Positive Diversity-productivity Relationship Research Articles

Systematic distributions of interaction strengths across tree interaction networks yield positive diversity-productivity relationships.

Understanding the mechanisms underlying diversity-productivity relationships (DPRs) is crucial to mitigating the effects of forest biodiversity loss. Tree-tree interactions in diverse communities are fundamental in driving growth rates, potentially shaping the emergent DPRs, yet remain poorly explored. Here, using data from a large-scale forest biodiversity experiment in subtropical China, we demonstrated that changes in individual tree productivity were driven by species-specific pairwise interactions, with higher positive net pairwise interaction effects on trees in more diverse neighbourhoods. By perturbing the interactions strength from empirical data in simulations, we revealed that the positive differences between inter- and intra-specific interactions were the critical determinant for the emergence of positive DPRs. Surprisingly, the condition for positive DPRs corresponded to the condition for coexistence. Our results thus provide a novel insight into how pairwise tree interactions regulate DPRs, with implications for identifying the tree mixtures with maximized productivity to guide forest restoration and reforestation efforts.

Animal and plant space-use drive plant diversity-productivity relationships.

Plant community productivity generally increases with biodiversity, but the strength of this relationship exhibits strong empirical variation. In meta-food-web simulations, we addressed if the spatial overlap in plants' resource access and animal space-use can explain such variability. We found that spatial overlap of plant resource access is a prerequisite for positive diversity-productivity relationships, but causes exploitative competition that can lead to competitive exclusion. Space-use of herbivores causes apparent competition among plants, resulting in negative relationships. However, space-use of larger top predators integrates sub-food webs composed of smaller species, offsetting the negative effects of exploitative and apparent competition and leading to strongly positive diversity-productivity relationships. Overall, our results show that spatial overlap of plants' resource access and animal space-use can greatly alter the strength and sign of such relationships. In particular, the scaling of animal space-use effects opens new perspectives for linking landscape processes without effects on biodiversity to productivity patterns.

Soil fungal communities contribute to the positive diversity–productivity relationship of tree communities under contrasting water availability

Abstract Plant diversity has often been linked to increased productivity; however, this apparent diversity–productivity relationship may rely on intertrophic interactions such as those between plants and soil microbes. Soil fungi can create complementarity between plant species via altered plant resource partitioning, facilitation via fungal networks or biotic feedbacks, thereby promoting plant diversity–productivity relationships. Furthermore, these relationships are likely to be context dependent in response to resource availability. We used a biodiversity–ecosystem function experiment with trees exposed to high and low water availability treatments to determine the contribution of soil fungal communities to the diversity–productivity relationship in tree communities. We used amplicon sequencing of soil fungi to assess fungal richness, community composition and richness of functional guilds. We then applied structural equation modelling to determine relationships between tree diversity, fungal communities and tree productivity and the role of water availability in these relationships. Tree species richness and functional diversity both increased above‐ground tree productivity and influenced soil fungal community composition. Fungal community composition had a direct impact on tree productivity and enhanced net diversity effects on productivity. Therefore, fungal communities mediated a positive, indirect effect of tree richness on productivity. While total fungal richness was not associated with tree diversity, pathogen richness decreased and mycorrhizal richness increased with tree richness. Pathogen and mycorrhizal richness had either no impact or a weak negative effect on productivity. Tree species traits strongly affected fungal communities and these changes promoted productivity. Finally, water availability greatly influenced fungal communities but did not interact with tree diversity to affect productivity; indicating possible resilience of tree communities to altered precipitation regimes and associated changes in fungal communities. Synthesis: Our study highlights the crucial role that fungal communities play in shaping the relationship between tree diversity, traits and productivity, and resilience to altered water availability.

Linking plant diversity-productivity relationships to plant functional traits of dominant species and changes in soil properties in 15-year-old experimental grasslands.

