A large number of tree-ring chronologies were devel-oped for the boreal zone and subarctic forest-tundra eco-tone in the northern hemisphere (Jacoby and D’Arrigo, 1989; Briffa
Further analyses in the Arctic are necessary due to sparse and short meteorological station records. Temperature has been observed almost continuously in Svalbard since 1898, however, in other parts of the archipelago continuous meteorological observations are shorter, e.g. since 1978 in southwestern Spitsbergen. Increase of temperature and changes of other climatic variables over the last few decades are evident by meteorological data in this area. According to the latest Global Climate Models (GCM) and Regional Climate Models (RCM) results, it is predicted that the air temperature in the Arctic will have increased between 5°C and 10°C in winter and 0°C and 2°C in summer during the next 100 years. New models simulate for almost all parts of the Arctic the increase of precipitation by 20–50% (Brönnimann, 2015; Przybylak, 2016). Dendrochronological research in different parts of the Arctic may be helpful for better understanding of temperature and moisture trends. Polar willow (
The study area is located in the southwestern part of Spitsbergen specifically the Svalbard Archipelago (Fig. 1). A large part of this High Arctic island is covered with glaciers, which are in the phase of accelerated retreat (Hagen
Location map showing the sampling sites in the southwestern Spitsbergen in the vicinity of the Polish Polar Station in Hornsund.
General view on the typical sampling area in northwestern shore ofHornsund fjord; flat raised marine terraces with tundra vegetation are visible in the foreground.
Spitsbergen has a periglacial climate often referred to as the "High Arctic climate" (Przybylak, 2016). Over the last 30 years, mean annual air temperature in the Hornsund area has varied from –7.4°C (1988) to –1.5°C (2006) with an average of–4.2°C (Marsz and Styszyńska, 2013; Hornsund GLACIO-TOPOCLIM Database, 2014). Inter-annual fluctuations of temperature are typical of this area but a distinct warming trend is evident. Nordli (2010) has calculated air temperature trends for about the last 100 years in Svalbard, which is +0.24°C per decade, but during the last 30 years the growth has been faster (+1.1°C per decade) (Marsz and Styszyńska, 2013) (Fig. 3). The annual total precipitation in this area is 434 mm/year (1979–2012). The extreme daily totals are observed in August and September (43–58 mm) (Hornsund GLACIO-TOPOCLIM Database, 2014). The growing season in this part of Spitsbergen is restricted to three months and usually begins in the first half of June when the ground temperature first exceeds 0°C (Fig. 4).
Course of annual mean air temperature and precipitation totals in the Polish Polar Station, Hornsund (77°N, 15°E) (source: Marsz and Styszyńska, 2013).
Summary of climate at the meteorological station in the Polish Polar Station, Hornsund (77°N, 15°E) (source: Marsz and Styszyńska, 2013).
Different tundra plant communities occur in the non-glaciated parts of Spitsbergen. Four plant zones are distinguished on which of the following species dominates (Rønning, 1996): white Arctic mountain heather (
Polar willow (
A. Small colony of Salix polaris (Wahlenb.) on the flat raised marine terrace surface (camera lens cap scale); B. Cross-section of 14-years old S. polaris, high variability of growth-ring width is clearly visible.
Complete individuals of
The collected samples were sectioned with GSL 1 sledge microtome. The 15–20 μm cross-sections were taken along each individual from four to seven different places (depending on the length of the root and wooden branches) in order to develop their chronology (Fig. 6). This method was described in detail in previous research (e.g. Bär
Individual plant chronology of 38 year old Salix polaris created on the basis of serial sectioning (after Owczarek et al. (2014a) modified).
The
In general, the raw data fluctuated around a horizontal straight line, without a distinct age trend. Therefore the measurements were transformed into chronology by averaging. The ARSTAN (ver.41d) standardisation software was used to calculate the chronology and its parameters (Cook, 1985). In order to compare the resulting chronology with other chronologies and to assess its reliability and relevance for the dendroclimatological research, the appropriate descriptive statistics, including mean sensitivity (MS), mean series inter-correlation (Rbar) and expressed population signal (EPS), were calculated (formulas after Wigley
Finally, 21 out of the 45 samples were included in the chronology and became the basis for further analyses. The remaining sequences were removed due to a large number of missing rings, eccentricity, presence of scars and clearly visible reaction wood. The oldest individuals of
Statistics for polar willow growth-ring chronology.
