Distribution, Composition and Community Characteristics of Seabuckthorn in Beichuanhe Basin Zheng Jiali, Hu Jianzhong




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Distribution, Composition and Community Characteristics

of Seabuckthorn in Beichuanhe Basin

Zheng Jiali, Hu Jianzhong

(College of Soil and Water Conservation, Beijing Forestry University;Key Laboratory of Soil and Water Conservation and Desertification Combating of the Ministry of Education,Beijing 100083)



Abstract: There exist a lot of natural seabuckthorn (Hippophae Rhamnoides ssp.Sinensis )stands in an altitude from 2300 to 3600 meters in the Beichuanhe basin, which is located on the upper reaches of the Yellow River. Picea crassifolia+Hippophae rhamnoides ssp. sinensis, Betula platyphylla+Hippophae rhamnoides ssp. sinensis, are two kinds of good mixed native stands for imitation of rehabilitation in high mountains of the elevation from 2700 to 3000 meters, while two kinds of good artificial stands, that is, Larix principis-rupprechtii+Hippophae rhamnoides ssp. sinensis, Populus cathayana+Hippophae rhamnoides ssp. sinensis, from more than 40 years practice, are suitable to afforest in the shady and semi-shady slopes of the low mountains with an elevation under 2700 meters. Pure seabuckthorn stands and mixed types of seabuckthorn forests and herbages, are eligible for high mountains and semi-shady and shady slopes in the low mountains. In the areas of which altitude is above 3000 meters, the natural seabuckthorn bushes are mainly protected according to the concerning laws. On the process of succession, the kinds of plant species not only change from intolerant, moderate to shade-tolerant species, but the number of the species gradually decrease as the stands grow and canopy coverage increases, with the Gleason richness presenting from small to large then small again along with the growth of seabuckthorn going on, and the Simpson dominance index presenting a contrary process. The succession in the areas trends to get to the climax community of Picea crassifolia+moss forest. The multiple eco-economical function of different seabuckthorn stands can be reached after a suitable allocation, scientific management and technology utilization based on the principle of imitating native seabuckthorn forests..

Key words: distribution; stand composition; community characteristics; Seabuckthorn (Hippophae rhamnoides ssp. Sinensis); the Baichuanhe basin

The eco-environment is the foundation for mankind’s survival and development. Improving and protecting the eco-environment is a fundamental national policy of China so as to implement the sustainable development1. The natural conditions on the upper reaches of the Yellow River are so harsh and the eco-environment is so fragile that it is difficulty to protect the ecological environment. In addition, people's ecological consciousness is so faint and the ecological construction input is so seriously insufficient that the eco-environment in this area trends to be worse and worse on the whole. The Central Party Committee, the State Council puts forward the develop-the-west strategy, and regards Natural Forests Protection Program, Land Converting Program, Three-north Shelter Forests Protection Program and Sand Combating Program, which are the main parts of the eco-environment construction, as an important component of implementing the develop-the-west strategy and the key construction contents2. Vegetation restoration and construction is the task of top priority when constructing the eco-environment in the upstream areas of the Yellow River, directly concerning several million people's survival and development, and flood disasters in the downstream.

Converted farmland is an incomplete degrading ecosystem with sparse vegetation, degraded soil and serious erosion. It is necessary to guarantee the constant existence of renewable resources and the dynamic equilibrium of the natural ecosystem so as to restore vegetation. Generally, the ecological restoration is designed artificially which is carried out on the eco-system and the main material is plants.3 Seabuckthorn is an important material of afforestation in the upper reaches of the Yellow River and Beichuanhe Basin, but reviewing on the research in advance, little is done on the distribution, type and community characteristics of Seabuckthorn stands.4 5 Since we do some research in this field so as to instruct the practice and offer scientific basis for implementing ecological construction.

