Explore the vast range of titles available.

Crop diversification has been put forward as a way to reduce the environmental impact of agriculture without penalizing its productivity. In this context, intercropping, the planned combination of two or more crop species in one field, is a promising practice. On an average, intercropping saves land compared with the component sole crops, but it remains unclear whether intercropping produces a higher yield than the most productive single crop per unit area, i.e., whether intercropping achieves transgressive overyielding. Here, we quantified the performance of intercropping for the production of grain, calories, and protein in a global meta-analysis of several production indices. The results show that intercrops outperform sole crops when the objective is to achieve a diversity of crop products on a given land area. However, when intercropping is evaluated for its ability to produce raw products without concern for diversity, intercrops on average generate a small loss in grain or calorie yield compared with the most productive sole crop (−4%) but achieve similar or higher protein yield, especially with maize/legume combinations grown at moderate N supply. Overall, although intercropping does not achieve transgressive overyielding on average, our results show that intercropping performs well in producing a diverse set of crop products and performs almost similar to the most productive component sole crop to produce raw products, while improving crop resilience, enhancing ecosystem services, and improving nutrient use efficiency. Our study, therefore, confirms the great interest of intercropping for the development of a more sustainable agricultural production, supporting diversified diets.

Although the relationship between species diversity and biomass productivity has been extensively studied in grasslands, the impact of tree species diversity on forest productivity, as well as the main drivers of this relationship, are still under discussion. It is widely accepted that the magnitude of the relationship between tree diversity and forest stand productivity is context specific and depends on environmental conditions, but the underlying mechanisms of this relationship are still not fully understood. Competition reduction and facilitation have been identified as key mechanisms driving the diversity–productivity relationship. However, contrasting results have been reported with respect to the extent to which competition reduction and facilitation determine the diversity–productivity relationship. They appear to depend on regional climate, soil fertility, functional diversity of the tree species involved, and developmental stage of the forest. The purpose of this review is to summarize current knowledge and to suggest a conceptual framework to explain the various processes leading to higher productivity of species-rich forests compared with average yields of their respective monocultures. This framework provides three pathways for possible development of the diversity–productivity relationship under a changing climate.

1. There is a growing interest in understanding the relationship between diversity and below-ground productivity due to the critical contribution of below-ground systems to overall terrestrial productivity. Yet, the temporal (seasonal and developmental) changes in diversity effects on below-ground productivity and their underlying mechanisms remain unclear. 2. We hypothesized that (i) diversity effects on fine root productivity increase with stand development, and (ii) increased diversity effects associated with stand development result from augmented horizontal soil space utilization, increased forest floor depth for rooting, enhanced effects in nutrient-poor soil layers and/or foraging towards high nutrient availability. 3. We investigated the effects of tree species diversity on fine root productivity by sampling 18 stands dominated by single species and their mixtures in post-fire boreal forests of two stand ages (8 and 34 years following stand-replacing fire). Species evenness was significantly higher in species mixtures than in single-species-dominated stands at both age classes, while species richness did not differ across stand types and age classes. 4. We found that the annual fine root production was higher in mixtures than the mean of single-species-dominated stands in both stand ages, with a significantly higher magnitude of effects in the 34-year-old than 8-year-old stands. Mixtures had higher horizontal soil volume filling than single-species-dominated stands with a more pronounced increase in the 34-year-old than 8-year-old stands. Compared with the 8-year-old stands, the 34-year-old stands had increased forest floor depth and greater overyielding with soil depth, and their fine root productivity was more responsive to the vertical variation in soil phosphorus concentrations among soil layers. 5. Synthesis. Our results provide evidence for increasing positive diversity effects on fine root productivity with stand development in heterogeneous natural forests. Moreover, our results indicate that the increased positive diversity effects with stand development was the result of multiple mechanisms, including higher horizontal soil volume filling, a thicker forest floor layer for rooting, a higher magnitude of complementarity in nutrient-poor deep soil layers and stronger nutrient foraging towards soil layers with high nutrient concentrations in older than younger stands.

