Global simulation of bioenergy crop productivity: analytical framework and case study for switchgrass

A global energy crop productivity model that provides geospatially explicit quantitative details on biomass potential and factors affecting sustainability would be useful, but does not exist now. This study describes a modeling platform capable of meeting many challenges associated with global‐scale agro‐ecosystem modeling. We designed an analytical framework for bioenergy crops consisting of six major components: (i) standardized natural resources datasets, (ii) global field‐trial data and crop management practices, (iii) simulation units and management scenarios, (iv) model calibration and validation, (v) high‐performance computing (HPC) simulation, and (vi) simulation output processing and analysis. The HPC‐Environmental Policy Integrated Climate (HPC‐EPIC) model simulated a perennial bioenergy crop, switchgrass (Panicum virgatum L.), estimating feedstock production potentials and effects across the globe. This modeling platform can assess soil C sequestration, net greenhouse gas (GHG) emissions, nonpoint source pollution (e.g., nutrient and pesticide loss), and energy exchange with the atmosphere. It can be expanded to include additional bioenergy crops (e.g., miscanthus, energy cane, and agave) and food crops under different management scenarios. The platform and switchgrass field‐trial dataset are available to support global analysis of biomass feedstock production potential and corresponding metrics of sustainability.     

Evapotranspiration of annual and perennial biofuel crops in a variable climate

Eddy covariance measurements were made in seven fields in the Midwest USA over 4 years (including the 2012 drought year) to estimate evapotranspiration (ET) of newly established rain‐fed cellulosic and grain biofuel crops. Four of the converted fields had been managed as grasslands under the USDA's Conservation Reserve Program (CRP) for 22 years, and three had been in conventional agriculture (AGR) soybean/corn rotation prior to conversion. In 2009, all sites were planted to no‐till soybean except one CRP grassland that was left unchanged as a reference site; in 2010, three of the former CRP sites and the three former AGR sites were planted to annual (corn) and perennial (switchgrass and mixed‐prairie) grasslands. The annual ET over the 4 years ranged from 45% to 77% (mean = 60%) of the annual precipitation (848–1063 mm; November–October), with the unconverted CRP grassland having the highest ET (622–706 mm). In the fields converted to annual and perennial crops, the annual ET ranged between 480 and 639 mm despite the large variations in growing‐season precipitation and in soil water contents, which had strong effects on regional crop yields. Results suggest that in this humid temperate climate, which represents the US Corn Belt, water use by annual and perennial crops is not greatly different across years with highly variable precipitation and soil water availability. Therefore, large‐scale conversion of row crops to perennial biofuel cropping systems may not strongly alter terrestrial water balances.     

Bioenergy crop productivity and potential climate change mitigation from marginal lands in the United States: An ecosystem modeling perspective

Growing biomass feedstocks from marginal lands is becoming an increasingly attractive choice for producing biofuel as an alternative energy to fossil fuels. Here, we used a biogeochemical model at ecosystem scale to estimate crop productivity and greenhouse gas (GHG) emissions from bioenergy crops grown on marginal lands in the United States. Two broadly tested cellulosic crops, switchgrass, and Miscanthus, were assumed to be grown on the abandoned land and mixed crop‐vegetation land with marginal productivity. Production of biomass and biofuel as well as net carbon exchange and nitrous oxide emissions were estimated in a spatially explicit manner. We found that, cellulosic crops, especially Miscanthus could produce a considerable amount of biomass, and the effective ethanol yield is high on these marginal lands. For every hectare of marginal land, switchgrass and Miscanthus could produce 1.0–2.3 kl and 2.9–6.9 kl ethanol, respectively, depending on nitrogen fertilization rate and biofuel conversion efficiency. Nationally, both crop systems act as net GHG sources. Switchgrass has high global warming intensity (100–390 g CO₂eq l^?1 ethanol), in terms of GHG emissions per unit ethanol produced. Miscanthus, however, emits only 21–36 g CO₂eq to produce every liter of ethanol. To reach the mandated cellulosic ethanol target in the United States, growing Miscanthus on the marginal lands could potentially save land and reduce GHG emissions in comparison to growing switchgrass. However, the ecosystem modeling is still limited by data availability and model deficiencies, further efforts should be made to classify crop‐specific marginal land availability, improve model structure, and better integrate ecosystem modeling into life cycle assessment.     

