Sep 19, 2012
Assessing Hurricane Wilma's hidden damage
In October 2005 Hurricane Wilma raged across Mexico's Yucatan Peninsula before heading north to Florida. Now a researcher from the US has looked at Wilma's effect on "invisible" factors in a Mexican tropical forest, such as changes in soil carbon flux.
"Limited information is available on how extreme climatic events, such as hurricanes, will influence ecosystem processes," Rodrigo Vargas, formerly at the Center for Scientific Research and Higher Education at Ensenada (CICESE), Mexico, and now at the University of Delaware, US, told environmentalresearchweb. "We took advantage of a hurricane disturbance that made landfall on an instrumented study site during 2005. The 2005 hurricane season was the most intense on record for the Atlantic Basin, with hurricanes Katrina and Wilma among others."
Vargas studied the El Eden Ecological Reserve in the northeast of the Yucatan Peninsula, using soil carbon-dioxide efflux measurements from two soil-sensor nodes, and estimates of gross primary productivity from satellite data.
"There are sharp ecological, climatic and socioeconomic differences among the three countries conforming North America, but there is limited information on carbon-cycle science in Mexico," he said. "Thus, it is fundamental to incorporate and study the unique contributions of Mexico toward carbon dynamics across the region."
Wilma caused plant defoliation and biomass loss, reducing leaf-area index, killing roots and adding to plant litter on the forest floor. After the hurricane had passed, soil carbon-dioxide efflux – due to microbial and plant respiration – increased while gross primary productivity decreased. Annual soil carbon-dioxide emissions for 2006 were more than 3.9 kg of carbon per m2 but decreased to 1.7 kg for the second post-hurricane year. The increase in soil-carbon flux post-Wilma is likely to be due to tree damage depositing nitrogen-rich easily decomposable organic matter.
The forest's gross primary productivity, meanwhile, reduced by nearly 0.5 kg of carbon per m2 per year from its long-term mean of roughly 3 kg. Immediately after the hurricane, maximum values for gross primary productivity were similar to earlier measurements and trees at the study sites produced new fine roots using stored carbon up to 11 years old. But by April 2006 there was a sharp reduction in productivity, coinciding with a decrease in soil moisture.
"One possible explanation is that these plants rapidly invest in the production of new structures but with a cost of non-structural carbon reserves that could make them vulnerable to stressful conditions (i.e. drought events) in the following year," wrote Vargas in his paper in Environmental Research Letters (ERL). Drops in gross primary productivity were not seen for trees in all regions of the study area, however.
"Our observations open new research questions on how plants could survive extreme events: how and when plants allocate non-structural carbon reserves following stressful conditions, and does investment of non-structural carbon reduce resiliency to future stressful conditions?" said Vargas.
The researcher believes the findings not only show that carbon-dioxide dynamics are highly variable following hurricanes but also demonstrate the strong resilience of tropical forests. Indeed, such forests may be able to recover pre-hurricane carbon dynamics within two years.
By assuming a mean carbon value of US$43 per tonne, Vargas calculated that Wilma caused a potential cost of US$ 1160 per hectare of forest by increasing soil emissions to the atmosphere and reducing forest gross primary productivity. "These costs associated with a hurricane disturbance are generally not included when evaluating economic assessments of extreme weather events and should be taken into account for policy and ecological discussions on the influence of disturbances on terrestrial ecosystems," he said.
Now Vargas is working with other researchers to consolidate MexFlux, a Mexican carbon-cycle science network. "We are also developing high-spatial-resolution gross-primary-productivity estimates of Mexico for the last decade and we will be studying the effect of droughts, hurricanes and fires at the regional scale," he said. "This information will be valuable for better understanding of carbon dynamics across North America."
Vargas reported his findings in Environmental Research Letters (ERL).
About the author
Liz Kalaugher is editor of environmentalresearchweb.