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)

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 as Fodder

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 shoot’s 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.