Introduction

Declining availability of wood supplies and increasing demand have prompted interest in fast-growing plantations around the world. Woody biomass can be accumulated in short rotation coppicing systems through intensive management (Fenning and Gershenzon 2002). Populus × euramericana cv. ‘74/76’ (hereafter poplar 107) is among the new fast-growing varieties selected and bred from 331 lines introduced from 17 different countries by the Chinese Academy of Forestry. Poplar 107 has the advantages of rapid growth, good stem form, excellent wood properties, strong stress resistance, abundant asexual reproduction, and a short rotation period. It is an excellent hybrid for pulpwood and ecological protection forests. It is widely planted in northern China, Huanghuai, and other regions; in Hebei Province alone, poplar 107 covers 32.3% of the total area of poplar (Zhang and Wang 2014). Therefore, it is necessary to study the properties of poplar 107 to maximize its economic value and utilization.

A soil type and fertilization experiment revealed that sandy loam was more conducive to the growth of poplar 107 than a mixed soil and clay (He 2006). Fertilization had clear effects on height, diameter at breast height (DBH), and volume growth whereas the method of fertilization had no significant effects on height increases (He 2006). The results of comparative tests of poplar 107 and Populus × euramericana CL. ‘Zhonglin46’ in northern Henan Province conducted by Zhang et al. (2006), demonstrated that the growth of poplar 107 in alluvial aeolian sandy soil was significant. A study of 3-year-old poplar 107 saplings by He (2008), revealed that mean volume increment was greatest in sandy medium-thick soil. Zhang (2015) reported that the growth of poplar 107 differed significantly among soil types and was best in loamy fluvo-aquic soils, followed by sandy fluvo-aquic and clayey fluvo-aquic soils. In addition to research on the growth of poplar 107, studies have been conducted on sap flow (Li et al. 2013; Mo et al. 2014; Kong et al. 2020), pulping performance (Yang and Yao 2007; Zhang et al. 2009; Meng et al. 2010; Zhou et al. 2011), wood anatomy and wood properties (Zhou et al. 2010; Liu and Liu 2011; Zhao et al. 2014; Guan et al. 2021).

Wood properties are important factors in predicting the suitability of a product’s end use and therefore its value (Cassidy et al. 2013). These are affected by the environment such as temperature, precipitation, soil conditions, competition, planting and thinning density, slope position and direction, and interactions among these factors (Guo et al. 2002a, 2002b; Shi et al. 2011). Monteoliva and Senisterra (2008) found that site can significantly influence Populus growth traits and wood density. Some climatic site conditions have shown a weak influence on vessel variables and a strong influence on fiber and parenchyma tissues in other species (Fichtler and Worbes 2012). However, it is still not clear how site factors influence the wood properties of poplar 107. Therefore, in this study, climatic factors were examined and soil chemical properties quantified in poplar 107 plantations at different sites on Hebei Plain, China, to understand their effects on the growth and properties of heartwood and sapwood. The objective was to provide a reference for site selection and plantation management.

Materials and methods

Sampling locations

In July 2017, poplar 107 plantations were randomly sampled at 24 sites. The plantations were 6–9 years old and extensively managed, with an average stand density of 802 trees. ha–2. The information for each sampling site is shown in Fig. 1. Table 1 lists meteorological data for 2012–2017 from the Hebei and Tianjin Meteorological Bureau.

Fig. 1
figure 1

Distribution of sites

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Table 1 Meteorological data for the growth of poplar 107
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Measurement of soil chemical properties

Using a 2 cm diameter soil drill, samples (0–40 cm) were collected from five points in each site and mixed into a single sample of approximately 1 kg. Roots, debris, and gravel were removed and the samples stored in a cool location to dry naturally.

Organic matter content was determined using the potassium (K) dichromate volumetric (external heating) method. Total nitrogen (N) was assessed using the Kjeldahl method, and hydrolytic N using alkali hydrolysis diffusion. Total phosphorus (P) was measured using the molybdenum antimony resistance colorimetric method and available P extracted using 0.50 mol L–1 sodium bicarbonate and measured by the molybdenum antimony resistance colorimetric method. Total K was determined using the flame photometric method and available K was extracted with 1 mol L–1 ammonium acetate and measured by the flame photometric method. Soil pH was determined with an electronic pH meter (PHS-3C, Shanghai INESA Scientific Instrument Co., Ltd., Shanghai, China). Each sample was measured three times, and an average value taken.

