Subalpine Tree Species Research Articles

Soil Preferences in Germination and Survival of Limber Pine in the Great Basin White Mountains

In the Great Basin, limber pine is a sub-alpine tree species that is colonizing newly available habitat above treeline in greater numbers than treeline-dominating Great Basin bristlecone pine, especially on dolomite soil, where few plants are able to grow and where limber pine adults are rare. To examine the role of soil type on germination and establishment of limber pine, I sowed limber pine seeds in containers of the three main White Mountains soil types in one location while measuring soil moisture and temperature. I found that dolomite soil retains water longer, and has higher soil water content, than quartzite and granite soils and has the coolest maximum growing season temperatures. Limber pine germination and survival were highest in dolomite soil relative to quartzite and granite where limber pine adults are more common. While adult limber pines are rare on dolomite soils, young limber pines appear to prefer them. This indicates that limber pine either has only recently been able to survive in treeline climate on dolomite or that bristlecone pine has some long-term competitive advantage on dolomite making limber pine, a species with 1500 year old individuals, an early succession species in Great Basin sub-alpine forests.

Diameter-related variations in root decomposition of three common subalpine tree species in southwestern China

Despite the importance of root quality for its decomposition, the effects of forest conversion and diameter size on root decomposition still remains poorly understood. A two-year field experiment was conducted to examine the mass loss and nutrient release of three root diameter classes (0–2mm, 2–5mm and 5–10mm) in three subalpine tree species (Abies faxoniana, Picea asperata and Betula albosinensis) using a litter-bag method on the eastern Tibetan Plateau of China. The roots of B. albosinensis decayed faster compared to those of two conifer trees (A. faxoniana and P. asperata) in the 2–5mm and 5–10mm roots. Root diameter and decay rate exhibited a significantly negative correlation for two conifer trees. Regardless of tree species and diameter classes, all root litters experienced significant net nitrogen (N) and phosphorus (P) immobilization during the first winter. Both 2–5mm and 5–10mm roots tended to release more N than fine roots after 2-year incubation. Almost no obvious P release was observed for all root litters over the experimental period. Irrespective of tree species, both C:N and C:P ratios followed a trend of 0–2mm roots <2–5mm roots <5–10mm roots during the experimental period. The root decomposition and N release were strongly associated with initial root quality (e.g., N and C:N). Our results suggest that diameter-associated variations in substrate quality could be an important driver of root decomposition and nutrient dynamics. Moreover, the diameter effects are dependent on tree species and decomposition period.

Late Holocene expansion of Siberian dwarf pine (Pinus pumila) in Kamchatka in response to increased snow cover as inferred from lacustrine oxygen-isotope records

Holocene records of cellulose-inferred lake-water δ18O were produced from two lake-sediment sequences obtained in central and northern Kamchatka, Russian Far East. The sediment records share similar fluctuations in δ18O during the interval of ca. 5000–800calyr BP that correspond (inversely) with changes in K+ content of the GISP2 ice-core record from Greenland, a proxy for the relative strength of the Siberian High, suggesting control by climate-related variability in δ18O of regional precipitation. The dramatic expansion of Siberian dwarf pine (Pinus pumila) in northern and central Kamchatka between ca. 5000 and 4000calyr BP, as inferred from pollen records from the same and neighbouring sites, appears to have occurred at a time of progressively declining δ18O of precipitation. This development is interpreted as reflecting a regional cooling trend accompanied by increasing winter snowfall related to gradual intensification of the Siberian High from ca. 5000 to ca. 3000calyr BP. A thicker and more long-lasting snow cover can be assumed to have favoured P. pumila by providing a competitive advantage over other boreal and subalpine tree and shrub species in the region during the later part of the Holocene. These results, which are the first of their kind from Kamchatka, provide novel insight into the Holocene vegetational and climatic development in easternmost Asia, as well as long-term atmospheric circulation dynamics in Beringia.

Modeling Subalpine and Upper Montane Forest-Climate Interactions in Colorado

The correlation of the distribution of five subalpine and montane tree species with precipitation and temperature were modeled using GIS. The results were compared with data presented by Thompson et al. (2000). Distributions of subalpine fir (Abies concolor), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta), limber pine (Pinus flexilis) and bristlecone pine (Pinus aristata) were compared to estimated precipitation and temperature fields that had been constructed from the Parameter-elevation Regressions on Independent Slopes Model (PRISM), National Climatic Data Center (NCDC) station data, and Snowpack Telemetry (SNOTEL) system data. Plant distribution maps from Little (1971) and CoGAP (2001) were used to determine the temperature and precipitation associated with the selected tree species. The estimates from this study were compared to those of Thompson, Anderson &amp; Bartlein (2000). In many cases precipitation and temperatures values were higher than those of Thompson, Anderson &amp; Bartlein (2000). Suggestions are made to improve the predictive power of GIS analysis for mapping climate and plant variability.