Positive plant diversity-productivity relationships are known to be driven by complementary resource use via differences in plant functional traits. Moreover, soil properties related to nutrient availability were shown to change with plant diversity over time; however, it is not well-understood whether and how such plant diversity-dependent soil changes and associated changes in functional traits contribute to positive diversity-productivity relationships in the long run. To test this, we investigated plant communities of different species richness (1, 2, 6, and 9 species) in a 15-year-old grassland biodiversity experiment. We determined community biomass production and biodiversity effects (net biodiversity [NEs], complementarity [CEs], and selection effects [SEs]), as well as community means of plant functional traits and soil properties. First, we tested how these variables changed along the plant diversity gradient and were related to each other. Then, we tested for direct and indirect effects of plant and soil variables influencing community biomass production and biodiversity effects. Community biomass production, NEs, CEs, SEs, plant height, root length density (RLD), and all soil property variables changed with plant diversity and the presence of the dominant grass species Arrhenatherum elatius (increase except for soil pH, which decreased). Plant height and RLD for plant functional traits, and soil pH and organic carbon concentration for soil properties, were the variables with the strongest influence on biomass production and biodiversity effects. Our results suggest that plant species richness and the presence of the dominant species, A. elatius, cause soil organic carbon to increase and soil pH to decrease over time, which increases nutrient availability favoring species with tall growth and dense root systems, resulting in higher biomass production in species-rich communities. Here, we present an additional process that contributes to the strengthening positive diversity-productivity relationship, which may play a role alongside the widespread plant functional trait-based explanation.

Historical context modifies plant diversity–community productivity relationships in alpine grassland

Abstract While most studies yield positive relationships between biodiversity (B) and ecosystem functioning (EF), awareness is growing that BEF relationships can vary with ecological context. The awareness has led to increased efforts to understand how contemporary environmental context modifies BEF relationships, but the role of historical context, and the mechanisms by which it may influence biodiversity effects, remains poorly understood. We examined how historical context alters plant diversity–community productivity relationships via plant species interactions in alpine grassland. We also tested how historical context modifies interactions between plants and arbuscular mycorrhizal (AM) fungi, which can potentially mediate the above processes. We studied biodiversity effects on plant community productivity at two grassland sites with different histories related to grazing intensity—heavy versus light livestock grazing—but similar current management. We assembled experimental communities of identical species composition with plants from each of the two sites in disturbed soil from a contemporary heavily grazed grassland, ranging in species richness from one to two, three and six species. Moreover, we carried out a mycorrhizal hyphae‐exclusion experiment to test how plant interactions with AM fungi influence plant responses to historical context. We detected a significantly positive diversity–productivity relationship that was driven by complementarity effects in communities composed of plants from the site without heavy‐grazing history, but no such relationship in plant communities composed of plants from the site with heavy‐grazing history. Plants from the site with heavy‐grazing history had increased competitive ability and increased yields in low‐diversity communities but disrupted complementarity effects in high‐diversity communities. Moreover, plants of one species from the site with heavy‐grazing history benefitted more from AM fungal communities than did plants from the site without such history. Synthesis. Using the same experimental design and species, communities assembled by plants from two sites with different historical contexts showed different plant diversity–community productivity relationships. Our results suggest that historical context can alter plant diversity–community productivity relationships via plant species interactions and potentially plant–soil interactions. Therefore, considering historical contexts of ecological communities is of importance for advancing our understanding of long‐term impacts of anthropogenic disturbance on ecosystem functioning.

Changes in the direction of the diversity–productivity relationship over 15 years of stand development in a planted temperate forest

Abstract Experiments manipulating diversity in both forests and grasslands have often observed a positive diversity–productivity relationship (DPR) which tends to strengthen during plant community development. This pattern is generally attributed to an increase in niche complementarity or facilitation. Most analyses do not examine species dominance and density, which also change over time. Moreover, how neighbourhood‐scale interactions among tree species affect the DPR is not well‐understood. We analysed growth and mortality data from the Simplex experiment, a part of the BIOTREE tree diversity experiment. Simplex consists of 36 plots each planted with four common and commercially important tree species to create a gradient of tree species evenness at two tree densities (6667 and 3556 trees ha−1). We test whether: (a) the effect of evenness on total above‐ground biomass productivity increase with stand development (year), (b) the effect of evenness on productivity is stronger in dense plots; and (c) intraspecific competition from neighbours negatively affect the growth of dominant species more strongly compared to co‐dominant species, and whether this negative effect is stronger in denser plots. The direction of DPR was initially negative because the fast‐growing long‐lived pioneer Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) dominated. However, with time, shade‐tolerant Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.) increased in abundance (by biomass), and the relationship between evenness and biomass increment changed from negative to positive in high‐density plots. Neighbourhood analyses revealed that for Douglas fir and Norway spruce, conspecifics reduced individual growth rates across density levels and years. Synthesis. We observed a shift in the diversity–productivity relationship over 15 years in our experiment. Over time, increasing intraspecific competition limited the increment of the abundant Douglas fir in uneven plots, and a persistent increase in the abundance (by biomass) of shade‐tolerant Norway spruce and European beech in even plots led to a higher community biomass increment, which led to a positive DPR. Emergence of a positive DPR in temperate forest plantations requires significant time and is importantly promoted by diversity at the neighbourhood scale (intimate mixtures) as well as higher density planting.

