天山北坡不同海拔梯度山地草原生态系统地上净初级生产力对气候变化及放牧的响应

以天山北坡三工河流域为例，利用改进后的Biome-BGC模型分别模拟了仅气候变化（Clm）、气候变化与放牧联合作用（ClmGra）下研究区不同海拔梯度3种山地草原生态系统（低山干旱草原，LAG；森林草甸草原，FMG；高寒草甸草原，AMG）1959—2009年地上净初级生产力（Aboveground Net Primary Production，ANPP）的动态，并通过假设27种放牧强度情景（0—8 羊/ha）模拟了其ANPP随放牧强度增加的变化趋势。近50年气候变化致使研究区各海拔梯度草原生态系统ANPP整体均呈上升趋势，但在放牧联合作用下，不同草原类型ANPP变化趋势差异显著；放牧导致FMG和AMG的ANPP呈下降态势，分别减少30.0%和33.2%，对比之下，由于1980前较低放牧强度促进了LAG的ANPP，放牧导致其ANPP整体增加1.3%。随着放牧强度增加，LAG的ANPP呈先增后减趋势，且在干旱年份最为显著；而FMG和AMG的ANPP呈显著非线性递减趋势。这些结果表明，近50年气候波动可能有利于中亚干旱区山地草原生态系统生产力的提高，但日益增强的放牧活动导致其净初级生产力显著降低；放牧对FMG与AMG生产力的负面效应随放牧强度增加而增强，但适度放牧可能促进LAG净初级生产力，尤其在干旱年份。

Impacts of grazing and climate change on the aboveground net primary productivity of mountainous grassland ecosystems along altitudinal gradients over the Northern Tianshan Mountains, China

Grazing and climate change are the two key factors influencing the aboveground net primary productivity (ANPP) in the grassland ecosystems. However, the impacts of grazing on ANPP are still in considerable disagreement, and little is known about its combined effects with climate change in the arid mountainous grassland ecosystems. In this study, we simulated the ANPP dynamics of mountainous grassland ecosystems along altitudinal gradients (i.e. LAG: lower-mountain arid grassland, FMG: forest meadow grassland, and AMG: alpine meadow grassland) over the Northern Tianshan Mountains under the impacts of climate change only (Clm) and by the combined effects with grazing (ClmGra) during 1959-2009 using a modified Biome-BGC model, and modeled the responses of the ANPP to grazing intensities (GIs) by setting 27 different levels of GIs (ranging from 0 to 8 sheep/hm²) as well. Model results indicated that the ANPP increased generally with the warm-humid climate change only in all the three grassland ecosystems, whereas the impacts of grazing on ANPP varied with grassland type. Grazing decreased ANPP in FMG and AMG by 30.0% and 33.2% respectively, in contrast, it resulted in a 1.3% increase of ANPP in LAG because the lower GIs before 1980 generally promoted the ANPP over LAG. Moreover, the responses of ANPP to the increasing GIs varied with grassland type, and the ANPP in LAG decreased significantly after a moderate increase and this changing trend is especially significant during drought years. Nevertheless, the ANPP always decreased non-linearly with increasing GIs in both FMG and AMG. These results suggested that the climate change in the past 50 years may benefit the productivity of the mountainous grassland ecosystem in the arid region of central Asia, but the increasing GIs decreased the productivity substantially. The ANPP of FMG and AMG decreased non-linearly with elevating GIs, but the ANPP may increase under the impact of lower GIs in LAG, especially in drought years. Nevertheless, there are uncertainties in our simulations and the responses of the productivity to increasing GIs together with the underlying mechanisms in arid mountainous grassland ecosystems, more field experiments are needed to validate these preliminary results.