Collaborative Research: Tracking Divergent Warming and Tree Growth at Arctic Treeline

北极的快速变暖预计会对北部森林造成重大变化。虽然气候变暖可能导致受短而冷的生长季节限制的森林增长，但气候变化也可能导致意外。一个例子是在过去几十年中发现的北方森林树木的温度变化和年增长率之间的脱钩。这种“分化问题”表明，一些北极森林系统中的树木生长不再受到温度的限制。原因尚不清楚，而且由于影响生长的其他因素，例如气候变暖导致的干旱，可能难以检验。该项目将测试北美北部林木线森林的差异，特别是在阿拉斯加和加拿大西北部迅速变暖的地区。研究人员将利用从古树木材中获得的树木年轮记录，结合数学方法和建模，以及木材解剖分析，更好地了解与分化有关的森林生长模式。更广泛的影响：如果我们要了解北方森林如何与大气相互作用并与全球环境的其他特征反馈，新的测量方法至关重要。该项目将通过利用北方许多地点的树木年轮来估计气候和森林生长之间的关系，从而有助于理解这一点。该项目为学生、美国原住民和教师提供资金，让他们参与北极研究。北极的快速变暖预计将对北部森林系统产生深远的影响。虽然气候变暖可能会提高森林的生产力和生长，但气候变化的速度和幅度也可能导致意外。一个例子是自20世纪中叶以来在森林地点检测到的北方森林树木的温度变化与年增长率之间的脱钩。这种被称为“分化问题”的现象表明，一些北极森林系统中的树木生长不再主要受温度的限制。原因尚不清楚，并且由于共同变化的生物、生理和环境因素可能影响近期树木的生长，因此很难进行测试。一种假设是，一旦变暖超过生理阈值，干旱压力就会限制生长。另一种假说是“全球变暗”，即大气气溶胶的增加减少了光合作用所需的太阳辐射量。该项目将评估目前北美北部林木线森林的分歧假设，强调阿拉斯加和加拿大西北部的快速变暖地区，这些地区似乎是分歧型效应的关键地区。该项目将利用新的大型树木年轮数据汇编（已经在手），并更新关键地点，尽管最近气候变暖，但已经显示出异常生长下降的早期证据。检测生长参数的近期变化将使用一种新的、综合的统计技术、定量木材解剖方法和树木生长模型，以对差异的程度、原因和碳循环影响进行详细评估。更广泛的影响：北方森林动态的新测量对于理解北方系统如何与变化的大气相互作用以及耦合的地球系统域之间的反馈至关重要。该项目将通过在每天到百年的时间尺度和从细胞到大陆的空间尺度上对气候/生长相互作用进行定量估计，为建立北方森林环境变化和生产力模型提供有价值的新数据，从而有助于理解这一点。该项目为一名研究生提供三年的支持，为高中课程开发和当地教育环境的课程提供资金，并为一名教师提供实地考察的机会，让他们亲身参与北极研究。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The rapid warming of the Arctic is expected to cause major changes to northern forests. While warming may lead to increased growth in forests limited by short, cold growing seasons, climate change could also lead to surprises. One example is a decoupling between temperature variability and annual growth rates in boreal forest trees that has been detected over the past few decades. This ‘divergence problem’ suggests that tree growth in some arctic forest systems is no longer limited by temperature. The causes are not well understood and can be hard to test due to other factors impacting growth, such as drought due to the warming climate. This project will test for divergence across the treeline forests of northern North America, especially in rapidly warming regions of Alaska and northwestern Canada. The investigators will use tree-ring records obtained from the wood of ancient trees, a combination of mathematical methods and modeling, and wood anatomy analysis, to better understand forest growth patterns related to divergence. Broader Impacts: New measurements are critical if we are to understand how boreal forests interact with the atmosphere and feedback with other features of the global environment. This project will contribute to this understanding by estimating relationships between climate and forest growth using tree rings at sites spanning many northern locations. The project provides funding for students, Native American citizens, and for teachers to take part in Arctic research.The rapid warming of the Arctic is expected to impact profoundly northern forest systems. While warming may lead to greater productivity and growth in forests typically limited by short, cold growing seasons, the speed and magnitude of climate change could also lead to surprises. One example is a decoupling between temperature variability and annual growth rates in boreal forest trees that has been detected at forest sites since the middle of the twentieth century. This phenomenon, known as the ‘divergence problem’, suggests that tree growth in some Arctic forest systems is no longer primarily limited by temperature. The causes are not well understood and have been difficult to test due to the co-varying biological, physiological, and environmental factors potentially impacting recent tree growth. One hypothesis is that once warming has surpassed a physiological threshold, drought stress limits growth. Another hypothesis is ‘global dimming’, in which increased atmospheric aerosols decrease the amount of solar radiation available for photosynthesis. This project will assess current divergence hypotheses across the treeline forests of northern North America, emphasizing rapidly warming regions of Alaska and northwestern Canada, which appear to be key areas for divergence-type effects. The project will leverage a new, large compilation of tree-ring data (already in hand) and update key locations that have shown early evidence of unusual growth decline despite recent warming. Detection of recent changes in growth parameters will use a novel, integrated combination of statistical techniques, quantitative wood anatomy methods, and tree growth modeling to develop a detailed assessment of the extent, causes, and carbon cycle implications of divergence. Broader Impacts: New measurements of boreal forest dynamics are critical for understanding how boreal systems interact with the changing atmosphere and the feedbacks between coupled earth system domains. This project will contribute to this understanding by developing quantitative estimates of climate/growth interactions at daily to centennial time scales and spatial scales from cellular to continental, yielding valuable, novel data for modeling of environmental change and productivity in northern forests. The project provides three years of support for a graduate student, funding for high school curricular development and courses in native educational settings, and fieldwork-based opportunities for a teacher to take part in hands-on Arctic research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.