Biodiversity-productivity Relationship Research Articles

Neighborhood Diversity Promotes Tree Growth in a Secondary Forest: The Interplay of Intraspecific Competition, Interspecific Competition, and Spatial Scale

Understanding the biodiversity–productivity relationship (BPR) is crucial for biodiversity conservation and ecosystem management. While it is known that diversity enhances forest productivity, the underlying mechanisms at the local neighborhood level remain poorly understood. We established a 9.6 ha dynamic forest plot to study how neighborhood diversity, intraspecific competition, and interspecific competition influence tree growth across spatial scales using linear mixed-effects models. Our analysis reveals a significant positive correlation between neighborhood species richness (NSR) and relative growth rate (RGR). Notably, intraspecific competition, measured by conspecific neighborhood density and resource competition, negatively impacts RGR at finer scales, indicating intense competition among conspecifics for limited resources. In contrast, interspecific competition, measured by heterospecific density and resource competition, has a negligible impact on RGR. The relative importance of diversity and intra/interspecific competition in influencing tree growth varies with scale. At fine scales, intraspecific competition dominates negatively, while at larger scales, the positive effect of NSR on RGR increases, contributing to a positive BPR. These findings highlight the intricate interplay between local interactions and spatial scale in modulating tree growth, emphasizing the importance of considering biotic interactions and spatial variability in studying BPR.

Cross-domain diversity effects: linking diatom species richness, intraspecific richness, and biomass production to host-associated bacterial diversity

Abstract Interactions between bacteria and microalgae are important for the functioning of aquatic ecosystems, yet interactions based on the biodiversity of these two taxonomic domains have been scarcely studied. Specifically, it is unclear whether a positive biodiversity-productivity relationship in phytoplankton is largely facilitated by niche partitioning among the phytoplankton organisms themselves, or whether associated bacterial communities play an additional role in modifying these diversity effects. Moreover, effects of intraspecific diversity in phytoplankton communities on bacterial community diversity have not been tested. To address these points, we factorially manipulated both species and intraspecific richness of three diatoms to test effects of diatom species/strain diversity on biomass production and bacterial diversity in algae-bacteria communities. The results show that diatom intraspecific diversity has significant positive effects on culture biomass and the diversity of the associated free-living bacterial community (0.2-3 μm size fraction), which are comparable in magnitude to species diversity effects. However, there were little to no effects of diatom diversity on host-associated bacterial diversity (>3 μm size fraction), or of bacterial diversity on biomass production. These results suggest a decoupling of bacterial diversity from the diatom diversity-productivity relationship and provide early insights regarding the relations between diversity across domains in aquatic ecosystems.

Changes in Relationship between Forest Biomass Productivity and Biodiversity of Different Type Subtropical Forests in Southern China

Forest productivity is influenced by various factors, including biodiversity, environmental factors, functional traits, and forest types. However, the relative importance of these factors in determining the productivity of subtropical forests in southern China remains controversial. In this study, we analyzed a dataset of 24 forest plots from four subtropical forest types in the Nanling Mountains with the main goal of identifying and quantifying the relative contribution of the main driving factors of forest productivity in these forests. Generalized linear regression and structural equation modeling were used to examine the relationship between forest biomass productivity (aboveground, belowground and total), biodiversity (taxonomic diversity, phylogenetic diversity and functional diversity), and environmental variables (i.e., physiography and climate). The results indicated that both environmental factors and biodiversity played pivotal roles in explaining the biomass productivity of the Nanling subtropical forests. Environmental factors had the greatest influence on total productivity, while the impacts of different types of biodiversity on various productivity components (aboveground and belowground) varied notably. Taxonomic diversity showed the strongest positive effect on the aboveground and belowground biomass productivity. However, phylogenetic and functional diversity had negative effects on productivity. Furthermore, these relationships also exhibited variations when considering different altitude gradients, with low altitudes generally leading to negative biodiversity–productivity correlations. We contextualized our results regarding the three state-of-the-art theories about biodiversity–productivity relationships (selection probability, niche complementarity, and biomass ratio) and concluded that both selection probability and niche complementarity are the driving mechanisms of productivity in the subtropical forests of the Nanling Mountains. This study offers valuable insights into the functioning and biodiversity mechanisms of subtropical forest ecosystems in southern China.

