The relationship between mist exposure and reproductive success in Scaphyglottis acostaei Orchidaceae Rebecca Lara Department of Environmental Science, Northern Arizona University _________________________________________________________________________________ ABSTRACT I examined how the reproduction and fitness of the orchid, Scaphyglottis acostaei depends on the water resources derived from cloud mist in Monteverde, Costa Rica. Thirty six individuals of S. acostaei were selected to undergo treatments of mist exclusion, mist enhancement and light modification for 15 days. The pseudobulb width change d significantly over the 15 days for the four treatments, suggesting that mist precipitation is a limiting resource for S. acostaei Additionally, mist was collected on the northeast and southwest sides of four pasture trees, and a total of 139 individual orchids were sampled for number of fruits, racemes and leaves, and leaf length. I found a higher reproductive success of S. acostaei on the northeast sides of the sample trees, which is partially explained by the trend that those individuals were also expo sed to a higher more frequent amount of mist than individuals on the southwest sides. My findings suggest that even orchids with specialized mechanisms for coping with water limitation are sensitive to changes in mist frequency. RESUMEN Yo examine como la reproduccin y sobrevivencia de la orqudea, Scaphyglottis acostaei depende de los recursos de agua que viene en la neblina en Monteverde, Costa Rica. Treinta y dos muestras de S. acostaei fueron elegidos para tratarlos con de Excluidor es de Neblina, Aumentador e s de Neblina, y Modificacin de Luz por 15 das para los cuatro tratamientos, sugiriendo que la neblina es un recurso limitante en S. acostaei La neblina se recogi en el lado noreste y sudeste de cuatro rboles en el pasto, y el nmero de frutos, racimos y hojas de 139 orqudeas fueron contados. Yo encontr un xito reproductivo ms alto en el lado noreste de los rboles, que fue explicado por la tendencia que estas orqudeas eran expuest a s una cantidad de neblina ms alto y frecuente que las orqudeas en el lado sudeste. Mis resultados sugieren que las orqudeas con adaptaciones para compensar la limitacin de agua fueron susceptibles a cambios en la frecuencia de neblina. INTRODUCTION Tropical montane habitats enveloped in mist and cloud water support among the worldÂ€s richest orchid communities. Mist interception has been recognized as an important ecological factor for the fitness of epiphytic orchids, because they have no contact wit h the ground and must rely on water inputs captured from the atmosphere Nadkarni 1986. Orchids minimize the impact of microclimatic variability and environmental stress with adaptations for quickly absorbing transient water and nutrients in mist Dressle r 1990. However, orchid response to microclimatic heterogeneity in precipitation and temperature remains largely unknown with few studies focused on quantifying a speciesÂ€ optimal range
of tolerances. Limiting factors to survival and reproduction of orchi ds may differ among species and habitats due to variation in adaptive strategies and their interactions with the abiotic environment, host tree, and neighbor epiphytes. Since the mid 1970Â€s climate trends in Monteverde, Costa Rica have lead researchers to propose the lifting cloud base hypothesis, which states that higher surface water temperatures in the equatorial Pacific Ocean have caused an altitudinal shift of the montane cloudbank Pounds et al. 1999. A lifting cloud base may result in longer periods without mist and non precipitating water droplets during the dry season, having serious implications for orchids below the cloud line that derive water and nutrients from the mist Pounds et al. 1999. Variation in rainfall over the course of the year has a greater impact on epiphyte success than total annual rainfall Benzing 1995. Thus, the variability of mist frequency during the dry season may be closely linked to the survival and reproduction of certain orchid species. An increase in the number of dr y days during the dry season due to global warming may limit critical mist and nutrient inputs for orchids submitting them to a greater degree of environmental stress than for which they are adapted. Plants that must survive outside their optimum range of abiotic conditions are limited by resources and are less likely to reproduce. All orchids undergo a certain degree of stress in microhabitats where water is limiting. How an orchid copes with water stress is a function of morphological and physical adapta tions against the backdrop of phylogenetic constraints. For instance, some genera have pseudobulbs, which may help buffer water