干旱荒漠白刺灌丛植被演替过程的土壤种子库变化特征

Abstract

The seed bank is the collection of viable seeds present on or within the soil at any given time.Soil seed bank is an important indicator in response to changes in land utilization and climate,and also forms the basis for desert vegetation restoration and development.Nitraria vegetation is typical of desert vegetation and with nebkhas is widely distributed along the periphery of oasis in the arid regions of Northwest China,playing an important role in desertification control around oases.In recent decades,Nitraria vegetation in the oasis-desert ecotone has been seriously degraded due to the rapid decrease of the groundwater table caused by agricultural over development.However,there is little information regarding the change in characteristics of the soil seed bank of Nitraria nebkhas in arid desert areas.Using the method of substitutingspace for time,the species composition,distribution pattern and quantitative variation of soil seed banks of N.Tangutorum nebkhas in initial,stable,degraded and severely degraded stages were studied in the lower reaches of the Shiyang River.Results showed that the soil seed bank of N.Tangutorum nebkha in the lower reaches of the Shiyang River was composed of18 species,belonging to 9 families.Herb species accounted for 55% —80% and were the main body of soil seed banks.The soil seed density of the stable stage was 660.7 seeds / m2,and was the highest,being 5.6,14.5 and 6.2 times greater than those measured in the initial stage,degraded stage and severely degraded stage,respectively.The soil seed density of the degraded stage was 45.7 seeds / m2,and was the lowest,only accounting for 38.9%,6.9% and 42.5% of those measured in the initial stage,stable stage and severely degraded stage,respectively.The soil seed density of the N.Tangutorum population at the different succession stages was also significantly different,showing a decreasing trend after the initial increase.The soil seed density of N.Tangutorum population at the stable stage reached 9.33 seeds / m2.The soil seed density in different positions of N.Tangutorum nebkhas was significantly different and was mainly distributed in the central parts of the windward and leeward slopes of N.Tangutorum nebkhas,with seed densities of 329 seeds / m2 and 309seeds / m2,respectively.The mean soil seed density of the nebkhas was 1.9 times that in the depressions within the nebkhas.Similarity coefficients of soil seed banks between the different succession stages were less than 0.6,which were recognized as having a moderate similarity level except for the stable stage which showed a low similarity coefficient.There were more common species which accounted for over 65% of vegetation above ground and the soil seed bank.Similarity coefficients between vegetation above ground and the soil seed bank were more than 0.5,which showed a moderate to extreme similarity level,and demonstrated a decreasing trend after the initial increase.The findings have proven the importance of soil seed banks and Nitraria nebkhas in Nitraria vegetation and also suggest that human disturbance has impacted potential underground vegetation and its restoration capacity.Therefore,man-made soil seed banks or vegetation construction should be adopted to contain Nitraria nebkhas activation and restore degraded Nitraria vegetation in arid desert areas.