Examining the Density Gradient Along the Rhizophora mangle Propagule

Abstract

Mangroves grow in tropical regions around the globe, but these dense forests have been declining due to agriculture, industry, and climate change. Mangrove propagules are not seeds but the seedlings themselves that grow on the parent tree until they reach maturity and fall. They are water dispersed, floating horizontally in seawater and vertically in brackish water. The purpose of this study was to further investigate whether there is a gradient change in density longitudinally along the Rhizophora mangle propagule that results in this floatation behavior.  This study also explored whether there was a difference in density between the medulla and the cortex within the propagule.   Propagules were cut into one-centimeter sections and placed in a pycnometer to calculate their density. The average densities of these categories were graphed and had a correlation coefficient of 0.6036 showing a strong positive correlation. The medulla, or inner ring, was removed from the cortex and placed in water to determine if there was a difference in buoyancies. In the plumule end of the propagule, both the medulla and cortex floated. Towards the radicle end of the propagule, the cortex would sink while the medulla would continue to float. The research raises the question if there is a difference between the cortex in the radicle end and the cortex in the plumule end but supports the idea that there is a change in density along the propagule longitudinally that plays a role in the buoyancy of the mangrove propagules.