Distributions of epiphytic orchids and bromeliads on secondary forest and pasture trees Greta Brom Palkowski Departments of Biology and Anthropology, University of Wisconsin Madison ABSTRACT The locations and densities of epiphytic orchids and bromel iads growing on host trees are influenced greatly by differences in epiphyte requirements for light, moisture and nutrients. Because these factors differ greatly between minimally and highly d isturbed habitats, epiphytic composition may also be altered. This study examined changes in epiphytic orchid and bromeliad density between hosts in secondary forest and pasture. Fifteen trees in each habitat type were censused for the abundance of orchids and bro meliads in different parts of the trees. Results ind icated that the highest concentration of both orchids and bromeliads is in the pasture. In addition, in both forest and pastureland, the highest density of epiphytes was recorded in tree regions characterized by seco ndary to tertiary branching. Significa nt results were also obtained which highlighted a positive correlation between orchids and bromeliads in both habitat types. Overall, results indicate that land conversion for human use has a great impact on the quantity and loca tion of epiphytic orchids and bromeliads living on remnant trees. RESUMEN La ubicacin y densidad de orqudeas y bromelias epfitas creciendo en rboles depende de muchos factores diferentes con luz, humedad y nutrientes. La composicin de las epfitas puede ser alterada de ma nera importante ya que estos factores difieren mucho en hbitats con disturbios mnimos y en habitats muy perturbados. Este estudio examin los cambios en densidad de las orqud eas y bromelias epfitas entre rboles anfitriones en un bosque secundario y e n un potrero. Se consideraron quince rboles de los dos tipos de habitat y se anot la abundancia de orqudeas y bromelias en partes differentes de los rboles. Los resultados indicaron que la concentracin ms grande de orqudeas y bromelias se encuentr a en el potrero. Adems, las densidades ms altas de epfitas, tanto en el potrero como en el bosque, se encontraron en las reas de ramificaciones secundarias y terciarias. Tambin se obtuviero n resultados significativos en la correlacin entre las or qudeas y bromelias en los dos hbitats. En general, los resultados indican que la conversin de la tierra para uso humano tiene un impacto grande en la abundancia y ubicacin de las orqudeas y las bromelias epfitas que viven en rboles restantes. INTRODUCTION Epiphytic communities account for about ten percent of all vascular plants (Werneck & Marcos do Esprito Santo 2002). Epiphytic densities are especially high in the cloud forests of Monteverde, where Ingram et al. (1996) found over 200 spe cies in a four hectare plot. Plants from the families Bromeliaceae and Orchidaceae make up a considerable amount of this diversity with 22 and 92 species respectively. Further, epiphytic orchids and bromeliads grow in a great range of habitat conditions but also exhibit preference for growth in particular sites on a given tree (Gentry & Dodson 1987, ter Steege & Cornelissen 1989). Epiphytic orchids frequently have succulent leaves, thick cuticles and pseudobulbs, but are most limited by their ability t o accumulate adequate levels of rainwater and mist (Walter 1983). In addition, epiphytic orchids are better able to colonize trees that have soft, spongy bark or rough surfaces for water retention (Dressler 1990). Epiphytic bromeliads are characterized b y a rosette of trichome covered leaves, and may utilize Crassulacean Acid Metabolism. This arrangement provides plants with a reservoir for collecting water, detritus, and nutrients, and thus facilitates development in a wider variety of habitat types (Ut ley & Burt Utley 1983). The differences in ideal growth conditions for epiphytic orchids and bromeliads have lead some scholars surrounding t his idea (e.g. bark composition, epiphyte competition and environmental conditions) have not been
adequately tested, presumably, differences in the conditions required for growth combined with high competition where overlap does occur should lead to negati ve associations between epiphytic orchids and bromeliads (Walter 1983). Despite this hypothesis, there is evidence for orchid and bromeliad growth on the same trees in both tropical lowland and dry forests (ter Steege & Cornelissen 1989, Werneck & Marcos do Esprito Santo 2002). Thus, water, wind, light, and competition can be considered important factors in determining the type, quantity and arrangement of epiphytic orchids and bromeliads on a tree (Zimmerman & Olmsted 1992, Zotz & Hietz 2001, Graham & A ndrade 2004). Variation in light, moisture and nutrients has been shown to vary greatly between habitat types as well as among the microhabitats within a tree. In general, thinly wooded forests, disturbed habitats, and the forest canopy are subject to highly variable water availability, and receive higher levels of sunlight and wind than do dense, undisturbed forests and the inner crown (ter Steege & Cornelissen 1989, Ingram 2000). For instance, branch texture, humus deposits and humidity have been sho wn to decrease towards the outer branch, while light intensity and wind velocity increase (Johansson 1975). Changes in the abiotic parameters have been observed both between habitats and amongst different locations on tree. Because these factors are im portant for determining the type, location and amount of epiphyte colonization, I hypothesized that significant changes in orchid and bromeliad epiphyte density would be observable between forest trees and those in a pasture. Differences in these factors also create microhabitats along the five Zones of a tree (Johansson 1975) (Figure 1). Thus, it was predicted that each habitat would exhibit differences in density along the five Zones.
