Self-shading and physiological integration in Phragmites australis: Factors leading to division of labor

Phragmites australis (the common reed) is increasingly invading new sites and expanding into low salt marshes in the Northeastern United States. Previous research has focused on the role of clonal integration in supporting expansion into harsher environments. Established Phragmites stands grow very dense and experience light limitation. Therefore, we tested the hypothesis that expanding (daughter) ramets experience a release from light limitations, specialize in light gathering structures, and are engaged in basipetal (backwards) translocation of carbon to the benefit of the established (mother) ramets. Three studies were conducted: (I) Root/Shoot ratio was measured for inner and outer plots in Phragmites stands in the Meadowlands of New Jersey. Ramets on the edge of the main stand were found to invest proportionally more dry weight in above ground, light gathering structures. (II) A shading-severing study in outdoor tubs tested the impact of shading mother ramets when severed from or connected to unshaded daughter ramets. Connected ramets demonstrated division of labor with unshaded daughters allocating proportionally more dry weight to above ground structures than below. (III) Finally, the middle of three connected ramets was labeled with 13C to see if shading the most basipetal shoot induced increased translocation of photosynthetic assimilates. We found slight isotope enrichment in shaded ramets. These results suggest that light limitations can shift carbon sinks within Phragmites and that physiological integration induces these sinks to reflect division of labor. Expanding ramets may experience release from light competition and serve as profitable colonies for the main stand.