Biodiversity: Net primary productivity relationships are eliminated by invasive species dominance.

Experiments often find that net primary productivity (NPP) increases with species richness when native species are considered. However, relationships may be altered by exotic (non-native) species, which are hypothesized to reduce richness but increase productivity (i.e., 'invasion-diversity-productivity paradox'). We compared richness-NPP relationships using a comparison of exotic versus native-dominated sites across the central USA, and two experiments under common environments. Aboveground NPP was measured using peak biomass clipping in all three studies, and belowground NPP was measured in one study with root ingrowth cores using root-free soil. In all studies, there was a significantly positive relationship between NPP and richness across native species-dominated sites and plots, but no relationship across exotic-dominated ones. These results indicate that relationships between NPP and richness depend on whether native or exotic species are dominant, and that exotic species are 'breaking the rules', altering richness-productivity and richness-C stock relationships after invasion.

The structure of root-associated fungal communities is related to the long-term effects of plant diversity on productivity.

Root-associated fungi could play a role in determining both the positive relationship between plant diversity and productivity in experimental grasslands, and its strengthening over time. This hypothesis assumes that specialized pathogenic and mutualistic fungal communities gradually assemble over time, enhancing plant growth more in species-rich than in species-poor plots. To test this hypothesis, we used high-throughput amplicon sequencing to characterize root-associated fungal communities in experimental grasslands of 1 and 15 years of age with varying levels of plant species richness. Specifically, we tested whether the relationship between fungal communities and plant richness and productivity becomes stronger with the age of the experimental plots. Our results showed that fungal diversity increased with plant diversity, but this relationship weakened rather than strengthened over the two time points. Contrastingly, fungal community composition showed increasing associations with plant diversity over time, suggesting a gradual build-up of specific fungal assemblages. Analyses of different fungal guilds showed that these changes were particularly marked in pathogenic fungi, whose shifts in relative abundance are consistent with the pathogen dilution hypothesis in diverse plant communities. Our results suggest that root-associated fungal pathogens play more specific roles in determining the diversity-productivity relationship than other root-associated plant symbionts.

Relationship between the vertical distribution of fine roots and residual soil nitrogen along a gradient of hardwood mixture in a conifer plantation.

In forest ecosystems, understanding the relationship between the vertical distribution of fine roots and residual soil nitrogen is essential for clarifying the diversity-productivity-water purification relationship. Vertical distributions of fine-root biomass (FRB) and concentrations of nitrate-nitrogen (NO3 -N) in soil water were investigated in a conifer plantation with three thinning intensities (Control, Weak and Intensive), in which hardwood abundance and diversity were low, moderate and high, respectively. Intensive thinning led to the lowest NO3 -N concentration in soil water at all depths (0-100 cm) and highest FRB at shallow depths (0-50 cm). The NO3 -N concentration at a given depth was negatively correlated with total FRB from the surface to the depth at which NO3 -N concentration was measured, especially at shallow depths, indicating that more abundant fine roots led to lower levels of downward NO3 -N leaching. FRB contributed positively to nitrogen content of hardwood leaves. These findings demonstrate that a hardwood mixture in conifer plantations resulted in sufficient uptake of NO3 -N from soil by well developed fine-root systems, and translocation to canopy foliage. This study suggests that productivity and water purification can be achieved through a hardwood mixture in conifer plantations.

Soil aggregate microbiomes steer plant community overyielding in ungrazed and intensively grazed grassland soils.

