The Limiting Factors In Vegetation Recruitment In The Hilly-gullied Region Of The Loess Plateau: On Species Resources Aspect

The hilly-gullied region of the Loess Plateau is well known due to the serious soilerosion and ecosystem degradation. It is also the key region needing much hard work torestore ecological environments. The vegetation is the key factor to control soil erosion andrestore the degraded environments. However, the efficiency of vegetation restoration wasnot entirely satisfactory due to the simple planted species and high mortality rate.Compared with an artificial plant community, a natural vegetation community has a higherpotential for adaptability and stability; thus, natural vegetation rehabilitation has beenproposed to control soil erosion in the Loess Plateau region. But, as an ecological stress,the soil erosion can influence the process of vegetation development includingpost-dispersal movement of seeds, establishment of seedlings, growth and reproduction ofplant. However, in the study region with serious soil erosion, it is still not clear whetherenough species and propagule persist in this field for the vegetation natural recruitment andrestoration. Thus, the regional species pool was studied in the Yanhe watershed. The soilseed bank, seedling bank and the vegetative propagation were studied in a typical smallwatershed with different vegetation and erosion intensities. And then the limiting factor ofthe vegetation recruitment was analyzed on the basis of the species sources, seed dispersaland seedling establishment. The main results as follows:1. In the regional species pool,209species were recorded and belonged to45types ofcommunities which includes the communities during the different successional stages onabandoned land, such as Artemisia scoparia, Leymus secalinus communities in the earlystage, followed by Stipa bungeana, Lespedeza davurica communities and Artemisiagiraldii, Artemisia gmelinii, Bothriochloa ischcemum communities in the later restorationstage; the remnant communities on the gully slope, such as Sophora davidii, Ostryopsisdavidiana, Syringa oblate, Rosa xanthina, Vitex negundo var. heterophylla, Caragana microphylla, Periploca sepium, Ziziphus jujuba communities; and tree communities, suchas Quercus wutaishanica, Platycladus orientals, Populus simonii communities. Thedistribution habitats of these communities include the hilly top, sunny and shady hillyslope and gully slope. In the regional species pool, the dominant constructive species andcompanion species have a wide distribution and can provide seed sources for naturalrevegetation on the abandoned land with different soil erosion conditions.2. Characteristics of the soil seed bank changed with the ecological environments andstanding vegetation. The average soil seed bank density varied from1188~22560seed/m2.The soil seed bank mainly distributed in the top soil, but there was no significant seed losswith the soil erosion intensity increasing.91species were recorded in the soil seed bank,and the dominant abandoned successional species distributed widely. Most of the dominantspecies had persistent soil seed bank which can enhance their recolonization in the erodedenvironments.3. Soil water validity was the main factor influence the seedling changing in spaceand time. The density and species richness of seedling both had significant increase withthe improvement of soil water and temperature in the rain season. On the slope, themicrosite where more soil water can be retained could enhance the seed germination andseedling survival.91species were recorded in the seedling bank and57of these speciescoexisted in the soil seed bank, seedling bank and standing vegetation. This resultsuggested that these species can germinate and establish under the natural conditions, andthe vegetation can be recruited and succession in the eroded environments.4. In the species pool,24perennial grasses and40perennial herbs have the ability ofvegetative propagation. And many of the wood species have the ability of sprouting. Themain clone species had high density of bud bank. The density of bud bank was higher thanthe density of soil seed bank and seedling bank of the corresponding species. Thevegetative reproduction (versus sexual) is the most effective mechanism for species tocolonize and resist on such erosive degraded areas. The important value of the vegetativereproduction species increased with the slope angle increasing.5. During the abandoned land restoration, the early successional species always canproduce large number of seeds with small size and form high density soil seed bank whichcan recolonization quickly by the seedling press. The following species also can producelarge number of seeds which dispersal distance and persist in the soil seed bank. As thesame time, most of the dominant species can survival and spread under the harsh habitatsby vegetative reproduction. These results indicated that these species were not limited byseeds sources and dispersal. Due to fragment landform the remnant wood species distributed on the gully slope with low frequency. The seed dispersal faces the stress of soilerosion and gravity when they dispersed from the lower position (gully slope) to higherposition (hilly slope). Furthermore, the wood species always produce big seeds which cannot disperse distance without help. Thus the recolonization of these wood species waslimited by the seeds sources and dispersal.6. In the study region, the main herb and subshrub species can recolonize theabandoned slope land within decades and form typical steppe community without limit inseeds sources and dispersal. But the woody species recolonize slowly due to the limit ofseeds sources and dispersal. These results suggested that more studies should be done tofind out the limiting factor in seed dispersal, seedling establishment and growth of thenative woody species in order to enhance their restoration on the abandoned slope landwhich can improve the vegetation structure and enhance the ability of control soil erosionand ecological function.