We studied the activity of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), peroxidase (POD)) and polyphenol oxidase (PPO) across ontogenetic stages during the formation of normal (in silver birch, Betula pendula Roth var. pendula) and abnormal (in Karelian birch, Betula pendula Roth var. carelica (Mercl.) Hamet-Ahti) wood structure. This was the first study of this sort. The objects were the seedlings (6‑and 11‑weeksold) and adult plants of different forms of the silver birch (var. pendula and var. carelica). It was shown that at the age of 6 weeks, the seedlings of Betula pendula Roth var. pendula and Betula pendula Roth var. carelica (Mercl.) Hamet-Ahti had no external differences, but seedlings of Betula pendula Roth var. pendula showed a higher activity of secondary metabolism enzymes (POD and PPO) compared to Betula pendula var. carelica seedlings. We noticed a change in the metabolic strategies of antioxidant enzymes by the ageof 11 weeks. Growth processes began to dominate in the var. pendula seedlings, and their morphometric parameters reached higher values; on the contrary, var. carelica seedlings had higher activities of antioxidant enzymes. As we have reported previously, abnormal xylogenesis in adult Karelian birch plants is biochemically reflected in a higher POD activity. A new finding from this study is the higher activity of SOD, CAT and PPO in the process of abnormal xylogenesis. It is important that the antioxidant system is reoriented towards secondary metabolism already at the age of 11 weeks, when there are no visible signs of abnormal xylogenesis yet. We put forward a hypothesis that the activity of antioxidant enzymes can act as a test for the diagnosis of abnormal xylogenesis in Karelian birch at the earliest stages of the ontogeny.