Yang Zili2, Zhou Shouyi2, Zhang Weidong2, Yang Zixiang3
China has a distinctively rich flora of Salicaceae, and in particular of poplars. Out of more than 100 Populus species reported in the world, 53 have been described to occur in China (Xu, 1988; Zhao & Cheng, 1994), and 37 are distributed in North China4 (Ibidem; see Table 1). Among them, three native species are found in the driest and northernmost sites of the country: Populus simonii, occurring in dry areas all over North China, P. euphratica in the Northwest, and P. nigra along a few river beds of north-western Xinjiang. Balsam poplar P. cathayana, occurring along the river systems of north-central and north-west China, requires higher humidity and better soil conditions (Weisgerber, 1995; Yang and Wang, 1995). These native poplars in North China constitute a unique genetic pool, which is locally under anthropogenic pressure, and several authors have stressed the need for its conservation (Xu, 1988; National Poplar Commission, 1996; Weisgerber, 1995). Maintaining fitness and properly utilizing this valuable resource will condition the sustainability of large-scale poplar cultivation in dry-zone Northern China, as reviewed in this paper.
Poorly developed, North China is endowed with a vast and open landscape comprising fragile ecosystems and a difficult environment. The area between 35º and 45º N is mainly characterized by a semi-arid continental climate. Local climates vary from semi-humid conditions in the Northeastern Plain and near the Yellow Sea, to extreme aridity in the cold deserts of Taklimatan and Mu Us. Major constraints to plant growth include irregular (summer) rainfall, permanent wind, severe cold and dry winters and soils with only moderate to low fertility. Wind and water erosion of fragile topsoil, particularly in the form of sandstorms on the Mongolian steppes and the loess plateau of the Yellow River, have led to severe ecological damage and a widespread trend towards desertification.
According to historical records, most of North China, currently a steppe turning slowly to grassland, used to be covered by dense vegetation (Three North Bureau, 1988; Menzies, 1994). Increases in human population and pressures on land have periodically led to overfelling, clearing for agriculture and overgrazing, exposing the land and initiating a desertification process. Periods of intense pressure on the land have been regularly followed by periods of partial ecological recovery, after local human populations moved away to escape starvation.
The difficult ecological conditions explain why vegetation can easily be disturbed and destroyed. However, after a short period of land protection, due to large underground water reserves, pioneer species recover rather rapidly in the lowlands, in particular poplars and willows.
Several attempts to develop North China for human settlement have been made this century. However, development guidelines and cultivation practices paid too little attention to environmental limitations. Growing concerns over economic losses in crops and animal husbandry, and sand and water damage to cities and infrastructure all over North China, have prompted national, provincial and local initiatives of soil conservation and rehabilitation of degraded areas, in particular through afforestation.
In the Northeast, attention was rapidly focused on the use of poplars for soil rehabilitation. Poplars had for centuries been closely associated with human activities and used in agroforestry and silvopastoral systems. Before 1950, native poplars (quite exclusively P. simonii and related P. pseudosimonii) and willows, had been planted extensively along roads and railway lines, using local planting materials (Zheng & Ren, in Zhou & Weisgerber, 1997). In the 1950's and 1960's, soil conservation and rehabilitation were given high priority in the Northeast and in central Inner Mongolia, and public campaigns promoted the protection of agricultural fields, grazing lands, cities, villages and houses, by the establishment of green shelterbelts. These initiatives resulted in systematic plantation programmes for shelterbelts along streets, roads, fields, canals and railways, and use of trees in home gardens. The unique characteristics of black and balsam poplars (ease to vegetatively propagate them, fast initial growth, and availability of reproductive materials locally) made them very popular. Vegetative propagation was commonly used in native poplars, complemented by collection and exchange of seed when the amount of local materials was insufficient. Large quantities of seed (P. simonii and P. pseudosimonii) were collected in the 1960's in central Liaoning and forwarded to forest farms in the North-Eastern Plain. Seedlings were grown and culled in local nurseries. Little attention was paid to provenance site matching, or even to care and tending after planting.
