Dynamics of alpine treelines: positive feedbacks and global, regional and local controls
Dynamics of alpine treelines: positive feedbacks and global, regional and local controls
Journal of Ecology and Environment. 2015. Feb, 38(1): 1-14
Copyright © 2015, The Ecological Society of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial Licens ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : November 01, 2014
  • Accepted : December 29, 2014
  • Published : February 28, 2015
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About the Authors
Jong-Wook, Kim
Department of Biological Science, Mokpo National University, Muan-gun 534-729, Korea
Jeom-Sook, Lee
Department of Biology, Kunsan National University, Gunsan 573-701, Korea

Whilst it is clear that increasing temperatures from global environmental change will impact the positions of alpine treelines, it is likely that a range of regional and local scaled factors will mediate the overall impact of global scale climate drivers. We summarized 12 categories of abiotic and biotic factors as 4 groups determining treeline positions. First, there are global factors related to climate-induced growth limitation and carbon limitation. Second, there are seven regional and local factors related to treeline dynamics including frost stress, topography, water stress, snow, wind, fire and non-fire disturbance. Third, species-specific factors can control treeline dynamics through their influence on reproduction and life history traits. Fourth, there are positive feedbacks in structuring the dynamics of treelines. Globally, the commonly accepted growth limitation hypothesis is that growth at a treeline is limited by temperature. Meanwhile, positive feedbacks between canopy cover and tree establishment are likely to control the spatial pattern and temporal dynamics of many treelines. The presence of non-linear dynamics at treelines has implications for the use of treelines as barometers of climate change because the lagged responses and abrupt shifts inherent in non-equilibrium systems may combine to mask the overall climate trend.
Average temperatures have risen globally over the past century, with the most pronounced and rapid changes at high altitudes ( IPCC 2007 , 2013 ). Within these zones, treeline positions are widely thought to be temperature sensitive and potentially responsive to a warming climate ( Lenoir et al. 2008 , Harsch et al. 2009 , Kreyling et al. 2010 , Kullman 2010a ). For this reason, the dynamics of the upper altitudinal or latitudinal treeline have been studied around the globe with the aim of detecting change, understanding responses to temperature variation, and evaluating the threat to alpine biota in response to treeline movement ( Kong 1999 , 2000 , Walther 2003 , Holtmeier and Broll 2005 , Ihm et al. 2007 , 2012 , Case and Duncan 2014 , Hagedorn et al. 2014 , Smith-McKenna et al. 2014 ). Harsch et al. (2009) reported a global dataset of 166 sites for which treeline dynamics had been recorded since AD 1900: advance was recorded at 52% of sites with only 1% reporting treeline recession. Recent treeline expansion has been reported for many locations around the world: Russia ( MacDonald et al. 2008 ), Asia ( Zhang et al. 2001 ), Europe ( Kullman 2001 ), India ( Singh et al. 2012 ), North America ( Szeicz and MacDonald 1995 ) and New Zealand ( Wardle and Coleman 1992 ). An assessment of treelines for the Swiss Alps ( Gehrig-Fasel et al. 2007 , Díaz-Varela et al. 2010 ) found a decadal increment of 32 m of mean altitudinal increment for a 12-year period. The record of temporal treeline dynamics on a slope in the Austrian central Alps ( Wallentin et al. 2008 ) indicated a decadal advance of 28 m for the maximum elevation of the treeline and 17 m for the mean elevation in the period 1954-2006. Feeley et al. (2011) in a study of elevational shifts of the 38 Andean tree genera reported a mean migration rate of 2.5 m yr −1 .
The tree limit of upright tree growth is defined as the line connecting the uppermost upright trees with a minimum height of 2 m ( Holtmeier 2003 ). This definition of tree limits captures three important vegetation boundaries: “treeline” (the point where trees disappear), the “forestline” (the upper limit of closed-canopy forest) ( Wieser et al. 2009 , Harsch and Bader 2011 ), and the “treeline ecotone” (the transition zone from the uppermost closed forest to treeless subalpine and alpine vegetation)( Fig. 1 ) ( Körner and Paulsen 2004 , Autio 2006 , Fajardo et al. 2011 , Kim 2012 , Körner 2012 ). Four primary treeline forms have been distinguished ( Kong and Watts 1993 , Chang et al. 1998 , Cuevas 2000 , Moen et al. 2004 , Harsch et al. 2009 , Harsch and Bader 2011 , Green and Venn 2012 ): diffuse, abrupt, island and krummholz. Treeline form indicates the relative dependence of tree performance on various aspects of the external climate and other environmental factors (especially summer warmth versus winter stressors) and on internal feedbacks, thus allowing inferences on the type as well as the strength of climate-change responses ( Harsch and Bader 2011 ). For example, diffuse treelines may be more responsive to warming because they are more strongly growth limited. In contrast, abrupt or krummholz treelines may be more strongly influenced by stress factors associated with winter conditions that lead to plant damage and limit survival. Krummholz form, characterized by a stunted habit, is commonly attributed to damage associated with factors such as wind abrasion, snow and ice damage. Hence, advance in krummholz and abrupt treelines may occur only when winter warming is sufficient to ameliorate other constraints, or when temperatures increase sufficiently to compensate for those constraints.
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Treeline region: ideas and concepts mainly according to Heikkinen et al. (2002) and Körner and Paulsen (2004).
A range of regional and local scale factors will affect the overall impact of global scale climate drivers ( Grabherr et al. 1994 , Körner 1998 , Jobbagy and Jackson 2000 , Cullen et al. 2001 , Dullinger et al. 2004 , Walther et al. 2005 ). Assessment of the response of the treeline to environmental variability at regional and local scales is complex ( Batllori et al. 2009 ) and requires a solid organizational framework. We summarized 12 categories of abiotic and biotic factors as 4 groups of global, regional, and local factors, speciesspecific traits, and positive feedbacks to differentiate the effects of each factor on the treeline positions ( Fig. 2 and Table 1 ). Of prime importance are global factors related to climate. The thermal limitation of either carbon uptake (photosynthesis) or carbon investment (growth) can be explained by limitations dictated by a globally common isotherm: (1) growth limitation ( Tranquillini 1979 , Körner 1998 , Shi et al. 2006 ) and (2) carbon limitation ( Schulze et al. 1967 , Stevens and Fox 1991 ). Secondarily, there are six regional and local factors related to treeline dynamics: (1) frost stress ( Körner 1998 , Körner and Paulsen 2004 ), (2) topography ( Brown 1994a , 1994b , Leonelli et al. 2011 ), (3) water stress ( Grace 1989 , Richardson and Friedland 2009 ), (4) snow ( Walsh et al. 1994 , Gottfried et al. 2011 ), (5) wind ( Holtmeier and Broll 2007 , Richardson and Friedland 2009 ),(6) fire ( Shankman and Daly 1988 , Stueve et al. 2009 ), and (7) disturbance ( Gehrig-Fasel et al. 2007 , Tomback and Resler 2007 ). Thirdly, species-specific traits deal with the effects of treeline species on treeline positions including reproduction ( Sveinbjörnsson et al. 1996 , Körner 1998 ) and life history traits ( Szeicz and Macdonald 1995 , Motta and Nola 2001 ). Finally, there are positive feedbacks in structuring the dynamics of treelines ( Wilson and Agnew 1992 , Malanson et al. 2011 ).
Global, regional, and local factors, species-specific traits, and positive feedbacks determining treeline positions in high altitude environments
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Global, regional, and local factors, species-specific traits, and positive feedbacks determining treeline positions in high altitude environments
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Effects of global, regional, and local factors, species-specific traits and positive feedbacks on treeline migration. Double ended arrow indicates a feedback relationship between processes. Adapted from Tranquillini (1979), Smith et al. (2006) and concepts discussed in this paper.
