Comparison Selection and Progeny Test of Natural Populations of White Birch (Betula platyphylla Suk.) in Northern China

Shibo LAN

Abstract [Objectives] This study was conducted to rationally and effectively use natural genetic resources of white birch （Betula platyphylla Suk.）， and improve the scientific and technological content of improved varieties.

[Methods] With the progeny of the white birch populations naturally distributed in northern China as objects， based on genetic effect analysis and comprehensive evaluation， the genetic variation， heritability and genetic gain of the growth characters in the progeny forest were systematically analyzed， by which populations or individuals with good growth vigor and strong stress resistance and adaptability were finally selected.

[Results] There were extremely significant differences in the growth characters between the natural populations of white birch in northern China. The variation was the smallest in tree height， followed by crown width， and largest in DBH， and the average coefficients of variation were 20.56%， 32.25% and 33.00%， respectively. The differences in characters between progeny of the same population were also extremely significant. The growth characters of white birch were more controlled by genes， and the genetic gains were larger. The heritability of tree height， DBH and crown width was 0.971， 0.816 and 0.576， respectively， and the genetic gains were 15.97%， 21.54% and 13.87%， respectively.

[Conclusions] With the growth characters as the main evaluation indexes， taking various influencing factors into account， the Shanxi[SX]population， Hebei[HB] population and Liaoning[LN] population were selected to be excellent populations， and LN305， SX516 and HB403 were excellent individuals， which are advised to be widely used in multigeneration genetic improvement breeding.

Key words Betula platyphylla Suk.; Natural population; Comparison and selection; Progeny test; Genetic effect; Genetic gain

White birch （Betula platyphylla Suk.） is a widely distributed tree species and the main tree species that form natural forests in eastern Asia. It belongs to Betula Linn. of Betulaceae in the taxonomy of plant systems. White birch holds complex morphological variation and has good resistance to wind and drought[1]. It is widely distributed in Heilongjiang， Jilin， Liaoning， Inner Mongolia， Henan， Ningxia， Sichuan， Yunnan， Hebei， Shanxi， Gansu， Qinghai， southeastern Tibet and Shaanxi （autonomous region）， Russian Far East， East Siberia， East Mongolia， North Korea， and Japan[2]. White birch has strong vitality， strong ecological adaptability， and the biological and ecological characteristics such as deep rooting， strong budding power， good adaptability to intense light， cold tolerance and resistance to infertility， as well as the abilityto adapt various complex ecological environments by its own advantages， but it is not very demanding on soil conditions， and can grow on acid soil， marshland， arid sunny slope and wet shady slope. White birch is dense， moderate in hardness， straight in texture， uniform in structure， and high in material value， and is widely used in furniture manufacturing， interior decoration， artificial board processing and mechanical pulping. The litter of white birch is easy to decompose to form good humus， which can effectively improve soil fertility and achieve soil improvement. White birch can form natural birch juice， which is rich in carbohydrates， amino acids， organic acids and various inorganic salts necessary for human body， as well as essential oils， betulin， saponin compounds and cytokinins. Birch juice is the source of life of birch trees， and recognized as the most eutrophic physiologically active water in the world which has very obvious antifatigue and antiaging effects. Europeans call it "natural beer" and "forest drink". White birch is elegant with a beautiful posture and profound cultural connotation， and thus have very high ornamental characteristic. In recent years， the research of white birch mainly focused on seed source selection and zoning[3-5]， breeding and cultivation of improved varieties[6-8]， genetic diversity[9-10]， genetic variation[11]and genetic improvement[12]， growth patterns and growth processes[13]， ecological coupling[14]， the genetic effects of B. latyphylla variant， B. platyphylla var. mandshurica[15]and other related fields. Studies on the determination and genetic effects of white birch have not been reported. In view of this， with the progeny of the white birch populations naturally distributed in northern China as objects， based on genetic effect analysis and comprehensive evaluation， the genetic variation， heritability and genetic gain of the growth characters measured in the progeny test forest were systematically analyzed， by which populations or individuals with good growth vigor and strong stress resistance and adaptabilitywere finally selected. This will enrich the genetic diversity and genetic resources of timber species in northern China， improve the level of sustainable management and innovative utilization， and lay a foundation for the cultivation of artificially bred populations of this type of germplasms.

