Winter-dormant-season sap flow dynamics and the environmental conditions that drive them

Winter-dormant-season (WDS) sap flow is a well-known phenomenon in maples, birches, and walnuts, the three genera of trees important economically for syrup production. During the winter dormant period, sap flow still occurs despite trees being leafless, which decreases transpiration. During the WDS of 2018 and 2019, sap flow was monitored in several species of deciduous, woody angiosperms in New England. The species included in this study were London plane (Platanus × acerifolia), white ash (Fraxinus americana), shagbark hickory (Carya ovata), American basswood (Tilia americana), quaking aspen (Populus tremuloides), American beech (Fagus grandifolia), and American hophornbeam (Ostrya virginiana). WDS sap flow was observed in all seven study species. Changes in ambient temperatures - particularly freeze-thaw cycles - and, to a lesser extent, precipitation events and solar radiation, seemed to bring about WDS sap flow. These results suggest that the anatomical and physiological mechanisms that cause WDS sap flow in the xylem are similar to that of maples and walnuts, which also have WDS sap flow that is driven by freeze-thaw cycles. More research is needed to elucidate the anatomical and physiological mechanisms of WDS sap flow in these novel (i.e., other than maples, birches, and walnuts) species.