The findings provide preliminary evidence about how quickly crops bred for single-species, or 'monoculture', settings can adapt to growing with other crop species.
Growing multiple food crops together is a more sustainable farming practice mimicking highly productive wild plant communities. This process, known as intercropping, takes advantage of complementary features of different types of crops to maximise production and minimise the need for fertilisers and other environmentally harmful practices. For example, indigenous people in North America have long grown corn, beans and squash together to maximise the yield of each plant and reduce the need for watering or fertiliser.
"Most commercial crops, however, have been bred for traits that make them highly productive in single-crop settings," explains lead author Laura Stefan, a former PhD student at ETH Zurich and now a postdoctoral researcher at Agroscope, the Swiss Confederation's Institute for Agricultural Research. "These crops may not be well suited for growing in multi-crop systems, which may reduce the benefits of intercropping."
To learn more about different crops' ability to adapt, the team grew wheat, oat, lentil, flax, camelina and coriander species in small plots. The plots included 13 combinations of two species, four mixtures of four different species, plants growing individually or in single-species parcels, in fertilised or unfertilised plots. The team repeated the experiments for three consecutive years, each year using seeds collected from the plots of the previous year to assess the generational effects of growing in different systems. In the third year, they measured the plants' traits and productivity.
They found that plants grown in the same multi-crop setting for two generations adapted to compete less and cooperate more with each other. However, the yield advantage of these multispecies crops compared to monoculture crops was only increased in fertilised plots. Over two generations, plants grown together in either monocultures or mixed-species plots grew taller. They also produced "cheaper," or thinner leaves, indicating a growth strategy associated with rapid biomass production.
"Our study shows that annual crops rapidly adapt to be more cooperative over just two generations, but this doesn't lead to increased yield advantages without fertiliser," says co-author Nadine Engbersen, who worked on the study as a PhD student at the Institute of Agricultural Sciences at ETH Zurich, Switzerland. "Unexpectedly, the plants all grew to have more similar traits rather than specialising to fill a unique niche."
The authors suggest that the short time frame of the study -- over just three years -- may explain why more differentiation did not occur. It is unlikely that many genetic changes happened during that time. However genetic selection of particular genotypes might have occurred for those species with existing genotypic variation. Furthermore, epigenetic modifications that turn genes on or off may explain some of the observed plant adaptations. Microbes or nutrient resources passed from one plant generation to the next via seeds may also explain some of these rapid adaptations.
Longer-term studies may observe more adaptations caused by genetic mutations or genetic recombination, the rearrangement of plant DNA sequences. The current results suggest selective breeding could give rise to traits that optimise cooperation and yield in multispecies plots.
"Our findings have important implications for the shift to more diversified agriculture," concludes senior author Christian Sch?b, Head of the Agricultural Ecology Group, previously at ETH Zurich and now at the University Rey Juan Carlos. "They suggest breeding plants to grow in mixed-species plots may further improve yields and reduce the need for fertiliser and other harmful practices."