Measurement of growth traits

Ten trees were randomly selected at each site to measure DBH, total height, branch-free height, and crown width. A diameter ruler was used to measure DBH, an altimeter to measure total height, and a meter ruler to measure the height under the branches and crown width.

Measurement of wood chemical properties

A 5 cm thick disk was removed at breast height and heartwood and sapwood separated. The material was air-dried and cut into small slices, pulverized, passed through a 0.30 mm mesh screen, and stored for later use. The contents of cellulose, hemicellulose, and lignin of the heartwood and sapwood were measured using the method described by Yin et al. (2017). The content of cold/hot water and NaOH extractives were measured following National Standards of the People’s Republic of China GB/T 2677.4-93 (1993a) and GB/T 2677.5-93 (1993b), respectively. Each sample was measured three times, and the average taken.

Measurement of wood physical and mechanical properties

At each sampling site, one individual tree of average growth was selected. Sampling methods followed the standards of National Standards of the People’s Republic of China GB/T 1927–2009 (2009a). Rate of shrinkage of the heartwood and sapwood were measured in accordance with National Standards of the People’s Republic of China GB/T 1932–2009 (2009b).

Wood density of the heartwood and sapwood were determined following National Standards of the People’s Republic of China GB/T 1933–2009 (2009c).

Bending strength refers to the maximum degree to which wood can withstand gradually applied bending loads. The bending elastic modulus represents the rigidity or elasticity of the wood; a large value indicates hard wood. Compressive strength parallel to the grain represents the degree to which wood can withstand pressure loads along the direction of the grain, and is an important indicator of the mechanical properties of wood. Bending strength, bending elastic modulus, and compressive strength parallel to the grain of heartwood and sapwood were measured following National Standards of the People’s Republic of China GB/T 1936–1-2009 (2009d), GB/T 1936–2-2009 (2009e), and GB/T 1935–2009 (2009f), respectively.

The comprehensive strength of wood is a composite indicator based on the sum of bending strength and compressive strength parallel to grain, and it is an important strength indicator used to evaluate load-bearing capacity.

The quality coefficient of wood, which is the ratio between mechanical strength and basic density, is an important parameter for evaluating wood quality. The sum of the compressive strength parallel to grain and bending strength is the comprehensive quality coefficient of wood.

Data analysis

All data were entered into Microsoft Excel for analysis. SPSS 20.0 software (IBM, Armonk, NY, USA) was used for analysis of variance (ANOVA) and the Duncan test to analyze variations of soil chemical properties, and chemical, physical, and mechanical properties of heartwood and sapwood.

Coefficient of variation (CV) was calculated using the formula (Hai et al. 2008):

$$CV \, \; = \;\frac{SD}{\bar{X}}$$
(1)

where \(\bar{X}\) and SD are the mean and standard deviation of growth traits and wood properties, respectively.

Spearman’s correlation coefficients between growth traits and wood properties, and the Mantel test between wood properties and environmental factors were calculated using the Vegan package of R language. Stepwise regression was performed in SPSS 20.0 software to analyze partial correlation coefficients between environmental factors and growth traits and wood properties.

Results

Variations in soil nutrients

Soil organic matter was 3.8–21.2 g kg–1, total K 3.5–13.0 g kg–1, total P 0.1–0.9 g kg–1, total N 0.2–0.9 g kg–1, pH 6.5–9.0, available P 0.5–26.2 mg kg–1, available K 25.0–75.8 mg kg–1, and hydrolytic N 12.6–120.5 mg kg–1 (Table 2). The chemical properties of soil differed substantially among sites (p < 0.05).

Table 2 Soil chemical properties
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Variations in annual growth

Mean annual growth increments were standardized to create a heatmap showing annual average growth. Red indicates larger values and blue smaller ones. Growth indicators differed considerably among sites (Fig. 2). Mean annual height increment was 2.5 m, and the coefficient of variation (CV) 14.5%. Mean annual DBH increment was 2.8 cm, with a CV of 10.6%, and mean annual volume increment was 0.04 m3, with a CV of 22.4%. Differences in mean annual increments in volume among sites were greater than differences in heights and DBH. The analysis of variance showed significant differences in growth traits among sites (p < 0.05), indicating that poplar 107 could adapt to a wide range of conditions.

