# Do the biomass yields of 37 poplar varieties in a German short rotation coppice indicate a response to drought?

Dirk Landgraf, Simon Thomas, Markus Neupert

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Dirk Landgraf, Simon Thomas, Markus Neupert. Do the biomass yields of 37 poplar varieties in a German short rotation coppice indicate a response to drought?. Biomass and Bioenergy, 2026, 211, pp.109140. 10.1016/j.biombioe.2026.109140. hal-05528315

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# Do the biomass yields of 37 poplar varieties in a German short rotation coppice indicate a response to drought?

Dirk Landgraf<sup>a,∗</sup> <sup>†</sup>, Simon Thomas<sup>a</sup>, Markus Neupert<sup>b,‡</sup>

## ARTICLE INFO

**Keywords:**

Bioenergy plantation

Short rotation coppice

Woody biomass

Populus

Renewable energy

## ABSTRACT

Poplars are among the most frequently cultivated trees in European short rotation coppices in Europe for both energy and material use. In a randomized trial, 37 different poplar varieties were cultivated on a typical sand-dominated site under continental climatic conditions of the Northeast German Lowlands. Planted in 2012, the poplars were harvested every three years based on the mini-rotation system. Survival rates were determined after the first growing season in 2013 and again right before the three harvests in 2015, 2018, and 2021. Important plant physiological parameters (i. e. resprouting capacity, plant height, and diameter at breast height [DBH]) were also recorded for the three harvests, and the harvested biomass was calculated in Mg ha⁻¹ y⁻¹ DM (Megagramm dry mass per hectare and year). Survival rates decreased to varying degrees for all poplar varieties. The loss of individuals was particularly severe in the first year after planting and in the last rotation period. Resprouting capacity, however, increased continuously for all poplar varieties over the nine years. The yields of the first harvest in 2015 were not analyzed, since these results do not reflect variety-specific characteristics or site conditions, but rather depend on the technological framework. In the second rotation interval, yield increases attributed to plant height and DBH were generally recorded for all poplar varieties at the 2018 harvest. Seven poplar varieties achieved more than 11 Mg ha⁻¹ y⁻¹ DM and can be recommended for commercial use. However, many varieties only yielded around 8 Mg ha⁻¹ y⁻¹ DM. Six varieties, including newly bred varieties, even yielded less than 4 Mg ha⁻¹ y⁻¹ DM. At the third harvest, significant shifts in the ranking of the poplar varieties in terms of biomass yield were observed. For example, poplar varieties that had medium yields in the second harvest, such as AF 19, now achieved up to 17.13 Mg ha⁻¹ y⁻¹ DM. A total of 16 poplar varieties increased their yield compared to the second rotation interval, while 21 varieties produced poorer harvest results. The yield increases were primarily achieved by new Italian varieties (i. e. AF 19, AF 2, AF 13), whereas older, well-known varieties such as all Max varieties, Fritzi Pauley, Androscoggin, and Muhle Larsen suffered significant yield reductions. It was conspicuous that the varieties with the greatest yield increases had the lowest survival rates. Since exceptionally high temperatures coupled with significantly lower precipitation were recorded during the three years of the third rotation, it is reasonable to conclude that the poplar varieties with high survival rates and therefore high plant densities suffered from drought-induced growth stress. Therefore, when establishing or re-establishing SRC, lower plant densities and longer rotations should be favored, combined with new drought-tolerant poplar varieties.

## 1. Introduction

Woody biomass is required for a wide range of uses. In addition to the simple and millennia-old energetic use of wood through combustion in campfires and later in various fireplaces, willow and poplar biomass can also be used in modern combined heat and power processes or be

converted into biofuels such as ethanol, renewable diesel, and sustainable aviation fuels [1,2]. Furthermore, poplar and willow timber is increasingly being used for materials, i. e. for pulp and paper [3,4], growth substrate for mushroom cultivation [5] or as mulch material in landscaping. Their application in furniture construction or to produce veneer and wooden boxes for fruit transport has also been promoted in

<footnote><sup>a</sup> Forestry and Ecosystem Management, Erfurt University of Applied Sciences, Leipzigiger Strasse 77, 99085, Erfurt, Germany</footnote>

<footnote><sup>b</sup> ECODIV USC INRAE 1499, University of Rouen Normandy, Place Emile Blondel, Mont-Saint-Aignan, 76821, France</footnote>

<footnote>This article is part of a special issue entitled: Biomass Europe published in Biomass and Bioenergy.</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: dirk.landgraf@fh-erfurt.de (D. Landgraf), Markus.neupert1@univ-rouen.fr (M. Neupert).</footnote>
recent years [6,7].

Within the bioeconomy framework, not only a holistic approach to wood is sought, but also as the sustainable (cascade) use of biological resources in the circular economy [8–10]. The most sustainable benefit, both economically and ecologically, is achieved when the wood from short rotation coppices (SRC) can be utilized for both material and energy production, leaving almost no residues [3,11].

