Above-ground net primary production (ANPP) in tallgrass prairie is highly temporally variable, which has important consequences for ecosystem function. However, the extent to which biotic and abiotic factors drive this variation in ANPP remains unclear. Previous work has shown that climate can influence ANPP, yet much of the variation in ANPP remains unexplained. In tallgrass prairie, flower production in the dominant C4 grasses is an understudied component of ANPP, and changes in flowering may contribute significantly to variation in ANPP. Our objectives were to determine: (1) how flowering of the dominant grasses contributes to variation in ANPP; (2) the direct and indirect (via impacts on flowering) effects of climate on ANPP; and (3) how the effects of flowering and climate on ANPP are mediated by fire regime, a dominant driver of tallgrass prairie dynamics. To address these objectives, we examined 22 years of ANPP, flowering, and climate data from Konza Prairie LTER using multiple regression analysis. Data was collected from watersheds with varying fire regimes (1-, 4-, or 20-year return interval). Flowering data was separated into the three dominant grasses (Sorghastrum nutans, Andropogon gerardii, and Schizachyrium scoparium) and climate variables included monthly temperature and precipitation measures, as well as growing season and yearly precipitation.
Results/Conclusions
Across all burn regimes, ANPP was strongly positively correlated with flower production, as measured by flower stalk density and weight. This correlation between ANPP and flower production was also apparent at the species level but differed with burn regime. S. nutans flower production most strongly correlated with ANPP in annual burns, whereas ANPP most strongly correlated with A. gerardii flower production in 4- and 20-year burns. Climate had varying effects on flower production in each of the three dominant grass species. Both S. nutans and A. gerardii flower production were positively correlated with average July precipitation and total growing season precipitation (April-August). A. gerardii flower production was also positively correlated with late season (July and August) temperature in 4-year burns. S. scoparium flower production was negatively correlated with late season temperature in annual burns, however was not strongly correlated with any climate measures in 4- or 20-year burns. Our analysis shows that ANPP is strongly influenced by flower production in the dominant grasses, though the response of flower production to climate varies by species and burn regime. Overall, yearly climatic changes may play a large role in determining ANPP via total flower production in tallgrass prairie.