1 General situation

Beichuanhe River is the Second branch of the Yellow River, of which 3009km goes through Datong County, Qinghai Province. It is located in the transition area between Qilian mountain and the Lloess Plateau, 10051-10156E, 3643~3723N, about 2280~4622 meters above sea level. It belongs to continental climate, of which the annual sunlight hours, average temperature, non-frost day, precipitation, evaporation, humid index are 2605h, 2.8℃, 70~120d, 508mm,1290mm, 0.56~1.32 respectively. According to altitude from low to high, 3 main types, the River beach area, the low mountain, the high mountain is sequential. The main soil types are hilly lithosol soil, hilly meadow soil, brown cinnamon earth, chernozem, castanozem, moistened oasis soil, bog soil etc. The forest belongs to the cold warm evergreen coniferous trees and fallen broadleaf forest, etc. There are not only obvious slope tropism but vertical zonality with regard to its distribution situation.

2 Material and method

The test materials are mainly pure seabuckthorn stands and mixed ones in Datong County, including natural secondary forests and artificial forests (which includes converted farmland). The investigating methods include “route investigation”6 and “standard plots investigation”7. Route investigation is used to investigate the total characteristic of the distribution of seabuchthorn and to establish the foundation for standard plots investigation, which is mostly used to investigate the important seabuckthorn communities and its related characteristics.

2.1 Approaches to community indexes

There are a lot of index and computing approach of community8 9 10. According to the research region and the growth and regional characteristics of seabuckthorn, we have selected 4 representative indexes in this text:


  1. Gleason Richness index:

I=S/lnA

In this equation, S is the total amount of species, and A is the area of the plot.

(2)Simpon dominance index:

In this equation, Pi is relative importance value。

(3)Shannon-Weiner diversity index:

(4)Pielou Evennesss index:



E=H/lnS

In this equation, H is Shannon-Weiner index, S is the amount of species.

3 Analysis and Results

Seabuckthorn has a good adaptability, a wide distribution and a rather wide ecological range, which can forms plant communities with different levels and composition.

3.1 Regional distribution characteristic of seabuckthorn

The natural distribution regions of seabuckthorn in Beichuanhe Basin is mainly in the cold, semi-humid area which is in the high mountain and semi-arid area in the low mountain. The average temperature of the whole year is from -5.7 to – 4.2℃; the average temperature of January is from -17.5 to -8.4℃; the minimum temperature over the years is from -35.7 to -26.6℃; the average temperature of July is from 7.5 to 17.2 ℃.The accumulated temperature of which average daily temperature is ≥5℃ is from 430.7 to 2580.8℃, and the annual precipitation is from 368.2mm to764.4mm with remarkable seasonal variation which is mainly concentrated on the rainy season from June to August. Soil is mainly gray-drab, sandy silluvial soil and sandy meadow soil.

The natural forests of seabuckthorn can be probably found in the places such as hilly area, piedmont area, river beach, etc. Not only can it form pure large community, but also it can form all kinds of mixed ones combining with the main local arbor trees. In Datong County, the seabuckthorn forests have an area of more than 20,000 hm2 among which mixed accounts for 93%. According to the growing period and its distribution, middle-age of 11~25a takes up half of the whole forests; The proportion of male to female is about 1 to 1, and they are distributed nubbly with different size in the horizontal direction11.

Considering the vertical distribution, there are seabuckthorn forests from the elevation of 2300m to 3600m in Beichuanhe Basin. It is concentrated in the hilly area and river valley plain among 2700 to 3300m, which takes up more than 90%, and it is the main shrub species in this region. Above 3300m, it is gradually rarely, only grows sporadically with other bush along with bad growing. From 2700m up to 3300m, the average coverage dropped from 70% to 15%, and within the range from 3000 to 3100m, it is 32%. In the region of river beach and gully with quite good site factors above 3000m, the coverage can be above 40% with good growing circumstances. Limits of slope condition should be about 2700m, where below this elevation, seabuckthorn is apt to be planted in the shady and semi-shady slope12, and the district suitable for seabuckthorn in Beichuanhe Basin are mainly as follows:

DistrictⅠ: hilly area, shady and semi-shady slope below 2700m. It is the place where vegetation restoration is mainly relied on artificial measures.

