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.