Distribution patterns of the tree ferns Alsophila erinacea and A. polystichoides (Cyatheaceae ) in cloud forests of Monteverde, Costa Rica Stephen Gottschalk Department of Geograp hy, University of Colorado at Boulder Abstract The effects of elevation on the tree ferns Alsophila erinacea and A. polystichoides (Cyatheaceae) distributions were examined between 1550 m and 1800 m in Monteverde, Costa Rica. Alsophila erinacea and A. polystichoides prefer similar habitat types and ove rlap almost completely in their elevational ranges, making them ideal species for comparison. Six transects were made in each of five 50 m elevational blocks between 1550 m and 1800 m for a total of 30 transects. Transects along a ridge trail were compared with those on a trail on the side of a slope and were found to contain considerably less Alsophila tree ferns at all elevations (a total of 5 individu als on six ridge transects compared with 36 individuals on four slope transects). There was a slight decrease in these two species' abundances on an increasing elevati onal gradient. Both species of fern were found to be patchy based on an index of dispersi on, Poisson comparison, and Chi squared tests (X 2 = 710 and 33063; df = 9 and 18; P = 0.001 and 0.001 for A. erinacea and A. polystichoides respectively). Finally, microclimate variations likely did more to influence A. erinacea and A. polystichoides distr ibutions than did elevation. Resumen Se examinaron los efectos de la elevaciÃ³n en los hele chos Alsophila erinacea y A. polystichoides (Cyatheaceae) entre 1550 m y 1800 m en Monteverde, Costa Rica. Estos helechos prefieren tipos de hÃ¡bitat similares y se traslapan casi completamente en sus Ã¡mbitos altitudinales. Por esta razÃ³n A. erinacea y A. polystichoides son especies buenas para comparar. Se hicieron seis transectos en cinco bloques de elevaciÃ³n , de 50 m cada uno, entre 1550 m y 1800 m para un total de 30 transectos. Se compararon los transectos a lo largo de un sendero en una cresta con los de un sendero empinado y se encontrÃ³ muchos menos helechos Alsophila en todas las e levaciones de la cresta (cinco individuos en seis transectos de la cresta comparados con 36 individuos en cuatro transectos de la cuesta). Hubo una leve disminuciÃ³n en la abundancia de estas dos especies del helecho en un gradiente de elevaciÃ³n ascendente. Se encontrÃ³ que las dos especies de helecho estÃ¡n agrupadas con base en el Ãndice de dispersiÃ³n , el test de comparaciÃ³n Poisson, y el test Chi cuadrado (X 2 = 710 y 33063; df = 9 y 18; P = 0.001 y 0.001 para A. erinacea y A. polystichoides respectivamente) . Finalmente las variaciones en el microclima influyeron mÃ¡s las distribuciones que la elevaciÃ³n .
Introduction Elevation is known to have a significant influence on community composition and species abundances in tropical forests (Kappelle and Gomez 1992; Lee et al. 1986; Lieberman et al. 1996 ). In general, tree fern abundances tend to increase slightly with increa s es in elevation (Gomez 1983). For two Cyatheaceae ferns, A. erinacea and A. polystichoides, the role of elevation in their abundances and distributions is largely unknown. These two species have over lapping elevational ranges (800m 2100 m for A. erinacea, and 700m 2000 m for A. polystichoides) and both are forest understory or riverbank trees making them ideal species for comparison (Rojas 1999). By comparing these two species, one can determine how closely related tree ferns interact with each o ther. Direct competition, facilitation, and allelopathic inhibition are all possible interactions to consider between these two species. Direct competition would be a result of one species using a resource thereby limiting the resource availability for the other species (Ricklefs 1990). Facilitation would result from the activities of one species enhancing the population of the other species (Ricklefs 1990). Because these two tree fern species prefer the same habitats and have similar growth forms, one of t hese two interactions is likely to occur. Finally, allelopathy is a known mechanism in some ferns for eliminating other plants by inhibiting germination, growth and/or development (Cooper Driver 1980). Thus it makes sense that the presence of one species of tree fern may limit the presence of the other. The purpose of this study was to determine whether elevation affected the abundances of these two species and to determine the distributions of each. One wou ld predict that, based on some of the above mentioned experiments, abundances for these tree ferns would increase on an elevational gradient, and that they would limit each other's abundance due to similar habitat selection, similar growth habits, and alle lopathic inhibition. Study Site and Methods Thirty t ransects were made between 1550m and 1800 m in both leeward cloud forest and elfin forest behind the EstaciÃ³n BiolÃ³ gica de Monteverde (EBM) and the Centro de EducaciÃ³ n Creativa (CEC) in Monteverde, Costa Rica. Five 50 meter elevational block s were established between 1550m and 1800 m using a handheld altimeter. For each elevational block a total of six transects were made, three on CEC property and t hree on EBM property up to 1750m. For the 1750m 1800 m bl ock all six transects were made on EBM land as CEC trails did not reach this elevation. Both CEC and EBM transects included one on trail transect and two off trail transects. The CEC on trail transects all were located along a ridge whereas the EBM on trai l transects were on the side of a slope (except for the 1750m 1800m transect of course). The on trail transects were 125 meters long and extended two meters into the forest on either side of the trail for a total of 500m 2 . In each case 125 meters were me asured down slope
