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Precipitation events have been predicted and observed to become fewer, but larger, as the atmosphere warms. Water-limited ecosystems are especially sensitive to changes in water cycling, yet evidence suggests that productivity may either increase or decrease in response to precipitation intensification. Interactions among climate, soil properties, and vegetation type may explain different responses, but this is difficult to experimentally test over large spatial scales. Simulation modeling may reveal the mechanisms through which climate, soils, and vegetation interact to affect plant growth. We use an individual-based plant ecohydrological model to simulate the effects of 25%, 50%, and 100% increases in precipitation event sizes on water cycling and shrub, grass, and forb biomass in 200 shrub-steppe sites spanning 651,000 km2 of the Intermountain West, USA. Simulations did not change annual precipitation amounts and were performed for 0, 3, and 5 °C warming. Larger precipitation events decreased evaporation and 'pushed' water into shrub root zones in arid and semi-arid sites, but 'pushed' water below shrub root zones in mesic sites resulting in increased shrub biomass in arid and semi-arid, but not mesic, sites. Positive effects of precipitation intensification on shrub growth partially counteracted negative effects of warming. Grasses and forbs showed no consistent response to precipitation intensification. Results indicate that increased precipitation intensity creates a competitive advantage for shrubs in arid and semi-arid sites. This advantage results in greater shrub relative abundance and suggests that precipitation intensification contributes to woody plant encroachment observed globally in arid and semi-arid ecosystems. [ABSTRACT FROM AUTHOR]

Precipitation events are becoming more intense around the world, changing the way water moves through soils and plants. Plants that have, or create, roots that absorb more water under these conditions are likely to become more abundant (e.g., shrub encroachment). Yet, it remains difficult to predict species responses to climate change because we typically do not know where active roots are located or how much water they absorb. Here, we used water tracer injections in a field experiment to describe forb, grass, and shrub root distributions under low and high precipitation intensity treatments. To estimate how much water different active rooting distributions can absorb over time, we used a soil water flow model, and we compared our estimates of water uptake to aboveground plant growth. In low precipitation intensity plots, deep shrub roots were estimated to absorb the most water (93 mm yr-1) and shrubs had the greatest aboveground cover (27%). Grass root distributions were estimated to absorb an intermediate amount of water (80 mm yr-1) and grasses had intermediate aboveground cover (18%). Forb root distributions were estimated to absorb the least water (79 mm yr-1) and had the least aboveground cover (12% cover). In high precipitation intensity plots, shrub and forb roots moved in ways that increased their water uptake relative to grasses, predicting the increased aboveground growth of shrubs and forbs in these plots. In short, water uptake caused by different rooting distributions predicted plant aboveground cover. Our results suggest that detailed descriptions of active plant root distributions can predict plant growth responses to climate change in arid and semi-arid ecosystems. [ABSTRACT FROM AUTHOR]

As the atmosphere warms, precipitation events become larger, but less frequent. Yet, there is fundamental disagreement about how increased precipitation intensity will affect vegetation. Walter's two‐layer hypothesis and experiments testing it have demonstrated that precipitation intensity can increase woody plant growth. Observational studies have found the opposite pattern. Not only are the patterns contradictory, but inference is largely limited to grasslands and savannas. We tested the effects of increased precipitation intensity in a shrub‐steppe ecosystem that receives >30% of its precipitation as snow. We used 11 (8 × 8 m) shelters to collect and redeposit rain and snow as larger, more intense events. Total annual precipitation was the same in all plots, but each plot received different precipitation event sizes ranging from 1 to 18 mm. Over three growing seasons, larger precipitation event sizes increased soil water availability, sagebrush (Artemisia tridentata) stem radius, and canopy greenness, decreased new root growth in shallow soils, and had no effect on herbaceous plant cover. Thus, we found that increased precipitation intensity can increase soil water availability and woody plant growth in a cold semiarid system. Assuming that stem growth is positively correlated with shrub reproduction, establishment and spread, results suggest that increasing precipitation intensity may have contributed to the woody plant encroachment observed around the world in the past 50 yr. Further, continuing increases in precipitation intensity caused by atmospheric warming are likely to continue to contribute to shrub encroachment in the future. [ABSTRACT FROM AUTHOR]

It has become clear that plants can create soils that affect subsequent plant growth. However, because plant-soil feedbacks ( PSFs) are typically measured in monoculture experiments, it remains unclear to what extent PSFs affect plant growth in communities. Here we used data from a factorial PSF experiment to predict the biomass of 12 species grown in 162 plant community combinations. Five different plant growth models were parameterized with either monoculture biomass data (Null) or with PSF data ( PSF) and model predictions were compared to plant growth observed in communities. For each of the five models, PSF model predictions were closer to observed species biomass in communities than Null model predictions. PSFs, which were associated with a 28% difference in plant biomass across soil types, explained 10% more variance than Null models. Results provided strong support for a small role for PSFs in predicting plant growth in communities and suggest several reasons that PSFs, as traditionally measured in monoculture experiments, may overestimate PSF effects in communities. First, monoculture data used in Null models inherently includes 'self ' PSF effects. Second, PSFs must be large relative to differences in intrinsic growth rates among species to change competitive outcomes. Third, PSFs must vary among species to change species relative abundances. [ABSTRACT FROM AUTHOR]

