When adapting forest management practices to a changing environment, it is very important to understand the response of an unmanaged natural forest to climate change. The method used to identify major climatic factors influencing radial growth of Siberian spruce and Scots pine along a latitudinal gradient in north-western Russia is dendroclimatic analysis. A clear increasing long-term trend was identified in air temperature and precipitation. During the last 20 years, all meteorological stations experienced temperature increases, and 40 years ago precipitation began to increase. This is shown by the radial increment of Siberian spruce and Scots pine. Therefore, climate change could partly explain the increased forest productivity. The total variance explained by temperature varied from 22% to 41% and precipitation from 19% to 38%. The significant climatic parameters for radial increment in Komi Republic were identified, and the relation between temperature and precipitation in explained variance changes over time for Siberian spruce.
Climatic changes including a lengthening growing season (
Previous results indicate that northern forest ecosystems are among the regions at greatest risk from the impacts of climate change (
In Europe, most forests are managed, except for those in north-western Russia, where there is a dominance of old-growth natural forests. It is important to understand the response of unmanaged natural forest to changing climate because it is possible to adapt forest management practices to a changing environment (
The largest administrative region of North-western Russia (forest area of Komi Republic is 33% of total North-western Russia’s forest area) was selected for the assessment of long-term forest growth trends. Komi is situated at the eastern boundary of the European part of Russia, in the boreal region where large areas of natural forest still exist. Borehole temperature measurements in Komi Republic indicate strong subsurface warming, reflecting changes in the trends of both surface air temperature and solid precipitation (
The main objective of this research is to investigate the effect of climate change on radial growth of Siberian spruce (
Climatically, Komi (
The vegetation cover of Komi is dominated by Middle and Northern taiga forests, with the exception of mountainous parts of the republic where forest-tundra and tundra ecosystems have developed (
Forests with Siberian spruce in Komi reach 16 mln. ha,
Komi is the most forest-rich region in north-western Russia. The forested area includes about 300 000 km2, making up 4.1% of Russian forested areas. The total stock of wood in the forests of Komi reaches 2855.8 mln. m3. The mean volume of wood is about 80-100 m3 ha-1. In southern regions the mean standing volume is 300 m3 ha-1.
The material was collected along a transect from the south of Komi (south taiga sub-zone of boreal forests) to the Arctic spruce timberline. The sampled stands were at similar altitudes. The study stands were grouped into “sub-zones” according to their geographical position in the taiga sub-zones of boreal forests. Totally 261 trees were collected in 5 sub-zones of taiga forests in Komi (
The sites were selected using GIS datasets of forest management units, old forest inventory maps and satellite images TERRA ASTER (scene size 60x60 km) with a spatial resolution of 15 m. In the procedure for site selection the main aim was to find the most common site types and at the same time exclude possible forest management or any other human impact from the past. Sites with a low productivity index (5 class, according to the classification system for Russian forest productivity) represents 70% of the forest area of Komi Republic (
The stands were selected according the following criteria for site conditions:
spruce or pine dominating species;
low site index (5 class, according to the Russian forest productivity classification system (
multistoried mature stands represented by trees of 3-5 different age classes.
In most of the regions in Komi the forest stands are multistoried. Therefore the sample trees were chosen from among trees not dominated by older trees but rather located in openings within the stand. The sample trees were expected to reveal homogeneity in their tree-ring pattern; they showed no obvious signs of near-neighbour competition or forest management. Trees were chosen from different diameter classes, healthy looking with straight, unbroken stems and regularly shaped crown. Mature dominant trees without visible signs of damage were selected as sample trees. The sample trees in the stands were expected to have a common growth trend, which was influenced by a large portion of the climatic effects and other factors which differ among individuals and from site to site. At each site an averaging process, during building chronology, helped to minimize the influence of other factors.
Prior to felling, for visual assessment of the tree ring pattern, a core from each tree was extracted with an increment borer. This allowed exclusion of those trees affected by competition in the past. Siberian spruces and Scots pines were sampled at breast height (1.3 m above the ground). In most cases, discs were cut using a chain saw. If it was difficult to cut discs, cores were extracted from trees with the increment borer from two radii per tree (the first one oriented to the north, the others at 90°-120° to the first). Geographical coordinates of sample trees were measured using GPS.
Monthly precipitation sums and air temperature means from 5 climate stations (WMO numbers 22996, 23804, 23711, 23412, 23219) were obtained from the Center of Meteorology and Environment Monitoring of Komi Republic (
Radial increments were measured to an accuracy of 0.01 mm. During the process of measurement, the raw measurements of tree rings were cross-dated using visual control, by comparing the series graphically. Cross-dating and data quality were assessed using the computer program COFECHA (
To maximize the climatic signals in tree ring series, other factors should be minimized. For example, a typical sample might display exponentially declining growth with age, the classic biological growth curve. Standardizing the sample using a spline curve results in data values that represent a departure from the “expected” value for a given year. This departure from the expected mean value is then used to interpret a proxy environmental signal in the data. The above-mentioned procedure usually is an attempt to remove the growth trends due to normal physiological ageing processes and changes in the surrounding forest community. Therefore individual ring-width series were indexed using spline curves of 60 years with a frequency response of 50%. This approach was selected due to the high amount of variance in the dataset because of using the trees from different age cohorts for chronology building.