The most likely explanations for the formation of alpine treelines emphasize the temperature-factor characteristic of higher altitudes such as growing season length and summer temperatures ( Table 1 and Fig. 2 ). Any common treeline theory needs to account for higher- and lower-latitude conditions, but the traditional view of temperature-controlled tree growth at upper elevations is largely based on high-latitude treelines ( Handa et al. 2005 , Shi et al. 2008 ). The treelines of the world’s mountains seem to follow a common isotherm, but this is only supported by indirect evidence. Actually, high altitude climatic treelines are associated with a seasonal mean ground temperature of 6.7-13.0°C ( Cogbill and White 1991 , Körner and Paulsen 2004 ). This temperature-treeline relationship emphasizes the length and mean temperature of the growing season ( Hättenschwiler and Körner 1995 , Körner 1998 , Sveinbjörnsson 2000 , Daniels and Veblen 2003 , Hoch and Körner 2003 , Lloyd and Fastie 200 3, Camarero and Gutierrez 2004 , Wang et al. 2006 ). Air and soil temperature has recently been identified as the most important factor in controlling the treeline positions across the world ( Körner and Paulsen 2004 , Leonelli et al. 2011 ). Seasonal mean temperatures are higher (7-8°C) in the temperate and Mediterranean zone treelines, and are lower in equatorial treelines (5-6°C) and in the subarctic and boreal zone (6-7°C). While air temperatures are higher than soil temperatures in warm periods, and are lower than soil temperatures in cold periods, the daily means of air and soil temperatures are almost the same at 6-7°C, a physics driven coincidence with the global mean temperature at treelines ( Körner and Paulsen 2004 ). This translates to low soil temperatures that reduce soil nutrient availability by lowering microbial soil mineralization rates and nitrogen fixation and by reducing nutrient uptake capacity and reducing root growth and thus their exploitable soil volume ( Moen et al. 2008 ). Two hypotheses have been put forward to link temperature, plant metabolism and treeline positions: (1) the growth (i.e., sink) limitation hypothesis ( Körner 1998 ), a sink limitation through thermal limitation of meristematic activity with direct consequences for structural growth, and (2) the carbon (i.e., source) limitation hypothesis ( Stevens and Fox 1991 ), a source limitation through insufficient net carbon acquisition due to low daytime temperatures.
- Growth limitation
The commonly accepted hypothesis is that growth at a treeline is limited by temperature ( Hoch and Körner 2003 , Shi et al. 2006 , Susiluoto et al. 2007 , Shi et al. 2008 , Hoch and Körner 2009 ). Körner (1998) argued that the development of new tissues in treeline trees are less likely to be limited by the supply of photosynthetic assimilates, but rather by the rate at which the products can be utilized. This is known as the growth limitation hypothesis. In a comparison with different treeline Pinus species from three latitudes (Mexico, Swiss Alps, and Sweden), the concentration of non-structural carbonates and lipids (NSC) was not lower, but rather higher at the tree limit, compared to tall montane forests, supporting growth rather than carbon limitation ( Hoch et al. 2002 , Hoch and Körner 2003 , Handa et al. 2005 , Hoch and Körner 2005 , Fajardo et al. 2012 ). The studies have demonstrated that NSC concentration rather increases as one approaches the tree limit. This holds true for both deciduous and evergreen taxa and for the eastern (lower altitude) as well as the western (very high altitude) treelines of the eastern Himalayas ( Shi et al. 2008 ). Environmental stress affects growth (carbon investments) long before they affect carbon assimilation ( Körner 2003 ). The growth limitation hypothesis is consistent with the correlation of tree growth with mid-summer temperatures, frequently observed at the treeline, and can be evoked to explain why the altitude of treelines is usually well correlated with mid-summer temperatures ( Körner and Paulsen 2004 ).
- Carbon limitation
Carbon limitation because of a shortage of photoassimilates has long been regarded as the key to explain the upper altitudinal or latitudinal treeline on a global scale ( Schulze et al. 1967 , Stevens and Fox 1991 ). The carbon limitation hypothesis draws on the carbon balance of photosynthesis and respiration ( Körner 2003 ). Körner (1998) hypothesized that carbon gain is restricted by the growing season lengths, whereby low temperatures and short growing periods limit photosynthesis to the point that it is exceeded by respiratory demand. To date, there has been little evidence in support of the carbon limitation hypothesis, although no direct manipulative test has been carried out. A recent study did not find evidence for carbon processing (sink) limitations, but rather photosynthetic carbon gain (source) limitations in Himalayan trees at the treeline ( Li et al. 2008 ). Also, significant reductions in photosynthetic carbon gain in seedlings just older than the pre-establishment life stage (>3 years old) have been reported ( Johnson et al. 2004 ), although limitations in carbon gain versus processing have not been differentiated. Meanwhile, the photosynthetic carbon uptake in treeline trees does not appear to be highly sensitive to temperature ( Shi et al. 2008 ). Furthermore, studies of the mobile carbon pool, measured as non-structural carbonates and lipids, in trees across an altitudinal and latitudinal transect at the treeline have shown that the accumulated carbon reserves are not lower at high elevations compared to low elevations ( Hoch and Körner 2003 , Körner 2003 , Handa et al. 2005 ).
The second suite of factors determining treeline positions operate at regional and local scales ( Walsh et al. 1994 , Daniels and Veblen 2003 , Körner and Paulsen 2004 , Malanson et al. 2007 , Leonelli et al. 2011 ). All these factors interact with and, in some cases, supersede the influence of climate to explain treeline positions ( Table 1 and Fig. 2 ). Thus, research on climate impacts on treelines at regional and local scales needs to identify multiple potential sources of variation in the structure and dynamics of treelines ( Daniels and Veblen 2003 ).
Frost, frost desiccation or phototoxic effects may contribute to treeline formation ( Körner 1998 , Berdanier 2010 ). Frost damage does not threaten tree survival in the temperate zone treelines, but may lead to distorted growth by causing injury damages ( Tranquillini 1979 , Körner 2012 , Rixen et al. 2012 ). Frost desiccation occurs during late winter when the soil is frozen but skies are clear and solar radiation is high. As exposed frosted foliage warms in direct sun, a strong vapor pressure deficit is created, evapotranspiration from the leaf is high, and desiccation occurs ( Sakai 1970 , Richardson and Friedland 2009 ). Frost desiccation is mainly observed in young trees and diminishes with age and size of branches or trees ( Körner 1998 ). Although this frost desiccation may be a problem for young trees above the treeline in some parts of the temperate zone, this factor does not appear to be widespread for established trees.
The role of topographical factors in controlling future treeline positions is manifest in the interaction of climate with elevation, aspect and soil properties at higher altitudes ( Brown 1994a , 1994b , Gottfried et al. 1999 , Walsh et al. 2003 , Butler et al. 2007 , Bader and Ruijten 2008 , Leonelli et al. 2009 , 2011 , Scherrer and Körner 2011 ) ( Table 1 and Fig. 2 ). Mountains are characteristically conical in shape, and climate change impact scenarios usually assume that a smaller surface area will be available as species shift to higher elevations. However, as the frequency distribution of additional physiographic factors (e.g., slope angle) changes with increasing elevation (e.g., fewer gentle slopes available at higher elevation), upslope migrating species will encounter increasingly unsuitable conditions ( Pauli et al. 1996 , Guisan and Theurillat 2000 ). The masselevation effect describes variation in the treeline based on mountain size and location and was introduced to account for the observed tendency for temperature-related factors such as treeline and snowline to occur at higher elevations in the warmer and drier continental climate of the inner regions than on their outer margins ( Odland 2009 , Leonelli et al. 2011 ). Under warmer temperature conditions, treeline shifts are therefore expected to be more evident in the inner regions since the treelines are more likely to shift upward into the alpine environments ( Leonelli et al. 2011 ). Resler (2006) highlights the importance of surface geomorphic features, specifically terrace risers, increasing favorable local site conditions, largely by protecting seedlings from wind in the study of the role of surface geomorphic features in tree establishment at the alpine treeline in Glacier National Park, Montana. The sheltering effect of surface features enables initial seedling establishment, and in some cases survival, above current treeline locations, thereby initiating a positive feedback effect that encourages subsequent tree establishment. Geomorphic features are therefore important in linking scales of patterns and positions at the alpine treeline.
The effects of water stress limiting the altitude of a treeline has been reported ( Marchand and Chabot 1978 , Cochrane and Slatyer 1988 , Rada et al. 1996 , Cairns and Malanson 1998 , Smith et al. 2003 ). Decreasing moisture contents and osmotic potential values at wind-exposed treeline during the winter months can be explained by the fact that at such sites the soil remains frozen to a depth of 1 m. The soil thaws near the surface only at the end of April, but later in May it thaws out to a greater depth ( Aulitzky 1961 , Baig and Tranquillini 1980 ). Plants wintering on such sites are unable to absorb soil moisture, thus their survival depends upon tissue water reserves and drought resistance.
