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Paleoclimate reconstructions for the western US show spatial variability in the timing, duration, and magnitude of climate changes within the Medieval Climate Anomaly (MCA, ca. 900–1350 CE) and Little Ice Age (LIA, ca. 1350–1850 CE), indicating that additional data are needed to more completely characterize late-Holocene climate change in the region. Here, we use dendrochronology to investigate how climate changes during the MCA and LIA affected a treeline, whitebark pine (Pinus albicaulis Engelm.) ecosystem in the Greater Yellowstone Ecoregion (GYE). We present two new millennial-length tree-ring chronologies and multiple lines of tree-ring evidence from living and remnant whitebark pine and Engelmann spruce (Picea engelmannii Parry ex. Engelm.) trees, including patterns of establishment and mortality; changes in tree growth; frost rings; and blue-intensity-based, reconstructed summer temperatures, to highlight the terminus of the LIA as one of the coldest periods of the last millennium for the GYE. Patterns of tree establishment and mortality indicate conditions favorable to recruitment during the latter half of the MCA and climate-induced mortality of trees during the middle-to-late LIA. These patterns correspond with decreased growth, frost damage, and reconstructed cooler temperature anomalies for the 1800–1850 CE period. Results provide important insight into how past climate change affected important GYE ecosystems and highlight the value of using multiple lines of proxy evidence, along with climate reconstructions of high spatial resolution, to better describe spatial and temporal variability in MCA and LIA climate and the ecological influence of climate change. [ABSTRACT FROM AUTHOR]

Rexford Daubenmire's 1954 seminal review of alpine timberline in North America remains a valuable classic for timberline scholars. His article was published at a formative time for timberline studies and, as such, has affected the direction of timberline research for nearly 70 years. Daubenmire's definition of timberline, his review of climatological theories controlling timberline, and his additions to altitudinal variations of timberline across latitudes remain at the forefront of timberline research. [ABSTRACT FROM AUTHOR]

Current climate trends are expected to result in the northward expansion of the subarctic treeline leading to changes in vegetation cover and permafrost distribution, as they did during the Holocene Climate Optimum when the treeline was 150 km north of its current position. The impacts of these changes on the region's hydrology are still poorly understood. The grain-size distributions of treeline lake sediments provide an important proxy related to spring melt conditions that can be used to reconstruct hydroclimatic variability. End-member mixing analysis was used to model depositional end-members in 55 modern lake sediment samples and two sediment cores spanning the mid- to late Holocene collected from above and below the treeline in the central Northwest Territories, Canada. Cold climatic intervals (e.g. 'Dark Ages Cold Period', 'Little Ice Age') were characterised by an increase in the very coarse silt and the fine sand end-members. This was interpreted to be a response to degradation of vegetation cover and/or permafrost development. We observed increases in fine and coarse silt end-members during warmer climatic intervals (e.g. Medieval Climate Anomaly) and over the past c. 300 yr BP. This pattern is probably the result of extended melt seasons, with greater losses to evaporation and increased infiltration. The most pronounced palaeo-hydroclimatological change over the past c. 8000 yr BP was the abrupt increase in a very coarse silt end-member (mode = 50-200 μm) at c. 6300 yr BP. We interpreted the sedimentological change as an increase in winter precipitation and more energetic spring melt conditions, leading to the spring melt becoming the dominant lacustrine sediment delivery mechanism. These results place modern hydrological changes in a millennial context and show that analysis of temporal changes in the hydroclimatological system can provide insight into the future states of these sensitive subarctic ecosystems. [ABSTRACT FROM AUTHOR]

Two high-altitude lake-sediment sequences (Lake Lia, 1910 m a.s.l. and Lake Bucura, 2040 m a.s.l.) from the Retezat Mountains (South Carpathians, Romania) were analysed using multi-proxy methods to study responses of treeline, timberline and alpine/subalpine vegetation to climate change and human impact during the past 16,000 years. Woody species (Pinus mugo, Pinus cembra, Picea abies and Juniperus communis) reached Lake Lia between 12,000 and 11,800 cal. yr BP, whereas P. mugo colonised the shores of Lake Bucura at 9600 cal. yr BP. Lake Lia was in the timberline ecotone between 8000 and 3200 cal. yr BP, in semi-open P. cembra and Picea abies woodland, probably mixed with P. mugo on the steeper slopes. Lake Bucura was surrounded by the upper part of the krummholz zone during the mid-Holocene. The increase in P. cembra after c. 6000 cal. yr BP around Lake Lia suggests that the composition of the timberline forest changed. The disappearance of P. cembra and Picea abies around Lake Lia at ∼3000 cal. yr BP reflects descent of the timberline. A large mean July temperature decline between 3300 and 2800 cal. yr BP may have driven or at least contributed to the descent of the Picea abies-P. cembra forests. An increase in human indicator pollen types in Lake Bucura around 4200 cal. yr BP may reflect human impact in the naturally open alpine zone in the Late Bronze Age. In contrast, human impact likely appeared considerably later, around 2650 cal. yr BP (Early Iron Age) around Lake Lia in the upper subalpine zone. Human impact likely intensified after 2200 cal. yr BP at both sites that resulted in the lowering of the krummholz zone. We conclude that climate change and human impact both played an important role in the lowering of the treeline and timberline in the late-Holocene. [ABSTRACT FROM AUTHOR]

