Nutrient limitation in tropical secondary forests following different management practices

Secondary forests now make up more than half of all tropical forests, and constraints on their biomass accumulation will influence the strength of the terrestrial carbon (C) sink in the coming decades. However the variance in secondary tropical forest biomass for a given stand age and climate is high and our understanding of why is limited. We constructed a model of terrestrial C, nitrogen (N), and phosphorus (P) cycling to examine the influence of disturbance and management practices on nutrient limitation and biomass recovery in secondary tropical forests. The model predicted that N limited the rate of forest recovery in the first few decades following harvest, but that this limitation switched to P approximately 30-40 years after abandonment, consistent with field data on N and P cycling from secondary tropical forest chronosequences. Simulated biomass recovery agreed well with field data of biomass accumulation following harvest (R2=0.80). Model results showed that if all biomass remained on site following a severe disturbance such as blowdown, regrowth approached pre-disturbance biomass in 80-90 years, and recovery was faster following smaller disturbances such as selective logging. Field data from regrowth on abandoned pastures were consistent with simulated losses of nutrients in soil organic matter, particularly P. Following any forest disturbance that involved the removal of nutrients (i.e., except blowdown), forest regrowth produced reduced biomass relative to the initial state as a result of nutrient loss through harvest, leaching and/or sequestration by secondary minerals. Differences in nutrient availability accounted for 49-94% of the variance in secondary forest biomass C at a given stand age. Management lessons from this study are the importance of strategies that help retain nutrients on site, recognizing the role of coarse woody debris in immobilization and subsequent release of nutrients, and the potential for nutrient additions to enhance biomass growth and recovery in secondary tropical forests.

当前次生林已占全球热带森林总量的一半以上，其生物量积累的限制因素将在未来数十年内影响陆地碳（C）汇的强度。然而，在林龄与气候条件固定的情况下，热带次生林生物量的变异幅度较大，且目前学界对其背后成因的认知仍较为有限。本研究构建了陆地碳、氮（N）、磷（P）循环模型，用以探究干扰与经营措施对热带次生林养分限制及生物量恢复的影响。模型预测：采伐后的最初数十年内，氮会限制森林恢复速率，而该限制作用会在弃耕约30至40年后转为磷限制，这一结果与热带次生林演替时间序列样地中氮、磷循环的野外观测数据相一致。模拟得到的生物量恢复结果与采伐后生物量积累的野外观测数据拟合度良好（决定系数R²=0.80）。模型结果显示：若发生风倒这类严重干扰后全部生物量仍留存于样地，森林恢复将在80至90年内接近干扰前的生物量水平；而择伐这类轻度干扰后的恢复速率则更快。弃耕牧场植被恢复的野外观测数据，与土壤有机质（尤其是磷）的模拟养分流失结果相吻合。但凡涉及养分移除的森林干扰（风倒除外），由于采伐、淋溶或次生矿物固持造成的养分流失，森林恢复后的生物量会低于初始状态。在林龄固定的情况下，养分可利用性的差异可解释49%至94%的热带次生林碳生物量变异。本研究的经营启示包括：需重视有助于样地养分留存的经营策略，充分认识粗木质残体在养分固持与后续释放中的作用，以及通过养分添加提升热带次生林生物量生长与恢复的潜在可能。