Passerine dispersal of the nitrogen-fixing root endosymbiont Frankia on a Virginia barrier island

Background/Question/Methods

Poor soil quality is a major barrier to establishment by colonizing species in harsh coastal environments.  Acquisition of limiting nutrients is especially critical to survival of early successional plant species.  Though much of the required cation input may be provided by sea spray, nitrogen availability remains low in these environments.  On the barrier islands of Virginia, colonization of protected swales by an actinorhizal woody shrub, Morella cerifera, occurs following establishment of sand dunes accreted by burial-tolerant grasses.  The considerable success of M. cerifera is due to its infection by the nitrogen-fixing root endosymbiont, Frankia.  At the edges of expanding shrub thickets, local senescing root nodules provide infective propagules for newly establishing seedlings, but the means by which the bacteria arrive in recently-accreted soils prior to shrub establishment have not been determined.  Our hypothesis that Frankia is transported to uncolonized coastal soils by avian defecation was tested on the barrier islands of the Virginia Coast Reserve (VCR) Long-Term Ecological Research site.  In addition, we investigated whether bird species or seasonal migratory periods affected the likelihood of successful infection.  Fecal samples were passively gathered using collectors in the field at the VCR, as well as directly from banded birds at the Coastal Virginia Wildlife Observatory during migratory season banding operations.  We used an accepted bioassay method to test for the presence of ecologically relevant Frankia.
Results/Conclusions

Sterile, lab-grown M. cerifera seedlings inoculated with dilute solutions of bird fecal matter showed significantly greater incidence of nodulation than those not treated with feces.  Both native and non-native passerine birds appear to be viable dispersal vectors of infective and effective Frankia propagules.  By providing the initial source of bacteria, these birds facilitate the transition of grassy swales to woody shrub thicket at the VCR.  Avian input of Frankia is likely to be a significant source of Frankia propagules in other ecosystems as well.  Once established, the dense-canopied thickets of M. cerifera represent the first woody seral stage of succession, and may increase soil quality by providing an autochthonous, organic source of available nitrogen, contributing to further successional transitions.