Effects of sea level rise on a rare Pterocarpus forested wetland in Puerto Rico

Background/Question/Methods

In the next century, rare Pterocarpus officinalis freshwater-forested wetlands are likely to be lost in Puerto Rico, due to climate change-induced sea level rise if successful approaches to protecting them are not developed quickly.  The potential ecosystem services lost will include sources of food, income, and recreational opportunities in the Humacao Natural Reserve (HNR) area, the location of the largest remaining forest (150 ha).  Restoration, hydrological modification, or conservation of adjacent lands could save these ecosystems but the current hydrologic context is not well understood.  As part of a flood control project in 2002, the U.S. Army Corps of Engineers built a diversion channel that allowed a permanent connection the HNR's largest lagoon with the Caribbean Sea.  The result of this modification was a change in the system hydrology, an increase in the lagoon salinity, and the loss of a portion of the Pterocarpus forest.  Under these circumstances, we ask ourselves: what can we learn from the Humacao Natural Reserve recent salt-water intrusion?  Our specific questions are: how will sea level rise affect the Pterocarpus forest of the Humacao Natural Reserve?  What is the role of hydrology and freshwater inputs in determining the future of the forest?  We are using a combination of stable isotopes (δ²H, δ¹⁸O) and tidal/conductivity gauges to study wetland water source and hydrology.  Tidal influence on the forest is being quantified by a series of tidal/conductivity gauges that record water level and salinity in the wetland on an hourly basis across a 1.2 Km tidal creek that runs through part of the forest.  Water samples from the forest and its possible water sources (rainfall, creek runoff, lagoon and ocean) are taken on a monthly basis and groundwater and tree tissue sample are taken on a quarterly basis.    

Results/Conclusions

Preliminary results (6 months) revealed a decreasing salinity gradient towards the end of the tidal creek. Water levels in the forest seemed to be influenced by rainfall events, although analysis of the stable isotope data suggests a clear distinction between runoff, forest and ocean water samples.  With the integration of groundwater stable isotope and lagoon water level datasets, we could be able to understand the importance of freshwater inputs into the forest, thus, identifying critical management zones.  Results from this project will also contribute towards a global water isotope mapping initiative and aid in the preservation of this rare forested wetland.