FIGURE 1. Five tree zones from trunk base to canopy. Modified from Johansson (1975). MATERIALS AND METHODS This study was conducted at four sites at the Estacin Biologca in Monteverde, Costa Rica. Located between the elevations of 1485 and 1560 m and receiving approximately 2.5 meters of rainfall per yea r, all four sites are classified as Lower Montane Wet Forest in the Holdridge life zone system (Nadkarni & Wheelwright 2000). Data on the quantity and location of epiphytic plants from the families Orchidaceae and Bromeliaceae were collected from 15 trees in secondary growth forest (from three different sites) and a pasture. All three secondary forest sites were subject to clearing or selective logging at some point in time, but have been in regeneration for at least 35 years. The pasture was first clear ed 40 years ago at which time several (remnant) trees were left standing. In general, initial epiphyte colonization took place under a variety of conditions for trees in both the secondary forest and pasture. In both habitats, remnant trees are assumed t o have acquired epiphytes both before and after human disturbance. For the secondary forest, trees that germinated after this point were colonized under environmental conditions that changed as the forest was allowed to regenerate. It is for this reason that it was necessary to generate data from trees at similar successional stages. Thus, trees selected from both habitat types had a diameter at breast height (DBH) range of 39 59 centimeters. For all 30 trees, epiphytic orchids and bromeliads were quan tified throughout each of the five tree subdivisions first described by Johansson (1975) (Figure 1). To keep constant the total amount of surface area examined in tree trunks (Zones 1 2) versus tree crowns (Zones 3 5), calculations of estimated trunk heig ht times DBH were made. These values were subsequently matched with similar estimates of total branch surface area. This meant that for certain trees, especially those with shorter trunks, not every branch was censused. For all pasture trees and five of the trees in the secondary forest, binoculars were used, while standing on the ground, to view epiphytes growing in Zone three and beyond. For the remaining ten trees, epiphytes growing in these
zones were counted during one of two climbs to the canopy us ing the single rope climbing techniques described by Perry (1978). A Friedman Test was used to compare the mean number of orchids or bromeliads per zone for each habitat type. Wilcoxon Tests were used to compare the difference in the mean number of epiphy tic orchids or bromeliads for each tree Zone, and for all three Zones combined, in both habitats. Lastly, a regression analysis was conducted for the number of epiphytes summed over all five Zones per tree, as well as the number of epiphytes in Zone four in each habitat type. These tests examined the relationship between the number of orchids found and either the number of bromeliads or host tree DBH. RESULTS Mean epiphyte densities For both habitats examined, Friedman tests indicated that the highe st mean densities for both orchids and bromeliads were found in Zone four (p < 0.0001, p = 0.0129, p < 0.0001, p < 0.001 respectively) (Table 1 a, b). Wilcoxon tests showed that the mean number of epiphytic orchids growing in Zones one through five was hi gher in the pasture than forest (Z = 2.11, p = 0.04; Z = 1.50, p = 0.13; Z = 1.96, p = 0.05; Z = 3.34, p = 0.0008; Z = 1.60, p = 0.11 respectively) (Figure 2a, c, e, g, i). This trend was also true for pasture bromeliads in Zones one through five (Z = 2.6 2, p = 0.0088; Z = 3.85, p = 0.0001; Z = 3.85, p = 0.0001; Z = 3.37, p = 0.0002; Z = 2.75, p = 0.0060 respectively) (Figure 2b, d, f, h, j). Higher mean densities were also observed in the pasture when the sum of orchids and bromeliads across all five Zon es were tested (Z = 3.96, p < 0.0001) (Figure 2k). Orchid Bromeliad Correlations Regression analyses showed no correlation between forest orchids and tree DBH (F = 23.51, p = 0.26, N = 75, R 2 = 0.40) (Figure 3a). However, a positive relationship was obs erved between pasture orchids and tree DBH (F = 6.46, p = 0.03, N = 75, R 2 = 0.15) (Figure 3b). Further, positive associations were found between the number of orchids and the number of bromeliads in both habitats (F = 