Little is known about how soil properties control tree growth at its upper limit. This paper reviews the state of knowledge and discusses the results specifically related to ecozones, to the scale-dependent importance of single factors, and to new findings from a near-natural treeline ecotone in Rolwaling Himal, Nepal. This paper identifies gaps in literature and shows where new research is needed, both conceptual and geographical. The review shows that at a global scale and throughout diverse ecozones, growing season soil temperature is considered a key factor for tree growth. Soil temperatures differ greatly at a local scale, and are mainly determined by local climatic, edaphic, and topographic conditions. Our result of 7.6 ± 0.6°C for growing season mean soil temperature at treeline in Rolwaling is 1.2 K higher compared to the postulated 6.4 ± 0.7°C for alpine treelines. We suggest a broadening of the ±0.7°C error term to cover the wide range at a local scale. The role of major soil nutrients and soil moisture for treeline shift has been underestimated by far. In Rolwaling, significantly decreasing nutrient availability (N, K, Mg) in soils and foliage with elevation might explain why treeline shift and global warming are decoupled. Further, soil moisture deficits early in the year impede seedling and sapling establishment, which could be an important mechanism that controls treeline position. These findings question previous results which argue that alpine treelines are unaffected by soil nutrient availability and soil moisture. We assume that specific combinations of soil properties as well as single soil properties limit tree growth even below climatic treelines. [ABSTRACT FROM AUTHOR]

Using a combination of 23 radiocarbon ages and annual ring counts from 18 Rocky Mountain bristlecone pine (Pinus aristata) remnants above the local present-day limits, a period of higher treeline has been determined for two sites near the Continental Divide in central Colorado. The highest remnants were found about 30 m above live bristlecone pines of similar size. The majority of the remnants, consisting of standing snags, large logs, and smaller remains, are highly eroded, such that the innermost annual rings of all but one are missing. The radiocarbon ages obtained from the oldest wood recovered from each remnant indicate that the majority were established above the present-day limit of bristlecone pine from prior to 2700 cal. yr BP to no later than about 1200 cal. yr BP. These radiocarbon ages combined with the annual ring count from the corresponding remnant indicate that the majority of the sampled remnants grew above the present-day limit of bristlecone pine from sometime before 2700 cal. yr BP to about 800 cal. yr BP. Evidence of recent climatic warming is demonstrated at one of the sites by young bristlecone pine saplings growing next to the highest remnants; the saplings were established after AD 1965 and represent the highest advance of treeline in at least 1200 years. [ABSTRACT FROM AUTHOR]

Siberian arctic vegetation and lake water communities, known for their temperature dependence, are expected to be particularly impacted by recent climate change and high warming rates. However, decadal information on the nature and strength of recent vegetation change and its time lag to climate signals are rare. In this study, we present a 210Pb/137Cs dated pollen and Pediastrum species record from a unnamed lake in the south of the Taymyr peninsula covering the period from AD 1706 to 2011. Thirty-nine palynomorphs and 10 morphotypes of Pediastrum species were studied to assess changes in vegetation and lake conditions as probable responses to climate change. We compared the pollen record with Pediastrum species, which we consider to be important proxies of climate changes. Three pollen assemblage zones characterised by Betula nana, Alnus viridis and Larix gmelinii (1706–1808); herbs such as Cyperaceae, Artemisia or Senecio (1808–1879), and higher abundance of Larix pollen (1955–2011) are visible. Also, three Pediastrum assemblage zones show changes of aquatic conditions: higher abundances of Pediastrum boryanum var. brevicorne (1706–1802); medium abundances of P. kawraiskyi and P. integrum (1802–1840 and 1920–1980), indicating cooler conditions while less eutrophic conditions are indicated by P. boryanum, and a mainly balanced composition with only small changes of cold- and warm-adapted Pediastrum species (1965–2011). In general, compositional Pediastrum species turnover is slightly higher than that indicated by pollen data (0.54 vs 0.34 SD), but both are only minor for this treeline location. In conclusion, the relevance of differentiation of Pediastrum species is promising and can give further insights into the relationship between lakes and their surrounding vegetation transferred onto climatic conditions. [ABSTRACT FROM PUBLISHER]

