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Tamao relacionado con la compartimentacin de nichos de las orqudeas epifitas en el dosel del bosque nuboso
Size-related niche-partitioning of epiphytic orchids in Cloud Forest canopies
Ten recently fallen trees in the Lower Montane Wet Forest (1550-1650 m) and Lower Montane Rain Forest (1650 m to 1800 m) of Monteverde, Puntarenas, Costa Rica were each divided into four zones based on tree architecture. Each zone theoretically represents differences in substrate and abiotic conditions: Zone 1 was the main trunk, Zone 2 the inner third of the canopy, Zone 3 the second third of the canopy, and Zone 4 the outer-most third of the canopy. The epiphytic orchids of each tree were surveyed to determine the role of weight in epiphytic orchid distribution among the different zones of the tree. In total, 335 individual orchids were weighed and identified to genus and then distinguished by morphospecies and analyzed to test for variation in species richness and weight
among the four zones. Total species richness was equally distributed among the four zones, however, when size specific genera were analyzed for species richness distribution alone, there was variation in species richness between zones. In addition, there was an overall variation in weight among zones. The outer-most region of the canopy,
Zone 4, had the smallest orchids (18.25 +/- 67.63 g), while the inner-most region of the canopy, Zone 2, had the largest orchids (67.76 +/- 113.11 g). In conclusion, I found that weight does play a role in the niche-partitioning of epiphytic orchids in the Cloud Forest of Monteverde.
Diez rboles recientemente cados en el bosque nuboso montano bajo (1550-1660 m) y el bosque lluvioso montano bajo (1650 a 1800 m) Monteverde, Puntarenas, Costa Rica fueron divididos en 4 zonas basadas en la arquitectura del rbol. Cada zona teorticamente representa diferencias en el sustrato y las condiciones abiticas: Zona 1 era el tronco, Zona 2 el primer tercio del dosel, Zona 3 el segundo tercio del dosel, y Zona 4 el ltimo tercio del dosel. Las orqudeas epifitas de cada rbol fueron muestreadas para determinar el papel de las epifitas y su distribucin dentro de las diferentes zonas en el rbol. En total 335 individuos se identificaron a nivel de gnero y luego distinguidas en morfoespecies y se realiz una prueba de variacin en la riqueza de especies y el peso entre las cuatro zonas. La riqueza total de especies fue distribuida equitativamente entre las cuatro zonas, sin embargo, cuando se analiz los gneros especficos para la riqueza de especies, se encontr una diferencia en la riqueza entre las zonas. Adems hubo una variacin en el peso entre las zonas. La zona ms externa del dosel, zona 4, tiene las orqudeas ms pequeas (18.25 +/- 67.63 g), mientras que la regin mas interna, zona 2 tiene las orqudeas ms grandes (67.76 +/- 113.11g). En conclusin, encontr que el peso juega un papel importante en la particin del nicho de las orqudeas epifitas en el bosque nuboso de Monteverde.
Text in English.
