Height–Age Structure of Picea abies Undergrowth in a Middle Taiga Spruce Forest: Microsite Influence

We have analyzed the height–age structure of Norway spruce (Picea abies (L.) H. Karst) undergrowth in three canopy projection zones (intercrown, crown edge and undercrown zones) and five microhabitat types (flat microsites without xylolytic substrate (XS); microelevations not associated with XS; microelevations formed by XS; microsite generated by visually indiscernible, “hidden” part of XS; and XS zone of influence) in three old-growth middle taiga spruce forests. The undergrowth of Norway spruce reaches the height of 50 cm at the age of 19–24 years. The forest growth conditions, canopy, and microsite influence the young trees of Norway spruce that are taller than 50 cm. The undergrowth reaches the height of 100 cm at the age of 28–36 years and 150 cm at 31–39 years. An exponential model best describes changes in the undergrowth with age. In five microsite types, 18–26% of the undergrowth reaches the height category of 51–100 cm (compared to the amount of small (<50 cm) undergrowth). The height category of 101–150 cm is reached by only about 6–8% of the undergrowth in the blueberry spruce forest, 10–23% in wood-sorrel–blueberry spruce forest with Silver birch Betula pendula and aspen Populus tremula, and 5–25% wood-sorrel–blueberry spruce forest with Scots pine Pinus sylvestris. Only 3% of the undergrowth of P. abies from the microsites formed by the XS in the blueberry spruce forest reaches the height category of 151–200 cm. In the spruce stand with the involvement of deciduous species in all microsite types (except the “hidden” parts of XS), only 6% of the undergrowth reaches this height category. Similarly, 6% of the undergrowth in the pine–spruce stand reaches this height category, but only on microelevations formed by XS and in the XS influence zone. The results indicate that the mortality rate of Norway spruce with a height of more than 50 cm is generally comparable in all microsite types (including those formed by XS).