Earth Systems Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH, USA.
Thayer School of Engineering, Dartmouth College, Hanover, NH, USA.
Glob Chang Biol. 2017 Apr;23(4):1610-1625. doi: 10.1111/gcb.13517. Epub 2016 Nov 3.
Climate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire, USA, that concurrently monitored climate, snow, soils, and streams over a three-year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero-length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter-to-spring transition and throughout the rest of the year.
气候变化正在改变春天的开始期（vernal window）的时间和持续期，这个时期标志着冬季结束和生长季节开始，生态系统的能量、水、养分和碳动态发生快速转变。关于这个时期的研究通常只捕捉到生态系统转变的一部分，并且主要关注系统苏醒的日期。以前的工作没有解决代表能量、水、养分和碳流动延迟的转变之间的滞后问题。本研究的目的是确定春天的开始期期间的物理和生物地球化学转变和滞后的顺序，并了解气候变化可能如何改变它们。我们综合了美国新罕布什尔州全州传感器网络的观测结果，该网络在三年的时间里同时监测气候、积雪、土壤和溪流，并用气候再分析数据、积雪数据同化模型输出和卫星光谱数据补充这些观测结果。我们发现，春天的开始期内发生的一些转变是连续的，气温先于积雪融化而变暖，而积雪融化又先于森林冠层关闭。其他转变是相互同时发生的，没有滞后，例如积雪消失、土壤快速升温和峰值溪流排放。我们将滞后建模为冬季寒冷和积雪深度的函数，预计这两者都将随着气候变化而下降。较温暖、积雪较少的冬季导致滞后时间更长，春天的开始期更长。这种单个滞后和整个春天的开始期的延长对生态系统的热力学和生物地球化学都有重要影响，无论是在冬季到春季的过渡期间还是在一年的其余时间。
