Leaf epidermal UV transmittance in Hawaiian plants: Patterns among species and along elevational gradients

The high solar UV climate found in the tropical alpine has been thought to be an important ecological and evolutionary factor influencing plant performance, adaptation and colonization.  To what degree this high UV limits the establishment and growth of non-native species originating from lower UV/temperate regions is unclear.  In this study, we surveyed leaf epidermal UV transmittance in a variety of plant species found on the island of Hawaii to: 1) determine if dominant native and non-native species growing above 2,900 m elevation on Mauna Kea differed in their UV shielding capabilities; and 2) compare the degree to which epidermal UV transmittance varied along an elevational/UV gradient in the native shrub, Ohelo (Vaccinium reticulatum) and the introduced forb, common mullein (Verbascum thapsus).  For plants growing in the Mauna Kea alpine, we found that epidermal UV-A transmittance, measured with a UVA-PAM fluorometer, did not differ between native (mean=5.6%; n=9) and alien (mean=5.5%; n=6) species. Along an elevational gradient (spanning 3,860 m for mullein, 2,590 m for Ohelo) we found that epidermal UV-A transmittance was variable (mean range = 6.0-11.2%) and strongly correlated (r² = 0.96) with elevation in the exotic mullein, but was consistently low (3%) and did not vary with elevation in the native Ohelo (r² = 0.02).  Our findings suggest that high levels of UV shielding is not restricted to species native to high UV environments, and plasticity in epidermal UV transmittance is a mechanism employed by some, but not all species to cope with varying solar UV exposures.