Ecophysiological responses of two dominant subalpine tree species Betula albo-sinensis and Abies faxoniana to intra- and interspecific competition under elevated temperature

Seedlings of tree species Abies faxoniana and Betula albo-sinensis were grown under different competition regimes (single plant, monoculture or mixture) at ambient temperature (AT) and elevated temperature (ET) to evaluate the impact of inter- and intraspecific competition on photosynthesis capacity, N resorption efficiency, N uptake patterns, water use efficiency (foliar carbon isotope composition, δ13C) and root carbon reserve accumulation. Our results showed that ET did not have significant effects on relative height growth rate (RGRH), relative diameter growth rate (RGRD), chlorophyll fluorescence parameters (Fv/Fm, ΦPSII), light-saturated photosynthetic rate (A1400) and nitrogen uptake in A. faxoniana, but it significantly reduced the values of these parameters in B. albo-sinensis. On the other hand, RGRH of A. faxoniana was not influenced by intraspecific competition irrespective of temperature regimes, while in B. albo-sinensis intraspecific competition was severer under AT than under ET. Decreased photosynthetic capacity and reduced plant size detected in B. albo-sinensis under ET likely contribute to its low intraspecific competitive ability. However, we found positive effects of interspecific competition on A. faxoniana but negative effects on B. albo-sinensis under ET, as A. faxoniana exposed to interspecific competition showed higher RGRH, RGRD, A1400, chlorophyll fluorescence parameters, foliar δ13C, foliar N, and percentages of 15N-ammonium and 15N-nitrate recovery than did A. faxoniana exposed to intraspecific competition or no competition. The results indicate that A. faxoniana is a better competitor than B. albo-sinensis under ET. Further, A. faxoniana altered resource use patterns and had a high plasticity in its ammonium uptake in mixtures compared with monocultures under ET, while B. albo-sinensis did not change its N use pattern. In all, our results showed that high temperature stress tolerance (e.g., higher root carbon accumulation), high ability to keep carbon balance (e.g., higher water use efficiency, lower leaf respiration rate, higher Fv/Fm), as well as a better N acquisition ability make A. faxoniana a better competitor when compared to B. albo-sinensisg under ET.

Altitudinal differences in bud burst and onset and cessation of cambial activity of four subalpine tree species

This study investigated bud burst and onset and cessation of radial growth (i.e., secondary xylem cell production by cambium) along an altitudinal gradient (1,600–2,800 m a.s.l.) in central Japan, to examine whether timing of the three phenological events are controlled by thermal conditions alone, irrespective of altitude. Measurement was done at biweekly intervals during 3 years (2006–2008) for four subalpine tree species at their upper and lower distribution limits, except for bud burst in 2006. Although bud burst and onset of radial growth were later at higher altitudes in the combined data of the four species, cessation of radial growth occurred in mid-August, irrespective of altitude. Coefficients of variation of timing of the phenological events decreased in the order bud burst, onset of radial growth, and cessation of radial growth in combined data of the four species. Growing degree days (GDD) for bud burst did not differ greatly along the altitudinal gradient for each species. However, GDD for the onset of radial growth was lower at higher altitude. This study suggests that the effects of thermal conditions on the phenological events along the altitudinal gradient might decrease in the order bud burst, onset of radial growth and cessation of radial growth. Probably not only temperature, but also other factors such as day length, may affect onset and cessation of radial growth. However, further research is necessary to examine the altitudinal differences in timing of the phenological events more accurately, by measurements at shorter time intervals than the biweekly intervals of this study.