Trophic cascades are important drivers of plant and animal abundances in aquatic and aboveground systems, but in soils trophic cascades have been thought to be of limited importance due to omnivory and other factors. Here we use a meta-analysis of 215 studies with 1526 experiments that measured plant growth responses to additions or removals of soil organisms to test how different soil trophic levels affect plant growth. Consistent with the trophic cascade hypothesis, we found that herbivores and plant pathogens (henceforth pests) decreased plant growth and that predators of pests increased plant growth. The magnitude of this trophic cascade was similar to that reported for aboveground systems. In contrast, we did not find evidence for trophic cascades in decomposer- and symbiont-based (henceforth mutualist) food chains. In these food chains, mutualists increased plant growth and predators of mutualists also increased plant growth, presumably by increasing nutrient cycling rates. Therefore, mutualists, predators of mutualists and predators of pests all increased plant growth. Further, experiments that added multiple organisms from different trophic levels also increased plant growth. As a result, across the dataset, soil organisms increased plant growth 29% and non-pest soil organisms increased plant growth 46%. Omnivory has traditionally been thought to confound soil trophic dynamics, but here we suggest that omnivory allows for a simplified perspective of soil food webs - one in which most soil organisms increase plant growth by preying on pests or increasing nutrient cycling rates. An implication of this perspective is that processes that decrease soil organism abundance (e.g. soil tillage) are likely to decrease aboveground productivity. [ABSTRACT FROM AUTHOR]

Global circulation models and empirical evidence suggest that precipitation events are likely to become more extreme across much of the globe. As most plant roots are in shallow soils, small but pervasive changes in precipitation intensity could be expected to cause large-scale shifts in plant growth, yet experimental tests of the effects of precipitation intensity are lacking. Here we show that, without changing the total amount of precipitation, small experimental increases in precipitation intensity can push soil water deeper into the soil, increase aboveground woody plant growth and decrease aboveground grass growth in a savannah system. These responses seemed to reflect the ability of woody plants to increase their rooting depths and competitively suppress grass growth. In many parts of the world, woody plant abundance has multiplied in the past 50-100 years, causing changes in fire, forage value, biodiversity and carbon cycling. Factors such as fire, grazing and atmospheric CO2 concentrations have become dominant explanations for this woody encroachment and semi-arid structure in general. Our results suggest that niche partitioning is also an important factor in tree-grass coexistence and that the woody plant encroachment observed over the past century may continue in the future should precipitation intensity increase. [ABSTRACT FROM AUTHOR]

Abstract: Plant–soil feedbacks are gaining attention for their ability to determine plant community development. Plant–soil feedback models and research assume that plant–soil interactions occur within days to weeks, yet, little is known about how quickly and to what extent plants change soil community composition. We grew a dominant native plant (Pseudoroegneria spicata) and a dominant non-native plant (Centaurea diffusa) separately in both native- and non-native-cultivated field soils to test if these species could overcome soil legacies and create new soil communities in the short-term. Soil community composition before and after plant growth was assessed in bulk and rhizosphere soils using phospholipid fatty acid analyses. Nematode abundance and mycorrhizal colonization were also measured following plant growth. Field-collected, native-cultivated soils showed greater bacterial, Gram (−), fungal, and arbuscular mycorrhizal PLFA abundance and greater PLFA diversity than field-collected, non-native-cultivated soils. Both plant species grew larger in native- than non-native-cultivated soils, but neither plant affected microbial composition in the bulk or rhizosphere soils after two months. Plants also failed to change nematode abundance or mycorrhizal colonization. Plants, therefore, appear able to create microbial legacies that affect subsequent plant growth, but contrary to common assumptions, the species in this study are likely to require years to create these legacies. Our results are consistent with other studies that demonstrate long-term legacies in soil microbial communities and suggest that the development of plant–soil feedbacks should be viewed in this longer-term context. [Copyright &y& Elsevier]

Plants can change soil biology, chemistry and structure in ways that alter subsequent plant growth. This process, referred to as plant–soil feedback (PSF), has been suggested to provide mechanisms for plant diversity, succession and invasion. Here we use three meta-analytical models: a mixed model and two Bayes models, one correcting for sampling dependence and one correcting for sampling and hierarchical dependence (delta-splitting model) to test these hypotheses. All three models showed that PSFs have medium to large negative effects on plant growth, and especially grass growth, the life form for which we had the most data. This supports the hypothesis that PSFs, through negative frequency dependence, maintain plant diversity, especially in grasslands. PSFs were also large and negative for annuals and natives, but the delta-splitting model indicated that more studies are needed for these results to be conclusive. Our results support the hypotheses that PSFs encourage successional replacements and plant invasions. Most studies were performed using monocultures of grassland species in greenhouse conditions. Future research should examine PSFs in plant communities, non-grassland systems and field conditions. [ABSTRACT FROM AUTHOR]

Exotic plants have been found to use allelochemicals, positive plant–soil feedbacks, and high concentrations of soil nutrients to exercise a competitive advantage over native plants. Under laboratory conditions, activated carbon (AC) has shown the potential to reduce these advantages by sequestering organic compounds. It is not known, however, if AC can effectively sequester organics or reduce exotic plant growth under field conditions. On soils dominated by exotic plants, we found that AC additions (1% AC by mass in the top 10 cm of soil) reduced concentrations of extractable organic C and N and induced consistent changes in plant community composition. The cover of two dominant exotics, Bromus tectorum and Centaurea diffusa, decreased on AC plots compared to that on control plots (14–8% and 4–0.1%, respectively), and the cover of native perennial grasses increased on AC plots compared to that on control plots (1.4–3% cover). Despite promising responses to AC by these species, some exotic species responded positively to AC and some native species responded negatively to AC. Consequently, AC addition did not result in native plant communities similar to uninvaded sites, but AC did demonstrate potential as a soil-based exotic plant control tool, especially for B. tectorum and C. diffusa. [ABSTRACT FROM AUTHOR]