Age trends of genetic parameters and genotype-by-environment interactions for growth traits of Eucalyptus urophylla clones in South-China

Experimental sites

Three trials were established in Guangxi Zhuang Autonomous Region, China: Shankou (SK) Forestry Farm (21°35′N, 109°42′E), Shankou Township, Hepu County of Beihai City; Tiantang (TT) Village, Qinlian Forestry Farm (21°52′N, 109°21′E), Hepu County of Beihai City; and, Xiniujiao (XNJ) (21°44′N, 108°44′E) Township in Qinzhou City. All three trial sites were within 9 to 10 kilometers of the coast sea (straight-line distance). Details on the geography and climate of these three sites were acquired from local county weather bureaus and presented in Table 1. The geographical and climatic conditions for the three trial sites are very similar. The three sites have altitudes between 25–50 m, mean annual temperatures of 20.0–22.3 °C, mean annual rainfalls of 1,594–2,104 mm and mean annual evaporation of around 1,133–1,340 mm, and all three sites are affected by south subtropical monsoons in spring-summer.

The soils at TT and XNJ sites are calcareous soils developing from basalt according to the parental materials, and the soil at the SK site is epeiric sea sedimentary soil, furthermore, the soils at SK, TT, and XNJ are aeolian sandy, lime, and phosphor calcic soils, respectively. Based on the soil texture, however, the soil at SK is sandy, with a clay content of about 260 g/kg, the soil at TT is loamy soil with a clay content of 456 g/kg, and the soil at XNJ is clay soil in which clay content is about 705 g/kg. Some years ago, severe soil erosion had occurred at XNJ due to a long period of inappropriate plow and cultivation and this has resulted in a thin soil in which the average depth is about 0.30–0.50 m from surface to bottom of horizon. One soil sample of about 500 g was collected at the actual depth from 0 to 0.50 m according to the five-point sampling mode at each site in April 2011.

Information on key soil nutrients and properties at each of the sites are presented in Table 2, based on one mixed homogeneously sample and measurements. Obviously, soils at the three trial sites were lower in nitrogen, phosphorus, and potassium. Even so, the pH values at the three sites differed markedly with values of 6.7 at SK, 5.2 at TT, and 7.8 at XNJ, indicating weakly acidic soil at SK, acidic soil at TT, and alkaline soil at XNJ. The average depth from the surface to parent material at three sites is about 2 m at SK, 1.5 m at TT, and 0.5 m at the XNJ site. The dominant understorey vegetation within the trials consisted mainly of Leucas mollissima Wall. Var. Chinensis Benth., Rhodomyrtus tomentosa, Dicranopteris dichotoma Bernh., and other thorn shrubs. Previous vegetation was a plantation of E. urophylla and E. tereticornis natural hybridization progeny at SK and TT site, and a plantation of Pinus massoniana Lamb. at the XNJ site.

At each trial site, planting pits (50 cm long, 50 cm wide, and 40 cm high) had been dug in rows along contour lines. In each pit 0.25 kg superphosphate and 0.25 kg of a special eucalypt compound fertilizer mix (N:P: K =15:15:5) were applied before trees were planted to ensure every individual tree had adequate nutrients for at least the first three months after planting.

Genotypes

Twenty clones were planted at the three trial sites in April 2003. These had been bred or selected at the Research Institute of Tropical Forestry of the Chinese Academy of Forestry, the Leizhou Forestry Bureau of Guangdong Province in China, and the Dongmen Forestry Farm of Guangxi Province in China. These clones could represent five taxa: E. urophylla, E. urophylla × E. grandis, E. urophylla × E. camaldulensis, E. urophylla × E. tereticornis and E. leizhouensis No. 1 × E. urophylla. Details of the 20 clones are presented in Table 3.

Methods

The field experiments were designed in a randomized complete block with 10 replications with each clone represented by a row plot of five trees in each replicate. Plots were aligned along the contour line with a between-tree spacing of 2 m, and the distance between rows was 4 m. Thus, the trials were established with initial stockings of 1,250 trees per hectare.