Numerous studies in recent years have shown that the cultivation of fast-growing tree species in SRC and agroforestry systems (AFS) can generate a variety of ecological benefits by establishing a sustainable circular economy based on renewable resources. Besides the undeniable absorption of CO₂ from the atmosphere and the incorporation of carbon into wood (carbon sink), poplar cultivation on agricultural soils can also lead to a significant reduction in methane emissions from agricultural soils [12–14]. This type of cultivation and the associated soil dormancy can, on the one hand, reduce soil erosion and, on the other hand, lead to the accumulation of organic carbon in the soil compared to arable farming [12,15–18]. The deeper roots of fast-growing trees can break up the plow pan of previous agricultural use [19] and protect groundwater through water filtration [15,20,21]. The cultivation of trees reduces wind speed and, through the shading effect, also moderates solar radiation, which together leads to a reduction in evaporation. These effects help to stabilizing the soil water balance, which is particularly important for regions with low annual rainfall. Moreover, the cultivation of fast-growing tree species leads to an increase in biodiversity, especially in homogenized agricultural landscapes [15,22–24]. Fast-growing tree species can also contribute to the regeneration of contaminated sites due to their phytoremediative properties [5,25].

According to the recent report of the Food and Agriculture Organization (FAO), 2024 [26] and other authors [27] on the state of the world's forests, global wood demand is projected to rise significantly by 2050. In response, cultivation systems for fast-growing tree species will need to expand in both short rotation coppices with varying cycle lengths in agroforestry systems and traditional forests. Fast-growing species will likely be grown under different rotation regimes (mini-, midi-, maxi-rotations) to meet the demand for renewable raw materials. In Europe, the cultivation of fast-growing tree species focuses on poplar (Populus spp.) [6,18,28–31] and willow varieties (Salix spp.) [5,18,29, 32,33]. In regions with light, sandy soils and low rainfall, black locust (Robinia pseudoacacia L.) is also utilized [34–37].

Due to their vegetative propagation via cuttings, the cultivation of poplar and willow varieties is technologically simple and cost-effective, both in SRC and AFS. While willows are often harvested in shorter rotation cycles of 2 to 4 years (mini-rotation), poplars can also range in systems with medium rotation cycles of 6-9 years (midi-rotation) up to 20-year rotation cycles (maxi-rotation). While willows are often harvested in shorter rotation intervals of 2 to 4 years (mini-rotation), poplars can also be cultivated in systems ranging from medium rotation (6-9 years; midi-rotation) to 20-year rotation intervals (maxi-rotation). Multi-year studies on willow and poplar yields have shown that their productivity is determined by several factors. These include environmental, climatic, and habitat conditions, the selection of clones, agricultural practices, planting density, and harvest cycles, as well as the interactions between these factors. Numerous recent publications have shown that yields in the first three consecutive harvest cycles can either remain stable or triple. However, there are cases where yields decline due to fluctuations in climate and soil conditions, diseases, or pest infestations, which reduce plant survival [2,38–41]. Given the growing interest in woody biomass, it is essential to conduct further research on mapping site-specific interactions of hybrid poplar yields, as well as on clone performance and overall life cycle assessments for poplars in SRC systems.

In this context, the present study aims to answer the following research questions: I) How did the plant growth parameters of the 37 investigated poplar varieties develop during the third rotation period? II) How did the plant growth parameters affect changes in yield? III)

Were there any other characteristics that could have influenced the biomass yield?

## 2. Materials and methods

### 2.1. Site description

The study site is located near Großthiemig in Northeastern Germany (latitude 51°23'52.9"N, longitude 13°40'11.6"E, 43 m a.s.l.) (Fig. 1a). The local climate is primarily inland, with a noticeable transition to the continental. The long-term averages of the meteorological parameters are defined over a period of at least 30 years (Table 1). Using the data from the nearest Doberlug-Kirchhain weather station (latitude 51°65'N, longitude 13°57'E, 98 m a.s.l.) of the German Meteorological Service (DWD, 2025) [42], it is notable that the mean annual temperature increased from 8.5 °C (1961-1990), to 8.9 °C (1971-2000), to 9.2 °C (1981-2010), and finally to 9.7 °C (1991-2020). This is particularly evident in the individual annual mean temperatures, which rose continuously during the study period from 9.4 °C in 2012 to 11.0 °C in 2020.

While the long-term average annual precipitation remained relatively constant, ranging from 559 mm (1961-1990), to 561 mm (1971-2000), to 580 mm (1981-2010) and to 562 mm (1991-2020), the annual precipitation total fluctuated greatly during the study period, ranging from 391 mm in 2018 to 614 mm in 2013. However, the most relevant precipitation for poplar growth occurs during the growing season from April to October. When considering this parameter in relation to the individual rotation periods, it is obvious that there was a permanent decrease in the amount of precipitation. While in the first rotation from 2012 to 2015 a total precipitation of 1042 mm was recorded over the three growing seasons, it reached only 1007 mm in the second rotation from 2015 to 2018, and only 774 mm in the third rotation from 2018 to 2021.

