Diverging climate trends in Mongolian taiga forests influence growth and regeneration of Larix sibirica
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- Dulamsuren, C., Hauck, M., Khishigjargal, M. et al. Oecologia (2010) 163: 1091. doi:10.1007/s00442-010-1689-y
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Central and semiarid north-eastern Asia was subject to twentieth century warming far above the global average. Since forests of this region occur at their drought limit, they are particularly vulnerable to climate change. We studied the regional variations of temperature and precipitation trends and their effects on tree growth and forest regeneration in Mongolia. Tree-ring series from more than 2,300 trees of Siberian larch (Larix sibirica) collected in four regions of Mongolia’s forest zone were analyzed and related to available weather data. Climate trends underlie a remarkable regional variation leading to contrasting responses of tree growth in taiga forests even within the same mountain system. Within a distance of a few hundred kilometers (140–490 km), areas with recently reduced growth and regeneration of larch alternated with regions where these parameters remained constant or even increased. Reduced productivity could be correlated with increasing summer temperatures and decreasing precipitation; improved growth conditions were found at increasing precipitation, but constant summer temperatures. An effect of increasing winter temperatures on tree-ring width or forest regeneration was not detectable. Since declines of productivity and regeneration are more widespread in the Mongolian taiga than the opposite trend, a net loss of forests is likely to occur in the future, as strong increases in temperature and regionally differing changes in precipitation are predicted for the twenty-first century.
KeywordsClimate variability Drought Forest-steppe ecotones Global warming Tree-ring width
The southern limit of the Siberian taiga, the earth’s largest continuous forest area, is located in Mongolia. Southwards, the taiga is displaced by the vast Eurasian steppe belt. Whereas the position of the forest-steppe ecotone has been modified by human activities, including livestock breeding, logging, and arson, the lack of conifers in the steppe is principally due to drought (Gunin et al. 1999; Dulamsuren et al. 2009a, b). This becomes apparent from fluctuations of the forest-steppe borderline along with changes in temperature and precipitation throughout the Holocene (Tsedendash 1995; Gunin et al. 1999; Miehe et al. 2007). Siberian larch (Larix sibirica Ledeb.), Mongolia’s most common forest tree, covering not less than 80% of the forest area (Tsogtbaatar 2004; Dugarjav 2006), migrated northward by 2° latitude during the past 4300 years (Dinesman et al. 1989).
Mongolia’s average temperature has increased by 1.7°C since the 1940s (Batima et al. 2005). During the next 80 years, the temperature is predicted to increase by 2°C in summer and 1°C in winter (Sato and Kimura 2006). The frequency of heat waves increased during the past decades (Nandintsetseg et al. 2007), while approaches of cold fronts from Siberia became rarer, reducing the occurrence of storms especially in spring (Hayasaki et al. 2006; Sato and Kimura 2006). The permafrost area is currently reduced (Bohannon 2008) and likely to vanish from Mongolia during the twenty-first century (Stendel and Christensen 2002; Böhner and Lehmkuhl 2005). Permafrost is not only an essential buffer for the vegetation, alleviating summer droughts (Sugimoto et al. 2002), but also an important carbon pool (Nelson 2003; Zimov et al. 2006). Mongolia’s glaciers have declined by 10–30% in area during the past 60 years (Kadota and Davaa 2005). The response of precipitation to global warming is spatially heterogeneous (Morinaga et al. 2003; Endo et al. 2006), and therefore no significant twentieth century trend is found on a national scale (Batima et al. 2005). However, both regional decreases and increases of annual precipitation by several decimeters have been reported from individual weather stations during the second half of the twentieth century (Batima et al. 2005). For most of northern Mongolia’s taiga forest belt, precipitation and, it is thought, with it, soil moisture will decrease during the twenty-first century (Sato et al. 2007).
Materials and methods
Wood core sampling and processing
Analyzed wood cores of Larix sibirica
Age class (years)
The wood cores were sampled with increment borers of 5-mm inner diameter. The borer was driven into the wood parallel to the contour lines of the mountain slopes at 1 m above the ground to avoid compression wood. The wood cores were mounted on wooden strips and cut lengthwise with a scalpel; the contrast between annual tree rings was enhanced with chalk. The tree-ring widths were measured with a precision of 10 μm on a movable object table, the movements of which are electronically transmitted to a computer system equipped with time series analysis and presentation (TSAP)-Win software (Rinntech, Heidelberg).