Growth–climate relationships for six subalpine tree species in a Mediterranean climate

A better understanding of the growth–climate relationship for subalpine trees is key to improving predictions about their future distributions under climate change. In subalpine regions of Mediterranean mountains, drought is an annual event, yet many sites can have long-lasting snowpack. We analyzed the growth–climate relationship from 1896 to 2006 for the six most abundant subalpine tree species (red fir (Abies magnifica A. Murray bis), whitebark pine (Pinus albicaulis Engelm.), Sierra/Cascade lodgepole pine (Pinus contorta var. murrayana (Balf.) Engelm.), Jeffrey pine (Pinus jeffreyi Balf.), western white pine (Pinus monticola Douglas ex D. Don), and mountain hemlock (Tsuga mertensiana (Bong.) Carrière)) of the central Sierra Nevada, California, USA, a region with deep spring snowpack followed by strong summer drought. Chronologies for the six species exhibited a high degree of synchrony in their response to annual fluctuations in temperature and precipitation. For all six species, cool, wet conditions in the year prior to growth are conducive to good radial growth, as well as warm springs with sufficient moisture during the year of growth. For species more common on protected slopes, such as mountain hemlock, deep spring snowpack can limit growth. Although predictions of future precipitation trends in the region are uncertain, drought stress appears to already be increasing. If this trend continues, radial growth is likely to be inhibited for most or all species in our study. Trees growing where snowpack is deep may be least likely to suffer reduced growth.

Bark flammability as a fire-response trait for subalpine trees

Relationships between the flammability properties of a given plant and its chances of survival after a fire still remain unknown. We hypothesize that the bark flammability of a tree reduces the potential for tree survival following surface fires, and that if tree resistance to fire is provided by a thick insulating bark, the latter must be few flammable. We test, on subalpine tree species, the relationship between the flammability of bark and its insulating ability, identifies the biological traits that determine bark flammability, and assesses their relative susceptibility to surface fires from their bark properties. The experimental set of burning properties was analyzed by Principal Component Analysis to assess the bark flammability. Bark insulating ability was expressed by the critical time to cambium kill computed from bark thickness. Log-linear regressions indicated that bark flammability varies with the bark thickness and the density of wood under bark and that the most flammable barks have poor insulating ability. Susceptibility to surface fires increases from gymnosperm to angiosperm subalpine trees. The co-dominant subalpine species Larix decidua (Mill.) and Pinus cembra (L.) exhibit large differences in both flammability and insulating ability of the bark that should partly explain their contrasted responses to fires in the past.

Forecasting the effects of global warming on radial growth of subalpine trees at the upper and lower distribution limits in central Japan

Effects of global warming on radial growth were examined for the subalpine tree species Abies veitchii (1600–2200 m a.s.l.), A. mariesii (2000–2500 m a.s.l.) and Betula ermanii (1600–2500 m a.s.l.) in central Japan, by using dendrochronological techniques. Chronologies of tree-ring widths were examined for the three species and of maximum latewood densities for the two Abies species at their upper and lower distribution limits (total 10 chronologies). We developed multiple regression models to reproduce these chronologies from the monthly mean temperature and sum of precipitation. Of the 10 chronologies, growth-climate relations could not be modeled for tree-ring width chronologies of the three species at their lower distribution limits because of low correlation. Annual mean temperature and annual sum of precipitation will increase about 3 °C and 100 mm, respectively, by 2100 in central Japan, according to 18 climatic change scenarios (6 general circulation models ×3 greenhouse gasses emission scenarios). We predicted tree-ring widths and maximum latewood densities by substituting 18 climatic change scenarios into the growth-climate models. Maximum latewood densities and tree-ring widths of A. mariesii at the upper and lower distribution limits increased by 2100. The rates of the increase tended to be greater for scenarios with more greenhouse gas emission. By contrast, maximum latewood densities of A. veitchii and tree-ring widths of B. ermanii were unchanged by 2100, irrespective of the three greenhouse gas emission scenarios. This study showed that radial growth of the three species responds differently to global warming and their responses are predictable by dendrochronological models.

Combined tree-ring width and δ13C to reconstruct snowpack depth: a pilot study in the Gongga Mountain, west China

