Distribution of Norway Spruce Undergrowth in Primary Middle Taiga Spruce Forests

This paper discusses the distribution of Norway spruce (Picea abies (L.) H. Karst.) undergrowth in three types of communities: blueberry spruce forest, wood-sorrel–blueberry spruce forest with silver birch (Betula pendula Roth) and European aspen (Populus tremula L.), and wood-sorrel–blueberry spruce forest codominated by Scots pine (Pinus sylvestris L.). Woody detritus as a microsite is represented not only by lying logs and stumps that account for 37–72% of undergrowth, but also by adjacent spaces (10–27% of undergrowth) and visually indistinguishable deadwood contained in soil (1–11% of undergrowth). In the blueberry spruce forest, intercrown zone occupy one-third of the surveyed area, accounting for 46% of undergrowth; the pericrown zone occupies almost half of the surveyed area (41% of undergrowth). The occurrence of undergrowth is high in all canopy projection zones (84, 73, and 68%); undergrowth occurs in clusters on deadwood (37%) and in adjacent microsites (40%). The density of undergrowth depends on the cumulative effect of multiple factors: the highest values were recorded in intercrown zone on deadwood (1.0 individuals/m²) and medium values were in intercrown zone near deadwood (0.6 individuals/m²), in pericrown zone on deadwood (0.5 individuals/m²) and near deadwood (0.5 individuals/m²), and in undercrown zone without deadwood (0.5 individuals/m²); the lowest values were on deadwood “hidden” in soil (0.1–0.2 individuals/m²). In the wood-sorrel–blueberry spruce forest with aspen and birch, the canopy structure is more complex, and undergrowth is almost evenly distributed between the canopy projection zones whose ratio is 12 : 33 : 55%. The maximum undergrowth occurrence and density values were recorded in intercrown zone on deadwood (39% and 0.7 individuals/m², respectively), while under the canopy, they are lower (18% and 0.3 individuals/m², respectively). In the wood-sorrel–blueberry spruce forest codominated by Scots pine, undergrowth is distributed randomly: in all canopy projection zones, its occurrence (31, 33, and 30%) and density (0.6, 0.5, and 0.6 individuals/m²) values are equally low; the highest density values were recorded on deadwood under crowns (0.5 individuals/m²). Overall, the distribution of undergrowth in primary middle taiga spruce forests is determined by a combination of factors: forest growing conditions, stand composition, and diversity of microsites, including those formed by deadwood at different decay stages.