Plant and soil microbial community composition play a central role in maintaining ecosystem functioning. Most studies have focused on soil microbes in the bulk soil, the rhizosphere and inside plant roots, however, less is known about the soil community that exists within soil aggregates, and how these soil communities influence plant biomass production. Here, using field-conditioned soil collected from experimental ungrazed and grazed grasslands in Inner Mongolia, China, we examined the composition of microbiomes inside soil aggregates of various size classes, and determined their roles in plant-soil feedbacks (PSFs), diversity-productivity relationships, and diversity-dependent overyielding. We found that grazing induced significantly positive PSF effects, which appeared to be mediated by mycorrhizal fungi, particularly under plant monocultures. Despite this, non-additive effects of microbiomes within different soil aggregates enhanced the strength of PSF under ungrazed grassland, but decreased PSF strength under intensively grazed grassland. Plant mixture-related increases in PSF effects markedly enhanced diversity-dependent overyielding, primarily due to complementary effects. Selection effects played far less of a role. Our work suggests that PSF contributes to diversity-dependent overyielding in grasslands via non-additive effects of microbiomes within different soil aggregates. The implication of our work is that assessing the effectiveness of sustainable grassland restoration and management on soil properties requires inspection of soil aggregate size-specific microbiomes, as these are relevant determinants of the feedback interactions between soil and plant performance.

Enhanced light interception and light use efficiency explain overyielding in young tree communities.

Diverse plant communities are often more productive than mono-specific ones. Several possible mechanisms underlie this phenomenon but their relative importance remains unknown. Here we investigated whether light interception alone or in combination with light use efficiency (LUE) of dominant and subordinate species explained greater productivity of mixtures relative to monocultures (i.e. overyielding) in 108 young experimental tree communities. We found mixed-species communities that intercepted more light than their corresponding monocultures had 84% probability of overyielding. Enhanced LUE, which arose via several pathways, also mattered: the probability of overyielding was 71% when, in a mixture, species with higher 'inherent' LUE (i.e. LUE in monoculture) intercepted more light than species with lower LUE; 94% when dominant species increased their LUE in mixture; and 79% when subordinate species increased their LUE. Our results suggest that greater light interception and greater LUE, generated by inter and intraspecific variation, together drive overyielding in mixed-species forests.

Effects of genomic and functional diversity on stand-level productivity and performance of non-native Arabidopsis.

Biodiversity can affect the properties of groups of organisms, such as ecosystem function and the persistence of colonizing populations. Genomic data offer a newly available window to diversity, complementary to other measures like taxonomic or phenotypic diversity. We tested whether native genetic diversity in field experimental stands of Arabidopsis thaliana affected their aboveground biomass and fecundity in their colonized range. We constructed some stands of genotypes that we a priori predicted would differ in performance or show overyielding. We found no relationship between genetic diversity and stand total biomass. However, increasing stand genetic diversity increased fecundity in high-resource conditions. Polyculture (multiple genotype) stands consistently yielded less biomass than expected based on the yields of component genotypes in monoculture. This under-yielding was strongest in stands with late-flowering and high biomass genotypes, potentially due to interference competition by these genotypes. Using a new implementation of association mapping, we identified genetic loci whose diversity was associated with stand-level yield, revealing a major flowering time locus associated with under-yielding of polycultures. Our field experiment supports community ecology studies that find a range of diversity-function relationships. Nevertheless, our results suggest diversity in colonizing propagule pools can enhance population fitness. Furthermore, interference competition among genotypes differing in flowering time might limit the advantages of polyculture.

Responses of diversity, productivity, and stability to the nitrogen input in a tropical grassland.

Atmospheric nitrogen (N) deposition is a matter of serious concern for the structure and functioning of global ecosystems, but the effect of N application of species diversity (D), primary productivity (P), and stability (S) of tropical grassland ecosystems is not known. The present study reports the effects of different levels of N application on species composition, and the D, P, S, and their relationships in a tropical grassland. Within the experimental grassland, 72 1 × 1 m plots with 6 N-input levels and with 12 replicates, were established in 2013. For 3 yr, different doses of urea as a source of N were applied to the plots. Data on individuals and biomass of each species were recorded and statistically analyzed. The study revealed that the N applied caused variations in species composition, D, P, and S. Below the 90 kg N dose, D was positively related to P and S while, above this level, the relations were negative due to N-induced responses of species and functional group composition as well as biomass distribution among them. The optimum applied N levels for maximum D (50-60 kg N), P (120 kg N), and a positive relationship of S with D (up to 90 kg N treatment) suggested that the 90-kg N dose could be the maximum dose of N that the grassland can tolerate. Hence, N application should not exceed the 90-kg level for sustainability of the structure and functioning of tropical grassland ecosystems.

