Long-term effects of elevated atmospheric CO2 on species composition and productivity of a southern African C4 dominated grassland in the vicinity of a CO2 exhalation

Abstract

We describe the long-term effects of a CO 2 exhalation, created more than 70 years ago, on a natural C 4 dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether longterm CO 2 enrichment changes the competitive balance between plants with C 3 and C 4 photosynthetic pathways and how CO 2 enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO 2 on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO 2 alone. Species compositional data and soil d 13 C isotopic evidence suggest no detectable effect of CO 2 enrichment on C 3 :C 4 plant mixtures and individual species dominance. Contrary to many general predictions C 3 grasses did not become more abundant and C 3 shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO 2 concentrations averaging 610 lmol mol À1 . Leaf properties such as total N decreased in the C 3 but not C 4 grass under elevated CO 2 while total non-structural carbohydrate accumulation was not affected. Elevated CO 2 possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO 2 enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO 2 source. The conservative response of this grassland suggests that many of the reported effects of elevated CO 2 on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO 2 conditions.

The role of C4grasses in New Zealand pastoral systems

“…C 3 grasses are generally reported to be more nutritious than C 4 grasses, reflecting higher nitrogen and hence protein concentration, as well as lower dry matter contents (Barbehenn et al 2004). Nutritional quality is affected by increase in CO 2 , particularly in C 3 grasses, as the latter tend to have greater decreases in nitrogen concentration (Stock et al 2005) and greater increases in non-structural carbohydrates than C 4 grasses. Despite this, C 3 grasses tend to remain more nutritious than C 4 grasses.…”

Crop yield responses to climate change in the Huang-Huai-Hai Plain of China

“…A widely held view is that the relative response of C4 plants to elevated CO 2 is usually smaller than that for C3 species, as C4 appears to be CO 2 saturated at ambient CO 2 level and shows very low responsiveness to higher CO 2 concentration (Adriana et al, 1998;Parry et al, 2004;Mera et al, 2006). However, from the metaanalysis and long-term effect analysis, this is not always true for some wild C4 species, and the differences in CO 2 response between C3 and C4 grass species are not as large as the current perception (Wand et al, 1999;Stock et al, 2005). It is only absolutely true for growth under non-stressful environmental conditions (Ghannoum et al, 2000;Kim et al, 2007).…”

Predicting C3 and C4 grass nutrient variability using in situ canopy reflectance and partial least squares regression

“…Shifts in composition of grass species following the C3 and C4 photosynthetic pathways are presumed to be accompanied by global climate trends such as elevated temperatures and atmospheric CO 2 (Stock et al 2005). The C3 and C4 types of grasses differ in their forage nutrient status and will respond differently to a changing climatic factor (Barbehenn et al 2004).…”

“…Forage nutrient variables, which are of importance, include limiting nutrients such as nitrogen (N), crude protein (CP), moisture, and non-digestible fibres (neutral fibre) that constrain the intake rate of herbivores (Grant et al 2010). Moreover, differences in the forage nutrient concentration between C3 and C4 grass dominated landscapes will affect the conditions and capacity of an agro-ecological system to support herbivore populations (Stock et al 2005).…”