Morphological adaptation of the root system of silver fir responding to changes of local environmental conditions

The aim of the study was to verify possible allometric changes in the spatial structure of the root system of nearly 100-year-old silver fir (Abies alba Mill.) growing outside its natural range (Białowieża Forest). The traditional view of it as a taproot with 4–5 strong diagonal roots should be supplemented by the finding that, under certain conditions, it undergoes significant modification. This involves development of an extensive lateral root network reaching 41.8–44.0 m in diameter in the upper soil profile (0.2–1.0 m), markedly exceeding previously reported maxima (21.2 m). This adaptive change is a result of the lack of competition from the shallow roots of Norway spruce (Picea abies (L.) H. Karst.), which died due to drought, a drop in the groundwater level below 3 m and infestation by the European spruce bark beetle (Ips typographus L.). With increased availability of nutrients and water (including from horizontal precipitation) in upper soil layers enriched by decomposing organic matter, silver fir develops long roots in the niche previously occupied by spruce. The allometric shift favoring lateral roots corresponds to increased growth rates after spruce retreat, demonstrating considerable morphological plasticity despite the trees’ advanced age, confirming the climax nature of the species. Noteworthy spatial parameters include root depth of 1.95–2.40 m and lateral extension of 6.40–10.38 m, likely representing flattened root bundles growing parallel to the soil surface from the taproot tip. These findings offer insights for managing species composition and productivity in mixed stands containing silver fir, emphasizing long-term species management.