Spatio-temporal Variation Of Forestland GPP And Its Influencing Factors In The Loess Plateau In Recent 20 Years

As one of the most important"carbon sinks"on land,forestland plays a key role in global carbon balance.With the implementation of a series of major forestry ecological projects,the vegetation in the Loess Plateau has been greatly improved,forestland area is also increasing.Gross primary production（GPP）is the important component of global terrestrial carbon flux,which can reflect the quality of terrestrial ecosystem to a certain extent and is also sensitive to climate change.It is of great significance to comprehensively understand the change characteristics of forestland in the Loess Plateau and sustainable management in the future to explore the spatial-temporal distribution pattern of forestland,quantify the spatial-temporal variation characteristics of forestland GPP and the impact difference of climate factors on forestland GPP.Based on years of land use data,this study divided the forestland into four types:natural arbor forestland,natural shrub forestland,artificial arbor forestland and artificial shrub forestland（note:The forestland existing in 1980 was defined as natural forestland in this paper,and the forestland added later was defined as artificial forestland）,and divided dry and wet areas according to Aridity index（AI）in the last 20years,studied the change characteristics of forestland in the Loess Plateau in recent 20years,and the interannual change of four types forestland GPP in different dry and wet areas and the difference of climate on them.Firstly,based on the land use transfer matrix,the multi-stage forestland conversion was analyzed.Then,unary linear regression analysis was used to explore the interannual variation characteristics of GPP in different dry and wet areas and four types of forestland at the time scale of year and growing season.Finally,the characteristics of climate change in the Loess Plateau during the past 20 years were discussed,and the effects of climate on GPP of four forestland in the study area and different subareas were revealed by partial correlation analysis and generalized additive model.The main results are as follows:（1）In recent 20 years,the area of forestland in the Loess Plateau has increased significantly,and the net increase area of forestland is 256.58×10³ ha.Compared with shrub forestland（26.55×10³ ha）,the net increase area of arbor forestland is 230.03×10³ha.In space,shaanxi province has the largest net increase area,followed by eastern Gansu Province,northern Shanxi Province,central and southern Inner Mongolia Autonomous Region.In terms of time scale,the most significant increase is observed from 2000 to 2005,the net increase of forestland is 199.53×10³ ha,and the increase is mainly concentrated inⅢ（0.3<AI<0.4）andⅣ（0.4<AI<0.5）region,and the net increase of arbor forestland is mainly in the way of"returning farmland to forest".（2）At the time scale of year,the mean annual value of forestland GPP which is higher than 600 g C m^-2 is mainly distributed in the southeastern part of Qinling mountains and Luliang Mountains.The forestland GPP decreases gradually from theⅤregion（0.5<AI<0.65）toⅠregion（AI<0.2）.From 2001 to 2018,the annual GPP of four forestland in the whole region and in different dry and wet regions show the increasing trend.The annual growth rates(3.869-4.972 g C m^-2 yr^-1)of four forestland GPP inⅠregion are lower than the maximum growth rates（Ⅳregion）,the minimum and maximum differ about three times.Except for theⅤregion,the GPP growth trends of artificial forestland are lower than that of natural forestland.At the growing season scale,the spatial distribution of forestland GPP is similar to the annual time scale,and the growing season contributes to the main forestland GPP throughout the year.In addition,in the whole region and different subregions,the interannual variation trends and characteristics of the four forestland GPP in the growing season are similar to the annual scale.（3）In recent 20 years,the downward shortwave radiation is the most important factor affecting the annual GPP of four forestland in the Loess Plateau.Compared with the whole study area,the partial correlations between climate factors and annual forestland GPP are enhanced in different dry and wet areas,and the influences become more complicated.The air pressure has a significant negative effect on annual GPP of four forestland inⅠandⅡ（0.2<AI<0.3）regions,while inⅢ,ⅣandⅤregions,the factor with greater negative influence is the downward shortwave radiation.The air humidity ratio has a great and stable positive effect on the annual GPP of natural forestland inⅠ,ⅡandⅢregions,while the downward longwave radiation has a great positive effect on the artificial forestland.FromⅠtoⅤregion,the partial correlations between GPP of natural forestland and downward shortwave radiation increase,while the partial correlations between GPP of artificial forestland and downward shortwave radiation decrease fromⅢtoⅤregion.Compared with the scale of year,the effects of climate factors on forestland GPP in the growing season are more complex.In the whole region,the influences of wind speed on the GPP of the four forestland in the growing season are enhanced and play a leading role,while in different regions,the partial correlation coefficients between climate factors and GPP of the four forestland are hardly more than 0.25 in the growing season.In conclusion,the area of forestland in the Loess Plateau has increased significantly in the past 20 years,and the GPP of forestland show an upward trend.The growth trend of artificial forestland in the arid region is slightly lower than that of natural forestland.Meanwhile,at the time scale of year and growing season,the effects of climate factors on GPP of four forestland in the whole region and in different dry and wet regions are highly heterogeneous.This study can not only provide some data support for the effectiveness evaluation of China’s forestry ecological engineering and ecological environment monitoring in the arid region,but also provide some scientific and technological support for the improvement of relevant policies such as natural forestland protection,artificial forestland management and climate change response in the Loess Plateau.