|Table 1: Distribution of poplar species in North China (Based on Xu, 1988)|
|SPECIES||North East||Central North||North West||Altitude|
|P. afghanica||X||1400 - 2800|
|P. alba||X||450 - 750|
|P. amuyensis||X||X||600 - 800|
|P. canescens||X||600 - 700|
|P. cathayana||X||X||X||X||X||X||X||X||800 - 3200|
|P. charbinensis||X||300 - 500|
|P. davidiana||X||X||X||X||X||X||X||X||X||X||X||200 - 3800|
|P. euphratica||X||X||X||X||X||2500 - 2900|
|P. gansuensis||X||1800 - 2000|
|P. girinensis||X||X||300 - 400|
|P. hopeiensis||X||X||X||X||X||X||700 - 1600|
|P. hsinganica||X||X||X||300 - 700|
|P. iliensis||X||600 - 750|
|P. jrtyschensis||X||200 - 2000|
|P. koreana||X||X||X||400 - 1100|
|P. lasiocarpa||X||1300 - 3500|
|P. laurifolia||X||1200 - 1700|
|P. maximowiczii||X||X||X||X||X||X||X||400 - 2000|
|P. nakaii||X||X||X||X||X||600 - 900|
|P. nigra||X||400 - 800|
|P. ningshanica||X||X||600 - 1000|
|P. pamirica||X||1800 - 2000|
|P. pilosa||X||1600 - 2300|
|P. pruinosa||X||300 - 1500|
|P. przewalskii||X||X||X||500 - 1500|
|P.pseudomaximowiczii||X||X||1000 - 1600|
|P. pseudosimonii||X||X||X||X||X||X||X||X||X||300 - 2300|
|P. pseudotomentosa||X||300 - 1400|
|P. purdomii||X||X||X||700 - 3300|
|P. simonii||X||X||X||X||X||X||X||X||X||X||X||600 - 2300|
|P. suaveollens||X||X||200 - 400|
|P. szechuanica||X||X||1100 - 4000|
|P. talassica||X||500 - 1800|
|P. tomentosa||X||X||X||X||200 - 1800|
|P. tremula||X||700 - 2300|
|P. ussuriensis||X||X||X||300 - 1400|
|P. wilsonii||X||X||1300 - 3300|
|NUMBER BY PROVINCE||12||8||8||11||10||8||14||12||5||6||16||37|
|Legend: Heil: Heilongjiang; Jil: Jilin; Liao: Liaoning; Mon: Inner Mongolia; Heb: Hebei; Shan: Shanxi; Shaa: Shaanxi; Gan: Gansu; Nin: Ningxia; Qing: Qinghai; Xin: Xinjiang|
While successful in the best environmental conditions, and when grown in home gardens, the overall performance of native poplars in North China was barely satisfactory. Considering the large amount of resources devoted to these early large-scale plantings, survival rates of young saplings were below those which could be considered acceptable. In the most difficult plantation sites, surviving plants became known as "small old trees". In the meantime, valuable and unique experience was gained on nursery and planting techniques, and on natural distribution and ecological requirements of native species.
Introduction of exotic poplars has taken place in the past, including P. nigra var. italica and P. nigra var. thevestina, P. x canadensis, and P. deltoides (P. deltoides hybrids having been locally known in coastal Hebei as P. x shanhaiguan since the 1900's); however, use of introduced poplars was limited to amenity plantings in towns and botanical gardens (Zhang Jie & Zhang Qi Wen, in Zhu & Zhang, 1991). P. nigra, found in western Xinjiang, is even reported to have been introduced in ancient times by traders along the Silk Road. An arboretum was established in Gaixian, Liaoning, in 1930, comprising several poplar species.
Systematic introductions were initiated only after 1949. From 1950 to 1960, approximately 60 clones were imported by the Chinese Academy of Forestry and the Botanical Garden in Beijing, from East Germany, Poland, Romania and the former USSR. In the early 1970's clones such as "I-214", "Pioneer", "Polska-15", "Robusta", "Ruskii", "Sacrau-79", "Serotina" and "Stalinetz", along with some material of P. x canadensis, P. x berolinensis, P. laurifolia and P. nigra were available in China (Zhang and Zhang, 1981).
Availability of new germplasm and expectations on potential heterosis in hybrid progenies (F1) were driving forces in the development of dynamic poplar improvement programmes. Some of the first efforts in controlled pollination were reported in white poplar, P. tomentosa, by Prof. Xu Weixin in 1947 (Zhang & Zhang, 1981). These efforts were followed by activities undertaken by the Chinese Academy of Forestry, Nanjing Forestry University, and other research institutions in North China, up to the early 70's.