1. There is increasing evidence that species diversity enhances the temporal stability (TS) of community productivity in different ecosystems, although its effect at the population and tree levels seems to be negative or neutral. Asynchrony in species responses to environmental conditions was found to be one of the main drivers of this stabilizing process. However, the effect of species mixing on the stability of productivity, and the relative importance of the associated mechanisms, remain poorly understood in forest communities. 2. We investigated the way mixing species influenced the TS of productivity in Pinus sylvestris L. and Fagus sylvatica L. forests, and attempted to determine the main drivers among overyielding, asynchrony between species annual growth responses to environmental conditions, and temporal shifts in species interactions. We used a network of 93 experimental plots distributed across Europe to compare the TS of basal area growth over a 15-year period (1999-2013) in mixed and monospecific forest stands at different organizational levels, namely the community, population and individual tree levels. 3. Mixed stands showed a higher TS of basal area growth than monospecific stands at the community level, but not at the population or individual tree levels. The TS at the community level was related to asynchrony between species growth in mixtures, but not to overyielding nor to asynchrony between species growth in monospecific stands. Temporal shifts in species interactions were also related to asynchrony and to the mixing effect on the TS. 4. Synthesis. Our findings confirm that species mixing can stabilize productivity at the community level, whereas there is a neutral or negative effect on stability at the population and individual tree levels. The contrasting findings regarding the relationships between the temporal stability and asynchrony in species growth in mixed and monospecific stands suggest that the main driver in the stabilizing process may be the temporal niche complementarity between species rather than differences in species' intrinsic responses to environmental conditions.

Aim: Positive relationships between plant species diversity and above-ground productivity have been observed across a wide range of terrestrial ecosystems. Despite a critical contribution of below-ground productivity to overall terrestrial productivity, no consensus exists about the nature of the relationship between species diversity and below-ground productivity. Location: Global. Methods: We collected data from published studies conducted in natural and planted forests and experimental grassland, crop and pot systems that were purposely implemented to isolate the effects of plant species diversity from other factors, such as soil conditions and topographic features. We conducted meta-analyses of 170 observations for root biomass and 23 observations for root production, derived from 48 published studies, using weighted linear modelling with bootstrap procedures to reconcile the effects of diversity on fine root productivity. Results: We found that species mixtures had, on average 28.4% higher fine root biomass and 44.8% higher annual production than monocultures. Higher fine root biomass in species mixtures than in monocultures was consistent across natural forests, planted grasslands, croplands and pot systems, except for young planted forests. Transgressive overyielding was only evident for planted grasslands. The log response ratio of fine root biomass in species mixtures to that in respective monocultures increased with species richness across all ecosystem types, and also increased with experiment age in grasslands. Main conclusions: Our meta-analysis reveals positive effects of species diversity on below-ground productivity. Despite profound differences in environments among terrestrial ecosystems, our analysis demonstrated that below-ground productivity responds similarly to variations in species richness. Furthermore, our study also reveals shifts in the effects of diversity over time in both forests and grasslands. Future efforts are needed to further understand below-ground productivity-diversity relationships.

Stability is an important property of ecological systems, many of which are experiencing increasing levels of anthropogenic environmental changes. However, how these environmental changes influence ecosystem stability remains poorly understood. We conducted an 8‐year field experiment in a semi‐arid natural grassland to explore the effects of two common environmental changes, precipitation and nitrogen enrichment, on the temporal stability of plant above‐ground biomass. A split‐plot design, with precipitation as the main plot factor and nitrogen as the subplot factor, was used. Temporal stability was related to potential explanatory abiotic and biotic variables using regressions and structural equation modelling. Increase in growing season precipitation enhanced plant species richness and promoted temporal stability of plant above‐ground biomass. Nitrogen fertilization, however, reduced both plant species richness and temporal stability of plant above‐ground biomass. Contrary to expectations, species richness was not an important driver of stability. Instead, community temporal stability was mainly driven by water and nitrogen availability that modulated the degree of species asynchrony and, to a lesser extent, by the stability of dominant plant species. Synthesis. Our results highlight the importance of limiting resources for regulating community biomass stability and suggest that the projected increase in growing season precipitation may potentially offset negative effects of increased atmospheric nitrogen deposition on species diversity and community stability in semi‐arid grasslands.

• Variations in crown forms promote canopy space-use and productivity in mixed-species forests. However, we have a limited understanding on how this response is mediated by changes in within-tree biomass allocation. Here, we explored the role of changes in tree allometry, biomass allocation and architecture in shaping diversity–productivity relationships (DPRs) in the oldest tropical tree diversity experiment. • We conducted whole-tree destructive biomass measurements and terrestrial laser scanning. Spatially explicit models were built at the tree level to investigate the effects of tree size and local neighbourhood conditions. Results were then upscaled to the stand level, and mixture effects were explored using a bootstrapping procedure. • Biomass allocation and architecture substantially changed in mixtures, which resulted from both tree-size effects and neighbourhood-mediated plasticity. Shifts in biomass allocation among branch orders explained substantial shares of the observed overyielding. By contrast, root-to-shoot ratios, as well as the allometric relationships between tree basal area and aboveground biomass, were little affected by the local neighbourhood. • Our results suggest that generic allometric equations can be used to estimate forest aboveground biomass overyielding from diameter inventory data. Overall, we demonstrate that shifts in tree biomass allocation are mediated by the local neighbourhood and promote DPRs in tropical forests.

