Chlorophyll fluorescence and sap flow in eastern red cedar (Juniperus virginiana) in both the growing and nongrowing season in Kentucky

Red cedar chlorophyll fluorescence remains high during the nongrowing season, while transpiration is reduced.

Evergreen trees retain their leaves during the winter and thus can photosynthesize when conditions permit. Eastern red cedar (Juniperus virginiana) is widely distributed in the eastern USA and is known to transpire and photosynthesize outside of the growing season. However, most recent work has been done in the Great Plains, which red cedar has recently invaded, while little work has been done in its original range. I hypothesized that red cedar would behave like other conifers from summer drought-free areas and show reduced chlorophyll fluorescence and transpiration during the winter. Four red cedar trees at a site near the Ohio River in Kentucky were equipped with Granier sap flow probes, while solar irradiance, temperature, relative humidity, and soil volumetric water content (VWC) were measured at the site. Dark chlorophyll fluorescence (F_v/F_m) was measured on northern and southern aspects on an approximately weekly basis from early 2019 through mid 2021. High values of F_v/F_m were measured in both growing (April-September) and nongrowing (October-March) seasons; median values in the nongrowing season were 94% of those in the growing season, which did not support my hypothesis. F_v/F_m data were fit to a Michaelis–Menten curve that used minimum temperature of the previous two nights, aspect, and maximum irradiance on the day of measurement taken before fluorescence was measured. Sap flow was explained by maximum daily temperature, vapor pressure deficit (VPD), mean daily irradiance, and VWC. Sap flow in the nongrowing season was 74% of that seen in the growing season, due to lower values of temperature, VPD, and irradiance, supporting my hypothesis. Thus, red cedar remains physiologically active during the nongrowing season. However, it does not appear to behave like other conifers from summer drought-free areas.