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Los efectos de elevacin en la riqueza de epifitas y la densidad en los helechos arborescentes (Cyatheaceae y Dicksoniaceae) del bosque nuboso
The effects of elevation on epiphyte richness and density on cloud forest tree ferns (Cyatheaceae and Dicksoniaceae)
Epiphytes play an integral role in nutrient cycling in tropical rainforests and are often found on the trunks of tree ferns. This study examined the role of elevation on epiphyte load on tree ferns. Ninety-six tree
ferns were examined along an elevational gradient, and data were collected regarding the identity and dimensions of the trees as well as their elevations and the number and type of epiphytes on their trunks. Epiphyte abundance was not significantly correlated with elevation, but epiphyte load was significantly related to tree fern morphology. The Alsophila, whose spines likely provide a prime habitat for epiphytes,
possessed a significantly larger epiphyte load than Cyathea, which does not possess spines (P = 0.0062). Further study regarding tree fern morphologies and epiphyte load should be conducted.
Las epfitas juegan un rol integral en el ciclo de nutrientes en el bosque lluvioso tropical y a menudo se encuentran en los troncos de helechos arborescentes. Este estudio examin el rol de la elevacin en la carga de epfitas en los helechos arborescentes. Noventa y seis helechos arborescentes fueron examinados a lo largo de un gradiente altitudinal y se colect datos con respecto a la identidad y dimensiones de los helechos con respecto a sus elevaciones y al nmero y tipo de epfitas en sus troncos. La abundancia de epfitas no estuvo significativamente correlacionada con la elevacin, pero la carga de epfitas estuvo significativamente relacionada con la morfologa del helecho arborescente. Alsophila, cuyas espinas probablemente proveen un mejor hbitat para las epfitas, tuvo una carga significativamente mayor de epfitas que Cyathea, la cual no tiene espinas (P = 0.0062). Debe de estudiarse la morfologa de los helechos arborescentes y la respectiva carga de epfitas.
Text in English.
Monteverde Biological Station (Costa Rica)
Morfologa de plantas
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Summer 2004
Ecologa Tropical Verano 2004
t Monteverde Institute : Tropical Ecology
The effects of elevation on epiphyte richness and density on cloud forest tree ferns (Cyatheaceae and Dicksoniaceae) Chad Taylor Department of Biological Sciences, The University of Texas at Austin ABSTRACT Epiphytes play an integral role in nutrient c ycling in tropical rainforests and are often found on the trunks of tree ferns. This study examined the role of elevation on epiphyte load on tree ferns. Ninety six tree ferns were examined along an elevational gradient, and data were collected regarding the identity and dimensions of the trees as well as their elevations and the number and type of epiphytes on their trunks. Epiphyte abundance was not significantly correlated with elevation, but epiphyte load was significantly related to tree fern morpho logy. The Alsophila, whose spines likely provide a prime habitat for epiphytes, possessed a significantly larger epiphyte load than Cyathea, which does not possess spines (P = 0.0062). Further study regarding tree fern morphologies and epiphyte load shoul d be conducted. RESUMEN Las epÃfitas juegan un rol integral en el ciclo de nutrimentos en el bosque lluvioso tropical y a menudo se encuentran en los troncos de helechos arborescentes. Este estudio examinÃ³ el rol de la elevaciÃ³n en la carga de epÃfita s en los helechos arborescentes. Noventa y seis helechos arborescentes fueron examinados a lo largo de un gradiente altitudinal y se colectÃ³ datos con respecto a la identidad y dimensiones de los helechos asÃ como con respecto a sus elevaciones y al nÃºmer o y tipo de epÃfitas en sus troncos. La abundancia de epÃfitas no estuvo significativamente correlacionada con la elevaciÃ³n, pero la carga de epÃfitas estuvo significativamente relacionada con la morfologÃa del helecho arboroscente. Alsophila , cuyas espi nas probablemente proveen un mejor habitat para las epÃfitas, tuvo una carga significativamente mayor de epÃfitas que Cyathea , la cual no tiene espinas (P = 0.0062). Debe de estudiarse la morfologÃa de los helechos arborescentes y de la respectiva carga d e