Caroline Brophy

Ecosystem function enhanced by combining four functional types of plant species in intensively managed grassland mixtures: a 3‐year continental‐scale field experiment

1.A coordinated continental-scale field experiment across 31 sites was used to compare the biomass yield of monocultures and four species mixtures associated with intensively managed agricultural grassland systems. To increase complementarity in resource use, each of the four species in the experimental design represented a distinct functional type derived from two levels of each of two functional traits, nitrogen acquisition (N2-fixing legume or nonfixing grass) crossed with temporal development (fast-establishing or temporally persistent). Relative abundances of the four functional types in mixtures were systematically varied at sowing to vary the evenness of the same four species in mixture communities at each site and sown at two levels of seed density. 2.Across multiple years, the total yield (including weed biomass) of the mixtures exceeded that of the average monoculture in >97% of comparisons. It also exceeded that of the best monoculture (transgressive overyielding) in about 60% of sites, with a mean yield ratio of mixture to best-performing monoculture of 1·07 across all sites. Analyses based on yield of sown species only (excluding weed biomass) demonstrated considerably greater transgressive overyielding (significant at about 70% of sites, ratio of mixture to best-performing monoculture = 1·18). 3.Mixtures maintained a resistance to weed invasion over at least 3 years. In mixtures, median values indicate

Diversity–interaction modeling: estimating contributions of species identities and interactions to ecosystem function

We develop a modeling framework that estimates the effects of species identity and diversity on ecosystem function and permits prediction of the diversity-function relationship across different types of community composition. Rather than just measure an overall effect of diversity, we separately estimate the contributions of different species interactions. This is especially important when both positive and negative interactions occur or where there are patterns in the interactions. Based on different biological assumptions, we can test for different patterns of interaction that correspond to the roles of evenness, functional groups, and functional redundancy. These more parsimonious descriptions can be especially useful in identifying general diversity-function relationships in communities with large numbers of species. We provide an example of the application of the modeling framework. These models describe community-level performance and thus do not require separate measurement of the performance of individual species. This flexible modeling approach can be tailored to test many hypotheses in biodiversity research and can suggest the interaction mechanisms that may be acting.

Biodiversity effects on yield and unsown species invasion in a temperate forage ecosystem

BACKGROUND AND AIMS Current agricultural practices are based on growing monocultures or binary mixtures over large areas, with a resultant impoverishing effect on biodiversity at several trophic levels. The effects of increasing the biodiversity of a sward mixture on dry matter yield and unsown species invasion were studied. METHODS A field experiment involving four grassland species [two grasses--perennial ryegrass (Lolium perenne) and cocksfoot (Dactylis glomerata)--and two legumes--red clover (Trifolium pratense) and white clover (Trifolium repens)], grown in monocultures and mixtures in accordance with a simplex design, was carried out. The legumes were included either as single varieties or as one of two broad genetic-base composites. The experiment was harvested three times a year over three years; dry matter yield and yield of unsown species were determined at each harvest. Yields of individual species and interactions between all species present were estimated through a statistical modelling approach. KEY RESULTS Species diversity produced a strong positive yield effect that resulted in transgressive over-yielding in the second and third years. Using broad genetic-base composites of the legumes had a small impact on yield and species interactions. Invasion by unsown species was strongly reduced by species diversity, but species identity was also important. Cocksfoot and white clover (with the exception of one broad genetic-base composite) reduced invasion, while red clover was the most invaded species. CONCLUSIONS The results show that it is possible to increase, and stabilize, the yield of a grassland crop and reduce invasion by unsown species by increasing its species diversity.

An improved model to predict the effects of changing biodiversity levels on ecosystem function

Summary 1. The development of models of the relationship between biodiversity and ecosystem function (BEF) has advanced rapidly over the last 20 years, incorporating insights gained through extensive experimental work. We propose Generalised Diversity-Interactions models that include many of the features of existing models and have several novel features. Generalised Diversity-Interactions models characterise the contribution of two species to ecosystem function as being proportional to the product of their relative abundances raised to the power of a coefficient h. 2. A value of h < 1 corresponds to a stronger than expected contribution of species’ pairs to ecosystem functioning, particularly at low relative abundance of species. 3. Varying the value of h has profound consequences for community-level properties of BEF relationships, including: (i) saturation properties of the BEF relationship; (ii) the stability of ecosystem function across communities; (iii) the likelihood of transgressive overyielding. 4. For low values of h, loss of species can have a much greater impact on ecosystem functioning than loss of community evenness. 5. Generalised Diversity-Interactions models serve to unify the modelling of BEF relationships as they include several other current models as special cases. 6. Generalised Diversity-Interactions models were applied to seven data sets and three functions: total biomass (five grassland experiments), community respiration (one bacterial experiment) and nitrate leaching (one earthworm experiment). They described all the nonrandom structure in the data in six experiments, and most of it in the seventh experiment and so fit as well or better than competing BEF models for these data. They were significantly better than Diversity-Interactions models in five experiments. 7. Synthesis. We show that Generalized Diversity-Interactions models quantitatively integrate several methods that separately address effects of species richness, evenness and composition on ecosystem function. They describe empirical data at least as well as alternative models and improve the ability to quantitatively test among several theoretical and practical hypotheses about the effects of

Phylogenetically diverse grasslands are associated with pairwise interspecific processes that increase biomass

Biodiversity is an important determinant of primary productivity in experimental ecosystems. We combine two streams of research on understanding the effects of biodiversity on ecosystem function: quantifying phylogenetic diversity as a predictor of biodiversity effects in species-rich systems and the contribution of pairwise interspecific interactions to ecosystem function. We developed a statistical model that partitions the effect of biodiversity into effects due to community phylogenetic diversity and other community properties (e.g., average pairwise interaction, between- and within-functional-group effects, and so forth). The model provides phylogenetically based species-level explanations of differences in ecosystem response for communities with differing species composition. In two well-known grassland experiments, the model approach provides a parsimonious description of the effects of diversity as being due to the joint effect of the average pairwise statistical interaction and to community phylogenetic diversity. Effects associated with functional groupings of species in communities are largely explained by community phylogenetic diversity. The model approach quantifies a direct link between a measure of the evolutionary diversity of species and their interactive contribution to ecosystem function. It proves a useful tool in developing a mechanistic understanding of variation in ecosystem function.

