Development Polyvariance of the Siberian Fir’s Undergrowth in Dark-Coniferous Forests of the Northern Pre-Urals

To correctly evaluate forest renewal, it is necessary to know the patterns of undergrowth development in forest-forming tree species. The aim of this study is to characterize the development of Siberian fir undergrowth using the concept of biological age and assess the prospects for undergrowth of different vitality. Based on the analysis of morphometric traits and the height growth parameters of investigated Siberian fir saplings (up to 3.5 m in height), diagnoses were formulated for ontogenetic stages im1, im2, v1, and vitality levels in the last two stages. To assess the prospects of the saplings under investigation, their height growth was predicted using the individually fitted Mitscherlich functions and the prognoses were compared to the height and age of reproductive trees grown under the same conditions. It is shown that, to reach the height of reproductive trees at a comparable age, the saplings should have an average height increment of at least 15–20 cm/year that are typical values for saplings of normal vitality at im2 and v1 ontogenetic stages; those individuals are classified as high-prospect undergrowth. Another clear diagnostic trait in the habitus of high-prospect sapling is the presence of at least two living branches on each annual height increment of the stem. Saplings at the im2 (normal or reduced vitality) and v1 (reduced vitality) ontogenetic stages are classified as moderate-prospects, and low vitality saplings at im2 and v1 ontogenetic stages are classified as doubtful undergrowth with regards to prospects. The saplings at the ontogenetic stage im1 haven’t developed sufficiently, and their differentiation by vitality is not yet clearly expressed to predict their prospects.