Department of Natural Sciences, University of Alaska Southeast, 11120 Glacier Highway, Juneau, AK, 99801, USA.
USDA Forest Service, PNW Research Station, 11175 Auke Lake Way, Juneau, AK, 99801, USA.
Glob Chang Biol. 2017 Jul;23(7):2903-2914. doi: 10.1111/gcb.13555. Epub 2016 Nov 28.
Climate change is causing rapid changes to forest disturbance regimes worldwide. While the consequences of climate change for existing disturbance processes, like fires, are relatively well studied, emerging drivers of disturbance such as snow loss and subsequent mortality are much less documented. As the climate warms, a transition from winter snow to rain in high latitudes will cause significant changes in environmental conditions such as soil temperatures, historically buffered by snow cover. The Pacific coast of North America is an excellent test case, as mean winter temperatures are currently at the snow-rain threshold and have been warming for approximately 100 years post-Little Ice Age. Increased mortality in a widespread tree species in the region has been linked to warmer winters and snow loss. Here, we present the first high-resolution range map of this climate-sensitive species, Callitropsis nootkatensis (yellow-cedar), and document the magnitude and location of observed mortality across Canada and the United States. Snow cover loss related mortality spans approximately 10° latitude (half the native range of the species) and 7% of the overall species range and appears linked to this snow-rain transition across its range. Mortality is commonly >70% of basal area in affected areas, and more common where mean winter temperatures is at or above the snow-rain threshold (>0 °C mean winter temperature). Approximately 50% of areas with a currently suitable climate for the species (<-2 °C) are expected to warm beyond that threshold by the late 21st century. Regardless of climate change scenario, little of the range which is expected to remain suitable in the future (e.g., a climatic refugia) is in currently protected landscapes (<1-9%). These results are the first documentation of this type of emerging climate disturbance and highlight the difficulties of anticipating novel disturbance processes when planning for conservation and management.
气候变化正在导致全球森林干扰格局的快速变化。虽然气候变化对现有干扰过程（如火灾）的影响相对较好，但像雪量减少和随后的死亡等新兴干扰驱动因素的记录却要少得多。随着气候变暖，高纬度地区冬季从降雪向降雨的转变将导致环境条件发生重大变化，如土壤温度，这些条件在历史上一直受到积雪的缓冲。北美洲太平洋沿岸是一个很好的测试案例，因为目前冬季平均气温已经接近雪雨的临界点，并且自小冰期以来已经变暖了大约 100 年。该地区一种广泛分布的树种死亡率的增加与较暖的冬季和雪量减少有关。在这里，我们首次展示了这种对气候敏感的物种 Callitropsis nootkatensis（黄杉）的高分辨率范围图，并记录了在加拿大和美国各地观察到的死亡率的幅度和位置。与雪量减少相关的死亡率跨越了大约 10 个纬度（该物种原生范围的一半）和 7%的物种总范围，并且似乎与整个范围内的这种雪雨转变有关。受影响地区的死亡率通常超过基础面积的 70%，在平均冬季温度达到或高于雪雨临界点（>0°C 平均冬季温度）的地区更为常见。预计到 21 世纪后期，目前适合该物种生长的 50%的地区（< -2°C）的气温将超过这一阈值。无论气候变化情景如何，预计未来仍将适合该物种的范围（例如气候避难所）的一小部分（< -2°C）目前处于受保护的景观中（<1-9%）。这些结果首次记录了这种新兴气候干扰的类型，并强调了在规划保护和管理时，预测新的干扰过程的困难。