General Situation of the Experimental Field

Natural conduction of the provenances

The test materials originated from eight natural populations in Heilongjiang， Jilin， Liaoning， Inner Mongolia， Hebei， Henan， Shanxi and Ningxia in northern China. Northern China refers to the vast area located north of the Qinling-Huaihe line， south of the Inner Mongolia Plateau， east of Daxinganling and the Qinghai-Tibet Plateau. The area is bordered by the Bohai Sea and the Yellow Sea in the east， spanning the middle and warm temperate zones of China， covering the Northeast China， North China and Northwest China. The terrain is dominated by mountains， plains and plateaus， of which the mountains are humid areas， surrounded by water， the plains are semihumid areas with open and flat terrain having a low altitude， and the plateaus belong to the semihumid and semiarid areas where there are vertical and horizontal gullies and the soil erosion is serious. The climate in this region belongs to the temperate or subtropical monsoon climate. The four seasons are distinct， and there are obvious differences between the north and the south and between the east and the west. The spring is less rainy and dry; the summer is hot and rainy; the winter is cold and dry; and the ice period is gradually shortened from north to south. The average temperature in the coldest month is below 0 ℃; and the average annual precipitation is between 400 and 800 mm （except the Changbai Mountain）， and it is mostly concentrated in July and August， decreasing from east to west. The soil types are mainly dark brown soil， brown soil， cinnamon soil， loess， black earth and chestnut soil.

Natural conditions of the introduction site

The introduction site is located in Qingshan Improved Variety Breeding Base in Linkou County， Heilongjiang Province. It is located in the eastern mountainous area of Northeast China， belonging to the hilly area of the eastern slope of Zhangguangcai Mountain of Changbai Mountain， with following geographical coordinates： 130°20′-130°40′E， 45°17′-45 ° 30 ′N. The terrain is high in the northeast， low in the southwest， with an average slope of 10 ° -15 ° and a maximum slope of 40 °， and the altitude is 300-500 m. The climate in this region belongs to the north temperate continental monsoon climate. The winter is cold and dry and long; the summer is warm and humid and short， accompanied by more rain， with sufficient sunshine; and the frostfree period is 120-130 d. The annual average precipitation is 400-600 mm， and is mostly concentrated from June to August， during which the precipitation accounts for about 50% of the annual precipitation. The annual effective accumulated temperature （≥10 ℃） is 2 100-2 600 ℃. The zonal soil is typical dark brown soil with a deep soil layer， which is moist and fertile， and has strong permeability， which is conducive to the growth and development of forest trees.

Materials and Methods

Setting of the progeny test forest

The test materials were derived from eight natural northern Chinese white birch populations in Heilongjiang， Jilin， Liaoning， Inner Mongolia， Hebei， Henan， Shanxi and Ningxia， labeled as LJ， JL， LN， MG， HB， HN， SX and NX， respectively. In order to avoid the differences between environments， the original materials for population variation analysis， genetic effect evaluation， progeny determination and individual selection were selected from the same introduction site. The breeding materials were introduced in 2002， and the seedlings were raised in 2003. The progeny test forest was constructed in 2005， and the growth indexes were measured in 2017. The progeny test forest was designed according to the completely randomized block design. It included the progeny of 240 excellent individuals of the eight populations. In each plot， the row spacing was 1.5 m×2.0 m， and the land preparation specification was 50 cm×50 cm×40 cm. The test was carried out in 6 replicates， and the blocks were separated by Picea koraiensis Nakai.

Test indexes and determination methods

（1） Test indexes： The main test indexes include tree height， DBH， eastwest crown width and northsouth crown width.

（2） Determination methods： The caliper measurement method was applied to measure the growth of DBH in the forest （accuracy： 0.5 mm）. The height of trees was measured by the height gauge method （accuracy： 0.5 cm）. The eastwest crown width and northsouth crown width were measured by the tape measure method （accuracy： 0.5 cm）， and the average value of the two was taken as the crown width of the forest.

Statistic analysis of data

The statistical analysis SPSS 16.0 software was used to process the raw data[16]， and the variance analysis was performed using the mathematical model： Yijk=μ+Bi+Fj+BFij+εijk， where Yijk is the kthobserved value of the jthfamily in the ithblock; μ is the population mean; Bi is the block; Fj is the family; BFij is the interaction between the family and the block; and εijk is the random error. The genetic parameters and coefficient of variation of characters were estimated on the basis of the results of variance analysis[17-18]： （1） heritability （h2）=（MSf-MSe）/MSf， （2） genetic gain （ΔG）=Sh2/X， （3） selection intensity （i）=S/σP， and （4） coefficient of variation （CV）=100×σP/X.

