J For Res (2009) 14:147–154 DOI 10.1007/s10310-009-0123-y
ORIGINAL ARTICLE
Tree-ring-width chronology of Larix gmelinii as an indicator of changes in early summer temperature in east-central Kamchatka Masaki Sano Æ Fumito Furuta Æ Tatsuo Sweda
Received: 3 October 2008 / Accepted: 10 February 2009 / Published online: 11 March 2009 Ó The Japanese Forest Society and Springer 2009
Abstract We developed a 378-year tree-ring-width chronology based on 110 core samples from 55 individual trees of Larix gmelinii (Rupr.) Rupr. growing in a wide open forest close to the tree line in the Kronotsky National Park. Reflecting strong climatic control over tree growth not only within the study area but also more extensively over central Kamchatka, our chronology was well correlated with those from other larch sites. Response analysis with 10-day mean temperature revealed that the ring width was primarily controlled by the temperature of early summer, i.e., of late May through late June (40 days). While the regression models for a formal reconstruction failed to pass stringent verification tests commonly used in dendroclimatology, the relationship between tree growth and climate was statistically significant and credible. We therefore used our chronology as a proxy of early summer temperature. The chronology shows a cool period from the 1660s until the 1680s, followed by gradual warming until ca. 1800, then by a slight cooling trend extending to ca. 1910, and a warming trend continuing up to the present, with decadal fluctuations throughout the chronology. The warming trend found in our chronology over the twentieth century is generally consistent with the ones commonly appearing in higher latitudes. Keywords Climate change Dendrochronology Kamchatka Larch
M. Sano F. Furuta T. Sweda United Graduate School of Agricultural Sciences, Ehime University, Matsuyama, Japan M. Sano (&) Physical Research Laboratory, Ahmadabad 380 009, India e-mail:
[email protected]
Introduction To understand the effects of increasing anthropogenic greenhouse gases on the climate system, knowledge of past climate variability extending back into the pre-industrial era is required (Bradley and Jones 1992). However, the instrumental climate records are limited to the past 150 years (Jones et al. 1986). Therefore, a wide variety of studies have been conducted to reconstruct past climate for various regions of the world using such proxy records as tree rings, corals, ice cores, and boreholes. Tree rings have been used as one of the best natural archives of past climate. However, tree-ring chronologies originate mainly from North America and Europe. Considerable effort has therefore been devoted to regional expansion of chronology building over the last couple of decades. Siberia is a region which has received great attention in this regard (e.g., Briffa et al. 1995, 2002; Jacoby et al. 2000; Hughes et al. 1999; MacDonald et al. 1998; Shiyatov et al. 1996; Vaganov et al. 1999). However, there are very few tree-ring chronologies from far eastern Russia. The Kamchatka peninsula is one of the most suitable regions for dendroclimatology because of its pristine nature with virgin forests growing in a harsh environment, in which tree growth is highly sensitive to climate fluctuations (Fritts 1976). Another advantage is that glaciers are scattered along mountain ranges, enabling ice-core-based climate reconstruction (Shiraiwa and Tchoumitchev 2002), which can be utilized as a counterpart to dendroclimatic studies and thus be used to verify results. The first published tree-ring chronology from Kamchatka was constructed by Yadav and Bitvinskas (1991). They indicated that the chronology based on ring widths of larch can be used as a useful indicator of past volcanism in Kamchatka, with eruptions corresponding to growth reduction.
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Climatological studies were also conducted for Kamchatka, and revealed that ring widths of larch showed significant associations with climate variables (Gostev et al. 1996; Solomina et al. 1999, 2000; Takahashi et al. 2001). This climatic response was generally seen as a positive correlation with May and June temperatures. Based on the climatic response of larch growth, Gostev et al. (1996) reconstructed May–June temperature variations back to AD 1670. More recently, Solomina et al. (2007) developed a regional larch chronology as a proxy of summer temperature, based on ring widths of 144 samples from several sites in Kamchatka. An increasing number of studies indicate that the role of temperature on larch growth at the northern tree lines in Siberia is limited to less than 2 months (e.g., Hughes et al. 1999; Vaganov et al. 1999; Kirdyanov et al. 2003). In such cases, temperature records arranged on a 5- or 10-day basis are effective for clarifying climatic response of tree growth, whereas monthly climatic data typically apply to tree-ring records obtained from relatively lower latitudes. Response analysis with the shorter time intervals, which has not yet been conducted for Kamchatka, helps narrow down the influence of temperature on tree growth, possibly in relation to leaf development followed by photosynthesis. In the present study, we developed a ring-width chronology extending back to AD 1624 from larch with a robust sample depth and a chronology signal strength back to 1660. The chronology was then subjected to response analysis using 10-day mean temperature data in order to identify the most important interval contributing to tree growth. Based on these results, our chronology was used as an indicator of early summer temperature, and compared with other ring-width chronologies available for Kamchatka and higher latitudes.