Tree-ring width (TRW) and stable carbon isotope (δ13C) in tree-ring cellulose of subalpine fir (Abies fabri) were used to develop high-resolution climate proxy data to indicate snow-depth variations in the Gongga Mountain, west China. Tree radial growth- and δ13C-climate response analyses demonstrated that the TRW and δ13C at the timberline (3,400 m.a.s.l.) are mainly influenced by temperature and precipitation of previous growth seasons and current summer (June to August) under cold and humid conditions. Considering the analogous control factors on both tree growth and carbon isotope discrimination (Δ13C) and snow accumulation, the negative and significant relationships between tree-ring parameters (TRW and Δ13C) and mean monthly snowpack depth were found. Herein, by combining two tree-ring parameters, a primary snow-depth reconstruction (previous October to current May) over the reliable period A.D. 1880–2004 was estimated. The reconstruction explains 58.0% of the variance in the instrumental record, and in particular captures the longer-term fluctuations successfully. Except the period with extreme higher snowpack depth around 1990, the snowpack depth seems to fluctuate in a normal way. The reconstruction agrees with the nearby snowpack depth record in Kangding and the mean observed snowpack-depth variations of the stations on the Tibetan Plateau, particularly at long-term scales. The snowpack depth in low-frequency fluctuations, during the past century, agrees quite well with the Eastern India precipitation covering the period of previous October–current May. Our results suggest that combing tree-ring width and δ13C in certain subalpine tree species growing on the Tibetan Plateau may be an effective way for reconstructing regional snowpack variations.

Why does temperature affect relative uptake rates of nitrate, ammonium and glycine: A test with Eucalyptus pauciflora

Few studies have examined how temperature affects uptake of nitrate, ammonium and amino acids from soil. This study tests the hypothesis that cool temperatures favour uptake of the amino acid glycine while warm temperatures favour uptake of inorganic forms of N such as nitrate. We used glasshouse-grown ectomycorrhizal seedlings of the sub-alpine tree species Eucalyptus pauciflora Sieber ex Spreng. Seedlings were grown in soil (humic umbrosol, from species' habitat) that was dominated by amino acids and ammonium with only small amounts of nitrate. To examine if root physiology affects temperature responses of N uptake, we measured uptake from 15N-labelled hydrosolutions containing equimolar 100 μmol L −1 mixtures of ammonium, nitrate and glycine at temperatures from 5 to 35 °C. We also examined if the effect of temperature on uptake of N forms was due to plant–microbe competition by following the fate of equimolar amounts of labelled ammonium, nitrate and glycine injected into the soil at temperatures of 5 °C and 25 °C. Hydrosolution experiments showed that uptake of glycine was favoured by warm temperatures and inorganic N by cool temperatures. In contrast, when 15N was injected into soil the uptake of glycine was favoured by low temperatures and nitrate by warm temperatures. At 25 °C, glycine was 17% of the N taken up from soil and nitrate was 51%; whereas at 5 °C glycine was 30% of the N taken up from soil and nitrate was 23%. Microbes were better competitors than seedlings for all forms of N, but temperature did not affect microbial preference for the different N forms. Hence, while microbes limit N available for plant uptake, they do not seem to be the cause of the greater plant uptake of glycine at cool temperatures and nitrate at warm temperatures. Intact uptake of glycine by plants was suggested by the positive relationship between uptake of 13C and 15N and detection by GC–MS of intact C 2 13 , 15N glycine molecules in roots. In conclusion, uptake of glycine is favoured by cool temperatures and nitrate by warm temperatures, but this is apparently not a function of root physiology or competition with soil microbes.

Taking the Pulse of Mountains: Ecosystem Responses to Climatic Variability

An integrated program of ecosystem modeling and field studies in the mountains of the Pacific Northwest (U.S.A.) has quantified many of the ecological processes affected by climatic variability. Paleoecological and contemporary ecological data in forest ecosystems provided model parameterization and validation at broad spatial and temporal scales for tree growth, tree regeneration and treeline movement. For subalpine tree species, winter precipitation has a strong negative correlation with growth; this relationship is stronger at higher elevations and west-side sites (which have more precipitation). Temperature affects tree growth at some locations with respect to length of growing season (spring) and severity of drought at drier sites (summer). Furthermore, variable but predictable climate-growth relationships across elevation gradients suggest that tree species respond differently to climate at different locations, making a uniform response of these species to future climatic change unlikely. Multi-decadal variability in climate also affects ecosystem processes. Mountain hemlock growth at high-elevation sites is negatively correlated with winter snow depth and positively correlated with the winter Pacific Decadal Oscillation (PDO) index. At low elevations, the reverse is true. Glacier mass balance and fire severity are also linked to PDO. Rapid establishment of trees in subalpine ecosystems during this century is increasing forest cover and reducing meadow cover at many subalpine locations in the western U.S.A. and precipitation (snow depth) is a critical variable regulating conifer expansion. Lastly, modeling potential future ecosystem conditions suggests that increased climatic variability will result in increasing forest fire size and frequency, and reduced net primary productivity in drier, east-side forest ecosystems. As additional empirical data and modeling output become available, we will improve our ability to predict the effects of climatic change across a broad range of climates and mountain ecosystems in the northwestern U.S.A.

