Wyoming big sagebrush screens ultraviolet radiation more effectively at higher elevations

• We examined UV-screening effectiveness in Sagebrush along an elevation gradient.
• We measured epidermal transmittance of UV in leaves with a UVA-PAM fluorometer.
• Epidermal transmittance of UV decreased with increasing elevation.
• Levels of UV-absorbing compounds and trichome density increased with elevation.
• Environmentally-induced phenotypic plasticity is the likely cause of the ecocline.

The flux of biologically-effective ultraviolet radiation (UV) is diminished by atmospheric absorption which may cause physiological and morphological phenotypic differences among plant populations at different elevations. We examined UV-screening effectiveness of leaves of Artemisia tridentata ssp wyomingensis (Wyoming big sagebrush) along an 800-m elevation gradient in North-central Wyoming with a pulse-amplitude modulated fluorometer. Adaxial epidermal transmittance of UV increased at lower elevations; means ranged from 2.3% (high elevation) to 10.2% (low elevation). To provide a proximate explanation for this relationship, we collected leaves across the gradient and estimated concentrations of bulk-soluble UV-absorbing compounds (λ = 300 and 365 nm) and the density of trichomes on the adaxial surface. Concentrations of UV-absorbing compounds decreased with elevation and ranged from 0.64–2.25 A300 cm−2 and 0.43–1.35 A365 cm−2. Trichome density increased from 14,400 cm−2 at low elevation to 22,500 cm−2 at high elevation. Because the distance along the elevation gradient was only 18 km, gene flow likely prevents ecotypic differentiation; the ultimate cause of the ecocline in screening effectiveness is likely the evolution of environmentally-induced phenotypic plasticity in both biochemical and anatomical properties of leaves in response to variation in UV irradiance.