Evaluation of xylem maturation and among-site variation in the anatomical characteristics of Betula platyphylla growing naturally in Mongolia

Betula platyphylla Sukaczev, a fast-growing pioneer tree, is an important species for wood resources in boreal regions and steppe areas in Mongolia. We examined the anatomy and maturation of wood in 30 B. platyphylla trees growing in natural populations in three different sites in Mandal, Selenge, Mongolia to clarify the relationship between tree growth and wood formation. Core samples, 5 mm in diameter, were collected from 30 trees with the mean stem diameter at each site. The radial variations in the anatomical characteristics were evaluated using linear or nonlinear mixed-effects models. Almost all measured anatomical characteristics increased from the pith to the bark. Fiber length showed an annual increase of over 1% up to about the 20th annual ring from the pith, beyond which the increase was less than 1%, suggesting that xylem maturation started around the 20th annual ring. The growth rate at the early growth stage did not affect the annual ring number where xylem maturation began. The linear mixed-effects model revealed a site variation in wood fiber wall thickness. Thus, in combination with previous research results with this species, we believe that the selection of superior trees with thicker fiber walls might be possible for future tree breeding programs aimed at improving wood quality for pulp and paper raw materials and the production of solid wood and wood-based materials.