epÃfitas. INTRODUCTION Scientific research has recently begun focusing on the enormous biodiversity of canopy organisms and their contributions to the ecosystem. Epiphytes comprise a significant portion of the canopy biomass, and their integral role in uptake, storage and release of minerals play a noteworthy part in the mineral cycling of the ecosystem (Coxson and Nadkarni 1995). No other place on earth has as great a diversity or abundance of epiphytes as the Neotropics. Epiphytes are the most spe cies rich life form in the Monteverde flora, with 878 species including ferns, orchids, and bromeliads, among others (Haber 2000). Typically found in the canopy, epiphytes in the cloud forest are often associated with low growing tree ferns, which are abu ndant in the cloud forests of Monteverde and are often found in clearings as light gap pioneers (GÃ³mez 1983). The trunks of tree ferns
2 are composed of a fibrous layer known as the root mantle, which is a good substrate for many epiphytes. Moran et al. (2 003) found that some epiphytic ferns occurred exclusively on tree ferns. The limited maximum height of tree ferns (about ten meters) provides an opportunity for a unique study on the relationship of epiphytes and tree ferns. Due to epiphyte requirements for high aerial moisture and nutrient availability, it has been observed that epiphyte density and abundance drop dramatically with a decrease in elevation on the Pacific slope of the TilarÃ¡n mountain range (Haber 2000). High humidity and rainfall favor t he increased presence of vascular epiphytes and bryophytes in mid elevation cloud forests in the tropics (VÃ¡zquez 1998); however, the abundance of epiphytes on tree ferns along an elevational gradient has not yet been studied. The purpose of this study wa s to gain information on epiphyte abundance and richness on tree ferns. Epiphytic density (abundance related to available surface area for growth) and richness on different tree fern species was compared along an elevational gradient. It was hypothesized that epiphyte load on tree ferns would be positively correlated with elevation. Dimensions of tree size (DBH, height) and percent of canopy coverage were also hypothesized to be positively related to epiphyte abundance and richness. MATERIALS AND METHO DS This study was conducted in the forest adjacent to the EstaciÃ³n BiolÃ³gica de Monteverde in Puntarenas, Costa Rica from July 15 to August 6, 2004. The study area included primary forest located on the Pacific slope of the TilarÃ¡n mountain range between the elevations ranging from 1500 m to 1800 m above sea level, within the lower montane rain forest life zone. This area receives an average of 2.5 meters of precipitation per year, with an additional 25% meters in mist; the mean temperature is 18.8Â° C (Cl ark et al. 2000). Ninety six tree ferns located along trails were sampled along the elevational gradient. The tree ferns were identified to species, and data were collected only on those taller than 1.3 meters. The following data were recorded for each t ree: circumference at breast height (CBH), tree height, elevation and percent canopy coverage. Tree height was calculated from the base to the lowest frond. Surface area of the trunk was extrapolated from CBH and height. Elevation was measured using th e altimeter on a SUUNTO Vector watch. Epiphytes on the trunks were identified to morphospecies on each tree fern sampled and their individual abundance recorded. Percent coverage of the canopy was estimated by standing directly next to the trunk of the t ree fern and looking up at a 90Â° angle from the horizontal. Epiphyte richness and abundance along the elevational gradient were compared using simple regressions between the independent variables (CBH, height, surface area, elevation and canopy coverage) and the dependent variables (richness, abundance, density and adjusted species richness). Species richness was determined only for epiphytes on individual tree ferns. Density was calculated by dividing the number of individuals by surface area, and adjus ted species richness was calculated on individual tree ferns by dividing the species richness by surface area. The data for all the tree ferns were statistically analyzed by simple regressions; Cyathea caracasana, C. delgadii and C. onusta were analyzed i ndividually using simple regressions as well.
