青藏高原草地地上生物量和理论载畜量

Alpine Grassland Aboveground Biomass and Theoretical Livestock Carrying Capacity on the Tibetan Plateau

准确模拟和预测草地地上生物量(Aboveground biomass,AGB)和理论载畜量对于维持草地生态系统平衡、优化放牧管理至关重要。当前很多研究以围栏外草地AGB为基础,估算了青藏高原草地AGB的现存量。但是,牛羊啃食后的草地AGB现存量无法准确评估草地理论载畜量。围栏内草地不受家畜采食影响,其年际变率由环境因子驱动,可视为草地潜在AGB (potential AGB,AGB_(p)),更适用于草地理论载畜量的评估。本研究以青藏高原345个围栏内AGB观测数据为基础,结合气候、土壤和地形数据,利用随机森林算法构建草地潜在地上生物量估算模型,并对当前气候条件(2000-2018年)和未来20年(2021-2040年)4种气候变化情景(SSP1-2.6、SSP2-4.5、SSP3-7.0和SSP5-8.5)下的草地AGB_(p)和高寒草地理论载畜量进行模拟与预测。结果表明:(1)随机森林算法可准确模拟当前气候条件下的青藏高寒草地AGB_(p)(R^(2)=0.76,P<0.001);2000-2018年青藏高寒草地AGB_(p)平均值为102.4 g m^(-2),时间上增加趋势不明显(P>0.05);AGB_(p)年际波动和生长季降水显著正相关(R^(2)=0.57,P<0.001),和生长季温度日较差显著负相关(R^(2)=0.51,P<0.001)。(2)当前气候条件下,青藏高寒草地平均理论载畜量为0.94 SSU ha^(-1)(standardized sheep unit ha^(-1));在过去20年约有54.1%草地理论载畜量呈提升状态。(3)和当前相比,未来20年青藏高原中部和北部草地AGB_(p)和理论载畜量呈下降态势。因此,建议未来在厘清气候变化影响下草畜关系的基础上进行有针对性的草牧业规划和管理,以缓解区域气候变化引起的草畜矛盾。

The accurate simulation and prediction of grassland aboveground biomass (AGB) and theoretical livestock carrying capacity are key steps for maintaining ecosystem balance and sustainable grassland management.The AGB in fenced grassland is not affected by grazing and its variability is only driven by climate change,which can be regarded as the grassland potential AGB (AGB_(p)).In this study,we compiled the data for 345 AGB field observations in fenced grasslands and their corresponding climate data,soil data,and topographical data on the Qinghai-Tibetan Plateau (TP).We further simulated and predicted grassland AGB_(p)and theoretical livestock carrying capacity under the climate conditions of the past (2000-2018) and future two decades (2021-2040) based on a random forest (RF) algorithm.The results showed that simulated AGB_(p)matched well with observed values in the field (R^(2)=0.76,P<0.001) in the past two decades.The average grassland AGB_(p)on the Tibetan Plateau was 102.4g m^(-2),and the inter-annual changes in AGB_(p)during this period showed a non-significant increasing trend.AGB_(p)fluctuation was positively correlated with growing season precipitation (R^(2)=0.57,P<0.001),and negatively correlated with the growing season diurnal temperature range (R^(2)=0.51,P<0.001).The average theoretical livestock carrying capacity was 0.94 standardized sheep units (SSU) ha^(-1)on the TP,in which about 54.1%of the areas showed an increasing trend during the past two decades.Compared with the past two decades,the theoretical livestock carrying capacity showed a decreasing trend in the future,which was mainly distributed in the central and northern TP.This study suggested that targeted planning and management should be carried out to alleviate the forage-livestock contradiction in grazing systems on the Tibetan Plateau.