| Grassland ecosystems represent one of the most significant terrestrial carbon reservoirs,contributing to roughly 20%of the worldwide carbon stock,and playing a crucial role in mitigating greenhouse gas concentrations and promoting climate stability.Nonetheless,due to climate change and human activities,grasslands have been degraded to varying degrees,necessitating the addressal of the environmental problems associated with grassland degradation.Therefore,studying the response mechanisms of grassland ecosystems to global changes and human activities holds great scientific importance.This investigation employed several analytical tools,including Sen’s slope,Mann-Kendall trend tests and mutation tests,spatial autocorrelation,structural equation modelling,and spatio-temporal non-stationarity analysis,on the basis of the CASA model,for a scientific assessment of the spatio-temporal evolution factors and driving forces influencing the net primary productivity（NPP）of major grassland types in China between 2000 and 2019.The main results are as follows:（1）The NPP values of the eight major grassland types in China was calculated to be 196.14 g C m-2 a-1 on average over several years,with a spatial trend of being higher in the east and lower in the west mountain meadow regions.Among these varied grassland types,mountain meadow exhibited the highest multi-year NPP values followed by alpine meadow,temperate meadow grassland,lowland meadow,temperate grassland,temperate desert grassland,alpine grassland,and temperate desert,respectively.The NPP values of temperate desert grassland and alpine grassland were similar,though the multi-year mean NPP of each grassland type varied significantly.（2）The NPP of eight major grassland types in China exhibited an overall increasing trend from 2000 to 2019,with a rate of change of 3.24 g C m-2 a-1.However,there were notable differences in the NPP trend among different grassland types.The rate of change of NPP among the grassland types ranged from temperate meadow grassland（highest）to temperate desert（lowest）.Moreover,the highest proportion of areas showing an increasing trend of NPP was observed in temperate meadow grassland,followed by lowland meadow,alpine grassland,and temperate grassland,while the lowest was in temperate desert.In all major grassland type areas of China,there were significant mutations in NPP,with varying occurrences between grassland types:significant positive mutations in lowland meadow NPP were observed in 2008-2009,positive and negative mutations in montane meadow NPP were observed in 2012 and2013,respectively;for temperate meadow and temperate desert grassland NPP,multiple mutations were observed until 2010;negative mutations in temperate grassland NPP were observed around 2010,and temperate desert grassland NPP mutations mainly occurred in 2002-2003;significant positive mutations were observed in alpine meadow NPP in 2008-2009,and in alpine grassland NPP around 2007.（3）A strong positive correlation was found in the spatial distribution of different grassland types of NPP,showing clear clustering.These grassland types shared similar internal spatial structure characteristics.The area occupied by the"high-high"and"low-low"types of the four clustering types was significantly higher than that of the"high-low"and"low-high"types,and the latter two were scattered in the former two.The"high-high"type of NPP was mainly concentrated in the eastern region of each grassland type,while the"low-low"type was primarily distributed in the western region.Over time,a clear increasing trend was observed in the spatial distribution of the"high-high"type of NPP for each grassland type,while the"low-low"type showed a decreasing trend.The spatial distribution of other types remained mostly stable.（4）The NPP in China’s major grassland types can be attributed to the interplay between natural and human factors.Temperature,moisture,soil,and socio-economic factors exerted a positive impact on NPP dynamics,whereas atmospheric deposition and topography had negative effects.The topographic factor was linked indirectly to NPP changes by its influence on the temperature and socio-economic factors,while the moisture factor had an indirect relationship with NPP via its effect on soil and socio-economic factors.Of these factors,moisture was the most crucial driver of NPP dynamics in China’s main grassland types,although the dominant influence on NPP displayed significant spatial variability among different grassland types.This study aims to unveil the spatial and temporal patterns of evolution and the main driving factors that influence the carbon sequestration capacity of different grassland types in China.The findings of this study will provide scientific evidence for sustainable grassland development and production management.The results will also facilitate the protection of grassland ecosystems and provide insights into building a regional ecological security barrier. |
