Quo vadis C4? An ecophysiological perspective on global change and the future of C4 plants

Abstract

C(4) plants are directly affected by all major global change parameters, often in a manner that is distinct from that of C(3) plants. Rising CO(2) generally stimulates C(3) photosynthesis more than C(4), but C(4) species still exhibit positive responses, particularly at elevated temperature and arid conditions where they are currently common. Acclimation of photosynthesis to high CO(2) occurs in both C(3) and C(4) plants, most notably in nutrient-limited situations. High CO(2) aggravates nitrogen limitations and in doing so may favor C(4) species, which have greater photosynthetic nitrogen use efficiency. C(4) photosynthesis is favored by high temperature, but global warming will not necessarily favor C(4) over C(3) plants because the timing of warming could be more critical than the warming itself. C(3) species will likely be favored where harsh winter climates are moderated, particularly where hot summers also become drier and less favorable to C(4) plant growth. Eutrophication of soils by nitrogen deposition generally favors C(3) species by offsetting the superior nitrogen use efficiency of C(4) species; this should allow C(3) species to expand at the expense of C(4) plants. Land-use change and biotic invasions are also important global change factors that affect the future of C(4) plants. Human exploitation of forested landscapes favors C(4) species at low latitude by removing woody competitors and opening gaps in which C(4) grasses can establish. Invasive C(4) grasses are causing widespread forest loss in Asia, the Americas and Oceania by accelerating fire cycles and reducing soil nutrient status. Once established, weedy C(4) grasses can prevent woodland establishment, and thus arrest ecological succession. In sum, in the future, certain C(4) plants will prosper at the expense of C(3) species, and should be able to adjust to the changes the future brings.

Large regional-scale variation in C3/C4 distribution pattern of Inner Mongolia steppe is revealed by grazer wool carbon isotope composition

“…In grassland, the 13 C signal can vary considerably, which is primarily related to the presence of variable proportions of C3 and C4 photosynthetic types (Tieszen et al, 1997;Bird and Pousai, 1997;Collatz et al, 1998) and the large difference in carbon isotope discrimination ( 13 ) between them (Farquhar et al, 1989). Variation in the C3/C4 ratio has wide biogeochemical and land use implications: it affects the magnitude and seasonal distribution of biomass production, soil carbon storage, water use and nutrient cycling (Tieszen et al, 1997;Bird and Pousai, 1997;Epstein et al, 1998;Sage and Kubien, 2003;Semmartin et al, 2004). Hence, as it indicates the C3/C4 ratio, δ 13 C is a useful proxy of vital functions of grassland.…”

“…There is abundant evidence that the current distribution of C4 plants is primarily controlled by growing season temperature (Ehleringer et al, 1997;Collatz et al, 1998) and that this is related to the higher effective quantum yield of CO 2 fixation (Ehleringer and Bjorkman, 1977) or higher maximum photosynthetic rate of C4 plants at high temperature (Sage and Kubien, 2003;Still et al, 2003). Central East Asia has experienced one of the largest temperature increases on earth during the last decades (Chase et al, 2000;Yu et al, 2003), which should promote C4 plants.…”

“…On the other hand, rising atmospheric CO 2 stimulates C3 photosynthesis more than C4 (e.g., Ehleringer et al, 1997). Nutrient limitation due to increased photosynthesis may again favour C4 species (Sage and Kubien, 2003) leading to a complex web of interactions, which makes it difficult to predict the overall effect. Furthermore, the different influences act on different spatio-temporal scales.…”

“…C4 dicots predominate in hot arid, saline or highly disturbed habitats (e.g., Ehleringer et al, 1997). Presumably such secondary controls have their strongest effect on C3/C4 abundance in those regions which have a growing season mean temperature that is near the C3/C4 crossover temperature of light use efficiency (Ehleringer et al, 1997;Sage and Kubien, 2003;Still et al, 2003). In this respect, the grassland of Inner Mongolia is of particular interest, as the average temperature during the summer months (especially July) when most of the annual precipitation falls is close to the C3/C4 crossover temperature.…”

Can elevated CO2 modify regeneration from seed banks of floating freshwater marshes subjected to rising sea-level?

“…Nevertheless, many climate models use relatively simple assumptions of functional responses, often based on the carbon assimilation strategy of species (Cramer et al, 2001;Krinner et al, 2005), e.g., CO 2 enrichment benefits the growth of C 3 over C 4 species (Bazzaz, 1990;Sage & Kubien, 2003). In contrast, extensive study shows variable responses of species to CO 2 treatment depending on other environmental constraints (Körner, 2006).…”