Pollination biology of two winter-blooming giant columnar cacti in the Tehuacán Valley, central Mexico

In the neotropics, central Mexico and Venezuela, columnar cacti (mostly belonging to tribe Pachycereae) are pollinated mainly by bats. In the Tehuacán Valley 36 species of columnar cacti have their blooming peak almost simultaneously between April and June, and species with chiropterophilous pollination have been shown to be self-incompatible, fruiting only after bat pollination. Nectarivorous bats are abundant also during spring and summer, and migrate during autumn and winter. Two columnar cacti,Pachycereus weberi(tribe Pachycereae) andPilosocereus chrysacanthus(tribe Cereae), flower during winter and early spring facing an apparent scarcity of bats. We hypothesized that under this condition, these plants may resemble the columnar cacti in extratropical deserts where both bats and diurnal vectors can effect seed production. However, we found that these two plant species have white, long tubular flowers, are nocturnal, self-incompatible and produce fruits only after the visitation of four nectar-feeding bats and two frugivorous bats. We found that nectar-feeding bats have a small resident population in the Valley. In addition these cacti species grow along possible routes of bat altitudinal migration. A long, and possibly variable flowering season may be also related to low pollen vector density.