Above-Ground Biomass and Nutrient Accumulation in Ten Eucalyptus Clones in Leizhou Peninsula, Southern China

Abstract

Selecting suitable clones and regulating nutrients for Eucalyptus plantation are a key management practice for improving productivity and nutrient use. Therefore, this study evaluated growth performance, above-ground biomass, nutrient content (nitrogen, phosphorus, potassium, calcium, and magnesium) and nutrient use efficiency (NUE) of ten Eucalyptus Clones for three sites in Leizhou Peninsula. The present study showed a significant genetic variation among clones in growth parameters. Organs have different characteristics in biomass and nutrient content. Stemwood had the highest biomass but lowest total nutrient concentration. While, the stembark exhibited high contents of nutrients and biomass. On average, the NUE of clones was in the following order: phosphorus > magnesium > nitrogen > potassium > calcium. Taken together, among ten clones tested, clones LH1-9211, TH9224, DH32-13, M1 and DH32-22 showed consistently growth and production performance, as well, their NUE were superior in ten clones. Maximum amount of biomass was allocated to economically harvestable component (stemwood) and nutrients in non-harvestable components (stembark and foliage). While nutrients are removed from the planting area as part of the harvest, we can calculate nutrients loss by NUE and biomass. These findings provided useful insights for selection of Eucalyptus Clones and regulating nutrient export during the harvest of Eucalyptus Clones from a planted forest system.

1. Introduction

As global demand for wood increases and reduction in timber supply of natural forests, planted forests will play a vital role in shortage of wood raw material and protection of natural forests [1]. Without using superior genetic clones, the yield of traditional forest plantations was likely low and uneven quality. Genetic improvement of forest trees could obtain locally-adapted clones which character of higher production, stress tolerance etc. [2]. Therefore, studies of clonal propagation have been made toward in many tree species [3], including Populus [4,5], Eucalyptus [6,7], Pinus taeda L. [8], Pinus radiata D.Don [9], Picea asperata Mast. [10], Cunninghamia lanceolata (Lamb.) Hook. [11], Tectona grandis L.F. [12], Salix babylonica L. [13].

Quantification of forest biomass can determinate the production potential, predict forest yields [14]. Nutrients, one of largely foundations of biomass production, play a vital role in forestry system management [15]. Consequently, in order to develop proper approaches of nutrient management for improving biomass production, it is necessary to understand the nutrient accumulation and storage characteristics in planted forests. Meanwhile, much effort has been put into choosing species that achieve maximum biomass production for a given location by maximizing the uptake of nutrients has been made in many efforts [16,17].

Eucalypts, a kind of fast-growing trees, are the most widely cultivated forest trees in the world [1]. These fast-growing plantations, annual growth rates that routinely exceed 35 m³·ha⁻¹·year⁻¹, are widely used in pulp, fuel and solid wood products [18]. China, the third largest area of Eucalypt plantations, produced about 30 million m³ of Eucalypt wood, for a grand total of 27% annual timber output [19]. Eucalyptus sp. highest productivity rates up to 49 m³·ha⁻¹·year⁻¹, while the mean yield in China is about 20 m³·ha⁻¹·year⁻¹ [20]. Therefore, proper genotype selection and planting proper trees in proper areas are effective ways to improve productivity of Eucalyptus in China. Wood harvest, the main nutrient loss in the Eucalyptus forests land, may lead to declining Eucalyptus productivity and sustainability [18,21]. Biomass productivity and site nutrient removal must be appreciated. Thus, researchers in Brazil, India, Australia, Congo and elsewhere have done a lot of researches in order to maintain or increase productivity of Eucalyptus in a sustainable approach through the selection of genotypes and using suitable strategies of plantation management [17,22,23,24]. Related researches have also been done by Chinese researchers [25,26]. However, research in Leizhou Peninsula, the main planting area in China, has not been reported.

