Explaining the low-temperature limit of trees requires in situ information on the actual temperature conditions, the trees' phenology and freezing tolerance. Here, we report the seasonal course of freezing resistance and its response to local temperature in species of Larix and Abies, forming the treeline (4350-4400 m) in the south-eastern Himalayas, and Quercus and tree-size Rhododendron, reaching their upper limit in the immediate neighborhood (4270-4290 m). By linking tree-crown temperatures with on-site air temperature, we can utilize long-term meteorological records to hindcast critical freezing events. Deciduous Larix flushes earliest and exhibits the greatest freezing tolerance and largest safety margin. The evergreen species flush 2 months later and operate near their freezing damage limit. The conifers can compensate for freezing damage by rapid resprouting from small auxiliary buds, not an option for Quercus and Rhododendron with their bigger buds. Freezing damage does not appear to constrain the two treeline conifers. Their range limit most likely reflects the general low-temperature growth limit of the tree life form. Freezing tolerance is most likely controlling the upper limits of the evergreen broad-leaved species. The results offer a mechanistic explanation of the range limit of these high-elevation tree species.

Forests stock up to 90% of the global terrestrial plant biomass carbon (C). Any rise or fall of that stock, but also its utilization for substituting fossil resources can influence the rate of atmospheric CO2 enrichment. By employing the term 'C sequestration', the ongoing debate suffers, however, from an implicit confusion between (1) processes, rates or fluxes of C (e.g. tree growth) with (2) pools, stores or stocks of forest biomass C. Stock formation is driven by turnover, C duration, residence time, or tree demography, and not by the rate of influx of C, including tree growth. Enhanced tree growth must not be treated as a rise in C stock, without accounting for turnover, also removing often assumed benefits of CO2 fertilization for stock formation, should tree growth be C limited, another questionable assumption. A carbon 'sink' is a potential volume that can be filled with C, but it does not represent a stock either, without accounting for C residence time. 'Buying time' by lengthening rotation has a cost in terms of reduced utilization of forest products for substitution of fossil resources. Finally, management cessation for biodiversity benefits, should be qualified by its conservation value, rather than by making a case for C storage benefits, without accounting for natural forest gap dynamics, and again, without pricing-in the inevitable cost of the cessation of the substitution of fossil C by renewable C. All this calls for a strict separation of the meaning of carbon fluxes and carbon stocks, and avoiding ambiguous terms such as C sequestration and C sink.

The environmental conditions on present‑day Mars are far outside the range tolerated by known complex terrestrial life. Conceptual climate studies have suggested that, in hypothetical terraforming scenarios, artificially enhancing the greenhouse effect could restore Mars to more habitable surface conditions. Early colonizing terrestrial life on a warming Mars would plausibly consist of lichens and high‑alpine or high‑arctic plants. Here, we consider a later, more demanding step and investigate the thermal conditions under which the first tree could, in principle, grow on the Martian surface. Based on empirical treeline studies, we adopt thermal thresholds for a representative high‑elevation conifer: a growing season of at least 110 sols during which daily minimum temperatures exceed -6 °C, daily means exceed 6 °C, and daily maxima remain below 40 °C. In addition to liquid water and suitable substrates, O₂ at ~1 hPa and non‑toxic CO₂ levels are likely required; however, these non‑thermal constraints are not explicitly modelled and make the temperature thresholds necessary but not sufficient for tree viability. We use a high‑resolution surface energy balance model of Mars, assuming a pure CO₂ atmosphere with prescribed grey infrared opacity and neglecting the coupled water cycle, full atmospheric dynamics, photochemistry, and surface radiation, to estimate spatio‑temporal thermal windows for potential tree growth as a function of CO₂ surface pressure and additional greenhouse forcing. For a 100 hPa CO₂ atmosphere, near‑surface temperatures satisfying the treeline thresholds first appear when the added grey infrared opacity is ~ 0.39 optical depths. In our simulations, these thermal criteria are initially met not in the tropics (±25°), but in the low‑lying Hellas Basin. As either the CO₂ surface pressure or the imposed grey opacity is increased beyond the values required to open the thermal window, large regions of the southern hemisphere subsequently become thermally overheated and thus unsuitable for tree growth. In this sense, the thermal windows identified in our simulations mark conditions under which temperature would no longer be the primary limiting factor for tree growth, assuming that other essential environmental constraints (such as water availability, radiation environment, substrate properties, and atmospheric composition) are satisfied. We emphasize that this study deals with temperature only, which is an important factor in tree growth on Mars. Other factors that affect tree growth, including water, CO₂ limits, O2 limit, UV and ionizing radiation, and soil nutrients and microbial population, are not considered explicitly here.