A high-altitude lake sediment sequence (Buhăiescu Mare, 1918 m a.s.l.) in the subalpine zone of the Rodna Mountains was analysed through a multi-proxy approach to determine the sensitivity of high mountain habitats to climate, fire and land use changes. The early Holocene regional forests were dominated by Pinus (sylvestris and mugo) and replaced by Picea abies from 9800 cal. yr BP. After an extended hiatus in the profile (c. 9800–4200 cal. yr BP), probably because of the physical removal of sediments through avalanche or high-flow events, P. abies, Abies alba and Fagus sylvatica forests developed after 4200 cal. yr BP. The timberline and treeline reacted sensitively to past changes in climate and human impact. The site was probably situated above the treeline throughout most of the investigated period. However, a treeline ecotone or krummholz zone may have sporadically reached the lake’s elevation in the early Holocene. A decline in timberline and treeline elevation was noted during the last 1200 years, and more evidently over the past 200 years, with replacement by subalpine shrubs (Alnus viridis) and alpine herbaceous communities. Because these vegetation changes were associated with an increased prevalence of pollen-based anthropogenic indicators, charcoal particles and abiotic indicators, human-induced fires and clearance and resultant erosion inputs to the lake are implied. Effects of current warming on the altitude range of trees are not yet visible, probably because land use has more strongly contributed to changes in land cover than the climate fluctuations of the last millennium in the Rodna Mountains. [ABSTRACT FROM PUBLISHER]

This study presents evidence for treeline advance in the central Rocky Mountains, United States, between 1800 and 800 cal. yr BP. This evidence was generated by systematic survey, sampling, and accelerator-mass spectrometer (AMS) dating of remnant Pinus albicaulis above the modern treeline ecotone of Union Peak, in the Wind River Range of western Wyoming. The survey identified 257 tree remnants, of which AMS dates were generated on 15 samples. Based on AMS results, it appears that treeline was approximately 100 m higher in elevation and covered an additional 1 km of horizontal distance for approximately a millennium, 1800–800 cal. yr BP. These findings are surprising given the tendency of remnant forests across the region to date to the middle Holocene but are consistent with regional evidence for increases in effective moisture that are contemporaneous with the Union Peak treeline advance. The age of death for most of the sampled trees occurred, however, near 800 cal. yr BP, 200 or more years prior to the onset of the type of regional and hemispheric temperature decreases that might be expected to result in treeline decline. Given the contemporaneity of the Union Peak treeline decline with a substantial regional megadrought 820–780 cal. yr BP, it is hypothesized that declining moisture availability played a fundamental role in this phenomenon. If true, the late-Holocene advance and subsequent decline of treeline on Union Peak would emphasize the role effective moisture plays in the complex interactions between temperature, precipitation, and local-scale factors conditioning treeline elevation across the central Rocky Mountain region. [ABSTRACT FROM PUBLISHER]

Lake sediments from Lauenensee (1381 m a.s.l.), a small lake in the Bernese Alps, were analysed to reconstruct the vegetation and fire history. The chronology is based on 11 calibrated radiocarbon dates on terrestrial plant macrofossils suggesting a basal age of 14,200 cal. BP. Pollen and macrofossil data imply that treeline never reached the lake catchment during the Bølling–Allerød interstadial. Treeline north of the Alps was depressed by c. 300 altitudinal meters, if compared with southern locations. We attribute this difference to colder temperatures and to unbuffered cold air excursions from the ice masses in northern Europe. Afforestation started after the Younger Dryas at 11,600 cal. BP. Early-Holocene tree-Betula and Pinus sylvestris forests were replaced by Abies alba forests around 7500 cal. BP. Continuous high-resolution pollen and macrofossil series allow quantitative assessments of vegetation dynamics at 5900–5200 cal. BP (first expansion of Picea abies, decline of Abies alba) and 4100–2900 cal. BP (first collapse of Abies alba). The first signs of human activity became noticeable during the late Neolithic c. 5700–5200 cal. BP. Cross-correlation analysis shows that the expansion of Alnus viridis and the replacement of Abies alba by Picea abies after c. 5500 cal. BP was most likely a consequence of human disturbance. Abies alba responded very sensitively to a combination of fire and grazing disturbance. Our results imply that the current dominance of Picea abies in the upper montane and subalpine belts is a consequence of anthropogenic activities through the millennia. [ABSTRACT FROM PUBLISHER]

