Temporal dynamic of wood formation in Pinus cembra along the alpine treeline ecotone and the effect of climate variables

We determined the temporal dynamic of cambial activity and xylem development of stone pine (Pinus cembra L.) throughout the treeline ecotone. Repeated micro-sampling of the developing tree ring was carried out during the growing seasons 2006 and 2007 at the timberline (1,950 m a.s.l.), treeline (2,110 m a.s.l.) and within the krummholz belt (2,180 m a.s.l.) and the influence of climate variables on intra-annual wood formation was determined. At the beginning of both growing seasons, highest numbers of cambial and enlarging cells were observed at the treeline. Soil temperatures at time of initiation of cambial activity were c. 1.5°C higher at treeline (open canopy) compared to timberline (closed canopy), suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth. The rate of xylem cell production determined in two weekly intervals during June through August 2006–2007 was significantly correlated with air temperature (temperature sums expressed as degree-days and mean daily maximum temperature) at the timberline only. Lack of significant relationships between tracheid production and temperature variables at the treeline and within the krummholz belt support past dendroclimatological studies that more extreme environmental conditions (e.g., wind exposure, frost desiccation, late frost) increasingly control tree growth above timberline. Results of this study revealed that spatial and temporal (i.e., year-to-year) variability in timing and dynamic of wood formation of P. cembra is strongly influenced by local site factors within the treeline ecotone and the dynamics of seasonal temperature variation, respectively.     

Plant species distribution across two contrasting treeline ecotones in the Spanish Pyrenees

We describe the structure of two contrasting (elevation, topography,climate, vegetation, soil) alpine forest–pasture ecotones located in theCentral Pyrenees (sites Ordesa, O, and Tessó, T). We define ecotonestructure as the spatial distribution of trees of different size classes andgrowth-forms and the relationship between these aspects and the spatialdistribution of understory vegetation and substrate. The studied ecotones aredominated by Pinus uncinata Ram. and have been littleaffected by anthropogenic disturbances (logging, grazing) during this century.One rectangular plot (30 × 140 m) was located within eachsite with its longest side parallel to the slope and encompassing treeline andtimberline. The distribution of size and growth-form classes at site O followeda clear sequence of increasing size downslope from shrubby multistemmedkrummholz individuals to bigger arborescent trees. At site O, regeneration wasconcentrated near the krummholz area and over rocky substrates. This suggeststhat krummholz may modify microenvironment conditions and increase seedlingsurvival. At site T, regeneration was abundant above the treeline where thecover of the dominant understory shrub (Rhododendronferrugineum) decreased. In both ecotones the diversity of plants washigher above the treeline than in the forest and decreased going downslopecoinciding with the increase of P. uncinata cover. Thereduction of plant diversity appeared above the current timberline. At site O,the decrease was steep and spatially heterogeneous what may be due in part tothe edaphic heterogeneity. At site T the change was abrupt though smaller. Therelationships between the plant community and tree regeneration should be takeninto account in future ecological studies of treeline pattern.     

Implications of climate change for Daphnia in alpine lakes: predictions from long-term dynamics, spatial distribution, and a short-term experiment

Alpine lakes may be particularly useful as sentinels of climate change because they are highly sensitive to environmental conditions. To explore the potential biotic consequences of climate change in these systems, we conducted paleo- and neoecological observational studies, as well as a short-term experiment to examine Daphnia responses to changing environmental conditions in Rocky Mountain alpine lakes. Our analysis of a sediment core from Emerald Lake representing two periods from the Holocene revealed a significant positive relationship between the abundance of Daphnia remains and fossil Aulacoseira lirata, a diatom associated with deeper mixing depths. In addition, we detected a significant increase in mean Daphnia density in the long-term record (1991–2005) from Pipit Lake, a trend that correlated well with increases in mean surface temperature. In our survey of Daphnia in 10 lakes in the Canadian Rocky Mountains, Daphnia abundance was positively correlated with both dissolved organic carbon concentration and temperature. Finally, our short-term incubation experiment demonstrated significant effects of physical conditions (i.e., temperature and/or UV radiation) and water chemistry on the juvenile growth rate of Daphnia. Overall, our findings highlight the sensitivity of Daphnia to changes in mixing depth, water temperature, and dissolved organic matter, three limnological variables that are highly sensitive to changes not only in air temperature, but also to precipitation and location of the treeline in alpine catchments. Thus, we conclude that Daphnia abundance could serve as a powerful sentinel response to climate change in alpine lakes of the Rocky Mountains.     