According to Hartmann [43], soils are Gleyic Cambisols, consisting of 4.8 % clay, 2.9 % silt, and 92.3 % sand. The pH value measured in H₂O is 5.9 on average, which is considered slightly acidic. The soil contains 1.5 % organic carbon.

### 2.2. Experimental design and plant material

The randomized trial was established in spring 2012 using 27 common poplar varieties available on the market and 10 new poplar varieties that are not available on the market. These originated from the P & P forest nursery in Germany, Alasia Franco Vivai (AFV) in Italy, and the Thünen Institute in Germany. All poplar varieties used in this study along with their parentage are shown in Supplemental Table S1.

Three Thünen Institute varieties (P1, 4x Göttingen, and Esch 5) were supplied as rooted cuttings, whereas the other 34 poplar varieties were planted as unrooted cuttings. Three plots of 33 plants (n = 99 per variety) were established for each variety in a randomized block design. With a planting distance of 0.5 m within the row and a row spacing of 1.5 m, this results in a theoretical number of 13,333 poplars per hectare. The plots were arranged on the experimental area according to a randomized procedure (Fig. 1b). Details of the randomized experimental design are described in Landgraf et al. [39]. With the exception of the two mechanical maintenance measures in the establishment year, no further maintenance measures, neither mechanical nor chemical, were carried out in subsequent years. No fertilization whatsoever, neither organic nor inorganic, was carried out.

### 2.3. Determination of growth parameters

Survival was assessed one year after planting (Jan 2013) and again before each harvest (Jan 2015, 2018, 2021). At each harvest, we recorded the number of shoots per stool and measured the height and DBH of each shoot. Harvesting was carried out using a motor-manual
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Location of the study site in Germany (a); with a schematic representation of the randomized experimental setup (b) and an exemplary photograph from the establishment in 2012 (c).</figcaption></figure>

brush cutter with a saw blade. To determine the dry matter content (DMC) and calculate the dry matter biomass yield in megagramm absolute dry mass per hectare and year (Mg ha⁻¹ y⁻¹ DM), the same parameters and functions were used as for the previous harvests, as described in Landgraf et al. [39].

## 2.4. Data analysis

All data collected were tested for normal distribution using the

Shapiro-Wilk test (α = 0.05). As none of our data was normally distributed, the differences between the poplar varieties were analyzed using the non-parametric Kruskal-Wallis test (α = 0.05). Fisher's least significant difference method was used as a post-hoc test. The Pearson product-moment correlation coefficient (α = 0.05) was applied to analyze the correlation between the six parameters. Furthermore, the correlation between the parameters and the actual plot position was examined, but did not reveal any evidence of spatial autocorrelation. All analyses were performed using the R 4.3.1 [44] software with the
<caption>**Table 1**</caption>

Local annual average temperature, annual precipitation, and growing season precipitation (April to October) for the study period from 2012 to 2020 for the DWD meteorological measuring station at Doberlug-Kirchhain located 25 km from the study site (DWD, 2025).

<table><thead><tr><th rowspan="2"></th><th rowspan="2">Mean annual temperature (°C)</th><th colspan="3">Precipitation</th></tr><tr><th>Annual</th><th>April-October</th><th>Sum of the three growing seasons</th></tr></thead><tbody><tr><td>Long-term average (1961–2020)</td><td>9.1</td><td>580</td><td>-</td><td>-</td></tr><tr><td>2012</td><td>9.4</td><td>549</td><td>308</td><td>-</td></tr><tr><td>2013</td><td>9.1</td><td>614</td><td>417</td><td>-</td></tr><tr><td>2014</td><td>10.8</td><td>427</td><td>318</td><td>1042</td></tr><tr><td>2015</td><td>10.6</td><td>545</td><td>327</td><td>-</td></tr><tr><td>2016</td><td>10.0</td><td>562</td><td>345</td><td>-</td></tr><tr><td>2017</td><td>10.0</td><td>553</td><td>336</td><td>1007</td></tr><tr><td>2018</td><td>11.1</td><td>391</td><td>216</td><td>-</td></tr><tr><td>2019</td><td>11.2</td><td>444</td><td>240</td><td>-</td></tr><tr><td>2020</td><td>11.0</td><td>494</td><td>318</td><td>774</td></tr></tbody></table>

tidyverse [45] and ggcorrplot [46] packages.

# 3. Results

## 3.1. Survival rate

In the first year after establishment 33 varieties showed high survival rates of over 80 % (Table 2). Survival was notably high for varieties planted as unrooted cuttings (e.g., Max 4). Particularly excellent results were achieved by those varieties planted as unrooted cuttings (e. g. Max

<caption>**Table 2**</caption>

Survival rate of the poplar varieties at the end of the first growing season (January 2013), after three years (January 2015), six years (January 2018), and nine years (January 2021). The varieties are displayed in the order of their biomass yield in 2021, see Fig. 6.

