Floral ecology of Puya ctenorhyncha (Bromeliaceae) an endemic plant of Bolivia

Abstract

Background:
The terrestrial bromeliad Puya ctenorhyncha is a near-threatened endemic species that grows between 2,500-4,050 m asl.

Hypotheses:
Hummingbirds are the most important visitors of this plant given its morphological and floral traits.

Study site:
The study was carried out along the “Death Road”, located in the Yungas montane cloud forest, La Paz, Bolivia.

Methods:
We monitored an average of 216 individuals monthly for two years (2018-2019). Besides we conducted direct observations of focal plants to quantify the visits of animals.

Results:
The reproductive season lasted from April to August in the first year and from May to June in the second. The mean nectar volume was 8.09 ± 1.59 μL per flower and its composition included fructose, glucose, and 86 % of water. With 341 hours of observations, we determined that Coeligena torquata (Trochilidae) is the main visitor and putative pollinator, followed by C. violifer. Males of both species were more frequent visitors than females. Coeligena torquata showed the highest activity during the afternoon while C. violifer was more active in the morning.

Conclusions:
Floral phenology seems to be annual and unimodal. Although the main pollinators seem to be hummingbirds, the nectar concentration and composition is more similar to bromeliad species of other genera which are pollinated by bats or passerine birds. It is important to evaluate, in future studies, the dependence of both hummingbirds and their behavioural response towards the plant.

Full text

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
68
Floral ecology oF Puya ctenorhyncha (Bromeliaceae) an endemic plant oF Bolivia
paola velásquez-noriega1,2,3*, thorsten Krömer3 and luis F. pacheco4
1 Herbario Nacional de Bolivia, Instituto de Ecología, Carrera de Biología, Facultad de Ciencias Puras y Naturales, Universidad
Mayor de San Andrés, La Paz, Bolivia.
2 Museo Nacional de Historia Natural, La Paz, Bolivia.
3 Centro de Investigaciones Tropicales (CITRO), Universidad Veracruzana, Xalapa, Veracruz, Mexico.
4 Colección Boliviana de Fauna, Instituto de Ecología, Carrera de Biología, Facultad de Ciencias Puras y Naturales, La Paz,
Bolivia.
*Author for correspondence: paola.vn19@gmail.com
Botanical Sciences 102 (1): 68-82. 2024
DOI: 10.17129/botsci.3393
Received: August 24, 2023, Accepted: October 9, 2023
On line rst: November 22, 2023
Ecology / Ecología
This is an open access article distributed under the terms of the Creative Commons Attribution License CCBY-NC (4.0) international.
https://creativecommons.org/licenses/by-nc/4.0/
Abstract
Background: The terrestrial bromeliad Puya ctenorhyncha is a near-threatened endemic species that grows between 2,500-4,050 m asl.
Hypotheses: Hummingbirds are the most important visitors of this plant given its morphological and oral traits.
Study site: The study was carried out along the “Death Road”, located in the Yungas montane cloud forest, La Paz, Bolivia.
Methods: We monitored an average of 216 individuals monthly for two years (2018-2019). Besides we conducted direct observations of focal
plants to quantify the visits of animals.
Results: The reproductive season lasted from April to August in the rst year and from May to June in the second. The mean nectar volume was
8.09 ± 1.59 μL per ower and its composition included fructose, glucose, and 86 % of water. With 341 hours of observations, we determined that
Coeligena torquata (Trochilidae) is the main visitor and putative pollinator, followed by C. violifer. Males of both species were more frequent
visitors than females. Coeligena torquata showed the highest activity during the afternoon while C. violifer was more active in the morning.
Conclusions: Floral phenology seems to be annual and unimodal. Although the main pollinators seem to be hummingbirds, the nectar concen-
tration and composition is more similar to bromeliad species of other genera which are pollinated by bats or passerine birds. It is important to
evaluate, in future studies, the dependence of both hummingbirds and their behavioural response towards the plant.
Keywords: Ecological interactions, oral syndrome, hummingbirds, nectar, reproductive phenology, Yungas.
Resumen:
Antecedentes: La bromelia terrestre Puya ctenorhyncha es una especie endémica casi amenazada que crece entre los 2,500-4,050 m snm.
Hipótesis: Los colibríes son los visitantes más importantes de esta planta dadas sus características morfológicas y orales.
Sitio de estudio: El estudio se realizó en el “Camino de la Muerte”, situado en el bosque nublado montano de los Yungas, La Paz, Bolivia.
Métodos: Monitoreamos un promedio de 216 individuos mensuales durante dos años (2018-2019), mediante observaciones de su biología
fenológica. Asimismo, realizamos observaciones directas en plantas focales para cuanticar las visitas de animales.
Resultados: La época reproductiva duró de abril a agosto en el primer año y de mayo a junio en el segundo. El volumen medio de néctar fue
de 8,09 ± 1,59 μL por or y su composición incluía fructosa, glucosa y 86 % de agua. Con 341 horas de observaciones, determinamos que
Coeligena torquata (Trochilidae) es el principal visitante y polinizador putativo, seguido por C. violifer. Los machos de ambas especies fueron
visitantes más frecuentes que las hembras. Coeligena torquata mostró la mayor actividad durante la tarde mientras que C. violifer fue más activa
por la mañana.
Conclusiones: La fenología oral parece ser anual y unimodal. Aunque los principales polinizadores parecen ser los colibríes, la concentración
y composición del néctar es más similar a especies de bromelias de otros géneros que son polinizadas por murciélagos o aves paseriformes. Es
importante evaluar, en estudios futuros, la dependencia de ambos colibríes y su respuesta de comportamiento a la planta.
Palabras clave: Colibríes, fenología reproductiva, interacciones ecológicas, néctar, síndrome oral, Yungas.