Materials and methods Study site and sample trees To develop a climatically sensitive chronology, sampling was conducted in Kronotsky National Park where the alpine tree line is widely spread and human disturbances have been minimal due to the limited accessibility (Fig. 1). Distribution of tree species in the park is conventionally described thus: alluvial flat valleys and gradual ridges are dominated by Larix gmelinii (Rupr.) Rupr., whereas steep slopes are covered by patches of Pinus pumila (Pallas) Regel, Betula ermanii Chamisso, Salix sp., or Alnus sp. Of these species L. gmelinii is the oldest, and exhibits appreciable variations in ring width from year to year, indicating high sensitivity to climate. We therefore decided to collect samples from L. gmelinii.
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To obtain samples from the most sensitive trees, sampling was carried out at a site close to the tree line of 700 m a.s.l. on a wide and gentle ridge. The site was characterized by wide-open larch forest. L. gmelinii of 10–12 m in height grew coupled with P. pumila of 2–3 m, comprising patches of undergrowth represented by a mixed carpet of Vaccinium vitis-idaea L. and Empetrum nigrum L. var. japonicum K. Koch. Paired increment cores were bored at breast height (1.3 m above ground) from each of 55 trees for a total of 110 core samples. It should also be noted that the undergrowth carpet was dotted with larch seedlings ranging from 20 to 300 cm in height, suggesting that growing conditions have been improving in the past couple of decades, probably due to recent warming. Classification of those species was based on Kojima (1997) and Okitsu (2002). Chronology development All cores were polished with progressively fining sandpapers, and then were visually cross-dated using standard methodology (Stokes and Smiley 1968; Fritts 1976). Subsequently, ring widths were measured to an accuracy of 0.001 mm using a sliding stage micrometer interfaced directly with a computer for measurement capture.
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Ring-width measurement series were then assessed for dating accuracy using the software COFECHA (Holmes 1983), with which individual ring-width series were tested on the basis of correlation against a master series derived by averaging all the series. The mean ring-width series for each tree were obtained by averaging two series from a given tree, and then standardized using the software ARSTAN (Cook 1985) to detrend the declining growth pattern due to aging (Fig. 2), and at the same time to minimize the removal of long-term climatic variance. For this purpose, either a negative exponential curve or a straight line was fitted for the tree series. Each series measurement was then divided by the corresponding calculated value for each year. The resultant dimensionless indices with a mean of 1.0 enabled us to compare trees with different growth rates. The final chronology was built by combining all standardized series for each year. In order to evaluate the weakening representation of the chronology due to decreasing sample size back to the past, we calculated the moving expressed population signal (EPS) (Wigley et al. 1984), which indicates how well the chronology estimates a theoretically infinite population. The EPS is a function of the Rbar, which is the average correlation between tree-ring series (Cook and Kairiukstis 1990), and sample depth. The higher the Rbar and/or the greater the number of samples, the closer the EPS is to a value of 1. A value of 0.85, which has been proposed as an acceptable threshold for attaining a robust mean function by Wigley et al. (1984), was used to determine the cut-off point in the earlier portion of the chronology. Fig. 2 Measured ring-width series of 55 individual larch trees. To easily recognize the declining growth trends due to aging, the series were divided into three parts based on the series length
Climatic response of tree growth To investigate the climatic response of tree growth, first of all we chose one of two instrumental climate stations, i.e., Kljuci (AD 1927–1995) and Kronok (1939–2000), close to our sampling site (Fig. 1). The mean monthly temperature and precipitation are presented in Fig. 3 as the general climate calculated for the common period of both records. Judging from the temperature gradient and total amount of precipitation, the climate of Kljuci and Kronok can be characterized as featuring continentality and oceanicity, respectively. While the Kljuci station was farther from our study site, tree growth was correlated much better with climatic variables of the Kljuci records than with those of the Kronok counterparts. In addition, the results based on the Kljuci records were consistent with previous reports on the climatic response of larch in the interior of Kamchatka (Gostev et al. 1996; Solomina et al. 1999). We thus determined to use the Kljuci data in the present analysis. The Kljuci records obtained from online datasets of the All-Russian Research Institute of Hydrometeorological Information (http://www.meteo.ru/data/emdata.htm) consist of daily temperature and precipitation. We also determined to abandon a portion of the climate records because of an inherent difficulty encountered in response analysis. Our larch chronology showed abrupt growth reduction appearing in 1976 followed by growth recovery extending to the late 1980s, during which ringwidth variations cannot be explained by climatic factors, as will be discussed later. Therefore, climate data after 1975
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were excluded from the following analysis, resulting in the period of 1927–1975 being commonly available for climate and tree-ring data. Before conducting response analysis, the most suitable growth year was defined to distinguish it from the calendar year. Tree growth at the sampling site was considered to shut down by the end of September since the local mean temperature of the period is estimated to plunge below 5°C. Thus, the growth year was defined as beginning in October of a prior calendar year and ending in September of the given year. Then as response analysis, simple correlations between the larch chronology and monthly climatic variables (temperature and precipitation) of the current and prior growth years were calculated for the period 1927– 1975. As will be noted later, this monthly-based response analysis revealed that the most important climatic factor contributing to tree growth was the May–June temperature of the current growth year. Therefore, the chronology was also correlated with 10-day mean temperature spanning the period from early April through late July. The response analysis with the shorter time interval, basically following that by Hughes et al. (1999) and Vaganov et al. (1999) who cut the interval as short as 5 days for analysis of tree growth at high latitudes of Eurasia, turned out to be effective where the growing season is minimally short.