The proportion of trees suffering serious snow load damage has been reported to be 15-76% at the treeline ( Walsh et al. 1994 , Autio and Colpaert 2005 , Bebi et al. 2009 ), so the damage of this kind is a key factor controlling the treeline positions. Growing season length and moisture can be affected by snow cover. In some places, snow will suppress the treeline by shortening the growing season and in others, snow encourages an upward expansion of the treeline along the elevation gradient by providing more water ( Walsh et al. 1994 ). But these factors alone cannot explain why snow-free ridges at high elevations do not have trees, nor why some tropical mountains with little or no annual snowfall still have treelines ( Hättenschwiler and Smith 1999 , Richardson and Friedland 2009 , Barbeito et al. 2012 ). Snowload also plays an important role in the protection of young trees from frost and wind damage in treelines ( Holtmeier 2003 , Holtmeier and Broll 2007 , Devi et al. 2008 ). In this case, snow accumulation around trees might induce an insulation and protection from injurious climatic effects, which may allow establishing seedlings to grow into successful trees. In contrast, long-lying snow may curtail the growing season and increase snow fungus infection ( Phacidium infestans, Herpotrichia junipeperi, Gremmeniella abietaina ) of seedlings of evergreen conifers, mainly in wet years and in maritime regions ( Holtmeier and Broll 2007 ). The influence of wind on tree growth and positions in treelines can be interpreted within the context of stress and strain relationships ( Grace 1977 , Telewski 1995 , Richardson and Friedland 2009 ). The primary stress is the force of the wind applied to the tree. The fluttering of leaves and branches, the back and forth swaying motion of the stems, the displacement or wind-induced lean of the stems and failure of the stems or roots, resulting in windthrow, are the viable, mechanical strains manifested by the tree. Secondary stresses include the influence of desiccation by wind, abrasion by wind-driven snow and friction from strong winds ( Scott et al. 1993 ). As the magnitude of the stress (windspeed) increases, so do the resulting strains, resulting in a cascade of physiological strain responses. The physiological responses range from rapid changes in transpiration and photosynthesis at the foliar level to reduced translocation, callose formation and ethylene production in the phloem and cainbial zone. Long-term developmental and structural changes occur in canopy architecture, leaf, stem and root morphology, and modifications of cell structure and biomechanical properties of the xylem. Especially, wind appears to play a role in many northern hemisphere treelines because of common desiccation of high-elevation trees ( Scott et al. 1993 , Hättenschwiler and Smith 1999 , Sveinbjörnsson 2000 , Kullman 2005 , Richardson and Friedland 2009 ). Wind also plays a role in determining the tree heights and treeline positions at most of the world’s treelines ( Grace 1977 , Sveinbjörnsson 2000 ). Many tree species, particularly conifers with a distinct leader growth, take on flag-, hedge- or even matlike growth forms in the windiest situations.
Fire can affect local or regional treelines in several ways. Firstly, fire can cause the depression of altitudinal limits of treelines through the physical destruction of stands of trees ( Wilson and Agnew 1992 , Noble 1993 ). Under this scenario, particularly severe fires propagated in flammable treeless alpine environments or the treeline forests themselves can cause the mortality of large populations of trees at the treeline. For example, fires in the Colorado Front Range ( Shankman and Daly 1988 ) and the Cascades National Park ( Hemstrom and Franklin 1982 , Stueve et al. 2009 ) of the western USA, severely burned extensive areas of high-elevation forest, effectively lowering the existing treeline. Secondly, the destruction of seedlings established upslope by fire may prevent the advance of treelines. This mechanism is exacerbated by the slow growth rates of seedlings at higher altitudes such that they cannot escape the fire trap ( Murphy and Bowman 2012 ). Thirdly, the removal of upslope vegetation cover may reduce competition pressure and provide substrate for seedling establishment and treeline advance ( Noble 1980 , Green 2009 ). The impacts of fire on treelines are particularly relevant to regions with a climate and vegetation conducive to fire such as the Rocky Mountains of North America, Patagonia in South America and the Australian Alps. Anthropogenic climate change has the capacity to alter fire regimes fundamentally through changing seasonal patterns of temperature, wind and precipitation, especially the occurrence of extreme weather events ( Scott et al. 2014 ). Clearly, any consideration of the effect of global climate change on treelines will have to factor in a possible upregulation of fire in flammable alpine environments and their effect on tree populations and seedling recruitment.
Natural disturbance regimes related to herbivory, fungal pathogens, grazing and human disturbance such as land use changes and fragmentation are significant factors determining treeline dynamics ( Cullen et al. 2001 , Daniels and Veblen 2003 , Batllori and Gutiérrez 2008 , Brown 2010 , Leonelli et al. 2011 ) ( Table 1 and Fig. 2 ). Losses of a dominant treeline species by invasive forest pathogen may result in changes in vegetation patterns at the treeline ( Harvell et al. 2002 , Tomback and Resler 2007 ). Herbivores can limit the treeline below its potential at the landscape scale ( Cairns and Moen 2004 , Speed et al. 2010 , Herrero et al. 2011 ). The presence of large numbers of small trees above the current treeline at a site in northern Sweden that experiences limited reindeer populations ( Rangifer tarandus ) suggests range expansion at low levels of herbivory ( Cairns and Moen 2004 ). Other locations in the same region with higher reindeer populations have considerably fewer small trees, suggesting that range expansion of treelines is occurring much more slowly, if at all. The use of treelines as indicators of climate change is confounded by the activity of herbivores, which may either strengthen or nullify the impacts of a changed climate. Similar arguments are likely to be applicable to other ecotones ( Bale et al. 2002 , Cairns and Moen 2004 ).
A possible explanation for treeline changes is land use change ( Bolli et al. 2007 , Gehrig-Fasel et al. 2007 , Macek et al. 2009 , Chauchard et al. 2010 ). Most upward shift activities were found to occur below the potential regional treeline ( Gehrig-Fasel et al. 2007 ). Only 4% of the upward shifts were identified to rise above the potential regional treeline, thus indicating climate change. Land abandonment was the most dominant driver for the establishment of new forest areas, even at the treeline ecotone.
- Reproduction
Seedling establishment and subsequent growth are necessary for the formation of new forests at higher altitudes, and both appear to be particularly challenging for tree species in the upper treeline ecotone ( Black and Bliss 1980 , Daly and Shankman 1985 , Butler et al. 1994 , Cho 1994 , Hättenschwiler and Smith 1999 , Germino et al. 2002 , Kullman 2002 , Smith et al. 2003 , 2009 , Gworek et al. 2007 , Batllori et al. 2009 , Kullman 2010b ) ( Table 1 and Fig. 2 ). Regeneration success determines whether the treeline shifts or remains static in response to environmental changes. Seed development, dispersal, germination, and seedling establishment are all limited by cold temperatures at the treeline ( Körner 1998 , Hättenschwiler and Smith 1999 , Danby and Hik 2007 , Holtmeier and Broll 2007 , Dang et al. 2009 ). Some strategies such as extensive natural layering among Picea mariana (black spruce) stands at treelines in northern Quebec permit continued recruitment during prolonged cooling episodes ( Payette et al. 1989 ). Many trees in these stands survive such episodes by undergoing extreme reductions in growth rates and shifts in growth form from upright to krummholz, although these morphological changes can also delay or moderate responses to subsequent warming.
- Life history traits
The life history traits of tree populations at treelines have been closely linked to differences amongst species in relation to the influence of low summer temperatures on growth rates, phenology, cover/density and development of trees ( Motta and Nola 2001 , Grace et al. 2002 , Gamache and Payette 2004 , Kullman 2007 , Macek et al. 2009 , Hertel and Schöling 2011 , Xu et al. 2012 , Anadon-Rosell et al. 2014 ) ( Table 1 and Fig. 2 ). Understanding the interaction between environmental factors contributing to treeline formation and how these factors influence different life stages remains a major research challenge ( Barbeito et al. 2012 ). This studies of the spatial and temporal dynamics of tree mortality and growth at treelines in the Swiss Alps provide experimental evidence that tree survival and height growth require different environmental conditions and that even small changes in the duration of snow cover, in addition to changes in temperature, can strongly impact tree survival and growth patterns at treelines. Further, their results show that the relative importance of different environmental variables for tree seedlings changes during the juvenile phase as they grow taller.
