The Paradox of Leaf Phenology for Tropical Seasonal Forests: Will climate change affect their sustainability?

Forestry Administration, Cambodia

University of Tsukuba, Japan

FY 2019–2023 Grant-in-Aid for Scientific Research (KAKENHI)

IIDA, Shin'ichi (Hokkaido Research Center)

Tropical seasonal forests experience distinct dry seasons, during which no rainfall occurs for several months. Deciduous trees start to shed leaves early in the dry season and leaves completely fell by the middle of the dry season due to the limited soil water content available for the uptake by root system. However, early leaf flushing is generally found before the first monsoon rain. This early leaf flush during the dry season is called as “the paradox of tropical seasonal forests,” because it seems unlikely to occur under dry soil conditions. The source of water during the early leaf flush is currently unknown and could provide important information to promote best conservation plans and sustainable uses of tropical seasonal forests.

The objective of this research is to discover the water source available for the deciduous trees during the dry season in the Cambodian tropical forest.

We will elucidate the water source using the stable isotopes of water that has been taken up by deciduous trees. The stable isotope signature of the water is constant during the absorption by roots and movements from roots to branches. To do this analysis, we will collect the soil samples at various depths. Groundwater will also be sampled from observation wells in the forest. In the laboratory, we will sample water from cut branches. The stable isotopes of groundwater and water extracted from soil and branch samples will be analyzed. Finally, the source of water uptake by root system can be determined by comparison the stable isotopes compositions of soil water, ground water, and branch water.

Effects by recent climate changes resulted in the extremely small amounts of rainfall around Cambodia, which is located in Indochina Peninsula. The early flushed leaves could be threatened by the drought condition. Deciduous tree, which absorbed water at the depth of more than 1 m, was confirmed in this study. Nevertheless, it was clarified that transpiration was decreased in the extremely dry condition.

Data obtained in this study strongly implies that tropical seasonal forests would be degraded under the extreme condition of weather and climate. The results provide valuable insight into establishing suitable conservation plan, and contribute to accurate future prediction in hydrological cycle in this region.

Canopy of a tropical seasonal forest in wet season.

 Leaf shed trees in a tropical seasonal forest during dry season.

Observation of soil and root.

Shin'ichi Iida, Takanori Shimizu, Koji Tamai, Naoki Kabeya, Akira Shimizu, Eriko Ito, Yasuhiro Ohnuki, Sophal Chann, Nang Keth (2016): Interrelationships among dry season leaf fall, leaf flush, and transpiration: insights from sap flux measurements in a tropical dry deciduous forest. Ecohydrology. 9, 472-486.

Shin'ichi Iida, Takanori Shimizu, Yoshinori Shinohara, Shin'ichi Takeuchi, Tomo'omi Kumagai (2020): The necessity of sensor calibration for the precise measurement of water fluxes in forest ecosystems. In: Delphis F. Levia, Darryl E. Carlyle-Moses, Shin'ichi Iida, Beate Michalzik, Kazuki Nanko, Alexander Tischer (Eds.), Forest-Water Interactions. Ecological Studies Series, No.240, Springer Nature, Cham, Switzerland AG, 29-54.

Shin'ichi Iida, Takanori Shimizu, Koji Tamai, Naoki Kabeya, Akira Shimizu, Eriko Ito, Yasuhiro Ohnuki, Sophal Chann, Delphis F. Levia (2020) : Evapotranspiration from the understory of a tropical dry deciduous forest in Cambodia. Agricultural and Forest Meteorology, 295, 108170. https://doi.org/10.1016/j.agrformet.2020.108170

Shin'ichi Iida, Takanori Shimizu, Koji Tamai, Naoki Kabeya, Akira Shimizu, Makoto Araki, Yasuhiro Ohnuki, Eriko Ito, Tanaka Kenzo, Jumpei Toriyama, Tayoko Kubota, Tsutomu Yamanaka, Sophal Chann, Delphis F. Levia (2022) : Evapotranspirational processes in a dry deciduous forest in Cambodia: clarifying the respective contributions of overstory and understory vegetation to the hydrologic cycle. Journal of Japanese Association of Hydrological Sciences, 52, 65-72.