Planting Density Effects on Grow Rate, Biometric Parameters, And

agriculture

Article Planting Density Eﬀects on Grow Rate, Biometric Parameters, and Biomass Caloriﬁc Value of Selected Trees Cultivated as SRC

Adam Kleofas Berbe´c* and Mariusz Matyka

Department of Systems and Economics of Crop Production, Institute of Soil Science and Plant Cultivation-State Research Institute, Czartoryskich 8, 24-100 Puławy, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-81-4786-824

Received: 7 October 2020; Accepted: 25 November 2020; Published: 26 November 2020

Abstract: Agricultural land is mostly devoted to food production. Production of biomass is limited, as it competes for land with basic food production. To reduce land loss for growing food, biomass can be grown on marginal lands that are not usable for food production. The density of plantings have to be optimized to maximize yield potential. The presented study compares yield parameters end energy potential of six species of biomass plants (poplar, Siberian elm, black alder, white birch, boxelder maple, silver maple) cultivated in 18 planting densities from 3448 to 51,282 plants per hectare as short rotation coppice (SRC). Biomass yield parameters depended on both cultivated species and planting density. Green mass, dry mass, and shoot diameter was dropping with the increasing planting density for most tested species. Calculated yield of dry mass was dropping with increasing planting density for black alder, increasing for Siberian elm and boxelder maple. White birch and silver maple yields were optimal at moderate planting densities (25,000–30,000). White birch and boxelder maple had the highest average higher heating value (HHV). The optimal density of plantings should be chosen to best suit both the needs of cultivated species and to optimize the most important parameters of produced biomass.

Keywords: energy crops; planting density; caloriﬁc value; SRC

1. Introduction Although the main role of agriculture is food production, a part of agricultural land has always been devoted to non-food products, mainly within the framework of emerging technologies. Such uses include the production of bioenergy and various biomaterials [1]. The application of biomass for energy generation and for industry is of great importance and brings beneﬁts to: (i) energy independence, (ii) environmental protection, (iii) economy, and (iv) society [2,3]. Non-food crops cultivation should not compete with food and fodder cultivation on high-yielding, fertile soils of good agricultural quality. On the other hand, the number of available food and fodder species able to produce a satisfactory yield on light, sandy soils of rather poor agricultural quality is not wide [4,5]. The cultivation of tree species using the short rotation coppice technique of lignocellulose biomass production had spread following the ﬁrst oil crisis. Plants cultivated as short rotation coppices (SRCs) are characterized by a high growth rate, adequate sprouting of the stool bed, and an adaptation to sub-optimal environmental conditions [6]. Plantations for woody biomass production can be adapted depending on planting density and rotation length. Available experimental results indicate that a decrease in stem circumference is a commonly observed response to increasing planting density in poplar. However, studies also showed fast growing hardwoods tree height can increase, decrease, or remain unchanged with increasing planting density [7,8].

Agriculture 2020, 10, 583; doi:10.3390/agriculture10120583 www.mdpi.com/journal/agriculture Agriculture 2020, 10, 583 2 of 17

Agricultural biodiversity is of great importance nowadays. Plants of different genotype (species, varieties) including those cultivated as SRC, differ in habitat requirements for optimal growth and development and create different habitats for wildlife. Production conditions such as soil quality, water availability, harvest cycle, and technology (i.e., planting density) have an impact on biomass production, but the strength and direction of this impact could vary between species. The goal of this study was to the determine the grow rate, biometric parameters, and biomass caloric value of six trees species cultivated as SRC depending on planting density.

2. Materials and Methods

2.1. Site Characteristics and Experimental Design The experiment was conducted in the experimental farm of Institute of Soil Science and Plant Cultivation—State Research Institute in Osiny, Poland (N: 51◦28016.37”, E: 22◦305.11”). The experiment was established in spring 2010 on heavy black soil with a heavy clay granulometric composition. The following trees were included in the experiment:

Poplar (Populs L.), AF2 variety; • Siberian elm (Ulmus pumila); • Black alder (Alnus glutinosa); • White birch (Betula pubecsens); • Boxelder maple (Acer negundo); • Silver maple (Acer saccharinum L.). • Trees were planted in April 2010 at 18 ranges of density in a “Nelder wheel design” [9] (see Figure1). Eighteen concentric rings of trees were planted at radii ranging from 1.5 to 11 m as indicated in Table1. An additional outer ring was planted as a guard ring to minimize the edge eﬀect. The center of the circle was planted with a small ring to also form a guard. Each of the experimental rings contained 24 trees (six tested species in four replications) planted et equal distances around the circumference, giving a range from 3448 to 51,282 trees per hectare in equivalent planting density (Table1). Those 24 trees planted as a concentric ring of increasing diameter formed 24 rows of experiment. Each tested species was represented in four rows (replications). Two rows of the same species were always adjacent to each other, while the other two were on the opposite side of the circle—creating an experimental arrangement in the form of a mirror reﬂection (see Figure1).

Table 1. Planting distances and tree densities.

