Climate models project higher growing season temperatures and a decline in the depth and duration of winter snowpack throughout many north temperate ecosystems over the next century. A smaller snowpack is projected to induce more frequent soil freeze/thaw cycles in winter in northern hardwood forests of the northeastern U.S. We measured the combined effects of warmer growing season soil temperatures and increased winter freeze/thaw cycles on rates of leaf-level photosynthesis and transpiration (sap flow) of red maple (Acer rubrum) trees in a northern hardwood forest at the Climate Change Across Seasons Experiment at Hubbard Brook Experimental Forest in New Hampshire. Soil temperatures were warmed 5 °C above ambient temperatures during the growing season and soil freeze/thaw cycles were induced in winter to mimic the projected changes in soil temperature over the next century. Relative to reference plots, growing season soil warming increased rates of leaf-level photosynthesis by up to 85.32 ± 4.33%, but these gains were completely offset by soil freeze/thaw cycles in winter, suggesting that increased freeze/thaw cycles in winter over the next 100 years will reduce the effect of warming on leaf-level carbon gains. Soil warming in the growing season increased rates of transpiration per kPa vapor pressure deficit (VPD) by up to 727.39 ± 0.28%, even when trees were exposed to increased frequency of soil freeze/thaw cycles in the previous winter, which could influence regional hydrology in the future. Using climate projections downscaled from the Coupled Model Intercomparison Project, we project increased rates of whole-season transpiration in these forests over the next century by 42- 61%. We also project 52 to 77 additional days when daily air temperatures will be above the long-term average daily maximum during the growing season at Hubbard Brook. Together, these results show that projected changes in climate across both the growing season and winter are likely to cause greater rates of water uptake and have no effect on rates of leaf-level carbon uptake by trees, with potential ecosystem consequences for hydrology and carbon cycling in northern hardwood forests.

中文翻译：

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生长季节变暖和冬季土壤冻融循环增加了北部硬木林的蒸腾作用

气候模型预测，在下个世纪，许多北温带生态系统的生长季节温度会更高，冬季积雪的深度和持续时间会下降。预计在美国东北部北部阔叶林冬季，较小的积雪会导致更频繁的土壤冻融循环。我们测量了生长季节土壤温度升高和冬季冻融循环增加对叶片水平光合作用和在新罕布什尔州哈伯德布鲁克实验森林的跨季节气候变化实验中，北部硬木森林中红枫树的蒸腾作用（树液流）。在生长季节，土壤温度比环境温度升高 5°C，并在冬季诱发土壤冻融循环，以模拟下个世纪土壤温度的预计变化。相对于参考地块，生长季节土壤变暖使叶片水平光合作用的速率增加了 85.32 ± 4.33%，但这些增益被冬季土壤冻融循环完全抵消，表明冬季冻融循环增加了未来 100 年将减少变暖对叶级碳增加的影响。生长季节的土壤变暖使每千帕蒸汽压差 (VPD) 的蒸腾速率增加了 727.39 ± 0.28%，即使树木在前一个冬天暴露于土壤冻融循环的频率增加，这可能会影响区域水文将来。使用从耦合模型比对项目缩减的气候预测，我们预计这些森林在下个世纪的全季蒸腾速率将增加 42-61%。我们还预测了 52 到 77 天，在 Hubbard Brook 的生长季节，每日气温将高于长期平均每日最大值。总之，这些结果表明，生长季节和冬季的预计气候变化可能会导致更高的水分吸收率，并且对树木的叶级碳吸收率没有影响，对水文和碳循环具有潜在的生态系统影响在北部阔叶林中。我们还预测了 52 到 77 天，在 Hubbard Brook 的生长季节，每日气温将高于长期平均每日最大值。总之，这些结果表明，生长季节和冬季的预计气候变化可能会导致更高的水分吸收率，并且对树木的叶级碳吸收率没有影响，对水文和碳循环具有潜在的生态系统影响在北部阔叶林中。我们还预测了 52 到 77 天，在 Hubbard Brook 的生长季节，每日气温将高于长期平均每日最大值。总之，这些结果表明，生长季节和冬季的预计气候变化可能会导致更高的水分吸收率，并且对树木的叶级碳吸收率没有影响，对水文和碳循环具有潜在的生态系统影响在北部阔叶林中。