District Ⅱ:river beach, gully area, from 3000m to 3100m. It is the place where people mainly rely on natural force to restore vegetation;

District Ⅲ: hilly area of all slopes from 2700m to 3000m. It is the place where people rely on the natural force to restore vegetation along with artificial measures;

District Ⅳ: hilly area of all slopes from 3000m to 3300m. It the place where is enclosed to avoid disturbance from human and domestic animals.

Among these 4 distribution types mentioned above, the districts of Ⅰ,Ⅱand Ⅲ are the optimal sites for the natural distribution and the artificial afforestation of seabuckthorn, while in district Ⅳ, what we can do is only to breed it by enclosure, and in particular, we should restrict the cutting activity of seabuckthorn forests that have already glowed into forests so as to protect resources.

Artificial seabuckthorn forests occupy quite large an area in low mountain and high mountain. Pure forests and mixed forests with arbors or herbage are mainly the forest types. The soil in this area is mainly castanozem.

3.2 composition of seabuckthorn of different forests

The composition of seabuckthorn is described through the natural stands and the artificial stands as following.

3.2.1 The natural forests

In the natural environment, seabuckthorn mainly migrate with seeds, while after settling down, they rapidly expand the domain through sprout with roots, forming the appearance of the perfectly rotundity. If the seeds are migrated to the patch in the ecotone of secondary forests, they will grow mixed with arbor nubbly. After natural sparseness, they will often form the multiple stands of arbor and shrub with the invasion of secondary species, such as Betul platyphylla, Betul platyphylla and Betula albo-sinensis.

(1) Picea crassifolia+ Hippophae Rhamnoides ssp.Sinensis

This forest type generally lies in the fringe of Picea crassifolia forest. It is naturally formed after the invasion of seabuckthorn after the original Picea crassifolia forest is intensively deforested, or after the destruction by man or the natural disaster including fire, plant diseases and insect pests, etc. The age of Picea crassifolia in the investigating area is generally more than 50; the canopy is from 0.7 to 0.8; the height is from 15m to 20m; the diameter is from 25cm to 30cm.

The height of seabuckthorn is about 2-4m, forming the second layer by mixing with other bushes in the forests.

There are some other shrubs such as Berberis circumsterrata, Rosa sweginzowii, Caragana brevifoia, Rosa omeiensis, Lonicera syringantha, Contoneaster acutifolius, Spiraea mongolica, Sorbus koehneana, Euonymus przewalskii, Ribe pulchellum, Cerasus tomentosa, Potentilla glabra,etc, of which the coverage is about 20%.

Herb species include Carex allivescens, Fragaria orientalis, Ligularia przewalskii, Aconitum sinomontanum, Cimicifuga foetida, Paeonia veitchii), Triosteum pinatifidum, Ephedra sinica, Saussurea sylvatica, Polygonatum verticillatum, Pedicularis rudis, Thalictrum baicalense, Caltha palustris,etc, of which the coverage is below 10%。

This type trend to the climax community that is Picea crassifolia when stopping disturbance.

(2) Populus davidiana+ Hippophae Rhamnoides ssp.Sinensis

Generally, it lies in all slopes of lower part and the semi-sunny and semi-shady slope of middle and upper part of the hillside. In the lower part of the mountain, form the altitude of 2000m to 2700m, and the gradient 30°~40°, the forest land is dry, and the soil is relatively infertile, where the main specie is Populus davidiana, and there is also a small amount of Betul platyphylla and Betula albo-sinensis scattering there. Its composition is simple and it is relatively sparse, whose canopy is 0.4-0.6, the age is 25-45, the height is 8-12m, and the diameter is 8-14cm. In the middle and upper part, it is relatively steep with abominable growth condition, the forest grows slowly which is susceptible to be destroyed by wind and snow, and there are a lot of withered or fallen trees.

