Community Structure of Vascular Epiphytes in a Costa Rican Elfin Forest Duryea Delacroix Departments of Biology and Environmental Studies Oberlin College, Oberlin, Ohio ____________________________________________________________________ ABSTRACT Neotropical montane forests are characterized by their great abundance and high diversity of epiphytes. This study examines the distribution of epiphytic families in three zones of six Meliosma vemicosa trees using single rope climbing techniques to access the for est canopy. Nine hundred ninety seven vascular epiphytes from eight angiosperm families and division Pteridophyta occurred at the study site. Orchidaceae, Bromeliaceae, Araceae, Ericaceae, Piperaceae, and Gesneraceae were the most common angiosperm families. Pteridophytes accounted for 40% of the individuals present. Friedman's test for Shannon Weiner index and Orchidaceae showed a significant Chi squared value X 2 = 8.4, and X 2 = 7.4 and tied p value p = 0.02, and p = 0.02. Mode of seed dispersa l, type of growth form and structure, and methods of water absorption and storage are discussed as factors that affect vertical distribution of epiphytes. This study indicates relatively high epiphyte diversity at the family level, at a mid elevation site in Costa Rica. RESUMEN Los bosques nubosos neotropical e s tienen una alta abundancia y diversidad de epifitos. Este estudio investigÃ³ la distribuciÃ³ n de diferentes familias de epifitas en tres divisiones de seis arboles, Meliosma vemicosa. Se encontraron 997 epifitas vasculares de ocho familias angiosperma y la divisiÃ³n Pteridophyta. Orchidaceae, Bromeliaceae, Araceae, Ericaceae, Piperaceae, y Gesneraceae son las familias mÃ¡s abundantes. Pte ridophyta c onsta de 40 % de los individuos. Se encontrÃ³ una diferencia significativa entre la diversidad en los diferentes divisiones del Ã¡rbol y tambiÃ©n en la abundancia de los individuos en la f amilia Orchidaceae. La dispersiÃ³ n de semillas, la estructura y el almacenaje del agua puede a f ectar la dist ribuciÃ³n de epifitas. Esta investigaciÃ³n demostrÃ³ que la diversid ad de familias de epifitas es alt a en un bosque nuboso en Costa Rica. INTRODUCTION Epiphytic plants, which derive support but not nutrients directly from their host trees , reach their greatest diversity and abundance in Neotropical mid montane forests and elfin woodlands Madison 1977. In the most diverse tropical forests, epiphytes account for up to 35% of the total vascular plant flora and nearly half of the individual plants Gentry & Dodson 1987a. Documenting the epiphyte flora of epiphyte rich tropical forests will enhance our knowledge
of these communities and is the first step in understanding the biotic and abiotic conditions that affect epiphyte distribution Gentry & D odson 1987b. Epiphytes have been shown to affect the forest ecosystem as a whole through their remarkable abilities to absorb and retain atmospheric nutrients borne in rain, mist and dust Nadkarni 1986. Epiphytes may significantly contribute to overall nutrient cycling despite their small biomass, relative to the ecosystem as a whole. Investigations have shown that the presence of epiphytes enlarges the mineral capital of elfin forests Nadkarni 1984. Epiphytes must overcome greater extremes of tempera ture, wind, and humidity than their terrestrial counterparts, and have developed a wide variety of adaptations to survi ve in the canopy. Water stress in dry weather is more severe for epiphytes in exposed positions than for those in shade or site s where mo isture collects, as in branch forks. Thus, rapid and efficient water uptakes by roots, or by specialized trichomes in bromeliads, are important adaptations for exploiting microhabitats in the canopy. Neotropical cloud forests are characterized by a relatively high diversity and abundance of vascular epiphytes, yet despite their biological significance, information on their vascular epiphyte communities is relative ly incomplete Catling and Lefk ovitch 1 989. Examining the distribution of epiphytes within a single host species is necessary to understand epiphyte distribution and variation at a smaller scale. This study is an attempt to document the community structure of families of vascular epiphytes, an d draw attention to some of the ecological characteristics involved. MATERIALS AND METHODS The study site was located in the EstaciÃ³n BiolÃ³gica de Monteverde, in northwest Costa Rica. The site lies between 1720 1800m in the elfin forest along the Sendero D ivisiÃ³n, in the Lower