The common interval (1954 - 2003) adjusted for order of the pooled autoregressive model was used for analysis of climate-growth relationships. Indices were further prewhitened using Box Jenkins methods of autoregressive and moving average time series modelling (ARMA -
Residual ARSTAN chronologies containing high frequency variation were used to examine climate-growth relationships on an interannual basis.
Chronology confidence was determined using statistics of Expressed Population Signal (EPS -
Response function analysis (
Using collected samples 9 site chronologies of Scots pine and Siberian spruce radial increments were constructed for 5 sites. The expressed population signal (EPS) of chronologies was calculated for the common interval (1954 - 2003). Five of nine chronologies (
In western part of Middle taiga zone the statistically significant positive response of Scots pine to the monthly mean temperatures of November year prior to growth and May of the current growth season was found (
Response function analysis showed positive (
No statistically significant responses to temperatures and precipitations were found for Scots pine in Southern taiga zone (
The variance explained by the mean monthly temperature varied from 26% to 48% (
The variables identified in dendroclimatic analysis as significant for the radial increment of Scots pine and Siberian spruce in Middle taiga zone were retested for the temporal stability of the relationships using 25-year window correlation. In this step we were looking to disprove (p>0.05) a correlation between a monthly climate parameters and ring-width chronologies for a given 25-year period if r < |0.396| (for n = 25, the correlation is considered significant at the 5% significance level if its coefficient is greater than 0.396 in absolute value).
The statistically significant changes in correlations between climate variables and radial increment were identified for Siberian spruce (
The amount of variance of radial increment explained by the monthly mean temperatures and monthly precipitation sums changed over the period 25 years from 14% to 20% (
Fig. 5 shows the deviation in degrees C from the long-term mean (70 years) temperature and in millimetres from the mean annual sum of precipitation at the 3 meteorological stations included here with the longest period of observation. There is clear evidence during recent decades that the climate in the Komi Republic has changed. During the last 20 years the mean annual air temperature has increased, and over the last 40 years the annual precipitation sum at meteorological stations has also increased. But the impact of each month on the annual result differs over time (
In some months the trends are positive, and others have some negative trends. As shown above for the boreal zone, the radial growth of trees could be attributed to certain months. Deviations from means in temperature, which are significant for radial increment of pine and spruce in Komi, are shown on
The limit of this study is its spatial representativeness of the selected stands for different sub zones of taiga (
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 (
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 (
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,
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 (
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 (p < 0.05) of chronologies (
The amount of variance explained by the temperature is larger than the amount explained by precipitations (
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 (
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 (
A clear long-term trend in climate change was identified. At all meteorological stations the air temperature increased during the last 20 years, and total precipitation started to increase 40 years ago. This is reflected in the radial growth increment of Siberian spruce and Scots pine. Thus, climate change could partly explain the increased site productivity.
The total variance explained by temperature varies from 26% to 48% and precipitation from 26% to 44%. The climate factors influencing increased radial increment of Siberian spruce differ from those in Scots pine. The higher temperatures allow enhancement of evapotranspiration, which is why both temperature and precipitation affected radial increment positively. The temporal stability of dendroclimatic relationships has changed for radial growth of Siberian spruces in western and eastern parts of Middle taiga zone. The statistically significant climatic parameters influencing radial increment of spruce and pine in Komi were identified. The increased radial increment of spruce and pine in Komi is attributed primarily to the increase in temperature. But the correlation of precipitation sums and air temperature with radial increment of Siberian spruce in Middle taiga zone changes over the time. Climate change is causing increasing site productivity, but its direct influence, identified by means of response function analysis, explains only part of the high frequency variations in radial increment. Taking into account the correlation with climate variables and their temporal stability, it is possible to simulate the future development of forest resources in Komi under the changing climate. For purposes of forest management, new climate-sensitive growth models should be developed.
This study was supported by a CIMO fellowship grant TM-04-2734, NorFA grant “Network for Dendrochronological Research in Northern Europe”, travelling grants from the Graduate School in Forest Sciences (University of Joensuu), the project
Sampled stands and sub zones of taiga boreal forests. Borders of vegetation complexes according to
Standardized tree-ring chronologies of Siberian spruces smoothed with a 10-year moving average, shown as a deviations from the mean (n = number of trees).
