Trichocereus atacamensis (Phil.) W.T.Marshall & T.M.Bock (Cactaceae) shows decreasing germination fractions with seed age and differences in regeneration niche compared with a congeneric, sympatric species (T. tarijensis (Vaupel) Werderm.)

Abstract

Summary: The Cactaceae are a characteristic component of arid zones in the American continent. Some species are known to possess a seed bank, as a way to cope with environmental uncertainty, albeit seeds usually lose viability with time. Seed banks may respond with a constant, low germination proportion (delayed germination) or germination may depend on rainfall amount (predictive germination). On the other hand, several species of cacti usually co-occur in one locality. In the case of large, columnar species, we would expect competition as a corollary of their history of coexistence. We conducted two irrigation experiments (three watering levels: 40, 60, 80mm) to assess how seed viability in one columnar cactus is lost as years pass (three years), and to determine differences in germination/ early survival between two dominant columnar, sympatric cacti (Trichocereus atacamensis (Phil.) W.T.Marshall & T.M.Bock, T. tarijensis (Vaupel) Werderm.) from the arid puna. We predicted seeds from more recent years would have greater germination fractions and survival, and that the two columnar cactus species would show differences in their regeneration niche in order to decrease competition. We found a tendency for greater germination in younger seeds, and no effect of seed age on survival. Seeds from one of the species (T. atacamensis) had twice as many survivors as the other species (T. tarijensis). Both species showed a predictive response to water, having greater germination proportions with more irrigation. In conclusion, seed age affected germination, the columnar cactus species had a predictive germination, and showed differences in some aspects of their regeneration niche.