Evaluation of tree-ring data
Tree-ring data were analyzed following the methods applied by Sarris et al. (2007) and Dulamsuren et al. (2010a) using TSAP-Win software. During cross-dating, the tree-ring series were controlled for missing rings and false rings, which are quite frequent in the semiarid environment of Mongolia in drought-limited forests. Cross-dating was based on the coefficient of agreement [Gleichläufigkeit values (GL); Eckstein and Bauch 1969] and t values (Baillie and Pilcher 1973). Trees of the same site were pooled by calculating mean values of the annual increment. Tree-ring series used for the calculation of means had GL > 65% and t values > 3 (>6 in ca. 90% of samples). Trend lines were calculated using moving 5-year averages.
The interannual (high-frequency) variation of climate was extracted by removing the age-related information from tree-ring width series. This is principally achieved by dividing the observed tree-ring width (ri) by the expected annual increment. The expected annual increment is not constant, but declines throughout the lifetime of a tree for two reasons: the same amount of wood has to be distributed over a larger circumference of the trunk from year to year, trees tend to grow faster during the first decades of their lifetime. Finding the correct function for the age-related growth trend (for estimating the expected growth rate) is easier in semiarid environments, including our study area, than in regions with a good water supply, as the stand density in semiarid woodlands is relatively low and, thus, the individual trees are less influenced by changes in the stand structure than in dense and moist forests (Cook 1985). Therefore, the same type of standardization (i.e., of removing the age-related trend) could be applied for all tree-ring series studied. The annual tree-ring index (zi) of year i was calculated with the equation zi = 100 × ri/mi, where mi is the 5-year moving average of year i.
In addition to high-frequency variation, the long-term (low-frequency) trend of tree-ring width over time was analyzed to detect long-term changes in climate occurring during the lifespan of the tree. Long-term climate trends can be identified by removing the annual variation of climate from the tree-ring series and conserving the age-related trend (Sarris et al. 2007; Dulamsuren et al. 2010a). If the annual increment is not related to the calendar year, but to tree age, a mean age-related growth curve can be established for a given site, which is largely independent of the annual variation of climate. Such functions are called regional growth curves (RGC; Briffa et al. 1992; Helama et al. 2004; Naurzbaev et al. 2004). Climate trends can be deduced from the RGC by comparing it with tree-ring series from trees of different age. We prefer the comparison of several (partial) RGC for trees of different age classes with one another, because the comparison of an individual growth curve to the RGC calculated for all trees might blur existing trends, as the individual tree-ring series is also included in the RGC. Age is generally specified as the age of the oldest tree ring (cambial age) at the sampling height of 1 m; ca. 10 (to 20) years should be added to deduce tree age from these age specification (Körner et al. 2005; Sankey et al. 2006). Age classes distinguished in the analyses include trees with a cambial age >90 years (old trees), between 50 and 90 years (middle-aged trees) and trees <50 years (young trees). Assuming a difference of 10 years between the cambial age and the year of germination, these groups correspond to tree ages of >100, 60–100, and <60 years, respectively.
Influences other than climate and tree age, including internal (e.g., the natural death of a neighboring tree) and external disturbances (e.g., insect infestations, fire) of stands (Dulamsuren et al. 2010b), tree-specific characters caused by genetic variations or the small-scale variation of site parameters (Wilmking et al. 2004) were minimized due to the large sample size and the collection of wood cores on several mountain slopes per study area.
The temporal development of the establishment of larch trees was analyzed in all 90 plots. Past forest regeneration was deduced from the starting points of the individual tree-ring series. The years of establishment (i.e., germination) of the individual trees were inferred from the wood cores taken at 1 m above the ground by adding 10 years to the year of the oldest tree ring. There is some uncertainty inherent in this assumption, as the exact number of tree rings, which are not detectable at 1-m height, is not known and underlies some tree-to-tree variation. This has to be considered when interpreting the abscissas of Figs. 4a, 7 and 9a. However, the expected shifts along the abscissa would amount only to 1 year or a few years and would thus not affect the principle information which can be extracted from these analyses. It is improbable that less than 10 years would have to be added (Sankey et al. 2006). Therefore, any error inherent in this method would lead to a small overestimation of recent regeneration. In addition to wood core sampling, all sample plots were thoroughly searched for seedlings and saplings below 1-m height to collect stem cross-sections. These samples taken from seedlings and saplings at <1-m height were used for the determination of the age structure, but were not included in the tree-ring chronologies.
