POLLINATION BIOLOGY OF 2 WINTER-BLOOMING GIANT COLUMNAR CACTI IN THE TEHUACAN VALLEY, CENTRAL MEXICO

In the neotropics, central Mexico and Venezuela, columnar cacti (mostl y belonging to tribe Pachycereae) are pollinated mainly by bats. In th e Tehuacan Valley 36 species of columnar cacti have their blooming pea k almost simultaneously be tween April and June, and species with chir opterophilous pollination have been shown to be self-incompatible, fru iting only after bat pollination. Nectarivorous bats are abundant also during spring and summer, and migrate during autumn and winter. Two c olumnar cacti, Pachycereus weberi (tribe Pachycereae) and Pilosocereus chrysacanthus (tribe Cereae), flower during winter and early spring f acing an apparent scarcity of bats. We hypothesized that under this co ndition, these plants may resemble the columnar cacti in extratropical deserts where both bats and diurnal vectors can effect seed productio n. However, we found that these two plant species have white, long tub ular 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 populati on in the Valley. In addition these cacti species grow along possible routes of bat altitudinal migration. A long, and possibly variable flo wering season may be also related to low pollen vector density. (C) 19 97 Academic Press Limited.