The productive performance of intercropping.

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.

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.

Experimental evidence that evolutionarily diverse assemblages result in higher productivity.

There now is ample experimental evidence that speciose assemblages are more productive and provide a greater amount of ecosystem services than depauperate ones. However, these experiments often conclude that there is a higher probability of including complementary species combinations in assemblages with more species and lack a priori prediction about which species combinations maximize function. Here, I report the results of an experiment manipulating the evolutionary relatedness of constituent plant species across a richness gradient. I show that assemblages with distantly related species contributed most to the higher biomass production in multispecies assemblages, through species complementarity. Species produced more biomass than predicted from their monocultures when they were in plots with distantly related species and produced the amount of biomass predicted from monoculture when sown with close relatives. This finding suggests that in the absence of any other information, combining distantly related species in restored or managed landscapes may serve to maximize biomass production and carbon sequestration, thus merging calls to conserve evolutionary history and maximize ecosystem function.

Mixtures of macrophyte growth forms promote nitrogen cycling in wetlands.

The importance of aquatic plant diversity in regulating nutrient cycling in wetlands remains poorly understood. We investigated how variation in macrophyte growth form (emerging, submerged and bryophyte) combinations and species mixtures affect nitrogen (N) removal from the water and N accumulation in plant biomass. We conducted a wetland mesocosm experiment for 100 days during July-September 2015. Twelve species were grown in mono- and in two-species mixed cultures for a total of 32 single and two-growth form combinations. Nitrogen removal from the water was quantified on three occasions during the experiment, while N accumulation in plant biomass was determined following termination of the experiment. The number of species and growth forms present increased N removal and accumulation. The growth form combinations of emerging and bryophyte species showed the highest N accumulation and N removal from water, followed by combinations of emerging species. By contrast, submerged species growing in the presence of emerging or other submerged species showed the lowest levels of N accumulation and N removal. Temporal variation in N removal also differed among growth form combinations: N removal was highest for emerging-bryophyte combinations in July, but peaked for the emerging-submerged and emerging-bryophyte combinations in August. Indeed, the occurrence of complementarity among macrophyte species, particularly in combinations of bryophyte and emerging species, enhanced N removal and uptake during the entire growing season. Our study highlights the importance of bryophytes, which have been neglected in research on nutrient cycling in wetlands, for aquatic N cycling, especially given their worldwide distribution across biomes. Overall, our findings point towards the potential important role of the diversity of macrophyte growth forms in regulating key ecosystem processes related to N cycling in wetlands.