<table><thead><tr><th rowspan="2">Variety</th><th colspan="4">Survival rate [%]</th></tr><tr><th>2013</th><th>2015</th><th>2018</th><th>2021</th></tr></thead><tbody><tr><td>AF 19</td><td>57</td><td>37</td><td>37</td><td>33</td></tr><tr><td>AF 2</td><td>96</td><td>43</td><td>43</td><td>43</td></tr><tr><td>AF 13</td><td>90</td><td>54</td><td>54</td><td>54</td></tr><tr><td>AF 18</td><td>96</td><td>84</td><td>84</td><td>73</td></tr><tr><td>Max 1</td><td>97</td><td>95</td><td>95</td><td>86</td></tr><tr><td>AF 24</td><td>94</td><td>45</td><td>45</td><td>45</td></tr><tr><td>AF 27</td><td>95</td><td>62</td><td>62</td><td>62</td></tr><tr><td>Pannonia</td><td>93</td><td>64</td><td>63</td><td>63</td></tr><tr><td>AF 16</td><td>77</td><td>72</td><td>72</td><td>71</td></tr><tr><td>Heidemij</td><td>94</td><td>61</td><td>61</td><td>49</td></tr><tr><td>AF 28</td><td>81</td><td>52</td><td>52</td><td>45</td></tr><tr><td>Koltay</td><td>91</td><td>56</td><td>52</td><td>47</td></tr><tr><td>Monviso</td><td>93</td><td>61</td><td>61</td><td>55</td></tr><tr><td>Isière</td><td>98</td><td>97</td><td>97</td><td>88</td></tr><tr><td>AF 8</td><td>83</td><td>52</td><td>52</td><td>32</td></tr><tr><td>Robusta</td><td>90</td><td>75</td><td>75</td><td>42</td></tr><tr><td>Max 3</td><td>95</td><td>83</td><td>83</td><td>73</td></tr><tr><td>P1</td><td>100</td><td>96</td><td>96</td><td>92</td></tr><tr><td>Fritzi Pauley</td><td>84</td><td>80</td><td>80</td><td>80</td></tr><tr><td>Max 4</td><td>98</td><td>95</td><td>95</td><td>94</td></tr><tr><td>AF 17</td><td>95</td><td>86</td><td>86</td><td>83</td></tr><tr><td>Weser 6</td><td>96</td><td>87</td><td>87</td><td>80</td></tr><tr><td>Harff</td><td>94</td><td>80</td><td>76</td><td>66</td></tr><tr><td>I 214</td><td>96</td><td>62</td><td>62</td><td>46</td></tr><tr><td>Kopecky</td><td>96</td><td>61</td><td>60</td><td>49</td></tr><tr><td>AF 6</td><td>85</td><td>46</td><td>46</td><td>35</td></tr><tr><td>Matrix 24</td><td>91</td><td>72</td><td>67</td><td>47</td></tr><tr><td>Jacometti 78B</td><td>88</td><td>72</td><td>72</td><td>53</td></tr><tr><td>Rochester</td><td>95</td><td>82</td><td>82</td><td>74</td></tr><tr><td>NE 42</td><td>96</td><td>87</td><td>87</td><td>75</td></tr><tr><td>AF 20</td><td>56</td><td>17</td><td>17</td><td>17</td></tr><tr><td>AF 15</td><td>91</td><td>49</td><td>48</td><td>37</td></tr><tr><td>Matrix 49</td><td>89</td><td>81</td><td>81</td><td>81</td></tr><tr><td>4 x Göttingen</td><td>98</td><td>85</td><td>85</td><td>83</td></tr><tr><td>Androscoggin</td><td>83</td><td>59</td><td>59</td><td>58</td></tr><tr><td>Muhle Larsen</td><td>73</td><td>52</td><td>52</td><td>41</td></tr><tr><td>Esch 5</td><td>93</td><td>74</td><td>74</td><td>44</td></tr></tbody></table>

4). The lowest survival rates were recorded for AF 20 and AF 19, at 56 % and 57 %, respectively. By January 2015 (after the first rotation), survival rates of some varieties declined sharply – the Italian clones AF 2, AF 24, and AF 15 dropped by 53 percentage points (%pt), 49%pt, and 42%pt (relative to establishment), respectively. Isière, Max 1, and Max 4 showed only minimal declines in survival (1–3 %pt reduction from planting to 2021). However, some varieties (Isière, Max 1, Max 4) maintained high survival, losing only 1–3 %pt of individuals by 2015.