Parameter | |
---|---|
Time period (>3series) | 1951–2011 |
Chronology length (>3series) | 60 |
Number of series | 21 |
Mean series length | 38 |
Mean growth-ring width (Standard deviation) (mm) | 0.14 (0.08) |
Mean index (Standard deviation) | 0.96 (0.58) |
Mean series correlation with master | 0.44 |
Mean inter-series correlation (Rbar) | 0.32 |
Expressed population signal (EPS) | 0.82 |
Mean sensitivity | 0.56 |
Autocorrelation 1storder, unfiltered / filtered | 0.27/-0.01 |
Growth-ring chronology of Salix polaris (solid line) and its sample replication (dashed line).
Mean, minimum and maximum grow-ring widths variability of Salix polaris samples used in the constructed chronology.
Values of trend coefficients of climate and tree-ring variables and their statistical characteristics.
Variable | Trend coefficient | R | R2 | p< | N | Trend per decade |
---|---|---|---|---|---|---|
Mean growth ring width | 1.20 (± 0.50) | 0.33 | 0.11 | 0.019 | 50 | 12.0 μm |
Maximum growth ring width | 3.50 (± 1.10) | 0.42 | 0.17 | 0.003 | 50 | 35.0 μm |
Annual air temperature | 0.10 (± 0.02) | 0.70 | 0.49 | 0.000 | 34 | 1.1 °C |
Annual precipitation | 4.27 (± 1.51) | 0.45 | 0.20 | 0.008 | 34 | 42.7 mm |
The developed chronology shows very high inter-annual variations expressed by quite a high value of the mean sensitivity (0.56) (Table 1). The Expressed Population Signal values were slightly below the 0.85 threshold (Wigley
Consistent extreme years (narrow or wide ring widths) could be distinguished on many sections and in the resulting chronology. We found 13 extreme years in the Hornsund chronology, of which nine were negative: 1958, 1967, 1975, 1980, 1981, 1989, 1999, 2006 and 2010, and four were positive: 1963, 1966, 1976 and 1997. Positive years are usually the effect of regeneration after unfavourable conditions in the previous years. Negative years are much more important for dendroclimatological and ecological analyses, indicating unfavourable environmental conditions. Interpretation of extremely low growth or even a missing ring on the basis of existing meteorological data for the Hornsund station showed that there is no a single factor influencing growth of polar willow.
Detailed interpretation of the extreme years showed that very narrow rings occur when a few unfavourable climatic factors before or during the year of ring formation act together (Table 3). Despite the fact that there were not enough observational data for the years 1958, 1967 and 1975, we can trace the course of the weather in subsequent extreme negative years since 1978. In the year 1980 low annual air temperature of the previous year and the second largest absolute minimum of the air temperature were accompanied by a number of days with positive mean diurnal air temperature during winter. Such thermal fluctuations could have damaged the plant tissue and previous year's shoots. Another threat for the dwarf shrubs in 1980 was very low precipitation that spring (May-June). Similar meteorological situations, i.e. the occurrence of a number of days with positive temperatures in winter, were observed in 2006. That year had up to 13 days with positive temperature in January and the highest absolute maximum of air temperature in January (4.9°C) recorded for Hornsund. Despite the high precipitation in January 2006 (321% of the norm for the reference period, 1978–2011), the above average temperatures led to fast snow melt and consequently very low mean thickness of snow cover in January. The lack of any substantial snow cover along with the lowest recorded absolute minimum of air temperature (–35.9°C) that same month, could have caused damage to the dwarf shrubs, and thus explain the very narrow ring produced later that year. Additionally, the lowest recorded absolute minima of air temperature in May and June of that same year (2006) and the lack of snow cover at that time would have caused a water shortage for plants and thus also explain the formation of a very narrow ring. In 2010, a rapid thaw took place in January, an effect of nine days with positive temperature. Additional occurrence of liquid precipitation resulted in only 26 days with snow cover. After episodes of snow melt, a rapid temperature drop took place (Fig. 9). Lack of a deep and stable snow cover which isolates and protects
Climatic conditions of the negative pointer years.
Year | The annual air temperature of the previous year | Absolute minima of air temperature | Number of days with positive mean diurnal air temperature in winter (Jan-Feb) | Totals of atmospheric precipitation in May | Percentage of relative sunshine duration in July | Sunshine duration in July | Mean thickness of snow cover in January | Mean thickness of snow cover in May | Mean thickness of snow cover in June | Number of days with snow cover in June | |
---|---|---|---|---|---|---|---|---|---|---|---|
(°C) | (°C) | (mm) | (%) | (hours) | (cm) | (cm) | (cm) | ||||
Reference period means | 1978-2012 | -4.1 | -26.9 | 3.3 | 20.1 | 20.9 | 155.4 | 17.9 | 28.4 | 13.8 | 13.8 |
Negative pointer years | 1958 | - | -24.8 | - | - | - | - | - | - | - | |
1967 | - | - | - | - | - | - | - | - | - | ||
1975 | - | - | - | - | - | - | - | - | - | ||
1980 | -6.2 | -30.2 | 5 | 9.1 | 17 | 44 | 21 | 10 | |||
1981 | -5.8 | -35.9 | 0 | 26.3 | 19.5 | 145.2 | 7 | 40 | 35 | 30 | |
1989 | -7.3 | -28.5 | 0 | 9.8 | 22 | 7 | 0 | 0 | |||
1999 | -4.9 | -22.9 | 1 | 21.5 | 159.8 | 14 | 11 | 6 | 4 | ||
2006 | -2.6 | -20.5 | 45.6 | 7.1 | 5 | 0 | 0 | ||||
2010 | -2.7 | -23.7 | 9 | 20.3 | 78.9 | 21 | 13 | 5 | 10 |
Unfavourable climatic conditions marked grey
Annual course of daily mean air temperature and number of hours with liquid precipitation in 2010 in the Polish Polar Station, Hornsund (GLACIO-TOPOCLIM Database, 2014).