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In Bolivia, there are 65 species of Bromeliaceae in the genus Puya Molina, which shows a wide geographical
distribution from the tropical lowlands to the páramo vegetation of the Andes (500-4,500 m asl; Smith & Downs
1974, Jabaily & Sytsma 2013, Krömer et al. 2014). Fifty-six are endemic to the country, including Puya cteno-
rhyncha L.B. Sm. (Krömer et al. 1999, 2014). Puya plants are terrestrial or saxicolous (rarely epiphytic), have
a shrubby habit (Benzing 2000) and their spiny leaves usually form big rosettes, which develop large inorescences
with many showy owers (Varadarajan & Brown 1988, Benzing 2000). These owers are very attractive to hum-
mingbirds (Trochilidae) and represent an important food resource especially at high elevations (Kessler & Krömer
2000, Krömer et al. 2006, Kessler et al. 2020).
Puya owers are generally long, tubular, and wide, which allows for many hummingbird species to access the
nectar (Smith 1969, Krömer et al. 2006, Gonzalez & Loiselle 2016). Many researchers report that hummingbirds are
the main oral visitors and pollinators of several Puya species (García-Meneses & Ramsay 2012, Hornung-Leoni et
al. 2013, Restrepo-Chica & Bonilla-Gómez 2017, Gonzalez et al. 2019, Kessler et al. 2020, Velásquez-Noriega et al.
2020); however, a few species of bats and moths also consume their nectar and might act as pollinators (Hornung-
Leoni & Sosa 2005, Krömer et al. 2006, Aguilar-Rodríguez et al. 2019). Passerine birds were observed using the
plant’s inorescences as well, mainly as perches, but they also chew the corollas, often destroying the owers (Rees
& Roe 1980, Salinas et al. 2007, Hornung-Leoni et al. 2013, Velásquez-Noriega et al. 2020).
Phenological patterns and the abundance of resources during different phenological periods (e.g., the emergence
of owers and the amount of nectar available in a ower; Stiles 1978, Gonzalez & Loiselle 2016) shape when polli-
nators, granivores, herbivores, and seed dispersers exploit particular plants (Fenner 1998). Bromeliads usually have
a unimodal annual owering period (Benzing 2000, Machado & Semir 2006, Pool-Chalé et al. 2018) and Puya spe-
cies show a well-delimited owering period. However, not all plants ower each year, so the number of individuals
with owers varies annually (Janeba 2017, Restrepo-Chica & Bonilla-Gómez 2017, Velásquez-Noriega et al. 2020,
Franco-Saldarriaga & Bonilla-Gómez 2021). From the visitors’ perspective, nectar resources may not follow a direct
relationship with plant abundance, making multiyear phenological studies more adequate.
The composition and concentration of sugars in nectar are tightly associated with particular ower visitors, there-
fore, they are considered as being part of the oral syndrome (Freeman et al. 1984, Scogin & Freeman 1984, Baker
& Baker 1990). Hummingbirds are the primary pollinators of bromeliad species that have conspicuous inorescences
usually with red bracts and contrasting violet, orange, or yellow coloured tubular owers, which produce sucrose rich
nectar (Baker & Baker 1990, Kessler & Krömer 2000, Krömer et al. 2006, 2008, Ornelas et al. 2007, Kessler et al.
2020). The morphological characteristics of Puya owers thus indicate that their nectar must be a crucial resource
for hummingbirds.
The aim of this study was to describe the reproductive phenology of P. ctenorhyncha, its nectar production, and
sugar composition, as well as to determine the assemblage of its oral visitors and their activity pattern in the mon-
tane cloud forest of Bolivia. Considering a previous study on the closely related species P. atra L.B. Sm., which
also grows in this region (Velásquez-Noriega et al. 2020), we hypothesise that hummingbirds are the most important
visitors for P. ctenorhyncha because of similar morphological and oral characteristics between both species.
Materials and methods
Study area. The study was conducted in the Nor Yungas province, La Paz Department, Bolivia, along the so-called
“Death Road” (Camino de la Muerte) that connects the cities of La Paz and Coroico. Our study population grows
along a transect of the road between the villages of Chuspipata to Sacramento, between 2,500-3,000 m asl in the
humid montane cloud forest (16° 16’ 8.16” S, 67° 47’ 7.80” W - 16° 17’ 7.88” S, 67° 49’ 3.30” W; Figure 1). The
Yungas cloud forest is characterised by high humidity that results from the orographic shock of humidity-laden winds
hitting the mountains. The Yungas get torrential rains and frequent drizzles throughout the year (Ribera-Arismendi
1995). The study area shows a mean annual temperature of 10.1 ºC, average relative humidity of 97.5 %, and high
precipitation of 3,000 mm (Bach et al. 2003). The plant community is dominated by Cyatheaceae (tree ferns), Piper-

