Altitudinal variability of stand structure and regeneration in the subalpine spruce forests of the Pol'ana biosphere reserve, Central Slovakia

The structure of natural subalpine spruce forest in the Zadna Pol'ana massif of the Western Carpathians was analysed. We focused on the variability of different aspects of stand structure, tree decay and regeneration processes in altitudinal gradient. We used systematic sampling, covering an area of 2 km(2), to detect even subtle changes in stand structure within one forest type over a range of less than 200 m in elevation. Mean stand density was 290 trees (> 7 cm DBH) per hectare, average basal area was 41 m(2) ha(-1), and the volume accumulation in living trees amounted to 500 m(3)/ha(-1). Stand volume decreased by more than 50% between 1,260 and 1,434 m a.s.l. This means for an increase of altitude of 100 m that stand volume decreased by nearly 200 m(3). Neither stand density nor basal area was related to elevation. Maximum tree height was strongly correlated to elevation, and it decreased on average by 6 m for each 100 m increment of altitude. No significant changes in the maximum spruce diameter were recorded in relation to the elevation gradient. Spatial distribution of trees was biased toward regularity at lower altitudes. Tree clustering increased with increasing altitude. The stock of coarse woody debris (CWD) decreased slightly along the altitudinal gradient, but changes were not significant. Density of spruce saplings and their number growing on CWD significantly increased across the elevation gradient. Despite the fact that the analysed forest tract was relatively large, highly variable in respect to environmental factors, and that stand volume, spatial structure, and tree height displayed strong variability along the elevation gradient, the diameter structure of stands and regeneration measures were uniform. Our results suggest that the recruitment of new trees in the Zadna Pol'ana subalpine spruce forest is not temporally continuous even at a scale of several square kilometres.