The role of positive feedbacks in structuring the spatial pattern and temporal dynamics of treelines must be considered in models that put forward treelines as indicators of climate and environmental change. Positive feedbacks arise when vegetation communities actively modify their environment in a direction that enhances its own growth and survival, and simultaneously hinders or constrains other vegetation types ( Wood et al. 2011 ). Positive feedbacks are emerging as pivotal controls of the distribution of alternative stable states ( Scheffer et al. 2001 ) of plant communities at a variety of spatial scales from herbaceous sea beds, to regional dichotomies of treeless and woody vegetation in the temperate zone ( Warman and Moles 2009 , Odion et al. 2010 , Knox and Clarke 2012 , Wood and Bowman 2012 ), to savannahs and closed forests across the tropics ( Hirota et al. 2011 , Mayer and Khalyani 2011 , Staver et al. 2011 , Murphy and Bowman 2012 ). Identifying whether vegetation communities exist as alternative stable states is crucial because of the non-linear dynamics that dictate their past, present and future trajectories. Instead of gradual linear changes in response to gradual changes in climate, nutrient loading or habitat fragmentation, systems maintained as alternative stable states are characterized by lags in response to climate or abrupt switches to a contrasting state, as the resilience afforded by positive feedbacks breaks down ( Scheffer et al. 2001 ).
At the treeline, forest and treeless alpine vegetation communities can be considered as alternative stable states when the two opposing vegetation types are maintained by strong feedbacks driven by interactions between forest canopy cover, gradients of temperature and resource availability and the frequency and intensity of local stress factors such as wind, snow, frost and fire ( Fig 3 ). Through canopy cover, trees are able to modify environmental conditions to promote tree establishment and growth such that abrupt treeline boundaries are not related to patterns in substrate, topography or temperature, but are brought about by plant interactions alone ( Wilson and Agnew 1992 , Malanson et al. 2011 ). Field and modelling studies have shown that by modifying their microclimate, tree canopies can promote the perpetuation of their community through a range of mechanisms including shielding of wind ( Alftine and Malanson 2004 , Holtmeier and Broll 2010 ), exclusion of fire ( Bader et al. 2008 ), moderation of snow pack and frost events ( Wilson and Agnew 1992 , Bekker 2005 , Batllori et al. 2009 ), accumulation of nutrients ( Cairns 1999 ), or offering shade and protection from exposure to high levels of solar radiation ( Ball et al. 1997 , Germino and Smith 1999 , Bader 2007 ).
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Model of the combined effects of temperature/resource availability and local/regional stress factors (e.g., freezing temperatures, snow pack, wind or high solar radiation) on canopy cover at the treeline. In two alternative representations of the same model: (a) canopy cover is shown as a function of temperature and resource availability, and (b) canopy cover is shown as a function of local and regional stress factors. Alternative stable states of forest and treeless alpine vegetation exist under the same external environmental conditions (zone of tension: grey area) depending on the frequency and intensity of local and regional stress factors. Abrupt transitions from forest to alpine vegetation or vice versa can occur at threshold temperature/resource conditions or due to disturbance by stress factors. Adapted from Murphy and Bowman (2012) and Malanson et al. (2011).
The identification of positive feedbacks operating within treelines has important implications for their usefulness as barometers of climate and environmental change. These feedbacks ensure that an established forest can rejuvenate and persist even if the external climate deteriorates. Thus, treelines are not necessarily in equilibrium with climatic conditions and may exhibit a lagged response to climate change ( Scheffer et al. 2001 ) (see zone of tension in Fig. 3 ). A further emergent property of alternative stable state ecosystems is abrupt shifts from one state to another (i.e., tree to alpine) once a threshold in external conditions is reached or due to a disturbance ( Scheffer et al. 2001 ) ( Fig. 3 ). To date, field observations of the temporal trends expected in systems operating as alternative stable states (i.e., lags and catastrophic shifts) are lacking for alpine treeline environments (and indeed, for most long-lived forest ecosystems), although simulation modelling approaches have been used to explore the effects of positive feedbacks on the temporal dynamics at the treeline ( Malanson 2001 , Alftine and Malanson 2004 , Bader et al. 2008 ). Definitive evidence for treelines exhibiting the complex characteristics of alternative stable states is still nascent, but can be progressed though the adoption of the emerging research approaches undertaken in forest-treeless systems such as the spatial analyses of forests in tropical savannahs ( Staver et al. 2011 , Murphy and Bowman 2012 ) and the elucidation of temporal trends in vegetation states identified from sediments in temperate forests ( Jeffers et al. 2011 , Fletcher et al. 2014 ).
In general, treeline positions are the result of a combination of unfavorable conditions for tree regeneration, seedling establishment and tree growth ( Table 1 and Figs. 2 and 3 ). The commonly accepted hypothesis is that growth at a treeline is limited by growing season temperature ( Grabherr et al. 1994 , Shi et al. 2006 , Susiluoto et al. 2007 , Hoch and Körner 2009 ). This growth limitation hypothesis is consistent with the correlation of tree growth with mid-summer temperatures, frequently observed at the treeline, and can be evoked to explain why the altitude of treelines is well correlated with July temperatures in the northern hemisphere ( Körner 1998 ). The two factors determining the treeline positions are regional and local factors and species-specific traits. These factors interact with and, in some cases, supersede the influence of climate to explain treeline positions, structure and dynamics. Also, the role of positive feedbacks in decoupling the spatial pattern and process at the treeline from the underlying environmental variables is particularly important because it has implications for future trajectories of boundary shifts in the face of climate change. Thus, research on the climate change impacts on treelines at regional and local scales needs to identify multiple potential sources of variation in the structure and dynamics of treelines ( Daniels and Veblen 2003 ). In order to estimate and predict the effect of climate change on upward advance of alpine treelines and to conserve and restore the treeline species, the following suggestions should receive particular attention in the future ( Sveinbjörnsson 2000 , Dullinger et al. 2004 , Díaz-Varela et al. 2010 , Malanson et al. 2011 , Szerencsits 2012 , Carlson et al. 2013 ). First, additional experimental studies are required to differentiate between the growth limitation and carbon limitation hypotheses. Such experiments must be carried out in a wide range of environments and for multiple species before robust generalizations are made. Second, further exploration is required to elucidate how positive feedbacks and speciesspecific traits may affect future range dynamics and how they interact with variation in regional and local climate trends. Third, an effort should be made to obtain better landscape scale predictors controlling treeline positions through the use of the rapidly expanding availability of remote sensed information. Fourth, further research initiatives in high mountain environments are needed to establish an effective projection and conservation system. This should include international and interdisciplinary cooperation, and include long-term monitoring, ecophysiological and phenological studies and predictive modelling. Ergo, there is a pressing need for an effective network of monitoring and conservation programs such as LTER (Long Term Ecological Research) or worldwide research collaborations such as GLORIA (Global Observation Research Initiative in Alpine Environments) to be established throughout the treelines.
Dr. Samuel Wood, Professor David Bowman and Dr. Jennifer Sprent deserve to be acknowledged for their ideas, comments and helps on the manuscript at the University of Tasmania, Australia.
