As global climate change has a large effect on the structure and function of vegetation, it is very important to understand how forests in climate transition regions respond to climate change. The present study investigates the net primary productivity (NPP) of two planted forests (Robinia pseudoacacia and Pinus tabulaeformis) and one natural forest (Quercus wutaishanica) from 1951–2100 using the LPJ-GUESS model in the Shaanxi province of China, which is a typical transition region from humid to dry climates. We found that: (1) Future annual precipitation and mean temperature exhibited nonsignificant and significant increasing trend in the region, respectively, indicating a drier climate in future; (2) although precipitation would increase in the dry area and decrease in the humid area, the NPP of each species in the dry area would be lower than that of the humid area, possibly because increasing temperature and CO₂ concentration could restrain forest growth in dry areas and promote forest growth in humid areas; (3) of the three species, P. tabulaeformis forest exhibited the highest average NPP and R. pseudoacacia forest exhibited the highest NPP trend in both dry and humid areas, indicating these planted species may be adaptable to future climate change. Our results provide novel insights into the potential response of forest productivity to a changing climate in the transition region from humid to dry climates.

As global climate change has a large effect on the carbon cycle of forests, it is very important to understand how forests in climate transition regions respond to climate change. Specifically, the LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator) model was used to simulate net ecosystem productivity (NEP) and soil heterotrophic respiration (Rh) dynamics of two forest ecosystems of different origins between 1951 and 2100, to quantitatively analyze the carbon source and sink functions and potential changes in soil carbon dynamics in arid and humid regions under future climate change, simulate the dynamics of forest net primary productivity (NPP) under different climatic factors, and analyze the sensitivity of forests in arid and humid regions to temperature, precipitation, and carbon dioxide (CO₂) concentration. We found that: (1) in both the historical and future periods, the average NEP of both studied forests in the humid region was larger than that in the arid region, the carbon sink function of the humid region being predicted to become stronger and the arid zone possibly becoming a carbon source; (2) between 1951 and 2100, the forest soil Rh in the arid region was lower than that in the humid region and under future climate change, forest in the humid region may have higher soil carbon loss; (3) increasing temperature had a negative effect and CO₂ concentration had a positive effect on the forests in the study area, and forests in arid areas are more sensitive to precipitation change. We believe our research could be applied to help policy makers in planning sustainable forest management under future climate change.