Variable reproductive success in the Sonoran Desert cactus Stenocereus thurberi: Effects of timing and pollinator guild

Stenocereus thurberi (Organ pipe cactus) is a columnar cactus of the Sonoran Desert. Its flowering and fruiting is of paramount importance to migrant and resident species that rely on the nectar, pollen and juicy fruits as a staple during summertime. In this study, the spatial and temporal variations in the reproductive dynamics for three years in three distant populations along a latitudinal gradient are described. The northern and southern populations are representative of marginal populations. Flowering varied greatly. The southern population flowered for almost four months (late April-early September) while the northern population only for two months (early June-mid August). Although the flowers of S. thurberi show a chiropterophilous pollination syndrome, diurnal pollinators are also effective pollinators.  There are significant differences between sites in the time and type of pollinators. Exclusion experiments showed that self-pollination and autogamy treatments yielded no fruits, while cross-pollination almost invariably produced fruits indicating an obligate xenogamous reproductive system. Fruit set varied notably though time from 10% to 100% in a same season and site.  Seed set varied in a similar way, from a minimum of 20 to a maximum of 2933 seeds per fruit. This variation was probably due to temporal differences in pollinator and reproductive allocation limited by resource availability. Comparisons between natural and manual cross pollinated fruits indicated that the main factor affecting reproductive success was related to pollinator effectiveness rather than resource limitation.