Winters are Getting Warmer: Understanding the Lasting Effects of Record-breaking December Temperatures on Water Use of Mature Loblolly Pine Trees in East Texas

The potential for winter heat waves is projected to increase due to climate change in temperate evergreen forests in southeastern USA. In 2021, Texas experienced an unseasonably warm late fall and well above normal winter, reaching record maximums of 25 C° in December. This was followed by an unseasonably hot summer with both record-breaking daytime and nighttime temperatures in June and July. The goal of this study was to compare transpiration on warm winter days to similar days in the growing season. The study was conducted in a mature naturally-regenerated mixed pine forest in Montgomery, Texas. To estimate sap flux density (Js, kg m2 s-1) and transpiration, five loblolly pine trees (Pinus taeda L.) were instrumented with thermal dissipation sensors (n=10) from October 2021 to December 2022. Climate data was also collected from a nearby weather station. In December 2021, the top record-breaking temperatures averaged to 20 C° compared to the normal days which averaged to 12 C°. Sap flux density did not decline significantly into the winter season, reaching values as high as 1533 kg m2 day-1 in December. In the subsequent summer, on average the daytime (37 C°) and nighttime temperatures (24 C°), and below-normal rainfall triggered a precipitous decline in water use due to stomatal closure. Stand transpiration peaked in June and then declined sharply in July 2022 due to the consecutive heat waves. Transpiration rates were 10% lower in July 2022 (58 mm month-1, 38 C°) compared to rates in December 2021 (64 mm month-1, 25 C°). The observed water stress during the summer was exacerbated by the prior warm winter temperatures, resulting in a lasting effect on tree health. Future research should focus on assessing the impact of excessive transpiration during the cold season on loblolly pine forest growth and water balance.