摘要:紫花苜蓿作为黄土高原退耕还林还草工程的重要人工草本物种，对控制水土流失、植被固碳和生态环境改善等方面都发挥着重要生态作用；但是苜蓿草地在黄土高原普遍出现生长不可持续的特征，在第6年左右植被出现明显生长衰退现象。本研究在黄土高原4个降水梯度（280、400、500 mm和550 mm）的地点采集紫花苜蓿和自然对照草本物种：委陵菜的根部年轮样品，并分析了紫花苜蓿与委陵菜的年轮宽度和导管大小随年龄的变化趋势特征及与气候要素的相关关系。研究结果表明：在气候暖干化环境下，紫花苜蓿和委陵菜的年轮宽度随年龄增加都有不断下降的趋势，且在降水量偏低样点下降更为显著，这是由于植株在降水量偏低地点易遭受严重水分胁迫，以致无法满足生长所需的水分而造成根部生长速率明显减缓。紫花苜蓿根部导管大小随年龄增加均有明显下降趋势，在降水量偏低的样点下降趋势更加显著；这表明水分胁迫随年龄增加逐渐加剧，紫花苜蓿导管大小逐年下降以致不能满足自身生长的水分需求，植被生长呈衰退趋势，且在降雨偏低地区，苜蓿生长衰退出现时间较早。自然草本委陵菜根部导管大小随年龄增加均有明显上升趋势且在降水量偏低样点上升趋势更显著，这表明委陵菜通过增大根部导管大小、增加输水能力来满足随年龄增加不断上升的水分需求，这也说明委陵菜导管结构特征较好的适应了黄土高原干旱半干旱环境，生长处于可持续发展状态。



Abstract:Alfalfa (Medicago sativa L.) is an important artificial herb species planted widely in the Loess Plateau during the ‘Grain to Green’ large-scale revegetation program that been in operation since 1999. It plays an important ecological role in controlling soil erosion, increasing plant carbon sequestration, and improving ecological conditions. However, the growth of alfalfa grassland is unsustainable and shows signs of growth degradation after about 6 years in the Loess Plateau because of its high growth rate and deep root systems. The current understanding of the physiological mechanisms of the growth degradation of alfalfa grassland in the Loess Plateau is still limited. In this study, we collected samples of the main roots of alfalfa and a natural control, the herbaceous species Chinese cinquefoil (Potentilla chinensis) along a precipitation gradient (280 mm, 400 mm, 500 mm, and 550 mm) in the Loess Plateau, then analyzed the variations in radial incremental growth (growth rings) and xylem size, as well as the climatic factors that correlate with these characteristics, of the main roots of the two perennial forb species. The results indicated that the annual ring widths of alfalfa and Chinese cinquefoil both decreased noticeably with increasing age, and that the growth reduction was more pronounced at sites with lower precipitation. This shows that the perennial forbs in the Loess Plateau were confronted with deteriorating environmental conditions with increasing age, and the recent trend of a warmer and drier climate is further aggravating water deficits for these forb species. The vessel sizes of plants in the alfalfa grasslands along the precipitation gradient all declined monotonically with increasing age, and the vessel reduction was more obvious at sites with lower precipitation. With the gradually intensifying water deficits with increasing age, the vessel diameter of alfalfa decreased significantly and could not satisfy the self-growing water demand, and serious growth degradation had occurred in recent years. If the alfalfa grassland was located at a dry site with lower precipitation, the time of growth degradation occurred earlier. In contrast, the vessel size of the main roots of Chinese cinquefoil kept consistently increasing with increasing age, and the amplitude of the increase in vessel size at the relatively dry site was comparatively large. This indicated that Chinese cinquefoil could enlarge its vessel size to enhance water-use efficiency, then satisfy its increasing water demand with increasing age. The concurrently accelerated water acquisition ability of Chinese cinquefoil with increasing age also indicated that it can adapt to the semi-arid climatic conditions of the Loess Plateau, and maintain a state of sustainable growth development.





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