site and decreased at the arable site. The NEE of the forest site was an order of magnitude larger than that of the grassland or arable ecosystems, with large amounts of carbon stored in woody biomass and the soil. The average annual NEE, GPP and Reco values over the measurement period were -904, 2379 and 1475 g C m(-2) (forest plantations), -189, 906 and 715 g C m(-2) (arable systems) and -212, 1653 and 1444 g C m(-2) (grasslands), respectively. The average RMSE values were 3.8 g C m(-2) (forest plantations), 0.12 g C m(-2) (arable systems) and 0.21 g C m(-2) (grasslands). When these models were run with climate change scenarios to 2060, predictions show that all three ecosystems will continue to operate as carbon sinks. Further, climate change may decrease the carbon sink strength in the forest plantations by up to 50%. This study supports the use of the DNDC model as a valid tool to predict the consequences of climate change on NEE from different ecosystems.

Problems in the assessment of the package effect in five small phytoplankters

The magnitude of the package effect in five small phytoplankters [Thalassiosira sp., Clone 2601 (an unidentified eucaryote), Nannochloris atomus, Synechococcus ‘Syn’ and Synechococcus WH 7803] was assessed by comparison of the absorption spectra of intact and disrupted cells. The package effect was considerably reduced with reductions in cell size and this was broadly in agreement with theoretical predictions based on Mie theory. However, the quantitative assessment of the package effect is confounded by an inability to assign attenuation (apparent absorption) measurements at λ=750 nm to either scattering or absorption. The magnitude of the apparent absorption at λ=750 nm was greatest with the smallest picoplankton species examined, and was reduced, but not eliminated, after cell disruption. Whilst the apparent absorption at λ=750 nm is commonly thought to be due to residual scattering losses, the available evidence does not exclude the possibility that this may be due in part to absorption by cells or cell constituents and this requires further examination. Although these difficulties are particularly evident with the small picoplankton species, there is no reason to expect that they will not complicate the assessment of the package effect in larger phytoplankton cells.

Timing is everything: does early and late germination favor invasions by herbaceous alien plants?

Plant invasions represent a unique opportunity to study the mechanisms underlying community assembly rules and species distribution patterns. While a superior competitive ability has often been proposed as a major driver of successful plant invasions, its significance depends crucially on the timing of any competitive interaction. We assess whether a mismatch in germination phenology can favor the establishment of alien species, allowing them to exploit vacant niches where competition is low. As well as having important effects on the survival, growth and fitness of a species, asymmetric competition and potential soil legacies resulting from early or late germination can also impact on species recruitment. However, early or late germination comes at a cost, increases the risks of exposure to unfavorable conditions and requires an enhanced abiotic resistance if it is to lead to successful establishment. While there are several anecdotal accounts of early and late germination for invasive species, there are limited comparative data with resident species growing under natural conditions. Available evidence from grassland communities indicates that a short-term germination advantage or priority (few days/weeks) provides invasive species with a strong competitive advantage over native species and is a critical factor in many invasions. While the exploitation of periods of low competition is a plausible mechanism for the successful establishment of many invasive plants, direct evidence for this strategy is still scarce. This is particularly true with regard to the exploitation of late germination niches. Consequently, long-term comparative monitoring of the germination phenology of invasive and native plants in situ is needed to assess its significance in a range of ecosystems and its impact on community dynamics.