along the trail from the upper limit of the elevation block. The off trail transects were made on plots 25 meters long and ten meters wide, also for a total of 500m 2 . Each transect had to begin and end in the same elevational block, and t he off trail transects began two meters out from the trail so as not to overlap the trail transects. Slope, aspect, and nearest neighbor distances were measured for each tree fern found in a transect. Nearest neighbor distances were rounded to the nearest whole meter. An index of dispersion was taken for the nearest neighbor values. Chi squared tests were used to determine whether or not A. polystichoides and A. erinacea tended to occur in the presence or absence of one another. A Chi squared test was also used to compare each tree fern's frequency distribution against a Poisson distribution to determine whether or not distributions were random. Formulas for index of dispersion and Poisson distribution were taken from Southwood (1978). Results I found a total of 100 Alsophila tree ferns in my 30 transects (81 A. erinacea individuals and 19 A. polystichoides individuals. Both A. polystichoides and Alsophila erinacea abundances seemed to decrease along an elevational gradient (Figure 1). 1550 m 1600 m had the highest total Alsophila abundance (27 individuals) as well as the most A. erinacea (22 individuals). Alsophila polystichoides was most abundant in the 1600 m 16 50 m block with 7 individuals. The fewest individuals of each species were found in the 1650 m 1700 m block, 11 individuals for A. erinacea and one individual for A. polystichoides. Both species of Alsophila appeared to prefer moderate slopes (30 60 degre es), and aspects from South East (100 degrees from North) to South West (250 degrees from North) (Figure 2). Overall, transects made on land behind the EstaciÃ³ n BiolÃ³ gica de Monteverde contained more Alsophila of both species than the Centro de EducaciÃ³ n Creativa land. Specifically, ridge line transects, most of which were on CEC property, contained far fewer individuals (five individuals) than the trail transects on the slope of EBM property (36 individuals) despite being sampled more (six ridge transec ts compared to four slope transects). The index of dispersion for A. erinacea and A. polystichoides was 570 and 158 respectively, which makes both species significantly 'patchy', or non randomly dispersed (for A. erinacea and A. polystichoides respectively: df = 80 , 18 ; P < 0.001, 0.001). The Poisson distribution comparison also demonstrates this 'patchiness' (for A. erinacea and A. polystichoides respectively: X 2 = 710 and 33,063; df = 9, 18; P < 0.001, < 0.001). 88.9 percent (72 of 81 individuals) of A. erinacea and 73.7 percent (14 of 19 individuals) of A. polystichoides were within ten meters of a conspecific individual (Figure 3). Although A. erinacea was encountered separately from A. polystichoides in 14 of 30 tra nsects (47% of the time), A. polystichoides never occurred without A. erinacea in all 30 transects; these findings were significant (X 2 = 14.27; df = 3; P = 0.005).
Discussion I expected the distributions of Alsophila erinacea and Alsophila polystichoides to increase with elevation and to niche partition so that neither would be found in the same place as the other. The data I found do not support this hypothesis. There was an overall decrease in abundances for both species on an el evational gradient, and a much lower abundance for both in the 1650 m 1700 m block (Figure 1). While these findings do contradict my hypothesis and the general findings of Lee et al. 1986, they may be consistent with other studies that show 'life form' ab undance in general varies greatly over elevational gradients (Lieberman et al. 1996). There is also evidence that tree ferns are strongly dependent on microhabitat characteristics such as presence of light gaps, w ater availability, and slope (Gomez 1983). These dependencies may be more important than mere changes in elevation in encountering these two species of tree fern, and may explain the lower abundances of both in the 1650m 1700 m block (perhaps fewer ideal microhabitats were encountered on these transects). The co occurrence of A. polystichoides with A. erinacea throughout these transects suggests a lack of significant partitioning of space. If these species were partitioning space we would expect to find them separate from each other most of the time. The lower abundance of A. polystichoides in each elevation suggests it is outcompeted by A. erinacea for space, however, this study does not overwhelmingly support this hypothesis, nor does it rule out facilitation as a viable interaction. More studi es need to be done across these ferns' elevational ranges as well as in ideal Alsophila habitat (wet areas, light gaps) to better understand the dynamics of the relationship between A. erinacea and A. polystichoides. I determined both species of tree fern to be patchily distributed using index of dispersion , Poisson distribution, and the Chi squared tests, and that they tended to be within 11 meters of conspecifics. These results could give support to the hypothesis that microhabitat was perhaps more import ant than elevation in examining abundances. For example, light gaps or water drainages could cause A. erinacea or A. polystichoides individuals to clump together in the same habitat. It also appears that the habitat on ridges is far less favorable than hab itat on slopes (see Results). Ridges tend to have drier and warmer conditions than canyons on the Pacific side and are especially vulnerable to desiccating trade winds during the dry season (Haber 2000). We may expect this drop in tree fern abundances alon g the ridges considering they prefer wet environmen ts (Go mez 1983). It is clear that any future studies on an elevational gradient should consider these microclimate associations more closely. Acknowledgments I would like, to t hank Karen and Alan Masters for being helpful advisors and mentors. Andrew Rodstrom, Carmen Rojas, and Matt Gasner deserve much thanks for accompanying me to the field, answering loads of questions, and proofreading this
