Spatiotemporal Variation In Forest Net Primary Productivity And Its Response To Climate And Phenology In Southern Jiangxi Province

Forest net primary productivity（NPP）can directly reflect the status of forest quality and is also the key index to analyze forest carbon flux.Taking southern Jiangxi Province of eastern China as the study area,based on remote sensing data sets MODIS NPP,MODIS NDVI and basic data such as meteorology,land cover types and topography,using trend analysis,Hurst index,geodetector model,correlation analysis and other methods,the temporal and spatial variation characteristics of forest NPP from 2000 to2019 were analyzed.The temporal and spatial variation of climate and phenology and their effects on forest NPP were discussed.The research results can provide a reference basis for the evaluation of vegetation production capacity and the formulation of forest management policy in the study area.The main research results and conclusions are as follows:（1）Forest NPP showed obvious spatial differentiation,and its multi-year average value was 843.46 g C·m^-2·a^-1.The annual mean value of forest NPP was spatially higher in the south than in the north and higher in the west than in the east.With the increase of slope or elevation,forest NPP firstly increased and then decreased.The mean value of forest NPP in the area facing south and east was higher than that in the area facing north and west.The distribution of forest NPP in each season was also higher in the south than in the north.The multi-year average value of NPP of each forest type was broad-leaved forest(865.75 g C·m^-2·a^-1)>coniferous forest(825.96 g C·m^-2·a^-1)>coniferous and broad-leaved mixed forest(815.79 g C·m^-2·a^-1)>shrubbery(802.05 g C·m^-2·a^-1).There were some differences in NPP among different forest types.（2）At the annual scale,the annual forest NPP mean ranged from 780.93 to 904.49g C·m^-2·a^-1,with obvious interannual fluctuation.With the change of time,the areas with significant upward trend,significant downward trend and no significant change trend accounted for 19.43%,15.85%and 64.72%,respectively.The average variation coefficient was 0.07,reflecting that the forest NPP was comparatively stable.The change characteristics were mainly anti sustainability,reflecting that the forest NPP will accumulate benignly in the future.At the seasonal scale,forest NPP of spring and winter mostly showed an upward trend,while NPP of summer and autumn mostly showed a downward trend.The seasonal difference of forest NPP was obvious.At the monthly scale,forest NPP showed a trend of double peak curve,and the peak appeared from April to early May and October respectively.（3）Each climate factor had obvious spatial differentiation and interannual fluctuation.Forest NPP mainly had positive correlation with annual actual evapotranspiration and annual solar radiation,and had negative correlation with annual total precipitation and annual average temperature.The response of NPP of different forest types to climate factors were different.Based on the calculation of geodetector model,the impact of climate factors（P<0.01）on spatiotemporal differentiation of forest NPP from 2000 to 2018 was ranked as actual evapotranspiration>solar radiation>precipitation>temperature.Forest NPP was affected by the interaction of various climate factors.（4）The start of forest growing season was concentrated in the 80th to 110th days,showing a change trend of advance in the north and delay in the south.The end of forest growing season was mainly concentrated in the 280th to 320th days,which mainly showed a delayed trend.The length of forest growing season was mainly 170 to 210 days,which mainly showed an extended trend.The growing season of broad-leaved forest was the longest,followed by coniferous and broad-leaved mixed forest and coniferous forest,and shrubbery was the shortest.From 2001 to 2019,the start of forest growing season mainly had negative correlation with the spring NPP.The region of positive and negative correlation between the start of growing season and the summer NPP was similar.The end of growing season mainly had negative correlation with the autumn NPP.The region of positive and negative correlation between the end of growing season and the summer NPP was similar.The length of growing season mainly had positive correlation with the annual NPP.The response between forest NPP to phenology was complex,and it varied significantly with the geographical location and forest type of the forest.