Abstract Fraxinus mandshurica is one of the ancient relict flora species originated from the Tertiary period， which is mainly produced in the eastern mountains in Northeast China. It has an important scientific value for studying the Tertiary flora and the Quaternary glacial climate. In order to make rational use of the natural genetic resources of F. mandshurica， and to increase the scientific and technological added value of improved varieties， this study used the method of positional observation of fixed standard fields to discuss the genetic variations of tree height and DBH of the natural populations and its open??pollinated progenies， systematically analyze the heritability and genetic gains in growth characters based on the analysis of progeny test， growth dynamic and heredity effects. The populations with good growth， stress resistance and adaptability were selected. The results showed that there were extremely significant difference in the growth characters among the natural populations， and the coefficients of variance for tree height （22.99%） were smaller， while the coefficients of variance for DBH （41.37%） were larger. On the other hand， there were extremely significant differences in the growth characters among the open??pollinated progenies， and the interaction between these progenies and the blocks was significant. Meanwhile， the growth characters had strong heritability， high genetic gain， and remarkable heredity effects. The heritability of tree height and DBH were 0.825 and 0.781， and the genetic gains were 15.16% and 25.85%， respectively. Therefore， Shanhetun （SHT） and Xinglong （XIL） were determined to be excellent natural populations both in the growth， resistance and adaptation with growth traits as the main evaluation index， which could be widely applied in the advanced generation improvement breeding.
　　Key words Fraxinus mandshurica； Natural population； Excellent population； Progeny test； Heritability； Genetic gains
　　Fraxinus mandshurica Rupr. is an ancient Tertiary relict plant mainly produced in the eastern mountainous areas of northeastern China. It has an important scientific value for studying the Tertiary flora and the Quaternary glacial climate. F. mandshurica is widely distributed in China. The natural distribution regions have complex terrains， diverse ecological environment， and there are significant differences in the habitat factors such as climate， soil and topography. Affected by ecological isolation， genetic mutation and natural selection， the whole tree species cannot be randomly mated， thus differentiating and producing geographical ecological populations with different genetic structures[1]. F. mandshurica resources play an important role in maintaining and regulating the climate in China. Although the distribution is wide， the resources are very limited. Because of the early development history， it is subject to long??term predatory cutting and extensive management， and the traditional concept of simply pursuing production and profit but neglecting protection， resulting in narrower distribution， sharp decline in population， serious loss of recoverable resources， shrinking forest area， and depletion of natural resources. Therefore， in 1999， the State Council of China identified it as a key second??grade animal under national protection that is at risk in China. In the taxonomy of plant systems， F. mandshurica is a precious timber species of the Oleaceae or Fraxinus Linn. It is a fine tree species with fast growth， strong adaptability and high economic value in the broad??leaved trees[2]. F. mandshurica is the main associated tree species of the primitive Korean pine and broad??leaved mixed forest in Northeast China， and it is also the main forest tree species of the secondary forest in the region. It is especially common in the red pine forest in the valley. However， F. mandshurica formed from the succession in the secondary bare area from natural destruction has a moist type of community that is suitable for wet sites[3]. F. mandshurica is famous for its excellent material and has long enjoyed a good reputation in the world timber market. With hard and beautiful texture， F. mandshurica has high processing values， and is excellent in planing， bonding， painting and decoration. It is a high??grade material for civil， construction， industrial and military purposes. It is used to make high??grade furniture， instruments， stationery and musical instrument， sports equipment， vehicles， boats， machinery and parquet flooring. The tree is broad， tall and straight with strong adaptability and resistances to severe cold， drought， and smoke and pests and diseases， making it a good variety for greening and beautifying and ecological urban landscape construction. Moreover， F. mandshurica can form a forest ecosystem with composite structure with many tree species， which has significant and far??reaching significance for improving the ability of the entire forest to conserve water， maintaining soil and water， and preventing environmental degradation. In this study， the progeny test forest of the natural population of 8 a F. mandshurica was used as the study object. Based on the analysis and evaluation of the seasonal observation， population variation， Growth law and population genetic effects， the variation rules， heritability and genetic gains in growth characters were analyzed systematically by measuring the growth characters of the natural population and its open??pollinated progenies， and the populations with good growth， stress resistance and adaptability were selected， which provided genetic resources for multi??generation genetic improvement. 　　Geographical Distribution and Natural Conditions