No generality in biodiversity-productivity relationships along elevation in temperate and subtropical forest landscapes

An improved understanding of biodiversity-productivity relationships (BPRs) along environmental gradients is crucial for effective ecosystem management and biodiversity conservation. The stress-gradient hypothesis suggests that BPRs are stronger in stressful environments compared to more favorable conditions. However, there is limited knowledge regarding the variation of BPRs along elevational gradients and their generality across different landscapes. To study how BPRs change with elevation, we harnessed inventory data on 6,431 trees from 152 plots surveyed twice in eight to ten year intervals in mountain forests of temperate Europe and subtropical Asia. We quantified the relationship between aboveground productivity and different biodiversity measures, including taxonomic, functional, and phylogenetic diversity. To elucidate the processes underlying BPRs, we studied the variation of different functional traits along elevation across landscapes. We found no general pattern of BPRs across landscapes and elevations. Relationships were neutral for all biodiversity measures in temperate forests, and negative for taxonomic and functional diversity in subtropical forests. BPRs were largely congruent between taxonomic, functional and phylogenetic diversity. We found only weak support for the stress-gradient hypothesis, with BPRs turning from negative to positive (effect not significant) close to the tree line in subtropical forests. In temperate forests, however, elevation patterns were strongly modulated by species identity effects as influenced by specific traits. The effect of traits such as community-weighted mean of maximum plant height and wood density on productivity was congruent across landscapes. Our study highlights the context-dependence of BPRs across elevation gradients and landscapes. Species traits are key modulating factors of BPRs and should be considered more explicitly in studies of the functional role of biodiversity. Furthermore, our findings highlight that potential trade-offs between conserving biodiversity and fostering ecosystem productivity exist, which require more attention in policy and management.

Leaf trait variation within individuals mediates the relationship between tree species richness and productivity

In forest ecosystems, many ecosystem functions such as tree growth are affected by tree species richness. This biodiversity–productivity relationship (BPR) is mediated by leaf traits, which themselves are known to be influenced by tree species richness; at the same time, as the primary organs of light capture, they are an important factor for tree growth. However, how tree growth is influenced by a tree's ability to phenotypically adjust its leaf traits to the within‐individual light gradient has largely been unexplored. Furthermore, it is not known how such impacts of within‐tree leaf trait variation on individual tree growth sums up to productivity at the community scale. In this study we tested how tree species richness, a tree's mean leaf traits, within‐tree leaf trait variation and the light extinction coefficient within a tree crown influence tree growth. We measured these variables in the temperate forest plantation of the Kreinitz biodiversity experiment. We found that the relationship between tree species richness and tree growth is mediated via the leaf trait variation of the individual trees, which in turn was modified by light availability. In particular, trees in monocultures show a higher within‐individual leaf trait variation, which partly compensates for the lack in among‐species leaf trait variation, and thus affects the BPR. It seems that tree richness operates both through increased acquisitive trait values and within‐individual leaf trait variation, two processes that cancelled out each other and resulted in the absence of a significant effect of tree richness on productivity in our study. In conclusion, to understand the BPR, it is important to study the underlying processes and to know which ones reinforce or oppose each other. In particular, our study highlights the importance of including within‐individual leaf trait variation in ecological research as one important moderator in the BPR.

Relationships between species richness and biomass production are context dependent in grasslands differing in land-use and seed addition

Despite evidence from grasslands experiments suggesting that plant species loss reduces biomass production, the strength of biodiversity-ecosystem functioning relationships in managed grasslands is still debated. High land-use intensity and reduced species pools are often suggested to make relationships between biodiversity and productivity less positive or even negative, but concrete evidence is still scarce. We investigated biodiversity-productivity relationships over two years in 150 managed grasslands in Germany. Specifically, we distinguished between relationships of biodiversity and biomass production in managed grasslands (1) varying in land-use intensity (e.g. of mowing, grazing and/or fertilization), (2) where land-use intensity is experimentally reduced, and (3) where additionally to land-use reductions, species pools are enlarged by seed addition. Among grasslands varying in land-use intensity, we found negative biodiversity-productivity relationships. Land-use reduction weakened these relationships, towards neutral, and sometimes, even positive relationships. Seed addition reduced species pool limitations, but this did not strengthen biodiversity-productivity relationships. Our findings indicate that land-use intensity is an important factor explaining the predominantly negative biodiversity-productivity relationships in managed grasslands. While we did not find that species pool limitations weakened biodiversity-productivity relationships, our results are based on a two-year-old experiment, possibly such effects are only visible in the long-term. Ultimately, advancing insights on biodiversity-ecosystem functioning relationships helps us to understand under which conditions agricultural production may benefit from promoting biodiversity.

Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types.

Tree species diversity and mycorrhizal associations play a central role for forest productivity, but factors driving positive biodiversity-productivity relationships remain poorly understood. In a biodiversity experiment manipulating tree diversity and mycorrhizal associations, we examined the roles of above- and belowground processes in modulating wood productivity in young temperate tree communities and potential underlying mechanisms. We found that tree species richness, but not mycorrhizal associations, increased forest productivity by enhancing aboveground structural complexity within communities. Structurally complex communities were almost twice as productive as structurally simple stands, particularly when light interception was high. We further demonstrate that overyielding was largely explained by positive net biodiversity effects on structural complexity with functional variation in shade tolerance and taxonomic diversity being key drivers of structural complexity in mixtures. Consideration of stand structural complexity appears to be a crucial element in predicting carbon sequestration in the early successional stages of mixed-species forests.

Partitioning the biodiversity effects on productivity into density and size components.

Plant density and size - two factors that represent plant survival and growth - are key determinants of yield but have rarely been analysed explicitly in the context of biodiversity-productivity relationships. Here, we derive equations to partition the net, complementarity and selection effects of biodiversity into additive components that reflect diversity-induced changes in plant density and size. Applications of the new method to empirical datasets reveal contrasting ways in which plant density and size regulate yield in species mixtures. In an annual plant diversity experiment, overyielding is largely explained by selection effects associated with increased size of highly productive plant species. In a tree diversity experiment, the cause of overyielding shifts from enhanced growth in tree size to reduced mortality by complementary use of canopy space during stand development. These results highlight the capability of the new method to resolve crucial, yet understudied, demographic links between biodiversity and productivity.

High phytoplankton diversity in eutrophic states impedes lake recovery

AbstractAimThe widely observed positive response of productivity to biodiversity, known as the biodiversity‐productivity relationship (BPR), underpins nature‐based solutions for mitigating and adapting to global changes. Little is known of the role of such emergent processes in the restoration of damaged ecosystems. Here, we test for evidence of a BPR in degraded freshwater lakes, and its role in underpinning or undermining lake recovery from eutrophic states.LocationLower Yangtze and Huaihe river basins, China, East Asia.TaxaFreshwater phytoplankton; also macrophytes, fish, zooplankton and zoobenthos.MethodsA systematic review of biodiversity studies was conducted to enumerate freshwater biodiversity change during eutrophication, encompassing 572 samples from 155 lakes in East China. Empirical samples were then taken of 93 phytoplankton communities from 38 lakes in ecologically degraded states of eutrophication. Samples were analysed for niche overlap in relation to biodiversity and productivity, and metrics of network structure indicative of community stability.ResultsReviewed publications on eutrophication in East China lakes revealed losses in biodiversity of macrophytes, fishes, zooplankton and zoobenthos but not phytoplankton. Empirically sampled eutrophic lakes maintained high phytoplankton biodiversity through niche differentiation, and phytoplankton biodiversity correlated positively with biomass, in line with the expectations of BPR.ConclusionsThe positive effect of biodiversity on phytoplankton productivity in eutrophic lakes acts in opposition to the negative impacts on productivity induced by eutrophication‐management strategies of nutrient suppression. In degraded systems, the BPR contributes to sustaining productivity of lake phytoplankton communities, impeding recovery from a eutrophic state. This is a case of the BPR playing an undesirable role at the ecosystem level. Lake recovery programmes need to anticipate that the control of phytoplankton biomass will depend on reducing nutrients to a level sufficient to degrade the internal structure of the phytoplankton community.

