Philip A. Fay

Primary production underpins much of ecosystem functioning and is often limited by nutrient and water availability. Ongoing global increases in nutrient inputs and interannual precipitation variability are thus likely to impact primary production, with unclear consequences for interannual variability in plant biomass. We used data from 35 grasslands in the Nutrient Network coordinated fertilization experiment to test if nutrient addition alters the response of plant biomass to year‐to‐year variation in growing season precipitation, and whether the effects of nutrient addition vary with factors such as site aridity or plant community composition. Nutrient addition increased biomass (by an average of 46% across sites), rain‐use efficiency (by 48%), and the sensitivity of biomass to interannual variation in precipitation (i.e., larger increase in biomass in wet years; by 13%), but did not change the strength of the interannual precipitation‐biomass correlation. Both sensitivity and correlation strength were higher in more arid sites, regardless of nutrient addition. These results suggest that grassland ecosystem functioning may become more variable from year to year as nutrient enrichment increases biomass sensitivity to precipitation and precipitation becomes more variable.

Global change is enriching terrestrial ecosystems with multiple nutrients and amplifying interannual variation in precipitation. Grassland productivity may be co‐limited by combinations of nitrogen (N), phosphorus (P), and potassium (K). How these nutrients may interact with each other or with varying precipitation to influence the contributions of dominant species and functional groups to aboveground net primary productivity (ANPP) and species diversity is rarely considered. We fertilized a mesic grassland for 5 years with all combinations of N, P, and K+ micronutrients in the first year (Kμ) to test which nutrients (1) limited ANPP and functional group biomass, (2) reordered dominant species and impacted plant species diversity, and (3) interacted with annual precipitation to influence these responses. Adding N and P together disproportionately increased ANPP, but adding both N and P or N and Kμ disproportionately increased forb biomass to account for nearly all (90%) of ANPP. Grass biomass was correlated with light availability, not nutrients, and legume biomass decreased with added N, with or without other nutrients. Nutrient combinations (mainly NP and NPKμ) causing the greatest increases in forb biomass and ANPP also resulted in replacement of dominant species by an annual forb and decreased species diversity (Shannon index), evenness, and species richness. Nutrient combinations (P, Kμ, PKμ) not increasing biomass favored dominance by C4 grasses and increased species richness. N effects on ANPP, species diversity, and richness were greater in years with higher annual precipitation. Annual precipitation interacted with all three nutrients to exert sometimes positive and sometimes negative feedback on the abundance of the most dominant species. Dominant species drive nutrient effects on community productivity and species diversity. An expanded definition of nutrient limitation incorporating constituent responses will improve understanding of anthropogenic nutrient inputs on ecosystem productivity and related ecosystem services.

The generality of the worldwide leaf economics spectrum (LES) has made it a pillar of trait-based ecological research. Yet, few studies have examined the processes shaping the evolution of the LES within species, in part, because most species occupy only a small portion of the LES. To address this gap, we took advantage of the distinct leaf economics strategies present in different ecotypes of the phenotypically diverse perennial grass Panicum virgatum (switchgrass) to generate a genetic mapping population, which we planted in common gardens at three sites spanning 12 degrees of latitude in the central United States. With this genetic mapping population, we evaluated two potentially interacting causes of LES evolution: 1) genetic architecture, where multiple traits are influenced by either the same gene (pleiotropy) or by genes in close physical proximity (genetic linkage), and 2) correlational selection, where selection acts on traits in combination rather than in isolation. We found that shared genetic architecture influenced covariation between photosynthetic rate ( A _MASS ) and leaf nitrogen ( N _MASS ) and between A _MASS and leaf mass per area (LMA). We also found that correlational selection favored the trait combinations predicted by the LES (e.g., high LMA with low N _MASS or low LMA with high N _MASS ) and disfavored other, mismatched trait combinations at two of the three sites. Together, these results demonstrate how the evolution of an integrated LES within species can arise from multiple evolutionary causes.

