Although the eastern US is dominated by broadleaf deciduous forests, evergreen conifers are found throughout. One of the most widely spread of these trees is eastern red cedar (Juniperus virginiana L.). Although it is generally considered an early-successional and shade-intolerant species, it can persist in the forest subcanopy if was previously established, and it has been found to photosynthesize outside of the growing season. The latter ability may explain ability to persist in regions that favor deciduous trees and when overtopped by deciduous trees. Thus, the object of this study was to use sap flow and chlorophyll fluorescence measures to determine photosynthetic activity across the year. A study site was established at the Northern Kentucky University Research and Education Field Study (NKU REFS) in 2019. Four trees were equipped with Granier-type sap flow probes, and soil volumetric water content (VWC), air temperature, and relative humidity were also monitored. On an approximately weekly basis, dark chlorophyll fluorescence (Fv/Fm) was measured on four shoots of each tree, two on a southern and two on a northern exposure.
Results/Conclusions
Fluorescence responded significantly to minimum air temperature of the previous night when fit to a modified Michaelis-Menten curve; Fv/Fm fell to zero near -11°C, reaching half of the maximum value near -9°C. There was some indication, however, that temperatures below -10°C can depress fluorescence for several days. Shoots with northern exposures had significantly higher values of Fv/Fm. Sap flow rates increased with vapor pressure deficit (VPD), while soil VWC did not; the latter was expected, as the field site is located in a wetland, and this species is drought-tolerant. There was a significant though weak positive correlation between Fv/Fm and sap flow rates. Minimum daily air temperature fell below -9°C, when Fv/Fm was predicted to be half of its maximum, on only 3 occasions from April 2019 to February 2020, and so Fv/Fm was usually near its maximum value, indicating readiness for photosynthesis. Mean sap flow rates and VPDs outside the growing season were, respectively roughly 40-70% and 40% of growing-season rates, while VPD was roughly 40%. Assuming actual photosynthesis is proportional to sap flow rates, outside growing-season photosynthesis may account for up to 40-50% of the annual total.