Fig. 2
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Heatmap of the mean annual increments of poplar 107

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Variations in wood chemical properties

Cellulose, hemicellulose, and lignin contents in the heartwood were 56.7%, 23.3%, and 15.0%, respectively, with CVs of 4.3%, 10.7%, and 11.1% (Fig. 3, Table S1), and in the sapwood 58.1%, 21.8%, and 14.2%, respectively, with CVs of 3.2%, 8.6%, and 8.3%. Cold water, hot water, and NaOH extractives of the heartwood were 6.7%, 6.8%, and 19.6%, respectively, with CVs of 21.6%, 30.2%, and 10.6% (Fig. 3, Table S1), and of the sapwood 5.9%, 6.5%, and 20.8%, respectively, with CVs of 27.5%, 22.4%, and 7.8%.

Fig. 3
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Chemical properties of poplar 107; the bar is the mean value and the red dots represent individual data points

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Analysis of variance showed that there were significant differences and variations in the chemical composition of the heartwood and sapwood of poplar 107 among sites (p < 0.05) (Table S1).

Variations in wood physical and mechanical properties

The rates of radial, chordwise, and volume shrinkage of oven-dried heartwood were 5.2%, 6.8%, and 12.0%, respectively, with CVs of 23.4%, 16.8%, and 14.6% (Fig. 4a, Table S2) and for air-dried heartwood, 4.3%, 5.4%, and 10.0%, respectively, with CVs of 31.7%, 21.0%, and 21.5% (Fig. 4b, Table S2). The rates of radial, chordwise, and volume shrinkage of oven-dried sapwood were 4.6%, 6.7%, and 11.4%, respectively, with CVs of 23.1%, 13.8%, and 10.9% and for air-dried sapwood, 3.8%, 5.5%, and 10.0%, respectively, with CVs of 29.9%, 15.2%, and 15.4% (Fig. 4a, b, Table S2).

Fig. 4
figure 4

Physical and mechanical properties of poplar 107; a dry shrinkage rate of oven-dried, b dry shrinkage rate of air-dried, c density, d mechanical properties; the bar is the mean value, and the red dots represent individual data points

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Wood density is an important indicator of wood quality, and can include basic, air-dried, and oven-dried density; values for heartwood were 0.39, 0.42, and 0.40 g cm–3, respectively, with CVs of 9.0%, 8.6%, and 9.0% (Fig. 4c, Table S3). Values for sapwood were 0.40, 0.43, and 0.41 g cm–3, respectively, with CVs of 6.8%, 6.4%, and 6.4% (Fig. 4c, Table S3).

Bending strength and bending elastic modulus of the heartwood were 66.8 MPa and 9.0 GPa, respectively, with CVs of 15.4% and 15.0% (Fig. 4d, Table S4). The bending strength and bending elastic modulus of the sapwood were 76.2 MPa and 10.3 GPa, respectively, with CVs of 10.6% and 10.7% (Fig. 4d, Table S4). Compressive strength parallel to grain of the heartwood and sapwood were 35.7 MPa and 40.0 MPa, respectively, with CVs of 17.9% and 9.8% (Fig. 4d, Table S4). The comprehensive strength of the heartwood and sapwood were 102.5 MPa and 116.2 MPa, respectively, with CVs of 14.9% and 9.7% (Table S5). The comprehensive quality coefficients of the heartwood and sapwood were 263.3 and 288.3, respectively, with CVs of 10.6% and 6.0% (Table S5).

Analysis of variance showed that there were significant differences in the physical and mechanical indicators of the heartwood and sapwood of poplar 107 among sites (p < 0.05) (Tables S2, S3 and S4).

Correlations among growth, wood properties and environmental factors

Pairwise comparison of growth and wood property indicators revealed varying degrees of correlation: 97 pairs were highly significantly correlated (p < 0.01), while another 43 were significantly correlated (p < 0.05; Fig. 5). Among these, air-dried, oven-dried, and basic densities of the heartwood were highly significantly and positively correlated with bending strength and compressive strength parallel to grain, and significantly and positively correlated with the bending elastic modulus. Air-dried, oven-dried, and basic densities of the sapwood were highly significantly and positively correlated with bending strength, bending elastic modulus, and compressive strength parallel to grain.

