The limit of this study is its spatial representativeness of the selected stands for different sub zones of taiga (Lopatin et al. 2006). It is important to understand changes in forest productivity not only on a temporal scale, but also in spatial terms. Currently, due to the low accessibility and huge size of the territory, it is impossible to create systematic sample plots in Komi Republic. Therefore, to make conclusions about spatial response distributions other methodological approaches need to be used.
Another limit of this study for the climate reconstruction is the potential bias due to the different ages of the sampled trees. It was shown that the climatic signal is maximized in older trees and the higher amount of noise present in younger trees (Carrer & Urbinati 2004). But from the standpoint of forest management, for the purpose of understanding factors influencing radial increment of trees in the forest, represented by different age classes, this bias is not important.
The statistical relationships of Siberian spruce growth to climate in Middle taiga zone of Komi have changed over the last century. This is due to changes in temperature and precipitation or to changes in the sensitivity of trees to climate. The changes in tree-ring sensitivity could be attributed to ultraviolet radiation (Briffa et al. 1998), increased atmospheric CO2, nitrogen deposition (Kellomaki et al. 1997, Kellomaki & Vaisanen 1997, Kellomaki & Wang 1997a, Kellomaki & Wang 1997b), ozone exposure (Bartholomay et al. 1997, Wager & Baker 2003), pollution, and cycles in sunspot activity (Raspopov et al. 2004).
The method of analysing the factors influencing growth trends through the building chronology with standardization of raw tree ring measurements for the whole sub zone of taiga contains some limiting factors. At present it is impossible to find an ideal curve that removes variation in the radial increment caused by ageing, competition, stand dynamics and other factors reflected in tree rings and at the same time preserving the long-term growth trend. Furthermore, it is even more difficult to find an individual curve for each separate factor. However, the method could be used in case of a large number of sampled trees, and those limits could be minimized due to the process of averaging. In this study the low-frequency variation caused by the above-mentioned factors was removed, but with the understanding that standardization partly removes long-term growth trends, which belong to low- and medium-frequency growth variation, i.e. periods more than 30 years.
In Boreal forests, the climate change scenarios imply a great rise in temperature and in precipitation, especially in winter. Higher precipitation in summer may compensate for the enhanced evapotranspiration, making sufficient amounts of water available for forests. Longer growing seasons reduce the soil frost. In northern Europe under warmer conditions the Siberian spruce and Scots pine are likely to invade the tundra region and higher elevations (IPCC 2001). Therefore, it is probable that climatic warming will stimulate early initiation of photosynthesis in spring, resulting in better utilization of solar radiation and increased productivity.
On most of the studied sites there are increasing long-term trends in total monthly precipitation and mean monthly air temperatures that are important for radial growth. This increase could be attributed to a factor that causes increasing site productivity in Komi. But due to the big difference in climate conditions through the Komi Republic, there is no one single or clearly identified group of monthly climate parameters that affected the growth at all sites.
The analysis of the variables for the other months showed the increase in temperature and precipitation was due to the trends in months that are significant for radial increment. The importance for radial increment months with positive trends is higher.
The absence of statistically significant response (pFig. 4A, C, D, E) in dendroclimatic analysis to the air temperature does not show the absence of relation between radial increment and temperature, because relatively big amount of variance could be explained by the monthly temperature (Tab. 4). The absence of response in several chronologies testes using DENDROCLIM 2002 could be due to the possible non-linear relationships between temperature and radial increment or relatively short period (50 years) used for calculation response function coefficients.
The amount of variance explained by the temperature is larger than the amount explained by precipitations (Tab. 4) except for Siberian spruce in the eastern part of Middle taiga zone. However, the territory of Komi is characterized by surplus moisture; mean annual evapotranspiration is significantly lower than annual rainfall (Tab. 1). It was shown on pine growth in Sweden (Jonsson 1969) that response of ring-width to this parameter might be non-linear and therefore might be difficult to evaluate using methods assuming a linear weather-growth relationship (as for example, response function analysis - Drobyshev et al. 2004). In boreal regions the potential impact of precipitation is related to temperature.
The difference in response of spruce growth in different sub-zones is explained by the difference in climatic parameters, climate change trends and differences in the ecological characteristics of different provenances. The changes in air temperatures in Komi Republic are not distributed equally over the territory. The response to the temperatures in the western part of Middle taiga zone could be attributed to the increasing length of the growing season due to increasing temperature in May and November. We are assuming that during the recent decades cambial activity already started at the first weeks of May, because the temperature in this period was often above + 5°C.
The response of high frequency variation in radial increment of pine to summer temperatures was reported for different sites in Northern Europe (Helama et al. 2002, Kalela-Brundin 1999, Kirchhefer 2001, Linderholm et al. 2003, Lindholm 1996, Lindholm & Eronen 2000, Miina 2000). This pattern was also found in our study (i.e. Scots pine in western part of Middle taiga zone), but without the strongest response to the July temperature. The possible explanations are the selection of sites in multistoried forests and the influence of continental climate. In the Komi Republic there is a continental climate (Tab. 1) with more extreme climate conditions than those in Finland, Sweden and Norway, which are greatly influenced by the Gulf Stream.
The general decrease in amount of variance explained by climate is in accord with the findings on reduced correlations between growth and temperature in subarctic Eurasia (Vaganov et al. 1999).