*3.8. Survival Rate*

The highest survival rate after 7 years was observed for boxelder maple (100%), silver maple, and Siberian elm (both 99%). Black alder survival rate was also at moderately good level (81%), while poplar AF2 and white birch survival rate was the lowest (69% and 54%, respectively). Survival rate in the presented study was dropping with increasing planting densities (for whole experimental design), but some species were unaffected by increasing density (boxelder and silver maple, Siberian elm), while white birch, poplar AF2, and black alder survival rates were dropping with increasing densities (Figure 9). According to Trnka et al. (2008) [34], survival rate of 6-year-old poplar at dense stand (10,000 plants per hectare) varied between 37% and 73% depending on the genotype. Geyer et al. (1987) [18] found that survival rate of 7-year-old Siberian elm was "almost perfect" and did not vary on planting densities from 1400 to 7000 plants per hectare (stands of lower densities than in presented study). Moreover, Geyer (2006) [35] found that survival rate decreases substantially for most tested species at spacing distances less than 1 m (more than 10,000 plants per hectare), while 2 m spacing (2500 plants per hectare) was found optimal for high biomass production and high longevity of plantation. Authors also found that silver maple survival after five years of cultivation was at a level of 97%.

**Figure 9.** Survival rate after 7 years for different species and different planting densities.

#### **4. Conclusions**

Biomass yield parameters of six tested SRC plants strongly depended on both genotype (species) and planting density. The strength and direction of reaction of plants to increasing planting density varied between species. The green mass, dry mass, and shoot diameter of plants was dropping with the increasing planting density for almost all tested species (insignificant negative correlation of shoot diameter and planting density of poplar AF2). At the same time, function curve of calculated potential yield of dry mass and planting density was dropping with increasing planting density for black alder, increasing for Siberian elm and boxelder maple, and was concave for white birch and silver maple (with optimal planting density of about 25,000–30,000 plants per hectare). Planting density seemed to have no effect on calculated potential yield of dry mass for AF2 poplar.

White birch and boxelder maple had the highest average higher heating value (HHV) (19,509 and 19,158 J g<sup>−</sup>1, respectively). The lowest HHV was observed for poplar AF2 (17,908 J g−1). Dry mass of all other species had similar HHV value of around 18,500 J g<sup>−</sup>1. A tendency towards reduced higher heating value of plants in more dense stands was observed, but it was confirmed only for AF2 poplar.

Presented study showed that, in the study conditions, poplar AF2 was the most promising SRC plant, with calculated potential dry mass yield of about 15 t ha−<sup>1</sup> at wide range of planting densities, with boxelder maple being the only species able to match AF2 poplar's yielding performance (at 40,000 plants per hectare). The study showed the importance of testing and selection of best-performing species, to maximize biomass accumulation at local site conditions. The optimal density of plantings should be chosen to best suit the needs of cultivated species but also to meet the needs of the industry by optimizing the most important (for the industry) parameters of produced biomass. Cultivation of renewable resources, such as energy crops, must be optimized to site-specific conditions. This includes cultivation on poor soils or marginal soils to minimize their competitiveness against food crops. Optimization of energy crop yields can contribute to the goals of bioeconomy and sustainable development.

**Author Contributions:** Conceptualization, M.M.; methodology, M.M.; software, A.K.B. and M.M.; investigation, M.M. and A.K.B.; data curation, A.K.B. and M.M.; writing—original draft preparation, A.K.B.; writing—review and editing, M.M. and A.K.B.; visualization, M.M. and A.K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper is the result of a long-term study carried out at the Institute of Soil Science and Plant Cultivation, State Research Institute in Pulawy, and it was co-financed by the National (Polish) Centre for Research and Development (NCBiR), project: BIOproducts from lignocellulosic biomass derived from MArginal land to fill the Gap in Current national bioeconomy, No. BIOSTRATEG3/344253/2/NCBR/2017. This research acknowledges financial support from the Widening Program ERA Chair: project BioEcon (H2020), contract number: 669062.

**Conflicts of Interest:** The authors declare no conflict of interest.
