Analysis of growth of recruits of natural regeneration of Sorbus torminalis (L.) Crantz - a rare European forest tree species

We compared growth and survival of wild service tree (Sorbus torminalis [L.] Crantz) recruits of different origin (generative: seedlings; vegetative: root suckers) established in a fenced plot at the Forest District of Krzyz (north-western Poland). Total height, annual growth of the dominant shoot, stem diameter at root collar, number of first-order branches, and mortality were measured every year over the period 2011-2015 (5 years). In 2011, a total of 382 multi-age recruits originated both from seeds (212) and root suckers (170) were recorded. Five-year mortality was higher in the generative progeny (12.3% - only youngest seedlings) as compared with vegetative recruits (2.9%). The growth rate of individuals markedly increased with height as absolute values, but slightly decreased in terms of relative growth. Statistical analysis revealed that the effect of the recruit origin on growth was noticeably weaker than that of age, defined in terms of development (height) classes. The origin of recruits had a major effect on the annual growth of the dominant shoots and a minor (though significant) effect on stem diameter and the number of first-order branches. Overall, the analysis of growth rate showed that generative recruits grow faster than the vegetative ones. Our results highlight the importance of stimulating the generative regeneration and protecting seedlings as a conservation strategy for Sorbus torminalis