Results and Analysis

Variation analysis and index investigation

There is a specific adaptation range of white birch in northern China. There is a certain correlation between forest growth and ecological factor， so the introduction of germplasms from different ecological environments should focus on the important selection index of variation. In order to comprehensively evaluate the variation degree of the progeny of natural white birch populations in China， the characters of tree height， DBH and crown width of 905 freepollinated progeny in eight populations were determined， statistically described and analyzed by variance. The results are shown in Table 1 and Table 2. The results of variance analysis showed that the natural distribution of white birch was intermittent， and there was a lack of extensive gene communication among the populations， resulting in large genetic differentiation and rich genetic basis. The tree height， DBH and crown width of the natural population progeny were basically in normal distribution. The significant probability （0.000） of interaction between populations， between blocks and between populations and blocks was much lower than 0.05， and the differences were very significant at the 5% level. Under the premise of basically the same site conditions， the introduction of exotic tree species with different ecological requirements is greatly affected by the slope direction， slope position， direct light， temperature and moisture， which is directly reflected in the quantitative trait， crown width， thus affecting the growth and development of the forest. This laid a theoretical foundation for the introduction， domestication and genetic improvement of exogenous tree species. It can be seen from Table 1 that in the alpine regions of China， the variation of tree height， crown width and DBH of the natural populations of white birch had different degrees. The variation was the small in tree height， larger in crown width and the largest in DBH. The coefficients of variation were 20.56%， 32.25% and 33.00%， respectively; and the overall mean values were （8.840±1.817） m， （2.249±0.725） m and （7.301±2.409）cm， respectively， and the 95% confidence intervals were 8.353-9.007， 2.074-2.337 and 6.678-7.521 cm， respectively. In addition， as can be seen from Table 1， the crown width was closely related to diameter growth， and had a great effect on the growth of the DBH. Therefore， crown width， as an important index， should be fully considered during the comparison and selection of the excellent or good individuals of the white birch in northern China.

Growth performance and comparison

White birch is an important industrial timber species which is known throughout Europe for its natural birch juice. From the viewpoint of biology and ecology， following the principles of efficient management and innovative use， selecting superior genotypes or elite groups from differentiated populations can maximize ecological， social and economic benefits[19]. There were significant differences in tree height， DBH and crown width of white birch between populations. These variations directly affected the accumulation and yield of the stand， providing necessary and sufficient conditions and genetic basis for the comparison and selection of populations or individuals for timber. The comparison results of the growth characteristics are shown in Table 3. It can be seen from Table 3 that the tree height， DBH and crown width of all tested northern white birch were the worst in the Inner Mongolia[MG]population， which exhibited the average values of 6.700 m， 4.201 cm and 1.468 m， respectively. Through the multiple comparisons of the growth characters of the tested populations， the tree height and DBH were divided into 4 groups according to the growth potential and growth performance， and the crown width was divided into 3 groups. The Jilin[JL]， Liaoning[LN]， Hebei[HB] and Shanxi[SX]populations had the strongest growth potential of tree height， and exhibited the tree heights of 9.869， 9.618， 9.007 and 8.939 m， respectively， which were 47.29%， 43.55%， 34.43% and 34.22% higher than the Inner Mongolia[MG]population， respectively. The Shanxi[SX]， Hebei[HB]， Jilin[JL]and Liaoning[LN] populations were strong in growth potential of DBH， and showed the DBH values of 8.051， 7.999， 7.969 and 7.762 cm， respectively， which were 91.64%， 90.44%， 89.68% and 84.77% higher than the Inner Mongolia[MG]population， respectively. The Shanxi[SX]， Liaoning[LN]， Hebei[HB] and Jilin[JL]populations had the widest crowns， which were 2.444， 2.410， 2.373 and 2.346 m， respectively， increased by 66.49%， 64.17%， 61.65% and 59.81% compared with the Inner Mongolia[MG]population， respectively. The adaptability of white birch in northern China is strong， and it can adapt to the new environment and climate by virtue of its own advantages. Therefore， with DBH as the main factor， taking tree height and crown width into account， the progeny of Shanxi[SX]， Hebei[HB] and Liaoning[LN] populations were preliminarily determined as growing better. However， these character variations were affected by environmental factors or from genotypes， and the genetic effects of quantitative characters of the populations still need to be further analyzed.

Analysis on genetic effects of quantitative characters

Heritability and genetic gain are important genetic parameters for evaluating the genetic improvement of forest trees， of which the values and relative stability are affected by genes and environment. The natural distribution area of white birch in northern China spans the midtemperate zone and warm temperate zone of China. It has a temperate or subtropical monsoon climate， with complex ecological environment， diverse climatic conditions and various soil types， which enhances the adaptability and ecological coupling of this species. In order to explore the genetic effects of free pollination individuals， with family progeny as the object， variance analysis was carried out on the quantitative characters such as tree height， DBH and crown width， and the genetic parameters such as heritability and genetic gain were estimated based on the variance components. The results are shown in Table 4. As can be seen from Table 4， the heritability of tree height， DBH and crown width was 0.971， 0.816 and 0.576， respectively， and the genetic gains were 15.97%， 21.54% and 13.87%， respectively. The heritability of tree height and DBH was stronger， as well as the genetic gains， while the heritability and genetic gain of crown width were moderate. This fully shows that the crown widths of the natural populations of white birch in China are moderately controlled by genes， heritability and genetic gain， while tree height and DBH characters are highly controlled by genes， with small genetic differentiation and huge growth potential， laying a foundation for high generation genetic improvement. Therefore， with the growth characters as the main evaluation indexes， taking various factors such as ecological adaptability， stress resistance， genetic effect， heritability and genetic gain into account， the Shanxi[SX]， Hebei[HB] and Liaoning[LN] populations were determined as excellent populations. It is recommended to measure and comprehensively evaluate the progeny of these populations.