In this paper, we present the results of a study on growth performance, eucalypt above-ground biomass, nutrient content (nitrogen, phosphorus, potassium, calcium, and magnesium) and nutrient use efficiency (NUE) for three representative sites in Leizhou Peninsula. The objectives were to: (i) examine the clonal variation in growth and biomass production. (ii) understand the allocation pattern of biomass and nutrients in tree components. (iii) determine the nutrient use efficiency in ten Eucalyptus genotypes. (iv) identify possible Eucalyptus selections suitable for use in this region of Leizhou Peninsula. The study will provide useful insights for selection of Eucalyptus Clones and regulation of nutrient export during the harvest of Eucalyptus Clones from a plantation system.

2. Materials and Methods

2.1. Trial Description

Three plantations were established on Leizhou Peninsula, China. The locations and descriptions of the three clonal trials are presented in

Table 1. Study area description.

Site Characteristics	Sites
	Wuchuan	Tangjia	Suixi
Latitude	21°18′37′′ N	20°50′01′′ N	21°21′81′′ N
Longitude	110°31′13′′ E	109°51′76′′ E	110°01′66′′ E
Altitude (m)	34	31	40
Mean annual rainfall (mm)	1597.8	1787.5	1759.4
Mean temperature (°C)	22.5	23.7	23.5
Soil texture	Haplic acrisols	Haplic acrisols	Haplic acrisols
Soil pH	6.14	5.84	5.31
Soil Organic matter (g/kg)	4.22	3.14	5.67
Available N (mg/kg)	26.68	29.47	34.26
Available P (mg/kg)	1.27	1.85	1.9
Available K (mg/kg)	14.13	11.79	23.2

. Ten hybrid eucalypt clones were planted in April–May 2012 and most of them were widely used in southern China, especially for hybrids of E. urophylla and E. grandis. Clones were derived from a number of different species including E. urophylla, E. grandis, Eucalyptus camaldulensis Dehnh, Eucalyptus tereticornis Sm. (

Table 2. Parentage of studied Eucalyptus Clones.

Clone	Species	Origin
DH201-2	E. urophylla × E. camaldulensis	Dongmen Forestry Farm, China
DH32-13	E. urophylla × E. grandis	Dongmen Forestry Farm, China
DH32-22	E. urophylla × E. grandis	Dongmen Forestry Farm, China
DH32-26	E. urophylla × E. grandis	Dongmen Forestry Farm, China
DH33-27	E. urophylla × E. grandis	Dongmen Forestry Farm, China
LH1-9211	E. urophylla × E. Tereticornis	RITF/CAF
M1	E. urophylla × E. Tereticornis	Leizhou Forestry Bureau, China
SH1	E. leizhou No. 1	Leizhou Forestry Bureau, China
TH9224	E. urophylla × E. Tereticornis	RITF/CAF
U6	Nature hybrid	Zhangjiang Forestry Bureau, China

RITF/CAF represents Research Institute of Tropical Forestry, Chinese Academy of Forestry, China.

). The formerly widely planted U6 clone which was bred in early 1990s was used as a check (CK) comparison. Randomized complete block design with three replications and 10 treatments (Eucalypt clones) respectively, was used to set up at Wuchuan, Tangjia and Suixia. All saplings were planted at spacing of 3.0 m × 2.0 m and planting pits (50 cm × 50 cm × 40 cm) were prepared. In order to grow well, one kilogram Norwegian compound fertilizer (N15P15K15, Yara International, Oslo, Norway) was applied to each individual tree in the first two years.

2.2. Tree Growth and Biomass of Living Plants

Data on survival, Diameter at Breast Height (DBH), height and biomass were collected at the logging age of 6 years in 2018. Thirty trees per clone (three trees at least in three replicates distributed throughout nine basal area classes defined from the inventory) were cut down, and the major components were isolated: stemwood, stembark, branches, foliage. Fresh weights of these components of thirty trees were determined in Eucalyptus plantations. All component samples were oven dried at 70 °C to a constant weight. The hectare biomass was calculated by individual tree biomass [15].