Data collection

Growth traits, including total height (H in m), diameter at breast height over bark (DBH in cm), and survival were assessed at 6 (0.5 years), 12 (1 year), 18 (1.5 years), 30 (2.5 years), 42 (3.5 years), 56 (4.7 years), 68 (5.7 years) and 95 (7.9 years) months after the seedlings were planted. H was assessed using a hypsometer and DBH was measured with a diameter tape. Individual tree volume over bark (V in m³) was calculated using the following equation (Wu et al., 2011; He et al., 2012; Hodge & Dvorak, 2015): (1)${\displaystyle V=H\times DB{H}^{\text{2}}/30,000}$

Statistical analysis

The H, DBH and V values per clone for each year (ranged from 0.5 to 7.9 years) on each site were collected and applied to a linear model (Hansen & Roulund, 1996; Zhu, 2000; Wang, 2006): (2)${\displaystyle Y_{\mathrm{ijkl}}=\mathrm{\mu }+S_{\mathrm{i}}+B_{\mathrm{j(i)}}+F_{\mathrm{k}}+S{F}_{\mathrm{ik}}+B{F}_{\mathrm{jk}}+SB{F}_{\mathrm{iik}}+e_{\mathrm{ijkl}},}$ where i, j, k and l indicate the observed numbers of sites, blocks, clones, and plots, respectively,

Y_ijkl is the performance of the ramet of the kth clone of the lth plot in the jth block within the ith site,

µis the overall mean of the experimental trial, S_i is the fixed effect of the ith site,

B_j(i) is the fixed effect of the jth block within the ith site,

F_k is the random effect of the kth clone,

SF_ik is the random effect of the interaction between the kth clone and the ith site,

BF_jk is the random effect of the interaction between the jth block and the kth clone,

SBF_jik is the random effect of the interaction between the kth clone and the jth block within the ith site, and

e_ijkl is the random error of the lth individual in the kth clone within the jth block of the ith site.

Clone repeatability was calculated with the equation of broad sense heritability of family, using the following equation (Xu, 1988): (3)${\displaystyle {H_{\mathrm{c}}}^2={{\sigma }_{\mathrm{c}}}^2/\left({{\sigma }_{\mathrm{c}}}^2+{{\sigma }_{\mathrm{e}}}^2/r\right),}$ The single tree’s repeatability was estimated with the formula of broad sense heritability, which was calculated using the following formula (Xu, 1988): (4)${\displaystyle {H_{\mathrm{s}}}^2=4{{\sigma }_{\mathrm{c}}}^2/\left({{\sigma }_{\mathrm{c}}}^2+{{\sigma }_{\mathrm{e}}}^2\right),}$ where σ_c² and σ_s² indicate the variance of the clone and the single tree; σ_e² indicates the error of the variance; and r indicates the number of replicates.

Genetic gain was calculated using the following equation (Huang & Xie, 2001): (5)${\displaystyle \Delta G=H_{\mathrm{c}}^2\times S\times X^{-1}}$ (6)${\displaystyle S=\left(x-X\right),}$

where S is the skewness between the mean of the superior clones and that of the control clones, X is the mean value of the control clone, x is the mean value of the superior clones, and h_c² is equivalent to clone repeatability (broad sense heritability or H_c²) of the clones.

Variances of growth traits for H, DBH, and V across seven assessment ages at the three experimental sites were analyzed using GLM procedures in the SAS statistical software (SAS Institute Inc, 1990; Huang & Xie, 2001). Due to the unbalanced data, type-B genetic correlation parameters and genetic gains were estimated using GLM procedures in the SAS statistical software (Huber, White & Hodge, 1994; Huang & Xie, 2001). Curves of means of growth trait across ages at the three sites were simulated using SPSS statistics 21.0 (SPSS ver. 21.0; Armonk, NY, USA), with the equation used to describe the growth trends being selected based on maximizing R2, lower sums of mean squares residual errors, and standard errors.

Average trends of survival and growth traits

The trends in survival rates (%) from 0.5 to 7.9 years old at the three sites were similar (Table 4), with a moderate decrease from 96.9 to 72.5, from 94.8 to 77.6, and from 75.3 to 52.4 at SK, TT, and XNJ, respectively. Significant differences in survival rates were observed among the tree ages at each site. The survival rates reached a plateau from the beginning of 4.7 years old at XNJ. However, survival rates at SK and TT tended to be stable levels from 5.7 to 7.9 years old. Interestingly, the average survivals at SK and TT were similar and significantly higher than that of XNJ at every tree age.

The H at the three sites increased with age across nearly eight years, which can be expressed for SK, TT, and XNJ using the following equations, respectively:

y₁ = 7.0973ln(x) + 5.4412, R² = 0.9685;

y₂ = 7.3883ln(x) + 5.8401, R² = 0.9717; and

y₃ = 2.2964ln(x) +2.5609, R² = 0.9276.

