LTREB: Long-Term Consequences of Asymmetric Warming at the Tundra-Taiga Interface

北极树线的位置(最北端的树木)对碳循环、陆地表面能量交换、野生动物栖息地以及北极人类社区生存资源的可用性具有重要影响。目前流行的树线形成理论认为，树线树木的生长受到寒冷生长季节气温的严格限制。虽然这一观点表面上得到了夏季气温和树线高度之间的全球尺度相关性的支持，但只有大约50%的全球树线因人为气候变暖而提前的观察结果并不支持这一观点。越来越多的证据表明，其他因素可以决定树线的位置，特别是在北极。这个项目测试了这样一种假设，即随着积雪的加深，冬季变暖会增加树线树的生长和繁殖，从而推动树线向北极冻土带推进。结果改善了对未来树线在哪里以及在什么情况下可以预期前进的预测。该项目为阿拉斯加西北部的研究生和当地居民提供教育和培训机会。北极变暖的速度比全球其他地区快得多，而且变暖在季节上是不对称的，夏季温和变暖，冬季极端变暖。在北极的许多地区，极端的冬季变暖伴随着越来越多的降雪。极端的冬季变暖和更深的、隔热的冬季积雪将导致冬季土壤迅速变暖，对生态系统功能产生广泛的影响。较温暖的冬季土壤通常与越冬微生物活动增加、土壤有机质分解增加以及积雪融化后可供树木吸收的养分丰富有关。该项目利用了对阿拉斯加北极冬季积雪和相应树木生长的长期观察。它还扩展了一项雪栅栏实验，在三条对比鲜明的树线上，在每棵树周围建造更深的积雪。长期测量对于确定季节性不对称变暖对北极林线位置的影响至关重要，因为冬季天气的高年际变异性以及与土壤养分循环变化相关的时间滞后、树木资源的储存以及树木繁殖投资的间歇性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The location of the Arctic treeline (the northernmost extent of trees) has important implications for carbon cycling, land surface energy exchange, wildlife habitat, and the availability of subsistence resources to Arctic human communities. The prevailing theory of treeline formation states that growth of treeline trees is strictly limited by cold growing-season air temperature. While this perspective is superficially supported by global-scale correlations between summer air temperature and treeline elevation, it is not supported by the observation that only about 50% of global treelines have advanced in response to anthropogenic climate warming. There is increasing evidence that other factors can determine treeline position, particularly in the Arctic. This project tests the hypothesis that warming winters with deepening snowpacks increase the growth and reproduction of treeline trees, thereby fueling treeline advance into Arctic tundra. Results are improving predictions of where and under what circumstances future treeline advance can be expected. The project provides education and training opportunities for graduate students and native residents of northwest Alaska. The Arctic is warming much faster than the rest of the globe and the warming is seasonally asymmetrical, with modest warming during the summer months and extreme warming during the winter months. In many regions of the Arctic, extreme winter warming is coupled with increasing snowfall. The combination of extreme winter warming and deeper, insulating winter snowpacks will lead to rapid winter soil warming, with wide-ranging consequences for ecosystem function. Warmer winter soils are generally associated with greater overwinter microbial activity, enhanced breakdown of soil organic matter, and a flush of nutrients that may be available for uptake by trees following snowmelt. This project leverages long-term observations of winter snowpack and corresponding tree growth in the Alaskan Arctic. It also extends a snowfence experiment, which builds deeper snowpacks around individual trees at three contrasting treelines. Long-term measurements are essential to identify the effects of seasonally asymmetric warming on treeline position in the Arctic, because of high interannual variability in winter weather and time lags associated with altered soil nutrient cycling, storage of resources in trees, and the episodic nature of tree investment in reproduction.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.