2.3. Chemical Sample Analyses

The same organ samples from the same plot were mixed together as one sample. Nine replicate samples were applied to each clone. Before the chemical analyses, every sample was dried at 70 °C, ground and homogenised. Total N was analysed by acid-base volumetry after Kjeldahl mineralization [27]. P was determined by cold-colorimetry from the Murphy and Rileyreagents [28]. K, Ca and Mg were analysed by atomic absorption spectrophotometry [29,30].

2.4. Nutrient Use Efficiency (NUE)

The values of nutrient use efficiency (NUE) were obtained by dividing the amount of biomass of each component and the amount of nutrient from each biomass component, according to the equation [14]:

$$\mathrm{NUE}=\frac{(\mathrm{Amount} \mathrm{of} \mathrm{biomass})}{(\mathrm{Amount} \mathrm{of} \mathrm{nutrient})}$$

(1)

where: NUE; biomass (kg·ha⁻²); nutrient (kg·ha⁻²). Nutrient amount (kg·ha⁻²) was obtained by multiplying nutrient concentrations (kg·kg⁻¹) and biomass (kg·ha⁻²).

2.5. Statistical Analysis

All data on tree survival, Diameter at Breast Height (DBH), height and biomass were analyzed using a two-way analysis of variance (ANOVA). Whenever the ANOVA indicated a significant difference between the means (p < 0.05), these were compared using the least significant difference (LSD) starting from the multiple range test (Tukey HSD) for mean separation analysis. The statistical program used in the analyses was SPSS (SPSS software version 23.0, Chicago, IL, USA).

3. Results

3.1. Growth Performance

Diagrams of stem analysis showed the growth patterns of diameter and height for ten Eucalyptus Clones. The diameter and height growth performance were different among ten clones (Figure 1).

Furthermore, the results of growth performance evaluation of these clones at the time of felling were carried out for height, diameter at breast height and survival (Figure 2). As shown in Figure 2A, the highest value of height belonged to clone LH1-9211 (17.46 m) followed by TH9224 (16.51 m), DH32-13 (16.48 m) and SH1 (16.39 m). The lowest values belonged to clone DH32-26 (13.19 m), U6 (13.02 m), DH201-2 (12.47 m). Clones LH1-9211, TH9224, DH32-13, SH1, M1 and DH32-22 achieved significantly higher height compared to the other eucalypt clones.

Results showed that DBH varied between 9.15 and 12.57 cm (Figure 2B). Maximum DBH was attained by clone DH32-13. The lowest values belonged to clone DH201-2. The DBH of DH32-13, LH1-9211, DH32-22, M1 and TH9224 were significantly higher than others.

Survival rate ranging from 78.93 % to 98.09 % among ten clones (Figure 2C). Clones M1, LH1-9211, SH1, TH9224, DH32-13 and U6 had a survival rate of more than 90%.

3.2. Biomass Accumulation and Distribution

Total biomass differed significantly among clones, and ranged from 29.30 to 77.15 t·ha⁻² (Figure 3). The total biomass in different clones followed the order: LH1-9211, DH32-13, DH32-22, TH9224, M1, SH1, U6, DH32-26, DH33-27 and DH201-2. The best clone LH1-9211 produced 60.59%, 70.45%, 72.08%, 56.46% and 111.33% more than clones SH1, U6, DH32-26, DH33-27 and DH201-2, respectively. Clones LH1-9211, DH32-13, DH32-22, TH9224 and M1 achieved significantly higher total biomass compared to the rest of eucalypt clones.

In all clones, the biomass was mainly accumulated in the stemwood, which accounted for 49.83% to 72.05% of the total biomass (Figure 4). The five clones with the highest ratio of stemwood biomass to total biomass were DH32-13, DH33-27, SH1, M1 and DH32-26. The percentage contribution of stem bark to the total biomass ranged from 18.12% to 35.05%, while that of branches were generally low, 3.73–13.99%. The contribution of foliage to the total biomass was ≤4.76%. As a whole, the biomass distribution among different tree components was as follows: stemwood > stembark > branches > foliage.