Shifts from complementarity to selection effects maintain high productivity in maize/legume intercropping systems

Abstract Complementarity (CE) and selection effects (SE) have been either invoked to explain the positive diversity–productivity relationship in natural and semi‐natural ecosystems. Few studies have, however, separated selection and complementarity effects in economically valuable intercropping systems, which receive significant nutrient inputs throughout the growing season. We performed a 2‐year field experiment with five cropping systems (i.e. maize/peanut intercropping, maize/soybean intercropping, maize, peanut and soybean monocultures) under different combinations of nitrogen (N) and phosphorus (P) fertilization. Sequential harvest of subplots was performed five times during the growing season and the additive partitioning method was applied to determine complementarity and selection effects in the two intercropping systems. We found that the land equivalent ratio (LER) based on yield was greater than or close to 1 in both maize/soybean and maize/peanut intercropping systems, suggesting yield advantages in these intercropping systems compared to monocultures. The LER of the two intercropping systems without N fertilization was greater in 2018 than in 2017. Nitrogen fertilization reduced the LER of maize/soybean intercropping in 2018, and that of maize/peanut intercropping across 2 years. Nitrogen fertilization had much stronger effects on maize yield than on soybean or peanut yield. Intercropping increased maize yield but decreased soybean and peanut yield. The CE was more important for net biodiversity effects without N fertilization while the SE was more important with N fertilization in the two intercropping systems across 2 years. Phosphorus fertilization impacts on the LER and biodiversity effects were weaker compared to N fertilization. Finally, net biodiversity effects were greater at later growth stages within a growing season. Synthesis and applications. Our study demonstrates how yield advantages of species‐diverse intercropping systems can be maximized either through (a) CE in the absence of N fertilization thanks to complementarity in nutrient use between crops or (b) SE under N fertilization thanks to overyielding of highly productive species (i.e. maize). Yield advantages promoted by CE are more sustainable because they are associated with reduced chemical fertilizer use and greater monetary benefits to farmers. These findings have important implications for the design and management of species‐diverse intercropping food‐production systems.

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.

Factors controlling the productivity of tropical Andean forests: climate and soil are more important than tree diversity

Abstract. Theory predicts positive effects of species richness on the productivity of plant communities through complementary resource use and facilitative interactions between species. Results from manipulative experiments with tropical tree species indicate a positive diversity–productivity relationship (DPR), but the existing evidence from natural forests is scarce and contradictory. We studied forest aboveground productivity in more than 80 humid tropical montane old-growth forests in two highly diverse Andean regions with large geological and topographic heterogeneity and related productivity to tree diversity and climatic, edaphic and stand structural factors with a likely influence on productivity. Main determinants of wood production in the perhumid study regions were elevation (as a proxy for temperature), soil nutrient (N, P and base cation) availability and forest structural parameters (wood specific gravity, aboveground biomass). Tree diversity had only a small positive influence on productivity, even though tree species numbers varied largely (6–27 species per 0.04 ha). We conclude that the productivity of highly diverse Neotropical montane forests is primarily controlled by thermal and edaphic factors and stand structural properties, while tree diversity is of minor importance.

Can complementarity in water use help explain diversity-productivity relationships in semi-arid grasslands?

