While there is strong evidence of a positive biodiversity-productivity relationship, benefits of biodiversity are often underestimated as they ignore losses in productivity due to heterotrophic consumers. Moreover, perennial plants, for which stored carbon reserves play an important role in subsequent year productivity, may experience sustained costs from heterotrophic consumers that amplify across seasons. Thus, a multi-trophic analysis of productivity across successive seasons may reveal cumulative losses in ecosystem potential that are missed with single season assessments. Here we characterize effects of multi-year heterotroph removal on long-term accumulation of above and belowground productivity. We hypothesized that sustained heterotroph removal would result in successive increases in productivity over time across the plant diversity gradient.
Starting in 2009, we manipulated heterotrophs in experimentally-assembled perennial grassland communities within a long-running biodiversity experiment at Cedar Creek Ecosystem Science Reserve (University of Minnesota). Through regular pesticide applications, we removed arthropods, soil fungi, foliar fungi, and all heterotrophs (combination) from plots of varying plant diversities (1, 4, and 16 species). Across nine consecutive seasons, we measured biweekly plot reflectance (NDVI) to generate season-long metrics of aboveground productivity, coupled with annual root cores (5x30cm) as end of season estimates of belowground productivity.
Results/Conclusions
Within every season, aboveground productivity was greater for each heterotroph removal treatment vs. control. Moreover, across successive seasons, both foliar fungicide and all-combination treatments, but not insecticide or soil fungicides, showed increasing benefits in aboveground productivity. The combination treatment saw the greatest cumulative increases in productivity, averaging 1.22% increase in benefits of pesticides over successive seasons. This trend was likely driven by the foliar fungi removal (0.89% annual increases), as insecticide- and soil fungicide-treated plots exhibited neutral or declining benefits to productivity over time (0.17% and -0.54% respectively).
Relative benefits of heterotroph removal over time varied among plant diversity treatments. Greatest rates of increase in benefits were observed in the combination and foliar fungal removal treatments in monocultures (1.69 and 1.22% annual increases, respectively) while these rates of increase were greatly reduced in the 16 species plots (0.33% and 0.83%). Finally, while heterotroph removal on average increased root biomass, we found no increases in relative benefits of heterotroph removal across growing seasons.
This work showcases the successive, cumulative impacts of heterotrophs on primary productivity and their disproportionate effects across the diversity gradient. Our results highlight our lack of comprehensive understanding of year-to-year impacts of heterotrophs on ecosystem productivity and function.