Alftine KJ , Malanson GP 2004 Directional positive feedback and pattern at an alpine tree line J Veg Sci 15 3 - 12    DOI : 10.1111/j.1654-1103.2004.tb02231.x
Anadon-Rosell A , Rixen C , Cherubini P , Wipf S , Hagedorn F , Dawes MA 2014 Growth and phenology of three dwarf shrub species in a six-year soil warming experiment at the Alpine treeline PLoS ONE 9 e100577 -    DOI : 10.1371/journal.pone.0100577
Aulitzky H 1961 Die Bodentemperaturen in der Kampfzone oberhalb der Waldgrenze und imsubalpinen Zirben- Larchenwald Mitt Forstl Bund Mariabrunn 59 153 - 208
Autio J 2006 Environmental Factors Controlling the Position of the Actual Timberline and Treeline on the Fells of Finnish Lapland Oulu University Press Oulu
Autio J , Colpaert A 2005 The impact of elevation, topography and snow load damage of trees on the position of the actual timberline on the fells in central Finnish Lapland Fennia 183 15 - 36
Bader MY 2007 Tropical Alpine Treelines: how ecological processes control vegetation patterning and dynamics. PhD Dissertation Wageningen University Wageningen, The Netherlands
Bader MY , Rietkerk M , Bregt AK 2008 A simple spatial model exploring positive feedbacks at tropical alpine treelines Arct Antarct Alp Res 40 269 - 278    DOI : 10.1657/1523-0430(07-024)[BADER]2.0.CO;2
Bader MY , Ruijten JJA 2008 A topography-based model of forest cover at the alpine tree line in the tropical Andes J Biogeogr 35 711 - 723    DOI : 10.1111/j.1365-2699.2007.01818.x
Baig MN , Tranquillini W 1980 The effects of wind and temperature on cuticular transpiration of Picea abies and Pinus cembra and their significance in dessication damage at the alpine treeline Oecologia 47 252 - 256    DOI : 10.1007/BF00346828
Bale JS , Masters GJ , Hodkinson ID , Awmack C , Bezemer TM , Brown VK , Butterfield J , Buse A , Coulson JC , Farrar J , Good JEG , Harrington R , Hartley S , Jones TH , Lindroth RL , Press MC , Symrnioudis I , Watt AD , Whittaker JB 2002 Herbivory in global climate change research: direct effects of rising temperature on insect herbivores Glob Change Biol 8 1 - 16    DOI : 10.1046/j.1365-2486.2002.00451.x
Ball MC , Egerton JJG , Leuning R , Cunningham RB , Dunne P 1997 Microclimate above grass adversely affects spring growth of seedling snow gum (Eucalyptus pauciflora) Plant Cell Environ 20 155 - 166    DOI : 10.1046/j.1365-3040.1997.d01-61.x
Barbeito I , Dawes MA , Rixen C , Senn J , Bebi P 2012 Factors driving mortality and growth at treeline: a 30-year experiment of 92000 conifers Ecology 93 389 - 401    DOI : 10.1890/11-0384.1
Batllori E , Camarero JJ , Ninot JM , Gutiérrez E 2009 Implications and potential responses to climate warming Glob Ecol Biogeogr 18 460 - 472    DOI : 10.1111/j.1466-8238.2009.00464.x
Batllori E , Gutiérrez E 2008 Regional treeline dynamics in response to global change in the Pyrenees J Ecol 96 1275 - 1288    DOI : 10.1111/j.1365-2745.2008.01429.x
Bebi P , Kulakowski D , Rixen C 2009 Snow avalanche disturbances in forest ecosystems-State of research and implications for management For Ecol Manage 257 1883 - 1892    DOI : 10.1016/j.foreco.2009.01.050
Bekker MF 2005 Positive feedback between tree establishment and patterns of subalpine forest advancement, Glacier National Park, Montana, USA Arct Antarc Alp Res 37 97 - 107    DOI : 10.1657/1523-0430(2005)037[0097:PFBTEA]2.0.CO;2
Berdanier AB 2010 Global treeline position Nat Educ Knowl 3 11 -
Black RA , Bliss LC 1980 eproductive ecology of Picea mariana (Mill.) BSP., at the tree line near Inuvik, Northwest Territories, Canada Ecol Monogr 50 331 - 354    DOI : 10.2307/2937255
Bolli J , Rigling A , Bugmann H 2007 Regeneration dynamics of Norway spruce (Picea abies L.) on a subalpine meadow near the treeline in Sedrun, Kt. Graubünden, Switzerland Silva Fenn 41 55 - 70
Brown CD 2010 Tree-line dynamics: adding fire to climate change prediction Arctic 63 488 - 492
Brown DG 1994 Comparison of vegetation-topography relationships at the alpine treeline ecotone Phys Geogr 15 125 - 145
Brown DG 1994 Predicting vegetation types at treeline using topography and biophysical disturbance variables J Veg Sci 5 641 - 656    DOI : 10.2307/3235880
Butler DR , Hill C , Malanson GP , Cairns DM 1994 Stability of alpine treeline in Glacier National Park, Montana, USA Phytocoenologia 22 485 - 500    DOI : 10.1127/phyto/22/1994/485
Butler DR , Malanson GP , Walsh SJ , Fagre DB 2007 Influences of geomorphology and geology on alpine treeline in the American West - more important than climatic influences? Phys Geogr 28 434 - 450    DOI : 10.2747/0272-3646.28.5.434
Cairns DM 1999 Multi-scale analysis of soil nutrients at alpine treeline in Glacier National Park, Montana Phys Geogr 20 256 - 271
Cairns DM , Malanson GP 1998 Environmental variables influencing the carbon balance at the alpine treeline: a modeling approach J Veg Sci 9 679 - 692    DOI : 10.2307/3237286
Cairns DM , Moen J 2004 Herbivory influences tree lines J Ecol 92 1019 - 1024    DOI : 10.1111/j.1365-2745.2004.00945.x
Camarero JJ , Gutiérrez E 2004 Pace and pattern of recent treeline dynamics: response of ecotones to climatic variability in the Spanish Pyrenees Clim Chang 63 181 - 200    DOI : 10.1023/B:CLIM.0000018507.71343.46
Carlson BZ , Randin CF , Boulangeat I , Lavergne S , Thuiller W , Choler P 2013 Working toward integrated models of alpine plant distribution Alp Bot 123 41 - 53    DOI : 10.1007/s00035-013-0117-4
Case BS , Duncan RP 2014 A novel framework for disentangling the scale-dependent influences of abiotic factors on alpine treeline position Ecography 37 838 - 851    DOI : 10.1111/ecog.00280
Chang NK , Shim KC , Lee HU , Kang KM , So KH 1998 The theory of boundary distribution of plant and wave character of the timber line on Mt. Paektu Korean J Ecol 21 491 - 499
Chauchard S , Beilhe F , Denis N , Carcaillet C 2010 An increase in the upper tree-limit of silver fir (Abies alba Mill.) in the Alps since the mid-20th century: a land-use change phenomenon For Ecol Manage 259 1406 - 1415    DOI : 10.1016/j.foreco.2010.01.009
Cho DS 1994 Community structure, and size and age distribution of conifers in subalpine Korean Fir (Abies koreana) Forest in Mt. Chiri Korean J Ecol 17 415 - 424
Cochrane PM , Slatyer RO 1988 Water relations of Eucalyptus pauciflora near the alpine tree line in winter Tree Physiol 4 45 - 52    DOI : 10.1093/treephys/4.1.45
Cogbill CV , White PS 1991 The latitude-elevation relationship for spruce-fir forest and treeline along the Appalachian mountain chain Vegetatio 94 153 - 175    DOI : 10.1007/BF00032629
Cuevas JG 2000 Tree recruitment at the Nothofagus pumilio alpine timberline in Tierra del Fuego, Chile J Ecol 88 840 - 855    DOI : 10.1046/j.1365-2745.2000.00497.x
Cullen LE , Stewart GH , Duncan RP , Palmer JG 2001 Disturbance and climate warming influences on New Zealand Nothofagus tree-line population dynamics J Ecol 89 1061 - 1071    DOI : 10.1111/j.1365-2745.2001.00628.x
Daly C , Shankman D 1985 Seedling establishment by conifers above the tree limit on Niwot Ridge, Front Range, Colorado, USA Arct Alp Res 17 389 - 400    DOI : 10.2307/1550864
Danby RK , Hik DS 2007 Variability, contingency and rapid change in recent subarctic alpine tree line dynamics J Ecol 95 352 - 363    DOI : 10.1111/j.1365-2745.2006.01200.x
Dang H , Zhang K , Zhang Y , Tan S , Jiang M , Zhang Q 2009 Treeline dynamics in relation to climatic variability in the Shennongjia Mountains, Central China Can J For Res 39 1848 - 1858    DOI : 10.1139/X09-106