1 Ring No. Distance to the Center [m] Distance to Previous Ring [m] Density [Plants ha− ] 1 1.0 Guard ring 2 1.5 0.5 51,282 3 2.0 0.5 48,462 4 2.5 0.5 30,769 5 3.0 0.5 25,477 6 3.5 0.5 21,739 7 4.0 0.5 19,231 8 4.5 0.5 16,949 9 5.0 0.5 15,384 10 5.5 0.5 14,286 11 6.0 0.5 12,739 12 6.5 0.5 11,764 13 7.0 0.5 10,929 14 7.5 0.5 10,204 15 8.0 0.5 9569 16 8.5 0.5 9009 17 9.0 0.5 8510 18 9.5 0.5 8064 19 10.0 Guard ring 20 11.0 1.0 3448 21 12.0 Guard ring Agriculture 2020, 10, 583 3 of 17 Agriculture 2020, 10, x FOR PEER REVIEW 3 of 17

Figure 1. Distribution of species in the experiment design according to Nelder (1962) design (dots of

different color represents trees of different species). Figure 1. Distribution of species in the experiment design according to Nelder (1962) design (dots of 2.2. Biomassdifferent Analyses color represents trees of different species). Trees were harvested after 7 years of growth in February 2017. The harvest was made by hand. Shoot diameter at a height ofTable 15 cm 1. was Planting determined distances using and tree an densities. electronic caliper with an accuracy of 0.1Ring mm. No. Each Distanceplant was to cut the at Center a height [m] of 5 cmDistance above to the Previous ground level.Ring [m] Plant Densityheight was [Plants determined ha−1] from the1 cut place to the top1.0 of the plant with the accuracy Guard of 1 ring cm. Yield of green mass was determined by weighing2 whole plants1.5 immediately after harvesting (green0.5 mass included limbs 51,282 and/ or barks). Whole3 plants were chipped2.0 and carefully mixed. Representative 0.5 biomass samples (in 48,462 5 replications) were taken4 to determine the2.5 share of dry mass. In the laboratory, 0.5 biomass moisture was 30,769 determined by a drying5 method at a temperature3.0 of 80 ◦C for a period of 14 0.5 days. The dry mass yield was25,477 determined from the6 ratio of the green3.5 mass yield and its moisture content. 0.5 Dried samples of trees 21,739 were burned in calorimeter7 (KL-12Mn calorimeter,4.0 Precyzja-Bit, Bydgoszcz, 0.5 Poland) in order to assess 19,231 the higher heating8 value—HHV (or gross4.5 calorific value) of the biomass. 0.5 16,949 9 5.0 0.5 15,384 2.3. Statistical Analyses 10 5.5 0.5 14,286 Statistical11 analysis of results6.0 was performed using Statistica 0.5 10.0 software (StatSoft 12,739 Polska, Krakow, Poland)).12 Few clear extreme6.5 outliers (observations located 0.5 outside upper or lower 11,764 quartile) were removed13 from the dataset according7.0 to the software manual. 0.5 The level of significance for10,929 analysis was set to 14p = 0.05. 7.5 0.5 10,204 Subsequently,15 the normality8.0 of the distribution was 0.5 tested using the Shapiro–Wilk 9569 and Kolmogorow–Smirnow16 tests.8.5 The vast majority of the examined0.5 features were characterized 9009 by a non-normal17 distribution.9.0 Data transformation attempts 0.5 have not changed the data 8510 distribution. Therefore,18 nonparametric Kruskal–Wallis9.5 tests for comparison 0.5 of many groups of independent 8064 variables 19 10.0 Guard ring 20 11.0 1.0 3448 21 12.0 Guard ring

Agriculture 2020, 10, 583 4 of 17 were used to assess the significance of differences. Because of data distribution, all average values presented in the study are medians. The green mass, the share of dry mass, dry mass, and the biometric features of the examined trees, depending on the plant density, were characterized by using the trend equation. The criterion for selecting the trend equation was the highest value of the determination coefficient. In addition, for each tree species correlation coefficient for the tested parameters relationship was calculated. Woody plants intended for industry are, by definition, grown in long cycles, and therefore, their growth in subsequent years was not studied. Annual yields were not assessed as experimental design was not adapted to it (it would result in a complete failure of the basic methodological assumptions due to invasive nature of such measurements (annual harvesting)). However, potential 1 1 annual biomass of dry mass was calculated Ydm (t ha− year− ). To calculate the potential annual biomass yield of dry mass at given (chosen) density, the measured average dry mass of a single plant (Pdm) (kg) was multiplied by actual density of plantings (plants per hectare) (Dact). The obtained result 1 (t ha − ) was divided by the number of years of cultivation, which was 7. Potential annual biomass yield of dry mass:

P D Y = dm act dm 7

3. Results and Discussion

3.1. Green Mass The highest green mass of plants (GM) were observed in poplar AF2. On average, a single plant of poplar weighted 29.1 kg (median). Boxelder maple green mass was on average at a level of 9.3 kg per plant. Black alder, white birch, silver maple, and Siberian elm had a GM on a similar level (Table2). Nevertheless, the lowest GM was recorded for silver maple, and it was only 2.6 kg per plant. GM was positively correlated with plant height and shoot diameter for all tested plants. For Polar AF2 and Siberian elm, a negative correlation between the green mass of the plants and the share of dry mass was noted. The GM was dropping significantly with the increasing planting density for all tested species. The strength of response of species to the increasing stand density showed some differences. The strongest negative response was observed in black alder, while white birch and silver maple were the least sensitive species to increasing planting density. In the case of other species, the strength of dependence was at a similar level (Figure2, Table3). Wilkinson et al. [10] showed that yield of green matter of 1-year-old and 3-year-old willow (Salix ssp.) was increasing significantly with increasing plant density (density from 10,000 to 25,000 of plants per hectare).