δ13C signatures of tree-ring wood
Tree-ring wood from 1988 to 1997 and from 1998 to 2007 was dried at 105°C for 24 h, ground to a fine powder, and bulk samples for the 10-year periods (ca. 1 mg) were weighed in tin capsules for the determination of δ13C signatures. The analyses were conducted with a Delta V Advantage isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, Mass.), which was connected to an NA 1500 C/N Elementar Analyzer (Carlo Erba Strumentazione, Milan) via a Conflo III interface (Thermo Fisher Scientific). Acetanilide was used as an internal standard. Using this internal standard, the δ13C signature was related to the Pee Dee belemnite limestone standard using the equation δ13C (‰) = [(Rsample/Rstandard) − 1] × 1,000, with R = 12C/13C. Enrichment of 13C indicating drought stress results in high (less negative) values of δ13C.
Trends for annual, summer (June–August) and winter (December–February) mean temperatures in the Mongolian forest belt since 1961 (A), 1950 (B), 1937 (C) and 1942 (D)
y = −3.51 + 0.06x
y = 16.0 + 0.05x
y = −26.3 + 0.07x
Mean T (°C)
−2.0 ± 0.2a
17.1 ± 0.2
y = −4.07 + 0.08x
y = 14.9 + 0.04x
y = −25.6 + 0.14x
Mean T (°C)
−1.8 ± 0.2
15.9 ± 0.2
−21.5 ± 0.4
y = −0.60 + 0.03x
y = −13.1 + 0.02x
y = −15.2 + 0.04x
Mean T (°C)
−0.3 ± 0.1
13.9 ± 0.1
−14.0 ± 0.2
y = −1.23 + 0.02x
y = 17.6 + 0.00x
y = −22.2 + 0.04x
Mean T (°C)
−0.6 ± 0.1
17.7 ± 0.1
−21.0 ± 0.3
Trends for annual precipitation in the Mongolian forest belt since 1961 (A), 1950 (B), 1937 (C) and 1942 (D)
y = 330 − 2.36x
273 ± 11a
y = 251 − 0.27x
259 ± 10
y = 351 − 0.32x
340 ± 9
y = 207 − 0.76x
237 ± 7
Mongolia’s climate is characterized by the Asiatic anticyclone in winter, which typically has its center southwest of Lake Baikal and causes dry and cold winters. In summer, warm air masses from the south flow into northern Mongolia resulting in the formation of cyclones when they meet the cold air from Siberia. Therefore, most precipitation is received during summer. Mongolia’s average temperature shows a long-term trend of increase throughout the period since 1940 when sufficient instrumental data are available for the calculation of a regional average (Batima et al. 2005). Consistent with the global development of temperatures, the period from 1940 to 1960s was generally cooler than the subsequent decades.
In addition to declining growth of already established trees, failure of forest regeneration (Fig. 4a) is a threat for the persistence of the larch forests covering the mountain slopes of the north-western Khentey. After a period of rich regeneration in the 1930s and 1940s, the rejuvenation of larch strongly decreased concomitant to the decline of the annual increment of mature trees (Fig. 4a). Since the 1970s, larch seedlings have been virtually absent in the mountain forests (Fig. 4a), while they still occur in flood plains.
Spatio-temporal fluctuations of L. sibirica forest lines in Mongolia’s forest-steppe ecotone have often been discussed in the context of a potential encroachment of trees into the steppe, because L. sibirica was thought to be excluded from many steppe areas only by livestock grazing and not for climatic reasons (Korotkov and Dorjsuren 1988; Hilbig 1995; Sankey et al. 2006). Recent studies in the north-western Khentey suggested that natural site factors alone are sufficient to prevent L. sibirica from establishing in grasslands along the lower forest line at least in that particular area. Relevant natural site factors include water shortages and high soil temperatures as well as herbivory by insects and small mammals (Dulamsuren et al. 2008, 2009a, 2010a, b; Hauck et al. 2008). The present results indicate that the potential of L. sibirica to invade the steppe has continuously decreased during the late twentieth century. Declines in the annual stem increment and, even more importantly, in regeneration suggest that in parts of the Mongolian forest-steppe ecotone the conversion of forests into steppe is much more probable than the succession of grassland to Siberian larch forests.
