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.未来百年间，美国东北部地区的气温预计将升高至多5℃。由于蒸散发（evapotranspiration）是陆地生态系统水分损失的主导途径，树木对增温的生态生理响应将对森林水量平衡产生重要影响。本研究在美国马萨诸塞州哈佛森林（Harvard Forest）开展了大气增温（较环境温度最高提升5.5℃）与土壤增温（6厘米深度处较环境温度最高升高1.85℃）的耦合控制试验，测定了成熟北美红栎（Quercus rubra L.）在生长季的树干液流速率。主成分分析（Principal Components Analysis）结果显示，大气与土壤温度对树干液流的影响最为显著。整体而言，整个生长季内气温每升高1℃，树干液流速率平均提升约1100千克水·平方米边材面积⁻¹·天⁻¹；在生长早期，该提升幅度可达1200千克水·平方米边材面积⁻¹·天⁻¹。冬季寒冷日数的减少与生长早期夜间树干液流的提升呈正相关关系（采暖度日数每减少100，树干液流速率提升约5千克水·平方米边材面积⁻¹·天⁻¹）。试验增温下土壤湿度呈下降趋势，而土壤湿度每提升1个百分点，树干液流速率约增加360千克水·平方米边材面积⁻¹·天⁻¹。夜间时段，土壤湿度与树干液流速率呈正相关关系。本研究结果表明，冬季及整个生长季内更高的大气与土壤温度会提升树干液流速率，进而可能对全年的森林水量平衡产生影响。