COS 147-2 - Conditional drought resistance in mesic oak savanna under experimental nutrient resource availability

Abstract: Resource limitations of productivity vary among biomes and ecosystems, putting different constraints on communities within the same landscape. These long-term abiotic filters modifying community structure are now subjected to directional global change, with impacts such as increased nutrient availability and simultaneous extreme drought. Importantly, community-stabilizing metrics, such as high biodiversity, are threatened from predicted impacts from elevated nutrient availability. This creates the potential for interacting impacts of community structure on ecosystem function that are challenging to predict from single factor experiments. Addressing this complexity, we implemented a cross-factor experimental manipulation in a well-studied mesic oak savanna at the Cedar Creek Ecosystem Science Reserve. Treatments included growing season extreme drought (Drought: shelter, -43% precipitation; Control: no shelter; Inverted: shelter, no change in precipitation) and increased nutrient availability (Ambient: no nutrients added, NPK: nutrient additions 10 g m^-2y^-1 for nitrogen, phosphorus, and potassium; total n = 30). Drought manipulations started in 2017, and nutrients were added < 2018 growing season. We measured (2016-2020) aboveground biomass and production, as well as community composition, to address the question: how do drought and nutrient addition alter the relationship between biodiversity and ecosystem function?

Drought did not modify community structure through shifts in biodiversity (richness: F_1,20 = 2.7, p = 0.09; effective number of species [e^H]: F_1,20 = 2.1, p = 0.15) nor impact ecosystem function (aboveground biomass: F_2,20 = 1.5, p = 0.24; production: F_2,20 = 3.1, p = 0.07) throughout the four years of imposed precipitation manipulations. In contrast, nutrient addition had an immediate positive effect on community biomass (F_1,20 = 22.3, p < 0.01) and new growth (F_1,20 = 27.8, p < 0.01), for both 2018 and 2019 that was driven strongly by an increase in grass and a reduction of legume growth. Biodiversity was likewise impacted, with initial (2018-2019) reductions in species abundance as indicated by changes in effective number of species (e^H: F_1,20 = 8.5, p < 0.01) that transitioned into reductions in species richness for 2019-2020 (richness: F_1,20 = 9.2, p < 0.01). Importantly, there were no instances of interactions between the drought and nutrient treatments, and the comparative drought resistance exhibited by this mesic oak savanna, even under nutrient enrichment, indicates that there are community and ecosystem factors promoting stability despite extreme climate conditions. These results have important implications for conservation of this rare ecosystem, and the dominant global change threats likely to alter ecosystem function.