Abstract

Climate change is warming stream temperatures with significant implications for species that require cold temperatures to persist. These species often rely on headwater habitats in mountainous regions where elevation gradients in hydroclimatic conditions may induce differential patterns of long-term warming that affect the resistance of refugia. Forecasts from mechanistic and statistical stream temperature models diverge regarding whether this elevation dependence will cause above- or below-average warming in headwaters during warm summer periods, so we examined monitoring records for stream temperature (n = 271), air temperature (n = 690), and stream discharge (n = 131) across broad elevation gradients in a mountainous region of western North America to better understand potential future trends. Over a 40-year period characterized by rapid climate change from 1976–2015, air temperature stations exhibited below-average warming rates at high elevations while stream discharge declined at above average rates. Between climatically extreme years that involved summer air temperature increases >5 °C and discharge declines >70%, temperatures in high-elevation streams exhibited below average increases but otherwise showed negligible elevation dependence during intermediate climate years. In a subsequent example, it was demonstrated that elevation dependent stream warming has a minor effect on the amount of thermal habitat loss relative to the average water temperature increase within a mountain river network. We conclude that predictions of above average warming effects on headwater organisms for this region may be overly pessimistic and discuss reasons why different types of temperature models make divergent forecasts. Several research areas warrant greater attention, including descriptions of elevation-dependent patterns in other regions for comparative purposes, examination of long-term stream temperature records to understand how sensitivity to climate forcing may be evolving, use of new data sources to better represent key processes in temperature models across broad areas, and development of hybrid models that integrate the best attributes of mechanistic and statistical approaches.

摘要

气候变化导致溪流温度升高，对需要维持低温的物种产生重大影响。这些物种通常依赖山区的水源栖息地，那里的水文气候条件的海拔梯度可能会导致长期变暖的差异模式，从而影响栖息地的抵抗力。关于这种海拔依赖性是否会导致暖夏季期间源头变暖高于或低于平均水平，机械和统计河流温度模型的预测存在分歧，因此我们检查了河流温度（n = 271）、气温（n = 690）的监测记录）和北美西部山区大海拔梯度范围内的河流流量（n = 131），以更好地了解潜在的未来趋势。在 1976 年至 2015 年气候快速变化的 40 年间，高海拔地区气温站的变暖速度低于平均水平，而河流流量的下降速度高于平均水平。在夏季气温升高> 5°C且流量下降> 70%的极端气候年份之间，高海拔溪流的温度增幅低于平均水平，但在中间气候年份中表现出可忽略不计的海拔依赖性。在随后的例子中，事实证明，相对于山区河流网络内平均水温的增加，海拔相关的河流变暖对热栖息地丧失的影响较小。我们的结论是，对该地区水源生物的高于平均水平的变暖影响的预测可能过于悲观，并讨论了不同类型的温度模型做出不同预测的原因。一些研究领域值得更多关注，包括出于比较目的描述其他地区的海拔相关模式、检查长期河流温度记录以了解对气候强迫的敏感性如何演变、使用新数据源更好地表示关键过程广泛领域的温度模型，以及集成机械和统计方法最佳属性的混合模型的开发。