Daniels LD , Veblen TT 2003 Regional and local effects of disturbance and climate on altitudinal treelines in northern Patagonia J Veg Sci 14 733 - 742    DOI : 10.1111/j.1654-1103.2003.tb02205.x
Devi N , Hagedorn F , Moiseev P , Bugmann H , Shiyatov S , Mazepa V , Rigling A 2008 Expanding forests and changing growth forms in Siberian larch at the Polar Urals treeline during the 20th century Glob Change Biol 14 1581 - 1591    DOI : 10.1111/j.1365-2486.2008.01583.x
Díaz-Varela RA , Colombo R , Meroni M , Calvo-Iglesias MS , Buffoni A , Tagliaferri A 2010 Spatio-temporal analysis of alpine ecotones: a spatial explicit model targeting altitudinal vegetation shifts Ecol Model 221 621 - 633    DOI : 10.1016/j.ecolmodel.2009.11.010
Dullinger S , Dirnbock T , Grabherr G 2004 Modelling climate change-driven treeline shifts: relative effects of temperature increase, dispersal and invisibility J Ecol 92 241 - 252    DOI : 10.1111/j.0022-0477.2004.00872.x
Fajardo A , Piper FI , Cavieres LA 2011 Distinguishing local from global climate influences in the variation of carbon status with altitude in a tree line species Glob Ecol Biogeogr 20 307 - 318    DOI : 10.1111/j.1466-8238.2010.00598.x
Fajardo A , Piper FI , Pfund L , Körner C , Hoch G 2012 Variation of mobile carbon reserves in trees at the alpine treeline ecotone is under environmental control New Phytol 195 794 - 802    DOI : 10.1111/j.1469-8137.2012.04214.x
Feeley KJ , Silman MR , Bush MB , Farfan W , Cabrera KG , Malhi Y , Meir P , Revilla NS , Quisiyupanqui MNR , Saatchi S 2011 Upslope migration of Andean trees J Biogeogr 38 783 - 791    DOI : 10.1111/j.1365-2699.2010.02444.x
Fletcher MS , Wood SW , Haberle SG 2014 A fire driven shift from forest to non-forest: evidence for alternative stable states? Ecology 95 2504 - 2513    DOI : 10.1890/12-1766.1
Gamache I , Payette S 2004 Height growth response of tree line black spruce to recent climate warming across the forest-tundra of eastern Canada J Ecol 92 835 - 845    DOI : 10.1111/j.0022-0477.2004.00913.x
Gehrig-Fasel J , Guisan A , Zimmermann NE 2007 Tree line shifts in the Swiss Alps: climate change or land abandonment? J Veg Sci 18 571 - 582    DOI : 10.1111/j.1654-1103.2007.tb02571.x
Germino MJ , Smith WK 1999 Sky exposure, crown architecture, and low‐temperature photoinhibition in conifer seedlings at alpine treeline Plant Cell Environ 22 407 - 415    DOI : 10.1046/j.1365-3040.1999.00426.x
Germino MJ , Smith WK , Resor AC 2002 Conifer seedling distribution and survival in an alpine-treeline ecotone Plant Ecol 162 157 - 168    DOI : 10.1023/A:1020385320738
Gottfried M , Hantel M , Maurer C , Toechterle R , Pauli H , Grabherr G 2011 Coincidence of the alpine-nival ecotone with the summer snowline Environ Res Lett 6 014013 -    DOI : 10.1088/1748-9326/6/1/014013
Gottfried M , Pauli H , Reiter K , Grabherr G 1999 A finescaled predictive model for changes in species distributions patterns of high mountain plants induced by climate warming Divers Distrib 5 241 - 252    DOI : 10.1046/j.1472-4642.1999.00058.x
Grabherr G , Gottfried M , Pauli H 1994 Climate effects on mountain plants Nature 369 448 -
Grace J 1977 Plant Responses to Wind Academic Press London
Grace J 1989 Tree lines Phil Trans R Soc Lond B 324 233 - 245    DOI : 10.1098/rstb.1989.0046
Grace J , Berninger F , Nagy L 2002 Impacts of climate change on the tree line Ann Bot 90 537 - 544    DOI : 10.1093/aob/mcf222
Green K 2009 Causes of stability in the alpine treeline in the Snowy Mountains of Australia-a natural experiment Aust J Bot 57 171 - 179    DOI : 10.1071/BT09052
Green K , Venn SE 2012 Tree-limit ribbons in the snowy mountains, Australia: characterization and recent seedling establishment Arct Antarc Alp Res 44 180 - 187    DOI : 10.1657/1938-4246-44.2.180
Guisan A , Theurillat JP 2000 Assessing alpine plant vulnerability to climate change: a modeling perspective Integr Assess 1 307 - 320    DOI : 10.1023/A:1018912114948
Gworek JR , Vander Wall SB , Brussard PF 2007 Changes in biotic interactions and climate determine recruitment of Jeffrey pine along an elevation gradient For Ecol Manage 239 57 - 68    DOI : 10.1016/j.foreco.2006.11.010
Hagedorn F , Shiyatov SG , Mazepa VS , Devi NM , Grigor’ev AA , Bartysh AA , Fomin VV , Kapralov DS , Terent’ev M , Bugman H , Rigling A , Moiseev PA 2014 Treeline advances along the Urals mountain range - driven by improved winter conditions? Glob Change Biol 20 3530 - 3543    DOI : 10.1111/gcb.12613
Handa IT , Körner C , Hättenschwiler S 2005 A test of the treeline carbon limitation hypothesis by in situ CO2enrichment and defoliation Ecology 86 1288 - 1300    DOI : 10.1890/04-0711
Harsch MA , Bader MY 2011 Treeline form - a potential key to understanding treeline dynamics Glob Ecol Biogeogr 20 582 - 596    DOI : 10.1111/j.1466-8238.2010.00622.x
Harsch MA , Hulme PE , McGlone MS , Duncan RP 2009 Are treelines advancing? A meta-analysis of treeline response to climate warming Ecol Lett 12 1040 - 1049    DOI : 10.1111/j.1461-0248.2009.01355.x
Harvell CD , Mitchell CE , Ward JR , Altizer S , Dobson AP , Ostfeld RS , Samuel MD 2002 Climate warming and disease risks for terrestrial and marine biota Science 296 2158 - 2162    DOI : 10.1126/science.1063699
Hättenschwiler S , Körner C 1995 Responses to recent climate warming of Pinus sylvestris and Pinus cembra within their montane transition zone in the Swiss Alps J Veg Sci 6 357 - 368    DOI : 10.2307/3236235
Hättenschwiler S , Smith WK 1999 Seedling occurrence in alpine treeline conifers: a case study from the central Rocky Mountains, USA Acta Oecol 20 219 - 224    DOI : 10.1016/S1146-609X(99)80034-4
Heikkinen O , Tuovinen M , Autio J 2002 What determines the timberline Fennia 180 67 - 74
Hemstrom MA , Franklin JF 1982 Fire and other disturbances of the forests in Mount Rainier National Park Quat Res 18 32 - 51    DOI : 10.1016/0033-5894(82)90020-5
Herrero A , Zamora R , Castro J , Hódar JA 2011 Limits of pine forest distribution at the treeline: herbivory matters Plant Ecol 213 459 - 469
Hertel D , Schöling D 2011 Below-ground response of Norway spruce to climate conditions at Mt. Brocken (Germany)-A re-assessment of Central Europe’s northernmost treeline Flora 206 127 - 135    DOI : 10.1016/j.flora.2010.05.001
Hirota M , Holmgren M , Van Nes EH , Scheffer M 2011 Global resilience of tropical forest and savanna to critical transitions Science 334 232 - 235    DOI : 10.1126/science.1210657
Hoch G , Körner C 2003 The carbon charging of pines at the climatic treeline: a global comparison Oecologia 135 10 - 21    DOI : 10.1007/s00442-002-1154-7
Hoch G , Körner C 2005 Growth, demography and carbon relations of Polylepis trees at the world’s highest treeline Funct Ecol 19 941 - 951    DOI : 10.1111/j.1365-2435.2005.01040.x
Hoch G , Körner C 2009 Growth and carbon relations of tree line forming conifers at constant vs. variable low temperatures J Ecol 97 57 - 66    DOI : 10.1111/j.1365-2745.2008.01447.x
Hoch G , Popp M , Körner C 2002 Altitudinal increase of mobile carbon pools in Pinus cembra suggests sink limitation of growth at the Swiss treeline Oikos 98 361 - 374    DOI : 10.1034/j.1600-0706.2002.980301.x
Holtmeier FK 2003 Mountain Timberlines: Ecology, Patchiness and Dynamics Kluwer Academic Publishers Dordrecht
Holtmeier FK , Broll G 2005 Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales Glob Ecol Biogeogr 14 395 - 410    DOI : 10.1111/j.1466-822X.2005.00168.x
Holtmeier FK , Broll G 2007 Treeline advance - driving processes and adverse factors Landsc Online 1 1 - 33
