Barrier islands may represent an underestimated sink for atmospheric carbon because they combine potential for high above-ground primary productivity (ANPP) with young, infertile soils capable of sequestering significant amounts of carbon. Ecosystem ANPP on many barrier islands of the Virginia Coast Reserve (VCR) has been further enhanced by the rapid expansion of woody shrubs. Compared to adjacent grasslands, shrubs in coastal systems combine high leaf area index with high photosynthetic rates. Furthermore, the ability to refix respired C through corticular photosynthesis improves whole plant C use efficiency and water use efficiency providing many shrub species an even higher potential for ANPP. High productivity of shrubs on Hog Island, VA, part of the VCR, has resulted in C sequestration rates as high as 101 g m^-2 yr^-1, almost 25 times higher than C sequestration rates in adjacent grasslands. However, projected increases in relative sea-level, storm frequency and storm intensity may threaten continued C sequestration by woody plants. Woody plants are particularly susceptible to salt spray and saltwater flooding, which causes significantly lower rates of stomatal conductance and photosynthesis. Additionally, higher levels of tissue chlorides cause significant impairment to photosystem II, which can be detected with remote sensing imagery. Future sea-level rise is likely to increase exposure of shrub thickets to salt spray and disturbance and could substantially reduce woody ANPP reducing the role of barrier islands in global C cycles. However, such stresses can be detected and monitored at large scales and provide an early warning of potential losses of ecosystem function.