How forest age impacts on net primary productivity: insights from future multi-scenarios

Forest net primary productivity (NPP) constitutes a key flux within the terrestrial ecosystem carbon cycle and serves as a significant indicator of the forests carbon sequestration capacity, which is closely related to forest age. Despite its significance, the impact of forest age on NPP is often ignored in future NPP projections. Here, we mapped forest age in Hunan Province at a 30 m resolution utilizing a combination of Landsat time series stack (LTSS), national forest inventory (NFI) data, and the relationships between height and age. Subsequently, NPP was derived from NFI data and the relationships between NPP and age was built for various forest types. Then forest NPP was predicted based on the NPP-age relationships under three future scenarios, assessing the impact of forest age on NPP. Our findings reveal substantial variations in forest NPP in Hunan Province under three future scenarios: under the age-only scenario, NPP peaks in 2041 (133.56 Gg C yr^-1), while NPP peaks three years later in 2044 (141.14 Gg C yr^-1) under the natural development scenario. The maximum afforestation scenario exhibits the most rapid increase in NPP, with peaking in 2049 (197.95 Gg C yr^-1). However, with the aging of the forest, NPP is projected to then decrease by 7.54%, 6.07%, and 7.47% in 2060, and 20.05%, 19.74%, and 28.38% in 2100, respectively, compared to their peaks under the three scenarios. This indicates that forest NPP will continue to decline soon. Optimizing the age structure of forests through selective logging, afforestation and reforestation could mitigate this declining trend in forest NPP. Insights from the future multi-scenarios are expected to provide data to support sustainable forest management and national policy development, which will inform the achievement of carbon neutrality goals by 2060.