The diversity of plants and trophic groups in a food web (i.e., trophic diversity) can regulate ecosystem productivity. In terrestrial ecosystems, tests of these diversity-productivity relationships are typically based on a single measure of peak biomass late in the season. However, rates of productivity accumulation and senescence are lost in a single timepoint measurement. Productivity rates across the growing season may provide new insights into seasonal timing of diversity-productivity relationships that are not discernible from the quantity of standing biomass late in the growing season. Here we measured the effects of plant diversity and trophic groups on productivity across the growing season within the longest running plant diversity experiment at Cedar Creek Ecosystem and Science Reserve in East Bethel, Minnesota. In each plant diversity treatment (1, 4, 16 species), we experimentally removed arthropods, foliar fungi, soil fungi, or all 3 trophic groups using pesticides to assess the relative impacts of plant diversity and trophic groups on plant productivity. We measured productivity throughout the growing season for 8 years using biweekly reflectance (NDVI) measures as a surrogate for productivity, allowing for comparison of cumulative early and late season response to treatments. Using these data, we test whether plant diversity and trophic groups vary in their impacts on productivity across the growing season.
Results/Conclusions
Across the entire growing season, plots with higher plant species richness had 64% higher NDVI, a response that has been shown with plant biomass. However, plant diversity effects diminished late in the growing season with 16 species plot declining in productivity 80% quicker than monocultures. In contrast, trophic diversity effects on NDVI increased in strength late in the season, with foliar fungi removal increasing NDVI by 16%. Plant diversity also affected the timing of maximum productivity; 16 species plots reached maximum NDVI 25 days earlier than monocultures. Taken together, these results provide new perspectives on the role of plant and trophic diversity in shaping the seasonal trajectory of productivity (as NDVI) across the growing season. Our work uncovers the result that consumers have greatest impacts on NDVI late in the season, whereas plant diversity effects manifest largely in the early to middle parts of the season. These findings suggest that studies estimating productivity with end-of-season harvested biomass may be underestimating biodiversity effects on critical ecosystem rates.