Late Wisconsin and Holocene Subalpine Forests of the Markagunt Plateau of Utah, Southwestern Colorado Plateau, U.S.A.

The vegetation and climatic history of subalpine forests on the Colorado Plateau is documented from Lowder Creek Bog and Alpine Pond on the Markagunt Plateau. Pollen and macrofossil data demonstrate substantial changes at sites above 3150 m elevation during the last ca. 13,000 yr. During and after Late Wisconsin deglaciation, subalpine tree species (Picea engelmannii and Abies lasiocarpa) were rare or absent near the Lowder Creek Bog site, but nonarboreal species predominated. P. engelmannii–A. lasiocarpa forest became well-established there between 11,000 and 9800 yr BP and subalpine trees dominated this elevation throughout the Holocene. By ca. 8500 yr BP, however, Picea declined somewhat, with minimal pollen and macrofossil deposition occurring between ca. 8500 and 6400 yr BP. Picea trees may have thinned during this time or Picea may have expanded to higher elevations during this part of the Holocene, while Pinus ponderosa expanded at lower elevations. These changes were probably driven by warmer conditions with variable precipitation. Alternative explanations include hydroseral changes within the bog, or insect infestation on Picea. After ca. 6400 yr BP, more consistent Picea pollen influx suggests renewed importance around the bog, which has been maintained until the present. Abies lasiocarpa became more important after ca. 2700 yr ago.The pollen sequence supports an interpretation of high effective precipitation during the early Holocene, followed by warmer temperatures and somewhat variable precipitation after ca. 8500 yr BP. This sequence is most similar to other sites on and near the western, southern, and northeastern Colorado Plateau, where the maximum influence of monsoon activity may have declined by the end of the early Holocene. Declines in summer insolation probably contributed to Late Holocene cooling, with increasing effective precipitation at most sites, including Lowder Creek Bog.

Late Wisconsin and Holocene Subalpine Forests of the Markagunt Plateau of Utah, Southwestern Colorado Plateau, U.S.A.

The vegetation and climatic history of subalpine forests on the Colorado Plateau is documented from Lowder Creek Bog and Alpine Pond on the Markagunt Plateau. Pollen and macrofossil data demonstrate substantial changes at sites above 3150 m elevation during the last ca. 13,000 yr. During and after Late Wisconsin deglaciation, subalpine tree species (Picea engelmannii and Abies lasiocarpa) were rare or absent near the Lowder Creek Bog site, but nonarboreal species predominated. P. engelmannii–A. lasiocarpa forest became well-established there between 11,000 and 9800 yr BP and subalpine trees dominated this elevation throughout the Holocene. By ca. 8500 yr BP, however, Picea declined somewhat, with minimal pollen and macrofossil deposition occurring between ca. 8500 and 6400 yr BP. Picea trees may have thinned during this time or Picea may have expanded to higher elevations during this part of the Holocene, while Pinus ponderosa expanded at lower elevations. These changes were probably driven by warmer conditions with variable precipitation. Alternative explanations include hydroseral changes within the bog, or insect infestation on Picea. After ca. 6400 yr BP, more consistent Picea pollen influx suggests renewed importance around the bog, which has been maintained until the present. Abies lasiocarpa became more important after ca. 2700 yr ago.The pollen sequence supports an interpretation of high effective precipitation during the early Holocene, followed by warmer temperatures and somewhat variable precipitation after ca. 8500 yr BP. This sequence is most similar to other sites on and near the western, southern, and northeastern Colorado Plateau, where the maximum influence of monsoon activity may have declined by the end of the early Holocene. Declines in summer insolation probably contributed to Late Holocene cooling, with increasing effective precipitation at most sites, including Lowder Creek Bog.