The coverage of the shrub layer whose main species is seabuckthorn is about 50%-70%, and the height is about 2-3m.There are some other shrub species such as Contoneaster acutifolius, Caragana brevifoia, Potentilla glabra, Berberis circumsterrata, etc.

The coverage of the herb layer is about 30%-50%, of which the species include Thermopsis licentiana, Anaphalis lacteal, Thalictrum rutifolium, Thalictrum przewalskii, Paeonia veitchii, Rubia cordifolia, Artemisia sacrorum, Drynaria sinica, Calamagrostis pseudophragmites, Adenophora stenanthina,etc. Clematis macropetala exists in the interbedded layer.

(3) Betul platyphylla+ Hippophae Rhamnoides ssp.Sinensis

This type distributes comparatively widely, and by and large, its distribution status is the same as that of Populus davidiana + Hippophae Rhamnoides ssp.Sinensis forest we have mentioned above. It is a very important local natural community. The age of Betul platyphylla is 30-50; its canopy is 0.8; its tree height is 7m; and the diameter is 9cm.

The height of seabuckthorn is 2~3m. Rosa omeiensis, Lonicera hispida, Spiraea alpine, Salix oritrepha,etc also exist in the shrub layer, of which the coverage is up to 40%~60%.The main species in the herb layer are Polygonum viviparum, Carex allivescens, Fragaria orientalis, Geranium pylzowianum, Astragalus datunensis, Thalictrum baicalense, Ephedra sinica, Caltha palustris,etc and its coverage is 25%-40%.

(4) bush forest of seabuckthorn

The bush forest of seabuckthorn usually has two layers, that is shrub layer and herb layer. Seabuchthorn takes up an absolute dominant status. The incidental species are Contoneaster mwltiflorus, Myricaria paniculata, Contoneaster.adpressus, Potentilla fruticosa, Potentilla glabra, Salix cheilophila, Lonicera syringantha, Caragana brevifoia, ,Ribes stenocarpum, Berberis circumsterrata, etc. All of the species are in the same layer as seabuckthorn except Salix cheilophila. The height of the constructive species is 2-4m and the coverage is 60%-90%.

The composition of species, height and coverage differ from river beach area to the hilly area with regard to local conditions. The main characteristic is that there isn’t any dominant species and the common species can be listed as Stachys sieboldii, Galeopsis bifida, Thalictrum petaloideum, Rubia cordifolia, Capsella bursa-pastoris, Lancea tibetica, Caltha palustris, Phlomis dentosa, Urtica cannabina, Fragaria orientalis, Artemisia sacrorum, Elymus nutans, Potentilla anserine, Poa alpigena, Plantago asiatica, Taraxacum mongolicum, Ranunculus tanguticus,etc. The height is 20-60cm and the coverage is 40%-60%.

At a few locations, seabuckthorn often grows into the form of little arbor along with the good site factors, and its height is 4-5m. In this way, it form 3 layers which are little arbor, shrub and herb.

3.2.2 The artificial forest

In Beichuanhe Basin, seabuckthorn is a kind of shrub species which grows fast, growing into forest in the next 4-6 years after afforestion, and forming stands with the height of more than 2m. It is the predominant or concomitant specie for forests of water resource protection and soil and water conversation in high and low mountain in practice. The afforestation is imitative of the natural stands which are mainly distributed in barren hill, waste valley, barren beach and converted farmland. And the names of many artificial stands are the same as those of the natural stands.

(1) Picea crassifolia+ Hippophae Rhamnoides ssp.Sinensis

The forests are mainly linal, zonal of nubby inter-planted. It is a kind of stands which can bring seabuckthorn into full play of improving the soil, assisting the arbor and covering the clearing. It can be often found in vegetation imitation of rehabilitation in barren hill and converted farmland in high mountain.

