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factor of pine trees is induced by overstory pine trees. According to Milios (2000a), the density of overstory pines in the development stages where beech trees have been established under the shade of pines (and grow in the understory and middle story) is the lowest in site type A (143 trees/ha) and seems to be the highest in site type B (290 trees/ha in the first development stage and 640 trees/ha in the second). In site type C the density of overstory trees is 335 trees/ha. These data explain the values of form factors of sampled pine trees in the different site types; 0.41 for site type A, 0.47 for site type B and 0.45 for site type C (Table 3).
In conclusion, from the central part of the Rhodope Mountains, selecting 158 Pinus sylvestris trees from 3 site types, by applying Neyman’s random stratified sampling, we developed volume estimation models for each site type and for the whole study area. Selected models were:
For site type Α: , R2 = 0.7653, standard error = 0.3096
For site type B: , R2 = 0.8146, standard error = 0.3379
For the total area: , R2 = 0.8377, standard error = 0.3039.
For site type C none of the tested models was selected. There is not a clear effect of the distance of the nearest trees on the form factor of sampled trees since the species and dimensions of the nearest trees have different influence in the form of trees. As for site types A and B and for the study area as a whole, they seem to be analogous regarding volume – dimensions (D, H) relationship with a similar study in Sweden.
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