Effects Of Slope-Gully Erosion Environment On The Propagule And Seedling Establishment

Soil erosion can result in seed loss in the surface soil seed bank, and also affects soil nutrients and water conditions, thus has impacts on seed germination and establishment, growth and reproduction of the vegetation. Soil erosion is frequent and serious in the the hill-gullied area on the Loess Plateau. However, there has been no report about the interaction between slope-gully erosion and plant reproductive strategy, soil seed bank removal and seedlings establishment. So it is important to study this problem to provide some theories for the vegetation restoration and rebuilding in this area.The study area is based in Zhifanggou Ansai, the typical hill-gully area on the Loess Plateau. Three representative slope-gully erosion units were selected: short slope and deep gully erosion unit with natural restoration (UnitⅠ), long slope and shallow gully erosion unit with natural restoration (UnitⅡ), managed slope and deep gully erosion unit (UnitⅢ). The plots were selected along the route of runoff by the order: watershed divide, slopes of inter-gullies, the margin between gully and inter-gully, and slope of gully. Totally 20 plots with area of 5m×5m were arranged. The soil cores were collected from soil layer 0-2cm, 2-5cm, and 5-10cm to indentify the seed density and species composition of the soil seed bank in three different microinstructions (the area under shrub/herbosa, the gap between shrub/herbosa, and the silted area). The density and composition of species of soil seed bank were detected by germination method in laboratory, and tracking investigation on the seedling bank and clone propagulum bank were also conducted to analyze the interaction of soil erosion and seedling establishment and vegetation reproduction. The main results were obtained as follows:(1) 6597 seedlings belonging to 50 species were germinated in the soil samples from the 20 sampled plots, which belong to 17 families. The annual species takes up larger proportion in the plots with sparse vegetation and serious soil erosion. While the perennial species takes up larger proportion in the plots which has been restoring for long period with slight soil erosion. (2) The density and species composition of soil seed bank are affected by the surface soil erosion. The result shows that the density of soil seed bank in the lower silted place is significantly larger than that in the gap between shrub/herbosa with the sequence of the silted area > the area under shrub/herbosa > the gap between shrub-herbosa. The density of soil seed bank in silted area is 1585-21585 seed/m2, under the shrub/herbosa is 2170-10736 seed/m2, and in the gap between shrub/herbosa is 1129-6585 seed/m2, the density of seed bank in the UnitⅢwith managed slope is 2007-9381 seed/m2.(3) The species of soil seed bank is affected by the soil erosion too. The vegetation tussock can hold up the seed of most species efficaciously, and then is the silted area, while the gap between the shrub/herbosa loss the seed of many species.(4) 35 species belong to 9 families have the ability of clone regeneration. About 77% of them are belonged to composite and gramineous grass. UnitⅢhas the largest number of this kind of species. In the UnitⅠand UnitⅡthis kind of species are mainly growing in the gully slope, where has been restoring for a long period.(5) Some of the main species with the ability of clone regeneration such as Artemisia gmelinii, Artemisia giraldii, Ixeris Chinensis, Viola dissecta mainly grow in the gully slope in the natural restoring unit with steep slopes, intense erosion, low soil moisture content, but higher content of organic matter, total N, available N and available K. The other clone species such as Bothriochloa ischcemum, Stipa bungeana, Cleistogenes chinensis, Leymus scalinus, Potentilla bifurca, Heteropappus altaicus mainly grow in inter-gully slopes, where has the high frequency of surface erosion, lower soil nutrient and soil moisture. The species reproduction only by seed such as Artemisia scoparia, Ixeris denticulate, Youngia japonica, Euphorbia humifusa, Potentilla tanacetifolia, Setaria viridis, Lespedeza davurica, Scorzonera divarieata, Gueldenstaedtia stenophylla, Astragalus scaberrimus, Oxytropis discolor, Poa sphondylodes grow in the plot with serious erosion, sparse vegetation.(6) There are 56 species in the seedling bank, 60% of them reproduced only by seed. The density of seedling bank is changing with time, i.e. 31st July >> 11th September > 13th October > 27th June. The margin area between gully and inter-gully has larger density of seedling bank, but the gully slope has lower density.(7) The density of seedling bank is less than the seed bank, but there are more species in the seedling bank. The species similarity coefficient of seedling bank and seed bank is lower in the eroded area but higher in the place with covered vegetation in the UnitⅠand UnitⅡ. But in the UnitⅢwith planted Caragana intermedia in inter-gully slope, has the lower similarity coefficient. The species both in the seed bank and seedling bank with high frequency are Artemisia scoparia, Artemisia gmelinii, Heteropappus altaicus, Lespedeza davurica, Dracocephalum moldavica, Potentilla tanacetifolia, Euphorbia humifusa, Cleistogenes chinensis, and Bothriochloa ischcemum.(8) The establishment of the seedling of the main species shows that the seedling of Artemisia gmelinii, Artemisia giraldii, Heteropappus altaicus, Caragana intermedia, Periploca sepium need the habitat with higher nutrient and covered vegetation. Dracocephalum moldavica, Bothriochloa ischcemum, Cleistogenes chinensis, Sophora viciifolia, Artemisia scoparia, Lespedeza davurica, Euphorbia humifusa, Salsola collina Pall, Stipa bungeana can grow in the habitat with sparse vegetation and erious erosion, and some seedlings can established successfully.