Table 2. Median values of tested plants and their biometric features.

Green Share of Dry Plant Shoot Higher Heating Potential Yield Specification Mass Dry Mass Mass Height Diameter Value of Dry Mass 1 1 1 [kg] [%] [kg] [m] [mm] [J g− ] [t ha− yr− ] Poplar AF2 var. 29.1 a * 42.8 a 13.1 a 10.6 a 113.5 a 17,908 a 15.8 a Siberian elm 2.8 b 58.5 b 1.7 b 4.7 d 41.8 b 18,664 bc 5.2 b Black alder 5.3 bc 48.2 c 2.5 bc 6.6 bc 60.4 c 18,366 ab 3.9 b White birch 5.5 bc 52.4 c 2.9 bc 7.0 b 59.4 bc 19,509 c 2.2 b Boxelder maple 9.3 c 50.8 c 4.7 c 6.8 b 63.8 c 19,158 c 9.8 c Silver maple 2.6 b 51.8 c 1.4 b 5.2 cd 39.2 b 18,726 bc 4.7 b * Data marked with the same letter do not differ significantly between species (α = 0.05). Agriculture 2020, 10, 583 5 of 17

Table 3. Correlation matrix for the studied features.

Share of Dry Plant Shoot Higher Heating Potential Yield Green Mass Dry Mass Plant Speciﬁcation 1 Mass 1 Height Diameter Value of Dry Mass [kg Plant − ] [kg Plant − ] 1 1 1 [%] [m] [mm] [J g− ] [t ha− yr− ] Plant density [plants ha 1] 0.619 * 0.152 0.651 0.384 0.367 0.485 0.025 − − − − − − Green mass [kg] 0.542 0.993 0.822 0.815 0.435 0.377 − Share of dry matter [%] 0.454 0.650 0.389 0.350 0.131 − − − − − Poplar AF2 var. 1 Dry mass [kg plant − ] 0.797 0.796 0.428 0.399 Plant height [m] 0.676 0.443 0.525 Shoot diameter [mm] 0.346 0.130 1 Higher heating value [J g− ] 0.086 Plant density [plants ha 1] 0.660 0.457 0.663 0.402 0.628 0.248 0.414 − − − − − Green mass [kg] 0.504 1.000 0.774 0.923 0.065 0.309 − − Share of dry matter [%] 0.479 0.376 0.439 0.356 0.087 − − − − Siberian elm Dry mass [kg plants 1] 0.777 0.925 0.061 0.306 − − Plant height [m] 0.793 0.023 0.518 − Shoot diameter [mm] 0.124 0.234 − 1 Higher heating value [J g− ] 0.235 Plant density [plants ha 1] 0.833 0.409 0.804 0.775 0.874 0.115 0.539 − − − − − − Green mass [kg] 0.357 0.929 0.616 0.884 0.166 0.437 − − Share of dry matter [%] 0.025 0.257 0.293 0.317 0.257 − − − − − Black alder Dry mass [kg plant 1] 0.603 0.850 0.304 0.437 − − Plant height [m] 0.841 0.129 0.346 − Shoot diameter [mm] 0.129 0.280 − Higher heating value [J g 1] 0.149 − − Agriculture 2020, 10, 583 6 of 17

Table 3. Cont.

Share of Dry Plant Shoot Higher Heating Potential Yield Green Mass Dry Mass Plant Speciﬁcation 1 Mass 1 Height Diameter Value of Dry Mass [kg Plant − ] [kg Plant − ] 1 1 1 [%] [m] [mm] [J g− ] [t ha− yr− ] Plant density [plants ha 1] 0.589 0.301 0.589 0.188 0.639 0.095 0.175 − − − − − − − Green mass [kg] 0.278 0.998 0.598 0.679 0.139 0.728 − − Share of dry matter [%] 0.227 0.457 0.175 0.520 0.185 − − − − − White birch Dry mass [kg plant 1] 0.588 0.684 0.170 0.727 − − Plant height [m] 0.563 0.062 0.434 Shoot diameter [mm] 0.169 0.233 Higher heating value [J g 1] 0.288 − − Plant density [plants ha 1] 0.684 0.486 0.670 0.642 0.706 0.321 0.493 − − − − − − Green mass [kg] 0.185 1.000 0.587 0.956 0.388 0.068 − − Share of dry matter [%] 0.245 0.119 0.229 0.109 0.360 − − Boxelder maple Dry mass [kg plant 1] 0.580 0.957 0.395 0.095 − − − Plant height [m] 0.630 0.163 0.121 − Shoot diameter [mm] 0.313 0.130 − − 1 Higher heating value [J g− ] 0.196 Plant density [plants ha 1] 0.590 0.209 0.601 0.124 0.621 0.064 0.378 − − − − − − − Green mass [kg] 0.041 0.994 0.567 0.865 0.112 0.326 − Share of dry matter [%] 0.144 0.450 0.140 0.172 0.281 − − − − Silver maple Dry mass [kg plant 1] 0.518 0.839 0.121 0.295 − − Plant height [m] 0.668 0.087 0.549 − Shoot diameter [mm] 0.013 0.197 − Higher heating value [J g 1] 0.539 − − * Signiﬁcant correlations are in bold (α = 0.05). Agriculture 2020, 10, x FOR PEER REVIEW 5 of 17

