Distribution and Flowering Ecology of Bromeliads Along Two Climatically Contrasting Elevational Transects in the Bolivian Andes1

BIOTROPICA 38(2): 183–195 2006 10.1111/j.1744-7429.2006.00124.x Distribution and Flowering Ecology of Bromeliads along Two Climatically Contrasting Elevational Transects in the Bolivian Andes1 Thorsten Kromer¨ 2, Michael Kessler Institute of Plant Sciences, Department of Systematic Botany, University of Gottingen,¨ Untere Karspule¨ 2, 37073 Gottingen,¨ Germany and Sebastian K. Herzog3 Institut fur¨ Vogelforschung “Vogelwarte Helgoland,” An der Vogelwarte 21, 26386 Wilhelmshaven, Germany ABSTRACT We compared the diversity, taxonomic composition, and pollination syndromes of bromeliad assemblages and the diversity and abundance of hummingbirds along two climatically contrasting elevational gradients in Bolivia. Elevational patterns of bromeliad species richness differed noticeably between transects. Along the continuously wet Carrasco transect, species richness peaked at mid-elevations, whereas at Masicur´ı most species were found in the hot, semiarid lowlands. Bromeliad assemblages were dominated by large epiphytic tank bromeliads at Carrasco and by small epiphytic, atmospheric tillandsias at Masicur´ı. In contrast to the epiphytic taxa, terrestrial bromeliads showed similar distributions across both transects. At Carrasco, hummingbird-pollination was the most common pollination mode, whereas at Masicur´ı most species were entomophilous. The proportion of ornithophilous species increased with elevation on both transects, whereas entomophily showed the opposite pattern. At Carrasco, the percentage of ornithophilous bromeliad species was significantly correlated with hummingbird abundance but not with hummingbird species richness. Bat-pollination was linked to humid, tropical conditions in accordance with the high species richness of bats in tropical lowlands. At Carrasco, mixed hummingbird/bat-pollination was found especially at mid-elevations, i.e., on the transition between preferential bat-pollination in the lowlands and preferential hummingbird-pollination in the highlands. In conclusion, both richness patterns and pollination syndromes of bromeliad assemblages varied in distinct and readily interpretable ways in relation to environmental humidity and temperature, and bromeliad pollination syndromes appear to follow the elevational gradients exhibited by their pollinators. RESUMEN Comparamos la diversidad, composicion´ taxonomica´ y s´ındromes de polinizacion´ de comunidades de bromeliaceas´ con la diversidad y abundancia de colibr´ıes a lo largo de dos gradientes altitudinales climaticamente´ diferentes en Bolivia. Los patrones de elevacion´ de riqueza de especies de bromeliaceas´ difirieron notoriamente entre los transectos. A lo largo del transecto permanentemente humedo´ de Carrasco, la riqueza de las especies alcanzo´ su punto maximo´ a elevaciones intermedias, mientras que en Masicur´ılamayor´ıa de las especies se encontraron en tierras bajas, calientes y semiaridas.´ Las comunidades de bromeliaceas´ estuvieron dominadas por grandes bromeliaceas´ epifiticas tanque en Carrasco y por pequenas˜ tillandsias atmosfericas´ epifiticas en Masicur´ı. En contraste a los taxones epifiticos, la distribucion´ de bromeliaceas´ terrestres fue bastante similar en ambos transectos. En Carrasco, la polinizacion´ por colibr´ıes fue la manera de polinizacion´ mas´ comun,´ mientras que en Masicur´ılamayor´ıa de las especies fueron entomofilas.´ La proporcion´ de especies ornitofilas´ aumento´ con la elevacion´ en ambos transectos, mientras que la entomofila´ mostroelpatr´ on´ contrario. En Carrasco, el porcentaje de especies de bromeliaceas´ ornitofilas´ estuvo correlacionado significativamente con la abundancia de colibr´ıes pero no con la riqueza de especies de colibr´ıes. La polinizacion´ por murcielagos´ se relaciono´ con condiciones humedas´ y tropicales, coincidiendo con la alta riqueza de especies de murcielagos´ en las tierras bajas tropicales. En Carrasco, polinizacion´ mixta de colibr´ıes/murcielagos´ fue encontrada principalmente a elevaciones intermedias, es decir, en la transicion´ entre polinizacion´ preferencial por murcielagos´ en tierras bajas y polinizacion´ preferencial por colibr´ıes en las montanas.