Two cacti and the first rain. Sequential flowering in Acanthocalycium and Parodia

Two sites and four seasons

Urs Eggli and Mario Giorgetta described populations near Angastaco in Salta Province. Cerro Negro lay at 1980 m and Cerro Bayo at 1925 m, approximately 3.5 km apart. The region has a prolonged dry season from Southern Hemisphere autumn to spring, followed by summer rain. Even across this small area, a shower can wet one population while leaving another in dry soil. This spatial variation allowed the researchers to compare plants starting their season on different dates [Eggli and Giorgetta 2017].

Detailed field observations covered four summers, from 2012/2013 to 2015/2016. They totalled 108 days, documented by approximately 5300 photographs and 17 videos. During 2013/2014, the observer concentrated on fruits and seed dispersal, leaving the flowering record incomplete. Rainfall was classified as slight, medium or strong, without measurements in millimetres. The study therefore reconstructs the sequence of events and the association between flowering and rainfall while retaining the qualitative nature of the information on water supply [Eggli and Giorgetta 2017].

Buds prepared before rain

Both species have small, usually solitary stems and fibrous roots spreading through shallow soil. At the end of the dry season, their bodies were shrunken and spines from adjacent areoles interlocked. Acanthocalycium occurred more often on sandy, relatively level ground, whereas Parodia was more frequent in rocky microsites. Both species grew in exposed positions and beneath shrubs. Their shared landscape therefore contained a mosaic of conditions, with partial separation of the places occupied by the two cacti [Eggli and Giorgetta 2017].

Buds were already visible before the first summer rain. In Acanthocalycium, they formed a ring around the stem apex and reached approximately 14 mm in length and 12 mm in diameter. Parodia buds were smaller. After reaching a characteristic stage, development paused for as long as a month. Early floral development drew on resources already stored by the plant. The availability of new water was associated with completion of bud development and the start of anthesis, the period when the flower is open and functional [Eggli and Giorgetta 2017].

A sequence measured from rainfall

Parodia microsperma began mass flowering on the sixth or seventh day after rain. Acanthocalycium thionanthum began on the eighth or ninth day. Each species had a flowering flush lasting about three days, with one day of overlap. In years with typical rainfall patterns, approximately 90% of the buds present on flowering-sized plants opened synchronously. This figure describes buds in a particular population at the start of the season. Flowers produced later followed a different schedule [Eggli and Giorgetta 2017].

The 2015/2016 season provided a particularly useful comparison. The first rain at Cerro Negro fell on 30 December, and Parodia began flowering on 5 January. Cerro Bayo received rain on 6 January, followed by Parodia flowering on 12 January. The difference in calendar dates thus preserved the relationship with local rainfall. In each case, Acanthocalycium entered its flowering flush later. The repeated sequence across seasons indicates a stable rhythm in the populations studied, with its immediate physiological mechanism requiring a separate experiment [Eggli and Giorgetta 2017].

A brief shower could be sufficient to initiate this sequence. At Cerro Negro, the rain on 30 December 2015 lasted about 20 minutes and wetted soil to approximately 6 cm. By the afternoon, the uppermost 2 cm were already dry. Most roots of both species lay within the superficial 2–6 cm. Root placement therefore coincided with the depth temporarily reached by water. The researchers documented this field correspondence, while the quantitative rainfall threshold needed to initiate flowering remained undetermined [Eggli and Giorgetta 2017].

An individual flower within the brief flush

On sunny days, both cacti opened their flowers at about 10 in the morning. Acanthocalycium began closing around 14, whereas Parodia usually closed between 16 and 17. An individual flower could last three days, with earlier closure after successful pollination. Flower longevity therefore influenced the actual overlap between the two flushes. Longer-lasting Parodia flowers could remain available when the first Acanthocalycium flowers opened, even though their own opening had begun earlier [Eggli and Giorgetta 2017].

Colour supplied additional information about flowering progress. Acanthocalycium thionanthum flowers were predominantly sulphur yellow and retained their colour throughout anthesis. In the observed Parodia populations, flowers that opened yellow later became orange and red. In a photograph taken on the shared flowering day, colour differences could therefore reflect both species identity and flower age. Interpreting the image required attention to the observation date and the sequence in which buds opened [Eggli and Giorgetta 2017].

Insects and the significance of temporal separation

Bees, flies, butterflies and beetles visited the flowers. Shared visitors included local solitary bees and bee flies. The introduced honeybee was observed on Acanthocalycium. Visitor composition changed among seasons. Observations focused on selected plants, and the scattered populations made standardized counts difficult. The insect list consequently documents floral contacts, while comparison of each visitor species’ contribution to effective pollination requires additional measurements [Eggli and Giorgetta 2017].

