Sensitivity of three grassland communities to simulated extreme temperature and rainfall events

Three grassland communities in New Zealand with differing climates and prop ortions of C3 and C4 species were subjected to one-off extreme heating (eig ht hours at 52.5 degrees C) and rainfall (the equivalent of 100 mm) events. A novel experimental technique using portable computer-controlled chambers simulated the extreme heating events. The productive, moist C3/C4 communit y was the most sensitive to the extreme events in terms of short-term commu nity composition compared with a dry C3/C4 community or an exclusively C3 c ommunity. An extreme heating event caused the greatest change to plant comm unity species abundance by favouring the expansion of C4 species relative t o C3 species, shifting C4 species abundance from 43% up to 84% at the produ ctive, moist site, This was observed both in the presence and absence of ad ded water. In the absence of C4 species, heating reduced community producti vity by over 60%. The short-term shifts in the abundance of C3 and C4 speci es in response to the single extreme climatic events did not have persisten t effects on community structure or on soil nitrogen one year later. There was no consistent relationship between diversity and stability of biomass p roduction of these plant communities, and species functional identity was t he most effective explanation for the observed shifts in biomass production . The presence of C4 species resulted in an increased stability of producti vity after extreme climatic events, but resulted in greater overall. shifts in community composition. The presence of C4 species may buffer grassland community productivity against an increased frequency of extreme heating ev ents associated with future global climate change.