Spruce-fir growth form changes in the forest-tundra ecotone of Rocky Mountain National Park, Colorado, USA

Tree regeneration has traditionally been used as a measure of the response of treeline to climate Changes in growth form of krummholz trees may also indicate whether treeline is responding to changes in climate The purpose of this study was to determine whether krummholz trees m the forest-tundra ecotone of Rocky Mountain National Park, Colorado have experienced significant vertical stem growth, in the absence of mortality, and if this growth occurred in response to recent changes in climate We sampled and dated Engelmann spruce and subalpine fir krummholz leaders stratified by height class at three sampling locations to determine the dates leaders initiated growth above mean snow depth At one sampling location, 215 additional leaders were sampled to construct an age structure of leader release dates Dates of leader release taken from the age structure were compared with seasonal temperatures, seasonal precipitation, winter snow depths, and annual runoff using t-tests Dates of leader release were also compared to proxy climate records for the southern Rocky Mountain region Based on historical photos as well as the data presented here, both spruce and fir krummholz trees experienced significant height growth as early as the 1850's and continued to grow vertically, at least through the 1970's This vertical stem growth occurred in the absence of significant mortality Running mean annual temperature and May snow depth are both positively associated with years of leader release, suggesting that a warmer, wetter climate, possibly following the end of the Little Ice Age ca 1850, may have induced these changes in the ecotone     

Nitrogen and carbon source-sink relationships in trees at the Himalayan treelines compared with lower elevations

No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.     

高山林线变化的更新受限机制研究进展

全球林线位置对气候变暖的响应表现为上升、无变化或下降等截然不同趋势,表明影响林线位置及动态的因子十分复杂,除了较普遍认为的低温调控机制外,还存在其它控制林线位置变化的机制。林线向上迁移开始于种子向林线以上的传播及幼苗在林线以上的定居,这些过程中的限制因子均会影响林线的位移,因此研究更新过程及其限制因子对理解高山林线对气候变化的响应具有重要的科学意义。主要从种子和幼苗两个关键阶段综述高山林线森林更新的研究进展。在种子阶段,夏季积温不足导致种子产量和活力下降,风速过低和浓密灌丛限制种子向林线以上传播,近地表的霜冻/水分胁迫和灌木释放的化感物质会阻碍种子在林线以上萌发。在幼苗阶段,除冬季低温外,生长季内较大的温度日振幅和偶然出现的冻害事件也是导致幼苗死亡的重要原因,而低温环境下的强烈光照引起的低温光抑制会显著降低生长季的光合作用;土壤低温、由土壤温度昼夜变化引起的冻举事件、夏季土壤干旱可能会导致幼苗光合作用下降和死亡率上升;积雪太浅会导致生长季早期幼苗水分供应的严重缺乏,但积雪太深会导致幼苗感染真菌的可能性增加;浓密的灌木和草本植物以及植食动物的啃食也会降低林线以上的幼苗存活率。气候变暖对林线幼苗定居的影响复杂且具有很大不确定性,需要进一步研究气候变暖导致的环境因子变化对林线更新各关键阶段的影响。未来气候变暖无疑会导致生长季起始日提前,结束日推迟,这很可能会增加生长季期间尤其是早期的低温冻害事件,对高山林线树种幼苗的存活具有重要影响。在未来研究中,需要找出定义生长季冻害事件的温度阈值,利用长期气象观测数据分析增温背景下生长季早期冻害事件特征的变化趋势,并进一步开展野外模拟增温实验以深刻理解林线树种的种子萌发和幼苗定居与生长季冻害事件的关系,加强对不同地区林线树种的繁殖策略研究,这将有助于人们进一步理解不同区域林线的形成机制并预测未来气候变化条件下林线的动态变化趋势。     

Prehistoric human influence on the abundance and distribution of deadwood in alpine landscapes

Scientists have long inferred the locations of past treelines from the distribution of deadwood above modern tree boundaries. Although it is recognized that deadwood above treeline may have decayed, the absence of such wood is routinely taken to imply the absence of trees for periods ranging from the past few millennia to the entire Holocene. Reconstructed treeline histories are then explained in terms of such variables as slope, drainage, temperature, solar insolation, and precipitation. While these variables certainly help determine where deadwood is to be found above treeline today, we suggest that they cannot always explain where it is not to be found. In the alpine environments of the western United States, archeological work has established a human presence during nearly the entire Holocene in portions of the Rocky Mountains and for over 5000 radiocarbon years in the Great Basin and Sierra Nevada. We suggest that prehistoric occupations may have stripped deadwood from the landscape in all of these areas. To the extent that this is true, reconstructions of past treelines from deadwood may reflect the human prehistory of an area as much as it reflects treeline history itself. We encourage evaluation of this hypothesis in areas of active dendrochronological and archeological research.     

