Methyl Chloride Emission from Tropical Plants

We studied CH₃Cl emissions from tropical plants in Tropical Rainforest Glasshouse (25 m x 20 m x 10-24 m high) in Tsukuba Botanical Gardens, where more than 200 representative species from lowland tropical forests of Southeast Asia grow. CH₃Cl concentrations were always higher in the glasshouse than outside and increased significantly when the windows were closed. The fluxes of CH₃Cl from the tropical rainforest system in the glasshouse were calculated from the averages of their accumulation rates when the windows were closed (average; 142 pptv”h^-1) with the dimension of the glasshouse. Emission rates per unit area for CH₃Cl was 5.4 mg m^-2 h^-1. In order to determine which of the plants or whether the soil is responsible for the increase of CH₃Cl, flux measurements were done by using an enclosure method. The soil was found to take up CH₃Cl at a small rate. On the other hand, some plants from the Marattiaceae, Cyatheaceae (tree fern), Dicksoniaceae, and Dipterocarpaceae families were found to significantly emit CH₃Cl. The first three families are ferns commonly growing in tropical forests, and Dipterocarpaceae species are dominant in the tropical rainforests of Southeast Asia. The average CH₃Cl emission rate from the 9 plants in these families was around 0.5 mg (g dry leaf)^-1”h^-1. As for Cyatheaceae, we conducted a flux measurement from Cyathea lepifera E.Copel. in a subtropical forest in Okinawa and detected high emissions of CH₃Cl amounting to 1.1 mg (g dry leaf)^-1”h^-1. Strong emissions of CH₃Cl from tropical forests raises questions about the trends of chlorine compounds in the future and in the past.