Distribution and Germination Patterns of an Epiphytic Orchid

Orchids are known to have small, scattered populations, although the reasons for such distribution are not known. Orchids produce up to millions of seeds (depending on the species) from one single successful pollination event. Still, many orchids are rare. What stops orchids from covering the surface of the Earth?

Orchids have very particular symbiotic relationships. For one, they have a plethora of pollination mechanisms that have resulted in many orchids having a few or even a single pollinator. And secondly, producing so many seeds does not come without a price. To control the production expenses, the seeds produced by the orchids are minute, and lack endosperm (food source of the seed). In order to germinate in the wild, orchid seeds need to be infected by an Orchid Mycorrhizal Fungi (OMF). This mycorrhizal association is unique in the sense that the orchid will consume the OMF to obtain carbohydrates and nutrients to germinate and develop. But like with their pollinators, orchids are not always promiscuous with their OMF. It has been found that not all species of fungi can form a mycorrhizal relationship with orchids, and not all orchids will form the relationship with the same OMF. Hence, the distribution of orchids could be limited by the distribution and availability of their OMF.

This applies to all orchids, but another factor must be taken into consideration when studying epiphytic orchids. Epiphytic orchids are orchids that live on top of a tree for their entire life cycle. Although the orchid is only using the tree as a substrate, and it is not further connected to the tree, characteristics of the host tree determine the microclimatic conditions for both the orchid and the available OMF. Although host-tree specificity is rare, it has been found that preference for a subset of available host-tree species is common among epiphytes. Nonetheless, these preferences seem to be context dependent, suggesting that the preference for a host-tree species is not attached to the taxonomy of the tree, but rather a result of the climatic conditions of the ecosystem. That is to say, that depending on the environmental conditions of the geographic area in which the orchid species is, it could show preference for different trees with physical and chemical characteristics that are favorable for the orchid in that specific context.

My Master's thesis centered on studying the symbiotic relationships of the endemic Psychilis kraenzlinii an how these could be influencing its distribution and germination patterns in the Susúa State Forest in Puerto Rico. I concentrated on host-tree specificity and OMF.

To study the phorophyte preference I conducted a survey in the forest to take note of the available host-tree species and on which trees the orchids were actually growing on. I used statistics to determine whether the distribution of the orchid was independent of the host-tree species or not. To further analyze the relationship of the orchid and the host-tree species, I conducted in situ seed germinations to evaluate if the germination patterns mirrored the distribution of the established orchids in the forest. I found that P. kraenzlinii showed a preferene for the endemic and rare shrub Machaonia portoricensis in the study area, and that this same species had a high number of seed germinations. Nonetheless, seed germination was not dependent on the species of the host tree. Still, some host-trees had a higher number of seeds in later development stages. The fact that we found that established orchids were more common on M. portoricensis and that a high number of germinations were found on this tree species suggests that M. portoricensis should be considered when developing conservation and management strategies for P. kraenzlinii.

Which physical characteristics of the tree could be influencing the preference by P. kraenzlinii? There are a number of characteristics of the trees mentioned in the scientific literature tha could influence host-preference by epiphytes. Here I concentrated on Water Storage Capacity (WSC) and roughness of the bark of the trees. For the WSC I collected bark samples and weighed them when dry and when wet to calculate how much water they could storage. I measured water holding capacity (WHC), which is the water storage capacity at saturation; and water retention capacity (WRC), which is the capacity to retain water after 24 hours. To measure roughness, I used image analysis to calculate a ratio of smooth:rough bark. I found that orchids were more commonly found on trees with high WHC and low WRC, and that these characteristics were more commonly found on trees with smoother barks. This could be due to the climatic conditions at the Susúa State Forest, a moist tropical forest on the southwest of Puerto Rico. A high WHC could result in enough water to jumpstart germination, while a low WRC prevents the seed from drowning.

To understand how the distribution of the OMF could be influencing the distribution of the orchid, it is necessary to identify the OMF of P. kraenzlinii. I used two methods to try to isolate the OMF, isolation from adult roots and seed baiting. The first method consists of collecting roots of the orchid, confirming the presence of the fungi under the microscope and placing small thin slices of root on fungal isolation media (FIM). The second method uses the seeds as bate to trap the fungi in situ. Placing seeds in seed packets in the forest and leaving them for a few months to let fungal colonization occur. When seeds have started developing, this suggests the presence of the OMF, one takes the developing seeds to the lab and places them in FIM. I spent three years repeating these methodologies but could never isolate an OMF. I am not the first person to try to isolate the OMF of a Psychilis in Puerto Rico without success. This suggests that the OMF might not be cultivable in the lab, or the growth media did not have the nutrients needed by the fungi. Since we could not isolate the fungi at the lab, we did an ITS amplicon analysis of the root of 10 individuals to describe the fungal community and identify possible OMF. These results are still being analyzed. Meanwhile, to hypothesize about where the OMF could be found in the forest, we used the development stages of the in situ germinations as a proxy for the presence of the OMF and used statistics to determine if the OMF had a higher probability to be found in any of the studied host-tree species. We found that there were higher probabilities to find the OMF on Swietenia mahagoni and Randia aculeata, using development stages of the seed as a proxy for OMF presence. Further research is needed on the OMF of this and many other orchids.

We found that the distribution of established P. kraenzlinii in the Susúa State Forest is not independent of the host-tree species. The orchid showed preference for the endemic shrub, M.portoricensis. Germination varied among host trees, but later stages of germination, and therefore higher probability to find the OMF, where found on Swietenia mahagoni and Randia aculeata, suggesting that germination sites are not necessarily the best sites for development. The results from this research lay the foundation for proper conservation and management of P. kraenzlinii. The orchid could benefit from the protection of the also endemic M.portoricensis, as well as the protection of the area where the population is found. Since the pollinator and OMF of the orchid are unknown, protection of this population and its surroundings is imperative. The population studied here is unusually large, and fruit production and seedling recruitment was observed. This suggests that both the pollinator and the OMF are present in the studied area.

NOTE: This is not meant to be a scientific report, but rather a friendly account of my research project. For more technical details or the scientific report, reach out to me and I will gladly provide the information.