Environmentally friendly approaches to increasing food production include using the positive effects of plant biodiversity, such as in intercropping. Functional traits are key drivers of these positive effects, in part because variation in functional traits can increase niche partitioning. However, we know little about how variation in functional traits affects the long-term stability of yield in agroecosystems. We conducted a five-year field experiment with five different cropping systems; maize/peanut, maize/soybean, and maize, peanut and soybean monocultures. We compared the productivity of monocultures to intercrops and then compared plasticity in functional traits at different rates of nitrogen supply between the cropping systems. Intercropping generated plasticity, measured here as the inverse of the coefficient of variation, in three functional traits of maize (height, stem diameter, ear height), which probably increases aboveground spatial niche differentiation, and decreases the intraspecific competition of maize. Intercropping also increased the stability of grain yield and aboveground biomass. Plasticity in functional traits of maize correlated positively with year-to-year temporal stability (CV-1) of grain yield and biomass of maize and with the total productivity of the agroecosystem. Synthesis and applications. Our study provides evidence of the greater productivity and temporal stability of species-diverse intercropping systems. Interspecific interaction-induced plasticity suggests a unique mechanism for biodiversity effects on ecosystem function, which adds to our understanding of fixed, or inherent, variation in traits among genotypes or species. Theoretically, our findings provide novel insights into how interspecific interactions contribute to ecosystem service, especially in yield temporal stability, by increased trait plasticity of the dominant crop, maize. The results also had implications for applying intercropping in the sustainable management of food-production systems with the use of more crop species. Greater stability in production has the potential to provide a stable income for farmers.

Year to year stability in crop production is a crucial aspect of feeding a growing global population. Evidence from natural ecosystems shows that increasing plant diversity generally increases the temporal stability of productivity; however, we have little knowledge of the mechanisms by which diversity affects stability. In fact, understanding the drivers of stability is a major knowledge gap in our understanding of biodiversity and ecosystem function in general. We varied resource inputs into crop monocultures and intercropping of maize/pea and maize/rapeseed for three years in field experiments to create a wide range of values for temporal stability, complementarity effects, selection effects, competition, and facilitation. We correlated whole-system temporal stability in productivity with these values and the stability of competitively subordinate species and competitively dominant species in the intercrops. We then used structural equation modeling (SEM), which combines complex path models with latent variables, to estimate how interspecific interactions for water, nitrogen, and phosphorus affected the relationships between stability and these values. Intercropping treatments did not increase stability, but the wide range of stability created by our experiments allowed us to explore the relationship of many factors with stability. Complementarity correlated positively with the temporal stability of grain yield and aboveground biomass, suggesting that either facilitative interactions or niche partitioning shifted over time in ways that promoted stability. Furthermore, the temporal stability of total productivity of intercropping relied most on the stability of more productive species. However, facilitation tested by relative interaction index (RII) independently did not correlate with stability, but the temporal stability of the whole system increased as the competitive effects of competitively dominant species (pea and rapeseed) on competitively subordinate species (maize) decreased, and was highest when these competitive effects were virtually zero. SEM indicated that as competition for soil nitrogen from competitively dominant species on competitively subordinate species decreased, the overall temporal stability of whole-system aboveground biomass increased. This stability then led to greater stability in grain production. Our findings indicate that complex shifts in complementarity and competitive intensities are likely to be key mechanisms that maintain temporal stability in species-diverse agriculture, and potentially in natural systems.

Aim: Drought stress, and its effects on the biogeography of vegetation, has focused primarily on water availability during the growing season, thus focusing primarly on summer. However, variation in rainfall continentality (i.e., the continental interior being insulated from oceanic influences) can produce striking vegetation differences. We aim to disentangle summer water balance from the influence of rainfall continentality on winter rainfall, to better understand how climate regulated the distributions of woody plants in the Western USA. Location: Western USA. Time period: Actual. Major taxa studied: Angiosperms and Conifers. Method: We used Redundancy Analysis (RDA) to investigate correlations between rainfall continentality, summer water balance, minimum winter temperature and length of growing season on the distributions of 130 tree and shrub species in 467 plots. Rainfall continentality was calculated using the Gams (1932) index, modified for winter precipitation, and summer water balance with the ratio of summer precipitation to temperature. We estimated Actual EvapoTranspiration (AET), Deficit (DEF), mean annual temperature and rainfall from global gridded datasets and correlated them with RDA axes. Results: Rainfall continentality measured with the Gams index and minimum temperatures best explained the contrast between oceanic vegetation in the Pacific Coast Ranges and continental vegetation in the Intermountain Region and Rocky Mountains. Growing Season Length (GSL) was the second strongest factor correlated with vegetation distributions. Summer water balance, despite being the most widely used climatic factor to assess drought stress in biogeography, was the third strongest factor correlating with vegetation classes of the western US. AET was equally correlated with RDA axes 1 and 3, and, thus, could not discriminate between the contrasts in the RDA. Main conclusions: Rainfall continentality measured with the winter Gams index provides a more precise metric than summer water balance for understanding how the biogeography of woody plants in the western USA is regulated by climate. Broadly integrating the Gams index of continentality into plant distributions may improve our understanding of biogeographical distributions, the evolution of subspecies in species that span coastal to interior regions, and predictions of responses to climate change.