Plant species diversity
Monteverde Biological Station (Costa Rica)
Diversidad de especies de plantas
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Fall 2009
Ecologa Tropical Otoo 2009
t Monteverde Institute : Tropical Ecology
Size related niche partitioning of epiphytic orchids in Cloud Forest canopies Blaine Marchant Department of Biology, University of Puget Sound Abstract Ten recently fallen trees in the Lower Montane Wet Forest (1550 1650 m) and Lower Montane Rain Fore st (1650 m to 1800 m) of Monteverde, Puntarenas, Costa Rica were each divided into four zones based on tree architecture. Each zone theoretically represents differences in substrate and abiotic conditions: Zone 1 was the main trunk, Zone 2 the inner thir d of the canopy, Zone 3 the second third of the canopy, and Zone 4 the outer most third of the canopy. The epiphytic orchids of each tree were surveyed to determine the role of weight in epiphytic orchid distribution among the different zones of the tree. In total, 335 individual orchids were weighed and identified to genus and then distinguished by morphospecies and analyzed to test for variation in species richness and weight among the four zones. Total species richness was equally distributed among th e four zones, however, when size specific genera were analyzed for species richness distribution alone, there was variation in species richness between zones. In addition, there was an overall variation in weight among zones. The outer most region of the canopy, Zone 4, had the smallest orchids (18.25 +/ 67.63 g), while the inner most region of the canopy, Zone 2, had the largest orchids (67.76 +/ 113.11 g). In conclusion, I found that weight does play a role in the niche partitioning of epiphytic orch ids in the Cloud Forest of Monteverde. Resumen Diez arboles recientemente caidos en el bosque nubos montano bajo (1550 1660 m) y Bosque lluvioso montano bajo ( 1650 to 1800 m) Monteverde, Puntarenas, Costa Rica fueron divididos en 4 zonas basadas en la arquitectura del arbol. Cada zona teorticamente representa diferencias en sustrato y condiciones abioticas: Zona 1 era el tronco, Zona 2 el primer tercer del dosel, Zona 3 el segundo tercer del dosel, y Zona 4 el ultimo tercer del dosel. Las orquideas ep ificticas de cada arbol fueron muestreadas para determinar el papel de las epifitas y su distribucion dentro de diferentes zonas en el arbol. En total 335 individuos se identificaron a nivel de genero y luego distinguidas a morfoespecies y se realizo una prueba de variacion en riqueza de especies y peso entre las cuatro zonas. La riqueza total de especies fue distribuida equitativamente entre las cuatro zonas, sin embargo, cuando se analizo generos especificos para riqueza de especies, se encontro una dif erencia en la riqueza entre zonas. Ademas hubo una variacion en peso entre zonas. La zona mas externa del dosel, zona 4, tiene las orquideas mas pequenas (18.25 +/ 67.63 g), mientras que la region mas interna, zona 2 tiene las orquideas mas grandes (6 7.76 +/ 113.11 g). En conclusion, encontre que el peso juega un paple importante en la particion de nicho de las orquideas epifitas en el bosque nuboso de Monteverde. Introduction Of the many plant families that make up tropical epiphyte communities, n o other is as diverse as that of Orchidaceae. With over 22,000 recorded species ranging from Alaska to the tip of South America, orchids are most diverse in the tropics, where their diversity can exceed 16% of total plant species. In the Cloud Forest of Monteverde, Costa Rica alone there are over 500 known species (Atwood 2000), 88% of which are epiphytic (Walter 1983). Epiphytes are plants that have no contact with the ground, using other plants as substrate (Richards 1996). This may create water and nutrient limitations since epiphytes have no access
to the nutrient rich, absorbent soil (Richards 1996). In most boreal and temperate forests, epiphytes are therefore restricted to a few lichen species, bryophytes, or occasional ferns (Lyons et al. 2000) Tropical Moist/Wet/Rain Forests have many more vascular epiphytic species because they offer more above ground nutrients and more consistent and copious precipitation (Ingram 2000). Even among tropical evergreen forests, Cloud Forests represent particul arly favorable conditions for epiphytes due to adiabatic cooling and cloud formation that present a dense mist that bathes epiphytes in moisture (Nadkarni 2000). Although less so than temperate epiphytes, Cloud Forest epiphytes must tolerate a wide range of stresses from lack or overabundance of light, to the rainless periods during the dry season. These abiotic factors have led to a wide range of morphological features among epiphytes, particularly orchids, to deal with their environment. An example of this can be seen in the large portion of epiphytic orchids that have pseudobulbs to store nutrients and water when there is an abundance of these resources and are able to draw from these stores when resource inputs are low (Dressler 1993, Richards 1996). The wide range of morphologies and enormous diversity can be attributed to the specialization of their particular habitats. Within a single canopy tree, one can find varying microclimates due to large variation in sunlight, wind, and precipitation. (Johan nson 1975). For example, the edge of the canopy has much higher light, wind, and moisture levels compared to