After the second rotation harvest in 2018, only slight survival decreases were observed for a few varieties (e.g., Koltay: 56% to 52%, Harff: 80% to 76%, and Matrix 24: 72% to 67%). However, survival rates declined far more severely for several varieties after the harvest of the third rotation in 2021. The survival rate of Robusta dropped from 75% to 42% (a 33 percentage-point decrease), and Esch 5 from 74% to 44% (30 %pt). AF 8 and Matrix 24 each lost 20 %pt. Other varieties showed only a very slight decrease in survival rate (e. g. Max 4, Androscoggin, and AF 16 with 1 %pt and 2 %pt respectively. Overall, a total of 15 varieties had less than 50 % of the established plants surviving after three rotation periods of three years each (e. g. AF 2, AF 8, AF 19, and AF 24).

## 3.2. Resprouting capacity

Resprouting capacity increased over time. While about 99% of poplar stools had only a single shoot after the first rotation (2012–2015), the second rotation saw an average of 1.59 shoots per stool. The maximum was 2.59 shoots (median) in clone Max 3 [39]. During the third rotation, the average shoots per stool increased further to 2.25, with Max 1 achieving the highest average (3.12 shoots), followed by AF 13 (3.09) and Max 3 (2.89). Although extremely high shoot numbers were rare, 32 of the 37 varieties had at least some stools with ≥4 shoots, and 14 varieties even had stools with ≥6 shoots (Fig. 2). According to a Kruskal–Wallis test with post-hoc grouping, the 37 varieties fell into 11 statistically distinct groups in terms of shoot number, though many groups overlapped considerably. Max 1 and AF 13 still exhibited significantly higher shoot numbers than most other varieties in the trial.

## 3.3. Plant height

Compared to the median height of 6.29 m across all varieties at the end of the first rotation, only a marginal increase of 0.19 m to 6.48 m was detected at the end of the second rotation [39]. In the third rotation, the median height across all varieties decreased by 1.33 m to 5.15 m (Fig. 3). The tallest trees were associated with AF 2 with a median value of 6.76 m, followed by AF 19 and AF 24 (both 6.39 m). With a median height of 3.63 m, Androscoggin showed the lowest growth, followed by Esch 5 (3.64 m) and Muhle Larsen (3.86 m). Notably, AF 20 had a relatively high median height of 5.16 m even in 2021, whereas all other varieties showed their lowest heights in the third rotation.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Average (± standard error) number of shoots of the poplar varieties in the winter of 2021, nine years after planting and six years after the first harvest. Statistical analysis was carried out using the non-parametric Kruskal–Wallis test (α = 0.05, n ∈ [25; 262]) paired with Fisher's least significant difference procedure to highlight statistically different varieties. The letters on the right-hand side of the figure represent the statistical groups to which each variety belongs. The varieties are displayed in the order of their biomass yield, see Fig. 6.</figcaption></figure>

### 3.4. Diameter at Breast Height (DBH)

The DBH across all poplar varieties reached a median of 3.3 cm in 2015 (Fig. 4). After the second rotation in 2018 the median DBH decreased by 0.9 cm to 2.4 cm. This decrease affected all poplar varieties. The AF varieties were the best performers in terms of DBH across all three rotation periods. In addition to the variety AF 13 with 4.7 cm, the varieties Fritzi Pauley with 4.4 cm and Max 1 with 4.3 cm were identified as the varieties with the strongest DBH growth in the first

rotation. In the second rotation in 2019 the best-performing varieties were AF 24 (3.9 cm), AF 2, and AF 28 (both 3.6 cm). In the third rotation in 2021, AF 2 (3.94 cm) performed best in terms of DBH followed by AF 19 (3.79 cm) and AF 24 (3.43 cm) performing the best in terms of DBH. The lowest values in the third rotation were recorded for Esch 5 (1.38 cm), Androscoggin (1.51 cm), and Muhle Larsen (1.79 cm) (Fig. 4).
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Plant height of the poplar varieties in the winters of 2015, 2018, and 2021. For each variety the difference between the height in 2015, 2018, and 2021 was analyzed using the non-parametric Kruskal-Wallis test (α = 0.05, n = 3). Statistically significant differences are marked with an asterisk. The varieties are displayed in the order of their biomass yield, see Fig. 6.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Diameter at breast height of the poplar varieties in the winters of 2015, 2018, and 2021. For each variety the difference between the diameter in 2015, 2018, and 2021 was analyzed using the non-parametric Kruskal-Wallis test (α = 0.05, n = 3). Statistically significant differences are marked with an asterisk. The varieties are displayed in the order of their biomass yield, see Fig. 6.</figcaption></figure>
## 3.5. Dry Matter Content (DMC)

The average dry matter content (DMC) across varieties was 45.2% in 2015, with substantial variation among clones. Rochester had the highest DMC (50.7%), followed by NE-42 (49.5%) and Androscoggin (48.0%). The lowest values were found for AF 28, Koltay and Max 1, at 42.2%, 43.3% and 43.3% respectively. After the second rotation, the average DMC decreased slightly to 44.5%. Androscoggin (54.6%), NE 42 (49.5%), and Rochester (50.7%) again achieved the highest DMC values. With 38.1% the lowest DMC was recorded for AF 27, followed by AF 17 with 40.5%, and AF 2 with 40.2%. After the third rotation in 2021, the average DMC decreased slightly again, reaching 42.1%. Androscoggin achieved the highest DMC (48.9%), followed by 4 x Göttingen (48.2%), and Matrix 49 (45.8%). AF 27 had the lowest DMC content at 38.0%, followed by Esch 5 and AF 2 (both 39.3%).

