*3.5. Height of Plants*

Plants of poplar AF2 were, on average, the highest of all tested species (10.6 m). Black alder, boxelder maple, and white birch's height was on a similar level of 6.6–7.0 m. Silver maple and Siberian elm had, on average, the lowest plants of both 5.2 and 4.7 m, respectively (Table 2). Siberian elm plants in Geyer, Argent, and Walawender [18] were, on average, higher (6.3 m) than in presented study (4.7 m). Geyer, Barden, and Preece [24] found that 6-year-old silver maple clones of dense stands (no accurate data available) were 7.3 m high. Plant height for all tested species was positively correlated with their green and dry mass and shoot diameter. In addition, for poplar AF2, plant height was negatively correlated with share of dry mass, and, for black alder, with plant density (Table 3). Height of tested plants varied on planting density. In most cases, the decrease in plant height with increasing density of planting started from the very begging—from about 3500 plants per hectare (poplar AF2, black alder, boxelder maple, and Siberian elm) (Figure 6). Geyer, Argent, and Walawender [18] also found this relationship for the 7-year-old Siberian elm, whose height was, on average, 6.4 m for the stand density of 1400 plants per hectare and was dropping to 6.2 m for the density of 7000 plants per hectare. A similar relationship was presented by Perez et al. [25] for a 3-year-old Siberian elm. In addition, it was confirmed by Geyer and Walawender for silver maple [19] and black locust [20]. On the other hand, Toillon et al. [8] found that height of poplar increases with increased planting density in favorable

site conditions (as an effect of increased competition for light), while in less favorable conditions, height of plants remained unaffected by increasing planting density. Benomar et al. [7] showed that the relationship between the density and height of plants is also strongly influenced by the genotype.

**Figure 6.** Relationship between plant height (m) and planting density (plants ha<sup>−</sup>1).
