Seasonal Signal of Photosynthesis in the Forests of North Eurasia

The seasonal course of the water potential in branch xylem reflects the dynamics of photosynthetic СО₂/Н₂О exchange in the forest stand. Long-term figures of water potential in the branches of major wood species growing in East Siberia—larch (Larixcajanderi Mayr), pine (Pinussylvestris L.) and birch (Betulaplatyphylla Sukaczev)—have been analyzed. The seasonal dynamics of the water potential showed its minimum in the first half of the vegetation period. The time when the minimum of water potential was observed concurred with the earlier determined peak in seasonal dynamics of net ecosystem exchange (NEE) of C/СО₂ in the forests of East Siberia. Statistical analysis of a long-term time series of atmospheric СО₂ concentration along the latitudinal zone (43°07′–55°45′ N) of the transcontinental transect from Moscow (55°45′ N, 37°34′ E) to Vladivostok (43°07′ N, 135°54′ E) corroborated the existence of the seasonal minimum of atmospheric СО₂ in the first half of the vegetation period. However, this transcontinental Eurasian minimum of atmospheric СО₂ was reached a month before such a minimum in the region of East Siberia (Yakutsk, 62°05′ N, 129°33′09″ E). It was concluded that the minimum of atmospheric СО₂ concentration is a seasonal signal of photosynthesis in the regional forest ecosystems, and time shift of this signal might serve as an indicator of modification in the regional biogenic cycle of carbon caused by climate fluctuation.