Most common species: Orchids, bromileads, and ferns. The least amount of epiphytes are found in highlands.
Functions in this climate: In this climate with less rain, epiphytes are particularly important. Their ability to direct water to the trunk of the host is crucial to the survival of the trees. For the epiphytes, the ability to reach sunlight is just as crucial for their growth and nutrient intakes.
Specializations in this climate: Compared to the wetter rain forests, there is less diversity of epiphytes in the mountains. Though there is a large population of epiphytes, the ones in the park are not as prevalent as those found in other rain forests.
Impacts on environment around them: The mosses pictured below are beneficial not only to the tree that they are on, but also the plants on the forest floor below them. The mosses are also food for many organisms, including birds, but also habitats for ants and other insects (jrscience). However, mosses can sometimes overgrow and block sunlight from reaching the trees, which has lead to some trees having protective measures, like shedding their bark (Viva Costa Rica).
Future in this climate: Because mosses are generally found in places with moderate to high precipitation, the biggest issue mosses are facing is the possible decreased precipitation, especially as species are forced to move up slopes to find their niche again, or disappear.
References:
http://www.flowersgrowing.com/easy-to-grow-orchids-cattleya/
http://jrscience.wcp.miamioh.edu/fieldcourses07/PapersCostaRicaArticles/Epiphytes.Anecosystemcont.html
http://www.vivacostarica.com/costa-rica-information/costa-rica-flora.html