Relationship between net primary productivity and stand age in typical plantation forests in China

Plantation forests play an important role in carbon sink in terrestrial ecosystems. Based on tree rings of five main plantation tree species (Robinia pseudoacacia, Quercus variabilis, Cunninghamia lanceolata, Pinus sylvestris var. mongolica, and Pinus tabuliformis) at 25 sites in China, we calculated the average annual NPP of standard trees in each study area by the biomass equations and extended to the stand scale. The relationships between NPP and stand age were fitted by the InTEC and Law models. The results showed that NPP of R. pseu-doacacia, C. lanceolata, and P. tabuliformis plantations increased to a peak and then leveling off with stand age, while that of Q. variabilis and P. sylvestris var. mongolica plantations reached a peak and then showed a decreasing trend. The inflection points of NPP-stand age curve for different planatations was 11 years for P. sylvestris var. mongolica, 14 years for C. lanceolata, 16 years for P. tabuliformis, and 20 years for R. pseudoacacia. The NPP peak was 6.65, 7.58, 4.70 and 2.59 t·hm^-2·a^-1, respectively. Both the InTEC and Law NPP-stand age models had high fitting accuracy at a large scale, with the lowest R² of 0.95 and RMSE of 0.55 t·hm^-2·a^-1 for the P. sylvestris var. mongolica InTEC model and the highest R² of 0.99 and RMSE of 0.26 t·hm^-2·a^-1 for the C. lanceolata InTEC model. The construction of NPP-stand age relationship for major plantations in China provided mechanistic support for the estimation of carbon sinks in plantations at long time scales and provided a reference for the diversification of afforestation tree species selection.