Factors influencing birch (Betula spp.) age distribution in Norway: Insights from tree ring data

The age-class distribution of forests is a key indicator of both carbon stock potential and biodiversity conservation, playing a vital role in sustainable forest management. In Norway, birch (Betula spp.) is the most abundant tree species, covering 42% of the forest area. Understanding the factors that shape the age structure of birch is essential for developing management practices that balance timber production, carbon sequestration, and biodiversity conservation. Using data from the Norwegian National Forest Inventory (NFI), we examined the age-class distribution of birch trees across various site conditions. Our analysis revealed that middle-aged trees (50-100 years) were prevalent in most regions, while older trees were notably scarce, particularly in highly productive areas. This pattern reflects management strategies prioritizing younger, fast-growing trees to maximize economic returns. In contrast, less productive sites, which are often managed less intensively, tend to support older trees. Additionally, younger birch trees revealed significantly greater radial growth than older generations when evaluated at the same biological age (e.g., at 10 years old), particularly under favorable site conditions. These findings underscore the combined effects of site productivity, forest management, and environmental factors on growth dynamics and age-class distribution.