Questions: Grassland conversion to agriculture is a major driver of plant biodiversity loss. Repeated soil disturbances (e.g., tilling) and nutrient application (e.g., fertilization) common in many agricultural practices are thought to impose barriers limiting initial community reassembly. We experimentally disturbed and applied nutrients in a remnant tallgrass prairie to determine how these drivers influenced growing season trajectory of plant species richness and community composition.Location: Texas Blackland Prairie remnant grassland, Southern Great Plains, USA.Methods: We performed annual tilling and nutrient addition with a combination of nitrogen (N), phosphorus (P), and potassium (K) in a full‐factorial experiment that included (n = 5 per treatment): untreated controls (C), nutrient addition only (NPKμ), tilling only (D), and tilling plus nutrient addition (NPKμ + D). Plant species cover was surveyed monthly during two growing seasons to determine plant species richness and composition change. Differences in community composition among treatments were identified by ordination.Results: After 2 years of monthly growing season (April–August) plant community sampling, we found that species richness was significantly lower in tilled plots compared to untilled plots. The plant community in the D and NPKμ + D treatment shifted toward annual forb dominance, whereas populations of perennial grasses and forbs persisted in the C and NPKμ treatment. Annual forb dominance was most pronounced in the NPKμ + D treatment, where Ambrosia trifida established in near monocultures and seemingly prevented colonization of other species throughout the growing season. In these plots, species loss was greatest, whereas species gains and rates of composition change were lowest throughout the growing season.Conclusions: Our results show annual soil disturbance and nutrient addition controlled the trajectory of growing season plant species richness and community composition by favoring a single dominant annual forb in near‐monoculture, which prevented colonization by other species. The early application of practices that reduce soil nutrient availability and increase desired species propagule sources may be needed to prevent monoculture establishment and increase intraannual grassland plant diversity.

Summary: In coevolving species, parasites locally adapt to host populations as hosts locally adapt to resist parasites. Parasites often outpace host local adaptation since they have rapid life cycles, but host diversity, the strength of selection, and external environmental influence can result in complex outcomes. To better understand local adaptation in host–parasite systems, we examined locally adapted switchgrass (Panicum virgatum), and its leaf rust pathogen (Puccinia novopanici) across a latitudinal range in North America. We grew switchgrass genotypes in 10 replicated multiyear common gardens, measuring rust severity from natural infection in a ‘host reciprocal transplant’ framework for testing local adaptation. We conducted genome‐wide association mapping to identify genetic loci associated with rust severity. Genetically differentiated rust populations were locally adapted to northern and southern switchgrass, despite host local adaptation to environmental conditions in the same regions. Rust resistance was polygenic, and distinct loci were associated with rust severity in the north and south. We narrowed a previously identified large‐effect quantitative trait locus for rust severity to a candidate YELLOW STRIPE‐LIKE gene and linked numerous other loci to defense‐related genes. Overall, our results suggest that both hosts and parasites can be simultaneously locally adapted, especially when parasites impose less selection than other environmental factors.

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass–MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass–MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass–MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass–MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass–MAP relationship.

Summary: The leaf economics spectrum (LES) describes a suite of functional traits that consistently covary at large spatial and taxonomic scales. Despite its importance at these larger scales, few studies have examined the major drivers of intraspecific variation in the LES – phenotypic plasticity and standing genetic variation. Using experimental precipitation manipulations, we examined whether covariation among leaf economics traits and selection on leaf economics traits and trait combinations change as diverse genotypes of the widespread perennial grass Panicum virgatum are exposed to differences in precipitation. We also used RNA‐Seq to examine whether groups of co‐expressed genes that align with leaf economics traits function in processes hypothesized to underlie the LES. Water availability impacted leaf economics trait covariation in important ways – covariation between leaf economics traits and selection on covariation between traits (i.e. correlational selection) tended to be strongest when water availability was high. Additionally, many genes associated with leaf economics traits functioned in processes that may explain how the LES originates, such as chloroplasts, cell walls, and nitrogen metabolism. Water availability is likely an important modulator of selection and evolution of the LES in P. virgatum that can be better understood by examining gene expression.

The timing of vegetative and reproductive growth in plants (“phenological timings”) depend on genetic effects (G), environmental (e.g., weather) cues, and their interaction. Here, we measure phenological timings in two highly divergent switchgrass (Panicum virgatum) subpopulations using repeated plantings of cloned individuals at eight sites across the central United States. The timing of vegetative growth for the two subpopulations reversed between their two natural ranges and had strong negative correlations between these regions; in contrast, the timing of flowering was positively correlated between gardens. We expect that these phenotypic correlations consist of polygenic effects on phenology which have distinct patterns of GxE segregating at different mapped loci. Thus, we infer the mixture of ways genetic effects impact phenological timings, such as across common gardens (GxE) or with weather cues (GxWeather). We demonstrate that we can identify genetic variation with GxWeather and assign genetic loci to specific weather-based cues or other patterns. For example, in the Gulf subpopulation, 65% of genetic effects on the timing of vegetative growth covary with daylength 14 days prior to green-up date, and 33% of genetic effects on the timing of flowering covary with cumulative rainfall in the week prior to flowering. However, most variation in genetic effects cannot be attributed to variation in weather variables. Selective breeding for particular alleles at GxWeather loci could alter flowering responsiveness in a photoperiod or rainfall-specific way. More broadly, our approach refines the characterization of genotype-by-environment interactions and can be implemented in any species phenotyped in multiple environments.