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
70
aceae, Rubiaceae, Ericaceae, Orchidaceae, and Melastomataceae, and there is a high dominance of climbers, vascular
epiphytes, and mosses.
Figure 1. Location of the study area along the “Death Road” from Chuspipata to Sacramento in the Yungas montane cloud forest in La Paz Department,
Bolivia.
Study species. Puya ctenorhyncha is endemic to Bolivia, where it has a restricted distribution between 2,500-4,050
m asl in the Department of La Paz (Krömer et al. 1999, 2014), and has been categorised as Near Threatened by the
IUCN in the Red List of Threatened Species (Mercado Ustariz et al. 2020). It is a shrubby plant with a basal rosette
of 1-1.2 m in diametre and 1.2 to 2 m in height (Smith 1969, Smith & Downs 1974). It grows on rocky slopes either
in isolation or in dense patches (Figure 2). Inorescences are about 1 m long, pendulous, densely white pubescent,
and somewhat woolly (Krömer 2000). The sepals are free, green, triangular-subulate, 26-30 mm long, 6-7 mm wide
in the base; petals are free, oblanceolate, about 41-46 mm long, 10-20 mm wide with a light greenish-yellow colour;
stamens are all equal in length, shorter than the petals; laments whitish, liform, 27-32 mm long; anthers yellow,

