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 ​Tg ​C·yr−1), while NPP peaks three years later in 2044 (141.14 ​Tg ​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 ​Tg ​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. Controlling the age structure of forests through selective logging, afforestation and reforestation, and encouraging natural regeneration after disturbance could mitigate this declining trend in forest NPP, but implications of these measures on the full forest carbon balance remain to be studied. 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.

中文翻译：

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林龄如何影响净初级生产力：来自未来多情景的见解


森林净初级生产力 （NPP） 是陆地生态系统碳循环中的关键通量，是森林碳固存能力的重要指标，与林龄密切相关。尽管森林年龄对 NPP 的影响很重要，但在未来的 NPP 预测中经常被忽视。在这里，我们利用 Landsat 时间序列堆栈 （LTSS）、国家森林清查 （NFI） 数据以及高度与年龄之间的关系，以 30 m 的分辨率绘制了湖南省的森林年龄图。随后，NPP 从 NFI 数据中得出，并针对各种森林类型构建了 NPP 与年龄之间的关系。然后，根据 3 种未来情景下 NPP 与年龄的关系预测森林 NPP，评估林龄对 NPP 的影响。我们的研究结果揭示了在三种未来情景下湖南省森林 NPP 的巨大差异：在仅年龄情景下，NPP 在 2041 年达到峰值 （133.56 Tg C·yr−1），而在自然发展情景下，NPP 在 2044 年后达到峰值 （141.14 Tg C·yr−1）。最大造林情景表现出 NPP 增长最迅速，在 2049 年达到峰值 （197.95 Tg C·yr−1）。但随着森林老化，预计 2060 年NPP 将分别比 3 种情景下的峰值下降 7.54%、6.07% 和 7.47%，到 2100 年将分别下降 20.05%、19.74% 和 28.38%。这表明森林 NPP 很快将继续下降。通过选择性采伐、植树造林和再造林来控制森林的年龄结构，并鼓励干扰后的自然更新可以缓解森林 NPP 的这种下降趋势，但这些措施对整个森林碳平衡的影响仍有待研究。 来自未来多情景的见解有望提供数据以支持可持续森林管理和国家政策制定，这将为到 2060 年实现碳中和目标提供信息。