Several hybrid combinations were tested in controlled crosses, including P. nigra x P. simonii or P. simonii x P. nigra (later released under the names of P. x simonigra, P. x xiaozhuanica, P. x opera, P. x popularis, P. x simopyramidalis, etc.), P. nigra x P. cathayana (P. x beijinensis), and many other species and clones. A summary of the work carried out by the Poplar Research Institute of Gaixian, Liaoning, is decribed in detail in Zhang & Zhang (1981). Some selection programmes, for example in Baicheng Forestry Institute, Jilin, or in railway nurseries, also used mass selection based on bulk seed from open pollination collected from P. simonii mother trees; progenies displayed hybrid vigour. Seed was sown in a nursery or greenhouse, seedlings culled on the basis of vigour and frost resistance, and families tested in the field. As a result, several hybrids displaying valuable features such as straight form and fast initial growth, when compared with native balsam poplars, were reproduced. In North China, the most promising hybrid combinations included P. simonii x P. nigra, P. cathayana x P. nigra, and P. pseudosimonii x P. nigra.
These early programmes were carried out in an overall empirical way, using mainly seed sources available locally, and relying on few parent trees. It is suspected that the genetic base of the new materials was very limited and several clones are likely to have been closely related (see Table 2). The need for a rational approach to poplar improvement, starting with systematic exploration, collection and evaluation of parent trees, was progressively recognized, and exploration and conservation activities were initiated for P. nigra in Xinjiang and P. simonii in Jilin by the Baicheng Forestry Research Institute.
Results of, and information on, the selection and propagation work carried out by scientists in the 1960's and 1970's were progressively passed to forest managers and technicians. However, many research activities, including tree improvement, and several plantation programmes, were drastically reduced or fully abandoned during the Cultural Revolution. The period also saw an upsurge in tree felling and deforestation following the Great Leap Forward-policy. In North China, dramatic increases in human population contributed to worsen environmental damage.
Scientific work was re-activated in forestry academies, universities and research institutions after the Cultural Revolution, in the late 70's, at a time when political commitment again emphasized the need for large-scale mobilization of efforts and resources to combat desertification, improve or restore agricultural capability and increase wood production. Several national, regional and local programmes were launched, and a specialized agency in charge of reforestation in arid areas, the Three-North Shelterbelt Development Programme, was established within the Ministry of Forestry5.
Since its establishment in 1978, the Three-North Shelterbelt Development Programme of the Three North Bureau (Sanbei in Chinese) has initiated and coordinated a systematic plantation programme carried out through state, provincial/regional and local forestry authorities. Reporting to the State Council, the Programme encompasses 551 counties/districts/cities of 13 provinces, autonomous regions and municipalities in north-west, central north and north-east China covering 4.069 million square kilometres or 42.4% of the country's total land area. This is the world's largest ecological programme; it was started in 1978 and shall be completed by 2050, covering a planned area of 35.08 million hectares, including 23.03 million hectares of plantations.
By 1994, over 13 million hectares had been planted under the first and second phases of the programme. During 1996-2000, the objective is to plant 6.18 million hectares. and 4.04 million hectares between 2001-2010. The programme presently also aims at increasing the quality and marketability of the wood produced and enhancing economic benefits from the plantations.
Other major forestry-related programmes include the Shelterbelt Development Programmes along the Liaohe, Yangtze and Yellow rivers, the Plain Afforestation Programme, the Taihang Mountains Afforestation Programme and the National Programme to Combat Desertification (Shi et al, 1997). Most programmes are located in North China.
Poplar remains the main afforestation genus. Out of 33 million hectares of planted forests in China, the area under poplar is estimated at 6.7 million hectares (including commercial and protection forests). In the Three North Shelterbelt Programme and the Plain Afforestation Programme, poplar is reported constituting 60% of the total of trees planted (National Poplar Commission, 1996). 80% of wind-break shelters of the Three North have been afforested with poplar (Han & Grosscurth, in Zhou & Weigerber, 1997).
Map 1. Situation of the Three North Shelterbelt Development Programme. Source: Three North Bureau 1989.