Soil microbiome mediates positive plant diversity-productivity relationships in late successional grassland species.

Which processes drive the productivity benefits of biodiversity remain a critical, but unanswered question in ecology. We tested whether the soil microbiome mediates the diversity-productivity relationships among late successional plant species. We found that productivity increased with plant richness in diverse soil communities, but not with low-diversity mixtures of arbuscular mycorrhizal fungi or in pasteurised soils. Diversity-interaction modelling revealed that pairwise interactions among species best explained the positive diversity-productivity relationships, and that transgressive overyielding resulting from positive complementarity was only observed with the late successional soil microbiome, which was both the most diverse and exhibited the strongest community differentiation among plant species. We found evidence that both dilution/suppression from host-specific pathogens and microbiome-mediated resource partitioning contributed to positive diversity-productivity relationships and overyielding. Our results suggest that re-establishment of a diverse, late successional soil microbiome may be critical to the restoration of the functional benefits of plant diversity following anthropogenic disturbance.

Plant domestication disrupts biodiversity effects across major crop types.

Plant diversity fosters productivity in natural ecosystems. Biodiversity effects might increase agricultural yields at no cost in additional inputs. However, the effects of diversity on crop assemblages are inconsistent, probably because crops and wild plants differ in a range of traits relevant to plant-plant interactions. We tested whether domestication has changed the potential of crop mixtures to over-yield by comparing the performance and traits of major crop species and those of their wild progenitors under varying levels of diversity. We found stronger biodiversity effects in mixtures of wild progenitors, due to larger selection effects. Variation in selection effects was partly explained by within-mixture differences in leaf size. Our results indicate that domestication might disrupt the ability of crops to benefit from diverse neighbourhoods via reduced trait variance. These results highlight potential limitations of current crop mixtures to over-yield and the potential of breeding to re-establish variance and increase mixture performance.

Biodiversity-ecosystem functioning relationships in a long-term non-weeded field experiment.

Many grassland biodiversity experiments show a positive relationship between biodiversity and ecosystem functioning, however, in most of these experiments plant communities are established by sowing and natural colonization is prevented by selective weeding of non-sown species. During ecosystem restoration, for example on abandoned fields, plant communities start on bare soil, and diversity is often manipulated in a single sowing event. How such initial plant diversity manipulations influence plant biodiversity development and ecosystem functioning is not well understood. We examined how relationships between taxonomic and functional diversity, biomass production and stability develop over 16 yr in non-weeded plots sown with 15 species, four species, or that were not sown. We found that sown plant communities become functionally similar to unsown, naturally colonized plant communities. However, initial sowing treatments had long-lasting effects on species composition and taxonomic diversity. We found only few relationships between biomass production, or stability in biomass production, and functional or taxonomic diversity, and the ones we observed were negative. In addition, the cover of dominant plant species was positively related to biomass production and stability. We conclude that effects of introducing plant species at the start of secondary succession can persist for a long time, and that in secondary succession communities with natural plant species dynamics diversity-functioning relationships can be weak or negative. Moreover, our findings indicate that in systems where natural colonization of species is allowed effects of plant dominance may underlie diversity-functioning relationships.

Anthropogenic versus fish-derived nutrient effects on seagrass community structure and function.

