A climatically extreme year has large impacts on C4 species in tallgrass prairie ecosystems but only minor effects on species richness and other plant functional groups

P>1. The occurrence and intensity of climate extremes, such as extremely warm years, are expected to continue to increase with increasing tropospheric radiative forcing caused by anthropogenic greenhouse gas emissions. 2. Responses of terrestrial ecosystem processes and services - such as above-ground net primary productivity (ANPP) and maintenance of plant species diversity - to these extreme years for multiple years post-perturbation are poorly understood but can have significant feedback effects on net ecosystem CO2 uptake and ecosystem carbon sequestration. 3. We exposed six 12 000-kg intact natural tallgrass prairie monoliths to an extremely warm year (+4 degrees C in 2003) in the second year of a 4-year study (2002-2005) using the EcoCELL whole-ecosystem controlled-environment, gas exchange facility. Six control monoliths were not warmed in the second year but were maintained under average field conditions. Natural diel and seasonal patterns in air temperature were maintained in both treatments throughout the study. Thus, with the exception of the second year in the 'warmed' treatment, we created 4 years of nearly identical climate in all EcoCELLs. 4. Interannual ANPP (10 cm clipping height) responses of the entire plant community to the extreme year were largely determined by responses of the dominant C-4 grasses. These included large decreases in ANPP in 2003 followed by complete recovery to levels observed in the control ecosystems in the year following warming. Species richness and productivity of the nitrogen-fixing plant functional group appeared to play a role in defining overall plant community ANPP, however, even though this richness and productivity could not explain the decrease in community ANPP observed in warmed ecosystems in the second year (2003) of the study or its recovery in the year after (2004). Surprisingly, very few of the 67 species present in plant communities during the 4-year study responded to the warm year at any time during or after the treatment. 5. Synthesis. Results from this study indicate that as extreme climate years become more prevalent, their immediate and lagged impacts on collective ecosystem processes, such as whole-community ANPP, may be very pronounced, but effects on component ecosystem processes may be limited to the dominant plant functional group (ANPP).