Effects of high irradiance and moderate heat on photosynthesis of the tree-fern Dicksonia Antarctica (Labill., Dicksoniaceae) were examined in a climate chamber under two contrasting irradiance regimes (900 and 170 mmol photonsm–2 s–1) and three sequential temperature treatments (15_C; 35_C; back to 15_C). High irradiance led to decline in predawn quantum yield of photochemistry, Fv/Fm (0.73), maximal Rubisco activity (Vcmax; from 37 to 29 mmolm–2s–1), and electron transport capacity (Jmax; from 115 to 67 mmolm–2 s–1). Temperature increase to 35_C resulted in further decreases in Fv/Fm (0.45) and in chlorophyll bleaching of high irradiance plants, while Vcmax and Jmax were not affected. Critical temperature for thylakoid stability (Tc) of D. antarctica was comparable with other higher plants (c. 47_C), and increases of Tc with air temperature were greater in high irradiance plants. Increased Tc was not associated with accumulation of osmotica or zeaxanthin formation. High irradiance increased the xanthophyll cycle pigment pool (V+A+Z, 91 v. 48 mmol mol–1 chlorophyll–1), de-epoxidation state (56% v. 4%), and a-tocopherol. Temperature increase to 35_C had no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, b-carotene and a-tocopherols increased, potentially contributing to increased membrane stability under high irradiance.