Spatial-temporal Variations of Net Primary Productivity of Water Conservation Forests in the Upper Lijiang River Basin

In order to evaluate quantitatively the restoration of the water conservation forests in the upper Lijiang River basin, Boreal Ecosystem Productivity Simulator(BEPS) model was modified to estimate the net primary productivity(NPP) based on the ground forest measurements such as leaf area index and biomass in the study area.With the estimation of the leaf area index(LAI) from Radial Basis Function(RBF) artificial neural network combining multiply vegetation index, the spatial distributions of the annual NPP were calculated from Landsat remote sensing images by using BEPS model.The annual NPP values in the different years were normalized to the values between 0 and 1 so that they could be compared to detect their temporal changes.Results show that the annual NPP in the study area keep high levels ranging from 400 to 500g C/(m2•a)during 2000~2016.The NPP values of forests rank from highest to lowest in turn as follows: broadleaf, bamboo and conifer.As the NPP values are affected mainly by forest categories and atmospheric temperature, they increase first and then decrease with the rise of elevation, reach the maximum values at the height from 1300m to 1 400m above sea level.For every 100m rise in altitude, the annual NPP increases about 10g C/(m2•a) at the height from 400m to 1 200m above sea level, decreases about 15g C/(m2•a) from 1 600m to 1 800m and 20g C/(m2•a) from 1 800m to 2100m respectively.The area proportions with high normalized annual NPP in the range from 0.7 to 1.0 rise from 26.9% in 2000 to about 43.3% in 2010 and then drop a little.On the contrary, the area proportions with low normalized annual NPP(0~0.4) fall from 15.3% in 2000 to less than 10% from 2005 to 2010.The total normalized annual NPP values show an upward trend in the past years.The annual NPP values at high altitude are mainly influenced by the climate parameters, such as annual solar radiation and precipitation.However, the forest cover change caused by human activities has become another dominant factor affecting the annual NPP at low altitude.As the annual NPP rises with annual solar radiation increase and annual precipitation decrease, annual solar radiation have a greater impact on annual NPP than annual precipitation.The water conservation forest, especially part of the broadleaf forest, has been restored gradually in the study area. © 2021, The Editorial Board of Journal of Basic Science and Engineering. All right reserved.