　　Geographical distribution
　　F. mandshurica is widely distributed and has a discontinuity， which is concentrated in the mountains of Northeast Asia. The natural distribution area starts from North China to the south， and west to Shaanxi and Gansu of China， east to Shishima Island of Japan， north to the Great Khingan of China and the far east region of Russia. It is distributed across the northeastern part of China， the far east of Russia， the northern part of Japan and the northern part of the Korean Peninsula， and the northmost reaches Bei??an Township， the northernmost part of China （53??32??N， 123??30??E）[4-5]. The northeastern part of China is the main distribution area of F. mandshurica， while the northern mountainous area of Changbai Mountain in Jilin is the central distribution area， and the Lesser Khingan Mountains and the hilly area of Qianshan Mountains in Liaoning are marginal distribution areas. In mainland China， F. mandshurica is mainly produced in the eastern part of the Great Khingan in the northeast， Lesser Khingan Mountains and the north of Changbai Mountains in Jilin， west to the Qianshan Mountains in Liaoning and Yanshan Mountains in Hebei， and intermittently distributed to parts of Hebei， Henan， Shanxi， Shaanxi， Ningxia and Gansu[6-7]. At present， there are many introduced and cultivated varieties n in various parts of China， which are generally applied in the construction of ecological beautification projects in cities， and the number is very limited.
　　Natural conditions of the producing areas
　　Following the principle of similarity between the origin and the introducing area， the breeding materials of 12 natural populations， namely， Dongfangong （DFH）， Fangzheng （FAZ）， Huanan （HAN）， Hailin （HLN）， Huanren （HRN）， Huinan （HUN）， Linjiang （LNJ）， Lushuihe （LSH）， Shanhetun （SHT）， Wuchang （WCH）， Tangwanghe （TWH） and Xinglong （XIL）， were introduced to the natural distribution regions of F. mandshurica in the mountainous regions of northeast China， including Lesser Khingan， Changbai Mountains， Wandashan Mountains and Zhangguangcai Ranges. The natural conditions of the producing areas are shown in Table 1.
　　Natural conditions of the introducing areas
　　The introducing area was selected in Qingshan National Fine Breeding Base of Linkou County， Heilongjiang Province， which is located in the eastern mountainous area of northeastern China. It belongs to the hilly area of the eastern slope of Zhangguangcai Ranges of Changbai Mountains with the geographical coordinates of 130??20??-130??40??E， 45??17??-45??30 ??N. This area has high terrain in the northeast and low in the southwest with an average slope of 10??-15??， maximum slope of 40??， and altitude of 300-500 m. The area has a north temperate continental monsoon climate with cold and dry long winter， warm and humid short summer. Water and heat fall in the same season. Moreover， the area also has abundant sunshine with the frost??free period of 120-130 d， and an average annual precipitation of 400-600 mm. Most of the rainfall is concentrated from June to August， when the precipitation accounts for 50% of the whole year. The annual effective accumulated temperature of ??10 ?? is 2 100-2 600 ??. The zonal soil type in the area is typical dark brown soil with deep soil layer， moist and fertile soil texture， strong permeability， which is suitable for the growth and development of forest trees. 　　Materials and Methods
　　Test design
　　The breeding materials of F. mandshurica were from 12 natural populations and 1 artificial cultivated population in northeastern China. In order to discard the differences between ecological environments， the materials for progeny determination， analysis and evaluation were collected from the 8 a open??pollinated progeny test forest of the same introducing area. The test was conducted in Qingshan National Fine Breeding Base of Linkou County， Heilongjiang Province. The test forest had progenies of the 12 natural populations， and the progeny of the cultivated population Linkou （LNK） in Qingshan was set as the control， so there were a total of 13 treatments. Field experiment was conducted according to the completely randomized block design， and 4 repetitions were set up. The blocks were arranged with duplicate rows of 20 plants， and the land was prepared in cave shape （specification： 50 cm??50 cm??45 cm）. And Picea Koraiensis Nakai was planted between blocks.
　　Test indicators and determination
　　Test indicators The main test indicators were the quantitative characters of seed length， seed width， seed length??width ratio， fruit length， fruit width， fruit length??width ratio， tree height and DBH.
