Long-term effects of climate on Picea abies communities in the South European taiga

The effects of climate change on the Picea abies communities of the South European taiga have been analysed at three different scales: (i) the scale of long-term fluctuations in climatic patterns over thousands of years; (ii) the scale of climate variability over 100 years; and (iii) the scale of ecophysiological processes over a single growing season. On the basis of pollen-spore analysis, the role of Picea abies as a dominant species in the forests of the South European taiga region (Tver region, Russia) has been shown for the past 17,000 years. In the Holocene, the temperature of the region changed with 1000-1200 and 3000 year periods on the background of a nonstationary trend with a 14,000-year period; wetter periods occurred with 1000, 1500-1600 and 3000-year periods; and the vegetation itself changed with 1000-1100, 1500, 2500 and 14,000-year periods. The warmest period was observed 6000 years ago, but 1500 years later the temperature had decreased to the modem level. Climate warming results usually in intensive and deep reorganization of vegetation (successional replacement). The most intensive reorganizations occur about 500 years after the beginning of simultaneous and opposite oscillations in temperature (warming) and moisture (drying). The low stability of spruce forests to warming and drying can also result in 'catastrophic' reorganization of communities. On the basis of instrumental records over the past 100 years, a slight warming from the end of the 19th century and a slight cooling after the 1940s as well as a general increase in precipitation has been found for this region. In comparison with the beginning of the 20th century and the 1940s, the current climate tends to be less continental. The relative stability of the climate over this 100-year period contrasts with the relative instability of the vegetation communities. Fluctuations of radial increment and observations of stand destruction are consistent with water stress as the major factor. Intensive measurements of photosynthesis and evapotranspiration during individual seasons indicate that aggregate CO2 assimilation is reduced by 15-25% during dry periods.