tested species. The strength of response of species to the increasing stand density showed some differences. The strongest negative response was observed in black alder, while white birch and silver maple were the least sensitive species to increasing planting density. In the case of other species, the strength of dependence was at a similar level (Figure 2, Table 3). Wilkinson et al. [10] showed that

Agricultureyield of 2020green, 10 matter, 583 of 1-year-old and 3-year-old willow (Salix ssp.) was increasing significantly with7 of 17 increasing plant density (density from 10,000 to 25,000 of plants per hectare).

−1 −1 1 FigureFigure 2. 2.Relationship Relationship between between green green mass mass of of plants plants (kg (kg plantplant− )) and and planting planting density density (plants (plants ha ha−). ).

3.2. Dry Mass Table 2. Median values of tested plants and their biometric features.

Very similar relationships were found for the dry mass (DM) ofHigher plants as for green matter. Green Share of Dry Plant Shoot Heating Potential Yield of ThisSpecification shows that theMass dry massDry /Massgreen massMass ratio Height (or in otherDiameter words—moisture content) is at constant Value Dry Mass [t ha−1 yr−1] [kg] [%] [kg] [m] [mm] level at different plating densities and, therefore, have little or no effect on[Jyields. g−1] The highest dry mass (DM)Poplar of plants AF2 were observed in poplar AF2. On average, a plant of poplar weighted 13.1 kg (median). 29.1 a * 42.8 a 13.1 a 10.6 a 113.5 a 17,908 a 15.8 a Boxeldervar. maple green mass was on average at a level of 4.7 kg per plant. Black alder, white birch, silver maple,Siberian and elm Siberian 2.8 elmb had a58.5 dry b mass 1.7of plantb on 4.7 ad similar 41.8 level b (Table2 18,664). Nevertheless, bc the 5.2 lowest b dry Black alder 5.3 bc 48.2 c 2.5 bc 6.6 bc 60.4 c 18,366 ab 3.9 b massWhite of plantsbirch was 5.5 recorded bc for 52.4 silverc maple, 2.9 bc and 7.0 it wasb only 59.4 1.4 bc kg per plant. 19,509 Wallec et al. [11 2.2] comparedb the dryBoxelder mass of 4-year-old SRC of poplar (Populus trichocarpa deltoids), birch (Betula pendula Roth), 9.3 c 50.8 c 4.7 c 6.8 b 63.8 c× 19,158 c 9.8 c and maplemaple (Acer pseudoplatanus L.) cultivated with a density of 20,000, 6667, and 6667 plants per hectare, Silver maple 2.6 b 51.8 c 1.4 b 5.2 cd 39.2 b 18,726 bc 4.7 b respectively. The authors found that the average dry mass of plants grown under these conditions * Data marked with the same letter do not differ significantly between species (α = 0.05). was 831, 2007, and 738 g, respectively, which was a noticeably different result than in presented study; however, the growing conditions were also different to the plants tested by Walle et al. [11], which were also about 3 years younger than in the presented study. DM in the presented study was positively correlated with plant height and shoot diameter for all tested plants. For Siberian elm, a negative correlation between the dry mass of the plants and the share of dry mass was noted. The dry mass of plants dropped significantly with the increasing planting density for all tested species. The strength of response of species to the increasing stand density showed some differences. The strongest negative response was observed for black alder, while white birch and silver maple were the least sensitive species to increasing plant density. In the case of other species, the strength of dependence was at a similar level (Figure3, Table3). Other authors investigated the response of dry mass of plants to increasing density and found out that it was also dropping for willow (Salix ssp.) [12] and for oak (Quercus robur)[13]. Agriculture 2020, 10, x FOR PEER REVIEW 8 of 17

silver maple were the least sensitive species to increasing plant density. In the case of other species, the strength of dependence was at a similar level (Figure 3, Table 3). Other authors investigated the Agricultureresponse2020 of, 10 dry, 583 mass of plants to increasing density and found out that it was also dropping 8for of 17 willow (Salix ssp.) [12] and for oak (Quercus robur) [13].

