Management breaks the natural productivity-biodiversity relationship in forests and grassland: an opinion

Background: Two approaches mark the difference between the "ecological" and "agricultural" view of the biodiversity/growth relation. In ecology the trend is averaged by taking monocultures of all species as baseline to evaluate mixtures. This contrasts the "agricultural" view focusing on the most productive species or species combination as baseline to evaluate mixtures. The present study investigates the change of highest rates (maximum) productivities in grasslands and forests with increasing plant (or tree) diversity, and compares these with the average response. Methods: We base our analysis on existing published datasets relating the growth of plant stands (growth rate per land area) to the diversity on the same plot. We use a global dataset (Ellis et al. 2012 and MODIS-data, see Fig. 1), the grassland experiment in Jena (Buchmann et al. 2017), the regional study on forests in Romania and Germany by Bouriaud et al. (2016), and data from the German National Forest inventory (BWI 3, see Fig. 3). In all cases the average response of growth to changes in biodiversity as well as the boundary line of the maximum values was calculated. Results: In both vegetation types a decreasing trend of maximum productivity with any added species emerges, contrasting the average trend that was positive in grassland, but absent in forests. The trend of maximum values was non-significant in grasslands probably due to the fact that not all combinations of species mixtures were available. In temperate forests, maximum productivity decreases significantly by about 10% in regional studies and by 8% at national scale with each added species. Maximum biomass per area was the same for managed and unmanaged conditions. A global assessment of NPP and biodiversity could also not confirm a general positive biodiversity productivity relationship. Conclusions: Managed grasslands and forests reach highest productivity and volumes at low diversity. Also globally we could not confirm a biodiversity effect on productivity. Despite this, for long-living organisms, such as trees, the incentive for land managers exists to reduce the risk of failure due to climate extremes and diseases by taking a loss in productivity into account and to actively maintain a mixture of species.