We studied the effects of slightly elevated temperature (T), O(3) concentration (O(3)) and their combination (T + O(3)) on the antioxidant defense, gas exchange and total leaf area of Betula pendula saplings in field conditions. During the second year of the experiment, T enhanced the total leaf area, net photosynthesis (P (n)) and maximum capacity of carboxylation, redox state of ascorbate and total antioxidant capacity in the apoplast. O(3) did not affect the total leaf area, but P (n) was slightly and g (s) significantly reduced. The saplings responded to elevated O(3) level by closing the stomata and by developing leaves with a lower leaf area per mass, rather than by accumulating ascorbate in the apoplast. The effects of T and O(3) on total leaf area and P (n) were counteractive. Elevated O(3) reduced the saplings' ability to utilize the warmer growth environment by increasing the stomatal limitation for photosynthesis and by reducing the redox state of ascorbate in the apoplast in the combination treatment as compared to T alone.