Epiphytes as a bioindicator of climate in the Hawaiian Islands

Jonathan Price and Scarlett Kettwich, University of Hawai‘i at Hilo

alt textA variety of epiphytic ferns, orchids, mosses, and bromeliads are found in Hawai‘i.

When walking through a Hawaiian wet forest, it is not uncommon to encounter a fern or orchid perched on a tree, thriving despite any connection to the ground. These epiphytic plants grow physically supported by a host but rely only on their own specialized structures to gather water and nutrients. The downside of this self-contained lifestyle is that the surrounding air must supply all the moisture and nutrients that the epiphyte requires, making them exceptionally sensitive to air quality and moisture. Unfortunately, over the next 100 years, Hawai‘i is projected to experience overall increasing temperatures with stronger warming at higher elevations. Precipitation patterns including cloud and fog formation are expected to shift both temporally and spatially. The factors that make epiphyte species especially vulnerable to climate change also mean that they are excellent bioindicator species, allowing us to detect the effects of climate change rapidly and on small spatial scales. However, there are relatively few studies of epiphyte abundance and composition in Hawai‘i.

Tall ohia trees with smaller green plants growin off the trunkMultiple epiphyte species on an ‘ōhi‘a tree.

We assessed epiphyte abundance and species composition on windward Hawai‘i Island across both elevation and precipitation gradients, and analyzed stable isotopes to trace rainwater and fog uptake. Epiphytes increased in abundance and diversity as rainfall increased, and species composition varied significantly by elevation, with filmy ferns and bryophytes most abundant above 600 m elevation and larger species more common below. Stable isotope analysis suggests that fog is an important source of water for many epiphytes, and has a strong effect on community composition where elevation and rainfall are similar.

Epiphytes show responses to changes in climate at fine temporal and spatial scales, and our work provides a baseline that can be combined with future surveys. Our work also provides a reminder that many sensitive Hawaiian species in higher elevation wet forests rely on fog as a significant source of moisture, and so climate projections should consider changes to fog production conditions as well as rainfall abundances.

Quick Summary

      Epiphytes are dependent on the air for water and nutrients, so species generally show very tight habitat associations. Their sensitivity to changes in their environment may make them an effective bioindicator species, clearly illustrating shifts in climate patterns.
      We investigated patterns of epiphyte abundance and species composition across elevation and precipitation gradients on windward Hawai‘i Island, and found that there were significant differences across both rainfall and elevation gradients. Fog incidence seemed to also be an important driver of community composition.
      Our work provides a baseline from which changes in epiphyte communities can be monitored as a leading near term indicator of likely Hawaiian forest change.

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This project was supported by the Pacific Islands Climate Adaptation Science Center (Cooperative Agreement # G13AC00314 from the US Geological Survey). Contact Jonathan Price (jpprice@hawaii.edu) for more information on this project. To learn more about climate science at PI-CASC, contact David Helweg at dhelweg@usgs.gov or visit: https://nccwsc.usgs.gov/pacificislandscsc.