Sap Flow of Northern Red Oak Trees Under Ecosystem Warming at Harvard Forest 2011

Over the next century, air temperature increases up to 5 °C are projected for the northeastern USA. Because evapotranspiration dominates water loss from terrestrial ecosystems, tree ecophysiological response to warming will have important consequences for forest water budgets. We measured growing season sap flow rates in mature northern red oak (Quercus rubra L.) trees in a combined air (up to 5.5 °C above ambient) and soil (up to 1.85 °C above ambient at 6-cm depth) warming experiment at Harvard Forest, MA, USA. Principal components analysis found air and soil temperatures had the largest effects on sap flow. On average, each 1 °C increase in temperature increased sap flow rates by approximately 1100 kg H2O m-2 sapwood area day-1 throughout the growing season and by 1200 kg H2O m-2 sapwood area day-1 during the early growing season. Reductions in the number of cold winter days correlated positively with increased sap flow at night during the early growing season (a decrease of 100 heating-degree-days was associated with a sapflow increase of approximately 5 kg H2O m-2 sapwood area day-1). Soil moisture declined with increased treatment temperatures, and each soil moisture percentage increase resulted in an increase in sap flow of approximately 360 kg H2O m-2 sapwood area day-1. At night, soil moisture correlated positively with sap flow rate. These results demonstrate that warmer air and soil temperatures in winter and throughout the growing season lead to increased sap flow rates, which could affect forest water budgets throughout the year.

未来一个世纪，美国东北部地区的气温预计将升高至多5摄氏度。由于蒸散作用（evapotranspiration）是陆地生态系统水分损失的主导途径，树木对气候变暖的生态生理响应将对森林水分预算产生重要影响。我们在美国马萨诸塞州哈佛森林（Harvard Forest）开展了空气（较环境温度最高升高5.5℃）与土壤（6cm深度处较环境温度最高升高1.85℃）的联合增温实验，测定了成熟北美红栎（*Quercus rubra* L.）在生长季内的树干液流（sap flow）速率。主成分分析（Principal components analysis）结果显示，空气与土壤温度对树干液流的影响最为显著。平均而言，整个生长季内气温每升高1℃，树干液流速率提升约1100千克水·(平方米边材面积（sapwood area）)⁻¹·日⁻¹；在生长季早期，该增幅可达约1200千克水·(平方米边材面积)⁻¹·日⁻¹。冬季寒冷日数的减少与生长季早期夜间的树干液流提升呈正相关关系：采暖度日数（heating-degree-days）每减少100，树干液流速率提升约5千克水·(平方米边材面积)⁻¹·日⁻¹。实验增温导致土壤含水量下降，而土壤含水量每提升1%，树干液流速率约增加360千克水·(平方米边材面积)⁻¹·日⁻¹。夜间，土壤含水量与树干液流速率呈正相关关系。上述研究结果表明，冬季及整个生长季的空气与土壤温度升高会提升树干液流速率，进而可能全年影响森林水分预算。