the much more protected inner regions of the canopy. In 1975, Johannson did a study of orchid diversity in canopy trees in Liberia, Africa based o n these abiotic variations in different regions of the canopy. He divided the trees into five zones: (1) from the ground to the top of the buttress, (2) from the top of the buttress, along the trunk, to the first branch, (3) the inner third of the canopy radiating from the base of the first branch, (4) the middle third of the canopy, and (5) the outer third of the canopy. Although it was not one of the original determining factors of zone, size, orientation, and roughness of the substrate also highly vari ed among zones. The largest, sturdiest branches were in the inner region of the canopy, Zone 3, and got progressively smaller to the edge of the canopy, Zone 5. Johannson found distinct variations in orchid diversity between zones, demonstrating niche pa rtitioning even within the canopy, based on abiotic and biotic variations. The substrate plays a very large role in orchid diversity because it is both the support and the main source of available nutrients (Ingram 2000) and determines the chances of a see d landing and obtaining the necessary resources for germination. Among the branches of the innermost region of the canopy, one should find the most nutrients because the branches are largest and thus have the most surface area to accumulate organic matte r. The amount of nutrients should progressively decrease as the branches get smaller, extending further from the trunk (Ingram 2000) therefore varying among zones. This same gradient can be seen in wind, light, and precipitation levels as well. As one ge ts closer to the edge of the canopy, one will a higher input of light, wind, and precipitation (Ingram and Nadkarni 1993). The trunk of the tree may experience less variation in wind, light, and precipitation input compared to the canopy but because its s urface is completely vertical it is highly difficult most orchids to germinate. Orchid morphology should reflect the abiotic and biotic conditions of their habitat and therefore their zone. The purpose of this study was to survey the epiphytic orchid weig ht and determine whether or not weight plays a large role in zone determination, or niche partitioning. I expected that orchid weight would vary significantly among zon es, with the heaviest orchids in organic matter and largest branches. The outer third of the canopy will contain only the lightest
individuals, which are able to tolerate the high levels of wind stress with their small forms in return for the highest light and moisture levels. Methods Study Site This survey was conducted in the Lower Montane Wet Forest and Lower Montane Rain Forest (Holdridge 1966) behind the Es tacin Biolgica de Monteverde, Puntarenas, Costa Rica on the Pacific slope of the Tilarn Mountain Range between 1500 and 1750 meters from October 31 to November 18, 2009. The study site receives an average of 2000 2500 mm of rain per year and has an ave rage temperature of 18.8 C. Although there is a distinct dry and wet season in Monteverde, the study site is kept moist throughout the year due to continual cloud and mist cover providing an additional 500 2000 mm of precipitation per year (Ingram and N adkarni 1993). The consistent moisture and minimal temperature range allow an unrivaled diversity of epiphytes among the forest canopy (Richards 1996). Ten recently fallen canopy trees were used as sample sites, as surrogates of living, upright canopy tre es, to increase sample size and ease of data collection. The use of fallen trees as viable representations of living trees is a reasonable alternative because the majority of epiphytic orchids should remain attached to the tree or branches and living for a minimum of a year, at which point undergrowth typically shades out the original flora of the tree (Nadkarni et al. 2000). Fallen trees were all large canopy trees, chosen based on a minimum diameter at breast height of 70 cm, lack of decomposition, and lack of overgrowth from understory plants. Diameter at breast height (cm), elevation, and location were noted for each sample tree. Zonation Due to varying height and architecture, the sample trees were divided into four zones similar but different to those described by Johannson (1975) (Figure 1): (1) ranged from the base of the trunk to the division of the first branch, (2) was the inner third of the canopy (>13 cm branch diam.) radiating from the base of the first limb, (3) was the middle third o f the canopy (5.1 13 cm branch diam.), (4) was the outer third of the canopy (<5 cm branch diam.). Sampling The epiphytic orchid individuals of each sample tree were divided by zone, weighed, photographed, and their zone recorded. To prevent pseudore plication of clumped individuals of the same species, solely the largest individual was sampled if there were multiple individuals of the same species in a zone. Nearly all of the sampled individuals were identified to genus and categorized to morphospecie s (i.e. Pleurothallis 3 ), due to the enormous diversity of orchid Morph based on vegetative traits. Analysis Friedman tests were performed to detect significant differen ces in species richness among the four zones in the sample trees. In addition, One Way Analysis of Variation were used to test for Kramer tests were used to compare the weights between the zones.
