The Relative Importance Of Climate Change And Human Activities On The Change Of Vegetation NPP At Different Time Scales In Northwest China

The Belt and Road Initiative bring not only great chances for the economic development of the area along "the Belt and Road"(B&R),but also challenges for local vegetation and ecology.Net Primary Productivity(NPP)is an important parameter for vegetation growth in ecosystems and also a key indicator for the carbon cycle research.The six provinces in the northwestern China,the hub region of "B&R",have been changed greatly in vegetation coverage due to the natural factors and human activities.Studies on the changes in vegetation NPP at different scales and its correlation with driving forces will contribute to reveal the response mechanism of NPP to climate change and human activities in the context of global warming,and provide some scientific references for the economical and environmental sustainability.NPP in six provinces in northwestern China during 1982-2015 was estimated using modified parameter CASA model with the GIMMS NDVI data,vegetation type data and meteorological data.Temporal and spatial dynamics of NPP at different time scales were analyzed with the EEMD method,and the relative contributions of climate change and human activities to NPP dynamic changes at different time scales were calculated quantitatively through RESTREND residual method.The results showed that:(1)The average NPP values during the growing season from 1982 to 2015 existed obvious spatial difference.The characteristics of NPP values in the northwest,southeast and northeast were generally high while the middle was low.The NPP values in the vegetation growing season generally showed an increasing trend with a linear growth rate of 0.718 gC ·m-2 ·a-1.In most areas,NPP values will continuously maintain the current trend of change in the future,especially in southern Inner Mongolia,the Qinghai-Tibet Plateau and the edge of the Tarim Basin.(2)The dynamic characteristics of vegetation NPP values were mainly based on the 3-year,6-year cycle and long-term increasing trend.Northern Shaanxi,northern Xinjiang,northern Ningxia,northern Gansu,northern Qinghai,central Qinghai,and central and northern Inner Mongolia were dominated by 3-year periodic changes,while long-term changes were mainly observed in northern Shaanxi,southern Gansu,southern Ningxia,southern Xinjiang,and eastern Inner Mongolia.For different vegetation types,shrubs,grasslands,and farmland were dominated by 3-year cyclic and long-term growth trends,while coniferous forests,broad-leaved forests,and mixed forests were dominated by 3-year cycles.(3)The relationship between NPP change and temperature and precipitation became more and more significant with the increase of time scale.At 3 year time scale,the correlation between vegetation change and temperature and precipitation was not significant in most regions(p>0.05).At 6-year time scale,the positive correlation area between vegetation change and precipitation had a tendency tothe south.The negative correlation between alpine meadows in southern Qinghai was positively related.Furthermore,in the alpine meadows of southern Qinghai,the negative correlation between vegetation change and precipitation was transformed into positive correlation.At long-term trend,the relationship between vegetation NPP and temperature and precipitation was very significant.In the most study areas,the positive correlation between NPP and temperature and precipitation was greater than negative correlation.(4)The effect of climate change on vegetation changes was increasing.At 3-year time scale,the contribution of human activities to the dynamic change of vegetation NPP values was greater than climate change.At 6-year time scale,the area of vegetation affected by climate change had expanded,but human activity was still the dominant factor of NPP values.In the long-term trend,climate change was the dominant factor for long-term vegetation changes,and human activities existed little effect.As a whole,dynamic changes in NPP values were strongly influenced by climate change;for the entire time series,climate change contributed to vegetation growth,while human activities prevented vegetation growth.This study found that multi-time scale analysis can well reflect the changes of NPP and the influences of climate change and human activities on NPP changes at different time scales.In this paper,it has an important theoretical significance for assessing ecological environment risk under climate change and human activities,and developing coping strategies,which will provide scientific basis for the sustainable development and the ecological environment protection of the six provinces in the northwestern China.