Luquillo LTER 4

The Luquillo Long-Term Ecological Research (LTER) Program takes place in northeast Puerto Rico, including the Luquillo Mountains and surrounding urban and developed areas.  It began in 1988 and focuses on how changing natural and human disturbances, and changing climate, are altering tropical forest and aquatic ecosystems, as well as on the consequences of these changes for society in both the tropics and elsewhere.  The Luquillo LTER’s guiding research principle is that only long-term research will reveal the direction and causes of environmental trends that affect societal well-being.  In the funding period, 2006-2014 (LTER 4), Luquillo LTER has increased our understanding of environmental change in the tropics and engaged with society by recruiting students to conduct this research and by communicating its findings through a variety of public forums.

Changing disturbance regime

Natural and human disturbances shape the biological, biogeochemical, and climatic characteristics of tropical ecosystems.  This disturbance regime is changing and thus tropical ecosystems are changing and affecting peoples in and outside the tropics.

Hurricanes are the major natural disturbance in the Luquillo Mountains and in other tropical areas.  Hurricanes affect forest structure, species composition, and nutrient cycling.  In LUQ 4 our long-term observations and a hurricane simulation experiment, the Canopy Trimming Experiment, showed that soil nutrient cycling and stream chemistry return to pre-hurricane conditions relatively quickly, but forest structure, stream-water content of particulate matter, and abundances of some plants and animals recover more slowly.  The emission of carbon and loss of the important nutrient phosphorous from soils after hurricanes is high. Thus the predicted increase in severe hurricanes may change the abundances and diversity of plants and animals, increase the emission of greenhouse gases, and change soil fertility and the quality of stream water used by humans in Puerto Rico and other parts of the tropics.  Repeating the simulated effects of hurricane damage every ten years in the Canopy Trimming Experiment will address this idea.

Land use change is the major human disturbance in the Luquillo Mountains region.   Luquillo LTER has shown that changes in land use, especially urbanization, around stream headwaters significantly affect stream water quality and stream organisms such as shrimp and migratory fish, both of which affect economics and quality of life in Puerto Rico.  These results apply broadly, since both the tropics and other parts of the world are becoming increasingly urbanized.  Logging and other forest uses change trees in forests to fast growing, soft-wooded species, potentially degrading diversity and economic value of forests.

Changing climate

Climate partly determines the spatial distribution of species, quantity and quality of water, and emission of greenhouse gasses.  Data and models suggest that temperature is increasing, rainfall is decreasing, and droughts will be more frequent in the Luquillo Mountains.

Luquillo LTER showed how the distributions of some plant and animal species depend on the climatic conditions along the elevation gradient.  Rising cloud height may dry out the upper elevations, possibly eliminating some species, and reducing water supply collected for human use.  Luquillo LTER has revealed that a possible cause is that in heavily urbanized northeast Puerto Rico the extent of paved areas and reduced vegetative cover has produced an “urban heat island” of higher temperature that can affect nearby natural areas.  Urban heat islands are growing in the tropics and elsewhere.

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