Altitudinal gradients are among the most powerful 'natural experiments' for testing ecological and evolutionary responses of biota to geophysical influences, such as low temperature. However, there are two categories of environmental changes with altitude: those physically tied to meters above sea level, such as atmospheric pressure, temperature and clear-sky turbidity; and those that are not generally altitude specific, such as moisture, hours of sunshine, wind, season length, geology and even human land use. The confounding of the first category by the latter has introduced confusion in the scientific literature on altitude phenomena.

The use of ‘altitude’ in ecological research

Aim At a coarse scale, the treelines of the world`s mountains seem to follow a common isotherm, but the evidence for this has been indirect so far. Here we aim at underpinning this with facts. Location We present the results of a data-logging campaign at 46 treeline sites between 68degrees N and 42degrees S. Methods We measured root-zone temperatures with an hourly resolution over 1-3 years per site between 1996 and 2003. Results Disregarding taxon-, landuse- or fire-driven tree limits, high altitude climatic treelines are associated with a seasonal mean ground temperature of 6.7 degreesC (+/-0.8 SD; 2.2 K amplitude of means for different climatic zones), a surprisingly narrow range. Temperatures are higher (7-8 degreesC) in the temperate and Mediterranean zone treelines, and are lower in equatorial treelines (5-6 degreesC) and in the subarctic and boreal zone (6-7 degreesC). While air temperatures are higher than soil temperatures in warm periods, and are lower than soil temperatures in cold periods, daily means of air and soil temperature are almost the same at 6-7 degreesC, a physics driven coincidence with the global mean temperature at treeline. The length of the growing season, thermal extremes or thermal sums have no predictive value for treeline altitude on a global scale. Some Mediterranean (Fagus spp.) and temperate South Hemisphere treelines (Nothofagus spp.) and the native treeline in Hawaii (Metrosideros) are located at substantially higher isotherms and represent genus-specific boundaries rather than boundaries of the life-form tree. In seasonal climates, ground temperatures in winter (absolute minima) reflect local snow pack and seem uncritical. Main conclusions The data support the hypothesis of a common thermal threshold for forest growth at high elevation, but also reflect a moderate region and substantial taxonomic influence.

/pdf/a-world-wide-study-of-high-altitude-treeline-temperatures-5fuajeq9a7.pdf

A world-wide study of high altitude treeline temperatures

Treelines are temperature sensitive transition zones that are expected to respond to climate warming by advancing beyond their current position. Response to climate warming over the last century, however, has been mixed, with some treelines showing evidence of recruitment at higher altitudes and/or latitudes (advance) whereas others reveal no marked change in the upper limit of tree establishment. To explore this variation, we analysed a global dataset of 166 sites for which treeline dynamics had been recorded since 1900 AD. Advance was recorded at 52% of sites with only 1% reporting treeline recession. Treelines that experienced strong winter warming were more likely to have advanced, and treelines with a diffuse form were more likely to have advanced than those with an abrupt or krummholz form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.

Are treelines advancing? A global meta‐analysis of treeline response to climate warming

Climate change mitigation: a spatial analysis of global land suitability for Clean Development Mechanism afforestation and reforestation

Mountains are the water towers of the world, supplying a substantial part of both natural and anthropogenic water demands1,2. They are highly sensitive and prone to climate change3,4, yet their importance and vulnerability have not been quantified at the global scale. Here we present a global water tower index (WTI), which ranks all water towers in terms of their water-supplying role and the downstream dependence of ecosystems and society. For each water tower, we assess its vulnerability related to water stress, governance, hydropolitical tension and future climatic and socio-economic changes. We conclude that the most important (highest WTI) water towers are also among the most vulnerable, and that climatic and socio-economic changes will affect them profoundly. This could negatively impact 1.9 billion people living in (0.3 billion) or directly downstream of (1.6 billion) mountainous areas. Immediate action is required to safeguard the future of the world’s most important and vulnerable water towers.