23.51, p < 0.0001, N = 75, R 2 = 0.40 ; F = 6.46, p = 0.0028, N = 75, R 2 = 0.15 respectively) (Figure 3c, d). When orchid and bromeliad plants growing specifically in Zone four (the most dense Zone) were examined, no significant associations were detected in both forest and pasture habitats (F = 4.05, p = 0.07, N = 15, R 2 = 0.24; F = 0.0009, p = 0.98, N = 15, R 2 = 0.000073 respectively) (Figure 3e, f). TABLE 1. Results of Friedman Tests examining differences between means for five variables. (a) Mean epiphyte density and rank for both or chids (p < 0.0001) and bromeliads (p = 0.0129) growing on forest trees, and (b) Mean epiphyte density and rank for both orchids (p < 0.0001) and bromeliads (p < 0.0001) growing on pasture trees. (a) Epiphyte type zone Mean # of epiphytes p er tree Standard error rank Orchid 1 1.40 0.92 5 Orchid 2 5.13 2.09 3 Orchid 3 7.0 1.69 2 Orchid 4 11.27 3.09 1 Orchid 5 3.80 2.71 4 Bromeliad 1 1.26 0.63 5 Bromeliad 2 1.40 0.68 3 Bromeliad 3 2.6 1.10 2 Bromeliad 4 4.0 0.79 1 Bromeliad 5 1.47 0.87 4
(b) Epiphyte type zone Mean # of epiphytes per tree Standard error Rank Orchid 1 4.53 1.66 5 Orchid 2 11.67 3.48 3 Orchid 3 15.93 4.50 2 Orchid 4 43.33 8.55 1 Orchid 5 9.60 2.47 4 Bromeliad 1 5.33 1.77 5 Bromeliad 2 10.80 2.26 3 Bromeliad 3 14.27 2.76 2 Bromeliad 4 19.87 3.60 1 Bromeliad 5 8.0 2.47 4
DISCUSSION The results of this study a re consistent with the hypothesis that overall, epiphytic orchids and bromeliads grow most densely in Zone four of forest and pasture trees. This finding is consistent with past research and can be attributed to the high levels of wind and sun, but relati vely thick, humus collecting branches found there (Ingram & Nadkarni 1993, Steege & Cornelissen 1989). Exposure to more wind, water and sun also explain the greater quantity of epiphytes growing on pasture trees as well as the observed increase of growth in lower tree Zones (Johansson, 1975). Despite a body of theory suggesting the opposite, this study illustrates that there is not a significant negative correlation between orchids and bromeliads on a given host tree. Thus, while epiphytic orchids and
b romeliads must compete for the same space, the presence of individuals from one family cannot be considered the main factor in determining quantity or location of the other. Catling and Lefkovitch (1989) examined epiphytic associations and proposed that t he processes Although this model does not take the effects of human induced disturbances into account, it highlights the function of changin g processes, rather than purely biotic factors (e.g. competition) in producing the overall epiphyte composition throughout tree development. Because meta community dynamics such as immigration rates, host size and proximity to other epiphyte hosts can of ten be considered stochastic, these factors are considered important in early epiphyte colonization. For pasture trees, clearing resulted in greater exposure to sun, wind and water. These changes also created an increase in the space suitable for epiphyt e colonization. Though epiphyte densities and successional stages were different for individual host trees before this time, following clearing, stochastic processes became important. Thus, events such as high immigration from the offspring of existing s pecies can be considered essential for creating the increased epiphyte densities observed on pasture trees. In most cases, stochastic processes lead to epiphyte colonization in all spaces conducive to growth before deterministic processes occur. Because the results of this study indicate that distantly related epiphyte groups (orchids and bromeliads) are growing together in both the secondary forest and pasture, trees surveyed are considered to be in the later stages of succession, when competition become s important (Catling & Lefkovitch 1989). Thus, epiphytes that have already colonized a given space and can produce offspring easily, or those that can grow under the shade of larger epiphytes compete with greater success. Because the family orchidaceae c ontains many epiphytic species that are small and thrive in the shade (Dressler 1990), orchids may be favored under deterministic conditions. Therefore, the higher mean densities of orchids observed in both habitats and all Zones, except Zone one of the p asture, can be explained. While