## 3.6. Biomass yield

Fritzi Pauley achieved the highest yield in 2015 with 14.6 Mg ha⁻¹ y⁻¹ DM. Weser 6, Max 3, and Max 4 also attained high yields (14.3, 12.4, and 12.0 Mg ha⁻¹ y⁻¹ DM, respectively) (Fig. 5). The lowest yields were recorded for AF 20, 4 x Göttingen, and AF 19 with 2.4, 2.8 and 3.2 Mg ha⁻¹ y⁻¹ DM, respectively. After the second rotation (2018), Max 4 produced the highest yield (15.8 Mg ha⁻¹ y⁻¹ DM), followed by Fritzi Pauley (14.5) and Max 3 (13.3). In total, 19 varieties had higher biomass yield in the second rotation (2018) compared to the first, whereas 18 varieties' yields decreased. In contrast, Weser 6, Max 1, and Esch 5 had the greatest yield declines between the first and second rotations (yields lower by 5.4, 4.1, and 3.0 Mg ha⁻¹ y⁻¹ DM in 2018 compared to 2015, respectively). The third rotation in 2021 brought further significant shifts in the ranking of the individual varieties in terms of biomass yield. AF 19 achieved the highest yield (17.13 Mg ha⁻¹ y⁻¹ DM), followed by AF 2 (15.92 Mg ha⁻¹ y⁻¹ DM), and AF 13 (15.47 Mg ha⁻¹ y⁻¹ DM). These represents the highest biomass yield within the three rotation periods. In contrast, several once top-performing varieties (Max 3, Max 4, Fritzi Pauley) produced only about 8 Mg ha⁻¹ y⁻¹ DM in 2021, reflecting a steep drop in productivity. A total of 16 varieties displayed increased biomass yields in the third rotation compared to the second, while 21 varieties achieved lower yields. Although statistical analysis revealed no significant differences among the varieties, it did allow for their classification as high-yielding (a–c) or low-yielding (e–h) (Fig. 5).

With regard to the third rotation, a positive correlation between survival rate and the number of resprouting shoots was observed (Fig. 7). At 0.74, the correlation was significantly stronger than in the second rotation per stool (0.27). A clearly positive correlation of 0.93 was also confirmed between plant height and DBH (second rotation: 0.73). Furthermore, a positive correlation of 0.8 was found between DBH and biomass yield (second rotation: 0.75), and of 0.72 between plant height and biomass yield (second rotation: 0.69). Unlike the previous rotation periods, slightly negative correlations were apparent between survival rate and plant height (−0.15), survival rate and DBH (−0.27), and survival rate and biomass yield (−0.18) (Fig. 7).

# 4. Discussion

## 4.1. Management-related influencing factors

If, in addition to site-specific conditions (soil quality, rainfall, temperature, etc.), site management (e.g., site preparation, quality of cuttings, quality of planting, weed control) is also considered during the first rotation phase of a poplar plantation (SRC), the suitability of the various poplar varieties for the selected site can be determined in subsequent years [47]. The selected rotation system also plays a major role. In the present experiment, we deliberately focused on an SRC in the so-called mini-rotation, as this is the most common variant of rotation systems in Central Europe [39,48]. A mini-rotation requires rotation

<figure><img src="image_6.png" /><figcaption>Fig. 5. Biomass yield of the poplar varieties in the winters of 2015, 2018, and 2021. For each variety, the difference between the biomass yield in 2015, 2018, and 2021 was analyzed using the non-parametric Kruskal–Wallis test (α = 0.05, n = 3). Statistical differences between the three recorded yields per variety are indicated as follows: \* between 2015 and 2018, \# between 2015 and 2021, + between 2018 and 2021.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Biomass yield (± standard error) of the poplar varieties determined in the winter of 2021, at the third harvest after nine growing seasons. To statistically confirm significant differences, the Kruskal-Wallis test (α = 0.05, n ∈ [25; 262]) was performed in combination with the Fisher LSD test. The letters on the right-hand side of the figure represent the statistical groups to which each variety belongs.</figcaption></figure>

periods of three years. The poplar varieties used in this experiment exhibited very different plant survival rates in response to this management. In general, a constant decline in survival rates was observed across all varieties over the study period, with a significant decline during the establishment year [39]. Since there was a high-quality site-management during the establishment phase in the first year, the very drastic decline in survival rates for some varieties (i.e. AF 20, 19, and Muhle Larsen) may be variety-specific. This would be consistent with the results of Shifflett et al. [49]. The fact that the yield of poplar varieties at different sites can be highly variety-specific was described in

a large-scale study by Thevs et al. [47], among others.