The age of Picea crassifolia in the converted farmland in the low part of the high mountain of shady and gradient slope is generally more than 50; the average height is 6.8m; the diameter is 9.1cm; the production is 206m3/hm2 ; the canopy is up to 0.8. The height of seabuckthorn is about 1.5-2.5m, forming the second layer. The main herb species are Ephedra sinica, Saussurea sylvatica, Geranium pylzowianum, Carex allivescens, Poa pratensis, Ligularia sagitta, Fragaria orientalis, Lancea tibetica and so on. Its coverage is about 40%. As the canopy of Picea crassifolia goes bigger and bigger, seabuckthorn trends to wither and finish its assistant function which is usually cut, making into use of fire.

(2)Larix principis-rupprechtii+Hippophae rhamnoides ssp. sinensis

Larix principis-rupprechtii is an exotic species, and it was introduces to Datong County for experiment in 1957. Now it have become an important species of the artificial afforestation, distributed in low mountain, high mountain, barren hill and converted farmland. The construction of the artificial forest of Larix principis-rupprechtii is relatively simple, and it is mainly the pure planted or mixed planted with seabuckthorn..

The age of Larix principis-rupprechtii which is planted with seabuckthorn in the converted farmland in the low part of the high mountain of shady and gradient slope is18, the average height is 6.7m; the diameter is 7cm; the canopy is up to 0.8; the production is 49m3/hm2. There are some shrubs in the forest like Rosa omeiensis, Berberis dasystachya, Potentilla fruticosa, and some herbs like Polygonum viviparum, Fragaria orientalis, Ligularia przewalskii, Rubia cordifolia, Caltha palustris, Carex allivescens, Galeopsis bifida, Lepyrodiclis holosteoides, Aster diplostephioides, Carpesium lipsky, Geranium pylzowianum, Ligularia sagitta, Sonchus oleraceus, Microula sikkimensis, Cephalanoplos segetwm, Ephedra sinica, Poa alpigena, Anaphalis lacteal, etc. Its coverage is about 40%.

(3)Betul platyphylla+ Hippophae Rhamnoides ssp.Sinensis

It can be often found in barren hill and converted farmland in shady slope in low mountain and all slopes of high mountain. The mixed types of Betul platyphylla+ Hippophae Rhamnoides ssp.Sinensis is inter-planted in the form of band or line, which utilizes seabuckthorn’s characteristics of fast-growing and strong nitrogen fixation function, and well promotes Betul platyphyll’s growth, making it grow into forest relatively fast.

The age of Betul platyphylla is 23, diameter is 8.7cm, canopy is 0.6 and production is 73m3/hm2. And the average height of seabuchthorn is 3.9m, base diameter is 6.1cm, and the coverage is 35%. The coverage of herb layer is 50%, of which the species include Saussurea sylvatica, Ephedra sinica, Geranium pylzowianum, Primula tangutica, Galium aparine, Ligularia sagitta, Stachys sieboldii, Poa pratensis, Berberis vernae, Microula sikkimensis, Phlomis dentosa, Ligularia przewalskii, Cacalia deltophyllus, Carpesium lipsky, Caltha palustris etc.

(4)Populus cathayana+Hippophae rhamnoides ssp. sinensis

The afforestation of Populus cathayana has a long history in Beichuanhe Basin. Populus cathayana grows stable, and it is the main species in the plain and the side of farmland, pavement and highroad. Populus cathayana+Hippophae rhamnoides ssp. Sinensis is a common type of inter-planted forest in low mountain and the river beach, whose growth is superior to that of pure forest of Populus cathayana, and the rate of plant disease and insect pest is also lower13. It is also fountain in high mountain.

