The Influence Of Soil Erosion On Seed Removal And Seedling Establishment

The Loess Plateau is a area with degraded ecosystem and serious soil erosion in China. Vegetation restoration and reconstruction is the key factor to control and reduce the extent of soil erosion in the region. As propagules of seed plants, seeds is the basis for vegetation restoration. Early seedling stage is most sensitive to environmental conditions in plant life cycle, so seed and seedling stage have important influences to vegetation restoration. Not only soil particles and nutrients can be lost by soil erosion, surface runoff and sediment transport also can carry seeds and intensify soil drought, deteriorate the seedling living environment. Thus, simulated rainfall experiments were carried out for analyzing the features of seed loss and movement under different rainfall intensities, underlying surfaces and seed morphological characteristics, the characteristics of seedlings destruction by different rainfall types, and the influence of soil erosion in seedlings establishment. Main results are as follows:1) Under the different underlying surfaces, seed loss rate increased while the rainfall intensity increased during the rainfall events, seed loss rate increased fast on the bare slope, but slower on slopes with vegetation and hoof prints. Seed loss rate was close to seed displacement rate on the bare slope, but not the result on the slopes with vegetation and hoof prints. It suggested that vegetation and hoof print have positive impact on seed loss and movement while rainfall intensity increase. Vegetation and hoof print could intercept seed on the slopes, and the intercept benefit of vegetation and hoof print was between 10-20%, and less than 10% separately , and it was stable with rainfall intensity changes.2) All the seeds forn 60 species on the slopes translocated during the rainfall process, and the seed displacement rate was between 3.33% and 100%. 93.33% of all the species had seed displacement rate more than 50%, and 61.67% of all the species reached 100%. 91.67% of seeds in all the species lost in the rainfall experiments, only Rosa xanthina, Ziziphus jujube, Prinsepia utilis, Cirsium leo and Stipa grandis did not lost in any rainfall events, and the seeds of Rehmannia glutinosa, Astragalus discolor, Potentilla Chinensis and Salsola ruthenica wete all lost out of the slopes. There were three types of seed removal on slope, i.e., rolling, suspension and jumping, and most seeds moved with jumping.3) Seed characterastics which prone to loss mainly included: seed weight less than 10mg, seed volume less than 30mm3, spherical shape, smoothly surface, and no appendages. Seed features which hard to loss mainly included: seed weight more than 150mg, segregating mucilage when contact with water, long-narrow and flat shapes, rough surface, and having appendages as hairs, wings awns and so on.4) In the conditions of weak erosion, seedlings within 40 days damaged by the rainfall most obviously, while the amount of seedlings which was older than 40 days had no changes after the rainfall event. It indicated that the seedlings which was older than 40 days have the ability to resist erosion damage. The amount of Sophora davidii, Bothriochloa ischaemun and Artemisia scoparia in early phases(20 days)were 1.08, 1.42 and 3.62 times higher than that in the ending phases(120 days), separately.5) Extreme rainfall with short duration and high intensity(150mm/h lasted for 20min) and rainfall with long duration and low intensity(25mm/h lasted for 120min) led to highest soil erosion amount; and high death rate of Artemisia scoparia and Bothriochloa ischaemun were occurred corresponding to the same rainfall conditions, and even higher in thr rainfall with the long duration and low rainfall intensity. There mainly were three ways of seedling destory by rainfall: pull out of roots, beat down and lost.6) The sustained effects on the development of vegetation by soil erosion are lasted in the whole process directly and indirectly. Soil erosion can move the seeds on the slope by runoff and sediment transport and reduce the amount of germination seed; Part of the seedlings are hardly exposed to the soil surface because the seed may be buried deeper by sedimentation effects; Soil erosion can also deterioration the environment for seed germination by reducing soil nutrients and soil moisture content, leading to lower survival rate; Whenever part of the seeds which germinated successfully, the seedlings would be destroyed by raindrop hitting, sediment transport, leading to lower survival rate; Some of temporary survived seedlings would also be affected by deteriorated erosion environment conditions and intraspecific and interspecific competition factors; The seedlings which still living on the slope finally is established successful after these sustained effects by soil erosion.