The comparative study of four regions within the Mongolian forest zone showed that twentieth century climate trends and responses of L. sibirica to changes in temperature and precipitation underlie a striking spatial variation. The differences between the three areas in the Khentey Mountains are especially remarkable, because they are only separated by 140–200 km. Within this small area, locations both with increasing and decreasing aridity are found and result in diverging responses of L. sibirica. The increase by 4.4°C within 55 years in the south-western Khentey is notable for its scale, but agrees with the general high increase in temperature in central Asia during the second half of the twentieth century, which exceeded the global trend by a factor of 3–4 (IPCC 2007).
Increased drought during the growing season due to the simultaneous increase in summer temperature and decrease in precipitation, as in the north-western Khentey, is obviously most detrimental for L. sibirica, as its growth is strongly reduced and reproduction fails to occur. This conclusion agrees with the δ13C signatures as well as the results of climate response analyses of tree-ring widths in Larix sibirica from the north-western Khentey (Dulamsuren et al. 2010c) and Larix gmelinii in central Siberia (Sidorova et al. 2009). The present lack of larch seedlings on the mountain slopes of the north-western Khentey is not only attributable to an inhibition of germination by high soil temperatures and low soil moisture (Dulamsuren et al. 2008), but also to the conspicuous lack of cones of larch trees in this area. This contrasts with other forest regions of Mongolia, where Siberian larch is often richly fertile. Increased temperatures during the growing season at constant precipitation, as in the south-western Khentey or the eastern Khangay, lead to less rigorously reduced growth and regeneration of L. sibirica. This is consistent with the fact that the tree line to the steppe is drought limited (Dulamsuren et al. 2009a), in contrast to alpine tree lines (Jacoby et al. 1996; D’Arrigo et al. 2000). Increased annual precipitation at constant summer temperature, as found in the south-eastern Khentey, results in constant or even improved growth and regeneration. Even >300-year-old larch trees are capable of resuming increased growth rates as a response to improving climatic conditions. This is inferred from Fig. 8, where an increasing annual increment is observed after 80 years of depressed growth at the end of the Little Ice Age in the late nineteenth century (Pederson et al. 2001). This behavior is characteristic of most old trees established in the eighteenth century or earlier. Crucial for the increased growth and regeneration of L. sibirica in the south-eastern Khentey is the increase in annual precipitation, and not the simultaneously increasing winter temperatures, as can be inferred from the summer drought-induced reduced tree-ring widths and regeneration in the south-western Khentey, despite the remarkable increase in winter temperatures since 1950 by 7.1°C (December–February), which is far above that in the south-eastern Khentey or the Mongolian average (Jacoby et al. 1999; Batima et al. 2005).
The present results for the Khentey and Khangay Mountains indicate that the improvement of growth conditions for L. sibirica by climate change may occur much more rarely in Mongolia than their deterioration. Considering the dominance of L. sibirica in the Mongolian taiga, this suggests a future loss of forest area, though Siberian larch might locally encroach into the steppe in the south-eastern Khentey and potential areas with similar climate trend. In addition to the direct effect of climate, more frequent forest fires and insect calamities in areas with increased aridity (Oberhuber 2001; Chapin et al. 2004; Hauck et al. 2008) may deteriorate conditions both for mature trees and seedlings of L. sibirica. Insect herbivory and fire, however, are unlikely to be the cause of the observed variation in tree-ring widths and regeneration between the four study areas, as three to five mountain slopes per area were studied. Insects or fire probably cause considerable variation between slopes of the same study area and the individual replicate plots on each slope, which was not observed.