3 A one way ANOVA was used to compare species richness and density between the three aforementioned species. A Kruskal Wallis test was used to compare species richness and density among the following most commo n species: Alsophila erinacea, A. polystichoides, C. caracasana, C. delgadii and C. onusta . Unpaired t tests were used to determine if there was a significant difference in the mean epiphyte density and mean adjusted species richness of Alsophila sp. and C. caracasana. Finally, unpaired t tests compared mean epiphyte density and mean adjusted species richness between Alsophila sp. and 19 randomly selected Cyathea sp. individuals. RESULTS Ninety six total individuals of eight different species of tree ferns were identified in the study area (E. Cruz, pers. comm.), including 10 Alsophila erinacea , 9 Alsophila polystichoides , 26 Cyathea caracasana , 20 Cyathea delgadii , 20 Cyathea onusta , 3 Cyathea poeppigii , 3 Shaeropteris brunei and 4 Dicksonia gigantea (Table 1). When testing all tree ferns together, elevation was not found to be significantly correlated with epiphyte abundance or richness. A significant positive correlation was found between epiphyte abundance and tree fern surface area (R 2 = 0.324, P < 0.0001), tree height (R 2 = 0.211, P < 0.0001) and CBH (R 2 = 0.283, P < 0.0001). Similarly, a significant positive correlation was found between species richness and surface area (R 2 = 0.238, P < 0.0001), tree height (R 2 = 0.211, P < 0.0001) and CBH (R 2 = 0.202, P < 0.0001) (Figure 1). A significant positive correlation was found between surface area of C. caracasana (N = 26) and epiphyte abundance (R 2 = 0.244, P = 0.0088) as well as between CBH and abundance (R 2 = 0.302, P = 0.0030) (Figure 2). A sign ificant positive correlation was also found between the surface area of C. delgadii (N = 20) and species richness (R 2 = 0.228, P = 0.0333) in addition to elevation and CBH (R 2 = 0.3, P = 0.0124). Further analysis showed that elevation and adjusted species richness were positively correlated (R 2 = 0.203, P = 0.0463) (Figure 3). A significant positive correlation between adjusted species richness and elevation were also found for C. onusta (N = 20) (R 2 = 0.293, P = 0.0137), although this species was only fo und at elevations over 1730 m. Furthermore, abundance on this species was positively correlated with surface area (R 2 = 0.322, P = 0.0090) and tree height (R 2 = 0.454, P = 0.0011). In addition, species richness was positively correlated with tree height (R 2 = 0.348, P = 0.0061), surface area (R 2 = 0.487, P = 0.0006) and CBH (R 2 = 0.332, P = 0.0078) (Figure 4). There was no significant difference between either epiphyte density (one way ANOVA, P = 0.0957) or adjusted species richness (one way ANOVA, P = 0 .1106) on C. caracasana, C. delgadii and C. onusta. A significant difference was found among A. erinacea, A. polystichoides, C. caracasana, C. delgadii and C. onusta with regards to epiphyte density (Kruskal Wallis, P = 0.0055) but not adjusted species ri chness (Kruskal Wallis, P = 0.2239). Based on the means, the Alsophila sp. had the greatest epiphyte density. When comparing Alsophila sp. and C. caracasana, a significantly greater mean epiphyte density was found on the Alsophila (unpaired t test, t = 3 .062, P = 0.0037), but not in the mean adjusted species richness (unpaired t test, t = 0.364, P = 0.7174). Finally,
4 Alsophila sp . was found to have a significantly higher mean epiphyte density than Cyathea sp. (unpaired t test, t = 2.906, P = 0.0062), but this was not found in adjusted species richness (unpaired t test, t = 0.751, P = 0.4575). DISCUSSION Epiphyte abundance and richness were found to have no significant correlation with elevation of tree ferns. This is in conflict with prior studies rega rding increased epiphyte abundance and richness with increased elevation. Perhaps this has to do with the vertical distribution of epiphytes on their host plants. Epiphytes tend to concentrate in the canopy, but tree ferns are primarily found in the unde rstory and in light gaps. It seems that the elevational distribution of epiphytes may vary when vertical distribution is limited. A positive correlation was found between epiphyte abundance and species richness when compared to tree height, CBH and sur face area. This indicates that tree dimension is more important than elevation in the frequency of epiphytes on tree ferns. Taller, thicker trunks are colonized by more epiphytes. This may be because the tree itself is larger and therefore its surface c an sustain a larger population. Additionally, the larger trunks indicate increasing age. It may take time for epiphytes to become established on tree ferns, so those that are older would be more likely to possess a greater abundance of epiphytes as well as more species. Among the individual Cyathea species, tree dimension may also be more influential on epiphyte load than elevation. Larger trees with larger surface areas possess larger environments. There is less competition for resources so more