Changes in sub-alpine tree distribution in western North America: a review of climatic and other causal factors

Changes in the distribution of sub-alpine tree species in western North America have been attributed to climatic change and other environmental stresses. These changes include tree-line fluctuations throughout the Holocene and recent invasion of sub-alpine meadows by forest. Most palaeoecological studies suggest that the tree-line was higher during a period of warmer climate approximately 9000 to 5000 BP and lower during the last 5000 years, with short periods of local tree-line advance. Recent advances in sub-alpine tree distribution can be compared with weather records, allowing an examination of relationships between tree advance and climate at a finer resolution. In general, recent sub-alpine forest advances in western North America, based on studies representing three climatic zones (maritime, Mediterranean and continental), have been associated with climatic periods favouring tree germination and growth, although factors such as fire and grazing by domestic livestock have had an impact in some areas. Limitations to tree establishment (e.g., winter snow accumulation, summer drought) vary in relative importance within each climate zone, as do predicted consequences of anthropogenic climatic change. Recent increases in establishment of sub-alpine trees may continue if climatic change alleviates the limitations to tree establishment important in each climatic zone. However, factors such as topography and disturbance may modify tree establishment on a local scale.

東北山地における主要樹種の温度領域からみた「為高山帯」の成因

The subalpine zones on the mountains south of the Tohoku mountains in the Japanese Islands are normally occupied by coniferous forests dominated by Abies mariesii, A. veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia. But there are many mountains where the subalpine coniferous forest does not exist; the thermometrical subalpine zones on such mountains are replaced by subalpine and/or montane deciduous broad-leaved forests. Especially in the Tohoku mountains, the subalpine zone without A. mariesii forest is called the “pseudo-alpine zone”, where Quercus scrub usually dominates. The origin of the pseudo-alpine zone is discussed here based on the thermal conditions of subalpine main tree species and the rise in temperature during the Hypsithermal climax phase.The thermometrical zone for A. mariesii is divided into four ranges as shown in Fig. 5-b. The mountains whose peaks are thermometrically in zones B and C can be regarded as mountains where A. mariesii is alternatively indigenous or not indigenous under the present thermal conditions. On the mountains whose peaks are in zone A, A. mariesii is expected to be indigenous. Thus, the mountains in zone A but without A. mariesii forest are called the “A. mariesii-lacking mountains” and are examined here.For the “A. mariesii-lacking mountains”, the thermal conditions at their peaks were out of zone A and into zones B and/or C during the Hypsithermal climax phase, when the mean summer temperature rose to be 2.5°C higher than that at present in Japan. During the high-temperature period of the Hypsithermal, the habitat of A. mariesii was pushed over the mountain peak due to the upward migration of vegetation zones. Thus, the principal cause of the pseudo-alpine zone is the ‘pushing-out effect’ caused by the rise of temperature. The origin of the subalpine zones without subalpine coniferous forest in West Japan is also considered to be the same process as the pseudo-alpine zone in the Tohoku mountains. The lacking of a tree species on a mountain depends on a process in which the thermal conditions suitable for the species disappeared from the mountain during a period in the history of climatic changes and, since the event, the thermal habitat of the species has been isolated from the surrounding mountains with forests of the same species. The present features of distribution of the subalpine tree species which grow under cold climatic conditions on high mountain areas in Japan have been affected by the small changes (2 or 3°C) in temperature during the Holocene, reflecting the topographical mosaic built up by many small mountain blocks and inter-lowlands.

Holocene history of the forest–alpine tundra ecotone in the Scandes Mountains (central Sweden)

The Holocene history of the forest-alpine tundra ecotone in Central Sweden (Scandes Mountains) is inferred from radiocarbon analyses of subfossil wood remains. Pinus sylvestris was the dominant subalpine tree species during the early Holocene, when it ascended almost 200 m higher than currently. A short climatic episode (less than 100 years) is postulated to have triggered erosional processes around 6300 n.p., and extinguished the upper part of the subalpine pine woodland. Subsequently, a subalpine belt of Betula pubescens ssp. tortuosa and Alnus incana developed. A Holocene thermal optimum occurred around 6100 b.p., when the birch/alder belt nourished and the tree-limits probably reached their highest levels during the Holocene. Shortly after 6000 b.p., a long-term pine forest retrogression started and the birch/alder belt was disrupted by expanding snow-beds. Pine receded slightly at its tree-limit, but the uppermost belt of closed pine forest (presently dominated by birch) remained intact until c. 3300 b.p., when a severe climatic deterioration occurred. The present-day subalpine belt of pure birch forest developed successively and increased in vertical extent after c. 5300 b.p., when summer temperature declined. The evolution of the birch belt is postulated to have been ultimately a response to decreased seasonally, which favoured birch at the expense of pine. Because of the 'inertia' characterizing the highest pine forest, the birch belt was relatively narrow until a major thermal decline e. 3300 b.p., when it made a massive downslope expansion. The latest phase of pine recession was during the Little Ice Age, 800-300 b.p.