| As is well known,Changbai Mountains is an important ecological barrier in the national ecological pattern of“two screens and three belts”.Net primary productivity（NPP）is one of the significant indicators to measure environment quality,thus it is significant to study on the spatio-temporal evolution characteristics of NPP,phenological regularity and clarify the driving mechanism of the forest ecosystem changes at different scales in Changbai Mountains with serious forest degradation for protecting regional ecological environment quality and mitigating the climate change.Based on the Global Production Efficiency（GLO-PEM）model,with Changbai Mountains as the study area,this study firstly integrated multi-source data to estimate the NPP in Changbai Mountains from 2001 to 2015,and then analyzed its spatio-temporal evolution and its spatial agglomeration patterns,phenology regularity and future evolution trends,as well as probed its multi-level driving mechanism based on the Hierarchical Linear Model（HLM）.The results showed as follow.（1）Based on the construction of GLO-PEM model and the results of phased verification,it was evident that the distribution results of the stations basically conformed to the typical theoretical distribution model,and the simulated values of solar radiation in clear sky were consistent with the actual daily maximum values observed over many years.The results indicated that the NPP values estimated by the model were generally consistent with the ones of MOD17 NPP,and the correlation coefficients at the pixel scale in 2003 and 2008 were 0.9121 and 0.8902,respectively.Therefore,the accuracy of NPP estimated by this model,which could represent the distribution of forest NPP of Changbai Mountains,was relatively high.（2）The spatio-temporal evolution characteristics of forest NPP in Changbai Mountains from 2001to 2015 showed that the forest NPP had a fluctuated increasing trend overall.From the perspective of spatial evolution characteristics,the NPP showed a gradual decline from southeast to northwest.Besides,from the first phase（2001-2008）to the second phase（2008-2015）,the area with the low-value NPP(0-500 g C?m-2?a-1)distributed in the northwest and northeast of the study area shifted to that with the media-value NPP(500-700 g C?m-2?a-1),with an increasing rate of 24.18%in the middle-value area.Meanwhile,the area with high-value NPP(900-1200 g C?m-2?a-1)also had a descending rate of 4.25%.Therefore,the NPP still showed a slight downward trend in general during this period.The results showed that the ecological environment in some specific areas in Changbai Mountains was still gradually deteriorating,and more attention should be paid to the rational planning and distribution of high-medium-low value areas.For different forest types,the NPP of temperate deciduous shrubs and temperate deciduous broad-leaved forest showed a decreasing trend.However,the NPP of other forestland types all showed an increasing trend of different amplitude,especially in the case of the coniferous forest.（3）According to the spatial autocorrelation results,the Moran’s I was all above 0.8 during the period of 2001-2008 and 2008-2015.However,the index of the latter stage was slightly lower than that of the former stage,indicating that the spatial agglomeration of NPP in the study area decreased to some extent with time went by.Besides,the ecological risk regions,including Low-Low（L-L）and High-Low（H-L）cluster types,expanded from 27.56%during 2001-2008 to 28.21%during 2008-2015,suggesting the local departments should focus on optimizing these regions and strengthen the construction of complex forests with large age differences to make the ecological environment healthier.（4）By using the Sen-Mann-Kendall method,the overall NPP in the study area presented a not-significant increasing trend from 2001 to 2015,and the area with an increase and decrease trend accounted for 52.58%and 47.42%,respectively.In terms of declining trend,45.22%of the regions in Changbai Mountains showed a slight degradation trend.They were mainly distributed in the south of Zhangguangcai Mountain.The severely degraded areas accounted for only 2.20%,mainly distributed in the south of Longjing City,Helong City in the east of the Changbai Mountain,and the Jiahe City in the middle region.As for the future sustainability results by using Hurst index,the study on the sustainability of NPP in the future indicates that the reverse direction feature of NPP change will be slightly stronger than the co-directional feature.It means that the degraded area will be slightly bigger than the improved areas.About 30.68%of the study area will switch from improvement to degradation,especially in the north of Zhangguangcai Mountain,as well as in the territory of the Changbai Mountain.Remarkably,the regions with a trend of continuous degradation will also account for 22.22%of the total area and those areas will be mainly distributed in Jiaohe City,Dunhua City in the middle of the study area.Therefore,attention should be paid to optimize the pattern of forest land types in the severe degraded area in Changbai Mountains.（5）As for the phenological regularity,the accumulation period of NPP mainly occurred at 145-257days（mainly from May to September）when the water and heat were well matched.It can be seen that the growth date of forest had a significant trend of advance from the change trend of forest NPP every 8 days in the early growth.Likely,most of the forest NPP in Changbai Mountain showed a trend of slightly increasing NPP at almost the end growth date,indicating that the end date of its growth showed a lagging trend.Therefore,the growth period of forest had a certain was prolonged with the change of phenology in Changbai Mountains to a certain.（6）Results from the HLM suggested that key driving factors,e.g.,the precipitation had significant promoting effects on NPP at the grid level.At the same time,the ecological restoration ability of the areas with high vegetation coverage is relatively strong.Whereas the soil organic matter content,distance to the highway,irrigation rate,percentage of the disaster area had significant inhibitory effects（p<0.01）on ecological environment at the watershed level.Especially,the influence of soil organic matter content and sand content were more serious.The reason is likely to that the area with high soil fertility is gradually replaced by cultivated lands and other lands,which results in the decrease of NPP in this area.Finally,the increase of the total investment in ecological engineering might not only directly promote the ecological recovery at the county level,but also indirectly promote the ecological restoration through the impact on population and GDP.Those results could provide a reasonable scientific basis for the rational development and utilization of regional forest resources,and sustainable socio-economic development.In addition,to some extent,it also plays an important role in mitigating climate change and building a civilization country. |
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