黄土高原草地植被碳密度的空间分布特征

草地是世界上分布最广的植被类型之一,作为陆地生态系统的重要组成部分,参与了全球碳源/汇及碳循环过程,在全球气候变化中扮演着重要角色,并对其产生重大影响。以黄土高原草地植被为研究对象,结合国家退耕还林草与封山禁牧工程的实施,对封禁前后的天然草地和退化草地,采用样带多点调查与多年定位测定相结合的方法,分析了黄土高原不同类型草地植物活体、凋落物和根系碳密度分布格局与地带性规律,系统研究了黄土高原不同草地类型退化草地和封禁草地生物量与碳密度沿海拔及降水梯度的时空变异特征,阐述了影响草地碳密度分布的主要驱动因子及其作用机理。结果表明:4种草地类型3种处理的草地生物量和碳密度自西北向东南均与降雨量呈指数增长趋势,并随海拔降低而显著降低,且二者呈显著的线性回归关系;各草地类型地上/地下生物量与碳密度分布规律均为荒漠草原<丘陵典型草原<梁塬典型草原<草甸草原;封禁11a草地活体植物、凋落物和根系碳密度总量:荒漠草原为7.066 t/hm2,丘陵典型草原为8.080 t/hm2,梁塬典型草原为15.319 t/hm2,草甸草原为20.982 t/hm2,分别是退化草地的14.8、8.33、6.5倍和15.88倍。充分表明,封禁不仅能使草地植被恢复和生物量提高,而且也是草地生产力和碳密度增加的一条重要途径。由此可见,气候干旱和草地退化是影响草地生物量和碳密度的关键因素,系统研究黄土高原封禁草地生物量增长与碳密度变化过程,将会对未来全球气候变化分析作出重要贡献。

Spatial distribution of carbon density in grassland vegetation of the Loess Plateau of China

Grassland is an important and terrestrial ecosystem and one of the most widely distributed ecosystems in the world. In the context of climate change, grassland has a significant impact on global carbon source /sink dynamics and carbon cycling. The focus of the present study was the grassland vegetation of the Loess Plateau. We analyzed the effects of natural and degraded grassland grazing pre- and post-prohibition to combine the policy of returning farmland to forest or grassland and grazing prohibition. Using multi-point transect surveys and long-term fixed monitoring sites, plant distribution, leaf litter, and distribution and zonation of below ground carbon density of different types of grassland in the Loess Plateau were recorded. The temporal-spatial variability of biomass and carbon density with respect to the altitudinal and precipitation gradient was studied in several degraded grasslands and an enclosed grassland. The main driving factors of carbon density change and mechanisms underlying the distribution pattern of carbon density were also analyzed. The results indicated that the biomass and carbon density showed a tendency to exponentially increase with precipitation amount, and also significantly decrease with altitude from northwest to southeast in the degraded grassland, early enclosed grassland and enclosed grassland for 11 years among four types of grasslands. There was a significant linear relationship between biomass and carbon density. In each type of grassland, the amount of biomass of above ground and below ground carbon density was present in the following order: desert steppe< hills typical steppe< plateau steppe< meadow steppe. Carbon density of plants, leaf litter, and roots in grassland where grazing has been prohibited for 11 years was 7.066 t/hm² for desert steppe, 8.080 t/hm² for hilly steppe, 15.319 t/hm² for plateau steppe and 20.982 t/hm² for meadow steppe, which were 14.8 times, 8.33 times, 6.5 times, and 15.88 times higher than degraded grassland, respectively. Our results demonstrated that grazing prohibition can not only restore vegetation and increase biomass in the grassland, but also significantly improve grassland productivity and the potential for carbon sequestration. The arid climate and grassland degradation are critical factors that influence the biomass and carbon density of grassland. Study of biomass and carbon density change following grazing prohibition can make an important contribution towards the analysis of the effects of global climate change on grassland in the Loess Plateau.