Fig. 5
figure 5

Correlation heatmap showing pairwise comparisons of indicators of growth and wood properties, with color gradient denoting Spearman’s correlation coefficients. Soil chemical properties and climatic factors were related to each indicator by partial Mantel tests. Edge width corresponds to Mantel’s r statistic for the corresponding distance correlations, and edge color denotes the statistical significance based on 9,999 permutations

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Climatic factors and soil chemical properties were analyzed, and the bending elastic modulus of the sapwood was highly significantly correlated with climate. Chordwise shrinkage rates of oven-dried and air-dried heartwood, lignin content of the sapwood, radial and volume shrinkage rates of oven-dried sapwood, and chordwise shrinkage rate of air-dried sapwood were all significantly correlated with climatic factors. Heartwood contents of hemicellulose and cellulose were significantly correlated with soil chemical properties. Other indicators were not significantly correlated with environmental factors.

Effects of environmental factors on growth and wood properties

Stepwise regression (Table 3, Table S6) revealed that mean annual relative humidity and sunshine duration, pH, and total K had the greatest effects on growth. Soil pH had a negative effect on mean annual height increment. Total K, mean annual relative humidity, and sunshine duration had a negative effect on mean annual increment DBH.

Table 3 Partial correlation coefficients of the growth indicators of environmental factors
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For the chemical properties, the stepwise regression (Table 4, Table S7) revealed that total and available soil P were the primary factors affecting the heartwood. The effects of available P on hemicellulose and the hot water extractives of the heartwood were positive, whereas the effects of total P were negative. Available P and hydrolytic N were the primary factors affecting the chemical properties of the sapwood. Available P negatively affected the contents of lignin and cold-water extractives of the sapwood, but positively affected NaOH extractives. Hydrolytic N positively affected the lignin content of the sapwood but was negative to the NaOH extractives. Available and total P, and hydrolytic and total N, along with annual precipitation, were the main factors affecting the chemical properties of poplar 107.

Table 4 Partial correlation coefficients of the chemical indicators of environmental factors
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Effects of environmental factors on the physical and mechanical properties

The stepwise regression (Table 5, Table S8) showed that pH, total K, and annual ground temperatures were the primary factors affecting the heartwood physical and mechanical properties. Densities (air-dried, oven-dried, and basic density) of heartwood were negatively affected by total K, but positively affected by pH and annual ground temperatures. Soil organic matter, pH, and annual precipitation were the main factors affecting the sapwood. Soil organic matter had a negative effect on shrinkage rate but positive effects on basic density, bending elastic modulus, and compressive strength parallel to grain. The pH had positive effects on sapwood densities, and bending strength, and compressive strength parallel to grain, whereas annual precipitation had a negative effect. Soil pH, soil organic matter, total K, and annual ground temperatures were the main factors affecting the physical and mechanical properties of poplar 107.

Table 5 Partial correlation coefficients of the physical and mechanical indicators of environmental factors
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Discussion

Effects of environmental factors on growth

In this study, the CVs of the mean annual increments of height, DBH, and volume of poplar 107 in different regions were 14.5%, 10.6% and 22.4%, respectively. Higher phenotypic variation values indicated that there was a significant environmental effect on growth traits and the wide adaptability of poplar 107 on Hebei Plain. Site factors represent the basic conditions for the growth and development of the hybrids, exhibiting different patterns under different conditions. Soil factors were not only the basis of site conditions but also affect the provision of nutrients for survival. Studies have shown that soil N is closely related to growth and is involved in the synthesis of chlorophyll, enzymes, amino acids, and other substances. Zhang et al. (2016) demonstrated that hydrolytic N and available K promoted the growth of young Liriodendron chinense × tulipifera stands. Paoli et al. (2008) and Shang et al. (2019) found that total N and P played an important role in the growth of Larix gmelina var. principis-rupprechtii (Mayr) Pilg. Wu et al. (2004) demonstrated that hydrolytic N and available P were key soil nutrients affecting the growth of Eucalyptus grandis Hill. In this study, that the growth of poplar 107 was significantly and negatively correlated with soil K and pH, indicating that, within a certain range, a lower pH promoted better growth. Although N did not promote the growth of poplar 107, pH was closely related to the absorption of N, mostly in the form of NH4+ and NO3– (Zhang et al. 2010), but NH4+ was easily adsorbed by colloids and easily lost, and might possibly be fixed in the crystal lattice of clay minerals. Therefore, NO3– is the primary source of N for plants (Zhao et al. 2008). Absorption requires the secretion of OH– ions by roots to maintain the charge balance outside cell membranes, resulting in a large amount entering the soil and leading to increased pH around the root systems. Higher pH values were associated with weaker OH– extravasation, resulting in a weakening of the plant’s capacity to absorb N element. Additionally, excessively high pH inhibited the growth of microorganisms, reduced the activity of soil enzymes, and affected the conversion of soil nutrients, thus affecting growth (Rietz and Haynes 2003). It is important to note that K acts as an important electrolyte and plays a critical role in maintaining osmotic pressure inside and outside cells. The absorption of K by plants reduces osmotic pressure inside and outside the cell, weakens OH– extravasation and ultimately weakens nitrogen absorption capacity of roots.