Landscape structure influences grassland productivity through plant functional diversity

Agriculture intensification led to the gradual destruction of semi-natural habitats and landscape simplification in the North-Western Europe. All these profound changes affected plant assemblages, especially in semi-natural ecosystems such as grasslands. They could have negative effects on the plant diversity and the related ecosystem functions, such as primary productivity. At the landscape scale this biodiversity-productivity relationship still have to be tested in real-world systems. It could have important economic implications concerning fodder production. We used a large-scale sampling design (30 landscape windows – 1 ×1 km) to quantify landscape structure variables (semi-natural habitat amounts and landscape heterogeneity), grassland plant taxonomic diversity and functional diversity calculated from multidimensional functional spaces (functional richness, evenness, dispersion), and grassland productivity (mean, temporal stability, spatial variability) derived from satellite image time-series. Taxonomic dissimilarity, functional evenness and functional dispersion of grassland plant assemblages were related to semi-natural habitat amount. Functional evenness and dispersion decreased with grassland amount whereas taxonomic dissimilarity and functional evenness increased with hedgerow amount. Only taxonomic and functional richness depended on landscape heterogeneity, and with a positive relationship. Grassland plant assemblages with a higher functional evenness presented a higher and more stable mean productivity. In addition to the direct effect of landscape heterogeneity on plant productivity, we demonstrated plant trait-mediated effect of landscape structure on mean and temporal stability of grassland productivity at the landscape scale. Landscape management, especially through the conservation of hedgerows can help maintain high and resilient production of fodder in the future as a result of their positive effects on plant functional diversity.

A critical assessment of the biodiversity–productivity relationship in forests and implications for conservation

The question of whether biodiversity conservation and carbon conservation can be synergistic hinges on the form of the biodiversity-productivity relationship (BPR), a fundamental ecological pattern. The stakes are particularly high when it comes to forests, which at a global level comprises a large fraction of both biodiversity and carbon. And yet, in forests, the BPR is relatively poorly understood. In this review, we critically evaluate research on forest BPRs, focussing on the experimental and observational studies of the last 2 decades. We find general support for a positive forest BPR, suggesting that biodiversity and carbon conservation are synergistic to a degree. However, we identify several major caveats: (i) although, on average, productivity may increase with biodiversity, the highest-yielding forests are often monocultures of very productive species; (ii) productivity typically saturates at fewer than ten species; (iii) positive BPRs can be driven by some third variable, in particular stem density, instead of a causal arrow from biodiversity to productivity; (iv) the BPR's sign and magnitude varies across spatial grains and extents, and it may be weak at scales relevant to conservation; and (v) most productivity estimates in forests are associated with large errors. We conclude by explaining the importance of these caveats for both conservation programmes focussed on protection of existing forests and conservation programmes focussed on restoring or replanting forests.

Tree mycorrhizal association types control biodiversity-productivity relationship in a subtropical forest.

Mycorrhizae are symbiotic associations between terrestrial plants and fungi in which fungi obtain nutrients in exchange for plant photosynthates. However, it remains unclear how different types of mycorrhizae affect their host interactions and productivity. Using a long-term experiment with a diversity gradient of arbuscular (AM) and ectomycorrhizal (EcM) tree species, we show that the type of mycorrhizae critically controls the effect of diversity on productivity. With increasing diversity, the net primary production of AM trees increased, but EcM trees decreased, largely because AM trees are more effective in acquiring nitrogen and phosphorus. Specifically, with diversity increase, AM trees enhance both nutrient resorption and litter decomposition, while there was a trade-off between litter decomposability and nutrient resorption in EcM trees. These results provide a mechanistic understanding of why AM trees using a different nutrient acquisition strategy from EcM trees can dominate in subtropical forests and at the same time their diversity enhances productivity.

Tree mycorrhizal association types control biodiversity-productivity relationship in a subtropical forest

Tree mycorrhizal association types control biodiversity-productivity relationship in a subtropical forest

Tree mycorrhizal association types control biodiversity-productivity relationship in a subtropical forest

Tree mycorrhizal association types control biodiversity-productivity relationship in a subtropical forest

Anthropogenic disturbance mediates soil water effect on diversity-productivity relationships in a temperate forest region