1 1 FigureFigure 3. Relationship3. Relationship between between dry dry mass mass of plantof plant biomass biomass (kg plant(kg plant− ) and−1) and planting planting density density (plants (plants ha− ). ha−1). 3.3. Share of Dry Mass 3.3.The Share highest of Dry dry Mass to green mass (DM to GM) ratio was observed for Siberian elm (58.5%), while the lowestThe was highest observed dry for to poplargreen mass AF2 (DM (42.8%). to GM) White ratio birch, was ob silverserved maple, for Siberian boxelder elm maple, (58.5%), and while black alderthe DMlowest to GMwas ratioobserved was for on poplar a similar AF2 level (42.8%). (52.4, White 51.8, birch, 50.8, and silver 48.2%, maple, respectively) boxelder maple, (Table and2). black DM to GMalder ratio DM varied to GM depending ratio was on on the a similar stand density,level (52.4, but 51.8, also 50.8, on the and tested 48.2%, species. respectively) Share of(Table dry mass2). DM was negativelyto GM ratio correlated varied depending with green on mass the ofstand plants density, for three but also tested on speciesthe tested (poplar species. AF2, Share Siberian of dry elm mass and boxelderwas negatively maple). Sharecorrelated of dry with mass green also mass negatively of plants correlated for three withtested plant species height (poplar for poplarAF2, Siberian AF2 and withelm higher and boxelder heating valuemaple). for Share white of birch dry mass (Table also3). Poplarnegatively AF2 correlated and silver withmaple plant showed height no for reaction poplar of DMAF2 to GMand ratiowith higher to increasing heating density value for of plants,white birch while (Table Siberian 3). Poplar elm, black AF2 alder,and silver and maple white birchshowed showed no moderatelyreaction of positive DM to GM increase ratio into DMincreasing to GM density ratio with of plants, increasing while plant Siberian density. elm, Boxelderblack alder, maple and white showed negativebirch showed response moderately of DM to GM positive ratio increase to increasing in DM plant to GM density ratio (Figurewith increasing4). Wilkinson plant etdensity. al. [ 10 ]Boxelderfound dry massmaple of 1-year-old showed negative and 3-year-old response willowof DM to (Salix GM ratio ssp.) to not increasing dependent plant on density density (Figure of plantings 4). Wilkinson (similar et al. [10] found dry mass of 1-year-old and 3-year-old willow (Salix ssp.) not dependent on density to poplar AF2 and silver maple reaction in presented study). In addition, Stolarski et al. [14] and of plantings (similar to poplar AF2 and silver maple reaction in presented study). In addition, Kulig et al. [15] found no effect of planting density on the fresh–dry matter ratio of willow. This was Stolarski et al. [14] and Kulig et al. [15] found no effect of planting density on the fresh–dry matter also confirmed by Elfeel and Elmagboul [16] for other woody species—leucaena leucocephala. On the ratio of willow. This was also confirmed by Elfeel and Elmagboul [16] for other woody species— other hand, Achinelli et al. [17] found that dry to fresh matter content ratio in willow was higher in leucaena leucocephala. On the other hand, Achinelli et al. [17] found that dry to fresh matter content moreratio dense in willow stands was (similar higher e inffect more to Siberiandense stands elm, (similar black alder, effect and to Siberian white birch elm, black in the alder, discussed and white study). birch in the discussed study).

Agriculture 2020, 10, 583 9 of 17 Agriculture 2020, 10, x FOR PEER REVIEW 9 of 17

−11 Figure 4. Relationship between dry mass share (%) and planting density (plants ha− ).).

