Worldwide environmental productivity indices and yield predictions for a cam plant, Opuntia ficus-indica, including effects of doubled CO 2 levels

Abstract

The productivity of Opuntia ficus-indica was predicted for 253 regions on a worldwide basis using data from 1464 weather stations within 60° of the equator. First, the climatic data were used to calculate daily values of a PAR index, a temperature index and a water index. These indices, each of which has a maximum value of unity when that environmental factor is not limiting net CO 2 uptake by O. ficus-indica over a 24-h period, were multiplied together to give an environmental productivity index, which indicates how the three environmental factors limit net CO 2 uptake and hence productivity. The photosynthetically active radiation (PAR) index for a canopy of closely spaced plants that have a high productivity per unit ground area was ⩾0.20 for 25% of the earth's land surface. The temperature index annually was ⩾0.50 for 81% of the earth's land surface, indicating that local temperatures do not greatly limit net CO 2 uptake by this species. The water index was ⩾0.50 for 79% of the earth's surface for O. ficus-indica, which exhibits Crassulacean acid metabolism with its accompanying high water-use efficiency. Predicted productivities for O. ficus- indica without irrigation were at least 10 metric tons ha −1 year −1, the value for many important annual agronomic crops, for 41% of the earth's land area. Irrigation increased such high productivity regions to 77% of the earth's land surface area within 60° of the equator. For simulations that included the worldwide changes in PAR, temperature and rainfall patterns that will most likely accompany a doubling in the ambient CO 2 level, the high productivity of 10 tons ha −1 year −1 was predicted to occur for 54% of the earth's land surface area. Under elevated CO 2, the predicted productivity of O. ficus-indica increased for most of the U.S.A. and a productivity of 32 tons ha −1 year −1 was predicted for western South America.