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Methyl chloride (CH3Cl), the most abundant halocarbon in the atmosphere, has received much attention as a natural source of chlorine atoms in the stratosphere. The annual global flux of CH3Cl has been estimated to be around 3.5 Tg on the grounds that this must balance the loss through reaction with OH radicals (which gives a lifetime for atmospheric CH3Cl of 1.5 yr). The most likely main source of methyl chloride has been thought to be oceanic emission, with biomass burning the second largest source. But recent seawater measurements indicate that oceanic fluxes cannot account for more than 12% of the estimated global flux of CH3Cl, raising the question of where the remainder comes from. Here we report evidence of significant CH3Cl emission from warm coastal land, particularly from tropical islands. This conclusion is based on a global monitoring study and spot measurements, which show enhancement of atmospheric CH3Cl in the tropics, a close correlation between CH3Cl concentrations and those of biogenic compounds emitted by terrestrial plants, and OH-linked seasonality of CH3Cl concentrations in middle and high latitudes. A strong, equatorially located source of this nature would explain why the distribution of CH3Cl is uniform between the Northern and Southern hemispheres, despite their differences in ocean and land area.

We investigated seasonal changes in the photochemical reflectance index (PRI) and its relation to the diurnal profile of photosynthetic light use efficiency (LUE) in mature Japanese larch (Larix kaempferi Sarg.) forest throughout the growing season from June to October 2003. The daily mean value of needle PRI showed seasonal variation, strongly correlated with the chlorophyll concentration and carotenoid/chlorophyll ratio of the needles. During the green period from early June to late September, the hourly values of both PRI and LUE showed significant midday depression, and were positively correlated. In late October, however, because the PRI of yellowing needles tended to increase slightly at midday in contrast to the LUE, this correlation became negative. Even before autumn senescence, the sensitivity of PRI to LUE changed with the season. Correlation analysis indicated that the slope and intercept of the regression line of the PRI-LUE relationship increased during summer, with peaks in July and August, respectively. The seasonal change in slope was strongly correlated with the foliar photosynthetic pigment concentration, nitrogen concentration, air temperature and the daily mean value of the normalized difference vegetation index (NDVI). The value of the intercept was positively correlated with the daily mean PRI. These results suggest that although diurnal change in LUE cannot be estimated quantitatively from PRI on its own throughout the growing season, the combined use of PRI and other variables such as foliar pigments or NDVI could improve the remote evaluation of seasonal changes in LUE of deciduous tree leaves.

To evaluate the effects on CO2 exchange of clearcutting a mixed forest and replacing it with a plantation, 4.5 years of continuous eddy covariance measurements of CO2 fluxes and soil respiration measurements were conducted in a conifer‐broadleaf mixed forest in Hokkaido, Japan. The mixed forest was a weak carbon sink (net ecosystem exchange, −44 g C m−2 yr−1), and it became a large carbon source (569 g C m−2 yr−1) after clearcutting. However, the large emission in the harvest year rapidly decreased in the following 2 years (495 and 153 g C m−2 yr−1, respectively) as the gross primary production (GPP) increased, while the total ecosystem respiration (RE) remained relatively stable. The rapid increase in GPP was attributed to an increase in biomass and photosynthetic activity of Sasa dwarf bamboo, an understory species. Soil respiration increased in the 3 years following clearcutting, in the first year mainly owing to the change in the gap ratio of the forest, and in the following years because of increased root respiration by the bamboo. The ratio of soil respiration to RE increased from 44% in the forest to nearly 100% after clearcutting, and aboveground parts of the vegetation contributed little to the RE although the respiration chamber measurements showed heterogeneous soil condition after clearcutting.