˜ En conclusion,´ los patrones de riqueza y s´ındromes de polinizacion´ de comunidades de bromeliaceas´ var´ıan de maneras distintas y facilmente´ interpretables en relacion´ a condiciones de humedad y temperatura. Los s´ındromes de polinizacion´ de bromeliaceas´ parecen seguir los patrones exhibidos por sus polinizadores preferenciales. Key words: bats; Bolivia; Bromeliaceae; diversity; hummingbirds; pollination; tropical montane forest. THE TROPICAL ANDES ARE AMONG THE BOTANICALLY MOST DIVERSE related to differences in elevation, but few studies have covered en- regions worldwide (Barthlott et al. 1996). Due to the difficulties tire elevational gradients in enough detail to describe with precision of studying species-rich tropical plant communities, however, our how tropical forest characteristics change with elevation (Lieberman knowledge of the magnitude and distribution of this diversity is still et al. 1996). The majority of elevational studies have focused on fragmentary (Gentry 1995). The most conspicuous changes of com- species richness (e.g., Gentry 1995, Lieberman et al. 1996, Vazquez´ munity composition and richness in tropical mountains perhaps are & Givnish 1998, Kromer¨ et al. 2005) and elevational zonation of vegetation types (see Frahm & Gradstein 1991). Also, most stud- 1 Received 11 February 2005; revision accepted 26 April 2005. ies have been based on woody plants (e.g., Gentry 1988, 1995; 2 Corresponding author. Current address: Estacion´ de Biolog´ıa Tropical “Los Kitayama 1992; Lieberman et al. 1996), even though the major- Tuxtlas,” Universidad Nacional Autonoma´ de Mexico,´ Apartado Postal 94, San ity of plant species in humid tropical forests belong to nontree life Andres´ Tuxtla, Veracruz 95701, Mexico;´ e-mail: [email protected] 3 Current address: Asociacion´ Armon´ıa, BirdLife International, Casilla 3566, forms (Gentry & Dodson 1987, Ibisch 1996, Balslev et al. 1998). Santa Cruz de la Sierra, Bolivia. As a result, it is not yet known whether a general relationship exists C 2005 The Author(s) Journal compilation C 2005 by The Association for Tropical Biology and Conservation 183 184 Kromer,¨ Kessler, and Herzog between species richness and elevation, or even whether a universal explanation or model exists (Colwell & Hurtt 1994; Rahbek 1995, 1997). The family Bromeliaceae is almost exclusively restricted to the New World tropics. Nearly 50 percent of the estimated 3000 bromeliad species are epiphytic (Benzing 2000). These have leaf- trichomes of varied forms which function as moisture- and nutrient- absorptive appendages (Benzing 1990, 2000). Studies on the elevational distribution and ecology of bromeliads are rare: Pittendrigh (1948) examined the vertical distribution of epiphytic bromeliads in Trinidad, Gilmartin (1973) related the distribution of 17 species growing on both slopes of the Ecuadorian Andes to meteo- rological data, and Garc´ıa-Franco and Peters (1987) and Castano-˜ Meneses et al. (2003) analyzed the spatial distribution patterns of selected species of Tillandsia along an elevational gradient in Mexico. Bromeliads have a wide range of pollinators, including bats, birds, and insects, and also include autogamous taxa (Gardner FIGURE 1. Map of Bolivia showing the location of the study sites. 1986, Till 1992, Benzing 2000, Kessler & Kromer¨ 2000). Further- more, bromeliads are one of the most important food sources for hummingbirds in many Neotropical forest regions (Cruden 1972; Most precipitation falls between November and May, but even in Araujo et al. 1994; Sazima et al. 1995a, 1996; Dziedzioch et al. the “dry” season from June to October every month receives >100 2003). mm, thus creating a perhumid climate (sensu Lauer et al. 1996). In the present study, we compared the diversity, taxonomic There are no climatic stations at higher elevations, but since the composition, and pollination syndromes of bromeliad assemblages steep topography concentrates the convective precipitation on a nar- as well as the diversity and abundance of hummingbirds along row belt, precipitation is undoubtedly higher than in the lowlands. two climatically contrasting elevational gradients in Bolivia. Our Ibisch (1996) estimated >3500 mm mean annual precipitation at primary aim was to study qualitatively the relationship of bromeliad 2200 m elevation in the somewhat sheltered valley of Sehuencas, pollination syndromes and one pollinator group (hummingbirds) to but it is likely that slopes directly exposed to incoming clouds at temperature and rainfall. By using two transects differing strikingly mid-elevations (1000–3000 m) in the NW of the park receive an- in humidity, we hoped to be able to disentangle the relative effects nual precipitation in excess of 8000 mm (J. Bohner,¨ pers. comm.). of elevation as such from that of humidity, which often also varies Additional humidity is contributed by dew and fog condensation with elevation (Kessler et al. 2001). We did not specifically study ◦ (Ibisch 1996). Mean annual temperature is 24.6 C at Villa Tunari bats and insects as pollinators due to the difficulties of sampling ◦ and 12–15 C at 2200 m at Sehuencas (Ibisch 1996), with an annual them quantitatively. ◦ variability of about 5 C. Nocturnal frosts occur down to 2000 m (Ibisch 1996, T. Kromer¨ & M. Kessler, pers. obs.), especially during METHODS periodic influxes of southern polar winds during austral

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