Humans are altering nutrient dynamics through myriad pathways globally. Concurrent with the addition of nutrients via municipal, industrial, and agricultural sources, widespread consumer exploitation is changing consumer-mediated nutrient dynamics drastically. Thus, altered nutrient dynamics can occur through changes in the supply of multiple nutrients, as well as through changes in the sources of these nutrients. Seagrass ecosystems are heavily impacted by human activities, with highly altered nutrient dynamics from multiple causes. We simulate scenarios of altered nutrient supply and ratios, nitrogen:phosphorus (N:P), from two nutrient sources in seagrass ecosystems: anthropogenic fertilizer and fish excretion. In doing so we tested expectations rooted in ecological theory that suggest the importance of resource dynamics for predicting primary producer dynamics. Ecosystem functions were strongly altered by artificial fertilizer (e.g., seagrass growth increased by as much as 140%), whereas plant/algae community structure was most affected by fish-mediated nutrients or the interaction of both treatments (e.g., evenness increased by ~140% under conditions of low fish nutrients and high anthropogenic nutrients). Interactions between the nutrient sources were found for only two of six response variables, and the ratio of nutrient supply was the best predictor for only one response. These findings show that seagrass structure and function are well predicted by supply of a single nutrient (either N or P). Importantly, no single nutrient best explained the majority of responses-measures of community structure were best explained by the primary limiting nutrient to this system (P), whereas measures of growth and density of the dominant producer in the system were best explained by N. Thus, while our findings support aspects of theoretical expectations, the complexity of producer community responses belies broad generalities, underscoring the need to manage for multiple simultaneous nutrients in these imperiled coastal ecosystems.

Facilitation contributes to Mediterranean woody plant diversity but does not shape the diversity-productivity relationship along aridity gradients.

The diversity-productivity relationship (humped-back model (HBM)) and the stress-gradient (SGH) hypotheses may be connected when productivity is limited primarily by aridity. We analytically connect both hypotheses and assess the contribution of facilitation to woody plant richness along the aridity gradient of the Western Mediterranean floristic region. We monitored regeneration niches of woody plants, obtaining rarefied species richness and plant relative interaction indices in 54 forests and scrublands in a 1750-km geographical range across Spain, Morocco and the Canary Islands. We verified the monotonic increase in facilitation with aridity postulated by SGH and the humped-shape pattern of species richness expected from HBM, which became manifest after expanding the aridity gradient or crossing vegetation types. Along the gradient, interaction balance turned into facilitation earlier in forest than in scrublands. The effects of aridity and interaction balance on species diversity were additive rather than interdependent. Facilitation is an important driver of woody species richness at macroecological scales because it added up to diversity in most sites, with enhanced contribution with increased stress. The HBM was not shaped by species interactions. Results suggest that facilitation may act in Mediterranean vegetation buffering against critical transitions between states allowing woody plant communities to cope with the rise in aridity expected with global warming.

Intraspecific diversity buffers the inhibitory effects of soil biota.

Plant community productivity can increase with increasing intraspecific genotypic diversity. Previous studies have attributed the genetic diversity-productivity pattern to differential resource use among genotypes, as many studies have found for species. But here we ask whether suppression of productivity at low intraspecific diversity by soil biota might also drive a positive diversity-productivity relationship. In a previous study, we manipulated genetic diversity by varying the number of Pseudoroegneria accessions growing together in common garden plots, and used soil from that experiment to evaluate soil feedbacks. The total biomass of P. spicata plants grown in unsterilized soil increased with accession richness, specifically when comparing soil that had contained plants from 3 accessions to soil that had contained plants from either 8 or 12 population accessions. Furthermore, soil from high-richness (8 or 12-accession) plots drove neutral feedbacks, whereas soil in the 3-accession plots (3) drove negative feedbacks. However, within each level of richness, there was no relationship between relative yield and feedback. Our results suggest that soil biota might play an integral role in the emerging understanding of the relationship between intraspecific diversity and ecosystem productivity.

Intraspecific genetic diversity and composition modify species-level diversity-productivity relationships.

Biodiversity regulates ecosystem functions such as productivity, and experimental studies of species mixtures have revealed selection and complementarity effects driving these responses. However, the impacts of intraspecific genotypic diversity in these studies are unknown, despite it forming a substantial part of the biodiversity. In a glasshouse experiment we constructed plant communities with different levels of barley (Hordeum vulgare) genotype and weed species diversity and assessed their relative biodiversity effects through additive partitioning into selection and complementarity effects. Barley genotype diversity had weak positive effects on aboveground biomass through complementarity effects, whereas weed species diversity increased biomass predominantly through selection effects. When combined, increasing genotype diversity of barley tended to dilute the selection effect of weeds. We interpret these different effects of barley genotype and weed species diversity as the consequence of small vs large trait variation associated with intraspecific barley diversity and interspecific weed diversity, respectively. The different effects of intra- vs interspecific diversity highlight the underestimated and overlooked role of genetic diversity for ecosystem functioning.

