Thursday, August 5, 2010

OPS 6-5: Demographic results emphasize need for habitat manipulation and augmentation in a rare scrub plant

Sarah J. Haller and Eric S. Menges. Archbold Biological Station


Dicerandra christmanii is one of the most critically endangered plant species on the Lake Wales Ridge, an ancient sandy upland in peninsular Florida and a biodiversity hotspot home to several dozen endemic species.  Only five D. christmanii populations exist; all occur in gaps within shrub matrices in xeric oak-hickory scrub.  Over the last 16 years we have been studying D. christmanii demography in its only protected population, located on the Lake Wales Ridge National Wildlife Refuge (LWRNWR) Flamingo Villas tract.  Our goal is to investigate the life history of D. christmanii while also providing research-based conservation guidelines.  We collected demographic data in permanent plots located in 3 study sub-populations with varying disturbance histories: naturally occurring gaps with no recent record of disturbance, an old sand road abandoned over 20 years previously, and a sand road only recently blocked from use.  We marked all individuals and recorded survival and recruitment three times a year.  We also recorded size measurements (number of branch tips) and measured fecundity (number of reproductive branches) annually.


All three of our study sub-populations have declined since their inclusion in our research.  Most drastically, the abandoned road sub-population has decreased by 86%; in 2009, only 17 plants remained in our demography plots at this site.  Annual survival averaged ~ 70% across all sub-populations, but has declined over recent years; annual survival dropped below 50% for the first time in 2009 to 35%.  Recruitment has high annual variation (30-fold) with greater recruitment in areas of more recent disturbance and overall poor recruitment in the last few years.  These patterns are likely related to weather (e.g. precipitation effects on seedlings) and vegetation encroachment due to fire suppression.  Dicerandra christmanii prefers open microhabitats (<60% canopy cover, <80% litter cover, <2 cm litter depth, and <3 m shrub heights), conditions historically maintained by fire.  Mature oak-hickory scrub cannot support D. christmanii and individuals are mostly restricted to edge sites, fire lanes, and old roads at LWRNWR.  Our study sub-populations remain unburned despite an established fire management program.  We propose an experimental project to enhance D. christmanii habitat at the LWRNWR while strongly supporting continued prescribed fire efforts.  We plan to artificially create canopy gaps in combination with population augmentation via seeds and transplants.