epi phytes are able to become established. Elevation could only be correlated with adjusted species richness on two species of tree ferns, C. delgadii and C. onusta . The latter was only found over a 65 meter range from 1730 m to 1795 m which is not represent ative of the entire elevational gradient of the study site. Furthermore, there was no significant difference among the three Cyathea spp. regarding epiphyte density or adjusted species richness. Belonging to the same genus, the surface of the trunks are similar which is reflected by comparable epiphyte compositions. There is some indication that epiphyte abundance and richness may be more strongly correlated with morphological characteristics on various species of tree ferns. In order to distinguish epi phyte load according to possible morphological differences, the five most represented tree ferns were compared statistically at the genus level, Cyathea and Alsophila . There was a significant difference between the genera with regard to epiphyte density. These two genera exhibit strikingly unique morphologies which may play a role in their epiphyte load. Alsophila are easily recognized by its spiny trunk, whereas Cyathea to protect th e tree fern from herbivory, but their uneven surface may serve as a prime substrate for epiphyte growth. The spines offer increased surface area, protection and a horizontal surface on which the epiphytes can grow. There could also be a chemical reason for the reduced number of epiphytes on Cyathea. In Mexico, it has been found that while orchids and other epiphytes are
5 abundant on some oak species, they are virtually absent on others. Experiments showed that toxicity in the bark inhibited orchid grow th (Richards 1996). It could be possible that in some way the Cyathea produce a chemical response which help to limit the epiphytic load, but not much is known on this topic. The data indicates that possibly tree fern morphologies, rather than elevatio n, have a greater effect on epiphyte density, abundance and richness. For future study, it is suggested that a closer look be taken at various morphologies of tree ferns in cloud forests with respect to epiphyte abundance and richness, while at the same ti me holding elevation constant. Individual epiphytic ferns are often difficult to differentiate; their roots may extend throughout the tree fern trunk, tangling with one another, forming mats of the epiphyte. Additionally, many epiphytes such as mosses were overlooked in this study due to the difficulty in counting individuals. It would be interesting to perform a similar study measuring epiphyte coverage area of the tree fern trunk. Lianas and vines were also found to inhabit tree fern trunks, and per haps this variable could be incorporated into future studies as well. ACKNOWLEDGMENTS Estoy especialmente agradecido por la ayuda de Eladio Cruz para su tiempo y pericia con la identificacion de los helechos arborecentes, and to Karen Masters for her ti me and expertise identifying and describing orchids, as well as aiding in statistical analyses. These two gems of Monteverde are irreplaceable resources. To my advisor Carmen Rojas, usted es la mÃ¡s linda persona en el mundo . Thanks you for your help and for accompanying me on a memorable journey to the ridge. Carlos, I must warn you that I will keep your e mail address handy for future reference; I will stump you with a Snapple fact one day. You have passed an immense amount of knowledge on to me this summer, and for that I am indebted. Maria special. Katie, the late night company made this assignment bearable. And to my friends across the country fro m CIEE, I am thankful that I was able to share this experience with you. Finalmente, para las mujeres de la EstaciÃ³n BiolÃ³gica, gracias por su comida buena que cocinÃ³ con amor. LITERATURE CITED Clark, K. L., R. O. Lawton, and P. R. Butler. 2000. The physical environment. In N. M. Nadkarni and N. T. Wheelwright (Eds). Monteverde: ecology and conservation of a tropical forest, pp. 15 38. Oxford University Press, New York, New York. Coxson, D. S. and N. M. Nadkarni. 1995. Ecological roles of ep iphytes in nutrient cycles of forest ecosystems. In M. D. Lowman and N. M. Nadkarni (Eds). Forest Canopies, pp. 495 543. Academic Press, San Diego, California. GÃ³mez, L. D. 1983. Cyatheaceae and Dicksoniaceae (Rabos de Mico, Tree Ferns). In D. H. Jan zen (Ed). Costa Rican Natural History, pp. 225 228. The University of Chicago Press, Chicago, Illinois. Haber, W. A. 2000. Plants and vegetation. In N. M. Nadkarni and N. T. Wheelwright (Eds). Monteverde: ecology and conservation of a tropical cloud forest, pp. 39 70. Oxford University Press, New York, New York. Moran, R. C., S. Klimas, and M. Carlsen. 2003. Low trunk epiphytic ferns on tree ferns versus angiosperms in Costa Rica. Biotropica 35 (1): 48 56. Richards, P. W. 1996. The Tropical Ra in Forest. Cambridge University Press, New York, New York. Rojas, A. 1999. Helechos arborescentes de Costa Rica: arborescent ferns. Instituto Nacional de Biodiversidad.