3.3. Nutrient Concentration and Content

Nutrient concentrations varied according to tree components (

Table 3. Mean concentrations ± S.D. (g·kg⁻¹) of major nutrients in different tree components in ten Eucalyptus Clones.

Clone	Organs	N	P	K	Ca	Mg
DH201-2	Stem wood	0.91 ± 0.14 abD	0.05 ± 0.01 cC	0.80 ± 0.25 bD	0.48 ± 0.06 abcB	0.10 ± 0.02 bC
DH32-13		0.93 ± 0.17 abD	0.05 ± 0.01 cC	0.80 ± 0.26 bD	0.46 ± 0.12 abcC	0.10 ± 0.03 bD
DH32-22		0.98 ± 0.20 abD	0.06 ± 0.02 bcD	1.45 ± 0.73 aB	0.66 ± 0.11 aB	0.15 ± 0.02 aC
DH32-26		1.18 ± 0.60 aC	0.07 ± 0.02 bcC	0.99 ± 0.50 abC	0.50 ± 0.08 abcC	0.11 ± 0.02 abC
DH33-27		0.98 ± 0.28 abC	0.07 ± 0.02 bcC	0.93 ± 0.48 abC	0.45 ± 0.17 bcB	0.13 ± 0.05 abB
LH1-9211		0.78 ± 0.14 bD	0.05 ± 0.02 cB	0.48 ± 0.16 bC	0.34 ± 0.09 cB	0.09 ± 0.03 bB
M1		0.77 ± 0.11 bC	0.06 ± 0.02 bcC	0.49 ± 0.12 bC	0.40 ± 0.10 cC	0.10 ± 0.02 bC
SH1		0.83 ± 0.17 abB	0.17 ± 0.06 aC	0.67 ± 0.25 bC	0.65 ± 0.19 abB	0.09 ± 0.02 bB
TH9224		0.71 ± 0.13 bB	0.11 ± 0.03 abC	0.62 ± 0.11 bC	0.50 ± 0.13 abcC	0.10 ± 0.02 bB
U6		0.75 ± 0.14 bC	0.15 ± 0.06 aC	0.87 ± 0.22 bC	0.43 ± 0.15 cB	0.10 ± 0.03 bC
H201-2	Stembark	3.48 ± 0.42 abB	0.54 ± 0.36 bB	5.32 ± 1.66 cB	19.32 ± 9.16 abA	2.79 ± 0.61 aB
DH32-13		3.66 ± 0.43 aB	0.99 ± 0.43 abB	7.03 ± 1.44 abcB	21.42 ± 6.19 abA	2.62 ± 0.45 abA
DH32-22		3.62 ± 0.43 aB	0.41 ± 0.17 cB	9.41 ± 4.82 abA	31.65 ± 10.97 aA	2.56 ± 0.52 abB
DH32-26		3.28 ± 0.66 abcB	0.42 ± 0.17 cB	8.91 ± 3.37 abB	21.46 ± 6.66 abA	2.51 ± 0.44 abA
DH33-27		2.92 ± 0.38 bcdB	0.67 ± 0.20 abcB	5.89 ± 1.03 abcB	23.19 ± 20.04 abA	1.96 ± 0.67 bcA
LH1-9211		3.05 ± 0.27 abcdB	1.09 ± 0.58 abcA	4.36 ± 0.80 cB	6.96 ± 3.80 bA	1.50 ± 0.31 cA
M1		2.78 ± 0.36 cdeB	0.94 ± 0.37 abB	4.30 ± 1.55 cB	15.84 ± 9.12 abA	1.42 ± 0.40 cB
SH1		2.74 ± 0.32 cdeB	1.25 ± 0.61 aB	6.47 ± 1.01 abcB	26.42 ± 16.17 aA	2.02 ± 0.21 abcA
TH9224		2.25 ± 0.23 eB	1.16 ± 0.45 abB	5.28 ± 1.53 cB	19.47 ± 10.61 abA	1.87 ± 0.67 bA