Life in the cold is strongly controlled by the interaction of landscape topography (exposure to the sun) and plant life-form. With their morphology, low-stature plants engineer their microclimate and, thus, partly escape the otherwise low-temperature regime. A flexible phenology and/or seasonal life cycle add to this. Trees are less flexible than low-stature plants and cannot engineer substantial departures from ambient air temperature, given their lofty architecture. Structure, not physiology, is the reason why we have treelines at similar air temperatures worldwide. Provided there is enough moisture, alpine ecosystem processes (e.g., biomass production, and carbon, water, and nutrient cycles) strongly depend on the duration of the growing season, with little elevational variation per unit of time within the growing season.

Aspect and steepness with ridges and depressions of different spatial extent, hence, the topography of high-elevation terrain affects the incidence angle of sun rays, the influence of wind, the redistribution of snow and the sorting action of gravity for water, nutrients, diaspores and sediments. These factors jointly shape life in the treeless alpine world, beyond the influences related to the strong vertical profile of the air temperature with a mean lapse rate of 0.55 K 100 m−1. On top of diverse soil types, the resulting thermal habitat conditions within a given elevation belt substantially exceed the current (+2.9 K) and future (+4 K) climatic warming in the European Alps. To account for the full spectrum of life conditions, the spatial scale of analysis is essential. We show that pronounced topography effects on microclimate on nunatak-type cliffs above the trimline of glacial ice shields could most likely not fully counteract the then lower and hostile temperatures. Life conditions across snowbeds exemplify the effect of season length on alpine plant’s phenology and growth over short distances, with halved aboveground biomass production for a shortening of the season by two to three weeks. Furthermore, we highlight potential pitfalls in microclimate assessments of topography effects in open terrain, but also explain how to improve such measurements. Throughout this review, we emphasize the interactive effect of plant canopy structure (height, density) with topography, mutually creating life conditions not predictable from standard meteorological sources, but responsible for the over-proportional plant species richness of the alpine flora globally.

Forests stock up to 90% of the global terrestrial plant biomass carbon (C). Any rise or fall of that stock, but also its utilization for substituting fossil resources can influence the rate of atmospheric CO2 enrichment. By employing the term 'C sequestration', the ongoing debate suffers, however, from an implicit confusion between (1) processes, rates or fluxes of C (e.g. tree growth) with (2) pools, stores or stocks of forest biomass C. Stock formation is driven by turnover, C duration, residence time, or tree demography, and not by the rate of influx of C, including tree growth. Enhanced tree growth must not be treated as a rise in C stock, without accounting for turnover, also removing often assumed benefits of CO2 fertilization for stock formation, should tree growth be C limited, another questionable assumption. A carbon 'sink' is a potential volume that can be filled with C, but it does not represent a stock either, without accounting for C residence time. 'Buying time' by lengthening rotation has a cost in terms of reduced utilization of forest products for substitution of fossil resources. Finally, management cessation for biodiversity benefits, should be qualified by its conservation value, rather than by making a case for C storage benefits, without accounting for natural forest gap dynamics, and again, without pricing-in the inevitable cost of the cessation of the substitution of fossil C by renewable C. All this calls for a strict separation of the meaning of carbon fluxes and carbon stocks, and avoiding ambiguous terms such as C sequestration and C sink.

