Ecosystem warming increases sap flow rates of northern red oak trees

Abstract

Abstract Over the next century, air temperature increases up to 5°C are projected for the northeastern United States. As evapotranspiration strongly influences water loss from terrestrial ecosystems, the ecophysiological response of trees 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, Massachusetts, United States. Through principal components analysis, we found air and soil temperatures explained the largest amount of variance in environmental variables associated with rates of sap flow, with relative humidity, photosynthetically active radiation and vapor pressure deficit having significant, but smaller, effects. 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 during the early growing season (a decrease in 100 heating‐degree days was associated with a sapflow increase in approximately 5 kg H2O m−2 sapwood area day−1). Soil moisture declined with increased treatment temperatures, and each soil moisture percentage decrease resulted in a decrease in sap flow of approximately 360 kg H2O m−2 sapwood area day−1. At night, soil moisture correlated positively with sap flow. 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.

Keywords

• climate change
• mixed temperate forest
• Quercus rubra
• transpiration
• warming experiment
• water uptake