Spatial And Temporal Distribution Pattern Of Net Primary Productivity In The Yangtze River Economic Zone And Its Influencing Factors

Estimation of vegetation net primary productivity（NPP）is an important part of ecosystem service function assessment,and is also the basic data for the assessment of the carbon cycle and other services.The distribution pattern and change of NPP affect the carbon cycle of terrestrial ecosystems.The Yangtze River Economic Zone（YREZ）covers 11 provinces（cities）,with a large east-west span,vegetation has large regional differences.It has a prominent ecological strategic position.This paper analyzed the spatiotemporal dynamic change characteristics of land use/cover types and meteorological elements in the YREZ.Based on CASA model,NPP was estimated from 2001 to 2018 to analyze the spatial distribution and variation characteristics of NPP in the YREZ.The influence of land use and meteorological elements on NPP during the 18 years was further analyzed and the following conclusions were drawn:（1）The change of land use/cover types in the YREZ were as follows:the area of forests,construction lands and water bodies increased,while the area of croplands,grasslands,savannas and unused lands decreased.Forests,grasslands and savannas are the dominant land types in the YREZ,accounting for more than 75.86%of the total land area of the YREZ.（2）The solar global radiation of the YREZ as a whole showed a trend of fluctuation and slow decline in the past 34 years,with a decline rate of 1.47 MJ·m^-2·a^-1,and spatially the highest in the west,followed by the east,and the lowest in the middle.The annual temperature as a whole showed a fluctuation rising trend in the past 34 years,with a rise rate of 0.03℃·a^-1,and decreased from south to north.The annual precipitation of the YREZ as a whole showed a trend of fluctuation and slow rising in the past 34 years,with a rise rate of 1.20mm·a^-1,and spatially the highest in the southeast,the lowest in the northwest.（3）In terms of spatial distribution,the spatial pattern of NPP distribution in the YREZ was as follows:the highest in the upper reaches,followed by the middle reaches,and the lowest in the lower reaches.The total NPP in the middle and upper reaches of the YREZ was738.04 Tg C·a^-1,accounting for 60.82%.In terms of time change,the 18 years on average of mean NPP in the YREZ was 594.19 g C·m^-2·a^-1,and showed a slow increasing trend,with an increasing rate of 10.30 g C·m^-2·a^-1.In 18 years,the total NPP increased by 322.36 Tg C,with an average annual increase of 18.96 Tg C·a^-1.（4）The influencing factors of NPP are mainly land use change,climate elements and altitude,among which the climate elements include average temperature,precipitation and solar global radiation,etc.Different land use types have different NPP.Forests,savannas,croplands and grasslands were the main contributors to the total NPP of the YREZ,with an average contribution rate of 99.10%in 18 years.There was a significant correlation between landscape pattern index and NPP of the YREZ.The increase of vegetation area contributed to the increase of NPP to a large extent,and vegetation restoration over many years was the main reason for the accumulation of total NPP and the increase of mean NPP.The areas with significant positive correlation between NPP and temperature,precipitation and solar global radiation accounted for 27.32%,23.82%and 14.45%of the YREZ respectively.Areas with significant positive correlation between NPP and radiation and areas between NPP and precipitation coincide more.Taking climate elements into consideration,it is concluded that the productivity of regions with higher temperature and better precipitation conditions is better.The mean NPP of the YREZ increased first and then decreased with the increase of altitude.The land use in the YREZ was dominated by evergreen broad-leaf forests,mixed forests and savannas in 1500-4000m.The change of land use at different altitudes,especially vegetation restoration,drove the change of NPP.