The reason for extremely narrow growth of polar willow could have also been very low precipitation at the beginning of the growing season accompanied by a lack of snow cover, which severely restricted water availability to the shrubs and resulted in the formation of a very narrow ring. Low amount of spring precipitation is rather typical for this area but exceptional years with values, which are significantly below the average, can be distinguished. The growing periods for
In addition, most of the extreme years were characterised by very low (above average for the reference period) sunshine duration in summer (Table 3). Sunshine is not the growth limiting factor at this site, but its values are connected to temperature, which logically drives the tree growth during the polar summer.
What was received in the course of the study is a 61-year-long chronology, which is similar to the results from the other parts of the Arctic (Woodcock and Bradley, 1994; Zalatan and Gajewski, 2006; Forbes
The extensive warming and changes in precipitation of the Svalbard archipelago during the last few decades correlates with the increase of growth rings width. These data confirm the observations over the Arctic areas where the fast expansion of tundra shrubs is observed (Forbes
During the last few years there has been a significant progress in the development of the Arctic plants chronologies. However, synchronisation of these chronologies can be difficult due to ecological disturbances, for example the impacts of microtopography, microclimatic differences, soil conditions, influence of animals, insect defoliation (Büntgen and Schweingruber, 2010). Therefore there is a need for developing numerous site chronologies from different parts of the Svalbard archipelago in order to recognise common growth patterns that can be associated with climate variability. Southwestern Spitsbergen, due to its location on the southern edge of Svalbard archipelago, is an important area for climate reconstruction and prediction in the High Arctic. It should be added that some extreme pointer years reconstructed in this study are in accordance with those from different parts of the Arctic (Fig. 10). The negative years 1980–1981 in this study can also be observed in S. arctica chronology from Ellesmere Island (Woodcock and Bradley, 1994), S. polaris from the central part of Spitsbergen (Buchwal
Tree-ring chronologies for different location of North Atlantic sector, including southwestern Spitsbergen and north coast of Eurasia: A — Salix polaris from Svalbard (this study), B — Betula pubescens from Troms Region, northern Norway (Opala et al. 2014), C — Salix lanata from Nenets Autonomous Okrug, northwest Russia (ITRDB, 2015a and Forbes et al., 2010), D — Pinus sylvestris from Kola Peninsula, northwest Russia (ITRDB, 2015b and Gervais and MacDonald, 2000). Black circles indicate negative pointer years common within all chronologies. Red circles indicate negative pointer years present only in single locations (not observed in Hornsund site).
The results of this study support the "snow-shrub hypothesis" developed in earlier studies in the Arctic (Hal-linger
Strong positive correlation of growth ring width with summer temperature was not observed in the Hornsund area (Owczarek
Non-climate factors can also affect growth ring widths, and the resulting chronology curve, independent of climate. The influence of the geomorphological processes on the growth ring width of the dwarf shrub, reported by Owczarek
During the last decade there has been a significant increase in the number of the Arctic plants chronologies due to a high sensitivity of tundra vegetation to modern climate change. However, synchronisation of these chronologies can be difficult due to local site influences. Therefore, there is a need for developing numerous site chronologies from different parts of the High Arctic in order to recognise a common growth pattern that can be associated with climate variability. Consistent extreme pointer years (very narrow or wide ring widths) could be distinguished in many sections of constructed chronology. We found 13 extreme years in the Hornsund chronology, of which nine were negative: 1958, 1967, 1975, 1980, 1981, 1989, 1999, 2006 and 2010, and four were positive: 1963, 1966, 1976 and 1997. The occurrence of climate extremes for the Hornsund meteorological station is concurrent with the dendrochronological pointer years for the analysed polar willow. The presented results indicate the importance of deep snow cover for isolation, frost damage protection and also as a water reservoir during the growing season for