The distribution, characteristics, use and ecological functions of Caragana in China
Niu Xiwu1 Gao Hongwen2
1 (Shanxi Academy of Agricultural Sciences, Taiyuan, Shanxi 030006, China)
2(Grassland Research Center, Beijing Academy of Agricultural Sciences, Beijing, 100089, China)
[A longer version of this paper was presented at the International Conference on Grassland Science and Industry, Hailar City, China, 16-20 July 2001]
|Summary Caragana is an important genus of drought -
resistant leguminous shrubs, common in north - western China and on the Tibet Plateau.
Sixty species have been recorded in China; all develop typical xeromorphic features and
several life forms occur from tall tree - like shrubs in the forest-grassland zone to sub
- shrubs in the deserts. In addition to being very drought tolerant, Caragana can
overwinter safely where temperatures go down to - 400 C and withstand summer
heat above 450 C. They are excellent shrubs for sand - fixing, erosion control,
animal feed, wildlife-cover, fuel and fibre. Caragana has been widely planted in
watershed management and soil conservation projects in northern China; mainly C.
korshinskii and C. microphylla, are used. Cultivation and management techniques
are described and details of the feeding value of several species are given.
Key words: Caragana
Caragana Fabr., sometimes known as "Peashrub", is a widely distributed genus of leguminous, Papillinaceous shrubs which play important roles in ecological environment management in northern China. They are major components of many arid and semi-arid pastoral ecosystems, especially in cold areas, and are also widely used for rehabilitation of degraded land. China has great experience in their ecology and use, both in natural pasture and over vast areas of land improvement and reclamation projects, covering hundreds of thousands of hectares, which have been executed, especially in the Loess plateau zone, over the past quarter of a century.
These shrubs are useful as a main component of natural ecosystems and are cultivated for land reclamation, soil conservation and forage. They are extremely hardy in dry and cold situations and of easy cultivation. In addition to their pastoral qualities they are good sources of honey, can provide fibre and energy, encourage the reinstallation of native vegetation on degraded land after they are established and are good wildlife habitat.
This paper brings together many years of experience in studying and using Caragana in northern and north-western China where it has proved extremely useful, in the hope that the technology and genetic material can be used in areas with similar problems and ecological conditions.
Caragana are deciduous shrubs distributed widely in Eurasia. In China they are mainly found in the arid and semiarid area of the northern part of the Yellow River catchment, and the Tibet Plateau. Their global distribution range is from the Caucusus and Central Asia going east towards Russian Siberia, Korea and Japan, southwards to Nepal, Bhutan, Sikkim and northern India. There are more than 100 species of which 66 are recorded from China. Species and their distribution are listed in Appendix 1.
Shoots Adapted to their habitat, all species of Caragana develop obvious xeromorphic features. Their shoots may have hard thorns such as shoot thorns, stipule thorns, bark thorns and some petioles transformed into thorns are found. Caragana is a typical deciduous xerophilous shrub and dominant life form of desert vegetation and shrub grassland communities. For instance C. microphylla is a thorny xerophilous shrub grown in typical grassland; C. intermedia and C. brachypoda are dominant shrubs in deserts; and C. stenophylla is widely distributed in deserts; these species all have very hard stipule thorns. C. tibetica is a constituent species of steppic desert and also appears in arid cold alpine and sub-alpine regions; it is a low plant with narrow, curly leaves, fairly developed coarse hair and petiolar thorns; it has an obvious xeromorphic structure of a mat or semi-mat shaped thorny shrub; it is mostly a secondary species in deserts. As their leaves are hardened into thorns, these small shrubs are very drought-tolerant; C. roborovskyi grows in arid, denuded hills and usually forms a small strip along dry valley bottoms or watercourses. When its leaf stalk hardens into thorn it become a very small thorny xeromorphic shrub. C. korshinskii is a tall thorny shrub which forms open communities in deserts.
Caragana can be classified into several life forms, based on morphological features, from tall tree-like shrubs to sub-shrubs. There is white hair and wax layers on the shoot surface for protection against sunburn in summer and extreme cold in winter.
Caragana has a great ability for shoot regeneration. A survey in Shanxi showed that a single plant of C. microphylla had over 400 shoots and its crown cover was 3.28 x 3.17 metres. More new shoots sprout from the crown soon after pruning or cutback. When a shoot is buried in soil, new roots and shoots can sprout from it. Moderate grazing by animals is good for shoot regeneration.
