Response of a grassland cactus to frequency and size of rainfall events in a North American shortgrass steppe

1 Field experiments were conducted to define the types of rainfall that most effectively contribute to the water resource of a grassland cactus, Opuntia polyacantha in a shortgrass steppe. 2 The study indicated that O. polyacantha is capable of responding to rain events as small as 2.5 cm although a 10-mm event has considerably more value than a 2.5-mm or a 5-mm event. 3 Water content of the cladodes was closely related to recent rainfall, and shoot growth was greatly influenced by cladode water content. 4 Results indicate that this cactus utilizes small rainfall events (2.5-5 mm), and that its success in the short term depends more on the frequency of usable rain than on rain amount.