Mixed-species forests are known to provide enhanced ecosystem functioning when compared with monocultures. The strength of the positive diversity effect is, however, largely dependent on environmental conditions and the human footprint. The specific patterns of context-dependence effects of diversity on productivity and their causal relationships are not well known, especially regarding large-scale natural plant communities. Accordingly, the focus of this study is to evaluate effects of environmental variables on biodiversity-productivity relationships (BPR) in the temperate forest region of North-Eastern China. We used a structural equation modeling and hierarchical Bayesian model to assess individual and interactive effects of human disturbance, environmental factors, and species diversity on productivity. The SEMs showed that the negative interaction effect of human disturbance and mean annual temperature on species diversity was greater than that of human disturbance on species diversity. The positive diversity effects on forest production were severely reduced under stressful soil water conditions due to increased competition for soil water resources. The positive diversity effects were greater under harsh annual mean temperature conditions. The biodiversity effects on productivity were influenced by negative interactive effect of the human footprint and soil water availability. Our results suggest that current management strategies for diversity conservation may be countered by robust negative interactive effects and thus, may be inefficient in temperate forest regions. The variations in the strength of the relationships between tree species diversity and forest productivity are critically dependent on several environmental stressors. Our results highlight the important role of human disturbance in mediating the response of BPR to soil water deficit, and the crucial role of management strategy to prepare for future climatic challenges in our temperate forest region.

Rapid transgenerational adaptation in response to intercropping reduces competition.

By capitalising on positive biodiversity-productivity relationships, intercropping provides opportunities to improve agricultural sustainability. Intercropping is generally implemented using commercial seeds that were bred for maximal productivity in monocultures, thereby ignoring the ability of plants to adapt over generations to the surrounding neighbourhood, notably through increased complementarity, that is reduced competition or increased facilitation. This is why using monoculture-adapted seeds for intercropping might limit the benefits of crop diversity on yield. However, the adaptation potential of crops and the corresponding changes in complementarity have not been explored in annual crop systems. Here we show that plant-plant interactions among annual crops shifted towards reduced competition and/or increased facilitation when the plants were growing in the same community type as their parents did in the previous two generations. Total yield did not respond to this common coexistence history, but in fertilized conditions, we observed increased overyielding in mixtures with a common coexistence history. Surprisingly, we observed character convergence between species sharing the same coexistence history for two generations, in monocultures but also in mixtures: the six crop species tested converged towards taller phenotypes with lower leaf dry matter content. This study provides the first empirical evidence for the potential of parental diversity affecting plant-plant interactions, species complementarity and therefore potentially ecosystem functioning of the following generations in annual cropping systems. Although further studies are required to assess the context-dependence of these results, our findings may still have important implications for diversified agriculture as they illustrate the potential of targeted cultivars to increase complementarity of species in intercropping, which could be achieved through specific breeding for mixtures.

Grazing and global change factors differentially affect biodiversity-ecosystem functioning relationships in grassland ecosystems.

Grazing and global change (e.g., warming, nitrogen deposition, and altered precipitation) both contribute to biodiversity loss and alter ecosystem structure and functioning. However, how grazing and global change interactively influence plant diversity and ecosystem productivity, and their relationship remains unclear at the global scale. Here, we synthesized 73 field studies to quantify the individual and/or interactive effects of grazing and global change factors on biodiversity-productivity relationship in grasslands. Our results showed that grazing significantly reduced plant richness by 3.7% and aboveground net primary productivity (ANPP) by 29.1%, but increased belowground net primary productivity (BNPP) by 9.3%. Global change factors, however, decreased richness by 8.0% but increased ANPP and BNPP by 13.4% and 14.9%, respectively. Interestingly, the strength of the change in biodiversity in response to grazing was positively correlated with the strength of the change in BNPP. Yet, global change flipped these relationships from positive to negative even when combined with grazing. These results indicate that the impacts of global change factors are more dominant than grazing on the belowground biodiversity-productivity relationship, which is contrary to the pattern of aboveground one. Therefore, incorporating global change factors with herbivore grazing into Earth system models is necessary to accurately predict climate-grassland carbon cycle feedbacks in the Anthropocene.

Overyielding is accounted for partly by plasticity and dissimilarity of crop root traits in maize/legume intercropping systems