limitations. However, a long term reduction in the abundance of an already limiting resource such as water can ultimately result in reduced fitness and survival of the plant. In a study of Brassavola nodosa Orchidaceae, Murren and Ellison 1996 found that larger plants with unlimited resources are more likely to allocate their energy to reproductive structures. Orchids receiving less than the optimal amount of a limiting resource, allocated their total energy to survival and maintenance of vegetative structures, leaving no or little extra energy for investment in reproductive structures. My study examined how the reproduction an d fitness of the orchid, Scaphyglottis acostaei Orchidaceae, Figure 1 depends on the water resources derived from mist precipitation in Monteverde, Costa Rica. If mist is a limiting resource for S. acostaei then individuals in drier microhabitats should draw water from stores in the pseudobulbs to varying levels of water stress. Thus, I predict a reduction in the frequency and amount of contact with mist should result in decreased average pseudobulb size. Smaller water reserves should translate to a redu ced capacity of the orchid to cope with long term water stress. If so, then mist is a limiting resource for S. acostaei Because pseudobulb storage cannot indefinitely compensate for water stress, individuals with less exposure to mist over long term perio ds should exhibit lower fitness, investing their total energy in survival and the maintenance of vegetative structures. Individuals with a greater exposure to mist should have extra energy to invest in reproduction. In Monteverde, the majority of mist orig inates from the northeast trade winds. I proposed that orchids on the northeast facing direction of trees would intercept more mist at a higher frequency and would therefore exhibit higher
fitness than water stressed orchids on the southwest facing sides o f the same trees. I expected to find a direct relationship between mist frequency and fitness of S. acostaei METHODS Study Site and Species My study site was located north of La Estacin Biolgica de Monteverde at Monteverde, Costa Rica, in a 2.5 ha pasture surrounded by Lower Montane Wet Forest at elevation 1530 m. I conducted my study in April and May 2001 at end of the dry season, which is often characterized by nighttime and early morning misting events, and multiple days of no recordable mist. S caphyglottis acostaei is a cespitose epiphytic orchid of the subtribe Laeliinae found at 1500 2600 m throughout Costa Rica and the adjacent mountains of Panama. It grows in dense mats with many shoots of thin secondary stems forming narrow, elliptical pseu dobulbs 1 5 cm long Atwood 1989. The inflorescence is a terminal raceme, bearing one or more white to purple flowers with 3 4 mm sepals Dressler 1990. Each pseudobulb can bear up to four racemes. The ovary elongates after pollination, exceeding the sub tending bracts and forming ellipsoid capsule fruits 5 6 mm long Atwood 1989. Mist and Light Treatments I selected individuals to undergo mist exclusion, mist enhancement and light modification treatments. The length and width of ten pseudobulbs /orchid were measured at the beginning of the experiment and at the end of the 15 days in all 36 orchids. Over the course of the 15 days, a natural six day dry spell occurred with no measurable precipitation, which has been characteristic of the end of the dry sea son during the last decade in Monteverde. In order to focus solely on the relationship between mist frequency and the response of S. acostaei to environmental stress, a control group of nine individuals was selected. The control group was exposed to natur al conditions in the same microhabitats as the other treatment groups. For mist enhancement, nine individuals were misted with an aspirator between four and five oÂ€clock in the afternoon everyday over a period of 15 days to achieve zero dry days for the s tudy period. The mist did not contain added nutrients. Plastic umbrellas were placed over nine individuals in the mist exclusion treatment for duration of 15 days, in order to simulate predicted patterns of climate change with long dry periods. The mist b lockers were positioned to allow for study of the orchid under natural conditions of temperature, wind, and competition; however the plastic covering changed the natural ultraviolet light reaching the orchids. In order to calibrate for effects of changed ultraviolet light from the plastic, nine individuals were selected to undergo light modification treatments near the mist exclusion treatments. The light modification group was exposed to the natural amount and frequency of mist; however umbrellas were pla ced overhead to change the light reaching the orchids.