/pdf/importance-and-vulnerability-of-the-world-s-water-towers-4fk2ml8593.pdf

Importance and vulnerability of the world's water towers

This is the first quantitative attempt at a global areal definition of ‘alpine’ and ‘montane’ terrain by combining geographical information systems for topography with bioclimatic criteria (temperature) subdividing the life zones along elevational gradients. The mountain definition adopted here refrains from any truncation by low elevation thresholds, and defines the world’s mountains by a common ruggedness threshold (>200 m difference in elevation within a 2.5′ cell, 0.5′ resolution), arriving at 16.5 Mio km2 or 12.3% of all terrestrial land area outside Antartica being mountains. The model employed accounts for criteria of “mountainous terrain” for biological analysis, and thus arrives at a smaller land area fraction than hydrologically oriented approaches, and by its 2.5′ resolution, it includes less unstructured terrain (such as large plateaus, very wide valleys or basins) than earlier approaches. The thermal delineation of the alpine and nival biogeographic region by the climatic tree limit (the lower boundary of the alpine belt) arrives at 2.6% or 3.55 Mio km2 of the global land area outside Antarctica (21.5% of all mountain terrain). Seven climate-defined life zones in mountains facilitate large-scale (global) comparisons of biodiversity information as used in the new electronic ‘Mountain Biodiversity Portal’ of the Global Mountain Biodiversity Assessment (GMBA).

/pdf/a-definition-of-mountains-and-their-bioclimatic-belts-for-2ubt8yrquy.pdf

A definition of mountains and their bioclimatic belts for global comparisons of biodiversity data

Natural climatic treelines are relatively discrete boundaries in the landscape established at a certain elevation within an otherwise continuous gradient of environmental change. By studying tree rings along elevational transects at and below the upper treeline in the European Alps, we (1) determine whether radial stem growth declines abruptly or gradually, and (2) test climatic influences on trees near treeline by investigating transects for climatically different historical periods. While tree height decreases gradually toward the treeline, there is no such general trend for radial tree growth. We found rather abrupt changes which imply threshold effects of temperature which moved upslope in a wave-like manner as temperatures increased over the past 150 yr. Currently radial tree growth at treeline in the Alps is the same magnitude as at several hundred meters below current treeline. Over short intervals, tree-ring width is more dependent on interannual climatic variability than on altitudinal distance to treeline. We conclude that (1) the elevational response of tree-rings includes a threshold component (a minimal seasonal temperature) and that (2) radial growth is more strongly correlated with year to year variation in climate than with treeline elevation as such. Our data indicate that the current treeline position reflects influences of past climates and not the current climate.

Tree Growth near Treeline: Abrupt or Gradual Reduction with Altitude?

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.

/pdf/a-climate-based-model-to-predict-potential-treeline-position-4tdbcyid1g.pdf

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

Mountains are hotspots of biodiversity. Yet, evaluating their importance in global biodiversity inventories requires the adoption of a pertinent definition of mountains. Here, we first compare the well-established WCMC and GMBA definitions, which both use geographical information systems. We show that the WCMC approach arrives at twice the global mountain area and much higher human population numbers than the GMBA one, which is explained by the inclusion of (mostly) low latitude hill country below 600 m elevation. We then present an inventory of the world’s mountains based on the GMBA definition. In this inventory, each of the 1003 entries corresponds to a polygon drawn around a mountain or a mountain range and includes the name of the delineated object, the area of mountainous terrain it covers stratified into different bioclimatic belts (all at 2.5′ resolution), and demographic information. Taken together, the 1003 polygons cover 13.8 Mio km2 of mountain terrain, of which 3.3 Mio km2 are in the alpine and nival belts. This corresponds to 83.7% of the global mountain area sensu GMBA, and 94% of the alpine/nival area. The 386 Mio people inhabiting mountainous terrain within polygons represent 75% of the people globally inhabiting mountains sensu GMBA. This inventory offers a robust framework for the integration of mountain biota in regional and larger scale biodiversity assessments, for biogeography, bioclimatology, macroecology, and conservation research, and for the exploration of a multitude of socio-ecological and climate change-related research questions in mountain biota, including the potential pressure on alpine ecosystems.

/pdf/a-global-inventory-of-mountains-for-bio-geographical-34z4aywmse.pdf

Jens Paulsen

Papers

A world-wide study of high altitude treeline temperatures

A definition of mountains and their bioclimatic belts for global comparisons of biodiversity data

Tree Growth near Treeline: Abrupt or Gradual Reduction with Altitude?

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

A global inventory of mountains for bio-geographical applications