Abstract Positive biodiversity–productivity relationships have been found in biodiversity field experiments of grasslands, forests and natural terrestrial ecosystems, where diversity effects were separated by complementarity (CE) and selection effects (SE). However, we know little about how CE and SE are related to root traits and root dissimilarity. A 4‐year field experiment was carried out with a split‐plot design, where main plot was four nitrogen (N) applications (N0, N1, N2 and N3) and five cropping systems (maize Zea mays L./soybean Glycine max L. Merrill., maize/peanut Arachis hypogaea L. intercropped and the corresponding monocultures) with three replicates. Roots were sampled in the N0 and N2 treatments in 2 years. Intercropping effects were analysed based on grain yield for 4 years and roots were sampled down to 60 cm depth, and analysed with morphological parameters at different crop growth stages in 2 years. Intercropping significantly increased grain yield and above‐ground biomass in both intercropping systems under all N treatments. The partitioning of the net intercropping effects showed that yield advantage in intercropping was due to a positive CE under the N0 treatment, and to a positive SE with N application. Maize showed greater root morphological plasticity than the legumes did, with greater changes in root length density (RLD), root weight density (RWD) and total root surface (TS) in intercropping than in monoculture. Intercropped maize occupied a larger soil space, while lateral RLD distribution of legumes was decreased by maize. The RLD, RWD and TS of intercropped maize were constant or increased in later growth stages. SE showed a significantly positive relationship with root dissimilarity. Principal component analysis showed mean root depth and specific root length of legumes drove the positive CE in the absence of N fertilization. Root dissimilarity determined by maize explained the selection effects in overyielding. Complementarity effects under N0 were closely associated with specific root traits such as mean root depth and specific root length. Linking changes of root traits with intercropping effects above‐ground helps understand yield advantages in diverse agroecosystem. In general, a cereal species with strong phenotypic plasticity intercropped with a legume species with strong physiological plasticity can maximize the yield advantage of intercropping. Read the free Plain Language Summary for this article on the Journal blog.

Effects of enemy exclusion on biodiversity-productivity relationships in a subtropical forest experiment.

Interspecific niche complementarity is a key mechanism posited to explain positive species richness–productivity relationships in plant communities. However, the exact nature of the niche dimensions that plant species partition remains poorly known.Species may partition abiotic resources that limit their growth, but species may also be specialized with respect to their set of biotic interactions with other trophic levels, in particular with enemies including pathogens and consumers. The lower host densities present in more species‐diverse plant communities may therefore result in smaller populations of specialized enemies, and in a smaller associated negative feedback these enemies exert on plant productivity.To test whether such host density‐dependent effects of enemies drive diversity–productivity relationships in young forest stands, we experimentally manipulated leaf fungal pathogens and insect herbivores in a large subtropical forest biodiversity–ecosystem functioning experiment in China (BEF‐China).We found that fungicide spraying of tree canopies removed the positive tree‐species richness–productivity relationship present in untreated control plots. The tree species that contributed the most to this effect were the ones with the highest fungicide‐induced growth increase in monoculture. Insecticide application did not cause comparable effects. Synthesis. Our findings suggest that tree species diversity may not only promote productivity by interspecific resource‐niche partitioning but also by trophic niche partitioning. Most likely, partitioning occurred with respect to enemies such as pathogenic fungi. Alternatively, similar effects on tree growth would have occurred if fungicide had eliminated positive effects of a higher diversity of beneficial fungi (e.g. mycorrhizal symbionts) that may have occurred in mixed tree species communities.

What plant–pollinator network structure tells us about the mechanisms underlying the bidirectional biodiversity productivity relationship?

The Biodiversity – Ecosystem Functioning (B–EF) relationship remains a topic of ongoing debate with most studies focusing on primary productivity, and documenting that this relationship takes many forms. It remains unclear if biodiversity drives productivity or productivity shapes biodiversity or the relationship is bidirectional. B-EF studies explore almost exclusively the relationship between species richness and ecosystem functioning, while the role of biotic interactions, a key component of ecosystem functioning, has been neglected. Here, using data of 80 local plant–pollinator networks on 20 Aegean islands, and of gross primary productivity (GPP) from the MODIS satellite, we explored the bidirectional relationship between interaction network structure (nestedness and specialization), species richness (plants and pollinators) and mean and inter-annual variability of GPP. We found that nestedness and specialisation of plant–pollinator networks is driven by mean GPP. However, specialisation alone was a significant predictor of mean GPP, implying that networks tend to be more specialised in low-productivity areas. Pollinator species richness exerted a strong effect on mean GPP with the remaining factors playing a minor role, while the effect of mean GPP on pollinator species richness was weaker. Furthermore, the nestedness of plant–pollinator networks drives inter-annual variability of GPP with more nested networks displaying less variability, which is in accordance with the predictions of the insurance hypothesis. Plant and pollinator species richness were also associated with inter-annual variability of GPP.