A century of tree line changes in sub-Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

Aim Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub-Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location The study was conducted in the Tornetrask area in northern Sweden where climate warmed by 2.5 degrees C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year-1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year-1 in elevation and +2.7 m year-1 in actual distance), whereas maximum retreat was 120 m downslope. Counter-intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long-term stability of the tree line position, respectively. Main conclusions In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non-climatic climax tree lines, such as those in our study area, both climate-driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution

Factors Structuring Treeline Dynamics of the Nepal Himalaya

The alpine treeline ecotone is an important component of mountain ecosystems of the Nepal Himalayas; it plays a vital role in the livelihood of indigenous people and provides ecosystem services. However, the region faces a problem of paucity of data on treeline characteristics at the regional, landscape, and local scales. Therefore, I applied remote sensing and geographic information science approaches to investigate the treeline ecotone at the regional (entire Nepal) and landscape (Barun and Manang Valleys) scales. Treeline elevation ranges from 3300–4300 m. Abies spectabilis, Betula utilis, and Pinus wallichiana are the main treeline-forming species in the Nepal Himalayas. There is an east to west treeline elevation gradient at the regional scale. No slope exposure is observed at the regional scale; however, at the landscape scale, slope exposure is present only in a disturbed area. From the landscape scale study, I found that topography and human disturbance are the main treeline-controlling factors in Barun and Manang, respectively. Diverse treeline-forming species and treeline nature observed in the landscape and regional scale study suggested more investigation was needed at the local scale. Therefore, I established two transects of 20 m width and 120 m length (100 m above and 20 m below the forestline) in the Betula utilis sub-alpine forest of the Dhorpatan Hunting Reserve in western Nepal to understand the local scale treeline dynamics. Poor regeneration was observed above the forestline in both transects compared to below the forestline. Low regeneration at the treeline ecotone suggested site-specific biotic and abiotic controlling factors. Seedling and sapling establishment above the forestline is limited by a lack of moisture, an absence of suitable microsites, and the presence of herbivores. I found the treeline stable at the local scale. I used the Maxent species distribution modeling approach to predict the likelihood of treeline advance in the Nepal Himalayas by modeling the habitat suitability of three dominant treeline species—A. spectabilis, B. utilis, and P. wallichiana—under present and alternative future climates. Temperature-related climatic variables and elevation explained the greatest amount of variance in the distribution of the study species. Under future climate models, I found a regional increase in habitat suitability of all three treeline species that predicted a potential for northward and upslope advance

Pattern of extinction of the woolly mammoth in Beringia.

Extinction of the woolly mammoth in Beringia has long been subject to research and speculation. Here we use a new geo-referenced database of radiocarbon-dated evidence to show that mammoths were abundant in the open-habitat of Marine Isotope Stage 3 (∼45-30 ka). During the Last Glacial Maximum (∼25-20 ka), northern populations declined while those in interior Siberia increased. Northern mammoths increased after the glacial maximum, but declined at and after the Younger Dryas (∼12.9-11.5 ka). Remaining continental mammoths, now concentrated in the north, disappeared in the early Holocene with development of extensive peatlands, wet tundra, birch shrubland and coniferous forest. Long sympatry in Siberia suggests that humans may be best seen as a synergistic cofactor in that extirpation. The extinction of island populations occurred at ∼4 ka. Mammoth extinction was not due to a single cause, but followed a long trajectory in concert with changes in climate, habitat and human presence

The assessment of present, past and future climatic variability in the americas from tree-line environments

In this note we introduce one of 14 Collaborative Research Networks (CRN) funded by the Inter-American Institute for Global Change Research. It was established in 1999 and involves 15 principal investigators from 13 institutions in Canada, USA, Mexico, Bolivia, Chile and Argentina. The primary goals of the project are (i) to develop a network of tree-ring chronologies from climatically-sensitive treeline sites in the western American Cordillera and (ii) to use these data to reconstruct and compare regional interannual to decadal climate variability along the PEP-1 transect from Alaska to Tierra del Fuego. The project also seeks to enhance the development and utilization of dendrochronology for tropical mountain tree species and expand collaboration, training and the application of paleoenvironmental science within Latin America to address the issues of climate variability and change.Fil: Luckman, Brian H.. University of Western Ontario; CanadáFil: Boninsegna, Jose Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentin

Reconstructed warm season temperatures for Nome, Seward Peninsula, Alaska

[1] Understanding of past climate variability in the Bering Strait region and adjacent land areas is limited by a paucity of long instrumental and paleoclimatic records. Here we describe a reconstruction of May - August temperatures for Nome, Seward Peninsula, Alaska based on maximum latewood density data which considerably extends the available climatic information. The reconstruction shows warm conditions in the late 1600s and middle-20th century and cooler conditions in the 1800s. The summer of 1783, coinciding with the Laki, Iceland volcanic event, is among the coldest in the reconstruction. Statistically significant relationships with the North Pacific Index and Bering-Chukchi sea surface temperatures indicate that the Seward tree-ring data are potentially useful as long-term indices of atmosphere-ocean variability in the region.</p

Eurasian Arctic greening reveals teleconnections and the potential for novel ecosystems

Arctic warming has been linked to observed increases in tundra shrub cover and growth in recent decades on the basis of significant relationships between deciduous shrub growth/biomass and temperature. These vegetation trends have been linked to Arctic sea ice decline and thus to the sea ice/albedo feedback known as Arctic amplification. However, the interactions between climate, sea ice and tundra vegetation remain poorly understood. Here we reveal a 50- year growth response over a >100,000 km2 area to a rise in summer temperature for alder (Alnus) and willow (Salix), the most abundant shrub genera respectively at and north of the continental treeline. We demonstrate that whereas plant productivity is related to sea ice in late spring, the growing season peak responds to persistent synoptic-scale air masses over West Siberia associated with Fennoscandian weather systems through the Rossby wave train. Substrate is important for biomass accumulation, yet a strong correlation between growth and temperature encompasses all observed soil types. Vegetation is especially responsive to temperature in early summer. These results have significant implications for modelling present and future Low Arctic vegetation responses to climate change, and emphasize the potential for structurally novel ecosystems to emerge fromwithin the tundra zone.Vertaisarviointia edeltävä käsikirjoitu