Mist Collection and Fitness Two mist collectors were placed in two microhabitats at each of four host trees with one in the northeast facing direction and one in the southwest. Only pasture trees that had over ten individuals of S. acostaei on each the northeast and southwest side were chosen for sampling. The host trees were located in a partially wooded patch along the western edge of the pasture and included a wide range of mist exposure. Tree T wo T2 was located on the northeastern edge of the patch with more than 200 m of open space on its northeast side. Tree Four T4 was located deep within the patch and had northeast sides that were relatively exposed. However, surrounding trees did not sh elter the southwest side of T3 as in T1. The mist collectors were constructed with a funnel and collecting bottle attached to forty strands of monofilament run vertically between two silicon surfaces as described by Calla 1999. The amount of ppt collecte d ml was recorded for all mist collectors northeast and southwest every day for 11 13 days. I sampled a total of 139 individuals of S. acostaei on the northeast and southwest sides of four pasture trees. The length of the longest leaf, and the number of fruits, old racemes, and leaves were recorded for each individual as an indicator of plant size and reproductive success. Two ratios were calculated by dividing the number of fruits by the number of racemes and the number of leaves to determine an orchidÂ€ s investment in reproductive structures relative to vegetative structures. Data Analysis Differences in average collected mist and the total number of dry days between the northeast and southwest sides of the host trees were analyzed with Mann Whitney U tests. A two way analysis of variance was used to determine the effect of direction NE and SW and tree T1 Â T4 on plant size and fitness. The relationship between plant size and overall fitness was established with a simple regression. I analyzed the c hange and proportional change in pseudobulb size before and after the treatments with a Kruskal Wallis nonparametric analysis of variance. Change in pseudobulb width W was calculated as: original W final W. Proportional change in pseudobulb width was c alculated as: original W final W / final W. RESULTS Mist and Light Treatments Figure 2. The pseudobulb width decreased for all treatments except the mist enhancement treatment, which showed a mean increase of 0.022 mm. The pseudobulb length was not s ignificantly affected by the treatments with a p value > 0.9 H = .181, df = 3. Mist Collection and Fitness The average mist collected was significantly higher on the northeast side than on the southwest side of all four trees, whereas the number of dry days was not Figure 3. In general, the number of dry days was greater on the southwest side of the tree than on the
northeast. The highest average mist was collected at the northeast side of T2 with 11.5 ml/day n = 11. The southwest side of T4 received no mist over a continuous period of 11 days Figure 3. The amount of mist collected at the northeast side of T2 did not differ significantly from the amount collected at the northeast side of T4, which was highly sheltered by other pasture trees p value = 0.07, n = 22, Figure 3. The direction NE vs. SW and tree T1 Â T4 had a significant effect on the numb er of fruits, racemes, leaves, the fruits to racemes ratio and the fruits to leaves ratio Figures 4 and 5. In every case except for the mean number of leaves and racemes for tree three, the measured parameters were greater on the northeast side of the tr ee than on the southwest. Tree Four had the greatest mean number of fruits, racemes and leaves, but the lowest fruits to racemes and fruits to leaves ratio Figures 4 and 5. Leaf length did not differ significantly between northeast and southwest sides of the tree F value = 2.07, p value = 01.107 DF = 3. A direct relationship existed between number of fruits and number of leaves Figure 6. DISCUSSION The above results support my hypothesis. The difference in the mean fitness of individuals between the northeast and southwest sides of the sample trees suggest that exposure to mist during the dry season affects the fitness of S. acostaei and its investment in reproductive and vegetative structures and the ratio of its investment of reproductive to vegeta tive structures. The Role of Mist as a Limiting Resource for S. acostaei The reduction in pseudobulb width for individuals in all groups except for the mist enhancement group was most likely a result of the natural six day dry spell that occurred during t he experiment. Precipitation was lacking due to natural weather patterns, affecting the orchids in the control and light modification treatments as well as the mist exclusion treatment. Both light and mist affected the size of the pseudobulbs, suggesting t hat the sampled orchids drew water from their pseudobulbs. The only group that did not resort to water reserves for survival and maintenance were those orchids receiving regular daily inputs of mist during this dry period. The relationship between mean siz e of pseudobulbs and the presence of moss in a previous study of S. acostaei indicated that reduced water stress decreases the dependence on long term water storage Fisher 2000. My observations substantiate that water limitation forced individuals to use water reserves for survival and maintenance. These results suggest that mist precipitation is an overall limiting resource for S. acostaei Scaphyglottis acostaei cannot cope indefinitely with a shortage of water. In general, plants under stress will allocate resources to survival at the expense of f itness. Thus, it is expected that at some point S. acostaei will compensate for the cost of limited mist interception with a decrease in reproduction.
The Long Term Effects of Water Stress in S. acostaei The higher reproductive success of S. acostaei on the northeast sides of the sample trees is partially explained by the trend that those individuals were also exposed to a higher and more frequent amount of mist than individuals on the southwest sides Figures 2, 3 and 4. All four trees had orchids that produced more fruits, racemes, and leaves on the northeast facing side than on the southwest side, and all four trees collected more mist on the northeast. The differences in mist capture and frequency between the northeast and southwest sides of the sampl e trees caused the microclimates to change between the two sides. At some level, the magnitude of this change was strong enough to cause water stress, and reduce fitness in S. acostaei on the southwest sides of the sample trees. Although T4 received the l owest measurable amount of mist at less frequent intervals, it had the greatest number of fruits, racemes and leaves per orchid. In the orchid, B. nodosa the number of leaves per individual is directly related to flowering success, and individuals with la rge flowers on multi flowered racemes tend to have higher fitness Murren and Ellison 1996. However, large biomass, indicated by the number of leaves, does not necessarily confer higher fitness for S. acostaei as demonstrated by the variation between poin ts in Figure 6. The mean ratio of flowers assuming that each race me produced at least one flower that actually set fruit and the mean number of fruits relative to the mean number of leaves was significantly lower for T4 than any of the other trees Figur e 5, suggesting that individuals found on both the northeast and southwest sides of T4 actually had an overall lower fitness. Individuals on T4, which is surrounded by larger trees that block mist contact, may be water stressed explaining the reduction o f investment in reproductive structures relative to vegetative structures. I encountered many orchids on the southwest sides of the sample trees that did not produce any racemes or fruits, suggesting that they had fallen outside of their optimal range of conditions for reproduction. The above results show that the set of conditions affecting the reproduction of S. acostaei can change over a small geographic area and that within this set of conditions the minimum level of mist capture and frequency for reproduction is relatively high. Future Research and Management Implications The trend in varying fitness between individua ls of S. acostaei is undoubtedly affected by other abiotic factors, such as light and wind exposure, temperature and inter and intra specific competition that were not accounted for in this study. The finding that individuals on T3 had a greater number o f leaves and racemes on the southwest side may be related to such factors. Future studies should focus on the effect of mist amount and frequency on orchids within the same microhabitat. Additionally, treatments should be performed over the entire dry seas on prior to and during flowering to determine the effect of mist exclusion on the reproductive cycle of S. acostaei. The above trends lead to the question: How will a rising cloudbank affect orchid fitness and ultimately the composition and function of or chid communities in Monteverde? The models of future climate change predict longer periods without mist and non
precipitating water droplets during the dry season. If the maximum number of days that S. acostaei can rely on its pseudobulbs during dry spells is exceeded, before water can be absorbed after rains, then the fitness of individuals in particularly unfavorable microhabitats will decline depending on whether or not S. acostaei has additional adaptations to compensate for water loss. Scaphyglottis ac ostaei is a common species that possesses many adaptations for water storage, including pseudobulbs and a tendency to grow in mats near moss and will most likely be more resistant to climate change than smaller orchid species lacking pseudobulbs. However, my findings confer that even orchids with specialized mechanisms for coping with stress are sensitive to changes in mist frequency. Thus future models of orchid communities in Lower Montane Wet Forest must be developed under scenarios of decreased mist fr equency. ACKNOWLEDGEMENTS I am extremely grateful to Dr. Karen Masters, who helped develop many of the ideas presented in this paper and whose extensive knowledge on orchids saved me precious time and energy. This study would not have been possible witho ut the use of Arnoldo BecheÂ€s beautiful pasture. Finally I want to thank my dad, Martin Lara, whose enduring support has given me the courage to pursue my dreams. LITERATURE CITED Atwood, J.T. 1989. Icons Plantarum Tropicarum Fascicle 14: Orchids of Costa Rica Part I. The Marine Selby Botanical Gardens, Sarasota, FL. Benzing, D.H. 1995. Vascular epiphytes. In: Forest Canopies M.D. Lowman and N.M. Nadkarni, ed. Academic Press, New York, NY, pp.225 254. Calla, E. 1999. Acid dep osition in rain and mist and its effects on Bromeliad community richness. Tropical Ecology and Conservation, Council on International Educational Exchange, Fall 1999. Dressler, R.L. 1990. The orchids: Natural history and classification. Harvard University Press, Massachusetts. Fisher, A. 2000. Adaptive response of Scaphyglottis acostaei to microhabitat variability. Tropical Ecology and Conservation, Council on International Educational Exchange, Fall 2000. Harper, J.L. 1977. Population biology of plants. AP Academic Press, New York. Murren, C.J. and Ellison, A.M. 1996. Effects of habitat, plant size, and floral display on male and female reproductive success of the Neotropical orchid Brassavola nodosa Biotropica 28: 30 41. Nadkarni, N.M. 1986. The nutrit ional effects of epiphytes on host trees with special reference to alteration of precipitation chemistry. Selbyana 9: 44 51. Pounds, J.A., M.P.L. Fogden, and J.H. Campbell. 1999. Biological response to climate change on a tropical mountain. Nature 398: 611 615