Results Of the ten trees surveyed, I collected data on 335 epiphytic orchid individuals. Species richness ranged from 10 31 species on a tree. Ten genera were identified, in addition to 16 unidentified genera. Because the vast majority of individuals were not in flower, they were categorized based on vegetative traits. Therefore, the total number of distinct genera is likely a conservative figure. The weights of individual orchids ranged enormously from 0.3 grams, Morph 7 to 940 grams, Camaridium 1 Even within genera there was a wide variation in weights. For example, Pleurothallis spp ranged from 0.4 grams to 195 grams, despite their common perception as a smaller sized genus. On average, Pleur othallis was one of the lighter genera (17.14 +/ 30.35 g, n = 68), along with Epidendrum (9.92 +/ 10.93 g, n = 37). The Oncidium individuals were typically of an intermediate weight (43.41 +/ 53.17 g, n = 30), while Camaridum was the heaviest of the ge nera collected (187.88 +/ 199.96 g, n = 29). Tree zone was found to have a significant effect on the weight of orchid species found there (One way ANOVA, F 3, 331 = 5.360, p = 0.001). Of the four zones, Zone 2 was significantly heavier than Zone 4 (67.76 +/ 113.17, 18.25 +/ 67.63, Tukey Kramer test, p < 0.05), although there was a general increase in average orchid weight from Zone 1 to Zone 2, then a clear decrease to Zone 4 (Figure 2). There was no significant trend in total species richness among the four zones (Figure 3, Friedman test, x 2 = 6.03, df = 3, p value = 0.110), however certain genera had a higher richness in some zones than others. For example, Epidendrum richness differed among the four zones (Figure 4, Friedman test, x 2 = 24.36, df = 3, p value = 2.101e 05), with a considerably higher richness in Zone 4 than the remaining three zones. This trend toward higher richness in the outermost part of the canopy was also seen in Oncidium (Figure 5), although the trend is less pronounced (Frie dman test, x 2 = 8.6, df = 3, p value = 0.04). In comparison, Camaridium was most species rich in Zone 2 and 3 (Figure 6, Friedman test, x 2 =16.25, df = 3, p value = 0.001). de Stelis, Pleurothallis, Masdevallia, and Lepenthes (Dressler 1993) When combined, the species richness of all of these genera followed the same trend as solely Pleurothallis (Figure 7, Friedman test, x 2 = 1.91, df = 3, p value = 0.59), with species ric hness evenly distributed among the four zones (Figure 8, Friedman test, x 2 = 2.18, df = 3, p value = 0.54). When the weights of Pleurothallis individuals were tested for variance among zones, there was a general trend similar to that of the overall averag e weight per zone, with an increase from Zone 1 to Zone 2, then a steep decrease to Zone 4 (Figure 9), but with no statistically significant differences between zones (One way ANOVA, F 3, 64 = 1.76, p = 0.16). Discussion In addition to abiotic factors, su bstrate plays a large factor in orchid species occupancy among the various zones in the canopies of Lower Montane Wet and Rain Forests of Monteverde. This is best demonstrated by the average weight variation among the four zones (Figure 2). Extending fro m Zone 2 at the center of the canopy where the branches are largest, the average orchid weight greatly decreased to Zone 4, indicating that lighter orchids were in the outermost region of the canopy, while heavier, larger orchids were in the innermost regi on of the canopy.
This could be in part due to the fact that larger orchids naturally need more nutrients and thus a higher accumulation of organic matter to survive, and because larger branches accumulate more organic matter than smaller branches (Ingram 2000), larger orchids would need larger branches to grow. Another reason behind this size gradient could be explained by the overall weight tolerance or lack of surface area of the substrate. In other words, small branches cannot support large orchids w ithout breaking and falling to the forest floor, where they will likely die within the year (Matelson et al. 2000). By comparing the species richness of different genera in the samples and comparing their average weights, we can see that, in general, lar ger genera were most rich in Zone 2. Camaridium is a generally large sized genus that was recently divided from Maxillaria (Blanco et al. 2007, 2008 in Morales 2009). It was the heaviest (187.88 g) genus on average of the ten genera sampled. Camaridium was most species rich in Zone 2 and Zone 3 (Figure 6), the inner two thirds of the canopy, where the branches were largest and could most readily support their weight and nutrients requirement. Zone 1, the trunk, had a significantly lower species richness probably due to a lack of organic matter accumulation, while Zone 4 did not have a single Camaridium individual in any of the ten trees sampled. The lightest (9.92 g) of the genera examined, Epidendrum was most species rich in Zone 4 (Figure 4). These i ndividuals were typically much larger than most of the other small genera, such as Pleurothallis, but due to their slender form, small leaves, and lack of pseudobulbs these scraggly individuals were a good deal lighter than their smaller cousins. Their sm all leaves, thin architecture, and light weight appears to make them extremely tolerant to light and wind stress, allowing them to thrive in the upper most canopies, despite their larger size. Their low richness in the inner zones of the canopy could be d ue to low shade tolerance and the need for high levels of precipitation due to their lack of a pseudobulb for nutrient and water storage, but this hypothesis needs to be more thoroughly tested. Similar to Epidendrum Oncidium was most species rich in the o uter most zone of the canopy (Figure 5). These moderately sized (43.41 g) individuals indicate that the outer branches of the canopy are able to support relatively heavy individuals. The species of this genus typically require more sunlight than most orc hids (Dressler 1993), therefore, likely cannot tolerate the shade of the inner canopy and are most rich in the sunny outer canopy. Pleurothallidinae, were found to be equally species rich througho ut the four zones (Figure 6). These genera consist of over 4000 typically small sized, species that all lack pseudobulbs (Dressler 1993). Due to their small size they are able to inhabit even the thinnest of branches or even the trunk of the trees with e ase because of their minimal need for organic matter accumulation. The low competition among epiphytes (Dressler 1993) allows these smaller genera to not be outcompeted in the middle zones of the canopy, where the larger genera are most often found, allow ing equal distribution throughout the canopy. Their high speciation also allows for a wide variation in size and habitat tolerances, further enabling their widespread colonization of the canopy, as demonstrated by Pleurothallis The range of individual w eights and the even distribution of species could indicate that Pleurothallis had diversified to match the abiotic and biotic factors of each zone. Therefore, larger species were in the middle, where the most nutrients were, while the smaller species were in the outer regions of the canopy where they could take full advantage of their small size and low nutrient intake. However, due to the mild and consistent climate of the sampled forest, no
zone was deemed more or less preferable and therefore diversifi cation was equally distributed among the zones. The Lower Montane Wet and Rain Forests of Monteverde are relatively aseasonal, with small fluctuations in precipitation and temperature, but nearly constant moisture from mist and cloud cover. The minimal a mounts of abiotic stress from desiccation and temperature change can possibly explain the low variability of species richness throughout the canopy (Figure 3). While each zone of the canopy has its own abiotic and biotic variation (Johannson 1975) forming an assortment of niches for epiphytic orchids, I believe that none of these zones are overly stressful, enabling equal diversification between them. It is apparent from this survey that although orchid species richness is even throughout Cloud Forest tree s, there is obvious niche partitioning in different parts of the tree based on varying abiotic and biotic factors. By looking at the species richness of distinct genera among the different zones of the canopy, I was able to see that there were differences in composition despite an overall even distribution of species richness. After comparing the distribution of these genera with their average size, I found that weight played a role in niche partitioning. Specifically, I found that there was a general de crease in orchid weight as one extended from the trunk to the edge of the canopy. Whether this is due to direct size limitation, such as individuals breaking and falling from the tree because they are too heavy, or species specific stress tolerances, such as certain species not being able to germinate in the edge of the canopy due to too much light input, is difficult to determine. Simply because I did not encounter any juvenile or smaller individuals of larger genera, such as Camaridium I predict that t he size variation is due to species specific germination requirements, although this is simply a hypothesis. Further studies should look into this issue and be sure to include species abundance, as well as richness, to calculate species diversity. In addi tion, they should measure wind, light, and moisture levels in each of these distinct zones, to more fully evaluate the factors that shape the epiphytic orchid communities. Acknowledgments I would like to thank Alan Masters, Anjali Kumar, Karen Masters, a nd Yimen Araya for helping me develop and lost. In addition, I would like to thank the Estacion Biologica Monteverde and its entire staff for t he use of their beautiful forest and facilities. Literature Cited Atwood, J.T. 2000. Orchids. Monteverde Ecology and Conservation of a Tropical Cloud Forest. Ed. Nadkarni, N.M., N.T. Wheelright. Oxford University Press. New York. 74 75. Arditti, J., A .K.A. Ghani. 2000. Numerical and physical properties of orchid seeds and their biological implications. New Phytol. 145, 367 421. Blanco et al. 2007 in J.F. Morales. 2009. Orchids of Costa Rica. INBio. Dressler, R.L. 1993. Field Guide to the Orchids of C osta Rica and Panama. Comstock Publishing Associates: Ithaca.
