The distribution of interchangeable sucrose-cleaving enzymes’ activities in sink organs (stem, root) of silver birch (Betula pendula var. pendula) and Karelian birch (B. pendula var. carelica) seedlings was investigated. It is shown that the dominant sink organ in 1.5‑month-old seedlings is the root, and sucrose cleavage is mainly performed by sucrose synthase. Early in the ontogeny sucrose metabolism in Karelian birch sink organs is more intensive compared to silver birch. As confirmed by the high activity of sucrosecleaving enzymes in the stem, the bulk of metabolites in 5‑month-old silver birch plants were spent on the formation of the stem’s structure. We observed high activity of sucrose synthase in the stem of 5‑month-old silver birch seedlings and high activity cell-wall invertase in Karelian birch. Thus, biochemical and molecular differences between silver birch and Karelian birch originate from the early stages of the plants’ ontogeny, before the traits of abnormal wood structure become visible in Karelian birch. The effect nitrogen availability on the activity of alternative sucrose-cleaving enzymes is shown. Cultivation of experimental plants in nitrogen-deficient media suppressed sucrose metabolism in sink organs: through reduced activity of sucrose synthase in silver birch, and through reduced activity of acid invertase in Karelian birch. This means that low nitrogen availability to silver birch seedlings resulted in inhibition of xylogenesis, and all the plant’s resources were utilized to support root biomass, whereas in Karelian birch it caused a reduction in sucrose cleavage in the apoplast, thus leading to an increase in the amount of parenchyma cells.