In addition to being closely related to soil factors, growth in this study was also affected by the combined effects of tree characteristics, soil density, and climatic factors. Li et al. (2005) concluded that light was the dominant factor affecting the growth of Syringa oblata Lindl., followed by moisture. In a study of fast-growing, high-yielding Populus stands, Zou et al. (2001) found that the dominant climatic factor affecting height and volume growth was moisture followed by light, whereas warm temperatures had the least influence. Mean annual DBH increment was significantly and negatively correlated with moisture (relative humidity) and light (annual sunshine duration). This might be the result of multiple factors, such as light intensity, transpiration, and plant midday depression of photosynthesis.

Effects of environmental factors on wood chemical properties

During the process of wood formation, leaves absorb CO2 and convert it into sugars. Through a series of biochemical reactions, these sugars are converted into cellulose, hemicellulose, lignin, and other constituents that make up wood. Cellulose, hemicellulose, and lignin are the main load-bearing components of cell walls, and the basis for the value of raw materials. The cellulose, hemicellulose, and lignin contents of poplar heartwood and sapwood are important criteria for assessing its utilization (Guo et al. 2020). Higher content of cellulose and fiber crystallinity are associated with higher yield and quality of pulp. Hemicellulose has better swelling in water, beneficial for pulping and enhancing bonds between fibers in the pulp, thus improving its physical properties. Lignin is associated with wood hardness and durability. However, the oxidation of lignin during bioenergy conversion, pulping, and paper production causes yellowing, and high lignin levels increases energy consumption and pollution (Silva et al. 2009; Yin et al. 2010). Cellulose and hemicellulose contents of poplar 107 heartwood and sapwood were higher than those of clone Nanlin-895 (Zhang et al. 2021), Michelia fujianensis Q. F. Zheng (Li 2000), Quercus variabilis BI., Q. acutissima Carruth. (Chen et al. 2019), and L. gmelina Rupr. (Sun et al. 1994), whereas the content of lignin was lower, indicating that poplar 107 may be good for pulp and paper production.

Extractives occur mainly in the tissue fluid of cells and are the product of the physiological metabolism of trees. Their content is important for evaluating wood utilization and provides a basis for exploring plant metabolic processes. The content of extractives of the heartwood was generally higher than that of the sapwood. Contents of cold and hot water extractives of the heartwood were slightly higher than that of the sapwood, while the level of NaOH extractives of the heartwood was slightly lower than that of the sapwood. The presence of extractives increases the amount of chemicals required in the pulping of raw fiber materials, affect pulp color, cause resin barriers, and reduce the raw material utilization rate of fiberboard. The content of NaOH extractives indicate, to some degree, the content of ellagic acid polyphenols because these consume substantial amounts of alkali during kraft cooking. Therefore, NaOH extractives negatively affect pulping performance (Kong and Wei 2004). A study by Shebani et al. (2008) also reported that the presence of extractives could affect the thermal stability of wood.

