内蒙古锡林河流域植被退化的格局及驱动力分析

草地退化是中国北方草原面临的主要生态问题。该文以1984和2004年草原植被群落调查数据为主要依据, 以优势种、建群种和群落类型及其比例的变化作为主要指标, 并与20世纪80年代的植被类型图比较, 分析了内蒙古锡林河流域草地的退化趋势及其空间分布。根据流域内草地退化的实际情况, 将其分为未退化、轻度退化、中度退化、重度退化和极度退化5种退化类型。另外, 根据近20年前后草地植被变化的实际情况, 又划分了恢复和盐化两个类型。结果表明: 草地退化呈现明显的空间分布, 以锡林河为标志, 总体上从上游到下游退化程度逐渐加剧, 表现为浑善达克沙地进入锡林河的部分及沿河地区为重度退化, 锡林浩特市以北的区域, 尤其是流域的西北部, 已经达到了极度退化; 流域中部的白音锡勒牧场主要是中度退化; 轻度退化则均匀地分布于整个流域; 未退化类型多分布于锡林河的西部, 锡林河中下游地区由于农田退耕、草地围封出现了一定程度的恢复; 锡林浩特市以北沿锡林河植被的盐化程度较重。不同的植被类型退化程度也不同, 沙地灌丛植被轻度退化比例较高, 占植被的43%; 羊茅(Festuca ovina)草原和榆树(Ulmus pumila)疏林沙地中度退化草地的比重较高, 超过50%; 贝加尔针茅(Stipa baicalensis)草原和无芒雀麦(Bromus inermis)杂类草草甸的重度退化面积达50%以上; 极度退化比例较大的有克氏针茅(S. krylovii)草原和小叶锦鸡儿(Caragana microphylla)灌丛化草原。对于羊草(Leymus chinensis)草原, 以轻度和中度退化为主。导致不同区域和不同植被类型草地退化的原因也不尽相同, 从近20年的时间尺度来看, 过度放牧、不合理的居民点布局, 以及道路等是草地退化的主要人为驱动因子。研究结果还显示, 仅以生物量的变化来划分草地退化存在一定的局限性, 因此, 该文以群落优势种和建群种来表征草地的退化类型更具有客观性和实际的应用价值。

Patterns and drivers of vegetation degradation in Xilin River Basin,Inner Mongolia,China

Aims Overgrazing is a key factor driving grassland degradation in arid and semiarid regions. In this study, we explored how spatial and temporal patterns of grassland degradation were linked to vegetation sensitivity and socioeconomic drivers in the Xilin River Basin, Inner Mongolia, China. Methods We categorized grassland degradation by species composition and community characteristics by comparing a 1980’s vegetation map with data from field surveys done in 1984 and 2004 across the Xilin River Basin. Five types of grasslands were distinguished: non-degraded, slightly degraded, moderately degraded, heavily degraded, and extremely degraded. We also classified two additional types of grasslands based on their conditions 20 years ago: salinized and restored grasslands. Important findings The distribution of degraded grasslands exhibited an obvious spatial pattern across the Xilin River Basin. In general, the degree of grassland degradation increased from upstream in the southeast to downstream in the northwest of the basin. Moderately degraded grasslands were distributed mostly in the southeast, and heavily degraded and salinized grasslands were located in areas close to the river and over much of the Hunshandak sandland. Extremely degraded grasslands were distributed throughout the northern part of the basin where ecosystems were more vulnerable to over grazing. Non-degraded and slightly degraded grasslands were scattered patchily across the basin. In contrast, some abandoned farmlands and fenced pastures rested from grazing showed restoration over the past two decades. Grassland degradation in the basin also showed vegetation-type specific characteristics. About 43% of the shrubland was slightly degraded. More than 50% of Festuca ovina and Ulmus pumila dominated grasslands were moderately degraded, and half of the Stipa baicalensis and Bromus inermis dominated grasslands were heavily degraded. For both Stipa krylovii and Caragana microphylla dominated grasslands, more than 50% of the total areas were extremely degraded. For Leymus chinensis grassland, only small part of the total area had been degraded. Based on our analysis, overgrazing is the most important socioeconomic factor driving grassland degradation in the Xilin River Basin. The grassland degradation was positively correlated with increasing stocking rate from the 1970’s to 2004. Other factors, such as shifts in the density of villages and network of roads, were also attributed to the widespread grassland degradation in the Xilin River Basin.