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linear, 7-10 mm long; style linear, 37-40 mm long; stigma divided into 3, ca. 3 mm long, recurved, green-yellowish.
It is locally known as “Bear Flower” (Flor de Oso) since its inorescences and rosettes serve as food for the Andean
bear (Tremarctos ornatus; Figure 2). A voucher of the species was deposited at the National Herbarium of Bolivia in
La Paz (P. Velásquez 2, LPB).
Figure 2. A. Flowering plants of Puya ctenorhyncha growing on slopes along the “Death Road” trail, B. Group of plants of P. ctenorhyncha in different
phenological stages, C. Peduncle of inorescence eaten by an Andean bear, D. Rest of a rosette after being visited by the Andean bear.
Assessment of oral biology and reproductive phenology. We visited the study transect each month from January
2018 to December 2019 to record the reproductive phenology of the plants, following the methods described in
Velásquez-Noriega et al. (2020). We observed and recorded the phenophase of plants using binoculars and a digital
camera during each visit, monitoring about 216 owering plant individuals distributed unevenly on rocks and steep
slopes along the study transect. We did not individually mark each plant as the rocky terrain made it difcult to reach
them all.
We described the phenophases in the following categories: (1) Bud set, from the rst appearance of the young in-
orescence within the rosette until it reaches its maximum height of about 100 cm with peduncle, bracts, and buds of

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
72
the immature ower separated; (2) Flowers, when the green-yellow corollas appear in the inorescence; (3) Implant-
ed owers, when all corollas become twisted and the peduncle and the petals acquire a brown-yellow colour because
they begin to wilt; (4) Fruits, when the fruits are immature, green and non-dehiscent; and (5) Open fruits, when the
capsules are dehiscent and release seeds, and the whole plant turns dark brown (Figure 3). A plant phenology diagram
was elaborated based on all these data (Pereira & Quirino 2008, Rodrigues Marques & De Lemos Filho 2008).
Determination of nectar characteristics. To prevent oral visitors from accessing the oral nectar, four inorescenc-
es were completely covered with tulle bags. We could not cover more inorescences with bags, because the access to
most of the plants was difcult as they were growing on steep rocky slopes. We harvested the total oral nectar vol-
ume about one hour after the beginning of the anthesis, between 7:00-7:30 h, using 80 μL micro capillaries (Scogin
& Freeman 1984). We could only measure each ower once because owers were removed from the inorescence to
obtain the nectar. A total of 12 individual owers were measured to estimate the mean oral nectar volume produced
and to determine its sugar composition (García & Hoc 1998). The oral nectar sugar concentration was measured
from eight owers using a hand refractometre (Cole-Parmer RSA-BR90S, range: 0-42 %, United Kingdom) directly
in the eld, and the total amount of sugar was calculated following Corbet (2003). Additionally, we transported nec-
tar samples in a container with silica gel to the laboratory for nuclear magnetic resonance spectroscopy (Hölscher et
Figure 3. Phenological stages of Puya ctenorhyncha A. Bud set, B. and C. Flowers, D. Implanted owers, E. Fruits, F. Open fruits with seeds. Photo-
graphs: B. G. Archondo, E. and F. E. Cuba.

