Impacts Of Needle Age, Tree Architecture, And Drought Stress On Photosynthetic Physiology Of Fraser Fir (Abies fraseri) Christmas Trees

The purpose of this study was to determine the impacts of drought stress and annual trimming on the growth, photosynthetic physiology, and crown architecture of Fraser fir (Abies fraseri) Christmas trees on a choose-and-cut farm in the Southern Appalachian Mountains. The extremely wet winter of 2020-2021, combined with potential underground water movement, thwarted the rain exclusion set-up, even with 74% channel coverage since July 2020. No notable differences in growth, soil moisture, water potentials, or photosynthetic physiology were found between droughted and control trees. Direct light intensity on a sunny day for current year needles was measured at ~2000 µmol m-2 s-1 and decreased by 97% and 99% at typical interior canopy locations for 1 and 2 yr old needles, respectively. Needle density, chlorophyll a:b ratios, and photosynthetic capacity decreased with increasing needle age, while total chlorophyll content increased. Trimming increases canopy density, causing older needles to have low or even negative carbon budgets due to the low light in the interior portion of the tree canopy. Why these trees maintain the oldest, heavily shaded needles is unknown and suggests the need for further research.