Haber, W.A. 2000. Plants and Vegetation. Monteverde Ecology and Conservation of a Tropical Cloud Forest. Ed. Nadkarni, N.M., N.T. Wheelright. Oxford University Press. New York. 42 43. Holdridg e, L.R. 1966. The life zone system. Adansonia. 6: 199 203. Ingram, S.W. 2000. Epiphytes. Monteverde Ecology and Conservation of a Tropical Cloud Forest. Ed. Nadkarni, N.M., N.T. Wheelright. Oxford University Press. New York. 72 73. Ingram, S.W., N.M. Nad karni. 1993. Composition and distribution of epiphytic organic matter in a Neotropical cloud forest, Costa Rica. Biotropica 25: 370 383. Johannson, D.R. 1975. Ecology of epiphytic orchids in West African rain forests. American Orchid Society Bulletin. 44: 125 136. Lyons, B., N.M. Nadkarni, M.P. North. 2000. Spatial distribution and succession of epiphytes on Tsuga heterophylla (western hemlock) in an old growth Douglas fir forest. Can. J. Bot. 78: 957 968. Matelson, T.J., N.M. Nadkarni, J.T. Longino. 20 00. Longevity of Fallen Epiphytes. Monteverde Ecology and Conservation of a Tropical Cloud Forest. Ed. Nadkarni, N.M., N.T. Wheelright. Oxford University Press. New York. 344 345. Nadkarni, N.M., R.O. Lawton, K.L. Clark, T.J. Matelson, D. Schaefer. 2000. Ecosystem Ecology and Forest Dynamics. Monteverde Ecology and Conservation of a Tropical Cloud Forest. Ed. Nadkarni, N.M., N.T. Wheelright. Oxford University Press. New York. 312 313. Richards, P.W. Epiphytes. The tropical rain forest an ecological study. Cambridge University Press. New York. 135 149. Walter, K. S. 1983. Orchids. In: Costa Rican Natural History. D. H. Janzen (ed.) The University of Chicago Press, Chicago. pp. 282 292. Wieder, W. 2000. Patterns of orchid diversity in elfin forest. Tropica l Ecology and Conservation, CIEE Spring: 1 18. Zotz G, B Vollrath 2003. The epiphyte vegetation of the palm Socratea exorrhiza correlations with tree size, tree age and bryophyte cove. Journal of Tropical Ecology. 19: 81 90.
Figure 1. Zonation of th Figure 2. Average weight (g) of orchid individuals based on zone. Zone 2 was significantly heavier than Zone 4.
Figure 3. Average species richness per tree zone of epiphytic orchids. There was no significant variation among the zones. Figure 4. Average species richness of Epidendrum per tree zone. Zone 4 had significantly more species than the remaining three zones.
Figure 5. Average species richness of Oncidium per tree zone. Zone 4 ha d significantly more species than the remaining three zones. Figure 6. Average species richness of Camaridium per tree zone. Zone 2 and 3 had significantly more species than Zone 1 or Zone 4.
Pleurothallis, Stelis, Lepenthes, Masdevallia ) per tree zone. There was no significant variation in species richness among zones. Figure 8. Average species richness of Pleurothallis per tree zone. There was no significant variation in species ric hness among zones.
Figure 9. Average weight of Pleurothallis individuals per tree zone. There was no significant variation in weight among zones.