Temperature sensitivity of tree recruitment at alpine treelines increases along latitudinal gradients

Alpine treeline is highly sensitive to temperature change, and treelines worldwide have been rapidly responding to temperature warming by increasing tree density. However, often inconsistent or even conflicting results regarding the response of tree recruitment to temperature change across regions, still challenge our profound comprehension of warming-induced treeline dynamics. Here, we identified the temperature sensitivity of tree recruitment within alpine treeline ecotones (i.e., the strength of the relationship between tree recruitment and temperature change), and tested whether and how the temperature sensitivity changed along latitudinal gradients by examining 124 alpine treelines across North America and Asia. Our analyses demonstrated that temperature changes could significantly influence tree recruitment at treeline ecotones and the sensitivity of tree recruitment to temperature change was significantly and positively associated with latitude, showing that tree recruitment was more sensitive to temperature change at high latitudes than at low latitudes. Our findings highlighted the heterogeneous responses of tree recruitment at alpine treelines to temperature changes across latitudinal gradients, which should be incorporated into assessment models to improve future projections of global warming impacts across alpine ecosystems.

高山林线(alpine treeline)对温度变化高度敏感，全球范围内的林线正通过提升树木密度快速响应气候变暖。然而，不同区域间树木更新(tree recruitment)对温度变化的响应结果往往不一致甚至相互矛盾，这仍制约着我们对气候变暖驱动的林线动态的深入认知。本研究通过考察北美与亚洲的124处高山林线，明确了高山林线交错带(alpine treeline ecotones)内树木更新的温度敏感性（即树木更新与温度变化间关联的强度），并检验了该温度敏感性是否沿纬度梯度发生变化、以及具体的变化模式。分析结果表明，温度变化可显著影响林线交错带内的树木更新，且树木更新对温度变化的敏感性与纬度呈显著正相关，即高纬度地区的树木更新对温度变化的敏感性高于低纬度地区。本研究结果揭示了不同纬度梯度下高山林线树木更新对温度变化的响应存在异质性，该结论应被纳入评估模型，以优化未来全球变暖对高山生态系统影响的预测工作。