Future climate projections predict warming at high elevations that will impact treeline species, but complex topographic relief in mountains complicates ecologic response, and we have a limited number of long-term studies examining vegetation change related to climate. In this study, pollen and conifer stomata were analyzed from a 2.3 m sediment core extending to 15,330 cal. yr BP recovered from a treeline lake in the Rocky Mountains of Wyoming. Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance. The complex topography of the Rocky Mountains makes it challenging to identify regional patterns associated with short term climatic variability, but our results contribute to gaining a better understanding of past ecologic responses at high elevation sites. [ABSTRACT FROM AUTHOR]

A 1.70 m core extracted from the Lac des Lauzons, Haut Champsaur, French Alps, at 2180 m altitude, provided a detailed Holocene record of beetles, pollen and plant macrofossils, enabling the reconstruction of local palaeoenvironmental changes during the last 10 000 years. After an early phase of colonization by plants and insects, corresponding to the Lateglacial interstadial, a long phase of relative stability of the ecosystems (at least in the vicinity of the lake) is recorded. Strikingly, there is no evidence from beetle and plant macrofossils that the treeline reached the altitude of Lauzons during the Holocene climate optimum, although this period is characterized by major forest expansion in many high-altitude sites in the southern French Alps. The uppermost part of the record is blurred by the infilling of the lake, progressively turning into a peat bog. This sequence also provides an opportunity to compare the records of coprophilous fungal spores and coprophilous beetles and to improve the interpretation of these proxies in terms of their significance as proxies for pastoralism. [ABSTRACT FROM AUTHOR]

The study presented here from the southern French Alps demonstrates the reliability of soil charcoal analysis for the study of Holocene past treeline positions. The occurrence of charcoal in mineral soils along transects from 1950 m up to 2920 m demonstrates the role of fire in the establishment of the present vegetation patterns. The abrupt decrease of charcoal concentration at about 2400-2600 m (which varies across the study sites) corresponds to the modern transition between subalpine forest and alpine tundra. Charcoal particles formed in situ provide high spatial-resolution data for the reconstruction of past forest and treeline changes. Soil charcoal analysis indicated that: (1) treeline was 300 m higher around 6800 cal. BP than today; and (2) the uppermost forest belt up to 2810 m was colonized by larch (Larix decidua Mill.) and arolla pine (Pinus cembra L.). This pine is present today but patchily distributed: it is absent from the three areas studied. Radiocarbon dates, ranging from c. 6800 cal. BP to the modern period, along with historical and archaeological data, suggest that the present pattern of the uppermost forest belt, and the patchy distribution of arolla pine in the studied area are the results of anthropogenic fire (mainly agropastoral activities). The question of global warming consequences on treelines in this part of the French Alps is discussed. [ABSTRACT FROM AUTHOR]

Seven peat and 40 dead wood remains (mountain birch) were sampled within and above the present treeline ecotone on two mountains (Rodjanoaivi, Koahppeloaivi/Staloskaidi) along the Tenojoki in northernmost Finnish Lapland. The oldest peat samples ('summit peats') date back to about 2000 yr BP. They accumulated during cool and increasingly humid climatic conditions. The other peat samples taken from wind-eroded peat remains on convex topography 60–100 m above the present tree limit are about 700–1300 years old. These peat layers developed during a phase of increasing Sphagnum peat formation. An eroded peat-covered Podzol that had developed under former tree stands with dwarfshrub vegetation in the understorey also reflects this climatic change. The oldest birch wood samples date from the same period. These and the many younger samples are evidence for a general treeline decline since the Holocene climatic optimum until present. As a result of the decline of the upper forest stands, the alpine zone has extended downslope and wind erosion has increased considerably in the former and present treeline ecotone, removing most of the peaty layers and top soils from wind-exposed topography. Whilst the cooling climate was the main factor triggering treeline retreat, episodic mass-outbreaks of the autumnal moth (Epirrita autumnata), particularly when coincident with cold summers, must also be taken into consideration as a factor that enhanced or accelerated treeline decline during this period. As is evidenced by the wood samples, forest decline continued into modern times when overgrazing by reindeer (Rangifer tarandus) became an additional factor enhancing wind erosion and affecting the regeneration of birch in the present treeline ecotone. [ABSTRACT FROM AUTHOR]