Water-limited environments affect the association between functional diversity and forest productivity.

The link between biodiversity and ecosystem function can depend on environmental conditions. This contingency can impede our ability to predict how biodiversity-ecosystem function (BEF) relationships will respond to future environmental change, causing a clear need to explore the processes underlying shifts in BEF relationships across large spatial scales and broad environmental gradients. We compiled a dataset on five functional traits (maximum height, wood density, specific leaf area [SLA], seed size, and xylem vulnerability to embolism [P50]), covering 78%-90% of the tree species in the National Forest Inventory from Italy, to test (i) how a water limitation gradient shapes the functional composition and diversity of forests, (ii) how functional composition and diversity of trees relate to forest annual increment via mass ratio and complementarity effects, and (iii) how the relationship between functional diversity and annual increment varies between Mediterranean and temperate climate regions. Functional composition varied with water limitation; tree communities tended to have more conservative traits in sites with higher levels of water limitation. The response of functional diversity differed among traits and climatic regions but among temperate forest plots, we found a consistent increase of functional diversity with water limitation. Tree diversity was positively associated with annual increment of Italian forests through a combination of mass ratio and niche complementarity effects, but the relative importance of these effects depended on the trait and range of climate considered. Specifically, niche complementarity effects were more strongly associated with annual increment in the Mediterranean compared to temperate forests. Synthesis: Overall, our results suggest that biodiversity mediates forest annual increment under water-limited conditions by promoting beneficial interactions between species and complementarity in resource use. Our work highlights the importance of conserving functional diversity for future forest management to maintain forest annual increment under the expected increase in intensity and frequency of drought.

It takes a village to grow a tree: Most tree species benefit from dissimilar neighbors.

Scientific consensus is that diverse tree species positively impact forest productivity, especially when species are functionally dissimilar. Under the complementarity hypothesis, differences in species traits reduce competition among neighboring tree species. However, while this relationship has been extensively studied at the community level, there is a lack of understanding regarding how individuals of different species specifically respond to a functionally dissimilar neighborhood. In this study, we used permanent plots from Quebec, Canada, and 19 focal tree species to test whether: (1) tree growth response to neighborhood dissimilarity varies with their identity and competition intensity, and (2) focal tree species' traits explain their response to neighborhood dissimilarity. We demonstrate that: tree growth is primarily influenced by competition, species identity, and their interactions, but that dissimilarity, alone and in interaction with the main drivers of tree growth, explains an additional 1.8% of the variation in species growth. Within this context, (1) most species' respond positively to neighborhood dissimilarity, with magnitude being species and competition dependent, and (2) focal tree traits partly explain these dependencies, with shade-intolerant species benefiting most from dissimilar neighbors under high competition. Our study provides empirical support for the complementarity hypothesis, emphasizing the small but consistent positive effect of functional dissimilarity on tree growth in local neighborhoods. Our findings identify the species with the highest potential of benefiting from dissimilar neighbors but also demonstrate that the positive effect of neighborhood dissimilarity is not limited to a select few species with specific traits; rather, it is observed across a diverse range of species. The cumulative growth responses of individuals to functionally dissimilar neighbors may help explain the commonly observed higher productivity in more diverse communities.

Toward Unifying Evolutionary Ecology and Genomics to Understand Positive Plant-Plant Interactions Within Wild Species.