　　Determination method Vernier caliper was used to measure seed length， seed width， fruit length， fruit width and DBH growth （accuracy to 0.5 mm）. Electronic balance was used to measure the 100??grain weight of seeds and fruits （accuracy to 0.05 mg）. The height growth of the tree was measured by a tape （accuracy to 0.5 cm）.
　　Statistical analysis of data
　　The original data was processed by statistical analysis SPSS 13.0 software[8]， and the variance analysis was performed by the mathematical model： Yijk=??+Bi+Fj+BFij+??ijk， where Yijk is the kth observed value of the jth family of the ith block， ?? is the population mean value， Bi is the block， Fj is the family， BFij is the interaction between the family and the block， and ??ijk is the random error. The genetic parameters and variation coefficients of growth characters were estimated according to the results of variance analysis[9-11]， among which： ?? heritability （h2）=（MSf-MSe）/MSf； ?? genetic gain （?¤G）=Sh2/X； ?? selection intensity （I）=S/??p； ?? coefficient of variation （CV）=100????p/X， where， h2 is family heritability， ?¤G is genetic gain， CV is coefficient of variation， MSf is phenotypic variance， MSe is environmental variance， i is selection intensity， S is selection deviation， ??p is standard deviation， and X is the character mean. 　　Results and Analysis
　　Seasonal variation of F. mandshurica
　　There is a close correlation between the growth and development of plants and the seasonal changes. The in??depth understanding of the phenological characteristics of different growth stages has certain guiding significance for the selection of excellent groups and excellent individuals of F. mandshurica and the sustainable management of forest stand. Based on the long??term observed data of the natural distribution area in Lesser Khingan Mountains， the characteristics of seasonal variation of F. mandshurica in this area were revealed from the following 7 aspects.
　　Germination period From early April to mid??April， the sap begins to flow and the germination period begins.
　　Leaf??expansion period The bud bursting of F. mandshurica begins relatively late. The buds are inflated at the beginning of May， open in mid??May， and the leaf??expansion period falls from late May to early June. New shoots are formed in mid??July.
　　Growing period The height growth of F. mandshurica is of short cycle， which begins in mid??May， and the vigorous growth period is from late May to early July， accounting for 70% of the total growth. The capping falls in late August， and then the growth stops. The stem growth period is from early May to early September， in which growth begins in early May， enters the fast??growing period in late May， which ends in late August， and then the growth slows down until early September， when the growth stops.
　　Flowering stage F. mandshurica begins flowering and fruiting when growing to 6??8a. the early flowering season is in May， and the full??blossom period is in middle and late May， while June is the late flowering period.
　　Fruit ripening period May to October is the fruit ripening period， in which the fruit begins to set in late May， mature in late August， fully mature in late September， and falls in mid??October.
　　Leaf color changing period The leaves begin to change color from late August to early September， and the leaves completely change colors from mid??September to late September.
　　Leaf falling period The leaves of F. mandshurica fall early， which generally begins in early September， achieves the peak in late September， and all fall off in early October.
　　Agricultural Biotechnology 2018Growth law of trees in different habitats
　　The growth law of forest trees is an important manifestation of the biological characteristics of tree species. It is of great theoretical and practical significance for resource recovery and sustainable management and utilization to explore the growth dynamics of F. mandshurica. Among the broad??leaved tree species， the growth rate of F. mandshurica is relatively faster， but the growth period of tree height and DBH is not synchronized. The growth of tree height begins first and then the growth of DBH begins. In order to reveal the growth law of F. mandshurica in different habitat conditions， the free??growing trees and the intra??forest competitive trees in the stand gap were evaluated. In different ecological regions， 100 individual plants were randomly selected for trunk analysis to analyze the total growth， annual average growth and current annual increment of tree height and DBH， and the growth dynamic curves were drawn （Fig. 1， 2 & 3）. As shown in Fig. 1， 2 & 3， the growth of tree height and DBH was on the rise during the whole growth cycle， which was in the vigorous growth stage all the way， and the 3 measured indicators of the free??growing trees were far higher than those of the competitive trees. ?? For the free growing trees， the peak seasons for the average annual growth of tree height （0.51 m） and the current annual increment （0.57 m） were between 15 and 30 a， and the maximum growth appeared in 35 and 15 a of 0.52 and 0.63 m， respectively. And after 35 a， the annual average growth declined. The peak seasons for the annual average growth of DBH （0.57 cm） and current annual increment （0.69 cm） were in 30-50 a and 15-35 a， respectively， and the maximum growth appeared in 30 and 25 a of 0.59 and 0.83 cm， respectively. After 25 a， the annual average growth was accelerated， while after 45 a， the growth declined. ?? For the competitive trees， the peak seasons for the average annual growth of tree height （0.36 m） and the current annual increment （0.46 m） were in 30-50 a and 20-30 a， respectively， and the maximum growth appeared in 35 and 25 a of 0.37 and 0.58 m， respectively. The peak seasons for the annual average growth of DBH （0.37 cm） and current annual increment （0.61 cm） appeared after 35 a and in 35-45 a， respectively， and the maximum growth appeared in 45 and 40 a of 0.39 and 0.64 cm， respectively. After 35 a， the annual average growth became significantly larger. Therefore， after 25 a， the diameter growth of free growing trees was in the dominant state in the forest gap， and the current annual increment of DHB still maintained at a high level， while the current annual increment of tree height decreased. After 45 a， with the expansion of upper space， enhancement of competitiveness and improvement of light energy utilization rate， the growth of tree height of intra??forest competitive trees was in the dominant state， and the current annual increment still maintained a stable growth trend， while the current annual increment of DBH decreased significantly. 　　Variation analysis of seed and fruit characters