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al. 2008, Wenzler et al. 2008). Staff at the “Laboratorio de Biorgánica” of the Instituto de Investigaciones Químicas
at Universidad Mayor de San Andrés, La Paz performed the analyses.
Observation of oral visitors. We assessed both, diurnal and nocturnal ower visitors. We observed 29 owering
individuals in six areas between 8:00 and 19:00 h over 31 days, for a total of 341 hours in 2019 (Canela & Sazima
2005). Given the low availability of owering P. ctenorhyncha individuals, we chose an observation area if at least
one individual was in the ower phenophase. Observation areas were at least 100 m apart from each other (Gonzalez
& Loiselle 2016).
We divided the observation time into 11 one-hour intervals to analyse the activity patterns of oral visitors through-
out the day, measuring the number of visits per hour per species (Woods & Ramsay 2001). This interval assignment
allowed us to compare the daily activity patterns between oral visitors and P. ctenorhyncha. For hummingbirds,
we considered a “visit” event when an individual sipped nectar from a plant by inserting its bill into a ower within
a one-hour interval (Vázquez et al. 2005). For invertebrates, we recorded a “visit” event when the animal clearly
touched the reproductive structures of the plant. We calculated the frequency of visits for each animal species using
the number of visits in each hour interval.
Nocturnal visits of P. ctenorhyncha were evaluated by placing three camera traps (Denver WCT-8010, Denmark)
near owering plants, located, due to logistical and safety reasons, in slightly different observation areas from those
where diurnal observations were taken, for ve consecutive nights. The cameras were scheduled to take one picture
every ve seconds over a 12-hour period each night (18:00-6:00 h). Additionally, direct observations were made dur-
ing the night between 19:00-23:00 h, for a total of 20 hours in ve consecutive days (Aguilar-Rodríguez et al. 2014).
Results
Assessment of oral biology and reproductive phenology. Puya ctenorhyncha has an acropetal owering. During the two
years of observation, the 193 monitored inorescences had an average of 15 ± 7 open owers per day (CV: 49.94 %;
range: 1-33 owers). In the rst year the maximum number of owering individuals (69) was recorded in July, while
in the second year a maximum of 14 owering individuals were recorded in June. The owers always opened early
in the morning (~ 6:00 h) and remained open between 72 to 96 hours.
The complete phenological cycle of P. ctenorhyncha lasted about 12 months (Figure 4). The rst stage (devel-
opment of buds) started in March 2018 with a peak in May. Fully open owers were observed between April and
August 2018 showing a unimodal pattern, and the implanted owers were present mainly in September and October
2018. Fruits developed from November 2018 to February 2019 and open fruits started to release their seeds in March
2019, although seed dispersal lasted until December 2019. Interestingly, seed dispersal was also present from Jan-
uary to April 2018, as a result of owers from 2017. Although the highest peak of buds was in May 2018, another
small peak was observed in April 2019, likely as a new phenological cycle started and lasted until December 2020.
Similarly, lower owering and implanted peaks were observed in 2019 from May to June and from July to October,
respectively (Figure 4).
Determination of nectar characteristics. The total nectar volume per ower averaged 31.26 ± 19.08 µL (SD 17.43)
and 468.9 µL per inorescence per day (n = 12 owers from four individuals) with an average sugar concentration
of 8.56 ± 4.5 °Bx ranging from 2-16 %. Nectar composition included 2 % β-fructofuranose, 2 % β-fructopyranose, 3 %
α-glucopyranose, and 7 % β-glucopyranose, traces of sucrose, and 86 % of water.
Observation of oral visitors. During our diurnal observations, we recorded visits to 29 of the 193 owering individ-
uals of P. ctenorhyncha, by two hummingbird species (both sexes; Figure 5): the Collared Inca (Coeligena torquata
Boissonneau 1840) with 77 % of visits and the Violet throated Starfrontlet (Coeligena violifer Gould 1846) with 23 % of
visits. Only one male of Amethyst throated Sunangel (Heliangelus amethysticollis d’Orbignye and Lafresnaye 1838)

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
74
visited the plant once and sipped the owers twice. Additionally, some unidentied ies (Diptera) were observed in
the owers, but none of them touched either the anthers or stigma. During the night no ower visits by bats, moths,
or other nocturnal insects have been detected by camera traps and direct observations. The only recorded visitor we
observed in the camera trap recordings was C. torquata at 18:45 h.
Figure 4. Reproductive phenology stages of Puya ctenoryncha along the “Death Road” in the Yungas montane cloud forest of La Paz, Bolivia, during
2018 and 2019. Every point represents the percentage of sampled individuals per month belonging to each respective stage (bud set, ower, implanted,
fruit, and open fruit).

