Yogani Govender 1, Elvira Cuevas 1,2, Jorge Ortiz 2,3, Leonel Sternberg 4, Jesús Rodríguez 5 & Miguel Canals 6 1 Center 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. 3 Institute of Tropical Ecosystem Studies, University of Puerto Rico - Rio Piedras, San Juan PR 00931; 4 Department of Biology, University of Miami, Coral Gables, FL 33124; 5 US Geological Survey-Caribbean Water Science Center, Guaynabo PR 00965; 6 Department of Natural Resources and Environment, Guánica Dry Forest Reserve, PR

Neelin et al Tropical drying trends in global warming models and observations. Past 25 year PNAS 103: Image Courtesy NASA Visible EarthNASA Visible Earth We worry about hurricanes but drying should also be our main concern. 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.

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

Guánica Dry Forest: UNESCO Biosphere Reserve

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.

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

01/01/ /01/ /01/ /10/ /01/2007 Daily rainfall and evapotranspiration Caña Gorda, Guánica, PR

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.

Methodology We used the natural abundance of stable isotopes of water (δD and δ 18 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).

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 δ 18 O and δD isotopic signature of xylem sap with the 18 O and δ D isotopic signature the water sources.

Rainfall and Evapotranspiration during study period 183 mm Tropical Storm Kyle

Origin of rainfall in the Guánica Dry Forest

E. fruticosa Soils Groundwater Rainfall F. citrifolia Soils Groundwater Rainfall Results δ 18 O in xylem sap and potential water sources

Groundwater and storm event rainfall uptake by plants in the Guánica dry forest based on δ 18 O signature of xylem sap.

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.

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