　　Forest seeds are rich in nutrients， and their size and quality directly affect the content of nutrients. In order to explore the differences in the seed and fruit characters of F. mandshurica under different ecological conditions， the variance analysis was performed to the seed and fruit characters of the natural populations in the mountainous areas of Lesser Khingan Mountains， Changbai Mountains， Zhangguangcai Ranges and Wandashan Mountains （Table 2）. As shown in Table 2， there were certain differences in the 4 characters of seed length， width， length??width ratio and 100??grain weight between different populations. The significance probability was far below 0.05， and the difference was very significant at the level of 5%. The average coefficients of variation were 4.74%， 10.08%， 15.33%， and 19.11%， respectively. Great variation was found in the length， width， length??width ratio and 100??grain weight of the fruit between different populations， and the variation of 100??grain weight was the greatest. The average coefficient of variation was 16.78%， 7.80%， 16.59%， and 37.07%， respectively， indicating that this difference in the fruit characters reflected the differences in the seed transmission capacity， natural regeneration ability and fruit nutrient contents between different populations.
　　Genetic variation of growth characters
　　F. mandshurica is a precious species of broad??leaved trees with fast growth， strong adaptability and ecological coupling， and high economic value. In order to investigate the genetic variation trend of the growth traits of natural populations in different ecological environments， 520 individual plants （including controls） from 13 populations were tested and evaluated， and the tree height and DBH of open??pollinated progenies were determined. The results of the measurement and analysis of variance are shown in Tables 3 & 4， respectively. As shown in Table 3， the overall mean values of tree height and DBH were （1.818??0.418） m and （1.099??0.455） cm， respectively， and the fluctuation ranges in the 5% confidence interval were between 1.782-1.854 m and 1.061-1.139 cm， respectively. The variation of tree height in natural population was small， and the variation of DBH was larger. The average coefficient of variation was 22.99% and 41.37%， respectively. The variation of tree height and DBH was the greatest in the progenies of population HRN， for which the variation coefficients were 27.54% and 52.71%， respectively. The variation of tree height was the smallest in the progenies of population TWH， and the variation of DBH was the smallest in the progenies of population DFH. As shown in Table 4， the tree height and DBH of the progenies of different populations were basically in normal distribution， and there were abundant variations. The significance probability between the populations was far less than 0.05， and the difference was extremely significant at the level of 5%. 　　Progeny determination and population selection
　　F. mandshurica is famous for its excellent material. Excellent genotypes or production populations were selected from the differentiated populations by adhering to biological and ecological perspectives and following specific breeding objectives， with the aim to maximize the ecological， social and economic benefits of forests. There were large differences in the growth characters of the open??pollinated progenies of F. mandshurica， which had the necessary and sufficient conditions for comparison and the genetic basis for excellent population recommendation. The results of multiple comparisons are shown in Table 5. As shown in Table 5， the progenies of cultivated population LNK had good performances in tree height （2.19 m） and DBH （1.432 cm）， which increased by 20.46% and 17.93% from the progenies of the other tested 12 natural populations， presenting strong growth vigor. Moreover， the variation in tree height was small between the progenies， but affected by the environmental conditions， the variation of DBH was relatively larger， which indicated that after long??term cultivation and domestication， the species had fully adapted to the ecological and climatic conditions of the introducing area， and had strong ecological coupling. Among the 12 natural populations participating in the study， the populations with the DBH and tree height of open??pollinated progenies ranking top 5 included SHT， XIL， WCH and HRN， and the mean values of the 2 characters were 1.261， 1.250， 1.134， 1.116 cm， and 1.978， 1.951， 1.897， 1.834 m， respectively. Compared with population LSH which had the smallest DBH （0.927 cm）， the DBH of the 4 populations were increased by 36.03%， 34.84%， 22.33%， and 20.39%， respectively， and compared with population DFH which had the lowest tree height （1.667 m）， the tree height of the 4 were increased by 18.66%， 17.04%， 13.80%， and 10.02%， respectively. Therefore， the progenies of these 4 populations performed well according to the initial determination， but it still needed further analysis on the genetic effects of the progenies to determine whether the variation on these characters originated from genotypes or environmental factors， so as to provide theoretical references for the selection of excellent populations.
