Nearly all plants in the wild live in close physical association with beneficial root fungi, called mycorrhizal fungi, to obtain adequate nutrition. Orchids, however, represent an extreme case of dependence on fungi because suitable fungal partners are required for their germination, and growth, in nature. Coupled with the fact that orchids grow in the wild in almost all parts of the planet and represent about 10% of flowering plants in the world, their requirement of forming mycorrhizal partnerships for something as fundamental as seed germination makes them a distinctively suitable system to investigate if the extensive diversity of orchids is related to their interaction with the required fungi. It is particularly curious how multiple orchid species can germinate and grow together in harmony in small physical spaces. This phenomenon is especially bewildering in tropical regions of the world, where orchids show their highest diversity and concentration. For example, Costa Rica is home to almost 1,400 orchid species while being just a little smaller in size than the US state of West Virginia.

Through this project, we set out to understand and explain how so many plant species can compete for the same resources of water, light, and soil nutrients and still grow close together in the same small natural spaces. We predicted that it is their preference for distinct fungi that allows many orchids to grow crowded together on tree branches. Through several large-scale experiments in a tropical rainforest in Costa Rica, we discovered that communities of fungi, including beneficial fungi, living in inner tree bark, outer bark, mosses, and detritus on tree branches were extremely diverse. Samples about the size of a dime could contain 50 or more fungal species. The types of fungi also changed along the length of a tree branch, where locations less than a foot apart had very few species in common. This high diversity of fungi can offer opportunities for many orchid species to grow close together by using different suites of fungi. In fact, our results showed that obligate mycorrhizal orchid plants preferred different fungi even when growing close together while temperature, humidity and light resources were rather similar across space and time. We further discovered that the overall fungal diversity in roots of orchids from our study site is generally higher than in orchids that grow in the ground or in other tropical forests. At the same time, it also seems that young orchid plants prefer a unique set of fungi when compared to adults of the same species. This type of developmental-stage associated preference for unique fungal partners underscore the importance and necessity of conserving the high diversity present in natural ecosystems. Altogether, intricate patterns of dependence of a highly diverse plant group on another very diverse group of organisms (i.e., fungi) highlight the complexity of natural ecosystems and of mycorrhizal interactions between plants and fungi. They also call attention to the potentially damaging effects of habitat degradation on global biodiversity.

That being the case, our documentation of thousands of previously unnamed fungi from within and outside of orchid roots, and vouchering of orchids, mosses, ferns, and lichens from tropical tree branches provides foundational data for conservationists and scientists alike. Moreover, we developed a DNA-based reference sequence database for all orchid species encountered at the study site, and produced multiyear, microenvironment data from the study area. These biodiversity and environmental information resources are (or soon will be) publically available for further use in research and education. While our DNA-based databases are available online, the physical specimens are accessible through university affiliated herbaria in Costa Rica. Altogether, these resources will assist in addressing questions surrounding biodiversity distribution patterns, mechanisms that shape biodiversity, and species conservation in near as well as distant future.

Lastly, through this project, our international team trained a diverse group of postdoctoral, graduate, and undergraduate researchers both in the US and in Costa Rica. Further, we created special interactive projects to involve and train Costa Rican school children and indigenous peoples in orchid identification, ecology, and ecosystem conservation. By including members of groups under-represented in science, and promoting international collaboration, this project also facilitated cultural and scientific exchange, and broadened participation in science and conservation. Notably, directly as a result of this project, Costa Rica now has its first few local orchid-mycorrhizal biologists based at an institution that has an otherwise strong history of studying orchids for almost 50 years. This recent addition of one of the most fundamental aspects of orchid biology as a research field at a renowned botanical garden in a country that has striking orchid diversity is particularly significant considering the cultural and biological significance of orchids in the region.

Last Modified: 02/08/2022
Modified by: Jyotsna Sharma