Quo vadis C-4? An ecophysiological perspective on global change and the future of C4 plants

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 CO2 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 CO2 occurs in both C-3 and C-4 plants, most notably in nutrient-limited situations. High CO2 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.