Progeny test and selection

In order to evaluate the growth performance of the progeny of the natural populations of white brich， from the selected Shanxi[SX]， Hebei[HN]and Liaoning[LN] populations， 360 progeny plants of 18 individuals were randomly selected for the determination of tree height， DBH and crown width， and the determined values were analyzed by differential analysis and multiple comparisons. The results are shown in Table 5. As can be seen from Table 5， the tree heights of LN305， LN318， SX516， HB424 and HB403 were listed in the top 5， with an average height of 10.395， 10.310， 9.866， 9.660， and 9.540 m， respectively; the top 5 individuals in DBH included HB429， SX516， LN305， SX509 and HB403， and the average DBH values were 9.238， 8.738， 8.530， 8.380 and 8.205 cm， respectively; and the top 5 individuals in the crown width were HB429， LN305， HB424， SX503 and SX528， the mean values of which were 2.735， 2.711， 2.610， 2.522and 2.503 m， respectively. With DBH as the main factor， the intersection of the tree height subset and the breast diameter subset included three individuals， namely： LN305， SX516 and HB403. Compared with the Inner Mongolia[MG]population performing worst， the tree heights of the three individuals increased by 55.15%， 46.04% and 42.39% respectively， the DBH increased by 103.05%， 108.01% and 95.31%， respectively， and the crown width increased by 84.64%， 70.27% and 62.02%， respectively. Aiming at the breeding of timber species， comprehensively according to tree height， DBH and crown traits， taking various factors such as adaptability， growth potential， heritability and genetic gain into account， LN305， SX516 and HB403 were finally determined as excellent individuals， which had small intraindividual interprogeny genetic differentiation and strong growth potential， of which the average coefficients of variation were 13.08%， 13.54% and 12.13%， respectively. They are recommended to be widely used in production.

Conclusions and Discussion

The white birch field in northern China crosses the middle temperate zone and warm temperate zone in China. White birch has strong vitality， adaptability， stress resistance， cold resistance， drought resistance and resistance to sandstorm， and can adapt to various ecological environments by virtue of its own advantages， thus possessing the genetic basis for population and individual selection.

Based on the progeny test and the analysis and evaluation of heritability， genetic gain and genetic effects， it was considered that the variation of growth characters was abundant among populations. The variation was smallest in tree height， followed by crown width， and largest in DBH， and the average coefficients of variation were 20.56%， 32.25% and 33.00%， respectively.

According to the growth potential and growth performance， it can be confirmed by comparative analysis that the tree height， DBH and crown width of Inner Mongolia[MG]population were the worst; the Jilin[JL]， Liaoning[LN]， Hebei[HB] and Shanxi[SX]populations were strongest in the growth potential of tree height， and exhibited the tree heights 47.29%， 43.55%， 34.43%and 34.22% higher than that of Inner Mongolia[MG]population， respectively; the Shanxi[SX]， Hebei[HB]， Jilin[JL]and Liaoning[LN] population were strong in the growth potential of DBH， and their DBH values were 91.64%， 90.44%， 89.68% and 84.77% higher than that of the Inner Mongolia[MG]population; the Shanxi[SX]， Liaoning[LN]， Hebei[HB] and Jilin[JL]populations had the widest crowns， which increased by 66.49%， 64.17%， 61.65% and 59.81%， respectively， compared with the Inner Mongolia[MG]population.

The crown width of the natural populations of northern white birch was weakly controlled by genes， having moderate heritability and genetic gain， which were 0.576 and 13.87%， respectively. The tree height and DBH were highly controlled by genes， which had the heritability of 0.971 and 0.816 and the genetic gain of 15.97%and 21.54%， respectively.

By comparing and analyzing the growth characters of 18 individuals in the selected populations， three excellent individuals with stable growth and small genetic differentiation were selected， namely： LN305， SX516 and HB403. Compared with the worst Inner Mongolia[MG]population， these individuals increased the tree height by 55.15%， 46.04% and 42.39%， respectively， and the DBH by 103.05%， 108.01% and 95.31%， respectively， and the crown width by 84.64%， 70.27% and 62.02% respectively. Therefore， with growth characters as the main evaluation indexes， taking various influencing factors such as adaptability， stress resistance， heritability and genetic gain into account， Shanxi[SX]， Liaoning[LN] and Hebei[HB] populations were finally determined as excellent populations， which are advised to be widely used in multigeneration genetic improvement breeding.

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Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU