干旱荒漠白刺灌丛植被演替过程土壤种子库变化特征

Varying characteristics of soil seed banks during the succession process of Nitraria Tangutorum vegetation in an arid desert area

土壤种子库是反映气候与土地利用变化的指示器,也是荒漠植被更新与恢复的基础。典型荒漠白刺灌丛植被广泛分布于我国西北绿洲边缘地带,对保护绿洲免受风沙危害发挥着重要作用。但是,对干旱荒漠白刺灌丛植被土壤种子库仍未见研究报道。应用空间代替时间的方法,选择石羊河下游白刺灌丛植被演替的初始阶段、稳定阶段、衰退阶段与严重衰退阶段样地,研究了白刺灌丛植被演替过程的土壤种子库物种组成、时空分布和数量变化特征。结果表明:石羊河下游白刺灌丛植被演替过程中,土壤种子库共出现9科18种植物,其中草本植物占到55%—80%,是白刺灌丛植被土壤种子库的主体。不同演替阶段土壤种子库密度以稳定阶段最高,达到660.7粒/m2,是初始阶段、衰退阶段和严重衰退阶段的5.6、14.5倍和6.2倍。不同沙堆部位,土壤种子库密度以迎风坡中部与背风坡中部最高,分别达到329粒/m2和309粒/m2;沙堆土壤种子库密度是堆间地的1.9倍,土壤种子库主要分布于灌丛沙堆上。不同演替阶段土壤种子库间的相似性系数均在0.6以下,除稳定阶段外,其他阶段间均属于中等相似水平;土壤种子库与地上植被的相似性系数均在0.5以上,达到中等相似或极相似水平。该研究进一步证明了灌丛沙堆和土壤种子库在白刺灌丛植被中的重要性,也说明人为干扰损害了白刺灌丛地下潜在植被及其恢复潜力。因此,需要开展人工种子库或人工植被建设以控制干旱荒漠白刺灌丛沙堆活化,并实现退化白刺灌丛植被恢复。

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 rote 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 substituting space 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 of 18 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 309 seeds/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.