The relative contributions of sexual reproduction and clonal propagation in Opuntia rastrera from two habitats in the Chihuahuan Desert

1 The clonal cactus Opuntia rastrera shows predominantly sexual reproductio n in grasslands (GH) and clonal propagation in nopaleras (NH). We assessed the effects of light, herbivory, water availability and the habitat an offs pring came from on the survival and growth of sexual or clonal offspring (i .e, seedlings and cladodes), through 3- and 4-year common garden and short- term greenhouse experiments. 2 Shading by nurse plants increased seedling survival in the held by an ord er of magnitude, and a small additional advantage due to predator protectio n by grasses was observed. Strong herbivory transforms a facultative nurse- protege relationship for seedlings into an obligatory one. 3 In the greenhouse seedlings grew batter under shade, but in the held the production of the first cladode was delayed in seedlings in the more shaded GH, Competition for soil resources may be more intense under a dense grass tussock than under a open shrub, thus affecting the nurse-protege relation ship, Seedling survival under nurse plants was similar in GH and NH, but hi gher plant cover suggests that a larger number of seedlings will establish in GH in the long term, 4 Cladode survival was higher in NH. Cladodes were more successful than see dlings at establishing in intercanopy areas, possibly due to physiological differences as well as their ability to survive partial predation. Cladode survival in intercanopy areas may explain the enhanced clonal propagation i n the more open NH scrubland. together with their susceptibility to the flo oding which affects GH. 5 The high seedling and cladode survival in the greenhouse experiments cont rasted with that observed in the field, indicating that survival is determi ned by the interaction between herbivores, plants and abiotic conditions ra ther than the physiological aptitude of the plants.