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The role of plant diversity in reducing invasions has generated decades of debate. Diverse communities might be more resistant to invasion because the communities contain resident species that are functionally similar to the invader (limiting similarity), or multiple species use the range of available resources more effectively (complementarity) than single species. However, the correlation of native and exotic diversity often reverses, becoming positive, with increasing spatial and temporal scale, in a phenomenon called the invasion paradox. We addressed two groups of hypotheses related to this paradox, broadly that (1) functional diversity and identity resist invasion initially, via complementarity or limiting similarity; and (2) disturbance and propagule pressure weaken the effects of functional diversity and identity on invader success through time. Using long-term data from experimental serpentine grassland assemblages in California, we examined how the abundance of a high impact invader, yellow starthistle (Centaurea solstitialis), related to functional diversity, functional dissimilarity, pocket gopher disturbance, and propagule pressure. We also conducted a single-season experiment in which we seeded disturbed and undisturbed areas and quantified invader success the following year. Neither diversity, nor dissimilarity, nor disturbance significantly impacted the success of C. solstitialis during the years of this study. Instead, propagule pressurewas the singlemost important predictor of C. solstitialis abundance. We consolidated these findings into a novel conceptual model of invader success to illustrate how propagule input may outweigh community resistance through time, andwhat implications these dynamics have for the invasion paradox. [ABSTRACT FROM AUTHOR]

Community assembly processes are thought to shape the mean, spread, and spacing of functional trait values within communities. Two broad categories of assembly processes have been proposed: first, a habitat filter that restricts the range of viable strategies and second, a partitioning of microsites and/or resources that leads to a limit to the similarity of coexisting species. The strength of both processes may be dependent on conditions at a particular site and may change along an abiotic gradient. We sampled environmental variables and plant communities in 44 plots across the varied topography of a coastal California landscape. We characterized 14 leaf, stem, and root traits for 54 woody plant species, including detailed intraspecific data for two traits with the goal of understanding the connection between traits and assembly processes in a variety of environmental conditions. We examined the within-community mean, range, variance, kurtosis, and other measures of spacing of trait values. In this landscape, there was a topographically mediated gradient in water availability. Across this gradient we observed strong shifts in both the plot-level mean trait values and the variation in trait values within communities. Trends in trait means with the environment were due largely to species turnover, with intraspecific shifts playing a smaller role. Traits associated with a vertical partitioning of light showed a greater range and variance on the wet soils, while nitrogen per area, which is associated with water use efficiency, showed a greater spread on the dry soils. We found strong nonrandom patterns in the trait distributions consistent with expectations based on trait-mediated community assembly. There was a significant reduction in the range of six out of II leaf and stem functional trait values relative to a null model. For specific leaf area (SLA) we found a significant even spacing of trait values relative to the null model. For seed size we found a more platykurtic distribution than expected. These results suggest that both a habitat filter and a limit to the similarity of coexisting species can simultaneously shape the distribution of traits and the assembly of local plant communities. [ABSTRACT FROM AUTHOR]

Many plant invasion studies in grasslands suggest that resident plants that share functional traits with invaders can reduce invasion by competing for limiting resources. However, since invasion studies often occur in highly controlled plots or microcosms, it is unclear how heterogeneous site conditions alter competitive interactions under realistic scenarios. To explore how landscape heterogeneity affects biotic resistance provided by competitive resident plants, we conducted a field-based experiment across four sites in California grasslands. Plots contained naturally occurring populations of native Hemizonia congesta, but differed in other characteristics, including litter cover, annual grass cover, soil moisture, and species richness. We invaded plots with the functionally similar nonnative Centaurea solstitalis (yellow starthistle) and, at one site, supplemented one-half of the established plots with water to test the effects of increasing a limiting resource. As in simplified plots and microcosms, increasing H. congesta abundance reduced starthistle biomass by competing for limited soil moisture, but only in plots with high starthistle germination. We conclude that higher abundances of native H. congesta can reduce starthistle invasion in heterogeneous grasslands, but competition is also affected by both abiotic (soil moisture) and biotic (starthistle germination number) conditions that vary across sites., (© 2018 by the Ecological Society of America.)