Canopy gas exchange and growth of white spruce near the Arctic treeline: confronting measurements with models along natural and experimental resource gradients

北极树木线的位置对气候变化中的地表能量收支和碳循环具有重要意义。模拟工作表明，这些影响在区域和全球尺度上都是相关的。我们对北极林木线树木生长控制的认识是通过树木年轮研究发展起来的，它们在本质上是必然相关的，而不是机械的。这些树木年轮研究已经确定了20世纪下半叶对变暖的正向和负向径向生长响应。研究人员推测，负增长趋势反映了温度引起的干旱压力的重要性日益增加，随着未来气候变暖，在湿润地区，而在干旱地区，树木线可能会向前推进。最近的工作揭示了几个重要的复杂性，清楚地表明我们过度简化了北极树木线气候和树木生长之间的关系。现在需要对树木生理和生长的季节性变化进行详细测量，以响应资源可用性的变化，从而将我们的理解提升到一个新的水平。这项工作将协调白云杉冠层气体交换的连续测量与每周测量树枝气体交换以及沿母体材料深度梯度接受因子营养和水分补充的树木的先导、分支、径向和细根生长。研究结果将首次解决林系白云杉碳吸收和水分流失的季节性问题；比较各主要器官生长的季节性和幅度；阐明资源可用性变化对白云杉气体交换生理和生长的影响，资源可用性沿梯度自然变化；并确定一个基于过程的模型，准确地描述气候和北极树木线树木生长之间的关系。

The position of the arctic treeline has important implications for surface energy budgets and carbon cycling in a changing climate. Modeling efforts suggest these effects are relevant on regional and global scales. Our understanding of the controls on tree growth at the arctic treeline has been developed using tree ring studies, which are necessarily correlative and not mechanistic in nature. These tree ring studies have identified both positive and negative radial growth responses to warming in the later half of the 20th century. Investigators have speculated that negative growth trends reflect an increasing importance of temperature-induced drought stress and that treeline advance may be expected in mesic and wet areas, but not in dry areas, with future climate warming. Recent work has revealed several important complexities that clearly show we have oversimplified the relationships between climate and tree growth at the arctic treeline. Detailed measurements of seasonal changes in tree physiology and growth in response to changes in resource availability are now required to take our understanding to the next level. This work will coordinate continuous measurements of white spruce canopy gas exchange with weekly measurements of branch gas exchange and leader, branch, radial and fine root growth in trees receiving factorial nutrient and water supplements along a gradient of parent material depth. Results of the study will, for the first time: resolve the seasonality of C uptake and water loss in treeline white spruce; compare the seasonality and magnitudes of growth in all major organs; articulate the consequences of changes in resource availability for white spruce gas exchange physiology and growth along a gradient where resource availability varies naturally; and identify a process-based model that accurately describes the relationships between climate and tree growth at the arctic treeline.