Holtmeier FK , Broll G 2010 Wind as an ecological agent at treelines in North America, the Alps, and the European Subarctic Phys Geogr 31 203 - 233    DOI : 10.2747/0272-3646.31.3.203
Ihm BS , Lee JS , Kim JW 2012 Global warming and biodiversity model projections J Ecol Field Biol 35 157 - 166    DOI : 10.5141/JEFB.2012.022
Ihm BS , Lee JS , Kim JW , Kim JH 2007 Relationship between global warming and species richness of vascular plants J Plant Biol 50 321 - 324    DOI : 10.1007/BF03030661
2007 Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change IPCC Geneva
2013 Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change IPCC Geneva
Jeffers ES , Bonsall MB , Brooks SJ , Willis KJ 2011 Abrupt environmental changes drive shifts in tree-grass interaction outcomes J Ecol 99 1063 - 1070    DOI : 10.1111/j.1365-2745.2011.01816.x
Jobbagy EG , Jackson RB 2000 Global controls of forest line elevation in the northern and southern hemispheres Glob Ecol Biogeogr 9 253 - 268    DOI : 10.1046/j.1365-2699.2000.00162.x
Johnson DM , Germino MJ , Smith WK 2004 Abiotic factors limiting photosynthesis in Abies lasiocarpa and Picea engelmannii seedlings below and above the alpine timberline Tree Physiol 24 377 - 386    DOI : 10.1093/treephys/24.4.377
Kim JH 2012 Global Warming Seoul National University Press Seoul
Knox KJE , Clarke PJ 2012 Fire severity, feedback effects and resilience to alternative community states in forest assemblages For Ecol Manage 265 47 - 54    DOI : 10.1016/j.foreco.2011.10.025
Kong WS 1999 The vertical distribution of air temperature and thermal amplitude of alpine plants on Mt. Halla, Cheju Island, Korea J Korean Geo Soc 34 385 - 393
Kong WS 2000 Geoecology on the subalpine vegetation and landscape of Mt. Sorak J Korean Geo Soc 35 177 - 187
Kong WS , Watts D 1993 The Plant Geography of Korea with an Emphasis on the Alpine Zones Kluwer Academic Publishers Dordrecht
Körner C 1998 A re-assessment of high-elevation treeline positions and their explanation Oecologia 115 445 - 459    DOI : 10.1007/s004420050540
Körner C 2003 Carbon limitation in trees J Ecol 91 4 - 17    DOI : 10.1046/j.1365-2745.2003.00742.x
Körner C 2012 Alpine Treelines Springer Berlin
Körner C , Paulsen J 2004 A world-wide study of high altitude treeline temperatures J Biogeogr 31 713 - 732    DOI : 10.1111/j.1365-2699.2003.01043.x
Kreyling J , Wana D , Beierkuhnlein C 2010 Potential consequences of climate warming for tropical plant species in high mountains of southern Ethiopia Divers Distrib 16 593 - 605    DOI : 10.1111/j.1472-4642.2010.00675.x
Kullman L 2001 20th century climate warming and treelimit rise in the southern Scandes of Sweden Ambio 30 72 - 80    DOI : 10.1579/0044-7447-30.2.72
Kullman L 2002 Rapid recent range-margin rise of tree and shrub species in the Swedish Scandes J Ecol 90 68 - 77    DOI : 10.1046/j.0022-0477.2001.00630.x
Kullman L 2005 Wind-conditioned 20th century decline of birch treeline vegetation in the Swedish Scandes Arctic 58 286 - 294
Kullman L 2007 Tree line population monitoring of Pinus sylvestris in the Swedish Scandes, 1973-2005: implications for tree line theory and climate change ecology J Ecol 95 41 - 52    DOI : 10.1111/j.1365-2745.2006.01190.x
Kullman L 2010 A richer, greener and smaller alpine world: review and projection of warming-induced plant cover change in the Swedish Scandes Ambio 39 159 - 169    DOI : 10.1007/s13280-010-0021-8
Kullman L 2010 One century of treeline change and stability - experiences from the Swedish Scandes Landsc Online 17 1 - 31
Lenoir J , Gegout JC , Marquet PA , de Ruffray P , Brisse H 2008 A significant upward shift in plant species optimum elevation during the 20th century Science 320 1768 - 1771    DOI : 10.1126/science.1156831
Leonelli G , Pelfini M , di Cella UM 2009 Detecting climatic treelines in the Italian Alps: The influence of geomorphological factors and of human impacts Phys Geogr 30 338 - 352    DOI : 10.2747/0272-3646.30.4.338
Leonelli G , Pelfini M , di Cella UM , Garavaglia V 2011 Climate warming and the recent treeline shift in the European Alps: the role of geomorphological factors in highaltitude sites Ambio 40 264 - 273    DOI : 10.1007/s13280-010-0096-2
Li MH , Xiao WF , Wang SG , Cheng GW , Cherubini P , Cal XH , Liu XL , Wang XD , Zhu WZ 2008 Evidence for carbon gain limitation but not for growth limitation Tree Physiol 28 1287 - 1296    DOI : 10.1093/treephys/28.8.1287
Lloyd AH , Fastie CL 2003 Recent changes in treeline forest distribution and structure in interior Alaska Ecoscience 10 176 - 185
MacDonald GM , Kremenetski KV , Beilman DW 2008 Climate change and the northern Russian treeline zone Phil Trans R Soc B 363 2285 - 2299
Macek P , Macková J , de Bello F 2009 Morphological and ecophysiological traits shaping altitudinal distribution of three Polylepis treeline species in the dry tropical Andes Acta Oecol 35 778 - 785    DOI : 10.1016/j.actao.2009.08.013
Malanson GP 2001 Complex responses to global change at alpine treeline Phys Geogr 22 333 - 342
Malanson GP , Butler DR , Fagre DB , Walsh SJ , Tomback DF , Daniels LD , Resler LM , Smith WK , Weiss DJ , Peterson DL , Bunn AG , Hiemstra CA , Liptzin D , Bourgeron PS , Shen Z , Millar CI 2007 Alpine treeline of western North America: Linking organism-to-landscape dynamics Phys Geogr 28 378 - 396    DOI : 10.2747/0272-3646.28.5.378
Malanson GP , Resler LM , Bader MY , Holtmeier FK , Butler DR , Weiss DJ , Daniels LD , Fagre DB 2011 Mountain treelines: a roadmap for research orientation Arct Antarc Alp Res 43 167 - 177    DOI : 10.1657/1938-4246-43.2.167
Marchand PJ , Chabot BF 1978 Winter water relations of tree-line plant species on Mt. Washington, New Hampshire Arct Alp Res 10 105 - 116    DOI : 10.2307/1550660
Mayer AL , Khalyani AH 2011 Grass trumps trees with fire Science 334 188 - 189    DOI : 10.1126/science.1213908
Moen J , Aune K , Edenius L , Angerbjörn A 2004 Potential effects of climate change on treeline position in the Swedish mountains Ecol Soc 9 16 -
Moen J , Cairns DM , Lafon CW 2008 Factors structuring the treeline ecotone in Fennoscandia Plant Ecol Divers 1 77 - 87    DOI : 10.1080/17550870802246664
Motta R , Nola P 2001 Growth trends and dynamics in subalpine forest stands in the Varaita Valley (Piedmont, Italy) and their relationships with human activities and global change J Veg Sci 12 219 - 230    DOI : 10.2307/3236606
Murphy BP , Bowman DMJS 2012 What controls the distribution of tropical forest and savanna? Ecol Lett 15 748 - 758    DOI : 10.1111/j.1461-0248.2012.01771.x
Noble IR 1980 Interactions between tussock grass (Poa spp.) and Eucalyptus pauciflora seedlings near treeline in South-Eastern Australia Oecologia 45 350 - 353    DOI : 10.1007/BF00540204
Noble IR 1993 A model of the responses of ecotones to climate change Ecol Appl 3 396 - 403    DOI : 10.2307/1941908
Odion DC , Moritz MA , DellaSala DA 2010 Alternative community states maintained by fire in the Klamath Mountains, USA J Ecol 98 96 - 105    DOI : 10.1111/j.1365-2745.2009.01597.x
Odland A 2009 Interpretation of altitudinal gradients in South Central Norway based on vascular plants as environmental indicators Ecol Indic 9 409 - 421    DOI : 10.1016/j.ecolind.2008.05.012
Pauli H , Gottfried M , Grabherr G 1996 Effects of climate change on mountain ecosystems - upward shifting of alpine plants World Resource Review 8 382 - 390
Payette S , Filion L , Delwaide A , Begin C 1989 Reconstruction of tree-line vegetation response to long-term climate change Nature 341 429 - 432    DOI : 10.1038/341429a0
Rada F , Azócar A , Briceño B , González J , García-Núñez C 1996 Carbon and water balance in Polylepis sericea, a tropical treeline species Trees 10 218 - 222
