Abstract:
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.