In the experiment eight populations of Picea abies were chosen at 100 m intervals between 500 m and 1200 m altitude a.s.l.. In each population wood core samples were collected from 14–19 trees (126 cores total), and measured using a Corim Maxi device. At four of the eight sites (every 200 m in elevation between 500 m and 1100 m a.s.l.), the diversity of ground vegetation was evaluated, and temperature was recorded at every 100 m of altitude. The highest average radial increment of spruce occurred between the altitudes 800–1000 m a.s.l., which is probably the optimum for spruce. The larger increment indices observed at higher altitudes may signify a high growth potential of spruce. It may also suggest a recent upward shift of the optimum growth zone for this tree species. In 15 phytosociological records, the presence of 148 plant species forming plant associations: Dentario glandulosae- Fagetum typicum (sub-mountainous and mountainous form) and Abieti-Piceetum, and community Abies alba-Rubus hirtus, was documented. No relationship was found between ground vegetation species diversity (expressed by Shannon-Wiener index) and levels of stand diversity. The vegetation species diversity varied with the elevation above sea level: the highest plant diversity was found at 500 m a.s.l., and decreased with increasing altitude. The potential increase in air temperatures may result in changes to the altitudinal range of many plant species including trees, and consequently in an upward shift of the boundaries of plant zones; in this case the sub-mountainous and lower mountainous forest zone. In this region, the optimal zone for Norway spruce may be restricted to the highest elevations.