The Growth Habits of the Ornamental Gunnera in a Costa Rican Natural Habitat

The genus Gunnera is the only taxa having a symbiosis with a cyanobacteria, Nostoc, capable of fixing atmospheric nitrogen.  Gunnera is known worldwide as both a striking horticultural plant and a disturbance colonizer.  In Costa Rica it frequents mid-elevation landslides and road cuts.  We examined the Gunnera-Nostoc symbiosis anatomically in a controlled nursery setting and in situ populations at the Cloudbridge Nature Reserve in San Gerardo de Rivas, Costa Rica.  Organs of adult and juvenile Gunnera were dissected macroscopically; Nostoc was found solely as distinct colonies near the perimeter of stem/rhizome tissue.  To indirectly determine whether Nostoc supplies all nitrogen needs of Gunnera, field transplanted juveniles were grown in a nursery setting receiving three treatment strengths of urea nitrogen for eight weeks.  The results of a destructive harvest will be presented, indicating relationships between organ biomass under differing nitrogen treatments.  To examine the dynamics of seedling recruitment post-landslide, numerous transect samplings were conducted focusing on plant dimensions as a representation of relative age.  Our data will reflect whether seedlings are recruited continuously or as cohort pulses.  In a related study, adults from two distinct populations growing on landslides were compared demographically.  The data from this comparison will illuminate the similarity of Gunnera populations occupying landslides and serve as validation for the accuracy of prior studies.