The age of Populus cathayana which is located in the middle part of high mountain of sunny slope is 22, average height is 8.2m, diameter is 11.2cm, canopy is 0.7 and production is 95m3/hm2. And the average height of seabuchthorn is 1.5m, base diameter is 3cm. and the coverage is 20%. The coverage of herb layer is 45%, of which the species include Elymus nutans, Bromus inermis, Poa pratensis, Halenia corniculata, Halenia corniculata, Geranium pylzowianum, Gueldenstaedtia diversifolia, Pedicularis kansuensis, Ranunculus tanguticus, Dracocephalum heterophyllum, Taraxacum mongolicum, Inula japonica, Artemisia lavandulaefolia, Plantago asiatica, Anaphalis lacteal,Gentianopsis paludosa, Astragalus datunensis ,Medicago lupulina, Microula sikkimensi, Ligularia sagitta, Thalictrum petaloideum Oxytropis bicolor, etc.

(5)pure forest of seabuckthron

It is located in barren hill and converted farmland in the low mountain of steep and shady slope and the high mountain. In low mountain where the planting density is 6500 trees/hm2, the average height is 2.5m, the base diameter is 4.5, and the coverage is above 90%. There are more than 20 species in the forest including Galeopsis bifida, Microula sikkimensi, Pedicularis rhinanthoides, Halenia corniculata, Carpesium lipsky, Microula sikkimensis, Galium verum, Potentilla anserine, Poa pratensis, Carduus crispus, Phlomis dentosa, Urtica cannabina, Ephedra sinica, Rubia cordifolia, Saussurea sylvatica, Gentianopsis paludosa, Fragaria orientalis, Caltha palustris, Stachys sieboldii, Bromus inermis, Leontopodium leontopodioides, Chenopodium glaucum, etc. But its coverage is below 10%.

(6)Seabuckthorn + herbage

It is an important type in land converting program. It is located in the shady slope of low mountain and most sites of high mountain. The grazing herbage species include Medicago setiva, Melilotus suaveolens, Medicago setiva,Melilotus suaveolens, etc.

After making a general survey of the plant species in the above-mentioned natural and artificial stand of seabuckthorn, we have not found comparatively obvious species connection. There is no inevitable connection between the plants, and moreover, the plant types in the forest are more comparatively closed with those of the natural vegetation. The stands of seabuckthorn especially the species connection still remains to be studied in the future.

3.3 Community characteristics of Seabuckthorn

Hippophae L. Genus may have ever originated from middle and high ailtitude between the east Himalayas and Traverse Mountain. Its primitive species is a kind of sunny hardwood which is fond of water and sandy soil , able to bear atmosphere aridity and frigidity, and it is a member in the variable zone transiting from grassland to forest. Low illumination, high temperature and excessive low precipitation are the limited factors of distribution of Hippophae L. genus14. Seabuckthron is often the pioneer species of forest after fire and cutting action, forming the single community. With evolution goes on, the forest gradually recovers, especially when the illumination is insufficient, seabuckthorn declines rapidly and have finished its historical mission.

Tab. 1 Community characteristics of different Hippophae rhamnoides ssp. sinensis forests in the Beichuanhe basin



Item

QH8113

QH8091

QH7192

QH8111

QH8121

QH8041

QH8093

Species type

P.crassifolia+ Seabuckthorn.

P.crassifolia+Seabuchthorn

L.principis-rupprechtii+Seabuchthorn

B.platyphylla+ Seabuckthorn.

P.cathayana+Seabuckthorn.

Seabuckthorn.

Seabuckthorn.

Land use form

Converted farmland

Converted farmland

Converted farmland

Converted farmland

Converted farmland

Natural bush

Converted farmland

Converting time

13

27

17

23

22

30

10

Gleason

Richness index



6.881

5.926

8.411

10.897

15.626

15.533

16.155

Simpson

Dominant index



2.169

1.124

1.121

1.093

1.564

0.296

0.078

Shannon-Wiener

Diversity index



2.022

2.134

2.304

2.560

3.541

2.988

2.860

Sheldon

Evenness index



2.755

2.939

2.770

2.761

2.425

1.000

1.671

Community characteristic of seabuchthorn is showed in table 1. As to the richness index, the natural forest is 15.533, the artificial forest is 16.155, and it is generally higher than that of the other inter-planted forests; According to different mixed forests, the richness index of broad-leaved forest is 10.897-15.626, and it is higher than that of conifer forest which is 5.926-8.411. Seabuckthorn bush is at the primary stage of succession, which is mainly composed by sunny species of great tolerance. And the vertical height of forest is low and the illumination in the forest is comparatively sufficient, where many species are suitable to distribute. So its Richness index is higher than that of other inter-planted forests. Broad-leaved forest is mostly broad-brush with better illumination condition, and it can contain much more tolerant and moderate species. So its richness index is higher than that of conifer forest.