From the end of the first growing season to the end of the first rotation in 2015, there was a significant decrease in survival rates for all varieties. Since no biotic pest species during the entire growing period and no abiotic damage (e. g., windthrow) was observed, it is likely attributable to the harvesting stress [11] or site conditions. In the third rotation (2018–2021), a varying degree of survival rate decline was observed, which was particularly severe for Robusta (33 %pt) and for Esch 5 (30 %pt). Since no other adverse effects were recorded besides the harvesting measure, it can be assumed that the decline to well below
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Correlation matrix of identified phenotypic parameters and determined biomass yield at the third harvest after nine growing seasons using Pearson product-moment correlation. Only significant ($\alpha$ ≤ 0.05) correlation coefficients are shown.</figcaption></figure>

half of the originally established plant number is mostly due to the prevailing site conditions and the aforementioned harvesting measure. In 2018 and 2019, very low annual precipitation was observed, which coincided with above-average annual temperatures. This combination resulted in an exceptionally high evapotranspiration rate. This placed the poplar plants under extreme stress, leading to decreased survival rates in certain poplar varieties. Similar findings have been reported by results from Fischer et al. and Danila et al. [37,50].

Regardless of poplar variety, the number of shoots per stool increased with each harvest. Results from Schlepphorst et al. [51], Hartmann [43], and Landgraf et al. [39] describe that this fact does not only depend on the variety but also on planting density and site conditions. Zou et al. [33] also demonstrated in their study of willow clones that the increase in shoot number, the greater height gains and the larger diameters (DBH) are related to an increase in the stool diameter. After the third harvest in 2018, the tripling of the shoot number observed by Röhle et al. [52] was also present in some poplar varieties in this study. It should be noted, however, that a significantly reduced survival rate was observed in the varieties with the highest resprouting capacity. It can therefore be assumed that these poplar varieties utilize the greater supply of light, water, and nutrients to produce a larger number of shoots. Based on the results of Paris et al. [53], it can be concluded that poplars in SRC can compensate for losses of individual plants and increase yield.

A number of studies have described the biomass yields of poplar SRC as a function of stem and leaf traits (e. g. total leaf area and individual leaf area) as well as plant height and diameter [11,39,53–55]. These findings are in agreement with our results. After three rotations (nine years), positive correlations between biomass yield and growth parameters (height, DBH, etc.) were observed. Poplar plantations can be tailored to end-use: high-density SRC for energy (chips, combustion) vs. lower-density systems for material production (e.g., peel wood for veneer or pulp for cellulose). Our results suggest that management (rotation length, planting density) should be adjusted with these end uses in mind, especially under future climate conditions. Different cultivation systems are associated with these different purposes, e. g. AFS or SRC with varying rotation systems (mini-, midi- or maxi-rotation). Yields can range from 2.0 to 22.0 Mg ha⁻¹ y⁻¹ DM [1,11, 37,47,54–56]. The monetary returns from poplar wood from SRC differ considerably depending on the utilization (material or energetic production, or a combination) and are also subject to local and temporal fluctuations in the timber market (e. g. after damage events affecting

large forest areas) [57]. Nevertheless, an average yield of 8 Mg ha⁻¹ y⁻¹ DM has been established as a benchmark for economically viable SRC (e. g. Rae et al. [58], Schiberna et al. [59], Landgraf et al. [39]).

## 4.2. Site-related influencing factors

When interpreting the effects of a site on the survival and growth of different poplar varieties, climate change and associated meteorological events cannot be ignored. The first rotation primarily serves to establish the poplars at a site. Thus, the relatively low yields recorded at the first harvest are not concerning. A further yield increase during the second rotation was observed in only 50 % of the poplar varieties studied (i. e. Max 4, AF 18, and AF 16), as reported in the literature [60]. These varieties increased their yields from 11.5 to 15.6 Mg ha⁻¹ y⁻¹ DM under the given site conditions. However, other poplar varieties achieved significantly lower yield increases (Weser 6, Max 1, and AF 6). Most poplar varieties achieved yields of approximately 8 Mg ha⁻¹ y⁻¹ DM. Some of these varieties increased their yield in the second rotation, while others did not. Since no active management measures were carried out in the second rotation and no major biotic damage was observed, it is assumed that the recorded yields correspond to the variety-specific yields at the study site. Poplar varieties that had already exhibited low yields of approximately 4 Mg ha⁻¹ y⁻¹ DM in the first rotation (e. g. 4 x Göttingen, Androscoggin, Mühle Larsen, and Esch 5) experienced a further decline in yield during the second rotation. This decline was demonstrated by decreases in all observed phenological parameters (i. e. survival rate, number of shoots per vine, growth height, and DBH). Even in the third rotation, no yield increase was observed for these varieties and must therefore be considered unsuitable for SRC at this site.