3.4.3.4. Potential Potential Yield Yield of of Dry Dry Mass Mass 1 PoplarPoplar AF2 hadhad thethe highest highest calculated calculated potential potential of dryof dry mass mass yield yield (after (after 7 years) 7 years) of about of about15 t ha 15− t*hafor all−1 for tested all tested planting planting densities. densities. Boxelder Boxelder maple mapl was ablee was to able match to match yielding yielding potential potential that AF2 that poplar AF2 poplaronly at only planting at planting density density of about of about 40,000 40,000 plants plants per hectare. per hectare. Silver Silver maple maple reached reached about about 50% 50% of this of 1 1 thispotential potential (about (about 7.5–8 7.5–8 t ha t− ha),−1 while), while other other tested tested species species reached reached about about 5 5 t t ha ha−−1 of dry mass mass yield. yield. (Figure(Figure 33).). TheThe calculatedcalculated annualannual yieldyield ofof drydry massmass waswas positivelypositively correlatedcorrelated withwith plantplant heightheight forfor poplar AF2, SiberianSiberian elm,elm, andand silver silver maple. maple. There There was was also also a positivea positive correlation correlation with with plant plant density density for forboxelder boxelder maple maple and negativeand negative for black for alder. black In alder. the case In of the white case birch, of white a strong birch, positive a strong correlation positive was correlationfound with was green found and drywith mass green of plantsand dry (Table mass3). of Despite plants (Table the fact 3). that Despite the dry the mass fact of that individual the dry mass trees ofdecreased individual with trees increasing decreased density, with theincreasing calculated dens annuality, the yield calculated of dry matter annual of yield Siberian of dry elm, matter boxelder of Siberianmaple, and elm, silver boxelder maple maple, was increasing and silver with maple increasing was increasing density ofwith planting increasing (Figure density5). The of same planting was (Figurealso observed 5). The by same Geyer, was Argent, also andobserved Walawender by Geyer, [18] forArgent, 7-year-old and Walawender Siberian elm. [18] Authors for 7-year-old found that 1 Siberianannual yields elm. Authors of dry matter found of that Siberian annual elm yields were atof a dry level matter of 4.7 of t ha Siberian− with standelm were density at a of level 1400 of plants 4.7 t 1 haper−1 hectare,with stand while density theyincreased of 1400 plants signiﬁcantly per hectare, to 9.8 while t ha− theywith increased stand density significantly of 7000 plants to 9.8 pert ha hectare.−1 with 1 standIn the density presented of 7000 study, plants calculated per hectare. annual In dry the mass pres yieldsented ofstudy, Siberian calculated elm varied annual between dry mass 1.8 tyields ha− for of 1 Siberianthe lowest elm density varied and between 7.4 t ha 1.8− tfor ha− the1 for highest the lowest density. density Geyer and and 7.4 Walawender t ha−1 for the [ 19highest] reported density. that 1 Geyerannual and dry Walawender mass yield of [19] 7-year-old reported silver that mapleannual was dry increasing mass yield from of 7-year-old 5.3 t ha − atsilver 1400 maple plants was per 1 increasinghectare to 11.2from t 5.3 ha− t haat − 70001 at 1400 plants plants per per hectare. hectare In addition,to 11.2 t ha other−1 at 7000 authors plants found per thathectare. for some In addition, species othersuch asauthors willow found (Salix thatL.) [for10] some and blackspecies locust such ( Robiniaas willow pseudoacacia (Salix L.))[ [10]20] annualand black yield locust of dry (Robinia mass pseudoacacia)was increasing [20] with annual increasing yield plantingof dry density.mass was Niemczyk increasing et al.with [21 ]increasing found that annualplanting yields density. of a 1 Niemczyk7-year-old poplaret al. [21] can found reach that up to annual 8 t ha −yields. Truax of eta 7-year-old al. [22] also poplar found can a positive reach up correlation to 8 t ha− of1. Truax planting et al.density [22] also and found yield ofa positive 8-year-old correlation hybrid poplar. of planting Stolarski density et al. and [23 ]yield found of that 8-year-old annual dryhybrid mass poplar. yield Stolarski et al. [23] found that annual dry mass yield of willow planted with densities from 12,000 to 96,000 was increasing from 12,000 to 24,000 (optimal density) and decreasing with increasing density

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Agriculture 2020, 10, x FOR PEER REVIEW 10 of 17 of willow planted with densities from 12,000 to 96,000 was increasing from 12,000 to 24,000 (optimal fromdensity) 24,000 and to decreasing 96,000. Similar with increasingreaction was density found from in presented 24,000 to 96,000.study silver Similar maple reaction (optimal was foundplanting in densitypresented of about study 25,000–30,000 silver maple (optimal plants per planting hectare). density of about 25,000–30,000 plants per hectare).

1 1 FigureFigure 5. RelationshipRelationship between potential yieldyield ofof drydry massmass perper year year (t (t ha ha− −1r r−−1) and plantingplanting densitydensity 1 (plants(plants ha ha−1).). 3.5. Height of Plants 3.5. Height of Plants Plants of poplar AF2 were, on average, the highest of all tested species (10.6 m). Black alder, 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 boxelder maple, and white birch’s height was on a similar level of 6.6–7.0 m. Silver maple and elm had, on average, the lowest plants of both 5.2 and 4.7 m, respectively (Table2). Siberian elm plants Siberian elm had, on average, the lowest plants of both 5.2 and 4.7 m, respectively (Table 2). Siberian in Geyer, Argent, and Walawender [18] were, on average, higher (6.3 m) than in presented study (4.7 m). elm plants in Geyer, Argent, and Walawender [18] were, on average, higher (6.3 m) than in presented Geyer, Barden, and Preece [24] found that 6-year-old silver maple clones of dense stands (no accurate study (4.7 m). Geyer, Barden, and Preece [24] found that 6-year-old silver maple clones of dense data available) were 7.3 m high. Plant height for all tested species was positively correlated with their stands (no accurate data available) were 7.3 m high. Plant height for all tested species was positively green and dry mass and shoot diameter. In addition, for poplar AF2, plant height was negatively correlated with their green and dry mass and shoot diameter. In addition, for poplar AF2, plant height correlated with share of dry mass, and, for black alder, with plant density (Table3). Height of tested was negatively correlated with share of dry mass, and, for black alder, with plant density (Table 3). plants varied on planting density. In most cases, the decrease in plant height with increasing density Height of tested plants varied on planting density. In most cases, the decrease in plant height with of planting started from the very begging—from about 3500 plants per hectare (poplar AF2, black increasing density of planting started from the very begging—from about 3500 plants per hectare alder, boxelder maple, and Siberian elm) (Figure6). Geyer, Argent, and Walawender [ 18] also found (poplar AF2, black alder, boxelder maple, and Siberian elm) (Figure 6). Geyer, Argent, and this relationship for the 7-year-old Siberian elm, whose height was, on average, 6.4 m for the stand Walawender [18] also found this relationship for the 7-year-old Siberian elm, whose height was, on density of 1400 plants per hectare and was dropping to 6.2 m for the density of 7000 plants per hectare. average, 6.4 m for the stand density of 1400 plants per hectare and was dropping to 6.2 m for the A similar relationship was presented by Perez et al. [25] for a 3-year-old Siberian elm. In addition, density of 7000 plants per hectare. A similar relationship was presented by Perez et al. [25] for a 3- it was conﬁrmed by Geyer and Walawender for silver maple [19] and black locust [20]. On the other year-old Siberian elm. In addition, it was confirmed by Geyer and Walawender for silver maple [19] hand, Toillon et al. [8] found that height of poplar increases with increased planting density in favorable 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

