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Phenological patterns, which are molded by physical and biotic interactions, in marginal habitats could reveal peculiar characteristics for the permanence of a population. Phenology of Stenocereus thurberi, a columnar cactus from the Sonoran Desert, has shown geographic variation that is associated with the weather and pollinators, however, there is no information regarding its behavior in habitats that are more humid than its core distribution area. The aim of this study was to describe the reproductive dynamics and the size structure of three populations along a scrub-tropical dry forest transition 170 km in length. Plants were characterized by height, coverage and number of stems. Phenological stages were based on the Campbell scale and were registered fortnightly on 50 individuals from each population. We found differences in the individual architecture, plant size distribution and reproductive activity. Canopy structure in plants from the coastal population (Plan de Guadalupe, 244 mm) is more branched (23.3 +/- 3.0 stems/plant) and wider (4.6 +/- 0.7 m(2)) than the population from the deciduous forest (Las Cruces, 830 mm), on which plants showed higher vertical development (9.3 +/- 1.3 stems/plant, 1.2 +/- 0.2 m(2)). Populations exhibited differences in the size structure, with bigger plants on the coast than the other populations. Timing of the breeding season was delayed 15-30 days and was extended four more weeks in the coast than in the other populations. Production of reproductive structures in peak dates showed highly significant differences (F = 31.68, p< 0.001), with a mean of 69.5 +/- 8.7 in the coast, 28.3 +/- 3.2 in Buenavista and 16.6 +/- 5.4 in Las Cruces. Rainfall and temperature are key determinants in the production of reproductive structures and in the size structure of the local populations. It is likely that the low production of pitaya flowers and fruits in the deciduous forest be a consequence of high abortion rates, predation of structures and light competition, while pollination is improved by facilitation from other columnar cactus, Stenocereus montanus.