6 VÃ¡zquez G., J. A. and T. J. Givnish. 1998. Altitudinal gradients in tropical fore st composition, structure, and diversity in the Sierra de ManantlÃ¡n. Journal of Ecology 86 (6): 99 1020.
7 TABLES Table 1: Relative abundance of families and species of 96 tree ferns sampled between 1500 m and 1800 m in forest adjacent to the Esta ciÃ³n BiolÃ³gica, Monteverde. Family Species Abundance Cyatheaceae Alsophila erinacea 10 Alsophila polystichoides 9 Cyathea caracasana 26 Cyathea delgadii 20 Cyathea onusta 20 Cyathea poeppigii 3 Shaeropteris brunei 3 Dicksoniaceae Dic ksonia gigantea 4 Total 96
8 Figure 1. Simple regressions showing the significant relationships between three physical measurements of tree ferns (N = 96) and the two major indicators of epiphyte load (abundance and richness). A) Epiphyte abundance increased with tree fern trunk surface area (R 2 = 0.324, P < 0.0001) (Y = 1.74 + 0.002 * X). B) Epiphyte abundance increased with tree fern trunk height (R 2 = 0.211, P < 0.0001) (Y = 5.786 + 9.648 * X). C) Epiphyte abundance increased with tree fern CBH (R 2 = 0.283, P < 0.0001) (Y = 15.272 + 1.297 * X). D) Epiphyte species richness increased with tree fern trunk surface area (R 2 = 0.238, P < 0.0001) (Y = 1.984 + 2.722E 4 * X). E) Epiphyte species richn ess increased with tree fern trunk height (R 2 = 0.211, P < 0.0001) (Y = 0.852 + 1.21 * X). F) Epiphyte species richness increased with tree fern CBH (R 2 = 0.202, P < 0.0001) (Y = 0.078 + 0.147 * X). A D B E C F
9 Figure 2. Simple regressions for Cyathea caracasana (N = 26) showing the significant relationships. A) Epiphyte abundance increased with trunk height (R 2 = 0.244, P = 0.0088) (Y = 1.88 + 0.002 * X). B) Epiphyte abundance increased with trunk CBH (R 2 = 0.302, P = 0.0030) (Y = 14.83 + 1.132 * X). A B
10 Figure 3. Simple regressions for Cyathea delgadii (N = 20) showing significant relationships. A) Epiphyte species richness increased with trunk surface area (R 2 = 0.228, P = 0.0333) (Y = 0.886 + 0.001 * X). B) CBH increased with increasing elevation (R 2 = 0.3, P = 0.0124) (Y = 17.748 + 0.026 * X). C) Adjusted species richness increased with elevation (R 2 = 0.203, P = 0.0463) (Y = 0.002 + 1.772E 6 * X). A B C
11 Figure 4. Simple regressions for Cyathea onust a (N = 20) showing significant relationships. A) Epiphyte adjusted species richness increased with elevation (R 2 = 0.293, P = 0.0137) (Y = 0.027 + 1.552E 5 * X). B) Epiphyte abundance increased with trunk height (R 2 = 0.322, P = 0.0090) (Y = 5.839 + 0.0 02 * X). C) Epiphyte abundance increased with trunk surface area (R 2 = 0.454, P = 0.0011) (Y = 4.198 + 9.319 * X). D) Epiphyte species richness increased with trunk height (R 2 = 0.348, P = 0.0061) (Y = 0.439 + 1.509 * X). E) Epiphyte species richness i ncreased with trunk surface area (R 2 = 0.487, P = 0.0006) (Y = 1.246 + 4.935E 4 * X). F) Epiphyte species richness increased with trunk CBH (R 2 = 0.332, P = 0.0078) (Y = 1.393 + 0.242 * X). A D B E C F