Leaves Most species of Caragana are deciduous xeromorphic shrubs with leaves which are small and degenerated into strips or linear shapes. Anatomical studies indicate that Caragana leaves have a typical xeromorphic structure. On both their upper and lower epidermis there are dense and heavy epidermal-hairs which are white, inner circled at the base and of a semicircular pipe shape, and they stretch upward. The epidermis hairs assist in reflecting strong sunshine to lower the temperature, which is a typical character for xeromorphism. The epidermis of the upper and lower leaf surfaces have cuticles of 2.9 and 2.7 µm thickness respectively. On the upper epidermis, stoma area is 418.1 µm² and their density is 234.4 stomata/mm²; on the lower epidermis stoma area is 406.5 µm² and density 224.4 stomata/mm². All stomata on both the upper and lower epidermis are covered with hair. Mesophyll palisade cells have an area of 95.4 µm² and are tightly arranged. Both upper and lower sides are developed into palisade tissue. There is a total of 4-6 layers with a thickness of 106.0 µm. Since spongy tissue is completely degenerated, so the ratio of palisade tissue to spongy tissue is very high. On the cross plane there are 19.7 veins and the diameter of the main fibro-vascular bundle is 100.0-117.6 µm. The vessel cavity has a maximum diameter of 12.3 and minimum 3.3 µm, vessel thickness of 2.8 µm and primary veins 49.3. The conducting bundle for primary veins have a vessel-cup composed of sclerenchymatous cells. The diameter of the vessel cups is slightly bigger than those of primary veins and have a supporting function. As for the subsidiary veins, they have a well developed conducting bundle and obvious vessel cups, which are all typical characters for xeromorphism.
Table 1. Heat-resistance of C. korshinskii
Cold resistance Most species of Caragana can overwinter safely under conditions with extreme low temperature of 30 to 40 ºC. For example, in Xilingerle county where the mean annual temperature is 1.5 ºC, the minimum temperature - 42º C and the frozen soil layer 290 centimetres, C. korshinskii and C. microphylla grow well and overwinter safely. A survey in Wuqi Shaanxi showed that fifty - day - old plants of C. korshinskii, which were 2-3 cm tall with three leaves, had an over-wintering rate of 100 percent, where mean temperature in January was - 7.8 ºC and the minimum - 28 Cº. Some species, such as C. versicolor and C. jubata are found on the Qinghai-Tibet plateau at elevations above 4,000 metres where extreme minimum temperatures are below - 42 ºC.
Sand fixation and soil protection Caragana with their big root system and great crown cover are very good for sand fixation and soil protection. About 0.5-1 m³ of sand could be fixed by one plant. Species, such as C. microphylla and C. tibetica, are excellent sand fixers. The more they are buried by drifting sand, the more branches they sprout, the more vigorous they grow and the more sand they fix. One survey showed that a single plant of C. microphylla buried by sand, produces tens or even hundreds of new branches after a few years, crown cover was 13.5 x 11.0 metres and tens of cubic metres of sand were effectively fixed. Caragana is widely used as a windbreak in northern China, combined with trees. The effective windbreak range is about 10 - 15 times plant height. Caragana is also valuable for soil and water conservation in hilly country. It has been measured that 34 percent rain was saved and 78 percent runoff was reduced on land under Caragana.
Invasiveness Caragana does not cause any problems in China but C. pygmae is reported as invading badly-managed pastures in Mongolia.
Caragana is highly nutritious (Table 2). Nitrogen in twigs and leaves is as high as 2.574-4.211 percent. It is also rich in minerals, such as P, K, Ca, Si, Mg, Na, Fe, and Al. Eight amino acids essential for animal growth account for 45 percent of the crude protein. At the stage from blooming to seed set, its crude protein content is 14-19 percent and ether extract content is 4-7 percent. At flowering the content of digestible crude protein per kilo of branches is equivalent to that of 2.67 kg of maize or 0.88 kg of peas or 0.64 kg of beans. The young shoots, leaves and flowers are highly digestible and have good palatability. Caragana tolerates browsing well and is quality feed, especially for sheep, goat, deer and camels. On the basis of nutrition standard for fattening lamb, a goat with liveweight of 27 kg, a daily intake of 0.89 kg natural wind-dried shoots and leaves could meet its nutritional needs for a daily gain of 159 grams. During winter, over 20 percent of shoots of Caragana can still be used, so it is valuable winter feed for browsing animals and in snowy conditions it is valuable because it is accessible when herbaceous vegetation is covered. It also provides good shelter for livestock during hot summer as well as in the cold, windy winter-spring.
Table: 2. Nutritive value of Caragana at flowering
Energy source Caragana has very strong regeneration ability of shoots and needs to be cut back every 3 - 4 years for vigorous growth. Average shoot production is over 1,500 kg/ha/yr. Trunk shoots heat value of combustion is 4480 kcal/kg and leaf 4330.58 kcal/kg. The heat value of 1.63 kg dried shoot is equivalent to that of 1 kilogram standard coal. So it is a very good energy plant.
Soil improvement Soil can be greatly improved by planting Caragana due to their big root system, nitrogen fixation and the great amount of litter produced. Survey results showed that soil organic matter was increased 0.3-0.6 percent after 10 years. Soil also can be protected against erosion including wind erosion which is serious in the kind of country where Caragana is used. It also provides good cover for wildlife and, by its protection, encourages re-establishment of native flora on degraded sites.