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The following analysis of the activity patterns of oral visitors therefore only includes the two species of Coeligena.
Males of C. torquata had the highest visiting frequency (2,157 visits) with a peak of activity between 15:01-16:00 h
(Figure 6). Females of C. torquata visited owers 145 times, with a peak of activity between 13:01-14:00 h. Males
of C. violifer (483 visits) just realised the 22 % of visits of C. torquata males, and their peak of activity was between
9:01-10:00 h and 12:01-13:00 h; while females of C. violifer visited owers as frequently as female C. torquata (147
visits in total), with a peak between 9:01-10:00 h (Figure 6).
Discussion
Assessment of oral biology and reproductive phenology. The owering phenology of P. ctenorhyncha may be
classied as annual and unimodal, coincident with other species of Puya (Benzing 2000, Restrepo-Chica & Bo-
nilla-Gómez 2017, Velásquez-Noriega et al. 2020). At the population scale, however, the complete phenological
cycle, from buds until seed dispersal, lasts up to 24 months. Puya ctenorhyncha shows a similar pattern as P. atra
(Velásquez-Noriega et al. 2020), which is usually found at higher elevations (3,000-3,500 m asl) along the “Death
Road” (Krömer 2000), although both species occur sympatrically in a few sites in the Yungas montane cloud forest.
Figure 5. A. Female of Coeligena violifer in hovering ight visiting a ower of Puya ctenorhyncha, B. Male of C. violifer perching in the observation
area, C. Male of C. torquata perching in the observation area, D. Male of C. torquata visiting the plant. Photographs: A. B. Téllez-Baños, B. E. Cuba.

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
76
The owers of P. ctenorhyncha are available in the dry season when most other plant species in the Yungas ecoregion
are not owering (Serrudo-Gonzáles et al. 2012), which indicates that they represent an important food resource,
especially for hummingbirds (Krömer et al. 2006). Our results and other preliminary observations suggest that the
populations of owering individuals of several Bolivian Puya species are cyclical; when there were many individuals
in the owering stage, the owering period was longer and followed by a year with few owers and a short period
of blooming.
Phenological data suggest that P. ctenorhyncha shows a “cornucopia” strategy, where a large number of owers is
produced daily over a time span of several weeks (Kessler et al. 2020). In contrast, most Bromeliaceae, specically
epiphytic species, show a “steady state” owering strategy, with plants producing relatively few owers per day over
extended periods of several weeks or months (Kessler et al. 2020). The acropetal and asynchronous owering has
been previously identied as an advantageous strategy to attract ower visitors and pollinators due to the increase of
ower longevity and foraging resources (Knight et al. 2005). The same mechanism might occur in P. ctenorhyncha
where owers are available for about four months and thus offer stable resources for their pollinators. Our study
population had owers available for a long period, and even individual owers remained open for about 72 hours;
however, the only observed visitors were diurnal.
Figure 6. Temporal frequency of Puya ctenorhyncha visits by two species of hummingbirds: Coeligena torquata (left panel) and C. violifer (right panel).
F = female and M = male.
Determination of nectar characteristics. The mean volume of nectar produced by P. ctenorhyncha per inorescence
per day is higher than in P. chilensis Molina (358.27 µL; Hornung-Leoni et al. 2013), but similar to P. atra (432.4 µL;
Velásquez-Noriega et al. 2020). Lower values per day have been reported for other Puya species, for example, P.
alpestris Gay. (120.88 µL), P. coerulea Miers (62.78 µL), P. raimondii Harms (21.7 - 49.5 µL), P. venusta Phil.
(23.65 µL), and P. hamata (4.8-18.3 µL; Woods & Ramsay 2001, Hornung-Leoni et al. 2013). The concentration of
dissolved sugar in P. ctenorhyncha (8.56 ± 4.5 °Bx) was lower than reported for P. alpestris (12.16 ± 0.51 °Bx) and

