Forest structure outweighs climatic and edaphic controls of primary productivity in Quercus mongolica forests in Northern China

Forest structural attributes such as density, size, and age are important determinants of productivity; however, whether and how this relationship changes across a wide geographic range with varying climate and soil conditions requires validation. Quercus mongolica-dominated forests are widespread in Northern China; however, the spatial characteristics and mechanisms underlying their productivity remain poorly understood. We integrated tree-ring data with field plots from 17 sites and 47 plots spanning approximately 15.5° latitude and 25.0° longitude to evaluate the relative importance of climatic, edaphic, and forest structural factors (forest density, age, and tree size) on the forest productivity. Allometric equations were used to convert diameter growth into estimates of aboveground and belowground net primary productivity (ANPP and BNPP, respectively). Fixed-effect regression and structural equation modeling were used to assess the effects of forest structure and environmental variables on ANPP and BNPP. The sampled forest density ranged from 375 to 1700 trees ha⁻¹, age varied from 10 to 197 years, and diameter at breast height (DBH) ranged from 5.0 to 63.3 cm. ANPP ranged from 2.1 to 7.4 t ha⁻¹ yr⁻¹, whereas BNPP ranged from 0.2 to 2.4 t ha⁻¹ yr⁻¹. Forest density and maximum DBH emerged as the strongest positive predictors of ANPP and BNPP, whereas age was a significant negative predictor. The structural equation model showed that climatic and soil variables only indirectly affected BNPP through their effects on forest structure. This study highlights the need to consider forest age, in addition to density and tree size, to estimate primary productivity across a wide geographic range.