Altitudinal changes in composition and structure of mountain-temperate vegetation: a case study from the Western Carpathians

Changes in composition and structure of plant communities in relation tothe soil and snow cover variation were analyzed along an altitudinal transect(1150–1750 m) from the mountain-temperate forests to a woodyshrub community and alpine meadows on Mt Velký Gápel', Slovakia.The soils below the treeline (1510 m) had a more developedorganic layer above the mineral substratum. Generally, soil depth decreased asthe altitude increased, although the maximum values were recognized at a middlealtitude in a beech stand. Snow was redistributed by westerly winds from theridgeline down to the upper forest margin. Mean snow depth decreased withaltitude up to almost snow-free sites around the summit. In the 48 plots at 16sites we recorded 118 taxa including 6 tree, 7 shrub, 18 grass, 42 herb, 5fern,25 moss and 15 lichen species. The species diversity showed no distinctrelationship to altitude but declined with canopy consolidation. The TWINSPANfloristic classification distinguished five groups of community typescharacterised by different dominants, and a further three clusters of samplesfrom transition zones. Horizontal compositional heterogeneity increased inareaswhere trees were aggregated and tree basal area was smaller. Vegetationcomposition became more patchy at open-canopy Acerpseudoplatanus–Abies alba mixed forest at 1150 m,in Picea abies forest limit 1470 m, andin Pinus mugo krummholz at 1590 m. Speciesturnover of the entire transect was 6.1 half-changes as estimated by DCA.Despite this heterogeneity, none of the 15 elevational bands had significantaggregation of species' limits. Vegetation varied continuously, with individualspecies overlapping in transition zones delimited by dominant taxa. Thecoincident aggregation of up-slope and down-slope boundaries was found at abelt1430–1510 m. This discrete ecotone corresponds to a shiftfrom the closed coniferous forest to P. mugo krummholz.Thesecond inherent up-slope boundary aggregation indicated the P.mugo krummholz – alpine meadow vegetation transition at1700 m. Spatial analysis (K-function) of eight forest plots(0.12 ha each) showed that at lower elevation, adult trees of thebroad-leaf forest were closer to a random arrangement while at higherelevation,trees of evergreen coniferous stands became aggregated toward the forest limitwith the highest intensity from 2 to 4 m. Altitudinal gradient andrelated factors explained 35% of the variance in vegetation data.Canonical correspondence analysis also showed that main vegetation changesabovethe treeline area were associated with the topographic pattern of pine shrubsand snow cover.     

川西北高寒草甸不同放牧模式对土壤呼吸的影响

采用土壤二氧化碳(CO_2)通量自动测量系统,对不同放牧模式(全年禁牧、夏季放牧、冬季放牧和自由放牧)下川西北高寒草甸的土壤呼吸进行监测,比较了不同放牧模式下土壤呼吸的季节动态和温度敏感性。研究发现:1)放牧模式可以改变高寒草甸土壤呼吸的季节动态变化。禁牧、夏季放牧以及自由放牧样地的土壤呼吸在季节上的变化趋势基本相似,而冬季放牧样地的土壤呼吸最大值与前者相比明显向后推迟;2)放牧模式并不改变高寒草甸年平均土壤呼吸速率,但对不同季节土壤呼吸速率的影响不同;3)不同放牧模式可以改变土壤呼吸对温度的敏感性(Q_(10))。不同放牧模式下土壤呼吸Q_(10)值大小依次为:禁牧1a(8.13)冬季放牧(7.49)禁牧3a(5.46)夏季放牧(5.20)自由放牧(4.53)。该地区土壤呼吸的Q_(10)值均明显高于热带和其它温带草地土壤呼吸的Q_(10)值。结果表明,放牧模式是影响高寒草甸土壤碳排放的一个重要因素。此外,在未来全球气候变暖背景下,在生长季节无放牧干扰的高寒草甸可能比放牧干扰的高寒草甸释放出更多的CO_2到大气中。     

