AbstractThe results of stationary studies (1965–2017) on the regularities of the dynamics of the spatial structure of old-growth, cedar-pine forests are shown on the example of a natural-growth, cedar-pine sedge–tall herb–fern forest of the late succession stage (Western Sayan, 53°01′ N, 92°59′ E, individual test site area of 1.56 ha). It was found that a dynamic balance has been maintained in the structure of the edificator layer for a period of 50 years, while the subedificator layer (fir) has undergone more rapid changes in age structure and affects the relations between the synusiae of lower layer and their role in the regrowth of cedar pine (Pinus sibirica (Du Tour) and Siberian fir (Abies sibirica (Ledeb.). Each synusia is characterised by a relatively constant number of species, dominant-species structure, and ratio of ecocoenotic groups. Tall herb–fern, calamagrostis–dryopteris, aconite–sedge, sedge and oxalis synusiae of different complexities remain the constant structural elements of this type of forest. The roles of the individual herbaceous synusia in cedar pine regrowth have been shown for the tall grass–fern group of forest types. Cedar-pine regrowth is slow in the dominant tall grass–fern synusia and occurs mostly on microrelief, at sites of windfall and fallen trees. Conversely, the competition with the grass is lower in the sedge synusia, and periodical outbreaks of cedar-pine regrowth occur in years of high productivity. However, the probability of seedlings’ further survival is low due to competition with the parent trees. The results allow the prediction of the age dynamics of a natural-growth cedar pine forest for the next several decades, under the assumption of climate and weather trends that are typical for the latter century cycle and are without any catastrophic changes. The diversity of microtypes and synusiae supports the sustainability of the entire mountain ecosystem and will play an important role in the modeling of the regrowth and spatial distribution of individual trees during the later stages of fir–cedar pine forest development in the taiga zone of the Sayan mountains.