Montane Wet or Rain Forest Holdridge life zone Haber et al. 2000. Along the upper ridges of the Cordillera de TilarÃ¡n at Monteverde, strong trade winds coming from the Atlantic create narrow bands of elfin forest with short 3 10m canopies, which are enshrouded by cloud cover year round Ingram et al. 1996, Nadkarni 1984. A nearby site receives about 2000 2500mm of rain annually, and although little precipitation falls from December through May, windblown mist contributes an additi onal 500 2000mm per year Ingram and Nadkarni 1993. Epiphytic vegetation varies with the age and species of the host tree Richards 1996, so only individuals of Meliosma vernicosa Sabiaceae were sampled in order to minimize the effect of host tree specificity. This species was chosen because it is abundant in the elfin forest, has good architecture for epiphytes, and is relatively easy to rig. Six trees with the diameter at breas t height DBH of 1.0m Â€ 1.2m x = 1.1m were sampled in order to keep the size and age of the tree consistent. Single rope climbing techniques described in Nadkarni 1984 were used to gain access to and sample from the trees. Data were collected from Apr il to May, 2001. Each tree was divided into five zones according to the method proposed by Johansson 1974 Figure 1. Zone one included the bottom portion of the trunk up to three meters. Zone two was the remainder of the trunk up to the first major bra nching. The branches were divided into three sections of equal length: zone three was the basal part of each branch, zone four was the center section, and zone five the outer tips. At each tree, the vascular epiphytes in zones two, three, and four were ide ntified to family and counted. Zone five was not sampled due to restricted access from the rope, and zone one was not sampled due to a higher occurrence of accidental epiphytes. Most species in zone two and three could be accessed from the rope, however of ten it was not possible to reach zone four and species were identified using binoculars. To quantify the number of individuals in a zone, all visible individuals or clusters
of stems that appeared to comprise a single individual were counted. In zone two, three square meters were sampled of the vertical trunk. In zone three, 1.5 square meters were sampled from three primary branches 45Â° 60Â°. One square meter was sampled from five Â€ eight horizontal branches in zone four. RESULTS Nine hundred ninety seve n vascular epiphytes from eight angiosperm families and division Pteridophyta occurred on the host trees. Pteridophytes accounted for 40% of the individuals present Figure 2. Orchidaceae, Bromeliaceae, Araceae, Ericaceae, Piperaceae, and Gesneraceae were the most common angiosperm families, which is consistent with Ingram et al. 1996. Data on abundance of epiphytic families in zones two, three, and four were arranged in a 9 x 3 contingency table. The data were analyzed using a Chi squared test for each tree, as well as for all trees combined Tables 1 6. The abundance data for all trees combined, and for four of the five trees were significant P < 0.05. Araceae often showed a preference for zone two, and never avoided that zone. Ericaceae sometimes pr eferred zone three, and often avoided zone two. Piperaceae sometimes preferred zones three and four; however it sometimes avoided zones two and three. Gesneraceae occasionally avoided zone three. Data comparing each tree with the abundance of epiphytes in zones two, three, and four were analyzed using a two way non parametric ANOVA FriedmanÂs test. Only the FriedmanÂs test for Shannon Â€ Weiner index and Orchidaceae showed a significant Chi squared value X 2 = 8.4, and X 2 = 7.4 and tied p value p = 0.02 , and p = 0.02 Table 8. For most trees, the Shannon Â€ Weiner index HÂ was highest in zone three, followed by zone four, and finally zone two Figure 3.