1. Invasive species have the ability to rapidly adapt in the new regions where they are introduced. Classic evolutionary theory predicts that the accumulation of genetic differences over time in allopatric isolation may lead to reproductive incompatibilities resulting in decreases in reproductive success and, eventually, to speciation. However, experimental evidence for this theoretical prediction in the context of invasive species is lacking. We aimed to test for the potential of allopatry to determine reproductive success of invasive plants, by experimentally admixing genotypes from six different native and non-native regions of Centaurea solstitialis, an invasive forb for which preliminary studies have detected some degree of reproductive isolation between one native and non-native region. 2. We grew plants under common garden conditions and outcrossed individuals originating from different source populations in the native and introduced range to evaluate reproductive success in terms of seed to ovule ratio produced. We also assessed geographic and genetic isolation among C. solstitialis regions as a potential driving factor of reproductive success. 3. Experimental admixture generated mixed fitness effects, including significant increases, decreases, and no differences in reproductive success as compared to crosses within population (control). Centaurea solstitialis invasive populations in the Americas generated preponderantly negative fitness interactions, regardless of the pollen source, suggesting selection against immigrants and reinforcement. Other non-native populations (Australia) as well as individuals from the native range of Spain demonstrated an increase in fitness for between-region crosses, indicating inbreeding. These differences show an asymmetrical response to inter-regional gene flow, but no evidence of isolation by distance. 4. Synthesis. The speed of adaptation and the accumulation of reproductive incompatibilities among allopatric populations of invasive species might be more rapid than previously assumed. Our data shows a global mosaic of reproductive outputs, showcasing an array of evolutionary processes unfolding during colonization at large biogeographical scales.

AbstractArbuscular mycorrhizal (AM) fungi can exert a powerful influence on the outcome of plant–plant competition. Since some exotic plants interact differently with soil biota such as AM fungi in their new range, range-based shifts in AM responsiveness could shift competitive interactions between exotic and resident plants, although this remains poorly studied. We explored whether genotypes of the annual exotic Centaurea solstitialis (yellow starthistle), collected from populations across the native and non-native ranges, differed in responsiveness to AM fungi in the introduced range and whether range-based differences in mycorrhizal responsiveness affected how strongly C. solstitialis tolerated competition with the North American native bunchgrass, Stipa pulchra. Grown alone, C. solstitialis from both ranges derived only weak benefits from AM fungi. However, association with AM fungi was costly to plants when grown in competition with S. pulchra. The magnitude of the suppressive effect of AM fungi was greater for genotypes from native versus introduced populations. Synthesis. Many exotic invasive species are known to associate weakly with AM fungi, which may be beneficial in disturbed habitats where competition for resources is low. Our results indicate that reduced mycorrhizal associations may also benefit invaders in a competitive environment. Centaurea solstitialis were more strongly suppressed by established S. pulchra plants in the presence versus absence of AM fungi, but exotic genotypes were less suppressed than native genotypes. This suggests that AM fungi may contribute to invasion resistance in established native communities, but range-based shifts in the way exotic genotypes respond to AM fungal partners may counter such biotic resistance.

AbstractPlants interact simultaneously with each other and with soil biota, yet the relative importance of competition versus plant soil feedback (PSF) on plant performance is poorly understood. Using a meta-analysis of 38 published studies and 150 plant species, we show that effects of interspecific competition (either growing plants with a competitor or singly, or comparing inter- vs. intraspecific competition) and PSF (comparing home vs. away soil, live vs. sterile soil, or control vs. fungicide-treated soil) depended on treatments but were predominantly negative, broadly comparable in magnitude, and additive or synergistic. Stronger competitors experienced more negative PSF than weaker competitors when controlling for density (inter- to intraspecific competition), suggesting that PSF could prevent competitive dominance and promote coexistence. When competition was measured against plants growing singly, the strength of competition overwhelmed PSF, indicating that the relative importance of PSF may depend not only on neighbor identity but also density. We evaluate how competition and PSFs might interact across resource gradients; PSF will likely strengthen competitive interactions in high resource environments and enhance facilitative interactions in low resource environments. Finally, we provide a framework for filling key knowledge gaps and advancing our understanding of how these biotic interactions influence community structure.