As for Dominance index, conifer forest takes up the first place (which is 1.121-2.169), which is contrary to Richness index. Broad-leaved forest ranks two (which is 1.093-1.564), and seabuckthorn is the lowest (which is 0.078-0.296). In form 1, most of artificial forests are converted farmland, where row spacing is comparatively even, and at the same time, richness of conifer forest is comparatively low, so it is easy to understand why its dominance degree is high. However, seabuckthorn’s ability of spreading is so strong and species types are so various that it forms a situation in which dominance degree is comparatively small. Diversity index of the Populus cathayana+Hippophae rhamnoides ssp. sinensis forest is the only one that can arrive at the value of 3.541, the rest has a similar trend as that of richness, that is seabuckthorn bush is the comparatively high one, which is up to 2.860-2.988; that of the broad-leaved forest is medium, which is 2.560; and that of conifer forest is the lowest, which is 2.022-2.304. Through doing some comparison, we found that the trend of Evenness index is the same as that of dominance index. Evenness of the conifer forest is the highest, which is up to 2.755-2.939; that of broad-leaved forest is medium, which is 2.425-2.761; that of the seabuckthorn bush is the lowest, which is 1.000-1.671. We can also find out that after conversation, richness index of 2 kinds of forests, Picea crassifolia+Hippophae rhamnoides ssp. sinensis aged from 13 to 27, drops from 6.881 to 5.926, as it indicates that, with the canopy increasing, species in the forest not only gradually transits from sunny and neutral to shady, but also its quantity reduces constantly, which trends to reach the local climax community –Picea crassifolia+ lichen.

After conversion, species is comparatively single at the beginning, and as time passes, weeds and pioneer plant with strong spreading ability reach successively, which increases species type gradually. In the series of succession, the stage with highest richness is the middle one, in which there are comparatively much more life forms, which are generally arbor-shrub-herb, and even in which arbor and shrub layer can be divided into 2 inferior layers, and the species types increase acutely because of the low density. At this moment there are not only the remnants of pioneer stage, but also neutral species of the transitional stage, and there are even more shady species appearing in advance. The vegetation variety of the climax stage has a lot of similar characteristics with that of the pioneer stage, for example, the species is always seldom. But there is intrinsic difference between them. Diversity index of the Pioneer stage is relatively low, but species type is mostly sunny ones. It is easy to tolerate bad conditions and restore community, but it is in the earlier stage of transference with bad resistance, so the stability is comparatively bad and will finally be replaced by other species of later stage. However, as for the vegetation of the climax stage, though its variety is relatively low, it is almost shady species and though restored ability is not so good when it is interfered, especially by the artificial destruction, the resistance of community of this stage is the strongest, so the stability is the highest, and it can maintain itself and can survive continuously forever15. The vegetation of the topmost stage is the shady species, which is exactly the model of imitative community in land converting program. In this course, seabuckthorn gives play to covering the earth's surface, improving soil and assisting arbor species from the beginning to the end, and it withdraws from the stage "quietly " until the arbor grows to denseness. It paves the way for the forming of the top community of Picea crassifolia in Beichuanhe Basin. So we can say that seabuchthorn is the " key kind " in the succession in Beichuanhe Basin, and through its function in many aspects, it promote the community evolution. In practice, we should afforest many kinds of seabuckthorn forests according to its characteristic.