Subfossil samples of Scots pine (Pinus sylvestris) and evidence of pine stomata and pollen accumulation rates from previously studied lakes located beyond the present pine forest line in Finnish Lapland were used to reconstruct the extent of the past forest line and the corresponding minimum shift in mean July temperatures, compared with the present. The location of the present pine forest line follows the c. +12.2°C mean July temperature isoline (1961–1990). When minimum shifts in mean July temperatures during the Holocene were studied, the estimated glacio-isostatic land uplift and its regional differences were taken into account. Results indicate that pine reached its maximum distribution between 8300 and 4000 cal. yr BP. The inferred minimum shift in mean July temperature was at that time c. +2.5°C. Until 3000 cal. yr BP, the results indicate a shift of c. +1°C. Between 2538 and 1721 cal. yr BP, evidence for a wider distribution of pine in Finnish Lapland is lacking. During the 'Mediaeval Warm Period' the reconstructed minimum shift in mean July temperature was +0.5°C. The record of subfossil pines beyond the present pine treeline ceases during the initiation of the 'Little Ice Age', c. 700 years ago. [ABSTRACT FROM AUTHOR]

Ten potential proxy measures of past climate were recovered from Scots pine (Pinus sylvestris) at three sites on a latitudinal transect close to the pine limit in northern Finland (earlywood, latewood and annual ring width; earlywood, latewood and maximum density; stable carbon isotope ratio; height increment; needle production; pollen deposition). Cambium dynamics were also monitored. The aim was to determine how climate influences each potential proxy and to decide which proxies are potentially useful for reconstructing climate. Height increment, needle and pollen production are strongly influenced by the temperature of the previous July, which is when the bud forms, but needle and particularly flower development may also be influenced by spring frosts. Maximum and latewood densities provide proxies of net photosynthesis. δ[sup 13]C is controlled mainly by summer sunshine, reflecting the influence of photon flux on photosynthetic rate, and moisture stress which reduces stomatal conductance. By combining proxies, the strength of climate correlations is increased and the range of extractable parameters extended. The multiproxy approach provides a powerful means of extracting climatic information from long tree-ring chronologies. [ABSTRACT FROM AUTHOR]

A set of 40 subfossil Scots pines (Pinus sylvestris L.) was sampled in small lakes beyond the modern pine limit in the northwest Kola Peninsula, Russia. Radiocarbon dating suggests that the pine expanded north of its present-day limit between 7000 and 3500 yr BP. The most extensive occurrence of pine forests was reconstructed between 6250 and 5750 yr BP. Numerous samples collected from trunks and stubs in situ clearly indicate lower lake levels between 6200 and 4400 yr BP. Living and [sup 14]C dated subfossil pine samples were analysed for the mean carbon, oxygen and hydrogen stable isotope composition of their wood cellulose. The mean δ[sup 13]C, δ[sup 18]O and δ[sup 2]H values of the wood samples from the mid-Holocene time are c. 1.1‰, 0.9‰ and 10‰, respectively - more positive than those of living trees in this region. These results suggest that the mid-Holocene climate in the region was warmer and drier than nowadays. The data agree with previous palynological, macrofossil and limnological studies from northernmost Fennoscandia. During the late-Holocene climatic deterioration after c. 3500 yr BP, the decline of the northern pine limit was synchronous with other parts of northern Eurasia. Comparison with dated pine series from the north-central Kola Peninsula and Finnish Lapland suggests that the decline of the northernmost pine limit in the eastern part of the area may have occurred somewhat earlier than in the western part. [ABSTRACT FROM AUTHOR]

The stable-isotope stratigraphy of sedimentary organic matter and algal cellulose in cores from two lakes near treeline on the Kola Peninsula indicate changes in water and nutrient balance that correlate with inferred expansion of Pinus sylvestris during the early to mid-Holocene. Trends in cellulose-inferred lakewater δ[sup 18]O values for both lakes suggest that moist conditions after deglaciation were followed by progressive drying from about 9500 to between 8000 and 7500 [sup 14]C yr BP. Although pine appeared in the area by 8000 [sup 14]C yr BP, the maximum density of trees locally, as suggested by stomate evidence, occurred during subsequent moistening between 7500 and 6000 [sup 14]C yr BP. This is c. 1000 years later than in other regions of northern Russia and is possibly attributable to limitations on seedling establishment and survival due to root desiccation during dry winters. Changes in nutrient balance and productivity in the lakes, as inferred from variations in sediment carbon and nitrogen elemental abundances and their respective stable-isotope signatures, are also consistent with expected changes in hydrological and edaphic conditions. Development of moister conditions at about 7000 [sup 14]C yr BP on the Kola Peninsula is in harmony with evidence for increased oceanicity at this time in central Siberia, which is thought to reflect enhanced propagation of warm, moist air masses across northern Eurasia due to increased sea-surface temperatures and reduced sea-ice cover in the Nordic Seas. [ABSTRACT FROM AUTHOR]