Previous studies reported the positive diversity-productivity relationship could be explained by combinations of species with different rooting distributions and greater legume abundance. However, because plant production is limited by soil water availability in arid areas, we study the positive diversity-productivity relationship of Mongolian semi-arid grasslands and the complementary use of water in these diverse plant communities. Plant species richness influences productivity through different pathways involving root weight and soil water content or legume abundance. We have examined these variables in plant species in a controlled site in Mongolia, determined the significance of these different pathways using path analysis, and performed single linear regression analysis between the significant factors. The results of the path analysis backed the hypothesis that the diversity-productivity relationship was not explained by greater legume abundance but by complementary usage of soil water by the plant communities of the arid grassland. These results were further supported by a significant positive relationship between plant species richness and root weight in shallow soil layers, and a negative relationship between plant species richness and water content or water potential, and root weight and soil water content.

Biodiversity-ecosystem functioning relationships of overstorey versus understorey trees in an old-growth temperate forest

The results of this study, based on a large dataset with mapped tree locations, suggest that the relationship between biodiversity and ecosystem functioning (BEF) is different between the over- and understorey in the investigated forest community. Neither the functional nor the phylogenetic diversity showed a significant advantage in predicting aboveground biomass and biomass production (aboveground biomass increment) when compared with species richness. Consistent conclusions have still not been reached regarding the relative importance of the different diversity indices in predicting biomass production. Furthermore, the effects of certain forest strata on the BEF relationship remain unclear. The woody plant species in the study area were classified as two vertical strata and were referred to as the “overstorey” (trees with dbh > 10 cm) and “understorey” (trees with dbh ≤ 10 cm). The relationships between different metrics of tree diversity and biomass production were quantified using linear models. The set of the best predictors for tree biomass and productions were determined using a multi-model selection approach. The differences in diversity-ecosystem functioning relationships between the two forest vertical strata, and the effects of canopy trees on the understorey trees, were investigated using structural equation models. The relative importance of the species diversity, phylogenetic diversity, and functional diversity in predicting the aboveground biomass production (ABP) was examined to compare the potential of phylogenetic and functional diversity indices and species richness in predicting biomass productivity and compare the influence of niche complementarity, selection effects, or mass-ratio effects on aboveground biomass variables and to assess possible differences in the BEF relationships between the overstorey and understorey layers in the studied ecosystems. Also, this study investigated possible differences in the BEF relationships between the overstorey and understorey layers in the studied ecosystems. The results confirmed a positive diversity-productivity relationship in the natural coniferous and broadleaved mixed forest. However, the relationship in the overstorey stratum was statistically stronger than that in the understorey layer. The combination of diversity indices and functional traits could explain more of the variations in the biomass and productivity than when examined separately. The correlation between the richness of the overstorey and understorey species was found to be positive. However, the aboveground woody biomass of the overstorey layer had negative effects on the understorey biomass. In summary, this study found a stronger positive correlation between the woody plant diversity and productivity in the overstorey than in the understorey stratum. These findings indicate that the BEF relationships are hierarchically dependent, and supported by niche complementarity. Also, a much weaker relationship was observed between the functional diversity and biomass production than between the species diversity and biomass production due to a selection effect. The aboveground woody biomass was found to be enhanced in communities dominated by functional traits correlated to smaller leaf areas, greater maximum tree heights, and higher leaf carbon content. These findings suggest that the BEF relationships were driven by a mass-ratio hypothesis.

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.

Unimodal diversity-productivity relationship emerged under stressful environment through sampling effect

Researches on the context dependence of biodiversity and ecosystem functioning (BEF) reveal the variation of diversity-productivity relationship (DPR) under stressful environment. The “habitat sampling effect” (HSE) is proposed as the dominant species interaction mechanism at stressful environment, whereas its potential role in driving the DPR has never been testified before. We constructed an individual-based simulation model to explore the variation of DPR along environmental stress gradient, and evaluated the contribution of HSE in explaining this variation. Our results indicated that DPR changed from positive to negative along environmental stress gradients. An unimodal DPR curve emerged at stressful environment, which was caused by the counterbalance of two opposite impacts of HSE on community productivity. At low richness level, the positive impact of HSE on community productivity dominated through the promotion of community size, which resulted in the positive DPR. Whereas with the increase of richness, the negative impact of HSE dominated instead through the reducing of individual productivity, which caused the decreasing part of unimodal curve. Our results highlight the complex characteristic of BEF relationship at stressful environment, and emphasize the necessity of biodiversity in maintaining community's functioning at stressful environment which are often sparse in species richness.

