Zróżnicowanie bonitacji wzrostowej drzewostanów daglezjowych w Polsce

The objective of the study was to analyse the site index (SI) of Douglas fir (Pseudotsuga menziesii) stands in relation to the forest habitat type, soil type and relief characteristics. We utilised infor− mation available in the Information System of the State Forests in Poland that included selected parameters describing habitat conditions and dendrometric characteristics of Douglas fir stands. In total, processed data referred to 863 stands with a total area of 1,644.45 ha. As the SI distribution was not consistent with the normal one (Shapiro−Wilk test, p <0.0001) we applied Kruskal−Wallis or Mann−Whitney tests to assess the significance of observed differences. Site index of surveyed stands ranged from 19.9 to 47.8 m, 32.6 m on average (standard deviation of 3.3 m). Douglas fir in northern and southern Poland characterized by higher SI than in western and south−western part of the country (HK−W=83.5; p<<0.001; fig. 2). Forest habitat type significantly distinguished analysed stands (HK−W=30.1; p<<0.001). The highest SI values were observed for mixed deciduous habitats (fig. 3). For more fertile fresh deciduous habitats Douglas fir achieved slightly lower site index, while the least fertile mixed coniferous habitats are characterized by significantly lower SI. Habitat moisture variant had no significant effect on the average SI values (HK−W=2.6; p=0.269). The highest values were found in strongly fresh habitats (32.8 ±4.1 m), while the lowest in the humid variant (31.7 ±3.4 m). We found significant influence of soil type (HK−W=23.2, p<0.001) even though the average SI values for each category were relatively similar (fig. 4). The highest values were observed for Cambisols (33.3 ±3.7 m), while the lowest for Arenosols (32.1 ±3.0 m). Particle size distribution is an important factor for Douglas fir site index (tab.). We found that the heavier material (the higher the content of silt and clay), the significantly higher SI values (HK−W=16.9; p=0.001). Mean SI for stands growing on former farmlands (32.0 ±3.3 m) and on forest areas (32.7 ±3.3 m) were not significantly different (ZM−W=1.73; p=0.084). There was no significant effect of the relief forms on site index surveyed Douglas fir stands apart from the fact that stands growing on slopes are characterized by significantly higher SI (ZM−W=1.99; p=0.047).