4 Result and discussion

Beichuanhe Basin which lies in the upstream of the Yellow River has a very important ecological status. Afforestation of multiple functions forests through imitation and rehabilitation has already become an important component of 3-north shelter forest construction, land converting program and comprehensive management of small watershed. Seabuckthorn is a very important species in this region. In this article, we have studied the distribution, composition and community Characteristics of seabuckthorn in Beichuanhe Basin so as to offer the scientific basis for the eco-environment construction projects. The result shows as following:

(1)There distributes natural seabuckthorn forest in most areas of Beichuanhe Basin of the elevation from 2300m to 3600m. In the low mountain area under 2700m with arid climate, barren soil and dense population, the original secondary forest is mostly cultivated into farmland. In The shady and semi-shady slope of this area, we can breed pure seabuckthorn forests or inter-plated forests. There exists comparatively large seabuckthorn forests above 3000m where is already belong to highly cold area. Because of the limitations of coldness, it is very difficult to recover if it is destroyed. So, the main task at present is to enclose, manage and protect the forest. In the high mountain between 2700-3000m, the resources of the light, heat, water and soil are comparatively ideal, and all kinds of slopes is fit for different forests of seabuckthorn.

(2)Rare and single species is a remarkable characteristic of Beichuanhe Basin. There are mainly 4 kinds of natural forestsand and 6 kinds of artificial forests. In the practice of afforestation, we have corresponding artificial forest except forest of Populus davidiana+ Hippophae Rhamnoides ssp.Sinensis.Picea crassifolia+Hippophae rhamnoides.ssp.sinensis and Betula platyphylla+Hippophae rhamnoides ssp. sinensis, are two kinds of good mixed native stands for imitation of rehabilitation in high mountains while two kinds of good artificial stands, that is, Larix rincipis-rupprechtii+Hippophae rhamnoides ssp. sinensis, Populus cathayana+Hippophae rhamnoides ssp. Sinensis are suitable to afforest in the shady and semi-shady slopes of the low mountains and pure seabuckthorn stands and mixed types of seabuckthorn forests and herbages, are eligible for high mountains and semi-shady and shady slopes in the low mountains. The construction of all kinds of seabuckthorn is mainly to imitate and rehabilitate the first and second layer, and should have the third layer (small bush and herbaceous layer) take its course. We should try our best to protect the original vegetation in the course of land reclamation and afforestation, and promote covering of vegetation by managing and protecting.

(3)In forest grassland area, seabuckthorn forest transfers from herb and sub-shrub or is formed by settling down as the pioneer species after the arbor trees are destroyed. Then Seabuckthorn will create the conditions for arbor to invade progressively through improving the ecological environment constantly. As an tolerant species, seabuckthorn’s important function is to capture all kinds of slashes clear land fast, and then realize succession which is from shrub to arbor till the climax community by improving soil and assisting the arbor species. This point is effectively supported by the data of community characteristics of different seabuckthorn forests. In the process of succession, the kinds of plant species not only change from intolerant, moderate to shade-tolerant species, but the number of the species gradually decrease as the stands grow and canopy coverage increase, with the Gleason richness presenting from small to large then small again along with the growth of seabuckthorn going on, and the Simpson dominance index presenting a contrary process. It accords with the following rule. That is : in the process of forming a community from converting farmland, species should go through the course as spread、settle down、gather、compete、react and then stabilize. And in this course, at first, it is mainly controlled by the un-ecology factors, and then it is gradually controlled by the ecology factors. What is more, it will lead to the forming of a regular component of species finally. The climax community in this region is Picea crassifolia+ mosses.

(4) It takes a long time for the succession of seabuckthorn. In this course, we should also pursue its good eco-economic functions. So, it is inevitable to optimize and manage the structure of the artificial shrub forest. By adjusting density which includes choosing from thick to thin, from small to big, from weak to strong, we can guarantee the biggest amount of living beings of seabuckthorn. In the mixed forest, we can solve the power problems of masses by artificially cut the wither trees so as to exert its various eco-economic functions.

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