1. Forest ecosystem functioning generally benefits from higher tree species richness, but variation within richness levels is typically large. This is mostly due to the contrasting performances of communities with different compositions. Evidencebased understanding of composition effects on forest productivity, as well as on multiple other functions will enable forest managers to focus on the selection of species that maximize functioning, rather than on diversity per se. 2. We used a dataset of 30 ecosystem functions measured in stands with different species richness and composition in six European forest types. First, we quantified whether the compositions that maximize annual above-ground wood production (productivity) generally also fulfil the multiple other ecosystem functions (multifunctionality). Then, we quantified the species identity effects and strength of interspecific interactions to identify the “best” and “worst” species composition for multifunctionality. Finally, we evaluated the real-world frequency of occurrence of best and worst mixtures, using harmonized data from multiple national forest inventories. 3. The most productive tree species combinations also tended to express relatively high multifunctionality, although we found a relatively wide range of compositions with high- or low-average multifunctionality for the same level of productivity. Monocultures were distributed among the highest as well as the lowest performing compositions. The variation in functioning between compositions was generally driven by differences in the performance of the component species and, to a lesser extent, by particular interspecific interactions. Finally, we found that the most frequent species compositions in inventory data were monospecific stands and that the most common compositions showed below-average multifunctionality and productivity. 4. Synthesis and applications. Species identity and composition effects are essential to the development of high-performing production systems, for instance in forestry and agriculture. They therefore deserve great attention in the analysis and design of functional biodiversity studies if the aim is to inform ecosystem management. A management focus on tree productivity does not necessarily trade-off against other ecosystem functions; high productivity and multifunctionality can be combined with an informed selection of tree species and species combinations.

Light partitioning is often invoked as a mechanism for positive plant diversity effects on ecosystem functioning. Yet evidence for an improved distribution of foliage in space or time in diverse plant communities remains scarce, and restricted mostly to temperate grasslands. Here we identify the mechanisms through which tree species diversity affects community-level light capture in a biodiversity experiment with tropical trees that displays overyielding, i.e., enhanced biomass production in mixtures. Using a combination of empirical data, mechanistic models, and statistical tools, we develop innovative methods to test for the isolated and combined effects of architectural and temporal niche differences among species as well as plastic changes in crown shape within species. We show that all three mechanisms enhanced light capture in mixtures and that temporal niche differences were the most important driver of this result in our seasonal tropical system. Our study mechanistically demonstrates that niche differences and phenotypic plasticity can generate significant biodiversity effects on ecosystem functioning in tropical forests.

Past failures of monocultures, caused by wind-throw or insect damages, and ongoing climate change currently strongly stimulate research into mixed-species stands. So far, the focus has mainly been on combinations of species with obvious complementary functional traits. However, for any generalization, a broad overview of the mixing reactions of functionally different tree species in different mixing proportions, patterns and under different site conditions is needed, including assemblages of species with rather similar demands on resources such as light. Here, we studied the growth of Scots pine and oak in mixed versus monospecific stands on 36 triplets located along a productivity gradient across Europe, reaching from Sweden to Spain and from France to Georgia. The set-up represents a wide variation in precipitation (456–1250 mm year−1), mean annual temperature (6.7–11.5 °C) and drought index by de Martonne (21–63 mm °C−1). Stand inventories and increment cores of trees stemming from 40- to 132-year-old, fully stocked stands on 0.04–0.94-ha-sized plots provided insight into how species mixing modifies stand growth and structure compared with neighbouring monospecific stands. On average, the standing stem volume was 436 and 360 m3 ha−1 in the monocultures of Scots pine and oak, respectively, and 418 m3 ha−1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m3 ha−1 year−1 in the monocultures and 10.5 m3 ha−1 year−1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter (p < 0.05), a 7% larger dominant diameter (p < 0.01) and a 9% higher growth of basal area and volume in mixed stands compared with neighbouring monocultures. For Scots pine, the productivity advantages of growing in mixture increased with site index (p < 0.01) and water supply (p < 0.01), while for oak they decreased with site index (p < 0.01). In total, the superior productivity of mixed stands compared to monocultures increased with water supply (p < 0.10). Based on 7843 measured crowns, we found that in mixture both species, but especially oak, had significantly wider crowns (p < 0.001) than in monocultures. On average, we found relatively small effects of species mixing on stand growth and structure. Scots pine benefiting on rich, and oak on poor sites, allows for a mixture that is productive and most likely climate resistant all along a wide ecological gradient. We discuss the potential of this mixture in view of climate change.