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

Neelin et al. 2006. Tropical drying trends in global warming models and observations. Past 25 year PNAS 103:6110-6115

Image Courtesy NASA 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

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

01/01/2006

01/01/2008

01/01/2009

11/10/2009

01/01/2007

Daily rainfall and evapotranspirationCaña 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 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).

Sampling – every 30 daysWe 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.

Rainfall and Evapotranspiration during study period

183 mm

Tropical Storm Kyle

Origin of rainfall in the Guánica Dry Forest

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E. fruticosa Soils Groundwater Rainfall

F. citrifolia Soils Groundwater Rainfall

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Results δ 18O in xylem sap and potential water sources

• 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.

AcknowledgementsStaff 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