DISCUSSION The mode of seed dispersal may be very important for determining the vertical distribution of families. For example, one would expect bird and arboreal mammal dispersed families to be predominant in zone two. This is because bird and mammal dispersers ofte n perch in the crown and defecate seeds which fall to zone two and germinate. This is supported by Araceae and Gesneraceae which showed an abundance of 15% and 14% respectively in zone two Figure 2. One would also expect wind dispersed families to be abu ndant in zone four where there is more wind than in the sheltered inner crown. This is supported by the predominance of Orchidaceae 19% in zone four. This is also supported by the abundance of Bromeliaceae 7% in zone four. The role of growth form or st ructure may also be important for vertical distribution of families. For example, Bromeliaceae showed a preference for zones three and four. This could be due to structural limitations, as it would be more difficult for the rosette to attach to a vertical trunk. Ericaceae showed a preference for zone three, and this could also be related to structure in that the large, heavy plants of this family are restricted to zone three. Conversely, Orchidaceae and Piperaceae showed a strong preference for zone four an d this is most likely because to the small plants of this family are able to grow on the small branches in zone four. Araceae was predominantly found in zone two and this is because the scandent growth form of this family is able to exploit the vertical tr unk of zone two. Adaptations to water stress are also important for determining the vertical distribution of families. Ericaceae showed a preference for zone three and this could be due to their bulky roots which are adapted for water storage in the desicc ating environment of the crown. However, their heavy root structure also confines them to zone three where they can utilize branch forks to secure and stabilize themselves. Orchidaceae thrives in zone four, and many
species use pseudobulbs and velamin for water storage in the high wind and light conditions of the outer crown. These adaptations allow them to maintain their s mall size while maximizing their water use efficiency. Some orchids, bromeliads and ferns collect water in the rosette created by overla pping fronds or leaves, and this also allows them to exploit desiccating conditions in the canopy N adkarni et al 2001. It is of interest to note that the predominance of Orchidaceae, Araceae, and Bromeliaceae is consistent with the species richness of these taxa Kress 1986. It is possible that numbers of species of these families affected how abundant these families were. However, in my observations there were usually a few species of certain fam ilies that dominated the number of individuals in that family. Therefore, it is most likely coincidence that the relative abundance of the families I observed followed the species richness of these families. Ep iphytes are generally patchy in distribution a nd thus once a species colonizes an area it is likely to become common in the immediate vicinity. For most trees, the Shannon Â€ Weiner index HÂ was highest in zone three, followed by zone four, and finally zone two. This is consistent with the study by I ngram et al. 1996 which showed a significantly greater species richness in zone three. A surprising result of my study was that although the HÂ values were significant X 2 = 8.4, p = 0.02, neither evenness or species richness were significant X 2 = 0.33 , p = 0.84, and X 2 = 0, respectively Table 8. It is likely that the combined effects of evenness and species richness contribute to the significant HÂ value. Within a forest, microsite differences exist at many different spatial scales: within a single branch, between branches at different heights of the tree, between trees of different architecture, and within stands of differing topography and aspects Nadkarni et al 2001. Thus,
in additional studies it would be informative to sample additional zones of Meliosma vernicosa , additional Meliosma trees, as well as other host species in order to have a broader understanding of community structure of epiphytes. The relatively large numbers of epiphytic families present at this site demonstrate the need for more sampling to fully characterize family distributions in communities and to identify the underlying patterns and mechanisms. ACKNOWLEDGEMENTS Thank you Karen Masters for your guidance and support throughout this project. Tim and Andrew g et a big thank you for helping me rig trees. I thank Rigo for showing me how to climb and lending me his equipment. Thank you to Stephanie and Ally for moral support, and Alan for being the wonderful person he is. And I give thanks to EstaciÃ³n BiolÃ³gica fo r the gift of this amazing forest, and all the trees I climbed. _____________________________________________________________________________________________ LITERATURE CITED Catling, P. and L. Lefkovitch. 1989. Associations of vascular epiphytes in a Guatemalan cloud forest. Biotropica 21 1: 35 40. Gentry A. H. and C.H. Dodson. 1987a. Contribution of nontrees to species richness of a tropical rainforest. Biotropica 19: 149 156. Gentry A. H. and C.H. Dods on. 1987b. Diversity and biogeography of neotropical vascular epiphytes. Ann. Missouri Bot. Gard. 74: 205 233. Haber W.A., W. Zuchowski, and E. Bello. 2000. An introduction to cloud forest trees. Mountain Gem Publications, Monteverde, Costa Rica. Ingram, S .W., K. Farrell Ingram, and N.M. Nadkarni. 1996. Floristic composition of vascular epiphytes in a Neotropical cloud forest, Monteverde, Costa Rica. Selbyana 17: 88 93. 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. Johansson, D.R. 1975. Ecology of epiphytic orchids in West African rainforests. American Orchid Society Bulletin 44: 125 136. Kress, W.J. 1986. The systematic distribution of vascul ar epiphytes: an update. Selbyana 9: 2 22. Madison, M. 1977. Vascular epiphytes: their systematic occurrence and salient features. Selbyana 2: 1 13. Nadkarni, N.M., M.C. Merwin, and J. Nieder. 2001. Forest canopies, plant diversity In S.A. Levin Ed. Ency clopedia of Biodiversity 3: 27 40. Academic Press. Nadkarni, N.M. 1984. Epiphyte biomass and nutrient capital of a neotropical elfin forest. Biotropica 16: 249 256. Nadkarni, N.M. 1986. The nutritional effects of epiphytes on host trees with special refere nce to alteration of precipitation chemistry. Selbyana 9: 44 51. Richards, P.W. 1996. The Tropical Rainforest . Cambridge University Press, New York, New York.
