TOP Frontier Newsletter No.13 Jan.2001


Siberian Taiga -A Symbiotic System of Climate and Vegetation through Water Cycle- ?



Dr.Yasunari's Photo Program Director, Hydrological Cycle Research Program of FRSGC and FORSGC

Dr.Tetsuzo Yasunari


Fig1

It is well known that one climatic zone corresponds well with one dominated vegetation, as has been described in some climatic classification (e.g., of Koppen's). However, we should also note that the vegetation dominated over a broad region of continents may affect, at least to some extent, the atmosphere and surface climate over there. This implies that the vegetation and climate of a large spatial-scale, should be understood as an interactive system rather than a one-way relation from one to the other. We have been studying this old but new issue of the earth climate, focusing Siberian Taiga as a typical interactive system over the Eurasian continent.


The eastern Siberia is broadly covered by permafrost, and well correspondently the subpolar boreal forest called Taiga is distributed on this frozen soil zone. One group of the FRSGC (Suzuki et al. 2000), by using satellite-based vegetation index, has revealed that the photosynthesis activity and associated evapo-transpiration rate of this Taiga zone seems to be very high in the peak summer season,compared to other boreal forest zones.


Ohta, Hiyama and his colleagues of the FORSGC, cooperating with some University and Russian researchers, have conducted intensive energy and water flux measurements in the Larch forest, a typical forest of Taiga, near Yakutsk in Lena river basin for more than two years. This observational work was really a very tough work, including the field measurements under the severe winter climate of below minus 40. Through this invaluable field observations, they noticed some interesting features in the seasonal change of energy balance and associated phenological change (Ohta et al., 2001).


That is, snowmelt started in the middle of April, and snow cover completely disappeared in the beginning of May. The sensible heat flux was dominated, but the latent heat flux was very small or negligible throughout this stage in spite of rapid snow-melting. The latent heat flux (i.e., evapo-transpiration rate) suddenly increased and dominated in the surface energy balance in the beginning of June, when the foliation of larch trees suddenly started (see Fig. 1). Why did the evapo-transpiration suddenly started? The perfect answer to this question has not been given yet, but the melting process of permafrost and the root depth of the trees are likely to be closely related to this problem; i.e., when the depth of the melted layer reached to about 20 cm depth where large portion of the Larch roots are distributed in the soil, the foliation and transpiration from the foliated leaves may suddenly have started. During summer, most of the evapo-transpiration occurred as the transpiration from these trees.


On the other hand, Yasunari and Yatagai (2000), as a cooperative study of the FRSGC and NASDA/EORC, estimated the seasonal change of atmospheric water balance over the Lena river basin using the ECMWF global reanalysis data and the objectively analyzed precipitation data from CMAP. They concluded that most of the precipitation in summer over this area is occupied by evapo-transpiration from the surface of this basin, implying that most of water is recycling during summer between the vegetation and the atmosphere. It has also been suggested that the permafrost has a hydro-climatic memory effect that anomalous soil moisture condition, which was mostly produced by precipitation/evapo-transpiration process in summer, is retained for the soil moisture of the next summer.


These hydro-meteorological studies have strongly suggested a symbiotic system of climate and vegetation through the hydrological processes that during the short summer the Taiga maintains its life by recycling water between the atmosphere and the vegetated surface, while the permafrost itself is likely to be maintained by Taiga by suppressing sensible heating. An essential issue for the near future is to fully understand the mechanisms of this "symbiotic sysem" and to model it for predicting the impacts of global warming (increase of greenhouse gases) and anthropogenic deforestration on changes of the continental-scale climate, vegetation and hydrological cycle. These changes, in turn, should definitely affect the climate of Japan, located leeward side of the continent.



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