Results and discussion Chronology signal strength All cores were successfully cross-dated due to the frequent appearance of characteristic rings common to the majority of the cores. In reflection of the harsh climate, missing
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rings were also common. They appeared 18 times in the last 378 years, averaging one every two decades. In many cases, they appeared in a few trees, but in 1877, 1905, and 1976, some 20% of the trees sampled shared missing rings. The established chronology extending back to AD 1624 and its sample depth are presented in Fig. 4a and b, respectively. The standardized ring-width series were well correlated between trees as shown in Fig. 4c, which represents the running Rbar (the average correlation between trees). The running EPS attained the generally accepted threshold value of 0.85 or greater back to 1660, but fell below this criterion further back as sample size diminished (Fig. 4c). Accordingly, the earliest data prior to 1660 were disregarded in the following analysis. In manifestation of strong climatic control over tree growth not only within the study area but also more extensively over central Kamchatka, our larch chronology was very strongly correlated with that of Shiyatov (not published, but chronology available from International Tree-Ring Data Bank) from Esso (150 km to the northwest; see Fig. 1), with as high a correlation as 0.70 (P \ 0.00001) for the common interval of 1690–1983 (n = 294). As noted earlier, our larch chronology showed abrupt growth reduction appearing in 1976 followed by growth recovery extending to the late 1980s. In order to identify the cause, we scrutinized instrumental climatic records, local volcanic history, and large-scale forcing effects of Pacific Decadal Oscillation (Mantua et al. 1997). It turned out, however, that none of them sufficiently accounted for the reduced growth. These results coupled with clear evidence that growth suppression did not appear in the Shiyatov’s chronology from Esso suggest that an unknown local disturbance occurred in the present study site.
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The mean segment length of our chronology was 224.7 years with a standard deviation of 77.3 years, indicating that multi-decadal to centennial scale variability was adequately retained (Cook et al. 1995). Response to climatic variables Figure 5a shows response of tree growth to monthly mean temperature and total precipitation in terms of simple correlation. A significant positive correlation was seen with temperature in May and June of the current growth year, with the highest correlation appearing in June. A similar response of ring width to monthly temperature of the current growth year was found in larch trees from Tolbachik (100 km to North; Solomina et al. 1999) and Esso (150 km to the northwest; Gostev et al. 1996), indicating that May and June temperatures primarily govern radial growth of larch growing in central Kamchatka. Figure 5b, which is the same as Fig. 5a but for the 10-day mean temperature, highlights the relationship between tree growth and the early summer temperature. The significant correlation was limited to less than 2 months, i.e., from late May through to mid-June. This suggests that the temperature of this period plays an important role in promoting leaf development followed by photosynthesis, which is supported by the fact that larch in Kamchatka begins to sprout in May (Yadav and Bitvinskas 1991). The climatic response roughly shares not only the short interval contributing to tree growth but also the local temperature of the earliest period showing a
significant correlation with the climatic responses of several larch species from Siberia (Hughes et al. 1999; Vaganov et al. 1999; Kujansuu et al. 2007). The fact that several species respond in a similar fashion to the early summer temperature gives hope for large-scale reconstruction of temperature using multiple species from multiple locations across far northeastern Eurasia. The negative effect of August temperature in the previous growth year on ring width might be attributed to a higher rate of respiration. Net photosynthesis reduction induced by increasing respiratory rates with rising temperature (Kramer and Kozlowski 1960) results in growth suppression for the following year. We also found a positive correlation with precipitation in September of the current growth year. However, the trees are considered to lie dormant because radial growth of the larch, Larix leptolepis, growing even in Japan does not continue until September (Tadaki et al. 1994), and thus the rainfall effect is negligible. Based on the above analyses, we finally identified that the 40-day period (henceforth defined as early summer) from late May to late June optimized temperature signal in the chronology. While the regression models for a formal reconstruction failed to pass stringent verification tests commonly used in dendroclimatology (e.g., Fritts 1976; Cook and Kairiukstis 1990), the relationship between tree growth and climate was statistically significant and credible. We therefore interpret our chronology as an indicator of early summer temperature in the following section.
