Response to elevated CO₂ from a natural spring in a C₄-dominated grassland depends on seasonal phenology

Abstract

A South African C₄-dominated grassland was exposed to twice-ambient atmospheric CO₂ concentration using gas emitted by a natural CO₂ spring and distributed over a 7m x 7m plot. A similar control plot was established 20m away at near-ambient CO₂ concentrations. Photosynthetic CO₂ response curves were performed on three C₄ and one C₃ grass species under both treatments, in spring (post-fire), mid-summer and autumn. Photosynthetic efficiency of the post-fire dominants, Alloteropsis semialata subsp. eckloniana (C₃) and Andropogon appendiculatus (C₄) was significantly enhanced in high CO₂ only during the early season, when photosynthetic capacity was high. Thereafter, photosynthetic capacity decreased with advancing season in both species, and positive responses to high CO₂ were lost or reduced. In the mid- to late-season dominant Themeda triandra (C₄), photosynthetic capacity was maintained in elevated CO₂ during the mid- to late-season, while decreasing in ambient CO₂ relative to high CO₂. Eragrostis racemosa (C₄), a slow grower, showed no significant responses to high CO₂ during the late season. CO₂-responsiveness in the field generally followed the seasonal phenologies of each species, being greatest during the most active growth period.


Keywords: climate change; gas exchange; photosynthetic regulation; stomatal conductance


African Journal of Range & Forage Science: 2002 19(2): 81-91