Flowering Phenology and Observations on the Pollination Biology of South American Cacti. 3. Temporally Robust Sequential “Big Bang” Flowering of Two Unrelated Sympatric Globular Cacti in Northern Argentina

Abstract

Sequential flowering of sympatric plant species is a commonly observed phenomenon and is thought to contribute towards maintaining the pollinator community. Sequential flowering is also interpreted as a potential way to reduce competition for pollinators and to reduce interspecific pollen transport.Within-season sequential flowering is well-known for many species assemblages and usually shows a staggering over weeks and months. Here, we report on the flowering behaviour of two unrelated sympatric cacti (both Cactaceae — Cactoideae) from N Argentina, Acanthocalycium thionanthum (Cereeae — Trichocereinae) and Parodia microsperma (Notocacteae). At the start of the flowering season, the two taxa exhibit a temporally stable sequential mass-flowering that is triggered by the first rainfall event after the dry season: On the 6th or 7th day after the rainfall, Parodia microsperma starts a 3-day “big-bang” flowering period, and on the 8th or 9th day after the rainfall, Acanthocalycium thionanthum also starts a 3-day “big-bang” flowering period, with 1 day of overlap with that of Parodia. The sequence and duration, including the 1-day overlap, has been found to be temporally stable in all study years, with c. 90% of all flowers buds of a population opening synchronously in years with average spring rainfall. Later in the season, after the fruits derived from these initial flowerings have ripened, opportunistic flowering (including sporadic local small-scale mass flowerings) is exhibited by both species with no apparent synchronization or sequence. Only once was a second “big bang” flowering observed, likely caused by above-average dry conditions. The species pair is notable for the massive “big-bang” flowering triggered by rainfall with a tightly staggered synchronization, as well as for the change in flowering strategy later in the season.