Assessing the yield for the 2018 harvest, the poplar varieties can be classified into high-yielding and low-yielding varieties [39]. The high yields of Max 3, Max 4, and Matrix 24 are in accordance with results by other authors [61–63], as are the low yields of poplar varieties with *P. nigra* parentage (e. g. Robusta, Jacometti 78B, AF 2, AF 6, and I 214) [63]. The latter varieties are known to be susceptible to fungal diseases, particularly poplar rust, which can significantly impair their growth [64, 65]. The very poor growth of AF 15 and AF 20 varieties was attributed to very low survival rates. Since no biotic damage, neither fungal nor insect, was observed in either variety, we assume that they were not adapted to the specific site conditions. Surprisingly, the yield results for the 2021 harvest revealed that only 16 of the 37 poplar varieties, e. g. AF 19, AF 2, AF 13, and AF 18, showed yield increases reported in scientific literature. These varieties represent less than half of those tested [48]. Positive correlations were again found between biomass yield and plant growth parameters (e. g. plant height and DBH). However, it is important to note, that for many varieties that experienced a yield increase during this period, the survival rate decreased significantly. For example, after nine years, the survival rates were 32 % for AF 8, 33 % for AF 19, and 43 % for AF 2. This translates into more light, water, and nutrients available per remaining plant. Thus, the reduction in plant density resulted in increased growth of the surviving plants. The effect of stand density on yield has been previously described [66], and this study confirms it. Examining the yield decreases of many varieties, e. g. Max 3, Fritzi Pauley, Max 4, and AF 17, in the third rotation in 2021, it is evident that these varieties exhibit a relatively high survival rate alongside lower shoot height gains and DBH values. This yield decrease can likely be attributed to the three dry years of 2018, 2019, and 2020. The consecutive drought conditions likely led to a significant decrease in groundwater, impacting stand growth. However, this cannot be statistically verified due to the lack of pertinent data. We hypothesize that the very high survival (and thus high stand density) of these varieties, combined with their high water use (evapotranspiration), exacerbated water scarcity during the drought years. In other words, dense stands likely ran out of available water, which ultimately reduced yields. The dependence of biomass yield on water availability has been previously
described in the literature [37,50]. The highest biomass yields in 2021 were achieved by poplar varieties that had not been particularly note-worthy in the previous rotation. This is coupled with a permanent decline in the survival rate of these poplar varieties. It can therefore be concluded that a high number of poplars per unit area is not ideal for stable biomass yields in times of climate change and the associated increased risk of drought in the Northeast German Lowlands.

## 5. Conclusions

The yield data collected from the third rotation revealed significant differences compared to the data from the second rotation. Our findings underscore that the potential impacts of climate change in the Northeast German Lowlands and other regions should be a critical factor in selecting poplar varieties for SRC cultivation. Our results clearly reflect the three consecutive dry years from 2018 to 2020. Therefore, poplar short-rotation coppice plantations should only be established in loca-tions where sufficient water is available, either through sufficient rain-fall or a high groundwater level. It should also be noted that prolonged drought periods can lower the groundwater level. This means that pre-cipitation deficits limit poplar growth.

It also became evident that SRC with a lower number of poplar plants per unit area experience a less pronounced yield decline during droughts. Our results clearly indicate that the traditional high-density, short-rotation (3-year) approach (up to 10,000 stems per ha) is not suitable under these dry conditions. Instead, we recommend establish-ing fewer plants per hectare and employing longer rotations of 6–10 years (midi-rotation).

Some varieties (i. e. Androscoggin, Muhle Larsen, Esch 5, and 4 x Göttingen) failed to perform well in the third rotation and are therefore not sustainable for light sandy sites with such dry conditions in the Northeast German Lowlands. In contrast, the relatively newly bred Italian poplar varieties AF 2, AF 13, AF 18, and AF 19 were particularly successful in coping with the very warm and dry conditions during the study period. Their biomass increase was primarily attributed to the plant growth parameters of height and DBH. Well-known poplar vari-eties such as Max, Robusta, Monviso, and Heidemij showed average yields which are at least economically viable. Nevertheless, the conse-quences of climate change are becoming increasingly evident, high-lighting the need to consider the use of new poplar varieties in combination with reduced planting densities when establishing new SRCs.

## CRediT authorship contribution statement

**Dirk Landgraf:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Simon Thomas:** Resources, Methodology, Data curation. **Markus Neupert:** Writing – original draft, Visualization, Validation, Software, Formal analysis.

## Funding

This research was funded by the German Federal Ministry of Edu-cation and Research under the grant number 0315972G for the first three years. In the following three years it was financially supported by the registered association "Biomasse Schraden e. V."

## Conflicts of interest

The authors declare no conflicts of interest.

## Acknowledgments

We want to sincerely thank the employees of the P&P Forest Service

GmbH for their practical help in the field. Our personal thanks go to Christiane Helbig for her proofreading of the text. Furthermore, we would like to thank the two unknown reviewers who made a significant contribution to the qualitative improvement of this paper.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.biombioe.2026.109140.

## Data availability

Data will be made available on request.

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