An increase in mean annual precipitation increased the cellulose content of poplar 107, and that increased total P in soil led to a decrease in the contents of hemicellulose and hot water extractives of the heartwood. Insufficient precipitation might reduce the rate of cell division and shorten the time for cell differentiation, resulting in a reduction of the growing season, inhibiting cell expansion, and smaller lumen diameters, which in turn reduce the cellulose content (Cheng et al. 2015). Studies have reported that site conditions had a direct impact on chemical properties of trees: poor site conditions led to lower cellulose and holocellulose and higher levels of extractives (Sun et al. 1994; Wang et al. 1997). In addition, cell age is also a factor that affects the chemical properties of wood. There were higher contents of lignin and NaOH extractives in juvenile wood compared to mature wood (Pitre et al. 2007; Wu et al. 2011).

Effects of environmental factors on wood physical and mechanical properties

In this study, environmental factors mainly affected the chordwise shrinkage of poplar 107 wood, but had little effect on radial shrinkage. Due to the characteristics of the structure and chemical composition of wood, radial shrinkage is smaller than chordwise shrinkage, so environmental factors have little influence on radial shrinkage. Yang et al. (2012) found that reduced precipitation led to a reduction in wood shrinkage, a similar finding in this study. They also demonstrated that an increase in total K, available P, and pH increased in shrinkage rates. This contrasts with the results of this study.

Wood density is generally considered to be the most important indicator of wood quality, and is correlated with the physical and mechanical properties of wood (Macdonald and Hubert 2002; Gjerdrum and Eikenes 2014; Schimleck et al. 2018). At the same time, an increase in wood density has an important effect on the efficiency of pulp production (Niemczyk and Thomas 2020). There were significant or highly significant positive correlations between the density of the heartwood and sapwood and mechanical properties. Mean annual precipitation had a negative effect on sapwood density, whereas mean annual ground temperatures had a positive effect on heartwood density. Yang et al. (2012) found that a reduction in mean annual precipitation increased the basic density of wood. Vega et al. (2021) reported that precipitation, followed by temperature, was the most important climate variable affecting wood density. There was a negative correlation between precipitation and wood density in angiosperms (Drew et al. 2017; de Lima Costa et al. 2020) because reductions in available water resulted in smaller lumens and thicker-walled cells. This plasticity might reduce water transmission efficiency but it was considered an adaptive mechanism because it increased the resistance of the water transport system to cavitation, resulting in increased density (Mitchell et al. 2015; Nabais et al. 2018). Temperatures typically had a positive effect on wood density (Thomas et al. 2007; Drew et al. 2017; Zhang et al. 2022), the higher the temperature, the earlier cambial activity begins, and as cell division accelerates, more wood cells are produced which increases density (Xu et al. 2011). Density is also related to other factors such as species, location of tree trunks, age, annual growth, and genetic factors (Wang et al. 1984, 2011).

Jiang and Yang (2012) reported that reduced soil N and pH improved the bending elastic modulus and bending strength of wood. Bending strength decreased with an increase in soil K and P, mean annual precipitation, and mean annual sunshine duration. Soil pH promoted improved bending strength and compressive strength parallel to grain of the sapwood of poplar 107. Bending strength increased with an increase soil hydrolytic N and annual mean ground temperatures. The mechanical properties of wood are mainly affected by moisture, density, and atmospheric temperature and humidity. The effects of the latter two are not found in this study, but may be due to the complicated effects of temperature. In general, when the temperature change range is small, the impact on mechanical properties is small. Only in the case of high temperatures or extreme low temperatures will it have a greater impact on the mechanical properties.

In this study, the coefficients of variation of the chemical, physical, and mechanical properties of poplar 107 heartwood ranged from 4.3 to 31.6%, and the coefficients of variation of the sapwood ranged from 3.2 to 29.9%. Analysis of variance showed that there were significant differences in the three properties of the heartwood and sapwood among sites. Soil and climatic conditions are the basic factors that influence the growth and development of trees, which in turn affect wood quality (Chave 2004; Zha et al. 2005; Jiang and Yang 2012).

Conclusions

The results of one-way ANOVA showed that there were significant differences in growth traits and wood properties among sites. Soil pH, total and available phosphorus, total potassium, and soil organic matter were the key factors affecting growth and wood properties of poplar 107, whereas mean annual ground temperature and precipitation were the main climatic factors. To better cultivate poplar 107, an area with less annual rainfall, slightly higher temperature and soil pH close to neutral should be selected. By exploring the effects of different site conditions on the growth and wood properties of poplar 107 in Hebei Plain, this study identified the factors affecting the growth and wood properties, and provided a basis for the scientific management and quality improvement of poplar 107 plantations.