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Productivity of irrigated prickly pear cactus [Opuntia ficus-indica (L.) Miller] was studied over 3 years in central Chile using two planting densities. A low-density planting (0.25 plants/m2), traditionally favored for fruit production, had maximal fruit productivity in the 2nd year (6 Mg dry weight/ha per year). A high-density planting (24 plants/m2), which assured almost full interception of incident solar radiation, led to an extremely high shoot dry-weight productivity (50 Mg·ha-1·year-1) in the 2nd year and maximal fruit productivity (6 Mg·ha-1·year-1) in the 3rd year. Cladode dry weight tended to increase with cladode surface area. However, fruit production did not occur until the dry weight per cladode exceeded the minimum dry weight for a particular cladode surface area by at least 33 g. The year-to-year variation in fruit production apparently reflected variations in such excess dry weight and, hence, in the storage reserves of individual cladodes.

Summary• We studied the seasonal fluctuation of soil respiration (R S ), and its rootdependent (R R ) and basal (R B ) components, in a Vitis vinifera (Chardonnay) vineyard.• The R S components were estimated through independent field methods (y-intercept and trenching) and modeled on the basis of a Q 10 response to soil temperature, and fine and coarse root respiration coefficients. The effect of assimilate availability on R R was assessed through a trunk girdling treatment.• The apparent Q 10 for R R was twice that of R B (3.5 vs 1.6) and increased linearly with increasing vine root biomass. The fastest R R of fine roots was during rapid fruit growth and the fastest R R of coarse roots was immediately following fruit development. R S was estimated at 32.6 kg ha )1 d )1 (69% as a result of R R ) for the hottest month and at 7.6 kg ha )1 d )1 (18% as a result of R R ) during winter dormancy. Annual R S was low compared with other natural and cultivated ecosystems: 5.4 Mg ha )1 (46% as a result of R R ).• Our estimates of annual vineyard R S are the first for any horticultural crop and suggest that the assumption that they are similar to those of annual crops or forest trees might lead to an overestimation.

Opuntia ficus‐indica (L.) Mill., a prickly pear cactus cultivated worldwide for its fruits and stem segments, can have an annual dry weight productivity exceeding that of many crops. Using a recently introduced environmental productivity index (EPI), the influences of water status, temperature, and photosynthetically active radiation (PAR) on its productivity can be predicted. This investigation calculated the water index, the temperature index, and the PAR index, whose product equals EPI, for 169 sites distributed approximately uniformly across the contiguous USA for present climatic conditions as well as for those associated with an elevated CO2 concentration of 650 µL L−1. The effect of elevated CO2 on growth of O. ficus‐indica was directly measured, and low temperature limitations on productivity were considered. The dry weight gain of O. ficus‐indica during 6 mo in an environmental growth chamber was 23% greater at 650 compared with 350 µL L−1 CO2 and increased as the duration of the wet period increased, in agreement with predictions of the water index (the fraction of maximal net CO2 uptake during a 24‐h period for the prevailing plant water status). For closely spaced plants that lead to a high productivity per unit ground area, EFI averaged about 0.10, except in desert regions where the water index lowered EPI, in the far North or South and at high elevations where the temperature index lowered EPI, and in the Northeast and Northwest where the PAR index lowered EPI. The predicted annual dry weight productivity for O. ficus‐indica was 12.8 Mg ha−1 yr−1 under current conditions, and 16.3 Mg ha−1 yr−1 under those associated with 650 µL L−1 CO2. Both productivities are relatively high compared with other agronomic plants. The percentage of sites where temperatures fall below −15 °C at least once during the 10 years simulated, which would be lethal to most prickly pear cacti, was reduced frori 49 to 18% by the general warming expected to accompany an approximate doubling of the atmospheric CO2 concentration

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