the results of this study seem to indicate that forest clearing has a positive impact on the epiphytic orchids and bromeliads growing in disturbed areas, there are several other factors that can be examined. First is the fa ct that a greater amount of biodiversity loss results from human induced habitat destruction than is gained through increased epiphyte colonization in the aftermath of logging (Brokaw & Lent 1999). In addition, species richness of epiphytes may be negativ ely affected. While elevated orchid diversity has been recorded in two Monteverde pastures (Atwood 2000), a study conducted by Barthlott et al. (2000), found that the total epiphyte community structure in a disturbed forest showed a 50% decrease in specie s richness. Therefore, while human disturbances lead to an increase in the mean number of epiphytes per tree, this increase could be associated with a decrease in species richness. Further, as Barthlott et al (2000) have suggested, if taxonomic informat ion is combined with data on epiphytic tree composition from primary forests, this information could be used as a bioindicator for determining the amount of human disturbance in a given area. Lastly, scientists have suggested that high densities of epiphy tes may have a detrimental effect on host trees (Johansson 1975, Dressler 1990). For example, Middleton et al. (1989) found that as epiphytic bromeliad colonization increased, new shoot growth on trees decreased while the amount of dead shoots increased. Again, this emphasizes the argument that an increase in overall abundance does not always confer greater stability or production to the ecosystem as a whole. cycling. They also provide habitat and food for a variety of organisms (Ellwood & Foster 2004). These trends are especially prominent in tropical cloud forests, which are considered to support the largest epiphyte densities of any forest type in the wor ld (Nadkarni 1984). It is for these reasons that changes in epiphyte densities impact the functioning of entire ecosystems. Though the environmental consequences associated with land conversion are not entirely known, this study demonstrated that outcome s do not have to appear negative to generate ACKNOWLEDGEMENTS Thanks to Karen Masters for her help throughout the entirety of the project, Maria, Ollie and Carrington fo r their help with rigging and climbing, the Estacin Biologca for use of the amazing forest and pasture, and the Fuentes Martinez family for their hos pitality
and patience with my Spanish. Shout out to the rest of the CIEE crew and thanks for the laughs, late night entertainment and best last semester of college ever! LITERATURE CITED Atwood, J.T. 2000. Orchids. In N. Nadkarni and N.T. Wheelwright (Eds.). Monteverde, pp. 74 75. Oxford University Press, Chicago, Illinois. Barthlott, W., V. Schmit Nauerburg, J. Nieder, and S. Engwald. 2000. Diversity and abundance of vascular epiphytes: A comparison of secondary vegetation and primary montane rain forest in the Venezuelan Andes. Plant Ecology. 152:145 156. Brokaw, N.L., and R.A. Lent. 1999 Vertical Structure. In M.L Hunter (Ed.). Maintaining Biodiversity in Forest Ecosystems, pp. 373 399. Cambridge University Press, Cambridge, United Kingdom. Catling, P.M., and L.P. Lefkovitch. 1989. Associations of vascular epiphytes in a Gualemalan cloud forest. Biotropica 21(1):35 40. Dressler, R.L. 1990. The Orchids. Harvard University Press, Cambridge, Massachusetts. Ellwood, M.D.F. and W.A. Foster. 2004. Doubling the estimate of invertebrate biomass in a rainforest canopy. Nature. 429:549 551. Gentry A.H., and C.H. Dodson. 1987. Diversity and biogeography of neotropical vascular epiphytes. Ann. Missouri Bot. Gard. 74:205 233. Graham, E.A., and J.L. Andrade. 2004. Drought tolerance associated with vertical stratification of two co occurring epiphytic bromeliads in a tropical dry forest. Am. Journal of Botany. 91:699 706. Holdridge, L.R. 1967. Life Zone Ecology. Tropical Science Center, San Jose, Costa Rica. Ingram, S.W., and N.M. Nadkarni. 1993. Composition and distribution of epiphytic organic matter in a neotropical cloud forest, Costa Rica. Biotropica. 25:370 383. Ingram, S.W., K. Ferrell Ingram, and N.M. Nadkarni. 1996. Floristic composition of vascular epiphytes in a neotropical cloud forest, Monteverde, Costa Rica. Selbyana. 