Intensity of soil disturbance shapes response trait diversity of weed communities: The long-term effects of different tillage systems

Disturbances have a prominent role in structuring plant communities. However, in agroecosystems, the long-term effect of disturbances on determining trait distributions within weed communities remains little studied. We analyzed the effect of three tillage treatments, which differ in the intensity of soil disturbance, on the mean, the range and the distribution of four response traits within weed communities. We aim to test whether tillage acts as a filter restricting the range and the distribution of response traits within weed communities and leads to reduced response trait diversity or whether tilling may have a diversifying effect, creating opportunities for more phenotypes to coexist and increasing response trait diversity. To test this idea, we used data on weed abundance recorded over 24 years from an experiment in which conventional tillage (CT), minimum tillage (MT) and no-tillage (NT) systems were compared. We selected four response traits, maximum height, specific leaf area (SLA), seed weight and seed output, and computed the community weighted mean (CWM) of each trait, as well as four multi-trait metrics related to a different aspect of functional diversity. We found that soil disturbance increases available niche opportunities for weeds especially in terms of regenerative traits. CT, the greater soil disturbance, leads to a greater range and even distribution of the studied traits and that abundant weed species from CT plots hold more divergent trait values than those from MT and NT plots. Our results may be explained by the idiosyncrasy of our disturbance treatments that affect weed seed placement in the soil layers as well as the stratification and availability of soil nutrients. We also found that NT system selected for lower CWM of seed weight (and higher seed output) than MT and CT systems. NT places weed seeds mostly on the soil surface, where having a large seed output may be necessary to avoid the risk of decay or depredation. Conversely, MT and CT systems offer some advantage to other strategies such as larger seed sizes useful to germinate from depth. CWM of SLA was higher in NT and MT than in CT plots and this could be related to greater soil nutrient content in NT systems. In addition our results showed a general trend over experimental time for weed communities to increase in height (and slightly in SLA and seed production) while reducing in seed size. These features are generally associated with intensive farming systems.This work has been funded by FEDER (European Regional Development Funds) and the Spanish Ministry of Economy and Competitiveness funds (Projects AGL2012-33736 and AGL2012-39929-C03-01).Peer reviewe