Climate models project increased growing season air temperatures, leading to warmer soil temperatures, and decreased depth and duration of winter snowpack for the northeastern U.S., leading to greater frequency of soil freeze/thaw cycles (FTCs) in winter over the next century. Past work shows that warmer soil temperatures increase transpiration and carbon uptake by northern hardwood trees, but soil FTCs in winter damage roots, which impair water uptake the following growing season. To our knowledge, the combined effects of changes across both the growing season and winter on water and carbon dynamics have been yet to be determined.
We created the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire in 2012 to determine the effects of projected changes in climate on the functioning of northern hardwood forests. Two plots serve as reference, two are warmed 5°C during the growing season, and two are warmed in the growing season and have soils FTCs induced in winter. We measured rates of water uptake via sap flow and carbon uptake via photosynthesis in 2015 and 2017. We measured stable isotopes of soil water and xylem sap water three times throughout the 2018 growing season to partition potential sources of water for red maples.
Results/Conclusions
Growing season soil warming led to increased water uptake in both years and increased photosynthesis in 2015 compared to reference trees. Soil FTCs did not offset increases in sap flow in both years, but did for photosynthesis in 2015 compared to reference trees. Stable isotopes of soil water (deuterium excess; d) declined with soil depth, but this vertical variation was less pronounced later in the growing season. Using a stem water mixing model we determined that trees exposed to warmer soils, even those exposed to soil FTCs in winter, accessed deeper soil water compared to reference trees. Our data suggest that the combined effects of warmer growing season temperatures and greater frequency of soil FTCs projected for the northeastern U.S. will increase rates of water and carbon uptake by northern hardwood trees, and will increase the depth from which trees take up water. These results should be considered when projecting carbon retention in these forests and how water relations may affect this sink, as warmer temperatures may strengthen the sink, but also put trees at risk of desiccation if deeper water becomes less available in a changing climate.