Raw material fibre. Caragana is quality raw material for industry. The average fibre length is 0.41 mm, maximum length 0.8 mm and minimum 0.23 mm. Average width of Caragana fibre is 11.5 µm, maximum 18.6 µm and minimum 6.2 µm. Ratio of length to width of fibre is 37.5, its coarse pulp rate 51.2 percent, fine pulp rate 38.5 percent, rinse rate 4-7 percent and whiteness degree 6.02-7.02 percent. Caragana fibre is good raw material for many kinds of paper. It is also good raw material for fine quality fibreboard. It was estimated that about 900 pieces of standard quality fibreboard could be produced from 1 hectare of Caragana. In addition shoots can be used for basketry to increase farmers income.
Honey and herbal medicine. Caragana has a long blooming period and is highly melliferous, so it is also a good honey plant. Its root, flower, shoots, bark or seed can be used in herbal medicine.
Establishment. C. korshinskii and C. microphylla are the main species cultivated. In northern China low rainfall and a short growing period are major limiting factors for shrub establishment. Direct seeding during the rainy season is feasible, but since about 60 days of growing time is required for seedlings to overwinter safely, rearing seedlings in nurseries and transplanting is recommended for successful establishment. Direct seeding would mainly be used on very rough ground to provide some protection and where nursery facilities are difficult to provide. On sloping land careful land preparation should be carried out in advance of planting (which is done in early spring); this usually involves construction of water-collecting terraces (individual "fish scale" terraces are mainly used) and hole-digging. The usual care must be taken at planning to ensure that young plants are not dried out or exposed to direct sunlight during transport from the nursery and water should be given wherever possible. There is a positive linear relationship between precipitation and Caragana growth. Planting density should be based on local precipitation. Recommended planting density for random planting are shown in Table 3. When planting in rows recommended methods are: 3 - 4 metre spacing in zones, a pair of rows with each zone, one metre apart and 50 centimetres between plants.
Since Caragana is usually planted in areas of risky rainfall and its success is very dependent on the weather at and immediately after planting, it will often be necessary to gap up plantations in the second year. Revegetation with Caragana, is not therefore cheap and depends on the availability of abundant labour both for land preparation and at planting when farming tasks may also be demanding.
Management: Caragana seedlings are easily injured by wind, moving sand, grazing and winter cold. So during the seedling stage, grazing should be prohibited and protection measures taken. Plants take three years to mature. Moderate grazing stimulates shoot growth and sprouting. Cutting back every 3 - 4 years is good for shoot regeneration. Because of their windbreak function and the need for continuous forage supply, rotational cutting back, probably every three years, is recommended. Cutting back, to a height of 3 - 5 centimetres, is usually done at the end of winter, when soil is still frozen.
Caragana is a very useful genus both as a component of the wild vegetation and for revegetation and livestock feed in harsh, semi-arid to arid, cold climates with hot summers. Its ability to grow up through drifting sand makes it especially valuable for sand fixation.
It is one of the few easily-grown shrubs for revegetation in cold desert climates; several species, including C. korshinskii and C. microphylla are cultivated, are easily raised in nurseries and stand transplanting well. The great range of species has a potential for use over a very wide ecological range and other species may be suitable for domestication and use elsewhere and in semi-arid lands outside China. They have been very widely used for watershed management and sand control in China and their silviculture is well understood. With adequate management and regular cutting back plantations are long-lived; the protection which the bushes give to the soil along with their fertility-building through symbiotic nitrogen fixation encourages the recolonisation of degraded land by native plants.
In addition to its great qualities for land protection and soil improvement Caragana is a valuable forage, best suited to browsing stock such as camels and goats, and to browsing wildlife. It is highly nutritious providing a high-protein feed; it is doubly valuable in winter since its twigs remain edible and, in case of snow cover, are accessible when herbaceous forage is covered.
It has many secondary uses which include: a source of honey, with flowering over a long season; firewood; fibre for particle-board; and it has some applications in herbal medicine.
This genus merits attention in other areas of similar climate. It is, however, a crop which demands a large labour input for establishment, as well as protection during the three years after establishment and adequate management thereafter.
[This shortened and revised version of the paper originally presented at the Hailar City Conference in 2001 was prepared by J.M. Suttie in April 2003].
Some common names
Littleleaf Peashrub (C. microphylla)
Intermedia Peashrub (C. Intermedia)
Squatfoot Peashrub (C. brachypoda)
Narrowleaf Peashrub (C. stenophylla)
Burr Peashrub (C.tibetica)
Mongolia-Qinghai Peashrub (C. roborovskyi)
Korshinsk Peashrub (C. korshinskii)
Versicolorous Peashrub (C. versicolor)
Shagspine Peashrub (C. jubata).