U6		2.52 ± 0.25 deB	0.84 ± 0.43 abcB	5.61 ± 0.86 bB	20.63 ± 12.24 abA	1.65 ± 0.44 cB
H201-2	Branches	2.14 ± 0.47 abC	0.24 ± 0.08 cdeC	2.18 ± 1.10 cdC	1.59 ± 0.57 bcB	0.28 ± 0.08 bC
DH32-13		2.40 ± 0.36 aC	0.30 ± 0.16 bcC	1.99 ± 0.61 cdeC	1.70 ± 0.51 abcBC	0.49 ± 0.19 aC
DH32-22		2.03 ± 0.29 abcC	0.25 ± 0.09 cdC	2.54 ± 0.72 bcB	2.10 ± 0.83 abB	0.36 ± 0.12 abC
DH32-26		1.77 ± 0.38 bcC	0.49 ± 0.21 abB	3.84 ± 1.00 abC	2.98 ± 2.10 abBC	0.26 ± 0.10 bC
DH33-27		1.76 ± 0.36 bcC	0.54 ± 0.21 aB	4.91 ± 1.61 aB	3.04 ± 1.02 aB	0.38 ± 0.11 abB
LH1-9211		1.55 ± 0.34 cdC	0.28 ± 0.15 cB	2.54 ± 1.00 bcB	1.68 ± 0.80 abcB	0.27 ± 0.10 bB
M1		0.96 ± 0.10 eC	0.05 ± 0.01 eC	0.63 ± 0.12 efC	0.39 ± 0.12 cC	0.08 ± 0.02 cC
SH1		0.90 ± 0.14 eB	0.05 ± 0.01 eC	0.59 ± 0.12 fC	0.38 ± 0.06 cB	0.07 ± 0.01 cB
TH9224		0.94 ± 0.29 eB	0.05 ± 0.01 deC	0.70 ± 0.27 efC	0.51 ± 0.14 cC	0.09 ± 0.02 cB
U6		1.07 ± 0.32 deC	0.07 ± 0.03 deC	0.83 ± 0.57 defC	0.38 ± 0.14 cB	0.08 ± 0.03 cC
H201-2	Foliage	18.79 ± 1.95 bcA	1.56 ± 0.21 bcdA	8.64 ± 1.17 eA	4.73 ± 1.16 cdB	2.08 ± 0.31 abA
DH32-13		19.12 ± 0.92 abcA	1.40 ± 0.16 cdeA	8.87 ± 1.34 eA	4.18 ± 0.88 dB	1.93 ± 0.17 bB
DH32-22		17.46 ± 2.02 cdA	1.33 ± 0.11 cdeA	13.98 ± 3.87 bcdA	6.59 ± 1.63 abcB	1.88 ± 0.49 bA
DH32-26		14.00 ± 1.78 dA	1.09 ± 0.15 eA	13.00 ± 1.78 cdeA	8.64 ± 1.34 aB	1.84 ± 0.53 bB
DH33-27		17.31 ± 1.04 cdA	1.25 ± 0.24 deA	9.88 ± 1.17 deA	6.67 ± 1.66 abcB	1.89 ± 0.59 bA
LH1-9211		18.17 ± 2.49 bcA	1.38 ± 0.26 cdeA	11.99 ± 2.05 cdeA	5.99 ± 0.56 bcdA	1.81 ± 0.26 bA
M1		20.84 ± 2.02 abcA	1.63 ± 0.35 abcA	10.96 ± 2.00 deA	7.00 ± 1.46 abB	2.01 ± 0.20 bA
SH1		22.61 ± 3.33 aA	1.81 ± 0.34 abA	18.57 ± 6.25 abA	8.38 ± 1.57 aB	2.24 ± 0.61 aA
TH9224		21.43 ± 3.40 abA	1.67 ± 0.17 abcA	19.49 ± 4.01 aA	7.91 ± 0.87 abB	2.11 ± 0.43 abA
U6		22.55 ± 2.26 aA	1.98 ± 0.19 aA	16.43 ± 1.92 abcA	7.66 ± 1.44 abB	2.50 ± 0.49 aA