In a local environment, plant networks include interactions among individuals of different species and among genotypes of the same species. While interspecific interactions are recognized as main drivers of plant community patterns, intraspecific interactions have recently gained attention in explaining plant community dynamics. However, an overview of intraspecific genotype-by-genotype interaction patterns within wild plant species is still missing. From the literature, we identified 91 experiments that were mainly designed to investigate the presence of positive interactions based on two contrasting hypotheses. Kin selection theory predicts partisan help given to a genealogical relative. The rationale behind this hypothesis relies on kin/non-kin recognition, with the positive outcome of kin cooperation substantiating it. On the other hand, the elbow-room hypothesis supports intraspecific niche partitioning leading to positive outcome when genetically distant genotypes interact. Positive diversity-productivity relationship rationalizes this hypothesis, notably with the outcome of overyielding. We found that both these hypotheses have been highly supported in experimental studies despite their opposite predictions between the extent of genetic relatedness among neighbors and the level of positive interactions. Interestingly, we identified a highly significant effect of breeding system, with a high proportion of selfing species associated with the presence of kin cooperation. Nonetheless, we identified several shortcomings regardless of the species considered, such as the lack of a reliable estimate of genetic relatedness among genotypes and ecological characterization of the natural habitats from which genotypes were collected, thereby impeding the identification of selective drivers of positive interactions. We therefore propose a framework combining evolutionary ecology and genomics to establish the eco-genomic landscape of positive GxG interactions in wild plant species.

Plant history and soil history jointly influence the selection environment for plant species in a long-term grassland biodiversity experiment.

Long-term biodiversity experiments have shown increasing strengths of biodiversity effects on plant productivity over time. However, little is known about rapid evolutionary processes in response to plant community diversity, which could contribute to explaining the strengthening positive relationship. To address this issue, we performed a transplant experiment with offspring of seeds collected from four grass species in a 14-year-old biodiversity experiment (Jena Experiment). We used two- and six-species communities and removed the vegetation of the study plots to exclude plant-plant interactions. In a reciprocal design, we transplanted five "home" phytometers (same origin and actual environment), five "away-same" phytometers (same species richness of origin and actual environment, but different plant composition), and five "away-different" phytometers (different species richness of origin and actual environment) of the same species in the study plots. In the establishment year, plants transplanted in home soil produced more shoots than plants in away soil indicating that plant populations at low and high diversity developed differently over time depending on their associated soil community and/or conditions. In the second year, offspring of individuals selected at high diversity generally had a higher performance (biomass production and fitness) than offspring of individuals selected at low diversity, regardless of the transplant environment. This suggests that plants at low and high diversity showed rapid evolutionary responses measurable in their phenotype. Our findings provide first empirical evidence that loss of productivity at low diversity is not only caused by changes in abiotic and biotic conditions but also that plants respond to this by a change in their micro-evolution. Thus, we conclude that eco-evolutionary feedbacks of plants at low and high diversity are critical to fully understand why the positive influence of diversity on plant productivity is strengthening through time.

Microhabitats are associated with diversity-productivity relationships in freshwater bacterial communities.

Eukaryotic communities commonly display a positive relationship between biodiversity and ecosystem function (BEF) but the results have been mixed when assessed in bacterial communities. Habitat heterogeneity, a factor in eukaryotic BEFs, may explain these variable observations but it has not been thoroughly evaluated in bacterial communities. Here, we examined the impact of habitat on the relationship between diversity assessed based on the (phylogenetic) Hill diversity metrics and heterotrophic productivity. We sampled co-occurring free-living (more homogenous) and particle-associated (more heterogeneous) bacterial habitats in a freshwater, estuarine lake over three seasons: spring, summer and fall. There was a strong, positive, linear relationship between particle-associated bacterial richness and heterotrophic productivity that strengthened when considering dominant taxa. There were no observable BEF trends in free-living bacterial communities for any diversity metric. Biodiversity, richness and Inverse Simpson's index, were the best predictors of particle-associated production whereas pH was the best predictor of free-living production. Our findings show that heterotrophic productivity is positively correlated with the effective number of taxa and that BEF relationships are associated with microhabitats. These results add to the understanding of the highly distinct contributions to diversity and functioning contributed by bacteria in free-living and particle-associated habitats.