MSB-ECA: A Lengthening Vernal Window: How Vernal Asynchronies in Energy, Water, and Carbon Fluxes Impact Ecosystem Function

春窗是美国东北部冬春之间的过渡时期，此时气温高于冰点，雪开始融化，森林冠层完全脱落。在这段时间内，物理变化的顺序和时间对季节性积雪覆盖的生态系统的健康功能起着关键作用。在寒冷多雪的冬季之后，春季的窗口期会变短，但当冬季变暖，积雪较浅或在冬季多次融化时，窗口期会延长。当春季窗口变长时，可能导致关键生态系统过程的时间不匹配。例如，早春的融雪会在树木从休眠中苏醒之前将土壤中的养分冲走，从而阻止树木在早春生长期间获得这些重要的养分。考虑到有记录的冬季气候变暖趋势，春窗期预计会延长，在高排放情景下，在更南方的地区可能会完全消失。这项研究将结合对雪、河流和森林健康的观测来记录春窗长度的历史变化，并利用模拟来预测春窗的未来变化。研究结果将用于评估关键能源、水和碳转换对生态系统功能的影响，并以美国东北部森林为试验平台。通过与当地一所高中教室的合作，研究人员将开发一套低成本的仪器，由学生通过探究式实验课程构建，使学生能够跟踪学校院子里春天窗户的变化。一个符合下一代科学标准的课程，包括能量平衡、碳循环和水文学的单元，将伴随仪器套件，为户外课堂的环境探究奠定基础。先前的研究已经证明了冬季寒冷或积雪作为春季到来的驱动因素的重要性，然而，尚不清楚先前的冬季温度和积雪特性如何相互作用影响春窗的时间和持续时间。此外，以前的工作并没有明确研究在较长的春季窗口中，由于过渡之间的较长滞后而导致的生物地球化学不同步如何影响生态系统功能，这对于理解环境变化的影响至关重要。冬季条件的快速变化可能导致春季的不同步，这有可能改变能量、水和碳平衡。最极端的情况是进入无雪状态，在这种情况下，春季窗口不复存在，取而代之的是在衰老和凋零之间，陆地和水生生态系统的净辐射增加，这可能从根本上改变生态系统的功能。这里提出的工作汇集了大量现有数据集和基于过程的模型，将用于明确研究1)冬季寒冷和积雪在改变春季窗口中的相对重要性；2)春期滞后和窗期对生态系统能量、水和碳平衡的影响；3)未来冬季的寒冷和积雪特征如何导致春季窗口的额外变化。这一过程的结果将是对春季窗口期间能源、碳和水平衡变化的区域30年历史分析。从这一历史分析中获得的洞察力将提供预测未来变化的能力，不仅是春窗长度的变化，还有春窗期间能量、碳和水平衡变化对生态系统功能的影响。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 vernal window is the transitional period between winter and spring in the northeastern US, when air temperatures warm above freezing, snow melt begins, and forest canopies fully leaf out. The sequential order and timing of physical changes during this window plays a key role in the healthy functioning of seasonally snow-covered ecosystems. The vernal window is shorter following cold, snowy winters but lengthens when winters are warmer and snow packs are shallow or melt multiple times through the winter. When the vernal window gets longer, it can lead to a mismatch in the timing of key ecosystems processes. For example, earlier snowmelt in the spring can flush nutrients from soils before trees emerge from dormancy, preventing trees from accessing those vital nutrients during early spring growth. The vernal window is expected to lengthen given documented warming trends in winter climate, and, under higher emission scenarios may be eliminated altogether in more southerly locations. This study will integrate observations of snow, streamflow, and forest health to document historical changes in the length of the vernal window and use simulations to project future changes in the vernal window. Results will be use to assess the impact on key energy, water, and carbon transitions on ecosystem function, using the northeastern United States forests as a testbed. In collaboration with a local high school classroom, the investigators will develop a low-cost instrumentation suite constructed by students through an inquiry-based lab course that will allow students to track changes in the vernal window in their school yards. A Next Generation Science Standard compliant curriculum with units on energy balance, the carbon cycle, and hydrology will accompany the instrument kit, laying the groundwork for outdoor classroom inquiry on the environment. Previous studies have demonstrated the importance of winter coldness or snowpack as drivers of spring onset, however, it remains unclear how antecedent winter temperature and snowpack properties interact to influence the timing and duration of the vernal window. In addition, previous work has not investigated explicitly how biogeochemical asynchronies that result from longer lags between transitions in the longer vernal window can impact ecosystem function, which is essential for understanding the impact of environmental change. Rapid changes in winter conditions could lead to vernal asynchronies that have strong potential to alter energy, water, and carbon balances. The most extreme case is a shift into a snow-free regime in which the vernal window ceases to exist and is replaced by a prolonged period of increased net radiation into terrestrial and aquatic ecosystems between senescence and leaf out that may fundamentally alter ecosystem function. The work proposed here brings together large existing datasets and process-based models that will be used to investigate explicitly 1) the relative importance of winter coldness and snowpack in altering the vernal window; 2) how longer vernal lags and windows impact ecosystem energy, water, and carbon balance; and 3) how future winter coldness and snowpack characteristics might lead to additional changes in the vernal window. The outcome of this process will be a regional, 30-year historical analysis of changes to energy, carbon, and water balances during the vernal window. Insight gained from this historical analysis will provide the ability to project future changes in not only vernal window length, but also the impact of changing energy, carbon, and water balances during the vernal window on ecosystem function.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.