The role of grassland plant species composition in defining synchronous and lagged responses of NPP and net ecosystem CO2 balance to interannual temperature variability

Background/Question/Methods   Interannual climate variability is expected to increase in responses to growing radiative forcing caused by rising anthropogenic atmospheric carbon dioxide. Net annual ecosystem carbon balance is defined primarily by net ecosystem productivity (NEP), or the sum of the instantaneous net ecosystem CO2 exchange rate (NEE) during one year. Since NEP is the difference between net primary [plant] productivity (NPP,) and ecosystem heterotrophic respiration (Rh), understanding how individual plant species and plant functional groups contribute to NPP is essential to quantifying their role in defining ecosystem NPP and NEP responses to climate variability--the major objective of our. We exposed six of twelve replicate 12,000 kg intact tallgrass prairie monoliths located in four enclosed lysimeters (EcoCELLs) to an anomalously warm year (+4°C above air temperatures of the six “control” ecosystems) in the second year of a 4-year study and continuously quantified NEE, and once per year we measured ANPP of each species.

Results/Conclusions   Exposure to the warm year reduced ANPP in that year by 30%, followed by complete recovery to pre-warming levels in the following year under normal temperatures. Overall community ANPP responses followed the responses of the dominant C4 grasses. Forbs and nitrogen fixers remained unaffected by warming. While ANPP also was not related to community species richness, it increased with the number and mass of nitrogen fixing species present in the communities, even though N2 fixing species represented less than 13% of total community peak biomass. This result suggests a causal link between the availability of symbiotically-fixed nitrogen and the productivity of co-occurring non-fixing species. High VPDs and later reductions in soil water limited leaf CO2 uptake, plant growth and ANPP. Patterns in annual ANPP were generally mirrored in parallel changes in NEP, when observed over the four years of the study, even though the anomalously warm year significantly reduced NEP for one additional year. Thus, the species richness of nitrogen-fixing functional group seems to play a key role in defining interannual responses of ANPP and NEP in these grasslands.