民勤绿洲边缘梭梭林衰败过程中土壤种子库动态及其与地上植被的关系

Dynamic changes in soil seed banks and their relationships with aboveground vegetation during the decaying of Haloxylon ammodendron plantations at the edge of the Minqin oasis

为了探明民勤绿洲边缘梭梭林衰败过程中土壤种子库演变特征及其与地上植被的关系,以民勤西沙窝——民勤治沙综合试验站1987年建植的未衰败(ND)、轻度衰败(SLD)、中度衰败(MD)以及重度衰败(SD)梭梭林为对象,通过野外调查及室内萌发的方法对林地土壤种子库物种组成、密度、物种多样性及其与地上植被的关系进行了研究。结果表明:1)梭梭林由未衰败向重度衰败演变过程中,种子库草本植物物种数先急剧增大再缓慢减小(峰值在SLD阶段),木本植物物种数先急剧减小再保持恒定;种子库优势种呈现出由沙生植物向沙旱生植物演替的趋势;大部分林地种子库与地上植被物种间存在较高的相似性。2)梭梭林衰败过程中,草本植物种子库密度呈现出急剧增大后缓慢减小趋势,SLD、MD及SD的密度分别是ND的2.57、2.32、2.28倍;木本植物种子库密度处于持续增大状态,SLD、MD、SD的密度分别是ND的5.25、11.38、14.74倍;种子库与地上植株密度间存在正相关关系,其中,草本植物为对数曲线,木本植物为二次函曲线。3)梭梭林从衰败初期(SLD)演变到后期(SD),种子库中草本植物物种多样性指数D_(ma)、D_(sim)、H'、J_(sw)均呈现出先稳定后变化的状态,而木本植物的4种指数一直呈现出减小的趋势;种子库草本植物物种多样性指数变化趋势与其地上植被存在差异,但其变化幅度基本一致;种子库木本植物与地上植被的物种多样性指数变化趋势完全一致,其种子库的变化幅度小于地上植被。基于以上结果得出,民勤梭梭林衰败过程中林地植物群落发生了由沙生植物向沙旱生植物的演替,林地植被与土壤种子库间存在很强的依赖性,衰败梭梭林林地土壤种子库具有较高的植被恢复能力。

The Minqin oasis is located in the northwest Hexi Corridor, and the northern, western, and eastern edges are surrounded by the Tengger and Badain Jaran deserts. To manage sand and prevent sandstorms, large tracts of Haloxylon ammodendron plantations were previously planted on the fixed and semi-fixed sand dunes of the Minqin oasis fringe. At present, parts of the H. ammodendron plantations, especially those close to the oasis, have presented with a degradation phenomenon due to multiple affecting factors. To ascertain the changes in soil seed banks and their relationships with aboveground vegetation during the degradation process of H. ammodendron plantations, some tracts of H. ammodendron plantations planted in 1987 were divided into no degradation （ND）, slight degradation （SLD）, moderate degradation （MD）, and serious degradation （SD） stages, according to the growth status of these plants and their seedlings, as well as the coverage of biological soil crust. In the present study, the species composition, density, and species diversity of soil seed banks and their relationships with aboveground vegetation were studied, with field monitoring and indoor germination methods. The results showed that 1） with an increased degree of degradation of H. ammodendron plantations, the number of woody plants in the soil seed banks first decreased rapidly and then remained stable, and the number of herb plants in the soil seed banks, first increased rapidly and then decreased slowly, so that the peak emerged at the SLD stage. At the same time, dominant species in soil seed banks presented a succession trend from typical psammophytes to xerophytes, and the species similarity coefficients of soil seed banks and aboveground vegetation, as calculated by Jaccard＇s similarity coefficient, appeared to increase from ND to SLD and then decrease from SLD to SD. 2） The soil seed bank density of herb plants rapidly increased their stage of degradation from ND to SLD and slowly decreased from SLD to SD, while the soil seed bank density of woody plants has been increasing continuously and dramatically. The densities of herb plants and woody plants in the SLD, MD, and SD stages were 2.57 and 5.25, 2.32 and 11.38, and 2.28 and 14.74 times higher than that of ND, respectively. There was a positive correlation between the species in soil seed banks and aboveground vegetation, such that this relationship for herb plants can be described with a logarithmic curve of y=aln （x） ＋ b （a ≠ 0）, and for woody plants may be described with a quadratic curve of y=ax2 ＋ bx ＋ c （a ≠ 0）. 3） With the evolution from the SLD stage in H. ammodendron plantations to the SD stage, some species diversity indices, such as Dma, Dsim, H＇, and Jsw, of herb plants in soil seed banks were initially obviously stable and then showed significant change, while the indices of woody plants showed continuous change. In addition, some obvious differences have been presented between trends of these indices in soil seed banks and in aboveground plants, while the degrees of change in soil seed banks were similar to those of aboveground plants. The changes in trend in the indices of woody species in soil seed banks were completely consistent with those in the indices of aboveground plants; however, the degrees of change in soil seed banks were smaller than those in the aboveground plants. These findings suggest that the degradation of H. ammodendron plantations is a succession from psammophytes to xerophytes, there is a strong interdependence between soil seed banks and aboveground vegetation, and soil seed banks in the degrading plantations will have a stronger potential for vegetation recovery due to the increase in seed bank stocks of some species, which is a response to increase the stability of the plant communities.