H51C-1355: Leaf ecophysiology of rubber (Hevea brasiliensis) and its implication to stand-level transpiration in the Asian monsoon in mainland Southeast Asia

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Authors: Yoshiyuki Miyazawa1, 3, Nakako Kobayashi2, Ryan G Mudd3, Song Yin4, Tiva LimKhan4, Thomas W Giambelluca3, Tomo'omi Kumagai2, 1

Author Institutions: 1. Research Institute for East Asia Environment, Kyushu University, Fukuoka, Japan; 2. Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan; 3. Department of Geography, University of Hawai'i at Manoa, Honolulu, HI, USA; 4. Rebber Research Institute of Cambodia, Phnom Penh, Cambodia

In Indochina Peninsula, rubber (Hevea brasiliensis) plantation is expanding rapidly as the major commercial crop. Rubber, which is a native of Amazon rainforests, are thought to have transpiration characteristics different from other native species in this region, which is characterized by high rainfall occurs in rainy season and severe drought in dry season. Studies suggested high stand transpiration rate (E) in rubber plantations, possibly due to the water use even in dry seasons or to high leaf photosynthetic capacity. Despite recent advances about the leaf-level measurements in leaf gas exchange traits, knowledge has been collected without clear linkage to the seasonal trends in stand-level gas exchange and its seasonal trends. In this study, we monitored the leaf-level photosynthetic capacity (Vcmax25) and a parameter for stomatal control (m, Ball et al. 1987) to show whether changes in atmospheric environments and leaf gas exchange traits explain the high E and its seasonal trends (annual peak in mid rainy season). Results showed that rubber had Vcmax25 and m similar to temperate broadleaved tree species and higher than other species in Indochina Peninsula, indicating that high Vcmax25 would be responsible for the higher E than other forests in Indochina Peninsula. Vcmax25 reached an annual maximum soon after leaf expansion before the beginning of rainy season, keep the level until soon before leaf fall in mid dry season and did not supported the hypothesis that these parameters increased in mid rainy season. The m also showed slight seasonal variations without clear trends. Multilayer model well modeled the diurnal trends in measured E using sap flow measurements, except soon after leaf expansion. Constant leaf gas exchange rates and good reproduction of finer time scale E suggested that increased E in mid rainy season was not due to the increased evaporative demand or changes in leaf gas exchange rates, but to the increased LAI. Poor modeling soon after leaf expansion suggested the effects of soil drought on E, and prolonged dry season is thought to strongly modify the E of rubber plantation in this region.

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