Ecophysiological investigations of understory eastern redcedar in central Missouri.

Juniperus virginiana is a frequent component of the understory in mature oak-hickory forests in central Missouri, where light levels are typically <10% of full sunlight during much of the growing season. This is below eastern redcedar's reported optimum for photosynthesis. The competitive survival of understory eastern redcedar under such environmental conditions was apparently due to it being an evergreen conifer in a deciduous forest. Hence, its foliage was able to maintain a positive CO₂ balance throughout much of the year, with maximum net photosynthetic rates occurring during periods when the overstory was leafless. The greatest daily average net photosynthetic rates occurred during overstory leaf emergence when temperatures were moderate and light levels to the understory trees were annually the highest. Furthermore, since leaf temperatures and tree water deficits were relatively low at this time, daily gas exchange rates were not greatly limited by midday stomatal closure. During late autumn and winter, low leaf and soil temperatures combined to limit gas exchange severely. The major controlling factors seemed to be cold air temperatures and cold soil temperatures indirectly producing tree water deficits due to reduced water uptake at the soil-root interface.-from Authors