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Eco-hydrology of critical coastal habitats in the Guánica Dry Forest

Eco-hydrology of critical coastal habitats in the Guánica Dry Forest. Yogani Govender 1 , Elvira Cuevas 1,2 , Jorge Ortiz 2,3 , Leonel Sternberg 4 , Jes ús Rodríguez 5 & Miguel Canals 6

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Eco-hydrology of critical coastal habitats in the Guánica Dry Forest

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  1. Eco-hydrology of critical coastal habitats in the Guánica Dry Forest Yogani Govender1, Elvira Cuevas1,2,Jorge Ortiz2,3, Leonel Sternberg4, Jesús Rodríguez5& Miguel Canals6 1Center for Applied Tropical Ecology and Conservation, University of Puerto Rico, Rio Piedras, Puerto Rico; 2 Department of Biology, University of Puerto Rico, Rio Piedras, Puerto Rico. 3Institute of Tropical Ecosystem Studies, University of Puerto Rico - Rio Piedras, San Juan PR 00931; 4Department of Biology, University of Miami, Coral Gables, FL 33124; 5US Geological Survey-Caribbean Water Science Center, Guaynabo PR 00965; 6Department of Natural Resources and Environment, Guánica Dry Forest Reserve, PR

  2. We worry about hurricanes but drying should also be our main concern. Neelin et al. 2006. Tropical drying trends in global warming models and observations. Past 25 year PNAS 103:6110-6115 Climate models concur that: - The Caribbean is drying out at a rate of 5-30% of mean annual rainfall per hundred years, - Intra- and inter-annual rainfall is becoming more variable as it is affected by ENSO and North Atlantic Oscillation. Image Courtesy NASA Visible Earth

  3. Puerto Rico – an excellent model to study climate change effects Northeasterly trade winds Northern Karst Southern Karst Southeasterly trade winds

  4. Guánica Dry Forest: UNESCO Biosphere Reserve

  5. Coastal Plateau in the Guánica Dry Forest Plants live in islands creating their own microhabitat. Combination of environmental stressors: substrate (limestone outcrops, semi-arid conditions, high temperatures, salt spray and low nutrient availability (e.g. P) results in a Dwarf Forest.

  6. Rainfall can vary from extreme dry years to moist years Annual Average = 756 mm But extreme variability also occurs within same months among years

  7. 01/01/2006 01/01/2009 01/01/2007 01/01/2008 11/10/2009 Daily rainfall and evapotranspiration Caña Gorda, Guánica, PR

  8. Hypothesis We hypothesize that the long-term dynamics of the plant communities in the Guánica Dry Forest are driven by fluctuations (charge-recharge) in the ground water dynamics, whereas intra-annual dynamics are determined by precipitation.

  9. Methodology • We used the natural abundance of stable isotopes of water (δD and δ18 O) to trace its origin and uptake by five native tree species that represent different functional strategies: • Tabebuia heterophylla (facultative deciduous), Pisonia albida (deciduous), Coccoloba microstachya (evergreen leaf exchanger), Ficus citrifolia (evergreen leaf-exchanger) and Erithalis fruticosa (evergreen leaf exchanger).

  10. Sampling – every 30 days • We collected water from all potential water sources (surface substrate, rainfall, and ground water) • We collected 5 cm lengths of small branches from three trees for each species. • All samples were stored in vacuum-sealed containers and stored at 4°C until analysis at the University of Miami. • We compared the δ18O and δD isotopic signature of xylem sap with the 18O and δ D isotopic signature the water sources.

  11. Rainfall and Evapotranspiration during study period 183 mm Tropical Storm Kyle

  12. Origin of rainfall in the Guánica Dry Forest

  13. Results δ18O in xylem sap and potential water sources E. fruticosa Soils Groundwater Rainfall F. citrifolia Soils Groundwater Rainfall

  14. Groundwater and storm event rainfall uptake by plants in the Guánica dry forest based on δ18O signature of xylem sap.

  15. CONCLUSIONS • Rainfall in the Guánica Forest originates in the Caribbean (southeasterly trade winds) and reflects the warmer temperature and increased evaporation of the sea surface. • Intensity and duration of rainfall events determine groundwater recharge and surface water access to plants. • Rooting depth of the tree species will determine water source throughout the year. Deep-rooted plants access groundwater, shallow rooted plants rely on surface substrate, thus partitioning water resources rather than competing for water. • Under a climate change scenario, with extended drought periods, plant biodiversity could be reduced if only plant species that can access groundwater throughout the year survive.

  16. Acknowledgements • Staff of the Guánica Dry Forest, Department of Natural resources and Environment, Government of PR • Undergraduate and graduate students who have participated in our study • Larry Diaz, Laboratory Coordinator

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