Tolerance and suppression are distinct components of competition among plants, and recognizing how they affect competitive outcomes is important for understanding the mechanisms and consequences of competition. We used simulations informed by experimental trials to ask whether tolerance or suppression of competitors was more important for the survival of native plants experiencing competition with an exotic invasive species. When competition was pairwise, tolerance and suppression contributed equally to competitive rank in simulations. However, when multiple native genotypes competed together against an invader, the ability to tolerate competition was up to 50 times more important than the ability to suppress the invader. In two-competitor communities the chief advantage of suppressing competitors was a global decrease in their abundance, but this advantage did not exist in communities of multiple competitors—which is more representative of natural conditions —because decreased competitor abundance benefited all plants regardless of their competitive ability. We suggest that this concept is analogous to a ‘demolition derby,’ an automotive contest where participants attempt to have the last functional vehicle on the playing field. Because strong suppressors share the benefits of eliminating competitors with other remaining competitors, we propose that tolerance of competitors is more beneficial than suppression when competition occurs in a multiplayer scenario—in a demolition derby and in nature. This finding has implications for our understanding of how competition influences plant species coexistence, plant community structure, and invasion outcomes.

The enemy release hypothesis (ERH) attributes the success of some exotic plant species to reduced top-down effects of natural enemies in the non-native range relative to the native range. Many studies have tested this idea, but very few have considered the simultaneous effects of multiple kinds of enemies on more than one invasive species in both the native and non-native ranges. Here, we examined the effects of two important groups of natural enemies – insect herbivores and soil biota – on the performance of Tanacetum vulgare (native to Europe but invasive in the USA) and Solidago canadensis (native to the USA but invasive in Europe) in their native and non-native ranges, and in the presence and absence of competition. In the field, we replicated full-factorial experiments that crossed insecticide, T. vulgare-S. canadensis competition, and biogeographic range (Europe vs. USA) treatments. In greenhouses, we replicated full-factorial experiments that crossed soil sterilization, plant-soil feedback, and biogeographic range treatments. We evaluated the effects of experimental treatments on T. vulgare and S. canadensis biomass. The effects of natural enemies were idiosyncratic. In the non-native range and relative to populations in the native range, T. vulgare escaped the negative effects of insect herbivores but not soil biota, depending upon the presence of S. canadensis; and S. canadensis escaped the negative effects of soil biota but not insect herbivores, regardless of competition. Thus, biogeographic escape from natural enemies depended upon the enemies, the invader, and competition. By explicitly testing the ERH in terms of more than one kind of enemy, more than one invader, and more than one continent, this study enhances our nuanced perspective of how natural enemies can influence the performance of invasive species in their native and non-native ranges.

Comparing genetic diversity, genetic differentiation and performance between native and non-native populations has advanced our knowledge of contemporary evolution and its ecological consequences. However, such between-range comparisons can be complicated by high among-population variation within native and non-native ranges. For example, native vs. non-native comparisons between small and non-representative subsets of populations for species with very large distributions have the potential to mislead because they may not sufficiently account for within-range adaptation to climatic conditions, and demographic history that may lead to non-adaptive evolution. We used the cosmopolitan weed Conyza canadensis to study the interplay of adaptive and demographic processes across, to our knowledge, the broadest climatic gradient yet investigated in this context. To examine the distribution of genetic diversity, we genotyped 26 native and 26 non-native populations at 12 microsatellite loci. Furthermore, we recorded performance traits for 12 native and 13 non-native populations in the field and in the common garden. To analyze how performance was related to range and/or climate, we fit pedigree mixed-effects models. These models weighed the population random effect for co-ancestry to account for the influence of demographic history on phenotypic among-population differentiation. Genetic diversity was very low, selfing rates were very high, and both were comparable between native and non-native ranges. Non-native populations out-performed native populations in the field. However, our most salient result was that both neutral genetic differentiation and common garden performance were far more correlated with the climatic conditions from which populations originated than native vs. non-native range-affiliation. Including co-ancestry of our populations in our models greatly increased explained variance and our ability to detect significant main effects for among-population variation in performance. High propagule pressure and high selfing rates, in concert with the ability to adapt rapidly to climatic gradients, may have facilitated the global success of this weed. Neither native nor non-native populations were homogeneous groups but responded comparably to similar environments in each range. We suggest that studies of contemporary evolution should consider widely distributed and genotyped populations to disentangle native vs. non-native range-effects from varying adaptive processes within ranges and from potentially confounding effects of demographic history.

Plants interact simultaneously with each other and with soil biota, yet the relative importance of competition versus plant soil feedback (PSF) on plant performance is poorly understood. Using a meta-analysis of 38 published studies and 150 plant species, we show that effects of interspecific competition (either growing plants with a competitor or singly, or comparing inter- vs. intraspecific competition) and PSF (comparing home vs. away soil, live vs. sterile soil, or control vs. fungicide-treated soil) depended on treatments but were predominantly negative, broadly comparable in magnitude, and additive or synergistic. Stronger competitors experienced more negative PSF than weaker competitors when controlling for density (inter- to intraspecific competition), suggesting that PSF could prevent competitive dominance and promote coexistence. When competition was measured against plants growing singly, the strength of competition overwhelmed PSF, indicating that the relative importance of PSF may depend not only on neighbor identity but also density. We evaluate how competition and PSFs might interact across resource gradients; PSF will likely strengthen competitive interactions in high resource environments and enhance facilitative interactions in low resource environments. Finally, we provide a framework for filling key knowledge gaps and advancing our understanding of how these biotic interactions influence community structure.