A meta-analysis of plant responses to elevated CO2 and temperature

Interactive effects of rising atmospheric CO₂ and temperatures on plants have not been reviewed with consideration for both increasing mean temperatures and acute heat stress.  Here, we use meta-analysis of 55 individual studies to examine CO₂ and warming effects on total biomass, net photosynthesis (P_n), Photosystem II efficiency (PSII), stomatal conductance (g_s), total non-structural carbon (TNC) and nitrogen concentration (%N).  Data were categorized by photosynthetic type (C₃, C₄) and treatment temperatures (AM, ambient; EL, elevated, =AM+1-8^oC; HS, heat stress, =AM+>8^oC).  In C₄ plants, elevated (vs. current) CO₂ altered P_n by 9.4, 23.7, and -10.9% for AM, EL, and HS treatments, respectively, compared to 25.9, 20.1, and 24.4% for C₃ species.  PSII was not affected by CO₂ in C₃ species, but was decreased by high CO₂ in C₄ species at EL and HS.  High CO₂ reduced g_s in C₃ and C₄ species at all temperature treatments, with larger reductions for C₄.  Total mass was increased more by high CO₂ in C₃ (AM=36.0, EL=36.1, HS=23.5%) than in C₄ species (AM=-16.5, EL=10.6, and HS=7.0%).  TNC for C₃ species was decreased in roots, but increased in leaves, by elevated CO₂ at all temperature treatments, while the reverse was true for C₄ species.  At HS, elevated CO₂ decreased %N in leaves and roots for C₃ and C₄ species.  These results indicate that CO₂ can have positive, negative, or neutral effects on plant response to increasing temperatures, which may vary between C₃ and C₄ plants and for moderate warming vs. heat waves.