Response function analysis tree-ring chronologies for common interval 1954-2003. Lower case “p” denotes months in the year previous to the current year growth season. Filled bars of response function coefficients are significant at p < 0.05. A: Scots pine in Middle taiga zone (east); B: Scots pine in Middle taiga zone (west); C: Siberian spruce in Middle taiga zone (east); D: Siberian spruce in Middle taiga zone (west); E: Siberian spruce in Southern taiga zone.
Temporal stability of dendroclimatic relationships estimated for the common interval of 50 years (1954-2003). Lower case “p” denotes months in the year previous to the current year growth season. Upper case “T” and “P” denotes temperature and precipitation of the months. Confidence intervals are showed with dashed lines.
Deviation in degrees C from long-term mean annual temperature and in mm from mean long-term annual sum of precipitation on analysed meteorological stations, smoothed using 30-year running mean.
Absolute deviations from long-term means for mean monthly temperature (A) and monthly sums of precipitations (B, C) that are significant for radial increment of pine and spruce in Komi smoothed with the 30-years running mean.
Climatic characteristics in different taiga sub zones in Komi (
Sub zone of taiga boreal forest | Vegetation period, days | Precipitation, mm | Annual evapotranspiration, mm | |
---|---|---|---|---|
May - October | October - April | |||
Northern forest - tundra transition zone | 117 | 235 | 190 | 125 |
Northern taiga | 143 | 290 | 190 | 175 |
Middle taiga | 158 | 330 | 260 | 200-250 |
South taiga | 177 | 370 | 250 | 300 |
Tree ring data collected in Komi Republic in 2003-2005. (§) The Scots pine trees not found in forest-tundra transition zone.
Site | Forest zone | Location coordinates | Siberian spruce | Scots pine | Distance to meteo- station, km | Meteo-station, analysed period | ||||
---|---|---|---|---|---|---|---|---|---|---|
n trees | Time Span | Min-Max Mean | n trees | Time Span | Min-Max Mean | |||||
1 | Forest - tundra transition zone(§) | 66°41’260” N 56°49’142” E | 16 | 1812 2005 | 71-192 115.6 | - | - | - | 125 | Khoseda - Khard, 1933-1995 |
2 | Northern taiga zone | 65°59’697” N 57°48’820” E | 16 | 1878 2005 | 37-126 75.3 | 20 | 1924 2005 | 52-80 69.3 | 42 | Ust-Usa, 1936-2003 |
3 | Middle taiga zone (west) | 61°44’834” N 50°34’910” E | 40 | 1779 2005 | 38-225 104.3 | 45 | 1786 2005 | 34-218 93.6 | 30 | Syktyvkar, 1896-2004 |
4 | Middle taiga zone (east) | 63°25’294” N 57°57’597” E | 51 | 1826 2005 | 27-176 91.3 | 21 | 1842 2005 | 142-163 152 | 130 | Troitsko-Pechersk, 1893-2004 |
5 | South taiga zone | 60°33’615” N 49°26’945” E | 30 | 1917 2005 | 18-89 52.8 | 22 | 1877 2005 | 28-127 73 | 38 | Objachevo, 1944-2004 |
Total: | 153 | - | - | 108 | - | - | Mean 73 | - |
Expressed population signal of chronologies for the period 1954 - 2003.
Site | Forest zone | Siberian spruce | Scots pine |
---|---|---|---|
1 | Forest - tundra transition zone | 0.41 | - |
2 | Northern limit of the Northern taiga zone | 0.63 | 0.71 |
3 | Middle taiga zone (east) | 0.85 | 0.85 |
4 | Middle taiga zone (west) | 0.86 | 0.85 |
5 | South taiga zone | 0.85 | 0.61 |
Amount of variance (R2) explained by temperature (T) and precipitations (P) for the period 1954-2003.
Site | Forest zone | Siberian spruce | Scots pine | ||||
---|---|---|---|---|---|---|---|
P | T | P and T | P | T | P and T | ||
3 | Middle taiga zone (east) | 0.416 | 0.390 | 0.641 | 0.446 | 0.484 | 0.646 |
4 | Middle taiga zone (west) | 0.402 | 0.413 | 0.652 | 0.220 | 0.386 | 0.702 |
5 | South taiga zone | 0.220 | 0.264 | 0.435 | - | - | - |
Temporal stability of the amount of variance (R2) explained by temperature and precipitations for 2 subsequent 25-years periods.
Forest zone | Time span | Siberian spruce | Scots pine |
---|---|---|---|
Middle taiga zone (east) | 1954-1978 | 0.649 | 0.574 |
1979-2003 | 0.727 | 0.388 | |
Middle taiga zone (west) | 1954-1978 | 0.309 | 0.228 |
1979-2003 | 0.292 | 0.370 | |
South taiga zone | 1954-1978 | 0.639 | - |
1979-2003 | 0.420 | - |