Environ. Sci. Technol. 2000, 34, 373-376
Evaluating Possible Causes for the Decline of Japanese Fir (Abies firma) Forests Based on δ13C Records of Annual Growth Rings M A S A H I R O S A K A T A * ,† A N D KIYOSHI SUZUKI‡ Komae Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1 Iwato-kita, Komae-shi, Tokyo 201-8511, Japan, and Kanagawa Prefecture Forest Research Institute, 657 Nanasawa, Atsugi, Kanagawa 243-0121, Japan
Marked damage to Japanese fir (Abies firma) forests have been observed on Mt. Oyama, Tanzawa Mountains, Japan, which is located close to densely populated and heavily industrial areas, since the 1960s. We evaluated possible causes for the decline based on the δ13C records of annual growth rings sampled from trees without visual decline symptoms. For comparison, the δ13C records for Japanese cedar (Cryptomeria japonica) on Mt. Oyama for which decline has not been observed were also investigated. The results showed that the δ13C of rings from both tree species increased abruptly during the 1960s and then decreased after reaching a maximum value in the early 1970s, while the tree growth inversely changed with δ13C. The historical changes of δ13C of rings and tree growth were correlated with atmospheric SO2 concentrations with a delay of 5-10 years in their maximum responses. Exposure to SO2 at the levels estimated during the 1960s on Mt. Oyama can induce the increase of δ13C of rings and the reduction of tree growth with stomatal closure being the probable cause. Previous studies have shown that Japanese fir has a lower tolerance to SO2 as compared to Japanese cedar. These results suggest that the fir forests on Mt. Oyama were injured mainly by SO2 pollution during the 1960s causing the decline.
Introduction Forest decline that seems to be associated with individual and interacting environmental stresses due to air pollution and climate change has been increasingly apparent in Japan (1). However, there is little direct evidence for such stresses in mature forest trees. Thus, suitable assessment methods for stresses are required for evaluating possible causes for forest decline. Farquhar et al. (2) developed a carbon isotope fractionation model to explain the δ13C of C3 plants. In this model, the δ13C of plants is related to the concentration ratio of CO2 in the atmosphere and the leaf intercellular spaces. It has been reported that the δ13C of plants changes with various environmental stresses (3, 4). These stresses affect the CO2 * Corresponding author: phone: +81-3-3480-2111; fax: +81-33480-1942; e-mail:
[email protected]. † Central Research Institute of Electric Power Industry. ‡ Kanagawa Prefecture Forest Research Institute. 10.1021/es990301c CCC: $19.00 Published on Web 12/28/1999
2000 American Chemical Society
concentrations in leaf intercellular spaces that induce changes in δ13C through effects on the carboxylation rate and/or the supply of CO2 through the stomata. An increase in δ13C has been observed in cases of increasing carboxylation rate and/ or decreasing supply of CO2 through the stomata, whereas a decrease in δ13C has been observed in the opposite cases. For example, water stress (e.g., ref 4) and SO2 and O3 pollution stress (5-10) have been shown to increase δ13C due to reduction of intercellular leaf CO2 by stomatal closure. However, Matyssek et al. (10) reported that the δ13C of birch trees exposed to O3 was less affected by stomatal limitation than by changes in carbon fixation such as elevated PEPcarboxylase activity. On the other hand, low light levels (11, 12) and acid misting (13) induced a decrease in the δ13C of plants probably due to reduction of the carboxylation rate. These results make it possible to expect that change in δ13C is a useful index for the type and degree of stresses experienced by trees (14-16). Especially the δ13C record of annual growth rings may provide information on the history of stress related to environmental changes during the past 50-100 years (16). In fact, the δ13C of tree rings has been reported to respond sensitively to the changes in SO2 pollution and drought conditions (5, 7, 16-19). Thus, measurement of the δ13C levels of tree rings combined with conventional tree-ring thickness data offers a potentially useful research tool for evaluating possible causes for forest decline related to environmental changes. The decline of Japanese fir (Abies firma) forests has been observed on the Tanzawa Mountains, Kanagawa Prefecture, Japan (20-22), which are located about 50 km west of densely populated and heavily industrial areas such as Yokohama and Kawasaki (Figure 1), since the 1960s. Markedly damaged stands have been found on south-facing slopes and along eastern ridges with an altitude more than about 700 m on Mt. Oyama (1252 m) in the eastern part of the Tanzawa Mountains (Figure 1). Declining stands have been usually associated with visual symptoms such as thinning of the crowns and excessive mortality. Even for trees without visual decline symptoms, however, a significant growth reduction was observed during the 1960s based on tree-ring thickness measurements (20, 21). The causes for decline remain unclear, although several hypotheses such as SO2 and O3 pollution and acid fog have been proposed to account for the observed decline (20-23). In this study, we evaluated possible causes for the decline based on the δ13C records of annual growth rings sampled from trees without visual decline symptoms on Mt. Oyama. It was anticipated that those trees had satisfactorily preserved the time trend related to environmental changes. For comparison, the δ13C records of tree rings from Japanese cedar (Cryptomeria japonica) on Mt. Oyama were also investigated. Decline of Japanese cedar has not been observed at this location.
Experimental Section Sampling. Disk samples of Japanese fir and Japanese cedar were taken from trees cut down in June 1997 on the southeast slope at about 500 m altitude on Mt. Oyama. Soils (brown forest soils) on Mt. Oyama tended to have relatively low acidity [pH (H2O): 5.6-6.1; exchangeable Al:
