Polar Treelines Research Articles

Global fading of the temperature-growth coupling at alpine and polar treelines.

Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree-ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature-growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature-growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature-growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming.

Alpine and Polar Treelines in a Changing Environment

Concerns have been raised with respect to the state of high-altitude and high-latitude treelines, as they are anticipated to undergo considerable modifications due to global change, especially due to climate warming [...]

Treeline Research—From the Roots of the Past to Present Time. A Review

Elevational and polar treelines have been studied for more than two centuries. The aim of the present article is to highlight in retrospect the scope of treeline research, scientific approaches and hypotheses on treeline causation, its spatial structures and temporal change. Systematic treeline research dates back to the end of the 19th century. The abundance of global, regional, and local studies has provided a complex picture of the great variety and heterogeneity of both altitudinal and polar treelines. Modern treeline research started in the 1930s, with experimental field and laboratory studies on the trees’ physiological response to the treeline environment. During the following decades, researchers’ interest increasingly focused on the altitudinal and polar treeline dynamics to climate warming since the Little Ice Age. Since the 1970s interest in treeline dynamics again increased and has considerably intensified from the 1990s to today. At the same time, remote sensing techniques and GIS application have essentially supported previous analyses of treeline spatial patterns and temporal variation. Simultaneously, the modelling of treeline has been rapidly increasing, often related to the current treeline shift and and its implications for biodiversity, and the ecosystem function and services of high-elevation forests. It appears, that many seemingly ‘new ideas’ already originated many decades ago and just confirm what has been known for a long time. Suggestions for further research are outlined.

Remotely sensed estimation of vegetation shifts in the polar and alpine tree-line ecotone in Finnish Lapland during the last three decades

Global warming is predicted to affect ecosystems, particularly in high-latitude regions where polar amplification accelerates temperature rise and environmental changes. Here, where plants grow under adverse conditions, a warmer climate provides more favourable conditions for growth and regeneration. At the alpine and polar tree line in Finnish Lapland, rising temperatures are assumed to promote densification and expansion of conifers towards fell tops and treeless boreal heathlands beyond the recent tree-line position. In this study, we analysed vegetation changes in the pine treeline ecotone in six study sites in Finnish Lapland using multi-spectral satellite data during 1984–2017. All of the six sites were established in fell areas, covering the transition from closed forest stands of the lower elevations to the open fell tops beyond the treeline position. The southern sites were located in pine dominated-stands, where treelines were of alpine character. The northern sites were located in the polar treeline ecotone where mountain birch forests already dominate the landscape. We assessed shifts in the vegetation pattern of the fell sites using the normalized difference vegetation index (NDVI) and a RandomForest land-cover classification as indicators of potential change. We did not find clear trends for advancing coniferous tree lines towards open fell tops or treeless heath vegetation, neither by NDVI change detection nor by the land-cover classification. However, we found evidence for densification of open forest stands and sparse vegetation cover in lower elevations and the expansion of deciduous vegetation in higher elevations of previously vegetation-free or sparsely covered fell tops. Increasing stand density was detected mostly in the southern, pine-dominated sites, while the northern sites indicated increasing biomass near the fell tops. Prominent changes in vegetation patterns originated rather from human impact in the southern sites appearing as recent roads, clear-cuttings or infrastructure constructions in skiing areas. In the northern sites, distinctive changes arose from human impact or from biotic disturbance events such as moth outbreaks defoliating mountain birch stands at site Karigasniemi.

FTIR spectroscopy of early and latewood of Larix gmelinii growing along the polar treeline: the correlation between absorption bands and climatic factors

ABSTRACT This paper presents the results of FTIR spectroscopy of the 1988–1998 annual ring early and latewood of Larix gmelinii (Rupr.) Rupr.) found along the polar treeline (Taimyr Peninsula, 70°52′53″ N, 102°58′26″ E). We analyzed samples of early and latewood to identify absorption bands of groups of wood components, as well as the bands that characterized the interactions among the components. We studied bound water spectral characteristics for wood formed in different years. An analysis of the correlation between the values of spectral absorption bands and mean monthly air temperature and precipitation showed that May-August weather had the greatest influence on the synthesis of the main polymeric components of early and latewood. Air temperature had a considerable effect on polymer composition of cell walls forming in early and latewood, whereas precipitation influenced only earlywood. FTIR spectroscopy is a promising tool to develop information on the biochemical composition of the walls of early and late tracheids of annual rings and on weather and climate influences on cell wall synthesis.

Thermal analysis of earlywood and latewood of larch (Larix gmelinii (Rupr.) Rupr.) found along the Polar tree line

This paper deals with the results of a study of the thermodestruction of the earlywood and latewood in the 1988–1998 growth rings of Gmelin larch (Larix gmelinii (Rupr.) Rupr.) trees growing along the polar tree line (70o52′53″N, 102°58′26″E, Taimyr Peninsula). The study used methods of thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC). Thermogravimetric data obtained were analyzed by Ozawa–Flynn–Wall kinetic model. We determined macrokinetic parameters of thermodestruction. Earlywood and latewood showed no significant difference in cellulose/hemicellulose/lignin ratio, whereas they differed significantly in destruction average value of activation energy (197.55 and 55.90 kJ mol−1, respectively). Our analysis of the correlation of thermodestruction values with mean monthly air temperature and precipitation revealed that component composition of both earlywood and latewood was influenced more by the weather conditions at the beginning of the growing season (April–June). Air temperature had markable influence on the wood component composition during cell wall synthesis only in latewood, whereas precipitation influenced the composition in both latewood and earlywood. TG/DTG and DSC methods were found to be promising for developing information on biochemical composition of early and latewood tracheids in stems and for identifying the extent of the influence of climatic changes on cell wall synthesis.

Growth response of Scots pines in polar-alpine tree-line to a warming climate

Coniferous trees at the alpine and polar tree lines of the Northern Hemisphere represent the outermost limit of their ecological range. Under such conditions, even small temperature variations may cause growth responses, which therefore can be used as indicators for changing environmental conditions. In this study we analysed the radial growth of Scots pine (Pinus sylvestris L.) along a latitudinal and altitudinal gradient at six locations in the polar and alpine tree-line ecotone in Finnish Lapland. We evaluated the effect of climatic trends on pine growth in relation to tree age and region, specifying a northern and a southern region in the study area. We found a response of Scots pine to climatic variations until the 1980s, but not to the current warming period. Increasing growth trends could be detected since 2000 in the radial growth of southern located trees, predominantly of juvenile ages, while the northern trees did not respond significantly to the current warming. In the north recent warmer and wetter conditions during winter time, inducing snow loads, wind damages, diseases and frost damages possibly masked the benefits of warmer conditions. The missing link between warming and radial growth would affect the use of tree-rings as proxy for past climate and for predictions for forest extension in polar-alpine tree-line sites.

Layering in the Rocky Mountain treeline ecotone: clonal conifer groups’ distribution, structure, and functional role

Layering plays a major role in treeline stability. Growth ring pattern of clonal groups reflects both an individualistic and an only temporally common response to the harsh treeline environment. Clonal groups are common to alpine and polar treelines. Distribution, shape, architecture, and development of clonal Picea engelmannii and Abies lasiocarpa in an extremely windy Rocky Mountain treeline ecotone are considered. Clonal groups prevail (>70% of all tree groups). Most of them consist of A. lasiocarpa (65%). Number, growth form, and damage were documented along transects and field routes. Age structure and development of the clonal groups were assessed using increment cores and stem disks. The groups usually consisted of different clonal series with several clonal members in each. On wind-swept terrain, clonal groups exhibit wedge-like, hedge-like, or mat-like habit. One clonal A. lasiocarpa group was studied in detail and it serves as an example showing formation, architecture, and internal growth pattern of clonal series. The clone members in the clone series exhibited an individualistic growth pattern. Wind-control of growth occasionally overruled the often ‘chaotic’ internal group growth pattern. The effect of prevailing winds, however, temporally overlapped with a ‘force’ directed from the center of the group margin (e.g., competition for light, high stem density), as reflected in recurrent change of eccentricity within the clonal trunks. Clonal groups may be of major importance in treeline dynamics.

Benefits from and threats to European treeline ecosystem services: an exploratory study of stakeholders and governance

The concept of ecosystem services (ES) is being increasingly applied in environmental governance and science. To safeguard key ES in changing and complex social–ecological systems such as treeline areas, we need to (1) map key ES in different types of treeline landscapes, (2) identify the stakeholders benefiting from and threatening ES, and (3) examine how ES could be governed more sustainably. We explore these questions in European treeline areas by using quantitative and qualitative social science techniques to analyse responses from a survey of local scientific experts in 20 altitudinal and polar treeline areas in 15 European countries. In contrast to the prevalent consideration of treeline areas as a single type of a social–ecological system, we show that European treeline areas can be divided into two types that significantly differ in the delivery of ES. Our analyses allowed us to categorize stakeholders according to their benefits from and threats to ES; “Green key players” formed the most numerous group, while smaller number of stakeholder groups was categorized as “Harmless crowd”, “Occasional stressors”, and “Risky users”. However, behaviour of stakeholders is very much site-specific. Of 595 pairs of stakeholders analysed, we found <5 EU-wide “Allies” and “Opponents”. Recommendations for improved governance include adjusting governance instruments to specific problems in divergent treeline systems and creating participatory structures where stakeholders better interact with scientists and can genuinely influence management decisions.

A climate-based model to predict potential treeline position around the globe

In situ temperature measurements revealed that the position of the high-elevation treeline is associated with a minimum seasonal mean air temperature within a temperature-defined minimum season length across latitudes. Here, we build upon this experience and present the results of a global statistical analysis and a predictive model for low temperature treeline positions. We identified 376 natural treelines from satellite images across the globe, and searched for their closest climatic proxies using a climate database. The analysis included a snow and a water balance submodel to account for season length constraints by snow pack and drought. We arrive at thermal treeline criteria almost identical to those that emerged from the earlier in situ measurements: tree growth requires a minimum length of the growing season of 94 days. The model yields best fit when the season is defined as all days with a daily mean temperature >0.9 °C, and a mean of 6.4 °C across all these days. The resultant treeline model ‘TREELIM’ offers a robust estimation of potential treeline elevation based on climate data only. Error terms include imprecise treeline position in satellite images and climate approximations in mountainous terrain. The algorithm permits constraining low temperature limits of forest growth worldwide (including polar treelines) and also permits a bioclimatic stratification of mountain biota, for instance, for biodiversity assessments. As a side product, the model yields the global potentially forested area. The results support the isotherm theory for natural treeline formation. This completely independent statistical assessment of the climatic drivers of the global treeline phenomenon confirmed the results of a multi-year measurement campaign.

Regional tree line dynamics in response to global change in the Pyrenees

Summary 1. Many studies in northern Europe, North and South America, describe regional trends of population densification at altitudinal and polar tree lines during the 20th century. The purpose of this study was (1) to ascertain if this regeneration enhancement is present across the alpine ecotones of the Pyrenees, (2) if synchronous recruitment trends are common among the studied populations and (3) to determine the tree limit stability during recent decades. 2. Twelve Pinus uncinata tree line populations were studied on the Iberian eastern range of the Pyrenees. Rectangular plots ranging from 940 to 7600 m2 were set along the forest-alpine grassland transition; more than 3600 P. uncinata individuals were mapped. Tree size and age were used to establish the demographic structure at each stand, and to characterize abrupt or smooth transition patterns along the tree line ecotone. A new procedure for estimating missing rings in off-centre cores was developed to ensure a correct interval for the age-classes distribution analysis. 3. Past and recent synchronous recruitment trends (mid 19th century, second half of the 20th century) were apparent at the tree line over the studied area of the Pyrenean range. The ecotone densification since the 1950s occurred in the context of climatic warming and substantial land use abandonment. Both gradual and step-like transition patterns in tree age and size along the ecotone were observed. 4. Regeneration enhancement in the last approximately 30 years appears as an abrupt change in population age structures, which could indicate the importance of feedback mechanisms for tree line recruitment dynamics. In 50% of the surveyed tree lines ecotone densification has been coupled to tree limit shifts in the recent past. This indicates both great tree limit sensitivity to short-term climatic changes and the presence of differential tree line dynamics at a regional scale. 5. Synthesis. The observed past and recent synchronous recruitment trends suggest the presence of regional climatic factors modulating tree line structure and dynamics. However, tree line dynamics in the Pyrenees have been widely affected by local anthropogenic activities. We suggest that the presence of step-like tree line transitions in tree age can be considered an evidence of recent human induced disturbances when no other major natural disturbances affect the tree line dynamics.

Ecology of woodlands and forests: description, dynamics, and diversity

Taking a functional rather than an ecosystem or a utilitarian approach, Thomas and Packham provide a concise account of the structure of woodlands and forests. Using examples from around the world - from polar treelines to savannahs to tropical rain forests - the authors explain the structure of the soil and the hidden world of the roots; how the main groups of organisms which live within them interact both positively and negatively. There is particular emphasis on woodland and forest processes, especially those involving the flow and cycling of nutrients, as well as the dynamics of wooded areas, considering how and why they have changed through geological time and continue to do so. This clear, non-technical, 2007 text will be of interest to undergraduates, foresters, ecologists and land managers.

Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales

ABSTRACTThe sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change are increasingly discussed in terms of climate change, often forgetting that climate is only one aspect of environmental variation. As treeline heterogeneity increases from global to regional and smaller scales, assessment of treeline sensitivity at the landscape and local scales requires a more complex approach than at the global scale. The time scale (short‐, medium‐, long‐term) also plays an important role when considering treeline sensitivity. The sensitivity of the treeline to a changing environment varies among different types of treeline. Treelines controlled mainly by orographic influences are not very susceptible to the effects of warming climates. Greatest sensitivity can be expected in anthropogenic treelines after the cessation of human activity. However, tree invasion into former forested areas above the anthropogenic forest limit is controlled by site conditions, and in particular, by microclimates and soils. Apart from changes in tree physiognomy, the spontaneous advance of young growth of forest‐forming tree species into present treeless areas within the treeline ecotone and beyond the tree limit is considered to be the best indicator of treeline sensitivity to environmental change. The sensitivity of climatic treelines to climate warming varies both in the local and regional topographical conditions. Furthermore, treeline history and its after‐effects also play an important role. The sensitivity of treelines to changes in given factors (e.g. winter snow pack, soil moisture, temperature, evaporation, etc.) may vary among areas with differing climatic characteristics. In general, forest will not advance in a closed front but will follow sites that became more favourable to tree establishment under the changed climatic conditions.

Responses to recent climatewarming of Pinus sylvestris and Pinus cembra within their montane transition zone in the Swiss Alps

Abstract. Altitudinal and latitudinal distribution limits of trees are mainly controlled by temperature. Therefore climate warming is expected to induce upslope or poleward migrations. In the Swiss Central Alps, summers in the period 1982‐1991 were on average 0.8 °C warmer than those of the period 30 yr before. We investigated whether populations of conifers at the montane Pinus sylvestris‐Pinus cembra ecocline exhibit demographic trends in response to that warming. We found no evidence for this. Young seedlings of Pinus sylvestris, the species which is expected to expand its range upward in a warmer climate, were virtually absent from all sites, whereas large fractions of Pinus cembra populations were observed in the seedling and juvenile categories even below the present lower distribution limit of adult trees. This suggests that there are no major altitudinal shifts in response to the recent sequence of warmer summers. Germination and seedling survival trials with Pinus sylvestris suggest that temperature per se would not exclude this species even from establishing at the current treeline in the Swiss Central Alps. Similar results were found at the polar treeline. Phytotron tests of seedling survival showed much less drought resistance in Pinus sylvestris than in Pinus cembra which is in contrast to their phytogeographic distributions. Thus, the montane pine ecocline in the Swiss Central Alps seems to be stabilized by species interactions and may not be directly responsive to moderate climatic change, which needs to be taken into account in predictive attempts.

Climatic signals in British Isles tree-ring chronologies

INTERPRETATION of proxy records of past environmental conditions derived from dated geological or biological materials is of great importance for the extension of the climatic record1,2. Precisely dated, replicated tree-ring series have been particularly useful as they provide records (usually ring-widths) dated to the individual year, for hundreds or thousands of years. Each such series, or chronology, is derived from a particular known location. Thus a network of such chronologies may be developed for a particular region and used as a proxy record of spatial and temporal climatic variations. This has been achieved in North America by Fritts et al.3,4, using principally chronologies from semi-arid areas or from near altitudinal or polar tree-lines. We report evidence here that tree-ring chronologies from sites in the British Isles will provide suitable proxy records for the reconstruction of historical temporal and spatial variation of climate.

CLIMATIC ZONES OF THE PLEISTOCENE

Although much emphasis has been placed on the effects of Pleistocene ice sheet as the determinant of Pleistocene climate in Europe, only through the combined evaluation of all useful climatic indicators can Pleistocene climatic zones be differentiated. The relationships between a paleoclimatically determined snowline and polar treelines and extent of loess deposition, determined stratigraphically, botanically, and morphologically, also indicate climatic conditions of Pleistocene Europe. Five great climatic-morphologic and plant-geographic zones, namely, forest-rubble tundra, forest tundra, loess tundra, loess steppe, and loess-forest steppe, may be distinguished. Considering these factors and their characteristics and spatial and temporal extents, it is possible to chronologically present the differentiation of the Wurm glacial period independent of the retardation effect of ice sheets. --G. E. Denegar.