Significant differences within each variable among clones were tested using Tukey’s HSD test (0.05 level); different small letters denote differences in means by the same organ only among clones; different capital letters denote differences in means by the same clone only among organs. S.D. is standard deviation of the mean.

). At all clones, N, P, K, and Mg were concentrated in leaves, while Ca was concentrated in stembark. Total nutrient concentration is generally in the following order: foliage > stembark > branches > stemtwood. The concentrations of N, P, K and Mg in foliage were 21.85, 18.64, 16.17, and 18.80 times in the stemtwood, respectively. The amount of calcium in the stembark was 42.03 times that in the stemtwood. In general, there were little differences in nutrient concentrations among clones in each organ.

Nutrient contents varied between genotypes and between different components within genotype (Figure 5). On average, nutrient contents of above ground woody biomass were accumulated in the biomass 112.64 kg·ha⁻² of N, 18.76 kg·ha⁻² of P, 147.08 kg·ha⁻² of K, 316.89 kg·ha⁻² of Ca and 36.90 kg·ha⁻² of Mg, with the following order for nutrients Ca > N > K > Mg > P. The highest amounts of N and P were found in clone LH1-9211. The highest amounts of K, Ca and Mg were found in clone DH32-22, and the lowest amounts were found in clone DH201-2.

Analyzing the total amounts of nutrients allocated in the biomass (Figure 6), it is observed that, among the genotypes, the stembark had the highest concentrations of nutrients and the branches the lowest concentrations, while the stemwood and foliage exhibited intermediate values. The stembark had the highest concentrations of nutrients. The contents of each component in nitrogen were stembark (30.19–41.56%), foliage (17.94–44.14%), stem wood (15.46–39.42%) and branches (1.76–12.03%) in order. The contents of each component in phosphorus were as follows stembark (52.52–75.52%) > foliage (17.94–44.14%) > stem wood (15.46–39.42%) > branches (1.76–12.03%). The contents of each component in potassium were stembark (44.47–65.23%), stem wood (12.79–27.15%), foliage (9.60–24.36%) and branches (0.73–20.95%) in order. The contents of each component in calcium were as follows stembark (78.39–92.23%) > stemwood (2.75–7.77%) > foliage (1.47–9.10%) > branches (0.18–5.93%). The contents of each component in magnesium were stembark (68.35–82.30%), foliage (5.01–18.33%), stem wood (6.31–16.91%), and branches (0.41–7.11%) in order.

3.4. Nutrient Use Efficiency

Genotypes showed variations in different nutrients of NUE (

Table 4. Nutrient use efficiency for ten Eucalyptus Clones.