Water‐use efficiency in a semi‐arid woodland with high rainfall variability

As the ratio of carbon uptake to water use by vegetation, water‐use efficiency (WUE) is a key ecosystem property linking global carbon and water cycles. It can be estimated in several ways, but it is currently unclear how different measures of WUE relate, and how well they each capture variation in WUE with soil moisture availability. We evaluated WUE in an Acacia‐dominated woodland ecosystem of central Australia at various spatial and temporal scales using stable carbon isotope analysis, leaf gas exchange and eddy covariance (EC) fluxes. Semi‐arid Australia has a highly variable rainfall pattern, making it an ideal system to study how WUE varies with water availability. We normalized our measures of WUE across a range of vapour pressure deficits using g₁, which is a parameter derived from an optimal stomatal conductance model and which is inversely related to WUE. Continuous measures of whole‐ecosystem g₁ obtained from EC data were elevated in the 3 days following rain, indicating a strong effect of soil evaporation. Once these values were removed, a close relationship of g₁ with soil moisture content was observed. Leaf‐scale values of g₁ derived from gas exchange were in close agreement with ecosystem‐scale values. In contrast, values of g₁ obtained from stable isotopes did not vary with soil moisture availability, potentially indicating remobilization of stored carbon during dry periods. Our comprehensive comparison of alternative measures of WUE shows the importance of stomatal control of fluxes in this highly variable rainfall climate and demonstrates the ability of these different measures to quantify this effect. Our study provides the empirical evidence required to better predict the dynamic carbon–water relations in semi‐arid Australian ecosystems.     

Agave as a biofuel feedstock in Australia

The opportunity for commercial production of Agave in Australia stems from the substantial carbohydrate and fibre content of Agave, the nature of carbohydrates stored, the pre‐existence of an Agave ethanol‐producing industry in Mexico, demand for biofuel feedstocks, impressive water‐use efficiencies of plants with the CAM pathway, and legislation mandating the ethanol content of fuel in Australia, where an estimated 748 ML will be required for blending with petrol by 2010–2011, compared with about 440 ML in 2009. Agave has potential as a crop for areas of seasonally limited rainfall in Australia, a judgement based upon desktop analyses and agronomic experience of growing Agave in Australia before 1915. Development of a viable Australian Agave farming system requires production be located in suitable regions, efficient propagation methods, mechanized production, and viable business plans at grower, processor and marketing levels. Growers and processors agree that Agave will not be grown commercially until plants are grown, maintained and harvested in Australia, product is produced and tested, and yield and risks are evaluated. To this end, Agave were imported into Australia from Mexico and a tissue culture propagation technique developed. A trial crop of Agave tequilana was planted in northern Queensland and CO₂ exchange, nitrogen and water dynamics, carbohydrate content, and system inputs and outputs are being monitored. The experience will be used to fine‐tune the farming system, assess production costs and develop robust life‐cycle assessments. Processing of plants from trials will test harvesting and transport infrastructure and will provide material to processors for testing. Samples will be provided to potential producers of value‐added products. An Australian Agave industry should provide opportunities for stimulating agronomic, scientific and commercial exchange between Australia and Mexico. Successful integration of Agave into Australian agriculture will require a biofuels‐focussed breeding programme in collaboration with Mexican researchers.     