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P. chilensis (12.56 ± 1.63 °Bx), while other species such as P. hamata (18.3 ± 8.2 °Bx), P. raimondii (20.1 ± 0.60
°Bx), P. venusta (22.93 ± 2.93 °Bx), and P. coerulea (22.78 ± 0.35 °Bx) showed even higher values (Woods & Ram-
say 2001, Hornung-Leoni et al. 2013). Thus, the sugar concentration of P. ctenorhyncha is not consistent with other
trochilophilous Puya species, but is more similar to those of chiropterophilous bromeliads (e.g., Pitcairnia, Vriesea,
Werauhia; Baker & Baker 1990, Krömer et al. 2008).
The water content in the nectar of our study species was higher (86 %) than in P. atra (62 %; Velásquez-Noriega
et al. 2020). Thus, the nectar is relatively diluted, which matches with the preferences reported for other angiosperm
species with the hummingbird pollination syndrome (Baker & Baker 1990). Hexose solutions have higher osmolar-
ity, and therefore lower evaporation rates than sucrose solutions, however, these former sugars also tend to require
more water for their formation (Abrahamczyk et al. 2017). Besides, it is possible that the plant at this elevation with
high atmospheric humidity absorbs water from the environment. There is no data on water content available for other
Puya species, although hummingbirds depend on the nectar reward for both water and energy, albeit they comple-
ment their diet with arthropods (Wolf et al. 1976, Calder 1979).
The nectar of P. ctenorhyncha is mainly composed of fructose and glucose (i.e., hexose-rich) and had only a small
proportion of sucrose, which is similar to P. atra (Velásquez-Noriega et al. 2020) and other Puya and bromeliad spe-
cies pollinated by bats or passerine birds (Scogin & Freeman 1984, Baker & Baker 1990, Baker et al. 1998, Krömer
et al. 2008). In P. alpestris the presence of these three sugars has been reported as well, although in different propor-
tions and mostly classied as sucrose-rich, while this plant is mainly visited by passerine birds (Hornung-Leoni et
al. 2013). In contrast, P. venusta, P. coerulea, and P. chilensis were also shown to be hexose-rich and are visited by
passerine birds as well (Hornung-Leoni et al. 2013), consistent with the pollination syndrome for passerines. Howev-
er, we did not record any visits by bats or passerines, which might suggest that sugar composition may not always be
associated with a certain type of ower visitors, as observed in other plant groups, such as Alooideae, Papilionoideae,
Proteaceae, and Ericaceae (van Wyk 1993, Barnes et al. 1995, Nicolson & Fleming 2003). Abrahamczyk et al. (2017)
reported that pollinators are sensitive to the proportion of sugars constituted by sucrose, while hexoses do not provide
evidence of a specic pollinator syndrome.
Hummingbirds usually prefer sucrose-rich nectar (Freeman et al. 1984, Baker & Baker 1990, Galetto & Ber-
nardello 1992, Nicolson & Fleming 2003, Krömer et al. 2008) even though this sugar has been reported only in a
small proportion for P. ctenorhyncha. Physiologically they can also digest fructose and glucose as alternative energy
sources as they feed on a mixture of sugars (Chen & Welch 2014). Thus, it remains possible that a putative phy-
logenetic constraint on nectar features is, like other characters, more or less relaxed in different taxonomic groups
(Schmidt-Lebuhn et al. 2007). Nonetheless, more studies of other plant families and Bromeliaceae genera are needed
to get rm conclusions.
Observation of oral visitors. Although some of the ower characteristics of P. ctenorhyncha are consistent with the
oral syndrome of hummingbirds as its main visitors (e.g., tubular, scentless owers, exerted stamens, anthesis in the
morning, abundant nectar production), there are no red bracts with contrasting corollas. Nevertheless, hummingbirds
also visit Puya species with green or yellow owers and unshowy brown bracts or woolly inorescences, such as
P. trianae L.B. Sm. in Colombia (Restrepo-Chica & Bonilla-Gómez 2017), P. raimondii in Peru (Salinas et al. 2007),
and P. atra near our study area (Velásquez-Noriega et al. 2020). Hummingbirds can discriminate a wide range of
colours, including not spectral ones (Altshuler 2003, Stoddard et al. 2020), which would allow them to recognise
the untypical ower colours of Puya species even when they have been trained to associate red colours with nectar
reward (Maruyama et al. 2013). It is likely that the visits of the hummingbirds are more related to the big size of the
inorescence and their numerous owers than their colour, as their size may make them attractive relative to other
plants with small owers in the Yungas montane cloud forest.
Hummingbirds have been reported as the main visitors and putative pollinators for most Puya species, regardless
of bill morphology or body size (Woods et al. 1998, Salinas et al. 2007, García-Meneses & Ramsay 2012, Hornung-
Leoni et al. 2013, Restrepo-Chica & Bonilla-Gómez 2017, Aquino et al. 2018, Gonzalez et al. 2019), while passerine

