Luquillo LTER studying recent environmental changes

Network News Spring 2009, Vol. 22 No. 1
Site News

Tropical environments are changing fast due to deforestation and regrowth, urbanization, climate change, and other forces. The consequences are immense for the whole array of ecosystem services that people require. The Luquillo Long-Term Ecological Research Program (LUQ) is tackling these issues in Puerto Rico. LUQ began in 1988 and focused on natural disturbances (hurricanes, landslides, droughts, floods) and ecosystem responses to them. That work revealed patterns of resistance and resilience to cycles of natural disturbance. But how will the tropics respond to directional changes in land use and climate

The LUQ study region is well-suited to answering this question. First, urbanization has been rapid, and there is a strong gradient of land use from El Yunque National Forest to the city of San Juan with 1.3 million people. Along this gradient, for example, LUQ is studying how urbanization affects stream chemistry and organisms. Second, there is also a strong gradient in climate, from the coast to the peaks of the Luquillo Mountains at 1075 meters. Along this gradient, for example, LUQ is studying how trends in climate apparently affect the distribution of tree species. Understanding these stream and forest changes in space helps us predict changes in time.

LUQ takes four approaches to understanding environmental change: long-term observations to describe change in time, gradient analyses to describe change in space, experiments to understand mechanisms of change, and modeling to conceptualize and extend our results. Some examples follow.

Our long-term observations have shown how the Luquillo Mountains area has undergone deforestation, reforestation, and urbanization. By 2002, 19 per cent of the mountain area was urban. Over the past few decades, rainfall in the mountains has decreased between 1 and 2 mm a year, whereas the amount of water extracted by humans from Luquillo streams has increased by 190 mm/yr. Air temperatures have increased in nearby urban areas and may be changing in the Luquillo Mountains. The supply of water for humans and healthy streams is threatened.

Our gradient analyses have shown that tree communities change significantly at about 500, 700, and 900 m elevation, probably due to such factors as mean nighttime cloud level. At higher elevation there are restricted, endemic tree and other species. With drying or warming, these boundaries may shift upwards, and endemics may be literally driven off the top of the mountain.

A core LUQ project is the Canopy Trimming Experiment (CTE). The hurricanes that strike Puerto Rico have two big impacts that affect forest response-canopy damage, and resulting debris deposition on the forest floor. How do we distinguish the light and temperature impacts due to canopy damage versus the soil and nutrient impacts due to debris? By trimming the canopy of forest plots and creating different combinations of canopy removal, debris addition, and controls, the CTE separates the effects of these factors on plant, animal, microbe, and biogeochemical responses. We are repeating the trimming to simulate the effects of increased hurricane frequency. Treatment results are preliminary, but one result so far is that a slight, seasonal temperature increase elevates carbon dioxide (CO2) emission from the soil.

This result connects with the final example of LUQ's research approach-modeling. The CTE will test predictions of the Century Soil Organic Matter Model (CENTURY) of soil organic matter accumulation and nutrient dynamics, as parameterized for the study site under different hurricane disturbance regimes. The model indicates lower levels of aboveground carbon and higher levels of soil carbon, nitrogen and phosphorous mineralization, and organic soil phosphorous under a regime of frequent hurricanes. CTE results will test these predictions and have implications for ecosystems subject to a changing regime of cyclonic storms.

These examples show how LUQ science relates to environmental issues in Puerto Rico, similar tropical areas, and the globe. While its research addresses these issues, LUQ's education program produces scientists (many minority) to tackle them. The LUQ education program includes high school students who gather climate and vegetation data, undergraduates doing original research with LUQ mentors, and graduate students with LUQ advisors, including PhD students working in a new IGERT program focusing on natural-human ecosystems in the urbanizing tropics. LUQ also has designed a web-based middle school curriculum for teaching ecology. With both its research and training LUQ is addressing the challenge of changing environments in the tropics.