paper. Tha nks to the folks at the E staciÃ³n BiolÃ³gica de Monteverde and the Centro de EducaciÃ³ n Creativa for allowing me to use their land. Finally I would like to thank Matt Greenfield for helping me greatly with stats, Blake Audsley for proofreading, Melody Saeman and Katie Heckendorn and everyone else in the CIEE program for making all this work a lot more bearable. Literature Cited Cooper Driver, G. 1990. Defense Strategies of Bracken, Pteridium aquilinum. Annals of the Missouri Botanical Garden, 77:281 286. Go mez, L.D. 1983. Cyatheaceae and Dicksoniacea (Rabos de Mico, Tree Ferns). In: Costa Rican Natural History. D.H. Janzen, ed. The University of Chicago Press, Chicago, IL, pp 225 228. Haber, WA. 2000. Plants and Vegetation. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest. N.M. Nadkarni and N.T. Wheelwright, eds. Oxford University Press, New York, p. 44. Kappelle, M. and L.D. Gomez P. 1992 'Distribution and Diversity of Montane Pteridophytes of the ChirripÃ³ National Park, Costa Rica.' Brenesia 37:67 7 7. Ricklefs, R.E. 1990. Ecology. W.H. Freeman and Company, New York, 804 809. Rojas, A. 1999. Helechos arborescentes de Costa Rica. INBio, Santo Domingo, Costa Rica, pp. 32,38. Lee, M.A.B., P . A. Burrowes, J .E. Fauth, J.C. Koella, S.M. Peterson. 1986. The Distribution of Tree Ferns along an altitudinal gradient in Monteverde, Costa Rica.' Brenesia 25 26:45 50. Lieberman, D., M. Lieberman, R. Peralta, G.S. Hartshorn. 1996. 'Tropical Forest Structure and Composition on a Large Scale Altitudinal Gradient in Costa Rica Journal of Ecology, Vol. 84,137 152. Southwood, T.R.E 1984. Ecological Methods. New York: Chapman and Hall .
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Patrones de distribucin de los helechos Alsophila erinacea y A. polystichoides (Cyatheaceae) en los bosques nubosos de Monteverde, Costa Rica
Distribution patterns of the tree ferns Alsophila erinacea and A. polystichoides (Cyatheaceae) in cloud forests of Monteverde, Costa Rica
Digitized by MVI
The effects of elevation on the tree ferns Alsophila erinacea and A. polystichoides (Cyatheaceae)
distributions were examined between 1550 m and 1800 m in Monteverde, Costa Rica. Alsophila
erinacea and A. polystichoides prefer similar habitat types and overlap almost completely in their
elevational ranges, making them ideal species for comparison. Six transects were made in each of five-
50 m elevational blocks between 1550 m and 1800 m for a total of 30 transects. Transects along a ridge
trail were compared with those on a trail on the side of a slope and were found to contain considerably
less Alsophila tree ferns at all elevations (a total of 5 individuals on six ridge transects compared with 36
individuals on four slope transects). There was a slight decrease in these two species' abundances on an
increasing elevational gradient. Both species of fern were found to be patchy based on an index of
dispersion, Poisson comparison, and Chi-squared tests (X2 = 710 and 33063; df = 9 and 18; P = 0.001 and
0.001 for A. erinacea and A. polystichoides respectively). Finally, microclimate variations likely did
more to influence A. erinacea and A. polystichoides distributions than did elevation.
Se examinaron los efectos de la elevacin en los helechos Alsophila erinacea y A. polystichoides (Cyatheaceae) entre 1550 m y 1800 m en Monteverde, Costa Rica. Estos helechos prefieren tipos de hbitat similares y se traslapan casi completamente en sus mbitos altitudinales. Por esta razn A. erinacea y A. polystichoides son especies buenas para comparar. Se hicieron seis transectos en cinco bloques de elevacin, de 50 m cada uno, entre 1550 m y 1800 m para un total de 30 transectos. Se compararon los transectos a lo largo de un sendero en una cresta con los de un sendero empinado y se encontr mucho menos helechos Alsophila en todas las elevaciones de la cresta (cinco individuos en seis transectos de la cresta comparados con 36 individuos en cuatro transectos de la cuesta). Hubo una leve disminucin en la abundancia de estas dos especies de helecho en un gradiente de elevacin ascendente. Se encontr que las dos especies de helecho estn agrupadas con base en el ndice de dispersin, el test de comparacin Poisson, y el test Chi-cuadrado (X2 = 710 y 33063; df = 9 y 18; P = 0.001 y 0.001 para A. erinacea y A. polystichoides respectivamente). Finalmente las variaciones en el microclima influyeron ms en las distribuciones que la elevacin.
Text in English.
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Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Plantas--Efecto de la altitud en
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
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