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temperature at the sampling site, as estimated by applying the lapse rate (–0.5°C per 100 m) of temperature to Kljuci records. Asterisks indicate significance at 5% level
Comparison with other proxy records
continuing up to the present, with decadal fluctuations throughout the chronology. Our chronology was compared with a reconstruction of May–June temperature in Esso (Gostev et al. 1996) and a regional larch chronology representing summer temperature in central Kamchatka (Solomina et al. 2007). As a whole, the multi-decadal to century-scale warming and cooling trends appearing in our chronology are shared. More specifically, 20-year warm periods centered on ca. 1750 and ca. 1800, and cold periods of the 1760s–1770s and 1860s–1880s, are found in all records. The synchronicity among them indicates that our chronology represents temperature variations of a large area in the interior of central Kamchatka. These three tree-ring records also share an increasing trend in the twentieth century with d18O variations obtained
Figure 6a represents our tree-ring chronology and early summer temperature over the entire period of climate records available (1927–1995), in order to highlight the divergence problem as noted earlier. The chronology for the period from 1975 back to 1660 is shown with the early summer temperature in Fig. 6b. Since our chronology failed to capture a temperature trend for the last quarter of the twentieth century due to the divergence, the chronology connected to the instrumental records of 1976–1995 was used to evaluate temperature variations. It shows a cool period from the 1660s until the 1680s, followed by warming gradually until ca. 1800, then by a slight cooling trend extending to ca. 1910, and a warming trend
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from an ice core at Ushkovsky volcano in central Kamchatka (Shiraiwa and Tchoumitchev 2002). It should be duly noted, however, that the reconstruction of Gostev et al. (1996), the chronology of Solomina et al. (2007) and the ice core records (Shiraiwa and Tchoumitchev 2002) show decreasing trends for the last quarter of the twentieth century, whereas the instrumental records at Kljuci show a warming trend (Fig. 6). Although our chronology is influenced by disturbance after 1975, and therefore we cannot evaluate tree-ring-based temperature variations for the period in the long-term context, our chronology combined with the Kljuci records indicates that temperature at our study site increased during the twentieth century. Our finding of the twentieth century warming trend is further supported by temperature reconstruction derived from the inversion of borehole temperature profiles around Petropavlovsk-Kamchatsky (Cˇerma´k et al. 2006). In addition, the warming trend during the last century is broadly consistent with the ones appearing in many dendroclimatic reconstructions in higher latitudes over the last century or two (e.g., Jacoby and D’Arrigo 1989; Sweda 1994; Briffa et al. 1995; MacDonald et al. 1998; Hughes et al. 1999).
We duly note that prominent cold periods detected widely in higher latitudes in the nineteenth century (e.g., Jacoby and D’Arrigo 1989; Graybill and Shiyatov 1992; MacDonald et al. 1998) were not found in our chronology, whereas other chronologies from the interior of Kamchatka (Gostev et al. 1996; Solomina et al. 2007) reveal that the 1860s–1880s were the coldest interval in the last 350 years. In conclusion, we developed a 378-year ring-width chronology from Larix gmelinii growing in east-central Kamchatka. Response analysis revealed that ring width was primarily controlled by 40-day temperature spanning the period from late May through late June. This suggests that the temperature of the period plays an important role in promoting leaf development followed by photosynthesis. Perhaps the most striking feature of our chronology coupled with the instrumental records is a warming trend over the twentieth century. The warming trend is broadly consistent with the ones commonly appearing in higher latitudes. Continued effort toward the development of a multi-species tree-ring network combined with other proxy records will shed more light on the climate variability in Kamchatka.
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154 Acknowledgments We thank Y.D. Muravyev for his assistance in our sample collection in Kamchatka. We are also grateful to M. Yamano and T. Nagao for inviting us to this study as a part of their paleoclimate reconstruction project primarily based on inversion of borehole temperature profiles. We also thank three anonymous reviewers for valuable comments and suggestions for improving this manuscript. This study was funded by the 2000–2002 Grant-in-Aid for Overseas Scientific Research (B) 12573015 from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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