To achieve its ambitious goals, the Three North Programme, in line with other similar projects, identified genera, species and varieties which had proven their adaptability and reliability in Northern China. In site types where groundwater was easily available (lowlands and river basins, sandy lands, loess plateaux, sandy dunes), priority was given to poplar, especially to those clones obtained in the 1950's and 1960's, for which the selection process had been completed. In the most difficult sites devoted to poplar cultivation, hybrids of P. simonii x P. nigra were selected on the basis of local experience.
The country's new, open policies re-activated contacts in the poplar breeders' community. China formally joined the International Poplar Commission in 1980. Increased contacts with the outside world resulted in the introduction of an unprecedented amount of introduced poplar germplasm (several hundred clones are now recorded; by comparison, only 80 clones were imported in the 70's, and only from two countries, Italy and Yugoslavia). These include southern provenances of P. deltoides, a wide collection of P. x euramericana from Western Europe, and several P. deltoides x
Since the early 1980's, much research work undertaken by forestry academies and universities has focused on importing, evaluating and screening exotic poplar materials, mainly in the form of clones already selected in programmes in the countries of origin, and crossing these with local poplars or their hybrids. Strong heterosis and great potential has been found in P. deltoides x P. cathayana families, displaying good adaptation in the conditions of Central China (Tong and Han, 1991).
In regard to activities in follow-up to the breeding work in native poplars of the early 1960's and 1970's for North China, a range of P x euramericana "Zhonglin" and other "ZJ" and "ZX" hybrids were created at the end of the 1970's in Beijing. P. x popularis clones were crossed with a variety of exotic germplasm (including "Polska-15" and various clones of P. nigra and P. deltoides) in 1983 and potentials for high genetic gain were found. Results of several recent comparative trials in North China confirmed that in the best sites few foreign clones performed better than the P. simonii x P. nigra hybrids (Wang, 1995). A large-scale evaluation of the performance of poplar plantations in China was carried out using 23 clones. This resulted in a typology for poplar growing in the country (Chen, Zhao, Xu & Yang, in Wang, 1995). Work was also carried out on white poplars (Leuce Section) which displayed good site adaptation and low incidence of pests. Technical difficulties to propagate species of this Section vegetatively have, however, limited their use in large-scale afforestation schemes. The overall gap in the continuity of breeding programmes in the 70's explains why few new clones have been released for North China, and why most afforestation schemes still today rely on poorly diversified stock (see Table 2).
Table 2: Three successful poplar clones used in afforestation programmes in North China|
(from Zhu & Zhang , 1991)
|Generic Name||Parent Trees|
(F x M)
|Number of Accessions*||Year of Creation||Area Planted (ha)||Number of Trees Planted|
|P. x popularis||simonii x nigra||10||1956||1,000,000||51 million|
|P. x beijinensis||cathayana x nigra||13||1954 - 1957||66,000||200 million|
|P. x xiaohei||simonii x nigra||-||1960||1,000,000||-|
|*: number of different, although closely related, genotypes (alleged half-sibs or full sibs).|
Few continuing breeding programmes have been carried out in poplars in North China, but the situation is different in Central China. In south temperate central areas of China (from latitude 30º to 35ºN), several Euramerican poplars, introduced from Europe, display good behaviour along extensive river floodplains and have become a major component of a highly productive agroforestry system since the 1980's.
The high value and scarcity of wood in the Central Plains provide a strong incentive for a greater expansion of high-yielding poplars, and silvicultural systems developed by scientists are rapidly adapted by farmers. However, genetic diversity in plantations is still extremely reduced: the Genetics Group at Nanjing Forestry University estimates that about 60 percent of the plantings in Central China, with a total land area of roughly 600,000 square kilometres, today consist of clone "I-69", introduced from Italy (Farmer, 1992).
With the assistance of several institutions, including the Chinese Academy of Forestry and the Nanjing Forestry University, breeders and forest managers are working to broaden the genetic base of the plantation programme. The area is climatically similar to the lower Mississippi Valley, and several missions of exploration and collection by Chinese breeders to southern USA have lead to the establishment of a diverse pool of black cottonwood clones, which now serve as base collections. Since 1980, Nanjing University scientists have also engaged in a breeding programme using i.a. the introduced P. deltoides and the native P. simonii from Central China.
