草地沙化过程地上植被与土壤种子库变化特征

Characteristics of soil seed bank and standing vegetation change in sandy grasslands along a desertification gradient

研究了草地沙漠化过程地上植被与土壤种子库的变化特征,获如下结论:(1)草地沙漠化过程地上植被与土壤种子库物种多样性的衰减模式不同,土壤种子库植物种数从潜在沙漠化阶段(固定沙地)到中度沙漠化阶段(半流动沙地)变化很小,而从中度沙漠化到严重沙漠化阶段(流动沙地)衰减速度明显加快;地上植被种数随着沙漠化程度增加而下降,其中从中度沙漠化到严重沙漠化发展阶段衰减幅度最大。(2)地上植被与土壤种子库密度随着沙漠化程度增加而下降,但下降速率因沙漠化发展阶段不同而异,从固定到半固定沙地是地上植被与土壤种子库密度下降最快的时期。(3)地上植被与土壤种子库共有种数随着沙漠化程度的增加而减少,从而导致了地上与土壤种子库群落组成的相异性增大。(4)4种退化沙地土壤种子库组成的相似性要高于地上植被,表明在沙漠化过程中土壤种子库群落组成的稳定性要高于地上植被。(5)地上植被密度与土壤种子库密度存在显著相关性,其关系可用二次曲线来描述。

In sandy grasslands of the Horqin desert in Inner Mongolia of China, vegetation degradation usually occurs under improper management regimes in fragile environmental conditions. Over the last several decades, most sandy grasslands in this region have become shifting, semishifting and semifixed sandy lands corresponding to severe, moderate and light desertification. However, little is known about the effects of degradation level on the structure and performance of soil seed banks and standing vegetation and the interactions between aboveground and belowground communities in a semiarid desert environment. A field experiment was established in a sandy grassland steppe to test the effects of degradation level on the structure and performance of the seed bank and the established vegetation and their spatial interactions at the level of individual plant species.In early April 2002, four types of degraded sandy grassland (i.e. shifting, semishifting, semifixed and fixed) were selected as the experimental sites. On the sites representing the four sandy lands, four parallel 100m line transects (10 m apart) were established along the sand dune (across windward slope, dune crest and leeward slope). For each transect, 10 sampling points were set up at 10m intervals, in which a soil sample of 20 × 20 cm and 5 cm deep was collected from each sampling point. Soil samples were transported to the laboratory in open plastic bags. The soil samples were germinated in round pots (diameter 30 cm in diameter, 15 cm high). The pots were first filled with seedfree fine sand of approximately 7cm deep, and then the soil samples were spread evenly to form an about 3cm thick layer covered with about 1cm thick layer of seedfree fine sand. All pots were placed in an unheated greenhouse, and the seeds allowed germinating over a 10week period. Pots were hand watered daily with a very fine nozzle early in the morning. Emergent seedlings were identified to species and then carefully removed from the pots. A few unidentifiable seedlings after 10 weeks remained in a longer period in the pots until they were identified. The density of soil seed bank was expressed as the number of germinating seeds from the soil samples of the 0～5cm layer per square meter (viable seeds m-2). The subsequent vegetation (only herbaceous community) was surveyed in lateAugust using a total of forty 1m2 quadrats that were placed in close vicinity to the sampling points of the soil seed bank at each site. In each quadrat, the density and aboveground biomass of established plants were measured for individual species present in a quadrat. The frequency of species occurrence in the soil seed bank and the established vegetation were determined in terms of the 40 sampling points or the 40 quadrats.Several important conclusions can be drawn from this study. (1) There were marked differences in the patterns of species diversity change along the desertification gradient between seed bank and standing vegetation. Species diversity in the seed bank showed a slight decrease at the stage from the least degraded fixed sandy land to the moderately degraded semishifting sandy land but a significant decrease at the stage from the semishifting sandy land to the severely degraded shifting sandy land. However, species diversity in the standing vegetation followed a tendency for decreasing with desertification level, with the greatest decease at the stage from the semishifting sandy land to the shifting sandy land. (2) Both densities of the seed bank and the standing vegetation decreased with increasing level of desertification but there were differences in the relative rates of decrease among different stages of desertification. The greatest reduction in the densities of the seed bank and standing vegetation was found at the stage between the fixed and the semifixed sandy lands. (3) The number of cooccurring species both in the seed bank and the standing vegetation decreased with increasing level of desertification, thus leading to a decrease in composit