Agriculture 2020, 10, 583 11 of 17

Agriculture 2020, 10, x FOR PEER REVIEW 11 of 17 site conditions (as an effect of increased competition for light), while in less favorable conditions, heightdensity. of Benomar plants remained et al. [7] unashowedffected that by the increasing relation plantingship between density. the Benomardensity and et al.height [7] showed of plants that is thealso relationship strongly influenc betweened by the the density genotype. and height of plants is also strongly influenced by the genotype.

1 FigureFigure 6.6. Relationship betweenbetween plantplant heightheight (m)(m) andand plantingplanting densitydensity (plants(plants haha−−1).). 3.6. Shoots Diameter 3.6. Shoots Diameter Plants of poplar AF2 were, on average, of the greatest shoot diameter (113.5 mm). Boxelder maple, Plants of poplar AF2 were, on average, of the greatest shoot diameter (113.5 mm). Boxelder black alder, and white birch’s shoot diameter was on similar level (63.8, 604, 59.4 mm, respectively). maple, black alder, and white birch’s shoot diameter was on similar level (63.8, 604, 59.4 mm, Siberian elm and silver maple had the lowest shoot diameter of 41.8 mm both (Table2). Geyer, Argent, respectively). Siberian elm and silver maple had the lowest shoot diameter of 41.8 mm both (Table and Walawender [18] found 7-year-old Siberian elm to have, on average, stems of diameter of 109 mm 2). Geyer, Argent, and Walawender [18] found 7-year-old Siberian elm to have, on average, stems of at 10 cm. Walle et al. [11] found that shoots diameter (at 30 cm) of 4-year-old poplar (Populus trichocarpa diameter of 109 mm at 10 cm. Walle et al. [11] found that shoots diameter (at 30 cm) of 4-year-old deltoids) (336 mm) and maple (Acer pseudoplatanus L.) (310 mm) were lower than shoot diameter of ×poplar (Populus trichocarpa × deltoids) (336 mm) and maple (Acer pseudoplatanus L.) (310 mm) were birch (Betula pendula Roth) (493 mm). These authors conducted research on other clones/species of lower than shoot diameter of birch (Betula pendula Roth) (493 mm). These authors conducted research trees in different habitat conditions than in presented study; however, differences in results show the on other clones/species of trees in different habitat conditions than in presented study; however, importance of the selection of appropriate species/cultivars and proper density of planting to make the differences in results show the importance of the selection of appropriate species/cultivars and proper best use of habitat conditions ensuring high biomass accumulation. 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 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 shoot diameter. For all tested species, a negative correlation with the planting density was found (Table3). Shoot diameter of all tested plant species were decreasing with increasing planting density, (Table 3). Shoot diameter of all tested plant species were decreasing with increasing planting density, but the strength of this reaction varied between species (Figure7). The decrease in stem diameter is a but the strength of this reaction varied between species (Figure 7). The decrease in stem diameter is common response to increasing planting density [18,26–28], which can also be modified by species a common response to increasing planting density [18,26–28], which can also be modified by species genotype (clone) [29]. genotype (clone) [29].

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1 FigureFigure 7. Relationship 7. Relationship between between plants plants shoot shoot diamet diameterer (mm) (mm) and and planting planting density density (plants (plants ha− ha1). − ). 3.7. Higher Heating Value 3.7. Higher Heating Value Tested energy sources differed significantly in higher heating value (HHV). White birch and 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 1 o, respectively). The lowest boxelder maple had the highest average HHV value (19,509 and 19,158− J g−1 o, respectively). The HHV was observed for poplar AF2 (17,908 J g 1 o). Dry mass of Siberian elm, black alder, and silver lowest HHV was observed for poplar AF2 (17,908− J g−1 o). Dry mass of Siberian elm, black alder, and maple had a similar HHV value of around 18,500 J g 1 o (Table2). Geyer, Argent, and Walawender [ 18] silver maple had a similar HHV value of around− 18,500 J g−1 o (Table 2). Geyer, Argent, and found that HHV of 7-year-old Siberian was between 18,900 and 20,200 J g 1 (19,700 J g 1 on average), Walawender [18] found that HHV of 7-year-old Siberian was between 18,900− and 20,200− J g−1 (19,700 whichJ g−1 on is average), a higher HHV which value is a thanhigher in theHHV presented. value than Study in the stand presented. density hadStudy no stand effect ondensity most had species’ no HHV.effect Aon statistically most species’ significant HHV. A negative statistically correlation significant of HHV negative withthe correlation plant density of HHV was with found the only plant for poplardensity AF2 was and found white only birch for poplar (Table AF23, Figure and white8). In birch the case (Table of other 3, Figure species, 8). In no the relationship case of other between species, HHVno relationship and planting between density HHV was and found. planting Literature density was study found. shows Literature that HHV study is highly shows variablethat HHV and is dependshighly variable on both and genetic depends factors on (species, both genetic cultivars) factors [15 ,(species,30,31] and cultivars) cultivation [15,30,31] conditions and (soilcultivation quality, managementconditions (soil methods) quality, [ 30management,32] or even methods) plant age [30,32] [33]. or even plant age [33].