In North China, early man-made stands comprising the native poplars P. simonii and P. pseudosimonii, elms, oaks or pines, have long reached their rotation age and following harvesting they are replaced by hybrid poplars. In North China, the genetic diversity of new poplar stands is often limited to a single clone (mostly the hybrid P. simonii x P. nigra). The original infusion of P. nigra traits into the local gene pool has allowed foresters to widen the ecological range covered by native P. simonii, P. pseudosimonii and P. cathayana, towards drier sites, outside riverbeds and valleys. However, poor silvicultural management (high plantation density, careless pruning) and recurrent extreme climatic events have recently contributed to a weakening condition of poplar stands growing in difficult environmental conditions.
The greatest hazard to the above large-scale effort, which relies on only a few genotypes, lies in the risk of pest and disease outbreaks. The number of insect pests recorded on poplar is more than 200 in China, and their impact has increased dramatically in the last decades as poplar plantations extended. The damage by borers is the most serious threat to commercial and protection forests. To control the spread of longicorns like the Asian long-horned beetle Anaplophora glabripennis, 24 million trees were cut and burned in Ningxia, North Central China, in the winter of 1991 and the spring of 1992 (National Poplar Commission, 1996). The incidence of poplar diseases is reported low in North China, allegedly due to harsh winters and dry autumns and springs, unfavourable to fungal spread (Schmutzenhofer, Mielke, Luo, Ostry and Wen, 1996). While integrated pest management techniques are being promoted, there is also a growing awareness that selection of planting material for insect and disease resistance is of vital importance for the sustainability of large-scale plantations. Traits relating to tolerance/resistance to biotic factors have increasingly been taken into account in poplar improvement programmes.
Decision-makers and foresters have become increasingly aware of the magnitude of the challenge, and various initiatives have been launched to ensure the sustainability of poplar plantation forestry in North China. Action has concentrated on two main issues: getting a better knowledge of remaining native stands and species and their genetic diversity, and subsequent exploration and collection; and making efforts to diversify plantation forestry, both by increasing species diversity and increasing the diversity in poplar materials used.
The above issues have been addressed through various initiatives promoting better utilisation of local genetic material. Exploration and collection missions were carried out in 6 provinces and regions from 1985 to 1987 (Han, Wu & Wang, in Zhu & Zhang, 1991). In 1987, a country-wide exploration of P. cathayana was commissioned by the Ministry of Forestry to the Chinese Academy of Forestry (Li and Yang, 1997). At the same time, characterization of the diversity among provenances of P. ussuriensis was initiated. An extensive survey of the botany, distribution, ecology, utilization and potential of P. euphratica resulted in a monograph on the species in Northern China (Wang, 1996).
A Chinese-German cooperative project began in 1984, and through this project much exploration work was carried out, particularly in Northwest China. 650 native and 170 exotic poplar clones (including 22 clones of P. simonii and 230 clones of P. cathayana) were collected and established in ex situ stands in Shanxi. Intraspecific and interspecific controlled crosses were carried out using 400 different combinations (1984-1995) and concentrating on white poplars and northern balsams. Large amounts of new germplasm are still under investigation in the field, after the project was completed in 1997 (Weisgerber, Konwnatzki and Musson, 1995).
Twenty new clones were produced between 1990 and 1997 within the framework of a UNDP-funded development project executed by the Chinese Government in cooperation with FAO. The project assisted poplar scientists at the Chinese Academy of Forestry in transferring a single gene of Bacillus thurigensis into several poplar varieties (including a P. nigra), to confer resistance to gypsy moth. Trial sites of transgenic poplars were established in Xinjiang, Northwest China. Additional work is continued in various institutions to generate borer-resistance and to investigate tolerance to salinity (UNDP/FAO, 1998).