Clone	NUE
	N	P	K	Ca	Mg
DH201-2	405.90 ± 36.18 d	4126.13 ± 1063.62 abc	414.67 ± 68.15 abc	196.79 ± 51.78 bc	1143.61 ± 188.61 c
DH32-13	522.62 ± 25.97 abc	3903.66 ± 1381.95 abc	432.17 ± 60.006 abc	206.33 ± 46.05 bc	1473.34 ± 238.47 bc
DH32-22	418.05 ± 30.37 d	4753.30 ± 965.93 a	270.40 ± 85.69 d	122.29 ± 37.90 d	1178.22 ± 156.78 c
DH32-26	474.70 ± 60.26 bcd	4664.32 ± 1122.51 a	290.72 ± 81.08 d	170.51 ± 61.90 bc	1262.36 ± 180.37 c
DH33-27	583.47 ± 82.00 a	4258.00 ± 1007.61 ab	448.78 ± 124.77 ab	353.80 ± 228.67 ab	2047.84 ± 486.30 a
LH1-9211	458.84 ± 27.85 cd	2781.32 ± 1075.74 cd	458.57 ± 63.86 ab	445.67 ± 157.87 a	1764.94 ± 313.63 ab
M1	447.45 ± 33.52 d	2968.38 ± 663.24 bcd	536.92 ± 122.82 a	290.36 ± 156.31 abc	1975.79 ± 327.04 a
SH1	530.47 ± 54.32 ab	2320.25 ± 744.11 d	371.52 ± 27.57 bcd	186.62 ± 186.62 bc	1501.40 ± 128.61 bc
TH9224	577.89 ± 51.03 a	2347.03 ± 718.94 d	412.02 ± 84.99 abc	216.95 ± 146.92 bc	1606.64 ± 517.45 abc
U6	421.73 ± 25.90 d	2371.05 ± 729.27 d	315.08 ± 39.43 cd	184.24 ± 108.39 bc	1394.10 ± 314.22 bc

Significant differences within each variable among clones were tested using Tukey’s HSD test (0.05 level); different small letters denote differences in means among clones.

). On average, the NUE was in the following order: phosphorus > magnesium > nitrogen > potassium > calcium. In the case of nitrogen use efficiency, clones DH32-13, DH 33-27, SH1, TH9224 were significantly higher than others. The highest value of phosphorus use efficiency belonged to clone DH32-22, DH32-26, DH33-27. The clones of DH32-13, DH33-27, LH1-9211 and M1 recorded the highest value in potassium use efficiency. The magnitude of calcium use efficiency, in decreasing order, was as follows: LH1-9211 > M1 > DH33-27 > DH32-13 > DH201-2 > TH9224 > DH32-26 > U6 > SH1 > DH32-22. In the case of magnesium use efficiency, clones DH33-27, M1 were significantly higher than others.

4. Discussion

4.1. Growth Performance and Biomass Production of Ten Eucalyptus Clones

Genotype selection of Eucalyptus based on its growth performance and wood production. The present study showed a significant genetic variation among clones in growth and biomass parameters. Those different performance may be attributed to genotypes and interactions with environmental factors [22] and it can be candidate for future plantations by selecting excellent clones [15].

The tree height of 6-year-old Eucalyptus ranged from 12.47 to 17.46 m, DBH varied between 9.15 and 12.57 cm, survival rate ranged from 78.93 % to 98.09 %. Similar to the growth of Eucalyptus trees studied by Dlamini et al. [31]. and Guzmán et al. [32].

Among ten clones tested, clone LH1-9211 showed consistently higher growth, larger DBH and higher survival. Clones TH9224, DH32-13, M1 and DH32-22 also exhibited larger DBH and higher survival. Xu et al. [33] studied the growth performance of 19 clones of Eucalyptus at Hainan, China. Clones TH9113, TH9117, TH9211 and TH9224 were recorded significantly higher height growth as compared to other clones. Similarly, Cao et al. [34] evaluated different clones of Eucalyptus for different growth characters. Further, they identified Clone M1, Clone GL9 and DH32-22 as superior genotypes than others. Such hereditary stability is appropriate for further selection and application in Leizhou Peninsula [22], even in southern China.

Here, the above-ground biomass of 6-year-old Eucalyptus ranged from 29.30 to 77.15 t·ha⁻². Such report was also recorded by several researchers on different Eucalyptus species [35,36]. The above-ground portion of the Eucalyptus tree is the main part of the timber harvest [37]. The greater the abovementioned biomass, the more abundant the available timber resources for industrial use. Results showed that clones LH1-9211, DH32-13, DH32-22, TH9224 and M1 had a larger biomass production.

Overall, Clones LH1-9211, TH9224, DH32-13, M1 and DH32-22 exhibited well growth and high biomass. Cultivating these Eucalyptus Clones is an excellent option causing its outstanding growth, high biomass production and early harvest within six years.