The average H growth of E. urophylla clones at SK, TT, and XNJ increased sharply from 0.5 to 3.5 years old and then increased smoothly in the following years (Table 5). Significant differences in H growth were found among the tree ages at each site. Moreover, the monthly mean increments of H growth reached the highest levels at the three sites during 1−1.5 years old. The H growth of SK and TT was remarkably higher than that of XNJ during the investigation. The H growth of E. urophylla clones at the three sites at 4.7 years (around one rotation) varied from 15.46 to 18.04 m. DBH over bark across nearly eight years at the three sites increased with age, which can be expressed for SK, TT, and XNJ with the following equations, respectively:

y₄ = 4.9138ln(x) + 4.5095, R² = 0.9973;

y₅ = 5.4723ln(x) + 4.2311, R² = 0.9917; and

y₆ = 4.7483ln(x) + 4.4367, R² = 0.9914.

The average DBH growth of E. urophylla clones increased rapidly with age up to 2.5 years old and slowly from 2.5 to 7.9 years old (Table 6). An obvious difference in DBH growth at each site over these years. The DBH growth showed a remarkable difference among the three sites from 0.5 to 5.7 years old. Additionally, the average monthly increments in DBH growth exhibited the maximum during 1–1.5 years old at TT and XNJ, while its peak value of SK was from 0.5 to 1.0 years old. The DBH growth of E. urophylla clones at 4.7 years (about one rotation) differed significantly by the sites, with the highest in XNJ (12.77 cm), followed by TT (12.23 cm) and SK (11.92 cm).

For V over bark, the average trends over nearly eight years at the three sites followed a consistent pattern (Table 7). The curves for average V at SK, TT, and XNJ can be expressed with the following equations, respectively:

y₇ = 0.0215x - 0.0158, R² = 0.9921;

y₈ = 0.0235x - 0.0178, R² = 0.9906; and

y₉ = 0.0201x - 0.0141, R² = 0.9849.

The average V growth increased moderately with age from the beginning. There were striking differences in V growth at each site over these years. Apart from 5.7 years old, the V growth of the three sites showed a significant difference at the investigation ages. For individual tree volume over bark, the growth of E. urophylla clones at the three sites at 4.7 years (about one rotation) varied from 0.0920 (XNJ) to 0.1002 m³ (TT).

Trends in H_c² for growth traits

Trends in broad sense heritability (H_c²) for all the growth traits (H, DBH over bark, and V) followed consistent patterns (Fig. 1, Fig. 2, and Fig. 3). The values of all H_c²’s at TT were dramatically greater than those at SK or XNJ, and those at SK were greater than those at XNJ. H_c² curves for growth traits across ages at TT were relatively stable, except for a slight increase at early ages. The H_c²’s at SK increased at early ages significantly and after a short plateau, started to fluctuate slightly with age to 7.9 years old. In contrast, H_c²’s at XNJ undulated largely at a relatively low level from 0.5 to 7.9 years old.

For H, the H_c² for E. urophylla clones varied from 0.437 to 0.970 at SK, 0.940 to 0.973 at TT, 0.303 to 0.775 at XNJ. For DBH over bark, the H_c² varied from 0.452 to 0.962 at SK, 0.854 to 0.972 at TT, and 0.250 to 0.801 at XNJ. For V, the H_c² varied from 0.564 to 0.942, 0.915 to 0.985, and 0.200 to 0.732 at SK, XNJ and TT, respectively.

Genetic correlations

Genetic correlations among the same growth traits (H, DBH over bark and V) at the same age were assessed, pairwise, between the sites. For the growth traits, the peak values of genetic correlations were between SK and TT at each age (Table 8). The genetic correlations for the growth traits between pairs of sites increased at early ages, then decreased, and finally increased again with age, except for V between SK and TT, which increased at early ages before starting to decrease with age. The strongest genetic correlations between two sites in H, DBH over bark, and V were found in 2.5 years (0.96), 7.9 years (0.91), and 1.5 years (0.95), respectively.

Genetic gains

Estimated mean genetic gains for H, DBH over bark, and V, as would be achieved by selecting the top five of the 20 clones at the three sites, increased with age at first and then decreased after peaking between 1.5 and 4.7 years of age (Table 9). The highest values of genetic gains at the same age were reached at the TT site.