Trend Contribution Of Vegetation GPP Change And Its Attribution Analysis

Vegetation is an important component of the global terrestrial ecosystem and is closely related to the human living environment.Vegetation change is a complex nonlinear change with both long-term trends and interannual fluctuations,which is subject to the combined effects of climate and human activities.However,the contribution of long-term trends in vegetation change and its nonlinear response to climate and human activities are still unknown.In this paper,we used GLASS GPP(Gross Primary Productivity)data from 1982-2015,based on EEMD(Ensemble empirical model decomposition)model,to analyze the nonlinear trend changes of vegetation GPP and its relative contribution to vegetation change in ecological engineering areas in China;based on BRT(Boosted Regression Tree)model,we quantitatively analyzed the effects of natural factors and human activities on the nonlinear trend changes of vegetation GPP in typical ecological engineering areas in China and their relative importance.Results indicated that:(1)The trend of vegetation GPP change in China was mainly monotonically increasing(37%),mainly distributed in central,southern and southern coastal areas of China.There were 26%of the regions with trends shift,among which 14%of the regions showed an increasing to decreasing change in vegetation GPP,distributed in northern Xinjiang,northeastern Mohe River,middle and lower reaches of the Yangtze River and southwestern China;12%of the regions had a decreasing to increasing vegetation trend,distributed in the central of the Three Northern Protective Forests,the middle reaches of the Yellow River Protective Forest and a few areas in the Pearl River Basin.The distribution of vegetation GPP trend change types in the eight forestry ecological project areas in China was dominated by monotonically increasing in all regions except for the middle reaches of the Yellow River protective Forest,where the trend was dominated by decreasing to increasing.The years of abrupt changes in decreasing to increasing trends were concentrated in 1985-1990,1990-1995 and 1995-2000,which basically corresponded to the period of the initiation and gradual development of each ecological project between 1987 and 2000.(2)The proportion of area dominated by trend changes in vegetation GPP in China was about 30%,mainly distributed in North China,the middle reaches of the Yellow River,Central China and the southern coastal areas,dominated by forestry ecological engineering areas such as the Three Northern Protective Forests,the Liaohe River Basin Protective Forests,the Middle reaches of the Yellow River Protective Forests,the Huaihe-Taihu Basin Protective Forests and the Pearl River Basin Protective Forests,which were roughly the same as the distribution of monotonically increasing and decreasing to increasing vegetation trends.It indicated that the vegetation GPP changes in these areas were dominated by trend changes.The vegetation ecosystem was steadily restored and not easily disturbed by external disturbances.The regions were about 70%where vegetation GPP changes dominated by interannual fluctuations,and especially nearly 55%of the regions where interannual fluctuations contribute were twice as much as the trend,mainly in northern Xinjiang,northeast China,Qinghai-Tibet Plateau,Yangtze River basin and southeast coastal areas.Among them,the dominant role of interannual fluctuations was particularly obvious in the middle and upper reaches of the Yangtze River Protective Forests and the northeastern part of the Three Norths Protective Forests.Therefore,despite the positive restoration effect of ecological engineering on vegetation ecosystems,the ecosystems were vulnerable to external disturbances due to the small trend contribution,and it was still necessary to further strengthen the intensity of ecological engineering to better maintain the effectiveness of ecological engineering.(3)In the four typical forestry ecological engineering areas in China,natural factors and human activities influence the changes of vegetation GPP.Among them,the relative importance of natural factors was generally greater than that of human activities in the Three Northern Protective Forests ecological engineering zone.For trend monotonically increasing,the contribution of natural factors was slightly larger than that of human activities(55.65%vs.44.35%),but among the different driving factors,the influence of land use change(20.20%)and population density(14.52%)was still the most important,reflecting the ecological significance of the Three Norths Protective Forest Project on vegetation restoration;for vegetation trend from increasing to decreasing and from decreasing to increasing,the importance of natural variables was generally higher than that of human activities,and land use change,climatic factors and topographic factors were the main influencing factors.In the middle reaches of the Yellow River Protective Forest Project,for vegetation GPP monotonically increasing and decreasing to increasing the overall importance of natural factors was higher,dominated by precipitation and solar radiation,but in the trend monotonically increasing,population density(9.56%)and land use change(8.14%)had higher influence and were located in the third and fourth place;in the trend decreasing to increasing,the importance of land use change(17.19%)was much higher than other driving factors,indicating that ecological project had an important contribution to the vegetation restoration in the middle reaches of the Yellow River.In the middle and upper reaches of the Yangtze River Protective Forest,the monotonically increasing of vegetation GPP was dominated by human activities(59.57%),and the importance of population density(29.88%)and GDP(20.39%)was much higher than other driving factors.That is to say,the vegetation recovery in the middle and upper reaches of the Yangtze River Protective Forest engineering area was driven by human activities;for the vegetation trend from increasing to decreasing,it was driven by natural factors,mainly precipitation and solar radiation.In the Pearl River Basin Protective Forest,for both the monotonically increasing of vegetation GPP and the trend from decreasing to increasing,the importance of natural factors was much higher than that of human activities,but in the trend monotonically increasing,land use change(7.03%)and CO₂(6.95%)were more important and in the third and fourth place;in the trend from decreasing to increasing,land use change(8.74%)was the main driving factor,indicating that the construction of the Pearl River Basin Protective Forest had a positive impact on vegetation restoration.(4)The study on the nonlinear response of vegetation change trends to the main driving factors in four typical forestry ecological engineering areas in China showed that in the Three Northern Protective Forest ecological engineering area,human factors(increase in woodland and grassland,moderate increase in population density)and natural factors(low elevation,decrease in warmest month maximum temperature,increase in atmospheric moisture and moderate increase in seasonal variability of precipitation)was conducive to vegetation greening and restoration.But human factors(increasing urban and rural land area due to urbanization and farming activities)and natural factors(decreasing total annual precipitation,increasing solar radiation and drought of atmospheric moisture)contributed to the vegetation degradation.In the middle and upper reaches of the Yellow River Protective Forest ecological project area,human factors(moderate increase in population density and increase in woodland and grassland)and natural factors(increase in precipitation during the driest season,weakening of atmospheric aridity and moderate warming)promoted the vegetation greening and recovery.In the middle and upper reaches of Yangtze River Protective Forest ecological project area,human factors(moderate growth of population density,coordinated development of socio-economic and ecological environment and increasing forest land)and natural factors(lower altitude,increased atmospheric moisture and moderate warming of annual average temperature)would promote vegetation greening.In contrast,the expansion of urban and rural land use,the decrease of precipitation and the rapid enhancement of net solar radiation on the surface would shift the vegetation from recovery to degradation.In the Pearl River Basin Protective Forest ecological project area,human factors(increased grassland and woodland,synergistic economic and ecological development)and natural factors(increased precipitation,weakened net surface solar radiation,and reduced seasonal variability in temperature)promoted vegetation greening and recovery.By studying the contribution of long-term trends to vegetation GPP,we found that though the vegetation in the ecological engineering areas of China had been restored and improved,the contribution of vegetation trends in some areas was low,and the vegetation ecosystem was unstable and easily affected by external disturbances.Further ecological protection measures are still needed to consolidate the effectiveness of ecological engineering.Meanwhile,by separating the nonlinear effects of natural and human elements on vegetation GPP trends,the relative importance of the effects of natural and human elements on vegetation changes can be quantitatively assessed,and the driving mechanism of vegetation trend changes can be revealed.The study in this paper can well assess the ecological effectiveness of ecological engineering on vegetation change in China and its sustainability,and provide some theoretical references for the subsequent management of ecological engineering areas.