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

Atmospheric carbon dioxide (CO2) and global mean temperature are expected to be significantly higher by the end of the 21st century. Elevated CO2 (eCO2) and higher temperature each affect plant physiology and growth, but their interactive effects have not been reviewed statistically with respect to higher chronic mean temperatures and abrupt heat stress. In this meta-analysis, we examined the effect of CO2 on the physiology and growth of plants subjected to different temperature treatments. The CO2 treatments were categorized into ambient (<400 ppm) or elevated (>560 ppm) levels, while temperature treatments were categorized into ambient temperature (AT), elevated temperature (ET; AT + 1.4–6°C), or heat stress (HS; AT + >8°C). Plant species were grouped according to photosynthetic pathways (C3, C4), functional types (legumes, non-legumes), growth forms (herbaceous, woody), and economic purposes (crop, non-crop). eCO2 enhanced net photosynthesis at AT, ET, and HS in C3 species (especially at the HS level), but in C4 species, it had no effect at AT, a positive effect at ET, and a negative effect at HS. The positive effect of eCO2 on net photosynthesis was greater for legumes than for non-legumes at HS, for non-crops than crops at ET, and for woody than herbaceous species at ET and HS. Total (W T) and above- (W AG) and below-ground (W BG) biomass were increased by eCO2 for most species groups at all temperatures, except for C4 species and W BG of legumes at HS. Hence, eCO2 × heat effects on growth were often not explained by effects on net photosynthesis. Overall, the results show that eCO2 effects on plant physiology and growth vary under different temperature regimes, among functional groups and photosynthetic pathways, and among response variables. These findings have important implications for biomass accumulation and ecosystem functioning in the future when the CO2 level is higher and climate extremes, such as heat waves, become more frequent.