Corticular photosynthesis: A mechanism for shrub expansion in coastal environments

            Myrica cerifera, the dominant woody species on many barrier islands along the southeastern coast of the United States, continues to expand into grassy swales and forms dense thickets. The success of Myrica may be attributed to a symbiotic nitrogen fixing association with Frankia, an evergreen leaf habit and possibly, corticular photosynthesis. Corticular photosynthesis, or the refixation of respirational CO_2, may increase the water use efficiency (WUE) and carbon use efficiency (CUE) of Myrica. We observed chlorenchyma tissue beneath the periderm of Myrica stems. For our study, the percent of CO₂ refixed in first through fifth order stems was determined seasonally and related to stem order variations in incident PAR and corresponding pigment concentrations. Percent refixation decreased significantly from first to fifth order stems (p= 0.000), and percent refixation was significantly higher during the winter (p= 0.003). One explanation for the differences in percent refixation for branch order as well as for season is a changing light environment. Incident PAR was quantified during the summer and winter for each branch order. As branch order increased, PAR decreased (p= 0.012), but PAR was higher during the winter as compared to the summer (p= 0.000). Results from analyses of total chlorophyll content as well as chlorophyll a:b ratios for first through fifth order branches were consistent with shade adaptation as branch order increased. Increased WUE and CUE due to corticular photosynthesis may provide Myrica with a competitive advantage over other species and may facilitate rapid expansion in coastal environments.