PARENT SESSION
Oral Session # 12: Biogeochemistry I: Carbon Dioxide Cycling.
Presiding: D Markewitz
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 204.

Assemblages of annuals exhibit nonlinear responses to gradients of elevated air temperature and atmospheric CO₂.

Bazzaz, Fakhri^*,1, Wolfe-Bellin, Kelly¹, ¹ Harvard University, Cambridge, MA, USA

ABSTRACT- Two abiotic global change factors, air temperature and atmospheric CO₂, are expected to have considerable impact on future ecosystem function by reducing biodiversity. We conducted a study in which assemblages of nine annual species, representing three functional groups (C₃, C₄, and legumes), were exposed to gradients of either elevated air temperature alone or a combination of elevated air temperature and atmospheric CO₂. A relatively novel approach was employed, in which greenhouse tunnels were used to create the two environmental gradients. All plant assemblages were exposed to an air temperature gradient that ranged from ambient to approximately 3°C above ambient during daylight hours. Additionally, half the plant assemblages were exposed to a CO₂ gradient that ranged from 370 to 810 ppm. Species-specific, non-linear responses to the two gradient treatments were evident in plant height and biomass. In particular, Ambrosia artemisiifolia, a C₃ species, exhibited little response to increasing air temperature alone, but exhibited a strong positive response that quickly reached an asymptote when exposed to increasing air temperature and CO₂. In fact, a strong shift in species composition was evident by the end of the study. Two species of Setaria, both C₄ grasses, dominated assemblages under conditions of ambient CO₂ and high temperature, while A. artemisiifolia dominated assemblages grown under conditions of high CO₂ and high temperature. These results demonstrate that it is important to consider both elevated air temperature and atmospheric CO₂ when conducting experiments designed to predict ecosystem responses to global change, and that plants may exhibit important nonlinear responses to gradients of both factors.

Key words: air temperature, carbon dioxide, annual plants, environmental gradients