Long‐term exposure to enhanced UV‐B radiation is not detrimental to growth and photosynthesis in Douglas‐fir

Summary • The effects are reported here of enhanced UV-B radiation on photosynthesis, growth, and leaf pigments in Pseudotsuga menziesii (Douglas-fir) over 3 yrs in both glasshouse and field experiments. • Douglas-fir seedlings were grown at ambient, twice ambient (2X), or three times ambient (3X) biologically effective UV-B radiation (UV-BBE) from square-wave (glasshouse) or modulated (field) lamp systems. • Growth was slightly reduced by enhanced UV-BBE during the first year, but recovered in the second and third years to levels equal to, or exceeding, trees under ambient UV-BBE. Enhanced UV-BBE stimulated lateral branch production. Photosynthetic rates were reduced only by the 3X treatment in the glasshouse during the first year. There was no interaction between treatments and leaf age on gas exchange variables. Photosynthetic–light-response curves changed under enhanced UV-BBE with higher light compensation points associated with higher UV-BBE. There were no significant effects on apparent quantum yield. Chlorophyll and UV-absorbing compounds were not affected by enhanced UV-BBE. • Long-term exposure to enhanced UV-B radiation is unlikely to have detrimental effects on growth and gas exchange in interior Douglas fir.