Competitiveness of a native Rhizobium leguminosarum biovar trifolii strain for nodule occupancy is manifested during infection

The stages in the nodulation process that determined the competitiveness of R. leguminosarum bv. trifolii (Rlt) strain 20–15, which proved to be highly competitive for nodulation in Iceland fields tests over several years, is analysed. White clover (Trifolium repens L.) roots were inoculated with inoculum mixtures containing three strains (Rlt 20-15, Rlt 8-9 and Rlt 32-28) in different proportions and cell densities. Competitiveness in root colonization, formation of infection threads and nodule development was assessed for Rlt 20-15 and its weakest competitor, Rlt 32-28. ERIC-polymerase chain reaction (PCR) DNA fingerprinting was used to identify inoculated strains recovered from root surfaces and individual nodules. GFP or DsRed tagged strains were used to determine identity in root hairs and nodules. Both strains colonized the root equally at all inoculum ratios tested. But, Rlt 20-15 initiated significantly more infection threads and formed more nodules than Rlt 32-28. These results show that Rlt 20-15 expresses its nodulation competitiveness during infection, either at infection thread initiation or during successive growth in the infection threads. The data presented support earlier observations that this strain competed well in the field in spite of its inferior ability to survive in the soil.

Benefits of sward diversity for agricultural grasslands

Abstract A pan-European experiment carried out at 28 sites across Europe showed strong benefits of sward diversity in agricultural grasslands. We systematically varied the relative abundance of four agronomic plant species (sown species evenness), and found that 4-species mixtures yielded more forage than could be expected on the basis of the monoculture yields. Mixtures generally yielded more than even the best performing monoculture (transgressive overyielding). Mixtures strongly reduced the incidence of unsown species in the sward. These diversity effects were consistent over the wide range of environmental conditions and persisted over three harvest years and in highly fertilized conditions. These results indicate a strong potential for agronomic mixtures to contribute to more sustainable agricultural systems. Agronomic diversity can improve forage yield and reduce weed invasion in intensively managed grasslands, and may also enhance the provision of other ecosystem services.

The Agrodiversity Experiment: three years of data from a multisite study in intensively managed grasslands

Kirwan, L., Connolly, J., Brophy, C., Baadshaug, O. H., Belanger, G., Black, A., Carnus, T., Collins, R. P., Cop, J., Delgado, I., De Vliegher, A., Elgersma, A., Frankow-Lindberg, B. E., Golinski, P., Grieu, P., Gustavsson, A., Helgadottir, A., Hoglind, M., Huguenin-elie, O., Jorgensen, M., Kadziuliene, Z., Lunnan, T., Luscher, A., Kurki, P., Porqueddu, C., Sebastia, M. T., Thumm, U., Walmsley, D. & Finn, J. A. (2014). The Agrodiversity Experiment: three years of data from a multisite study in intensively managed grasslands. Ecology, 95 (9), [2680]

The Agrodiversity Experiment: three years of data from a multisite study in intensively managed grasslands Ecological Archives

Kirwan, L., Connolly, J., Brophy, C., Baadshaug, O. H., Belanger, G., Black, A., Carnus, T., Collins, R. P., Cop, J., Delgado, I., De Vliegher, A., Elgersma, A., Frankow-Lindberg, B. E., Golinski, P., Grieu, P., Gustavsson, A., Helgadottir, A., Hoglind, M., Huguenin-elie, O., Jorgensen, M., Kadziuliene, Z., Lunnan, T., Luscher, A., Kurki, P., Porqueddu, C., Sebastia, M. T., Thumm, U., Walmsley, D. & Finn, J. A. (2014). The Agrodiversity Experiment: three years of data from a multisite study in intensively managed grasslands. Ecology, 95 (9), [2680]

Catching up on global change: new ragweed genotypes emerge in elevated CO2 conditions

Resource uptake by neighboring plants can be an important driver of natural selection in a changing environment. As climate and resource conditions are altered, genotypes that dominate within mixed populations today may differ markedly from those in future landscapes. We tested whether and how the dominance of different genotypes of the allergenic plant, common ragweed, may change in response to projected atmospheric CO2 conditions. We grew twelve maternal lines in experimental stands at either ambient or twice-ambient levels of CO2. We then constructed a model that combines classical quantitative genetics theory with a set of a priori predictions about the relative performance of genotypes in the two treatments. Our findings show a complete reversal in the genotypic size hierarchy of ragweed plants in response to projected atmospheric CO2 conditions. Genotypes that are competitively suppressed in size at ambient levels become dominant under experimental doubling of CO2. Subordinated plants, in turn, boost their reproductive allocation to that of dominants, shrinking the fitness gap among all genotypes in high CO2. Extending our model to a contextual analysis framework, we further show that natural selection on size is reduced at elevated CO2, because an individuals position within the size hierarchy becomes less important for reproduction than it is in ambient conditions. Our work points to potential future ecological and evolutionary changes in this widespread allergenic plant.