The influence of paleoclimate in shaping current biodiversity pattern is widely acknowledged. However, it remains unclear how the upper paleo-range limit of trees, which dictated the habitat of endemic alpine species, affects the variability in endemic alpine species composition across space over the Tibetan Plateau. We integrated satellite-derived upper range limit of trees, dendrochronological data, and fossil pollen records with a paleoclimate dataset in a climate-driven predictive model to reconstruct the spatio-temporal upper range limit of trees at 100-year intervals since the Last Glacial Maximum. Our results show that trees distributed at the lowest elevations during the Last Glacial Maximum (~3426 m), and ascended to the highest elevations during the Holocene Climatic Optimum (~4187 m), a level ~180 m higher than the present-day (~4009 m). The temporal fluctuations in paleo-range limits of trees play a more important role than paleoclimate in shaping the current spatial pattern of beta-diversity of endemic flora, with regions witnessing higher fluctuations having lower beta-diversity. We therefore suggest that anthropogenic-caused climate change on decadal-to-centennial timescales could lead to higher fluctuations in range limits than orbitally-forced climate variability on centennial-to-millennium timescales, which consequently could cause spatial homogenization of endemic alpine species composition, threatening Tibetan endemic species pool.

Maintaining or increasing forest carbon sinks is considered essential for mitigating the rise in atmospheric CO2 concentrations. In contrast, harvesting trees is perceived as having negative consequences for both the standing biomass stocks and the carbon sink strength. However, the forest carbon sink needs to be examined from a forest stand canopy perspective, where assimilation predominantly occurs in temperate forests. Here we show that a threshold of leaf area exists beyond which additional leaves do not contribute to CO2 uptake. The associated biomass can be harvested without affecting the forest carbon uptake. Based on eddy covariance measurements, we show that CO2 uptake (gross primary production - GPP) and net ecosystem exchange (NEE) in temperate forests are of a similar magnitude in both unmanaged and sustainably managed forests, on the order of 1500-1600 gCm-2yr-1 for GPP and 542-483 gCm-2yr-1 for NEE. A threshold located between 3 and 4.5 m2m-2 LAI (leaf area index) can be used for sustainable harvesting with regard to CO2 uptake. Simulations based on the LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator) model reproduce the saturation of GPP and NEP and the convergence on the LAI threshold range. Accordingly, in temperate managed forests, trees can be harvested while maintaining a high tree biomass and carbon sink of the remaining stand. In this case, competition between neighboring trees in unmanaged forests is replaced by harvest management and provision of wood products. No difference in the LAI productivity response was observed between managed and unmanaged sites.

The frost resistance of new foliage and flowers and their relationship with the phenology of leaf-out and flowering are essential for explaining plant species distribution in seasonally cold climates. In this study, we performed a congeneric, elevational comparison of phenology with frost resistance in evergreen Rhododendron species in the Southeastern Himalayas. A comparison of the microclimate with long-term meteorological records of low temperature extremes permitted the calculation of a realistic, long-term margin of safety for 12 Rhododendron species. Surprisingly, frost resistance and phenological events were matching for leaf-out time (not flowering) in higher elevation species only. Flower-leaf sequence (FLS) and frost resistance were linked for species at higher elevation and the earliest flowering species at lower elevation only. Despite a selection of FLS by elevation, flowers (including petals, filaments and ovaries) were still prone to frost damage during the early growing season at both lower and higher elevations, while new leaves were generally safe on long-term scales, regardless of phenology and elevation. In contrast to lower montane elevation, where severe frost is rare in spring, treeline elevation species maintain safety margins over centennial time-scales by adjusting leaf-out phenology. Our data show an evolutionary priority of leaf survival over flower survival. Both, physiological acclimation and phylogenetic components contribute to these adjustments. Rare extreme frost events restrict the upper range limit of the examined Rhododendron species by affecting new foliage. It is essential to know the actual temperature extremes at organ level rather than relying on weather station records.