Covariation between tree size and shade tolerance modulates mixed-forest productivity

In tree communities, tree size inequality reduces productivity and interacts with tree shade tolerance to modulate stand productivity, with a higher productivity in stands where shade-intolerant species dominate shade-tolerant species in size. Positive diversity–productivity relationships have been reported in different plant communities, including tree communities. These effects may be strongly related to both structural diversity and functional diversity, but also to their interactions if there is a non-random distribution of species functional characteristics among canopy layers. We explore the relative effects on forest productivity of tree species diversity, tree size inequality, and species shade tolerance diversity, as well as the effect of the distribution of tree shade tolerance in the canopy. We used 11,054 mixed-species forest plots from the French Forest Inventory (IGN) distributed throughout France (2006–2011). We analyzed the effects of species richness, shade tolerance diversity, and height inequality on forest plot productivity, represented by basal area annual increment over a period of 5 years, while controlling for first-order structure characteristics (basal area and quadratic mean diameter) and environmental factors (soil water budget and sum of growing degree days). Using the covariance between tree height and shade tolerance in mixed species canopies, we also explored the effect of the distribution of species’ shade tolerance among canopy layers. The results showed a positive effect of species richness (effect size, 0.02) and a negative effect of height inequality (− 0.05) on mixed-forest productivity. We also showed that a negative covariance between shade tolerance and height (e.g., higher proportion of shade-tolerant species in lower height classes) increased productivity (0.01). Shade tolerance diversity did not affect productivity. In tree communities, as shown previously in monospecific forest stands, tree size inequality reduces productivity. This effect is modulated by the distribution of shade tolerance among canopy layers. Previous studies on species diversity effect have generally overlooked the importance of the size structure and the size hierarchy of functional characteristics. These effects are, however, crucial and deserve to be explored in greater detail.

Positive relationship between species richness and aboveground biomass across forest strata in a primary Pinus kesiya forest

Both biodiversity and biomass are important variables in forest ecosystems, and the relationship between them is critical for ecosystem functioning and management. The primary Pinus kesiya forest is increasingly threatened by human disturbance in Yunnan Province. We observed that species richness had a positive impact on aboveground biomass across all forest vegetation layers, and this relationship was strongest in the herb layer. The asymptotic relationship between cumulative species number and aboveground biomass suggested that individual of Pinus kesiya trees with relatively large diameters contributed the majority of the aboveground biomass in the tall tree strata due to their strong competitive advantage over other tree species. Although aboveground biomass increased with stand age in the tall tree strata, climate factors and the soil nutrient regime affected the magnitude of the diversity-productivity relationship. Stand age had no significant effect on species richness and aboveground biomass in the forest understory. The effect of the positive diversity-productivity relationship of the tall trees on the shrub layer was negligible; the diversity-productivity relationship in the forest understory was significantly affected by the tall tree aboveground biomass. The tall trees have increased the strength of the positive diversity-productivity relationship in the forest understory.

Diversity spurs diversification in ecological communities

Diversity is a fundamental, yet threatened, property of ecological systems. The idea that diversity can itself favour diversification, in an autocatalytic process, is very appealing but remains controversial. Here, we study a generalized model of ecological communities and investigate how the level of initial diversity influences the possibility of evolutionary diversification. We show that even simple models of intra- and inter-specific ecological interactions can predict a positive effect of diversity on diversification: adaptive radiations may require a threshold number of species before kicking-off. We call this phenomenon DDAR (diversity-dependent adaptive radiations) and identify mathematically two distinct pathways connecting diversity to diversification, involving character displacement and the positive diversity-productivity relationship. Our results may explain observed delays in adaptive radiations at the macroscale and diversification patterns reported in experimental microbial communities, and shed new light on the dynamics of ecological diversity, the diversity-dependence of diversification rates, and the consequences of biodiversity loss.

Tree Diversity Enhances Stand Carbon Storage but Not Leaf Area in a Subtropical Forest.

