*3.6. Shoots Diameter*

Plants of poplar AF2 were, on average, of the greatest shoot diameter (113.5 mm). Boxelder maple, black alder, and white birch's shoot diameter was on similar level (63.8, 604, 59.4 mm, respectively). Siberian elm and silver maple had the lowest shoot diameter of 41.8 mm both (Table 2). Geyer, Argent, and Walawender [18] found 7-year-old Siberian elm to have, on average, stems of diameter of 109 mm at 10 cm. Walle et al. [11] found that shoots diameter (at 30 cm) of 4-year-old poplar (*Populus trichocarpa* × *deltoids*) (336 mm) and maple (*Acer pseudoplatanus* L.) (310 mm) were lower than shoot diameter of birch (*Betula pendula* Roth) (493 mm). These authors conducted research on other clones/species of trees in different habitat conditions than in presented study; however, differences in results show the importance of the selection of appropriate species/cultivars and proper density of planting to make the best use of habitat conditions ensuring high biomass accumulation.

Shoot diameter for all tested species was positively correlated with their green and dry mass and shoot diameter. For all tested species, a negative correlation with the planting density was found (Table 3). Shoot diameter of all tested plant species were decreasing with increasing planting density, but the strength of this reaction varied between species (Figure 7). The decrease in stem diameter is a common response to increasing planting density [18,26–28], which can also be modified by species genotype (clone) [29].

**Figure 7.** Relationship between plants shoot diameter (mm) and planting density (plants ha<sup>−</sup>1).

#### *3.7. Higher Heating Value*

Tested energy sources differed significantly in higher heating value (HHV). White birch and boxelder maple had the highest average HHV value (19,509 and 19,158 J g−<sup>1</sup> o, respectively). The lowest HHV was observed for poplar AF2 (17,908 J g−<sup>1</sup> o). Dry mass of Siberian elm, black alder, and silver maple had a similar HHV value of around 18,500 J g−<sup>1</sup> o (Table 2). Geyer, Argent, and Walawender [18] found that HHV of 7-year-old Siberian was between 18,900 and 20,200 J g−<sup>1</sup> (19,700 J g−<sup>1</sup> on average), which is a higher HHV value than in the presented. Study stand density had no effect on most species' HHV. A statistically significant negative correlation of HHV with the plant density was found only for poplar AF2 and white birch (Table 3, Figure 8). In the case of other species, no relationship between HHV and planting density was found. Literature study shows that HHV is highly variable and depends on both genetic factors (species, cultivars) [15,30,31] and cultivation conditions (soil quality, management methods) [30,32] or even plant age [33].

**Figure 8.** Relation of higher heating value (J g<sup>−</sup>1) and planting density (plants ha−1).
