Measurements of ambient solar UV and PAR were made using a broadband UV 376
sensor (Skye UV-B; Skye Instruments, Ltd., Powys, UK) and quantum sensor (LI-185, Li-COR, Inc., Lincoln, NE, USA), respectively, at the Mauna Kea field site during the experiments. Clear-day data during the experiment were regressed against biologically-weighted estimates calculated from solar UV irradiance measured at the USDA UV-B monitoring station UVMFRS radiometer located at the Mauna Loa Meteorological Observatory (3,397 m elevation; ca. 30 km from our field site on Mauna Kea, http://uvb.nrel.colostate.edu/UVB). Biologically effective UV-B irradiances were reported for three generalized spectral weighting functions from Caldwell (1971; UV-BBE71), Flint and Caldwell (2003; UV-BBE03), and McKinlay and Diffey (1987; erythemal). We reported the erythemal (reddening of human skin) weighting as sometimes this is the only weighted irradiance available to researchers and is occasionally reported in plant studies. The UV-BBE data were adjusted for elevation differences between the Mauna Loa station and the Mauna Kea study site using a regression equation generated from data by Nullet and Juvik (1997). Reported integrated UV-A (321 nm - 400 nm) irradiances were not adjusted for elevation, as differences would be minimal.
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