Interactions between insects also occurred at individual flowers. Honeybees locally displaced smaller bees. Males of an observed colletid bee defended occupied flowers against other males and large visitors while admitting females of their own species. Such behaviour could alter the route of successive visits. The authors regarded its implications for pollen movement distances as a question for further investigation. The bee’s generic identification was uncertain in the paper, so the behavioural observation is retained at a broader taxonomic level [Eggli and Giorgetta 2017].

Synchronous opening within one species increases the simultaneous availability of potential pollen donors and recipients. Staggering two species may also reduce competition for visits and the deposition of foreign pollen on stigmas. This was the explanation proposed by the authors. Flowering dates were not manipulated, and reproductive success was not experimentally compared under different degrees of overlap. The ecological benefit of the sequence therefore remains an interpretation consistent with the observations, distinct from the well-documented schedule of flower opening [Eggli and Giorgetta 2017].

Temporal separation of shared pollinators also occurs in the study of Neolloydia conoidea and Ariocarpus kotschoubeyanus at Tolimán. There, the shift involved different parts of the year. Neolloydia flowered in spring and summer, and Ariocarpus in autumn and winter. Three of the four recorded bee species were shared. The authors’ figure of 75% describes overlap in pollinator species composition. Its denominator comprises the four recorded bee species [Arroyo-Pérez et al. 2021].

In the Argentine Acanthocalycium and Parodia populations, the sequence was linked to days following the same rainfall, and individual flowers had a short active period. At Tolimán, seasonal separation coexisted with two-day flowers and experiments determining the ability to use self-pollen and pollen from other plants. These timescales can produce a similar ecological outcome, with some shared insects serving the species at different times. In both papers, the benefit of reduced competition is the authors’ explanation based on flowering schedules and visitor composition. Its magnitude remains a separate question from the timing shift itself [Eggli and Giorgetta 2017][Arroyo-Pérez et al. 2021].

Fruits and later flowers

After the first flowering flush, plants proceeded to fruit maturation, lasting approximately four weeks. Open flowers and developing fruits occurred together very rarely, although new buds could appear earlier. Acanthocalycium fruits split from the base upwards, exposing juicy pulp. Ants were observed eating pulp and carrying seeds away. Parodia fruits were dry and opened at the base, allowing seeds to roll out or remain exposed. A shared early-season flowering rhythm was therefore followed by different mechanisms of seed release [Eggli and Giorgetta 2017].

Later in the wet season, individual flowers and small groups appeared with much weaker synchronization among plants. The authors interpreted this as an additional reproductive opportunity after the main flush. An observation on 24 January 2013 illustrates its possible importance, when Acromyrmex lobicornis ants locally consumed Acanthocalycium flowers. Later flowers could escape that same episode of damage. The observation periods were nevertheless too short to establish the full contribution of late flowering to final seed production [Eggli and Giorgetta 2017].

An exception in a particularly dry season

During 2015/2016, Cerro Negro had a second mass-flowering event slightly more than a month after the first. Approximately 70% of present buds opened in the initial flush. The remainder were smaller and less developed. After the first rain, they completed growth to the waiting stage, and subsequent rain preceded their opening. The authors linked this pattern to the exceptionally dry preceding winter. A second event at Cerro Bayo remained a supposition because time was insufficient for direct observation there [Eggli and Giorgetta 2017].

This case indicates that a population’s response also depends on bud stage when water becomes available. A common rain event can produce a shared flush when enough buds are ready for their final developmental phase. Distinguishing bud initiation, subsequent growth and flower opening helps organize the observed sequence. The importance attributed to developmental stage is an inference from the recorded events. Measuring water balance and reserves in individual buds would help test its physiological basis [Eggli and Giorgetta 2017].

What the observed rhythm explains

The initiation of buds remains a separate question. Eggli and Giorgetta considered a role for day length, referring to different bud-development dates in Argentina and in Northern Hemisphere cultivation. This was an observational hypothesis, without a controlled photoperiod experiment. The association between final bud development and rain was likewise established under field conditions. Separating light signals, temperature and hydration would require comparable plants maintained under differing, precisely measured conditions [Eggli and Giorgetta 2017].

The Angastaco observations document the behaviour of two local population assemblages in detail. A next step would examine other parts of both species’ ranges, including mixed stands and places containing only one of the two. Simultaneous measurements of soil moisture, bud stage, insect visits and mature seed production would be particularly useful. These data would connect the clearly documented flowering schedule with the processes responsible for it and the actual reproductive outcome across successive seasons [Eggli and Giorgetta 2017].