We monitored 11 populations of eight species of Eleutherodactylus in Puerto Rico from 1989 through 2001. We determined relative abundance of active frogs along transects established in the Caribbean National Forest (El Yunque), Carite Forest, San Lorenzo, and in the vicinity of San Juan. Three species (Eleutherodactylus karlschmidti, E. jasperi, and E. eneidae) are presumed to be extinct and eight populations of six different species of endemic Eleutherodactylus are significantly declining at elevations above 400 m. Of the many suspected causes of amphibian declines around the world, we focused on climate change and disease. Temperature and precipitation data from 1970–2000 were analyzed to determine the general pattern of oscillations and deviations that could be correlated with amphibian declines. We examined a total of 106 tissues taken from museum specimens collected from 1961–1978 and from live frogs in 2000. We found chytrid fungi in two species collected at El Yunque as early as 1976, this is the first report of chytrid fungus in the Caribbean. Analysis of weather data indicates a significant warming trend and an association between years with extended periods of drought and the decline of amphibians in Puerto Rico. The 1970’s and 1990’s, which represent the periods of amphibian extirpations and declines, were significantly drier than average. We suggest a possible synergistic interaction between drought and the pathological effect of the chytrid fungus on amphibian populations.

We excluded throughfall from humid tropical forests in Puerto Rico for a period of three months to determine how drought affects greenhouse gas emissions from tropical forest soils. We established five 1.24 m2 throughfall exclusion and five control plots of equal size in three sites located on ridges, slopes, and an upland valley dominated by palms (total of 30 plots). We measured weekly changes in carbon dioxide (CO2) and bi-weekly changes in nitrous oxide (N2O) and methane (CH4) in response to manipulation. We additionally measured the effects of throughfall exclusion on soil temperature and moisture, nutrient availability, and pH. Rainout shelters significantly reduced throughfall by 22 to 32 % and decreased soil moisture by 16 to 36% (top 10 cm). Rates of CO2 emissions decreased significantly in the ridge and slope sites (30%, 28%, respectively), but not the palm during the experimental drought. In contrast, the palm site became a significantly stronger sink for CH4 in response to drying (480% decline relative to controls), while CH4 fluxes in the ridge and slope sites did not respond to drought. Both the palm and ridge site became a sink for N2O in response to drought and the slope site followed a similar trend. Soil pH and available P decreased significantly in response to soil drying; however, available N was not affected. Variability in the response of greenhouse gas emissions to drought among the three sites highlights the complexity of biogeochemical cycling in tropical forested ecosystems, as well as the need for research that incorporates the high degree of spatial heterogeneity in experimental designs. Our results show that humid tropical forests are sensitive to climate change and that short-term declines in rainfall could result in a negative feedback to climate change via lowered greenhouse gas emissions and increased greenhouse gas consumption by soils.

Cloud water can be a significant hydrologic input to mountain forests. Because it is a precipitation source that is vulnerable to climate change, it is important to quantify amounts of cloud water input at watershed and regional scales. During this study, cloud water and rain samples were collected monthly for 2 years at sites on windward and leeward East Maui. The difference in isotopic composition between volume-weighted average cloud water and rain samples was 1.4% d18O and 12% d2H for the windward site and 2.8% d18O and 25% d2H for the leeward site, with the cloud water samples enriched in 18O and 2H relative to the rain samples. A summary of previous literature shows that fog and/or cloud water is enriched in 18O and 2H compared to rain at many locations around the world; this study documents cloud water and rain isotopic composition resulting from weather patterns common to montane environments in the trade wind latitudes. An end-member isotopic composition for cloud water was identified for each site and was used in an isotopic mixing model to estimate the proportion of precipitation input from orographic clouds. Orographic cloud water input was 37% of the total precipitation at the windward site and 46% at the leeward site. This represents an estimate of water input to the forest that could be altered by changes in cloud base altitude resulting from global climate change or deforestation.

Hurricane activity is predicted to increase over the mid-Atlantic as global temperatures rise. Nitrous oxide (N2O), a greenhouse gas with a substantial source from tropical soils, may increase after hurricanes yet this effect has been insufficiently documented. On September 21, 1998, Hurricane Georges crossed Puerto Rico causing extensive defoliation. We used a before–after design to assess the effect of Georges on N2O emissions, and factors likely influencing N2O fluxes including soil inorganic nitrogen pools and soil water content in a humid tropical forest at El Verde, Puerto Rico. Emissions of N2O up to 7 months post-Georges ranged from 5.92 to 4.26 ng cm
2 h
1 and averaged five times greater than fluxes previously measured at the site. N2O emissions 27 months after the hurricane remained over two times greater than previously measured fluxes. Soil ammonium pools decreased after Georges and remained low. The first year after the hurricane, nitrate pools increased, but not significantly when compared against a single measurement made before the hurricane. Soil moisture and temperature did not differ significantly in the two sampling periods. These results suggest that hurricanes increase N2O fluxes in these forests by altering soil N transformations and the relative availabilities of inorganic nitrogen.