Since climatic treelines track the elevational position of isotherms across the globe, it is not the question if, but when and how they will arrive at a novel steady-state position. After briefly recalling the essential difference between the edge of the realized and fundamental niche of the life form tree (not to be confused with species’ range limits), I will present data on recent climatic trends in the Alps based on long term meteorological records. Two years of in-situ temperature records from Pinus cembra trees growing right at the current upper edge of tree size individuals in Eastern Tyrol (supplemented with data from the Swiss Engadin region), make it obvious that the current high elevation record positions around 2500 m elevation are lagging substantially behind the upslope shift of the isotherm. This explains, why these trees grew so exceptionally rapid over the past 10 years, partly growing a meter in height in only 6-8 years. The locations with rapid tree radiation are all under nutcracker control. These data permit projections on forthcoming treeline shifts. For the Austrian Alps, the current uppermost trees represent all-time elevation records, and will soon out-range the uppermost fossil elevation records of trees that date back to the warmest period of the Holocene. Suggested reading: Körner C, Hoch G (2023) Not every high-elevation or high-latitude forest edge is a treeline. J Biogeography, open access.

While the position of alpine and arctic treelines can be predicted by climatic data, the underlying biological mechanisms are still unclear. In a recent paper in this journal (Körner C, Lenz A, Hoch G (2023) Chronic in situ tissue cooling does not reduce lignification at the Swiss treeline but enhances the risk of 'blue' frost rings. Alpine Botany https://doi.org/10.1007/s00035-023-00293-6 ) we presented results of an in situ stem-cooling experiment at a Swiss treeline site. The experiment provided answers to two entirely different questions, related to xylogenesis at treeline: (a) the absence of chronic effects of low temperature on lignification, and (b) a high time resolution insight into the rare occurrence of damages in young, still undifferentiated, and thus, non-lignified cells at the occasion of an exceptional early season frost event. In the last issue of Alpine Botany (August 7, 2023), our data had been re-interpreted by (Büntgen, Alpine Botany, 2023) by confusing (b) with (a). Cell death before secondary wall formation interrupts all metabolism, and thus, cannot exert a specific limitation of lignification. For the xylem to lignify, it requires a secondary cell wall in the first place. A frost damage in young tracheid cells is unsuitable for a dendrological treeline hypothesis based on a low-temperature threshold for lignification. Generally, the global pattern of treeline position is not associated with local freezing conditions.

As a result of the elevational compression of climatic life zones across short distances and the contrasts in life conditions associated with pronounced variation in exposure and topography, biological diversity in mountains is particularly high. Moreover, because mountain ecosystems often evolved under biogeographic isolation, levels of endemism are high as well. Beyond its intrinsic value, this richness is important as it contributes to the security of downslope settlements and infrastructure and to the well-being of millions of people. Here we summarize knowledge on the state of and trends in mountain biodiversity, on its drivers of change, and on the importance of mountain biodiversity for ecosystem functioning and the well-being of people living in or near mountains. We close this chapter by emphasizing the importance of mitigation strategies for long-term sustainability of life in and near mountains.

We used five mature Picea abies continuously labeled with 13C-depleted CO2 in a broadleaf-dominated Swiss forest to assess the spatial extent and lag time of carbon fluxes to ectomycorrhizal fungi differing in hyphal development and host association. We traced labeled carbon into ectomycorrhizal sporocarps collected for two seasons at different distances from labeled Picea. Picea-derived photosynthate reached conifer-specific sporocarps up to 6-12 m away and reached other sporocarps only 0-6 m away. At 0-6 m, genera of lesser hyphal development acquired more Picea-derived photosynthate than those of greater hyphal development, presumably from preferential fungal colonization of inner root zones by the former genera. Correlations of sporocarp 613C with daily solar radiation integrated for different periods indicated that carbon fluxes from Picea to sporocarps peaked 17-21 days after photosynthesis. Thus, these results provided rough estimates of the spatial extent and temporal lags of carbon transfer from Picea to ectomycorrhizal fungi.

Extended growing season lengths under climatic warming suggest increased time for plant growth. However, research has focused on climatic impacts to the timing or duration of distinct phenological events. Comparatively little is known about impacts to the relative time allocation to distinct phenological events, for example, the proportion of time dedicated to leaf growth versus senescence. We use multiple satellite and ground-based observations to show that, despite recent climate change during 2001 to 2020, the ratio of time allocated to vegetation green-up over senescence has remained stable [1.27 (± 0.92)] across more than 83% of northern ecosystems. This stability is independent of changes in growing season lengths and is caused by widespread positive relationships among vegetation phenological events; longer vegetation green-up results in longer vegetation senescence. These empirical observations were also partly reproduced by 13 dynamic global vegetation models. Our work demonstrates an intrinsic biotic control to vegetation phenology that could explain the timing of vegetation senescence under climate change.