Research about biodiversity–productivity relationships has focused on herbaceous ecosystems, with results from tree field studies only recently beginning to emerge. Also, the latter are concentrated largely in the temperate zone. Tree species diversity generally is much higher in subtropical and tropical than in temperate or boreal forests, with reasons not fully understood. Niche overlap and thus complementarity in the use of resources that support productivity may be lower in forests than in herbaceous ecosystems, suggesting weaker productivity responses to diversity change in forests. We studied stand basal area, vertical structure, leaf area, and their relationship with tree species richness in a subtropical forest in south-east China. Permanent forest plots of 30 x 30 m were selected to span largely independent gradients in tree species richness and secondary successional age. Plots with higher tree species richness had a higher stand basal area. Also, stand basal area increases over a 4-year census interval were larger at high than at low diversity. These effects translated into increased carbon stocks in aboveground phytomass (estimated using allometric equations). A higher variability in tree height in more diverse plots suggested that these effects were facilitated by denser canopy packing due to architectural complementarity between species. In contrast, leaf area was not or even negatively affected by tree diversity, indicating a decoupling of carbon accumulation from leaf area. Alternatively, the same community leaf area might have assimilated more C per time interval in more than in less diverse plots because of differences in leaf turnover and productivity or because of differences in the display of leaves in vertical and horizontal space. Overall, our study suggests that in species-rich forests niche-based processes support a positive diversity–productivity relationship and that this translates into increased carbon storage in long-lived woody structures. Given the high growth rates of these forests during secondary succession, our results further indicate that a forest management promoting tree diversity after disturbance may accelerate CO2 sequestration from the atmosphere and thus be relevant in a climate-change context.

Twenty-million-year relationship between mammalian diversity and primary productivity

At global and regional scales, primary productivity strongly correlates with richness patterns of extant animals across space, suggesting that resource availability and climatic conditions drive patterns of diversity. However, the existence and consistency of such diversity-productivity relationships through geological history is unclear. Here we provide a comprehensive quantitative test of the diversity-productivity relationship for terrestrial large mammals through time across broad temporal and spatial scales. We combine >14,000 occurrences for 690 fossil genera through the Neogene (23-1.8 Mya) with regional estimates of primary productivity from fossil plant communities in North America and Europe. We show a significant positive diversity-productivity relationship through the 20-million-year record, providing evidence on unprecedented spatial and temporal scales that this relationship is a general pattern in the ecology and paleo-ecology of our planet. Further, we discover that genus richness today does not match the fossil relationship, suggesting that a combination of human impacts and Pleistocene climate variability has modified the 20-million-year ecological relationship by strongly reducing primary productivity and driving many mammalian species into decline or to extinction.

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.

Positive species diversity and above‐ground biomass relationships are ubiquitous across forest strata despite interference from overstorey trees

Summary There is growing concern over rates of global species diversity loss and its implications on healthy ecosystem functioning. While positive relationships between tree species diversity and forest biomass production have been observed, forests are structurally complex, consisting of understorey vegetation layers that also contribute to ecosystem functioning as they often account for the majority of species richness. However, relationships between understorey vegetation diversity and function are largely unexplored. Further, few studies have simultaneously assessed how both overstorey and understory vegetation interact and contribute to overall ecosystem function. By analysing Canada's National Forest Inventory data base using structural equation modelling, we explored the relationships between species richness and above‐ground biomass production across forest vegetation strata while accounting for potentially confounding factors, including climate, physical site characteristics and forest ageing. We found positive relationships between species richness and biomass production across all forest vegetation layers, but the relationship was strongest for the overstorey layer. Species richness of the understorey tree, shrub and herb layers was positively related to overstorey species richness. However, overstorey biomass had a negative effect on the biomass production of all understorey layers. Our results suggest that resource filtering by overstorey trees might have reduced the strength of the positive diversity–productivity relationships in the forest understorey, supporting previous hypotheses that the magnitude and direction of diversity–productivity relationships is context specific and dependent on the conditions of the surrounding environment. Further, heterogeneity in understory resources, as affected by the overstorey, may promote niche complementarity as the main mechanism driving diversity–productivity relationships in understorey vegetation.