基于改进土壤冻融水循环的Biome-BGC模型估算青藏高原草地NPP

Biome-BGC模型被广泛用于估算植被净初级生产力（Net Primary Productivity, NPP）,但是该模型未考虑冻土区土壤冻融水循环过程对植被生长的影响。本文基于Biome-BGC模型,改进冻土区活动层土壤冻融水循环,估算了2000—2018年青藏高原高寒草地NPP。通过比较原模型和改进后的模型,并对NPP模拟结果的时空特征进行了分析,结果表明：① 增加冻融循环提高了NPP估算精度,青藏高原草地NPP均值由114.68 gC/(m²·a)提高到128.02 gC/(m²·a)。② 原模型和改进后NPP的空间分布差异较大,时间变化趋势差异不明显。③ 青藏高原草地NPP总量为253.83 TgC/a,呈东南向西北递减的空间格局,年均增速为0.21gC/(m²·a)（P=0.023）,显著增加的占17.85%,主要分布在羌塘高寒草原地带的大部分地区和藏南山地灌木草原地带的西部。④ 该冻融水循环改进方法简单可靠,具有在其他多年冻土区推广的价值。

Estimation of grassland NPP in the Qinghai-Tibet Plateau based on the improved Biome-BGC model considering soil freeze-thaw water cycle

Accurate estimation of Net Primary Productivity (NPP) of alpine grassland ecosystems on the Qinghai-Tibet Plateau is important to complementing global carbon cycle studies. The Biome-BGC model is widely used to simulate ecosystem vegetation NPP, but it ignores the impact of soil freeze-thaw process on vegetation growth in the permafrost zone. Using the Biome-BGC model, this paper firstly improved the freeze-thaw water cycle process in the active layer of the permafrost regions, and then estimated the NPP of alpine grasslands on the Qinghai-Tibet Plateau from 2000 to 2018. The NPP accuracy, spatial and temporal variation characteristics between the original model and the improved model were compared, and the spatio-temporal characteristics of the improved NPP was analyzed. The results show that: (1) Adding freeze-thaw cycles can improve the accuracy of NPP estimation, and the modelled NPP of the grassland on the study area besed on the original and improved models were 114.68 gC/(m²·a) and 128.02 gC/(m²·a), the improved average NPP values for all the four grassland types of alpine swamp meadow, alpine meadow, alpine grassland and alpine desert grassland increased by 38.08 gC/(m²·a), 20.08 gC/(m²·a), 6.46 gC/(m²·a) and 0.6 gC/(m²·a), respectively. (2) The spatial distribution of NPP was significantly different between the original and the improved models, and there were no significant time changing trends. (3) The total NPP of the alpine grassland was 253.83 TgC/a, decreasing from southeast to northwest, with an average annual increasing rate of 0.21 gC/(m²·a)(P=0.023). The areas with a significant increase accounted for 17.85% of the total area, which were mainly distributed in the vast areas in the Qiangtang alpine steppe region and the western part of the southern Tibet montane shrub steppe region, while 18.48% of the area showed a decreasing trend, mainly in the central part of Qilian Mountains steppe region of eastern Qinghai and the eastern part of the Golog-Nagqu Plateau montane alpine shrub-meadow region. (4) In the permafrost region of the Qinghai-Tibet Plateau, the accuracy of NPP estimation is modified by improving the freeze-thaw water cycle of the soil in the model, which is simple and reliable and has the value of popularization in other permafrost regions.