17:88 103. Johansson, D.R. 1975. Ecology of epiphytic orchids in West African rain forests. Am. Orch. Soc. Bull. 44:125 136. Nadkarni, N.M. 1984. Epiphyte biomass and nutrient capital of a neotropical elfin forest. Biotropica 16:249 256. Nadkarni, N.M and N.T. Wheelwright. 200 0. Introduction. In N. Nadkarni and N.T. Wheelwright (Eds.). Monteverde, pp. 3 13. Oxford University Press, Chicago, Illinois. Perry, D.R. 1978. A method of access into the crowns of emergent canopy trees. Biotropica 10:155 157. Steege, H., and J.H.C. Cor nelissen. 1989. Distribution and ecology of vascular epiphytes in lowland rain forest of Guyana. Biotropica 21:331 339. Utley, J.F., and K. Burt Utley. 1983. Bromeliads. In D. Janzen (Ed.). Costa Rican natural history, pp. 197 200. The University of Chicag o Press, Chicago, Illinois. Walter, K.S. 1983. Orchidaceae. In D. Janzen (Ed.). Costa Rican natural history, pp. 282 292. The University of Chicago Press, Chicago, Illinois. Werneck, S., and & M.d. Esprito Santo. 2002. Species diversity and abundance of vascular epiphytes on Vellozia piresiana in Brazil. Biotropica. 34:51 57. Zimmerman, J.K., and I.C. Olmstead. 1992. Host tree utilization by vascular epiphytes in a seasonally inundated forest (tintal) in Mexico. Biotropica 24:402 407. Zotz, G., and P. Hi etz. 2001. The physiological ecology of vascular epiphytes: current knowledge, open questions. Journal of Experimental Botany. 52:2067 2078.
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Distribuciones de las orqudeas epfitas y las bromelias en los bosques secundarios y en los rboles de los potreros.
Distributions of epiphytic orchids and bromeliads on secondary forest and pasture trees
Digitized by MVI
The locations and densities of epiphytic orchids and bromeliads growing on host trees are influenced greatly by differences in epiphyte requirements for light, moisture and nutrients. Because these factors differ greatly between minimally and highly disturbed habitats, epiphytic composition may also be altered. This study examined changes in epiphytic orchid and bromeliad density between hosts in secondary forest and pasture. Fifteen trees in each habitat type were censused for the abundance of orchids and bromeliads in different parts of the trees. Results indicated that the highest concentration of both orchids and bromeliads is in the pasture. In addition, in both forest and pastureland, the highest density of epiphytes was recorded in tree regions characterized by secondary to tertiary branching. Significant results were also obtained which highlighted a positive correlation between orchids and bromeliads in both habitat types. Overall, results indicate that land conversion for human use has a great impact on the quantity and location of epiphytic orchids and bromeliads living on remnant trees.
Los lugares y las densidades de las orqudeas epfitas y las bromelias que crecen en los rboles anfitriones se ven influidos en gran medida por las diferencias en los requisitos de las epfitas por la luz, la humedad y los nutrientes. La composicin de las epfitas puede ser alterada de manera importante ya que estos factores difieren mucho en los hbitats con disturbios mnimos y en los hbitats muy perturbados. Este estudio examin los cambios en densidad de las orqudeas y las bromelias epfitas entre rboles anfitriones en un bosque secundario y en un potrero. Se consideraron quince rboles de los dos tipos de hbitat y se anot la abundancia de orqudeas y bromelias en partes diferentes de los rboles. Los resultados indicaron que la concentracin ms grande de orqudeas y bromelias se encuentra en el potrero. Adems, las densidades ms altas de epfitas, tanto en el potrero como en el bosque, se encontraron en las reas de ramificaciones secundarias y terciarias. Tambin se obtuvieron resultados significativos en la correlacin entre las orqudeas y bromelias en los dos hbitats. En general, los resultados indican que la conversin de la tierra para uso humano tiene un impacto grande en la abundancia y ubicacin de las orqudeas epfitas y las bromelias que viven en los rboles restantes.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2005
Monteverde Biological Research Station
Ecologa Tropical Otoo 2005
Estacin biolgica de Monteverde
Distribucin de bromelias
Distribucin de orquideas
t Monteverde Institute : Tropical Ecology