　　Analysis and evaluation of genetic effects
　　Heritability and genetic gain are important genetic parameters for evaluating the genetic improvement of forest tree， and their sizes and relative stability are affected by genes and environment. In the mountainous areas of northeastern China， the natural distribution area of F. mandshurica has a continental monsoon climate. The complex and diverse ecological environment， cool and humid forest climate and moist fertile soil enhance the ecological adaptability and ecological coupling of the species. In order to investigate the genetic effects of open??pollinated individuals of F. mandshurica， the progenies of natural populations were used as the determining objects to perform targeted variance analysis to the quantitative traits， and the heritability and genetic gains of the open??pollinated individuals were estimated according to the variance analysis results （Table 6）. As shown in Table 6， for the progenies of natural populations of F. mandshurica， the heritability of the tree height and DBH was 0.825 and 0.781， respectively， and the genetic gains were 15.16% and 25.85%， respectively. The results fully showed that in the eastern mountainous areas of northeastern China， the quantitative traits of the progenies of natural populations of F. mandshurica were strongly controlled by genes， so they had strong heritability， small genetic differentiation， high genetic gains， and huge growth potential， making them available for advanced generation improvement breeding. Therefore， taking into account of the various factors of ecological adaptability， ecological coupling， stress resistance， genetic effects， heritability and genetic gain， SHT and XIL were determined to be excellent natural populations with the growth characters of open??pollinated progenies as the main evaluation indicators， which could be widely applied in the advanced generation improvement breeding. 　　Conclusion and Discussion
　　The mountainous area in northeastern China is a concentrated distribution area for F. mandshurica. The complex terrain， diverse ecological environment and cool and humid climate make it contain highly genetic diversity of forest plants. F. mandshurica is fond of the cool and humid forest climate， and can form complex forest ecosystems with many tree species， so it has enormous potential for population selection and genetic improvement. The analysis on the genetic effects of the growth variation， heritability and genetic gains of the open??pollinated progenies shows the follows： first， there are rich variations in tree height and DBH of the parental progenies of the natural populations of F. mandshurica， and the confidence intervals for the 2 characters were 1.782-1.854 m and 1.061-1.139 cm， respectively， which show extremely significant differences at the 5% level. Second， the open??pollinated progenies of F. mandshurica in the mountainous area of northeast China have different adaptability to new living environment， and the differences in tree height and DBH are extremely significant between progenies. Moreover， the variation of tree height is small， while the variation of DBH is great， and the average variation coefficients were 22.99% and 41.37%， respectively. Third， for open??pollinated progenies of F. mandshurica in the mountainous area of northeast China， the heritability of tree height and DBH are 0.825 and 0.781， respectively， and the genetic gains are 15.16% and 25.85%， respectively， which fully indicates that growth characters are under strong genetic control. So they have strong heritability， small genetic differentiation， high genetic gains， and huge growth potential. Fourth， taking into account of the various factors of ecological adaptability， ecological coupling， stress resistance， genetic effects， heritability and genetic gain， SHT and XIL are determined to be excellent natural populations with the growth characters of open??pollinated progenies as the main evaluation indicators， which is recommenced to be widely applied in the advanced generation improvement breeding.
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