Net ecosystem exchange of CO2 with rapidly changing high Arctic landscapes

High Arctic landscapes are expansive and changing rapidly. However, our understanding of their functional responses and potential to mitigate or enhance anthropogenic climate change is limited by few measurements. We collected eddy covariance measurements to quantify the net ecosystem exchange (NEE) of CO₂ with polar semidesert and meadow wetland landscapes at the highest latitude location measured to date (82°N). We coupled these rare data with ground and satellite vegetation production measurements (Normalized Difference Vegetation Index; NDVI) to evaluate the effectiveness of upscaling local to regional NEE. During the growing season, the dry polar semidesert landscape was a near‐zero sink of atmospheric CO₂ (NEE: ?0.3 ± 13.5 g C m^?2). A nearby meadow wetland accumulated over 300 times more carbon (NEE: ?79.3 ± 20.0 g C m^?2) than the polar semidesert landscape, and was similar to meadow wetland NEE at much more southerly latitudes. Polar semidesert NEE was most influenced by moisture, with wetter surface soils resulting in greater soil respiration and CO₂ emissions. At the meadow wetland, soil heating enhanced plant growth, which in turn increased CO₂ uptake. Our upscaling assessment found that polar semidesert NDVI measured on‐site was low (mean: 0.120–0.157) and similar to satellite measurements (mean: 0.155–0.163). However, weak plant growth resulted in poor satellite NDVI–NEE relationships and created challenges for remotely detecting changes in the cycling of carbon on the polar semidesert landscape. The meadow wetland appeared more suitable to assess plant production and NEE via remote sensing; however, high Arctic wetland extent is constrained by topography to small areas that may be difficult to resolve with large satellite pixels. We predict that until summer precipitation and humidity increases enough to offset poor soil moisture retention, climate‐related changes to productivity on polar semideserts may be restricted.     

Canopy scale measurements of CO2 and water vapor exchange along a precipitation gradient in southern Africa

Short‐term measurements of carbon dioxide, water, and energy fluxes were collected at four locations along a mean annual precipitation gradient in southern Africa during the wet (growing) season with the purpose of determining how the observed vegetation–atmosphere exchange properties are functionally related to the long‐term climatic conditions. This research was conducted along the Kalahari Transect (KT), one in the global set of International Geosphere‐Biosphere Program transects, which covers a north–south aridity gradient, all on a homogenous sand formation. Eddy covariance instruments were deployed on a permanent tower in Mongu, Zambia (879 mm of rainfall per year), as well as on a portable tower in Maun (460 mm yr^?1), Okwa River Crossing (407 mm yr^?1), and Tshane (365 mm yr^?1), Botswana for several days at each site. The relationships between CO₂ flux, F_c, and photosynthetically active radiation were described well by a hyperbolic fit to the data at all locations except for Mongu, the wettest site. Here, there appeared to be an air temperature effect on F_c. While daytime values of F_c routinely approached or exceeded ?20 μmol m^?2 s^?1 at Mongu, the magnitude of F_c remained less than ?10 μmol m^?2 s^?1 when the air temperature was above 27°C. Canopy resistances to water vapor transfer, r_c, displayed an overall decline from the wetter sites to the more arid sites, but the differences in r_c could be almost exclusively accounted for by the decrease in leaf area index (LAI) from north to south along the KT. Ecosystem water use efficiency (WUE), defined as the ratio of net carbon flux to evapotranspiration, showed a general decrease with increasing vapor pressure deficit, D, for all of the sites. The magnitudes of WUE at a given D, however, were dissimilar for the individual sites and were found to be stratified according to the position of the sites along the long‐term aridity gradient. For example, Mongu, which has the wettest climate, has a much lower WUE for like levels of D than Tshane, which historically has the most arid climate. Given the similar inferred stomatal resistances between the sites, the disparate carbon uptake behavior for the grass vs. woody vegetation is the likely cause for the observed differences in WUE along the aridity gradient. The short‐term flux measurements provide a framework for evaluating the vegetation's functional adaptation to the long‐term climate and provide information that may be useful for predicting the dynamic response of the vegetation to future climate change.     

Sweetcane (Erianthus arundinaceus) as a native bioenergy crop with environmental remediation potential in southern China: A review

Sweetcane (Erianthus arundinaceus [Retzius] Jeswiet) is an ecologically dominant warm‐season perennial grass native to southern China. It traditionally plays an important role in sugarcane breeding due to its excellent biological traits and genetic relatedness to sugarcane. Recent studies have shown that sweetcane has a great potential in bioenergy and environmental remediation. The objective of this paper is to review the current research on sweetcane biology, phenology, biogeography, agronomy, and conversion technology, in order to explore its development as a bioenergy crop with environmental remediation potential. Sweetcane is resistant to a variety of stressors and can adapt to different growth environments. It can be used for ecological restoration, soil and water conservation, contaminated land repairing, nonpoint source pollutants barriers in buffer strips along surface waters, and as an ornamental and remediation plant on roadsides and in wetlands. Sweetcane exhibits higher biomass yield, calorific value and cellulose content than other bioenergy crops under the same growth conditions, thereby indicating its superior potential in second‐generation biofuel production. However, research on sweetcane as a bioenergy plant is still in its infancy. More works need be conducted on breeding, cultivation, genetic transformation, and energy conversion technologies.     

Development and evaluation of ForestGrowth‐SRC a process‐based model for short rotation coppice yield and spatial supply reveals poplar uses water more efficiently than willow

Woody biomass produced from short rotation coppice (SRC) poplar (Populus spp.) and willow (Salix spp.) is a bioenergy feedstock that can be grown widely across temperate landscapes and its use is likely to increase in future. Process‐based models are therefore required to predict current and future yield potential that are spatially resolved and can consider new genotypes and climates that will influence future yield. The development of a process‐based model for SRC poplar and willow, ForestGrowth‐SRC, is described and the ability of the model to predict SRC yield and water use efficiency (WUE) was evaluated. ForestGrowth‐SRC was parameterized from a process‐based model, ForestGrowth for high forest. The new model predicted annual above ground yield well for poplar (r² = 0.91, RMSE = 1.46 ODT ha^?1 yr^?1) and willow (r² = 0.85, RMSE = 1.53 ODT ha^?1 yr^?1), when compared with measured data from seven sites in contrasting climatic zones across the United Kingdom. Average modelled yields for poplar and willow were 10.3 and 9.0 ODT ha^?1 yr^?1, respectively, and interestingly, the model predicted a higher WUE for poplar than for willow: 9.5 and 5.5 g kg^?1 respectively. Using regional mapped climate and soil inputs, modelled and measured yields for willow compared well (r² = 0.58, RMSE = 1.27 ODT ha^?1 yr^?1), providing the first UK map of SRC yield, from a process‐based model. We suggest that the model can be used for predicting current and future SRC yields at a regional scale, highlighting important species and genotype choices with respect to water use efficiency and yield potential.     

Productivity and water use efficiency of Agave americana in the first field trial as bioenergy feedstock on arid lands

Agave species are high‐yielding crassulacean acid metabolism (CAM) plants, some of which are grown commercially and recognized as potential bioenergy species for dry regions of the world. This study is the first field trial of Agave species for bioenergy in the United States, and was established to compare the production of Agave americana with the production of Agave tequilana and Agave fourcroydes, which are produced commercially in Mexico for tequila and fiber. The field trial included four experimental irrigation levels to test the response of biomass production to water inputs. After 3 years, annual production of healthy A. americana plants reached 9.3 Mg dry mass ha^?1 yr^?1 (including pup mass) with 530 mm of annual water inputs, including both rainfall and irrigation. Yields in the most arid conditions tested (300 mm yr^?1 water input) were 2.0–4.0 Mg dry mass ha^?1 yr^?1. Agave tequilana and Agave fourcroydes were severely damaged by cold in the first winter, and produced maximum yields of only 0.04 Mg ha^?1 yr^?1 and 0.26 Mg ha^?1 yr^?1, respectively. The agave snout weevil (Scyphophorus acupunctatus) emerged as an important challenge for A. americana cropping, killing a greater number of plants in the higher irrigation treatments. Physiological differences in A. americana plants across irrigation treatments were most evident in the warmest season, with gas exchange beginning up to 3 h earlier and water use efficiency declining in treatments with the greatest water input (780 mm yr^?1 water input). Yields were lower than previous projections for Agave species, but results from this study suggest that A. americana has potential as a bioenergy crop and would have substantially reduced irrigation requirements relative to conventional crops in the southwestern USA. Challenges for pest management and harvesting must still be addressed before an efficient production system that uses Agave can be realized.     

Four‐year measurement of net ecosystem gas exchange of switchgrass in a Mediterranean climate after long‐term arable land use

Switchgrass (Panicum virgatum L.) is a perennial lignocellulosic crop that has gained large interest as a feedstock for advanced biofuels. Using an eddy covariance system, we monitored the net ecosystem gas exchange in a 5‐ha rainfed switchgrass crop located in the Po River Valley for four consecutive years after land‐use change from annual food crops. Switchgrass absorbed 58.2 Mg CO₂ ha^?1 year^?1 (GPP—gross primary production), of which 24.5 (42%) were fixed by the ecosystem (NEE—net ecosystem exchange). Cumulated NEE was negative (i.e. C sink) even in the establishment year when biomass and canopy photosynthesis are considerably lower compared to the following years. Taking into account the last 3 years only (postestablishment years), mean NEE was ?26.9 Mg CO₂ ha^?1 year^?1. When discounted of the removed switchgrass biomass, ecosystem CO₂ absorption was still high and corresponded to ?8.4 Mg CO₂ ha^?1 year^?1. The estimation of the life cycle global warming effect made switchgrass an even greater sink (?12.4 Mg CO₂ ha^?1 year^?1), thanks to the credits obtained with fossil fuels displacement. Water use efficiency (WUE), that is the ratio of NEE to the water used by the crop as the flux of transpiration (ET), corresponded to 1.6 mg C g^?1 of H₂O, meaning that, on average, 170 m³ of water was needed to fix 1 Mg of CO₂. Again, considering only the postestablishment years, WUE was 1.7 mg C g^?1 of H₂O. In the end, about half of annual precipitation was used by the crop every year. We conclude that switchgrass can be a valuable crop to capture significant amount of atmospheric CO₂ while preserving water reserves and estimated that its potential large‐scale deployment in the Mediterranean could lead to an annual greenhouse gas emission reduction up to 0.33% for the EU.     

Highly efficient de novo mutant identification in a Sorghum bicolor TILLING population using the ComSeq approach

Screening large populations for carriers of known or de novo rare single nucleotide polymorphisms (SNPs) is required both in Targeting induced local lesions in genomes (TILLING) experiments in plants and in screening of human populations. We previously suggested an approach that combines the mathematical field of compressed sensing with next‐generation sequencing to allow such large‐scale screening. Based on pooled measurements, this method identifies multiple carriers of heterozygous or homozygous rare alleles while using only a small fraction of resources. Its rigorous mathematical foundations allow scalable and robust detection, and provide error correction and resilience to experimental noise. Here we present a large‐scale experimental demonstration of our computational approach, in which we targeted a TILLING population of 1024 Sorghum bicolor lines to detect carriers of de novo SNPs whose frequency was less than 0.1%, using only 48 pools. Subsequent validation confirmed that all detected lines were indeed carriers of the predicted mutations. This novel approach provides a highly cost‐effective and robust tool for biologists and breeders to allow identification of novel alleles and subsequent functional analysis.     

Climate change drives a shift in peatland ecosystem plant community: Implications for ecosystem function and stability

The composition of a peatland plant community has considerable effect on a range of ecosystem functions. Peatland plant community structure is predicted to change under future climate change, making the quantification of the direction and magnitude of this change a research priority. We subjected intact, replicated vegetated poor fen peat monoliths to elevated temperatures, increased atmospheric carbon dioxide (CO₂), and two water table levels in a factorial design to determine the individual and synergistic effects of climate change factors on the poor fen plant community composition. We identify three indicators of a regime shift occurring in our experimental poor fen system under climate change: nonlinear decline of Sphagnum at temperatures 8 °C above ambient conditions, concomitant increases in Carex spp. at temperatures 4 °C above ambient conditions suggesting a weakening of Sphagnum feedbacks on peat accumulation, and increased variance of the plant community composition and pore water pH through time. A temperature increase of +4 °C appeared to be a threshold for increased vascular plant abundance; however the magnitude of change was species dependent. Elevated temperature combined with elevated CO₂ had a synergistic effect on large graminoid species abundance, with a 15 times increase as compared to control conditions. Community analyses suggested that the balance between dominant plant species was tipped from Sphagnum to a graminoid‐dominated system by the combination of climate change factors. Our findings indicate that changes in peatland plant community composition are likely under future climate change conditions, with a demonstrated shift toward a dominance of graminoid species in poor fens.     

Engineering plants to reflect light: strategies for engineering water‐efficient plants to adapt to a changing climate

Population growth and globally increasing standards of living have put a significant strain on the energy–food–water nexus. Limited water availability particularly affects agriculture, as it accounts for over 70% of global freshwater withdrawals (Aquastat). This study outlines the fundamental nature of plant water consumption and suggests a >50% reduction in renewable freshwater demand is possible by engineering more reflective crops. Furthermore, the decreased radiative forcing resulting from the greater reflectivity of crops would be equivalent to removing 10–50 ppm CO₂ from the atmosphere. Recent advances in engineering optical devices and a greater understanding of the mechanisms of biological reflectance suggest such a strategy may now be viable. Here we outline the challenges involved in such an effort and suggest three potential approaches that could enable its implementation. While the local benefits may be straightforward, determining the global externalities will require careful modelling efforts and gradually scaled field trials.     

Environmental implications of the use of agro‐industrial residues for biorefineries: application of a deterministic model for indirect land‐use changes

Biorefining agro‐industrial biomass residues for bioenergy production represents an opportunity for both sustainable energy supply and greenhouse gas (GHG) emissions mitigation. Yet, is bioenergy the most sustainable use for these residues? To assess the importance of the alternative use of these residues, a consequential life cycle assessment (LCA) of 32 energy‐focused biorefinery scenarios was performed based on eight selected agro‐industrial residues and four conversion pathways (two involving bioethanol and two biogas). To specifically address indirect land‐use changes (iLUC) induced by the competing feed/food sector, a deterministic iLUC model, addressing global impacts, was developed. A dedicated biochemical model was developed to establish detailed mass, energy, and substance balances for each biomass conversion pathway, as input to the LCA. The results demonstrated that, even for residual biomass, environmental savings from fossil fuel displacement can be completely outbalanced by iLUC, depending on the feed value of the biomass residue. This was the case of industrial residues (e.g. whey and beet molasses) in most of the scenarios assessed. Overall, the GHGs from iLUC impacts were quantified to 4.1 t CO₂‐eq.ha^?1_demanded yr^?1 corresponding to 1.2–1.4 t CO₂‐eq. t^?1 dry biomass diverted from feed to energy market. Only, bioenergy from straw and wild grass was shown to perform better than the alternative use, as no competition with the feed sector was involved. Biogas for heat and power production was the best performing pathway, in a short‐term context. Focusing on transport fuels, bioethanol was generally preferable to biomethane considering conventional biogas upgrading technologies. Based on the results, agro‐industrial residues cannot be considered burden‐free simply because they are a residual biomass and careful accounting of alternative utilization is a prerequisite to assess the sustainability of a given use. In this endeavor, the iLUC factors and biochemical model proposed herein can be used as templates and directly applied to any bioenergy consequential study involving demand for arable land.     

Aphid–willow interactions in a high Arctic ecosystem: responses to raised temperature and goose disturbance

Recently, there have been several studies using open top chambers (OTCs) or cloches to examine the response of Arctic plant communities to artificially elevated temperatures. Few, however, have investigated multitrophic systems, or the effects of both temperature and vertebrate grazing treatments on invertebrates. This study investigated trophic interactions between an herbivorous insect (Sitobion calvulum, Aphididae), a woody perennial host plant (Salix polaris) and a selective vertebrate grazer (barnacle geese, Branta leucopsis). In a factorial experiment, the responses of the insect and its host to elevated temperatures using open top chambers (OTCs) and to three levels of goose grazing pressure were assessed over two summer growing seasons (2004 and 2005). OTCs significantly enhanced the leaf phenology of Salix in both years and there was a significant OTC by goose presence interaction in 2004. Salix leaf number was unaffected by treatments in both years, but OTCs increased leaf size and mass in 2005. Salix reproduction and the phenology of flowers were unaffected by both treatments. Aphid densities were increased by OTCs but unaffected by goose presence in both years. While goose presence had little effect on aphid density or host plant phenology in this system, the OTC effects provide interesting insights into the possibility of phenological synchrony disruption. The advanced phenology of Salix effectively lengthens the growing season for the plant, but despite a close association with leaf maturity, the population dynamics of the aphid appeared to lack a similar phenological response, except for the increased population observed.     

Water use and yield of bioenergy poplar in future climates: modelling the interactive effects of elevated atmospheric CO2 and climate on productivity and water use

Vegetation exerts large control on global biogeochemical cycles through the processes of photosynthesis and transpiration that exchange CO₂ and water between the land and the atmosphere. Increasing atmospheric CO₂ concentrations exert direct effects on vegetation through enhanced photosynthesis and reduced stomatal conductance, and indirect effects through changes in climatic variables that drive these processes. How these direct and indirect CO₂ impacts interact with each other to affect plant productivity and water use has not been explicitly analysed and remains unclear, yet is important to fully understand the response of the global carbon cycle to future climate change. Here, we use a set of factorial modelling experiments to quantify the direct and indirect impacts of atmospheric CO₂ and their interaction on yield and water use in bioenergy short rotation coppice poplar, in addition to quantifying the impact of other environmental drivers such as soil type. We use the JULES land‐surface model forced with a ten‐member ensemble of projected climate change for 2100 with atmospheric CO₂ concentrations representative of the A1B emissions scenario. We show that the simulated response of plant productivity to future climate change was nonadditive in JULES, however this nonadditivity was not apparent for plant transpiration. The responses of both growth and transpiration under all experimental scenarios were highly variable between sites, highlighting the complexity of interactions between direct physiological CO₂ effects and indirect climate effects. As a result, no general pattern explaining the response of bioenergy poplar water use and yield to future climate change could be discerned across sites. This study suggests attempts to infer future climate change impacts on the land biosphere from studies that force with either the direct or indirect CO₂ effects in isolation from each other may lead to incorrect conclusions in terms of both the direction and magnitude of plant response to future climate change.     

Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi‐arid ecosystem

Plant–plant interactions are among the fundamental processes that shape structure and functioning of arid and semi‐arid plant communities. Despite the large amount of studies that have assessed the relationship between plant–plant interactions (i.e., facilitation and competition) and diversity, often researchers forget a third kind of interaction, known as allelopathy. We examined the effect of plant–plant interactions of three dominant species: the perennial grass Lygeum spartum, the allelopathic dwarf shrub Artemisia herba‐alba, and the nurse shrub Salsola vermiculata, on plant diversity and species composition in a semi‐arid ecosystem in NE Spain. Specifically, we quantified the interaction outcome (IO) based on species co‐occurrence, we analyzed diversity by calculation of the individual species–area relationship (ISAR), and compositional changes by calculation of the Chao‐Jaccard similarity index. We found that S. vermiculata had more positive IO values than L. spartum, and A. herba‐alba had values between them. Lygeum spartum and A. herba‐alba acted as diversity repellers, whereas S. vermiculata acted as a diversity accumulator. As aridity increased, A. herba‐alba transitioned from diversity repeller to neutral and S. vermiculata transitioned from neutral to diversity accumulator, while L. spartum remained as diversity repeller. Artemisia herba‐alba had more perennial grass species in its local neighborhood than expected by the null model, suggesting some tolerance of this group to its “chemical neighbor”. Consequently, species that coexist with A. herba‐alba were very similar among different A. herba‐alba individuals. Our findings highlight the role of the nurse shrub S. vermiculata as ecosystem engineer, creating and maintaining patches of diversity, as well as the complex mechanism that an allelopathic plant may have on diversity and species assemblage. Further research is needed to determine the relative importance of allelopathy and competition in the overall interference of allelopathic plants.