Table 1. A 9 x 3 contingency table of abundance of epiphyte families for ALL TREES. Chi square = 87.0, p < .0001 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 130 132 143 Orchidaceae 53 56 66 Araceae 45 23 19 Gesneraceae 43 23 33 Cyclanthaceae 8 10 5 Bromeliaceae 15 24 24 Piperaceae 2 12 33 Ericaceae 8 41 17 Poaceae 5 13 14 Table 2 . A 9 x 3 contingency table of abundance of epiphyte families for TREE 1. Chi square = 70.9, p < .001 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 17 28 19 Orchidaceae 8 10 10 Araceae 8 1 0 Gesneraceae 6 5 11 Cyclanthaceae 3 0 1 Bromeliaceae 3 17 5 Piperaceae 0 3 10 Ericaceae 0 14 0 Poaceae 0 3 3 Table 3 . A 9 x 3 contingency table of abundance of epiphyte families for TREE 2 . Chi square = 45.3, p = .0001 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 34 28 22 Orchidaceae 14 10 14 Araceae 13 1 8 Gesneraceae 8 5 2 Cyclanthaceae 2 0 0 Bromeliaceae 4 17 5 Piperaceae 2 3 9 Ericaceae 1 14 0 Poaceae 5 3 5 Table 4 . A 9 x 3 contingency table of abundance of epiphyte families for TREE 3. Chi square = 29.5, p = .021 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 28 24 24 Orchidaceae 12 13 14 Araceae 1 4 1 Gesneraceae 7 1 7 Cyclanthaceae 1 2 0 Bromeliaceae 4 2 1 Piperaceae 0 3 0 Ericaceae 4 8 13 Poaceae 0 2 6
Table 5 . A 9 x 3 contingency table of abundance of epiphyte families for TREE 4. Chi square = 20.0, p = .15 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 23 16 24 Orchidaceae 5 8 9 Araceae 5 9 2 Gesneraceae 14 6 7 Cyclanthaceae 0 0 1 Bromeliaceae 3 1 1 Piperaceae 0 3 2 Ericaceae 2 0 1 Poaceae 0 0 0 Table 6 . A 9 x 3 contingency table of abundance of epiphyte families for TREE 5. Chi square = 27.0, p = .020 Division or family Abundance in zone 2 Abundance in zone 3 Abundance in zone 4 Pteridophyta 12 22 16 Orchidaceae 14 11 15 Araceae 12 4 2 Gesneraceae 8 7 4 Cyclanthaceae 2 5 2 Bromeliaceae 1 2 3 Piperaceae 0 0 3 Ericaceae 0 4 3 Poaceae 0 0 0 Table 7 . Results of FriedmanÂs test comparing each tree with epiphytes in zones two, three, and four. Chi square values corrected for ties, and tied p values are presented. Significant vales are in bold type, when p<0.05. X 2 P value HÂ 8.4 .02 Number of families 2.5 .29 Total epiphyte abundance 0 Evenness .33 .84 Ferns .09 .95 Orchidaceae 7.4 .02 Bromeliaceae .11 .95 Araceae 3.0 .20 Ericaceae 2.3 .31 Gesneraceae 3.9 .14