In the case of the north-western Khentey, livestock grazing can be ruled out as the cause of the present lack of larch regeneration, in contrast to many other places in Mongolia. The present and historic land use of the specific study area in the north-western Khentey is known in detail (Schlütz et al. 2008). Pastoral nomads traditionally avoid the region with their livestock because of the poor accessibility and the small size of good pastures as well as the abundance of carnivores, including wolves and bears (Schlütz et al. 2008). The population densities of large herbivores, including deer, are low due to widespread poaching. In the south-western Khentey, increased livestock grazing can also be ruled out as the cause of reduced regeneration in the late twentieth century, since the plots studied there are located in one of the world’s oldest nature reserves at Mt. Bogd Uul (Hilbig et al. 2004). Therefore, increased aridity is the most plausible cause of the recently reduced or lack of reproduction in L. sibirica on mountain slopes. Recent larch regeneration in floodplains, even in the north-western Khentey, supports this conclusion. The dependence of seedling emergence on climate is also evident from parallels of peaks in the annual increment of mature trees with peaks in seedling establishment in all study areas (Figs. 4a, 7, 9a). Rich regeneration of larch in the 1930s and 1940s coincides with above-average precipitation throughout north-eastern Asia (Quian and Zhu 2001; Zhang et al. 2003). Relatively low documented numbers of seedlings in years of high annual stem increment before 1920 (Fig. 4a) are probably due to timber logging on the plots of the north-western Khentey in the late 1970s and 1980s (Schlütz et al. 2008), whereas the relative significance of past logging activities or natural mortality are not documented for the other study areas.
Though Matveev and Usoltzev (1996) found regeneration peaks in Siberian larch after fire due to the degradation of litter and herbs as well as reduced competition for water and nutrients, Danilin (1995) emphasized that fire only enhances the regeneration of Siberian larch, but is not mandatory. This agrees with our long-standing field experiences from the north-western Khentey, where larch seedlings in the river valleys definitely establish without preceding fire. Furthermore, sowing experiments in the western Khentey showed the germination of Siberian larch seeds in the field without any influence of fire (Dulamsuren and Hauck 2008). The slower growth of young trees than of old trees at a given age in three out of four study areas is not attributable to the assumption that the old trees represent the most quickly growing trees of their generation. Such an explanation would not match with the faster growth of young than old trees at a given age in the south-eastern Khentey. If effects of stand density play a key role in the tree-ring width of young trees, the stand density in the south-eastern Khentey should be lower than in the other study areas. However, Fig. 2 shows that the plots of the south-eastern Khentey are evenly distributed along the range of stand densities observed in the present study. This supports the hypothesis that the rich regeneration in the south-eastern Khentey and the high growth rates of the young trees result from the increased precipitation and not from the forest structure.
The high regional variability of global warming responses on a small geographical scale highlights that Mongolia’s natural forest resources have to cope with a highly diverse spatial pattern of climate trends. Regionally contrasting perspectives for the Mongolian taiga should be incorporated into the country’s development policy. Since declines of productivity and regeneration are more widespread than the opposite trend, a net loss of forests is likely to occur in the future, as strong changes in temperature and precipitation are predicted for the twenty-first century. Taiga forests harbor an important part of Mongolia’s biodiversity and are already under increasing pressure by the fast-growing Mongolian population, which heavily depends on the country’s natural resources. Any change in Mongolia’s vegetation pattern is not only a threat for Mongolia’s biodiversity and weak economy, but is also likely to interfere with the traditional nomadic lifestyle in the country. Tree increment is correlated with pasture biomass (Liang et al. 2003; Batima 2006). Over decades reduced stem increments may, thus, indicate an ongoing reduction of livestock carrying capacities in important rangeland areas of Mongolia and forewarn of a future disaster, as the Mongolian economy and cultural identity still heavily rely on nomadic pastoralism. Most of the 40% of the Mongolian population working in the agricultural sector pursues a nomadic lifestyle, which includes not only the utilization of pastures, but also of timber and other forest products (Neupert 1999; Havstad et al. 2008).
The study was supported by a grant of the German Science Foundation (Deutsche Forschungsgemeinschaft) to Ch. Dulamsuren (Du 1145/1-1, 1-2). We are thankful to M. Runge (Göttingen) for constant advice. M. Mühlenberg (Göttingen) made available the facilities of Khonin Nuga Research Station in the north-western Khentey Mountains to us. S. Nyambayar (Ulan Bator) is thanked for his assistance in field work and with tree ring measurements in Göttingen. Prof. Dr D. Suran, L. Bazarragchaa, L. Jadambaa, and D. Osokhjargal (Ulan Bator) helped during field work.
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