Project GCP/CPR/009/BEL is jointly financed by the Governments of Belgium and China, supervised by the Three North Bureau, and implemented by FAO. Situated in the eastern part of Inner Mongolia, the project has since 1991 undertaken exploration and conservation of P. simonii and related species, has carried out a poplar breeding programme for the Korqing sandy lands, and gathered over 652 domestic and foreign poplar clones for conservation and evaluation. The project's investigations have confirmed previous results on the high breeding potential of P. simonii in North China. The species appears to rank first among cold and drought tolerant native poplars, and its variability over its vast natural distribution range is under investigation. Additional genetic gain in hybrids with P. nigra and P. deltoides could probably be easily obtained through careful selection of parent trees and recurrent selection. Intraspecific hybridization is also undertaken in this species (Table 3). In addition to working on P. simonii, the project has also engaged in the introduction of documented materials of black poplars from Xinjiang (P. nigra) and P. deltoides from northern USA and the Canadian Prairies. Results of preliminary tests at age 2, in the nursery, confirm hybrid vigour of P. deltoides x P. simonii progenies. Ex situ collections (mainly of P. simonii and its close relatives) comprise more than 2380 clones (Figure 1) and are being duplicated for major security. The second phase of the project is due to finish in 2002. In the meantime, project staff are coordinating the preparation and the publication of a monograph on P. simonii and will be pleased to receive information and contributions from scientists, researchers and foresters from all over the world.
The different cooperation projects have managed to coordinate their work and collaborate closely between them, as well as with other national institutions, through exchange visits, and through exchanging experiences and reproductive materials.
|Table 3: Controlled crosses undertaken (shown in parentheses), and planned, in the GCP/CPR/009/BEL provenance (as per August 1999)|
|Female||Male||Populus nigra||Populus simonii|
|Populus simonii||( 35 ) 59||( 29 ) 45|
|Populus deltoides||( 5 ) 9||( 9 ) 51|
Afforestation programmes launched in North China in the 1950's and intensified since 1978 have contributed to the establishment of a network of shelterbelts to reduce soil erosion and desertification. The importance of man-made forests for the provision of a wide range of goods and services is likely to grow in the future. Domestic supplies of timber and wood are likely to depend even more on plantation forestry since a country-wide reduction in logging in remaining natural forests has been implemented since 1998. The Chinese Government has shown a strong commitment to conserve biological diversity and to combat desertification, and plantation forestry can play a major role in this. In North China, plantation programmes will, in addition, yield direct economic returns.
Originally using a short list of indigenous species and varieties, including native poplars, and paying limited attention to the genetic quality of the materials, plantation programmes have become more sophisticated and rely nowadays on a few proven, selected species and clones, from which higher outputs are expected. Poplar is at the core of modern man-made forestry and a key element in rural development, and there are few alternatives to the genus.
Monoclonal poplar plantations have, nevertheless, already led to severe disease and insect damages and to considerable productive losses in several parts of the Three North. The limited number of clones available for large-scale programmes, and their narrow genetic base, advocate the need for continuity of poplar improvement in North China to ensure long-term sustainability of the genetic gains obtained so far. Such programmes are also necessary for a proper utilization of, and to optimize outputs from, biotechnologies that are being developed in the forestry field.
A prerequisite for a sustainable programme is the maintenance of a wide genetic base for breeding and selection work. The experience gained in China demonstrates the importance and uniqueness of the native genetic stock, not only for planting purposes, but also and mainly for infusion of genetic materials in customized and tailored breeding programmes. Since native gene pools are rapidly decreasing, urgent action is required to conserve them, especially for P. cathayana, P. euphratica, P. nigra and P. simonii.
Introduction of poplar clones developed in other countries under different conditions, can only complement long-term national breeding efforts. In North China, few exotic clones have proved adapted, and the main interest of foreign poplar germplasm lies in crossing it with native well-adapted species and provenances (from northern, continental provenances). Among the best candidate parents are P. nigra, P. deltoides and P. simonii.
The size of the Three North region is a great challenge and several complementary, short-term projects and initiatives have already begun to address the issues of conservation and poplar improvement. The work should be continued with a long-term perspective and must benefit from strong political commitment to be successful.
Raising awareness of decision makers, foresters, scientists and the general public at national, provincial/regional and local levels is a critical issue. International workshops, like the one organized by the Chinese-German collaborative project in 1997, or the one planned by this project in 2002, can be instrumental in this respect. They could also help disseminate the Chinese experience in poplar breeding, experience that could be useful to other countries with similar difficult ecological and climatic conditions, like the Canadian Prairies, Central Asian lowlands, and South-western Russia.
For more information about project GCP/CPR/009/BEL, please contact:
Afforestation, Forestry Research, Planning and Development
in the Three-North Region of China
Xilamutun Dajie, Zhelimumeng Forestry Division Building,
028000 Tongliao, P.R. of China
Tel: +86 475 8315009 Fax: +86 475 8315827
E-mail: [email protected]
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