The personal safety and well-being of one fifth, and water supply for almost half of all people depend directly or indirectly on the functional integrity of mountain ecosystems, the key component of which is a robust vegetation cover. The green 'coat' of the world's mountains is composed of specialized plants, animals and microbes, all nested in a great variety of microhabitats. Because a single mountain may host a series of climatically different life zones over short elevational distances, mountains are hot spots of biodiversity and priority regions for conservation. With their diverse root systems, plants anchor soils on slopes and prevent erosion. Both landuse and atmospheric changes such as elevated CO2 and climatic warming affect mountain biodiversity. Sustained catchment value depends on sustained soil integrity, which in turn depends on a diverse plant cover. Whether landuse in mountains is sustainable is a question of its consequences for water yield and biodiversity. Given their dependence on mountains, lowlanders should show concern for the highlands beyond their recreational value.

In the European Alps, mean temperature has risen by 2.5 K since the end of the nineteenth century. A 2 K warming of the growing season has taken place in the last 4 decades only. The 2.5 K warming should rise the position of the climatic treeline by about 400 m. Actual shifts in uppermost tree positions reported here for the Austrian Defereggen Valley and the Swiss Lower Engadine region of the Eastern Alps reach only around 140 m of elevation above the limit of old trees that date back to the nineteenth century. Uppermost Pinus cembra trees of > 2 m height currently occur at c. 2500 m, representing elevation records for the Eastern Alps. In situ temperature records for 2022–2023 revealed seasonal mean temperatures for uppermost trees that are 1–3 K higher than the equilibrium treeline isotherm of c. 6 °C in both regions (corrected for temperature anomalies from long-term records). The 2 K span reflects microhabitat differences and two ways to define the season. Thus, tree advances lag behind the upslope shift of the treeline isotherm, on average, by more than 200 m. The uppermost trees currently grow under quite warm conditions with annual shoot length increments frequently reaching 20 cm. Even without additional future warming, the new steady-state climatic treeline will exceed the Holocene maximum elevation in the Eastern Alps substantially.

Abstract High‐latitude vegetation experience different temperatures than the ambient air temperature. While lacking a regional plant temperature product, we drove the dynamic ecosystem model, LPJ‐GUESS, with widely used ERA5‐land surface temperature (Tsurf, at radiative equilibrium) and air temperature to understand ecosystem process responses to these two temperatures. We show that tundra plants' growth is stimulated by warmer Tsurf in the summer, but in the boreal forests, colder Tsurf in the non‐summer months constrains leaf development and enzyme activity the following growing season. Tsurf drives higher productivity of tundra plant individuals but leads to less productive individuals in the boreal forest, although with compensatory changes (almost 68%) in vegetation structure. We demonstrate the importance of forcing temperature in simulating high‐latitude ecosystem processes and call for a community effort to measure plant temperatures across canopy heights and seasons to reduce uncertainties in estimating high‐latitude plant responses and feedback to climate.

Global warming will reduce available land area for cold-adapted organisms and therefore it will be a threat to mountain plant species richness, especially in front ranges where alpine plants are restricted to small and isolated summits. At very high elevations, biodiversity diminishes gradually but so does land area, leading to very high biodiversity/land area ratios which often exceed those of lower elevations. The Rocky Mountains have higher community and landscape diversity than surrounding lowlands. Natural timberline elevation in the Bolivian Andes is at 3800–4200 m, but human activities have lowered it to 3300–3600 m, destroying over 95% of the original forest cover above 3500 m. The Caucasus seems to be richer in species than the mountains of central Asia, when related to surface area. Alpine vascular plant species richness across 43 European mountain areas decreases from south to north. Of all global change impacts on mountain biodiversity, land use is the most important factor.