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1 1 FigureFigure 8. 8.Relation Relation of of higher higher heating heating value value (J (J g g−−1) and planting density density (plants (plants ha ha−−1). ). 3.8. Survival Rate 3.8. Survival Rate The highest survival rate after 7 years was observed for boxelder maple (100%), silver maple, 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%), and Siberian elm (both 99%). Black alder survival rate was also at moderately good level (81%), while whilepoplar poplar AF2 and AF2 white and white birch birchsurvival survival rate was rate the was lowest the lowest (69% (69%and 54%, and 54%,respectively). respectively). Survival Survival rate ratein the inthe presented presented study study was was dropping dropping with with increasi increasingng planting planting densities densities (f (foror whole whole experimental experimental design),design), but but some some species species werewere unaunaffectedﬀected byby increasingincreasing density (boxelder and and silver silver maple, maple, Siberian Siberian elm),elm), while while white white birch,birch, poplarpoplar AF2,AF2, andand blackblack alderalder survival rates were dropping with with increasing increasing densitiesdensities (Figure(Figure9 9).). According to to Trnka Trnka et et al. al. (2008) (2008) [34], [ survival34], survival rate of rate 6-year-old of 6-year-old poplar poplarat dense at densestand stand(10,000 (10,000 plants plantsper hectare) per hectare) varied between varied between 37% and 37% 73% anddepending 73% depending on the genotype. on the genotype.Geyer et Geyeral. (1987) et al. [18] (1987) found[18 ]that found survival that survivalrate of 7-year-old rate of 7-year-old Siberian Siberianelm was elm“almost was “almostperfect” perfect”and did not and didvary not on vary planting on planting densities densities from 1400 from to 1400 7000 to 7000plants plants per hectare per hectare (stands (stands of lower of lower densities densities than than in inpresented presented study). study). Moreover, Moreover, Geyer Geyer (2006) (2006) [35] [35 ]found found that that survival survival rate rate decreases decreases substantially substantially for for mostmost tested tested species species at at spacing spacing distances distances less less than than 1 1 m m (more (more than than 10,000 10,000 plants plants per per hectare), hectare), while while 2 m2 spacingm spacing (2500 (2500 plants plants per hectare) per hectare) was found was found optimal optimal for high for biomass high biomass production production and high and longevity high oflongevity plantation. of plantation. Authors also Authors found also that found silver that maple silver survival maple aftersurvival ﬁve after years five of cultivationyears of cultivation was at a levelwas ofat 97%.a level of 97%.

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FigureFigure 9. Survival 9. Survival rate rate after after 7 7 years years forfor didifferentﬀerent species species and and different diﬀerent planting planting densities. densities.

4. Conclusions4. Conclusions BiomassBiomass yield yield parameters parameters of of six six tested tested SRC SRC plants strongly strongly depended depended on onboth both genotype genotype (species) (species) and plantingand planting density. density. The The strength strength and and direction direction of reaction of of plants plants to toincreasing increasing planting planting density density varied between species. The green mass, dry mass, and shoot diameter of plants was dropping with 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 the increasingdiameter and planting planting density density for of almostpoplar AF2). all tested At the species same time, (insignificant function curve negative of calculated correlation potential of shoot diameteryield and of dry planting mass and density planting of density poplar was AF2). dropping At the samewith increasing time, function planting curve density of calculated for black alder, potential yieldincreasing of dry mass for andSiberian planting elm and density boxelder was maple, dropping and withwas concave increasing for plantingwhite birch density and silver for blackmaple alder, increasing(with optimal for Siberian planting elm density and boxelder of about maple,25,000–30,000 and was plants concave per hectare). for white Planting birch density and silver seemed maple (withto optimal have no planting effect on densitycalculated of potential about 25,000–30,000 yield of dry mass plants for perAF2 hectare). poplar. Planting density seemed to have no eWhiteffect on birch calculated and boxelder potential maple yield had ofthe dry highes masst average for AF2 higher poplar. heating value (HHV) (19,509 Whiteand 19,158 birch J g and−1, respectively). boxelder maple The lowest had the HHV highest was observed average for higher poplar heating AF2 (17,908 value J (HHV) g−1). Dry (19,509 mass and of all other1 species had similar HHV value of around 18,500 J g−1. A tendency towards reduced1 higher 19,158 J g− , respectively). The lowest HHV was observed for poplar AF2 (17,908 J g− ). Dry mass of heating value of plants in more dense stands was observed, but 1it was confirmed only for AF2 poplar. all other species had similar HHV value of around 18,500 J g− . 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 1 plant, with calculated potential dry mass yield of about 15 t ha− at wide range of planting densities, Agriculture 2020, 10, 583 15 of 17 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.

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