Demographic and net primary productivity dynamics of primary and secondary tropical forests in Southwest China under a changing climate

Tropical forests are major carbon sinks on the Earth's land surface. However, our understanding of how the demographic rate and carbon sink capacities of tropical forests respond to climate change remains limited. In this study, we investigated the impacts of environmental drivers on forest growth, mortality, recruitment, and stem net primary productivity (NPP stem ) over 16 years at five tropical forest plots in Xishuangbanna, Southwest China. These plots are along a successional gradient spanning three tropical secondary forests (tropical secondary forest‐1 [TSF‐1], tropical secondary forest‐2 [TSF‐2], and tropical secondary forest‐3 [TSF‐3]) and two primary forests (tropical rainforest [TRF] and tropical karst forest [TKF]). Our results showed that early successional secondary forests (TSF‐2 and TSF‐3) had higher diameter growth rates and relative mortality rates. An extreme drought event during 2009–2010 reduced the growth rate, relative recruitment rate, and NPP stem for most plots while increasing mortality in early successional forest plots. We observed significant negative effects of maximum temperature (T max ) on NPP stem and diameter growth rate across all plots. Additionally, we found that precipitation had significant positive effects on diameter growth rate across all plots. Furthermore, tree mortality increased with rising T max , whereas precipitation significantly enhanced tree recruitment. Our findings highlight the vulnerability of tree growth, mortality, recruitment, and productivity in tropical forests to extreme drought events in Southwest China. Continued climate warming and more frequent droughts will induce higher mortality rates and impede growth, thus reducing the carbon sink capacity of tropical forests, especially in early successional stage tropical secondary forests. 热带森林是地球上陆地生态系统重要的碳汇。然而目前对于热带森林的群落动态和碳汇能力如何响应气候变化方面的研究较少。在本研究中，我们分析了环境因子对过去16年来西双版纳地区五个热带森林样地直径生长率、死亡率、更新以及树干净初级生产力的影响。这五个森林样地包括三个热带次生林（TSF‐1, TSF‐2和 TSF‐3）以及两个热带原始林（热带季节雨林TRF、热带喀斯特森林TKF）。结果显示演替早期的次生林（TSF‐2, TSF‐3）表现出更高的直径生长率和相对死亡率。2009‐2010年的极端干旱事件使得大部分样地的直径生长率、相对更新率和NPPstem下降，而处于演替早期的次生林样地的死亡率显著增加。发现最高气温（Tmax）与NPPstem和直径生长率呈显著的负相关关系，而降水与五个样地的直径生长率呈显著的正相关关系。树木的死亡率与Tmax呈显著的正相关关系，而更新率与降水呈显著的正相关关系。本研究强调了中国西南地区热带森林的生长、死亡、更新以及生产力对极端干旱事件的脆弱性。持续的气候变暖以及频发的干旱事件将会导致树木生长的下降和死亡率的增加，进而降低热带森林的碳汇能力，而这种影响对于演替早期的热带次生林更为严峻。【翻译:付培立】 Secondary tropical forests demonstrate superior growth rates. Policymakers and forest managers should prioritize these forests alongside primary forest