Resler LM 2006 Geomorphic controls of spatial pattern and process at alpine treeline Prof Geogr 58 124 - 138    DOI : 10.1111/j.1467-9272.2006.00520.x
Richardson AD , Friedland AJ 2009 A review of the theories to explain Arctic and alpine treelines around the world J Sustainable For 28 218 - 242
Rixen C , Dawes MA , Wipf S , Hagedorn F 2012 Evidence of enhanced freezing damage in treeline plants during six years of CO2 enrichment and soil warming Oikos 121 1532 - 1543    DOI : 10.1111/j.1600-0706.2011.20031.x
Sakai A 1970 Mechanism of desiccation damage of conifers wintering in soil-frozen areas Ecology 51 657 - 664    DOI : 10.2307/1934045
Scheffer M , Carpenter S , Foley JA , Folke C , Walker B 2001 Catastrophic shifts in ecosystems Nature 413 591 - 596    DOI : 10.1038/35098000
Scherrer D , Körner C 2011 Topographically controlled thermal- habitat differentiation buffers alpine plant diversity against climate warming J Biogeogr 38 406 - 416    DOI : 10.1111/j.1365-2699.2010.02407.x
Schulze ED , Mooney HA , Dunn EL 1967 Wintertime photosynthesis of bristlecone pine (Pinus aristata) in White Mountains of California Ecology 48 1044 - 1047    DOI : 10.2307/1934564
Scott AC , Bowman DMJS , Bond WJ , Pyne SJ , Alexander ME 2014 Fire on Earth: An Introduction John Wiley & Sons Hoboken, NJ
Scott PA , Hansell RIC , Erickson WR 1993 Influences of wind and snow on northern tree-line environments at Churchill, Manitoba, Canada Arctic 46 316 - 323
Shankman D , Daly C 1988 Forest regeneration above tree limit depressed by fire in the Colorado Front Range B Torrey Bot Club 115 272 - 279    DOI : 10.2307/2996159
Shi P , Körner C , Hoch G 2006 End of season carbon supply status of woody species near the treeline in western China Basic Appl Ecol 7 370 - 377    DOI : 10.1016/j.baae.2005.06.005
Shi P , Körner C , Hoch G 2008 A test of the growth-limitation theory for alpine tree line formation in evergreen and deciduous taxa of the eastern Himalayas Funct Ecol 22 213 - 220    DOI : 10.1111/j.1365-2435.2007.01370.x
Singh CP , Panigrahy S , Thapliyal A , Kimothi MM , Soni P , Parihar JS 2012 Monitoring the alpine treeline shift in parts of the Indian Himalayas using remote sensing Curr Sci 102 559 - 562
Smith WK , Germino MJ , Hancock TE , Johnson DM 2003 Another perspective on altitudinal limits of alpine timberlines Tree Physiol 23 1101 - 1112    DOI : 10.1093/treephys/23.16.1101
Smith WK , Germino MJ , Johnson DM , Reinhardt K 2009 The altitude of alpine treeline: a bellwether of climate change effects Bot Rev 75 163 - 190    DOI : 10.1007/s12229-009-9030-3
Smith-McKenna EK , Malanson GP , Resler LM , Carstensen LW , Prisley SP , Tomback DF 2014 Cascading effects of feedbacks, disease, and climate change on alpine treeline dynamics Environ Model Softw 62 85 - 96    DOI : 10.1016/j.envsoft.2014.08.019
Speed JDM , Austrheim G , Hester AJ , Mysterud A 2010 Experimental evidence for herbivore limitation of the treeline Ecology 91 3414 - 3420    DOI : 10.1890/09-2300.1
Staver AC , Archibald S , Levin SA 2011 The global extent and determinants of savanna and forest as alternative biome states Science 334 230 - 232    DOI : 10.1126/science.1210465
Stevens GC , Fox JF 1991 The causes of treeline Annu Rev Ecol Syst 22 177 - 191    DOI : 10.1146/
Stueve KM , Cerney DL , Rochefort RM , Kurth LL 2009 Postfire tree establishment patterns at the alpine treeline ecotone: Mount Rainier National Park, Washington, USA J Veg Sci 20 107 - 120    DOI : 10.1111/j.1654-1103.2009.05437.x
Susiluoto S , Peramaki M , Nikinmaa E , Berninger F 2007 Effects of sink removal on transpiration at the treeline: implications for the growth limitation hypothesis Environ Exp Bot 60 334 - 339    DOI : 10.1016/j.envexpbot.2006.12.015
Sveinbjörnsson B 2000 North American and European treelines: external forces and internal processes controlling position Ambio 29 388 - 395    DOI : 10.1579/0044-7447-29.7.388
Sveinbjörnsson B , Kauhanen H , Nordell O 1996 Treeline ecology of mountain birch in the Torneträsk area Ecol Bull 45 65 - 70
Szeicz JM , MacDonald GM 1995 Recent white spruce dynamics at the subarctic alpine treeline of north-western Canada J Ecol 83 873 - 885    DOI : 10.2307/2261424
Szerencsits E 2012 Swiss tree lines - a GIS-based approximation Landsc Online 28 1 - 18
Telewski FW , Coutts MP , Grace J 1995 Wind-induced physiological and developmental responses in trees;Wind and Trees Cambridge University Press Cambridge 237 - 263
Tomback DF , Resler LM 2007 Invasive pathogens at alpine treeline: complications and concerns Phys Geogr 28 397 - 418    DOI : 10.2747/0272-3646.28.5.397
Tranquillini W 1979 Physiological Ecology of the Alpine Timber-line Springer Berlin Tree Existence at High Altitudes with Special References to the European Alps (Ecological studies 31)
Wallentin G , Tappeiner U , Strobl J , Tasser E 2008 Understanding alpine tree line dynamics: an individual based model Ecol Model 218 235 - 246    DOI : 10.1016/j.ecolmodel.2008.07.005
Walsh SJ , Butler DR , Allen TR , Malanson GP 1994 Influence of snow patterns and snow avalanches on the alpine treeline ecotone J Veg Sci 5 657 - 672    DOI : 10.2307/3235881
Walsh SJ , Butler DR , Malanson GP , Crews-Meyer KA , Messina JP , Xiao N 2003 Mapping, modeling, and visualization of the influences of geomorphic processes on the alpine treeline ecotone, Glacier National Park, MT, USA Geomorphology 53 129 - 145    DOI : 10.1016/S0169-555X(02)00350-1
Walther G 2003 Plants in a warmer world Perspect Plant Ecol Evol Syst 6 169 - 185    DOI : 10.1078/1433-8319-00076
Walther GR , Beißner S , Burga CA 2005 Trends in the upward shift of alpine plants J Veg Sci 16 541 - 548    DOI : 10.1111/j.1654-1103.2005.tb02394.x
Wang T , Zhang QB , Ma KP 2006 Treeline dynamics in relation to climatic variability in the central Tianshan Mountains, northwestern China Global Ecol Biogeogr 15 406 - 415    DOI : 10.1111/j.1466-822X.2006.00233.x
Wardle P , Coleman MC 1992 Evidence for rising upper limits of four native New Zealand forest trees New Zeal J Bot 30 303 - 314    DOI : 10.1080/0028825X.1992.10412909
Warman L , Moles AT 2009 Alternative stable states in Australia's wet tropics: a theoretical framework for the field data and a field-case for the theory Landsc Ecol 24 1 - 13    DOI : 10.1007/s10980-008-9285-9
Wieser G , Matyssek R , Luzian R , Zwerger P , Pindur P , Oberhuber W , Gruber A 2009 Effects of atmospheric and climate change at the timberline of the Central European Alps Ann For Sci 66 402 - 412    DOI : 10.1051/forest/2009023
Wilson JB , Agnew ADQ 1992 Positive-feedback Switches in Plant Communities Academic Press London
Wood SW , Bowman DMJS 2012 Alternative stable states and the role of fire-vegetation-soil feedbacks in the temperate wilderness of southwest Tasmania Landsc Ecol 27 13 - 28    DOI : 10.1007/s10980-011-9677-0
Wood SW , Murphy BP , Bowman DMJS 2011 Firescape ecology: how topography determines the contrasting distribution of fire and rain forest in the south-west of the Tasmanian Wilderness World Heritage Area J Biogeogr 38 1807 - 1820    DOI : 10.1111/j.1365-2699.2011.02524.x
Xu Z , Hu T , Zhang Y 2012 Effects of experimental warming on phenology, growth and gas exchange of treeline birch (Betula utilis) saplings, Eastern Tibetan Plateau, China Eur J Forest Res 131 811 - 819    DOI : 10.1007/s10342-011-0554-9
Zhang YJ , Dai, LM , Pan J 2001 The trend of tree line on the northern slope of Changbai Mountain J For Res 12 97 - 100    DOI : 10.1007/BF02867204