Floral ecology of Puya ctenorhyncha (Bromeliaceae)
78
birds or bats might act as secondary visitors. Only two long-billed hummingbirds frequently visited the P. cterno-
rhyncha owers, even though ve other hummingbird species with shorter bills live in the study area: Heliangelus
amethysticollis (which visited the plant only once), Adelomyia melanogenys Fraser 1849, Aglaiocercus kingie Les-
son 1832, Metallura tyrianthina Lodiges 1832, and Chaetocercus mulsant Bourgier 1842 (Velásquez-Noriega et al.
2023). Thus, P. cternorhyncha owers may be accessible to short billed species.
The main oral visitor of P. ctenorhyncha is C. torquata, which seems to monopolise an abundant and stable
resource (“cornucopia”), as we observed that one individual could stay at one observation area all day. In our study,
C. torquata showed a territorial and aggressive behaviour compared to C. violifer, which acted more as a trapliner.
A traplining strategy involves hummingbirds feeding on renewable food resources from isolated plant patches along
reused routes (Feinsinger & Colwell 1978). It appears that body size determines interspecies dominance among
hummingbirds (Bribiesca et al. 2019) and given that C. torquata is the largest hummingbird in our study, it could
monopolise feeding sites by chasing, attacking, and remaining vigilance against the approach of other individuals. In
areas of the Yungas where C. torquata is absent, C. violifer is the most aggressive hummingbird (Serrudo-Gonzáles
et al. 2012) and can adopt the cornucopia pattern (Kessler et al. 2020). Regarding nocturnal oral visitors, none have
been reported, although the characteristics of the owers and its nectar, such as the greenish-yellow petals, the nectar
composition, and the concentration of sugar, would appear to be adaptations to attract nocturnal visitors. However,
only 7 % of all bromeliads in Bolivia are known to be pollinated by bats (Kessler & Krömer 2000), suggesting that
there might be many more undetected cases (Aguilar-Rodríguez et al. 2019).
Our study showed that Puya ctenorhyncha is visited almost exclusively by two species of hummingbirds (Coelige-
na torquata and C. violifer). Its owering period is annual, its nectar sugar is composed mainly of fructose, glucose,
and water, with only a small proportion of sucrose. The main oral visitor and putative pollinator of P. ctenorhyncha
is C. torquata, which guarded owers from other hummingbirds. Regarding the activity of visitors, the males of both
species ate at owers more frequently than females. Future studies should look at these plant-bird interactions in
more detail to determine if the plant´s reproductive success depends on these hummingbirds as pollinators and if the
plant´s oral phenology pattern leads to changes in foraging patterns of hummingbirds. This information would be
key to develop adequate conservation and management strategies for the protection of Puya ctenorhyncha, a species
under critical threat due to its restricted distribution in an endangered habitat.
Acknowledgements
We thank to Y. Flores for processing the nectar samples, J. Quezada for lending climbing equipment, E. Cuba for
helping with the map and photos, B. Téllez and G. Archondo for photos, to L. Porter, M. Villegas, and D. Larrea
for critical comments. To Rosember Hurtado for the oral description. To E. Garcia for the feedback in the study
idea and all volunteers that made this work possible: N. Gómez, E. Alanoca, Y. Murillo, A. Lanza, B. Huanca, and
I. Manrriquez.
Declaration of competing interests
The authors declare that there is no conict of interest, nancial or personal, in the information, presentation of data
and results of this article.
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Associate editor: Pedro Luis Valverde
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