Mechanisms of Overyielding and Coexistence in Diverse Tallgrass Prairie Communities

Plants compete for the same basic nutrient and water resources. According to the competitive exclusion principle, when a substantial overlap in resource pools exists, the best competitor for resources should drive all other species to extinction. The ability for plants to coexist in violation of the competitive exclusion principle is the “biodiversity paradox”. Coexistence is actually beneficial for plants: as species diversity increases, you typically see increases in plant biomass production (known as the biodiversity-productivity relationship). The mechanisms behind coexistence and the biodiversity-productivity relationship remain an ecological mystery. One hypothesis is that plants obtain water and nutrients from different places in the soil, which reduces competition and results in plants coexisting and thriving by exploiting more spaces in the soil. Another hypothesis is that plants alter the soil in which they grow to their own detriment by accumulating species-specific soil pathogens or reducing soil nutrient levels. These plant-altered soils reduce the growth of species that are becoming too dominant in a plant community, creating a plant-soil feedback (PSF) effect that maintains biodiversity and increases productivity. I explored the role of PSFs and niche partitioning in coexistence and the biodiversity-productivity relationship. I investigated 1) how PSFs affect the biodiversity-productivity relationship in controlled greenhouse experiments, 2) whether greenhouse experiments are the best method to measure the role of PSFs in biodiverse communities in the field, 3) how PSFs affect the biodiversity-productivity relationship in diverse plant communities in the field, and 4) how partitioning of soil nitrogen and soil water affect coexistence and plant productivity. Greenhouse experimentation suggested PSFs influence productivity and the biodiversity-productivity relationship, but PSFs when measured in the greenhouse were not correlated with PSFs that were measured in the field. This implies PSFs should be measured in the field when trying to predict coexistence or the biodiversity-productivity relationship as observed in the field. Our ability to predict coexistence and productivity in the field was slightly improved by the inclusion of PSFs. However, partitioning of soil water and soil nitrogen was strongly correlated with landscape productivity in the same system, indicating that PSFs are not the dominant mechanism of these phenomena

Plant-Soil Feedbacks Help Explain Biodiversity-Productivity Relationships

Species-rich plant communities can produce twice as much aboveground biomass as monocultures, but the mechanisms remain unresolved. We tested whether plant-soil feedbacks (PSFs) can help explain these biodiversity-productivity relationships. Using a 16-species, factorial field experiment we found that plants created soils that changed subsequent plant growth by 27% and that this effect increased over time. When incorporated into simulation models, these PSFs improved predictions of plant community growth and explained 14% of overyielding. Here we show quantitative, field-based evidence that diversity maintains productivity by suppressing plant disease. Though this effect alone was modest, it helps constrain the role of factors, such as niche partitioning, that have been difficult to quantify. This improved understanding of biodiversity-productivity relationships has implications for agriculture, biofuel production and conservation

Are Plant–Soil Feedbacks Caused by Many Weak Microbial Interactions?

We used high-throughput sequencing and multivariate analyses to describe soil microbial community composition in two four-year field plant–soil feedback (PSF) experiments in Minnesota, USA and Jena, Germany. In descending order of variation explained, microbial community composition differed between the two study sites, among years, between bulk and rhizosphere soils, and among rhizosphere soils cultivated by different plant species. To try to identify soil organisms or communities that may cause PSF, we correlated plant growth responses with the microbial community composition associated with different plants. We found that plant biomass was correlated with values on two multivariate axes. These multivariate axes weighted dozens of soil organisms, suggesting that PSF was not caused by individual pathogens or symbionts but instead was caused by \u27many weak\u27 plant–microbe interactions. Taken together, the results suggest that PSFs result from complex interactions that occur within the context of a much larger soil microbial community whose composition is determined by factors associated with \u27site\u27 or year, such as soil pH, soil type, and weather. The results suggest that PSFs may be highly variable and difficult to reproduce because they result from complex interactions that occur in the context of a larger soil microbial community

Calf and yearling prices in California and the western United States

This paper investigates spatial, quality and temporal factors impacting the pricing of calves and yearlings in the western United States using data from a satellite video auction and a hedonic regression framework. Results suggest that spatial price discounts received by western ranchers closely match reported shipping costs and, thus, are consistent with free-on-board pricing and competitive procurement. This study also identifies the presence of temporal price premiums, on average, for seller-offered forward contracts at video auctions. With respect to quality attributes, this study provides estimates of the marginal value associated with various quality attributes and management practices, including vaccination protocols, weaning, certified Angus beef candidates, and age and source verification. Finally, we show that the considerable year-to-year variability in estimated valuations for value-added attributes in hedonic regression models of cattle pricing can be linked to the stage of the cattle cycle, with premiums paid by buyers being attenuated when cattle inventories are high

Plant-soil feedbacks help explain plant community productivity

Plant community productivity tends to increase as species richness increases, but the mechanisms behind this biodiversity-productivity relationship are not fully understood. Plant-soil feedbacks (PSF) are a compelling potential mechanism of the biodiversity-productivity relationship because they can explain patterns of both underyielding and overyielding in diverse plant communities. To test the role of plant-soil feedbacks in the biodiversity-productivity relationship we measured all possible plant-soil feedbacks for sixteen species, and used the measured plant-soil feedbacks to predict plant community biomass production. We compared the predicted plant community biomass production to observed biomass production in a paired biodiversity-productivity experiment