What yields most: 4-species mixtures or the best monoculture?




One of our experimental sites with a clear checkerboard pattern illustrating the different plant communities that varied in richness (monocultures, 4-species mixtures) and evenness (relative abundance of the four species).

What yields most in intensive grassland systems: the best of four monocultures or a four-species mixtures? We undertook a major multi-site, multi-year experiment that investigated this question (among others)...
UPDATE: Open Access version at: http://hdl.handle.net/11019/438

The agricultural research community is looking for more sustainable means of production; intercropping research points the way to new agronomic systems that can help reconcile food production and environmental sustainability. As part of a large consortium, we recently published a paper in Journal of Applied Ecology on multispecies mixtures, which builds on the 2007 reporting of year 1 results from the same experiment. Our latest paper presents results over a three-year duration. Thus, our work on multi-species mixtures is timely, as we ask what yields most: the best of four monoculture or a four-species mixtures?
In principle, there are competing processes that result in the answer to this question being not so clear. The best of the component monocultures is obviously best-suited to the prevailing conditions. The mixture of different species offers the possibility of synergistic (and antagonistic) interspecific interactions that may positively (or negatively) affect yield. However, any synergistic effects have to be sufficiently advantageous to overcome the effect of diluting the yield of the highest-yielding monoculture species with that from lower-yielding species.

As part of EU COST Action 852 ‘Quality legume-based forage systems for contrasting environments’, we established a common diversity experiment that involved monocultures of four agronomic species (two grasses and two legumes) and a range of 4-species agronomic communities at each of 30 European sites and a Canadian site. A common field experiment of 30 plots was established at each site with two seed density levels of a design with four monocultures (single species) and 11 mixtures (with systematically varying proportions of four species).

Q. 1.      Were there yield benefits (overyielding) from mixtures and, if so, did the benefits persist over three years and across sites?
Answer: Definitely! Overyielding occurs when a mixture (open circles in Fig. 1 represent each of the 11 mixture communities at a site) performs better than the average monoculture performance (shaded square in Fig. 1). Note that each data point averages across years, and across the two seeding densities.  





Fig. 1. Mixtures generally yielded better than the best-performing monoculture across the 31 international field sites. Average annual yield (includes weed biomass) of 11 mixture communities that varied in their sown relative abundances (○), the maximum-yielding monoculture (▬) and mean monoculture yield (■) at each site. Sites arranged in order of biomass production. Symbols above sites indicate where mixtures significantly outyielded the best-performing monoculture. From Finn et al. (2013).

Overall, we found that the yield of 4-species mixtures exceeded that expected from monoculture performance alone, and generally exceeded that of the best-performing monoculture species.


Q. 2.      Were the yield benefits sufficiently large for mixtures to outyield the best-performing monoculture?
Answer: In general, yes. For total yield (including weeds) we found that 65% of mixture communities outyielded the best-yielding monoculture and was significant at 58% of sites, with an average yield ratio of mixture:monoculture of 1.07. Interestingly, our test of transgressive overyielding is quite conservative, and in many cases, the best-performing monoculture was very often not the choice of monoculture that farmers would make if they were planting monocultures (see Supporting Information in the original paper).

 

Acknowledgements

Co-ordination of this project was supported by the EU Commission through one of the Working Groups in COST Action 852 ‘Quality legume-based forage systems for contrasting environments’. (First photo courtesy of Michael Fothergill).

References

Finn et al. 2013. Ecosystem function enhanced by combining four functional types of plant species in intensively managed grassland mixtures: a 3-year continental-scale field experiment. Journal of Applied Ecology 50: 365-375.

http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12041/abstract

UPDATE: Open Access version at: http://hdl.handle.net/11019/438

Kirwan, L., Lüscher, A., Sebastià, M.T., Finn, J.A., Collins, R., Porqueddu, C., Helgadóttir, A., Brophy, C., Bélanger, G., Fothergill, M., Frankow-Lindberg, B., Garcia-Sarrion, R., Ghesquiere, A., Golinski, P., Jørgensen, M., Kadžiuliene, Z., Nyfeler, D., Nykänen-Kurki, P., Parente, G., Vasileva V. and Connolly, J. 2007. Evenness drives consistent diversity effects in intensive grassland systems across 28 European sites. Journal of Ecology 95: 530-539. See related post.

Kirwan, L., Connolly, J., Finn, J.A., Brophy, C., Lüscher, A. Nyfeler, D. and Sebastia, T. 2009. Diversity-interaction modelling - estimating contributions of species identities and interactions to ecosystem function. Ecology, 90: 2032-2038.