4.2. The Allocation Pattern of Biomass and Nutrients in Tree Components

In our study, the greatest contribution to total biomass was from the stemwood, followed by the stembark, branches, and foliage. The relative distribution of biomass, considering the same components, was in line with several previous findings [38,39]. As the canopy reaches a phase of relative stability, the stemwood becomes one of the main sink for photosynthetic products [40], it accounts for most of a tree’s nonstructural carbohydrates stock.

Nutrient concentrations (N, P, K and Mg) in different components of clones were in the order of: foliage > stembark > branches > stemtwood. While Ca was more concentrated in stembark. Higher nutrient concentrations mean vigorous growth in different organs. This same trend was also reported in E. urograndis at 1.5 years in Brazil [38]. Clones have differential nutrient requirements which result in significant variation in nutrient concentrations between eucalypts [15]. The standing state of N, P, K, Ca and Mg in different components of eucalypts clones are comparable to the broad range reported in different eucalypts clones by Couto et al. [17].

Soil nutrient condition and sustained production have become an important consideration in eucalypts plantations, where nutrients are removed through frequent harvests [41]. Heavy nutrient loss has an adverse impact on long-term site quality and sustained production [15]. The harvest of eucalypts clones at six years age will drain about 112.64 kg·ha⁻² of N, 18.76 kg·ha⁻² of P, 147.08 kg·ha⁻² of K, 316.89 kg·ha⁻² of Ca and 36.90 kg·ha⁻² of Mg, if above-ground trees are harvested. Among the genotypes, the stembark had the highest concentrations of nutrients and the branches the lowest concentrations, while the stemwood and foliage exhibited intermediate values.

As commercial component (stem) was harvested, and non-timber purposes (foliage, branches and stembark) were left in plantation, plenty of nutrients will be returned to the site. Nutrient management, logging slash remained will help to compensate for nutrient losses under eucalypts plantation [15].

4.3. The Nutrient Use Efficiency in Eucalyptus Clones

Our results showed the NUE of clones was in the following order: phosphorus > magnesium > nitrogen > potassium > calcium. Globally, phosphorus limits productivity of trees in many forests and plantations. The high efficiency means lower nutritional requirement by a species, therefore, improving NUE is one of the strategies for plants to adapt to nutrient deficiency [14]. In all clones, phosphorus use efficiency was more than two thousand, and magnesium use efficiency was beyond one thousand. Such results were reported by Santana et al. [42] and Santos et al. [14]. It is one of the important strategies for plants to adapt to nutrient deficiency. In our study, on the whole, clones DH32-13, DH32-22, DH33-27, LH1-9211, M1 and TH9224 had better performance in NUE.

In summary, in a short-rotation, high-yield eucalypts plantation system where nutrients removed may exceed natural nutrient inputs [43], those clones could become an important option of afforestation. Meanwhile, NUE provides an indication of the amounts of nutrients exported at the harvest [23]. Thus, this parameter become effective parameter for estimating nutrients requirements since biomass production is known [44].

5. Conclusions

Among ten clones tested, clones LH1-9211, TH9224, DH32-13, M1 and DH32-22 showed consistent growth and production performance, as well, their NUE were superior. Hence, it means that these several Eucalyptus Clones can be selected in the Leizhou peninsula, southern China. Maximum amount of biomass was allocated in economically harvestable component (stemwood) and nutrients in non-harvestable components (stembark and foliage). Such allocational patterns will reduce the nutrient loss from the site at the harvest of Eucalyptus. Further, it is suggested to retain foliage and branches and also debarking of stems should be done before removing the timber from site in order to minimize nutrient losses. Finally, nutrients are removed from the planting area as part of the harvest, we can calculate nutrients content by NUE and biomass. Forest managers could return soil nutrients through fertilization to maintain productivity of eucalypts plantation. These findings provide an applied work to be valuable for industrial forestry.