The interacting effects of temperature,ground disturbance,and herbivory on seedling establishment: implications for treeline advance with climate warming

Thermal control of treeline position is mediated by local environmental and ecological factors, making trends in treeline migration difficult to extrapolate geographically. We investigated the ecological dynamics of conifer establishment at treeline in the Mealy Mountains (Labrador, Canada) and the potential for its expansion with climate warming. Available seedbed and tree seedling emergence in the treeline ecotone were monitored, and seeds and seedlings of Picea mariana were planted along an elevational gradient from open-canopy forest through tree islands to alpine tundra. Experimental treatments included passive warming of daytime air, ground disturbance, and vertebrate herbivore exclosures. Responses in seed germination and seedling growth, damage, and mortality were monitored over two growing seasons, and re-surveyed after 5 years. While no tree seedlings were observed growing naturally above the treeline, planted seeds were able to germinate, develop and overwinter, and persist for 4 years in all habitats examined. Disturbance of the seedbed was important for seedling emergence in the forest and tree islands. While temperature enhancement alone had little impact on emergence, even moderate temperature increases had significantly disproportionate effects on emergence of seedlings in the alpine habitat when combined with soil disturbance, indicating that future climate warming could lead to treeline advance if viable seed and suitable substrate for recruitment are available. The positive effect of excluding herbivores suggests that herbivory may be an important filter modifying future species distribution. While seedbed conditions and herbivory would control the rate of individual species advance, the results indicate potential upslope migration of the treeline in the Mealy Mountains, with consequent loss of alpine ecosystems.     

Seasonal dynamics of mobile carbon supply in Quercus aquifolioides at the upper elevational limit

Many studies have tried to explain the physiological mechanisms of the alpine treeline phenomenon, but the debate on the alpine treeline formation remains controversial due to opposite results from different studies. The present study explored the carbon-physiology of an alpine shrub species (Quercus aquifolioides) grown at its upper elevational limit compared to lower elevations, to test whether the elevational limit of alpine shrubs (<3 m in height) are determined by carbon limitation or growth limitation. We studied the seasonal variations in non-structural carbohydrate (NSC) and its pool size in Q. aquifolioides grown at 3000 m, 3500 m, and at its elevational limit of 3950 m above sea level (a.s.l.) on Zheduo Mt., SW China. The tissue NSC concentrations along the elevational gradient varied significantly with season, reflecting the season-dependent carbon balance. The NSC levels in tissues were lowest at the beginning of the growing season, indicating that plants used the winter reserve storage for re-growth in the early spring. During the growing season, plants grown at the elevational limit did not show lower NSC concentrations compared to plants at lower elevations, but during the winter season, storage tissues, especially roots, had significantly lower NSC concentrations in plants at the elevational limit compared to lower elevations. The present results suggest the significance of winter reserve in storage tissues, which may determine the winter survival and early-spring re-growth of Q. aquifolioides shrubs at high elevation, leading to the formation of the uppermost distribution limit. This result is consistent with a recent hypothesis for the alpine treeline formation.     

长白山林线过渡带树岛土壤微生物群落结构和生态功能

林线过渡带是指从郁闭森林上限到树种分布上限之间的区域,过渡带内生物多样性丰富,生态系统结构、功能和生态过程在很小的海拔梯度内发生剧烈变化,因此对全球气候变化和人类活动极为敏感。树岛是在林线过渡带内出现的斑块状或条带形不连续分布的树木集群,树岛内生存的树木通常能达到与较低海拔郁闭森林同样的高度和胸径,因此揭示树岛这一特殊生境的生态特征及其形成机制,对于预测未来气候变化下林线动态具有重要意义。以长白山岳桦林线过渡带一大型树岛作为研究对象,测定了土壤理化性质和土壤酶活性,采用宏基因组测序技术分析了微生物群落结构组成和功能基因丰度,通过与同海拔的开阔区生境进行对比,揭示了树岛这一特殊生境的土壤微生物群落结构特征和潜在生态功能,从土壤养分和土壤微生物学角度,阐明树岛形成的可能驱动机制。结果表明,树岛土壤的含水量、总碳、总氮和微生物生物量显著高于同海拔开阔区（P<0.05）,与微生物r-策略相关的生理生化和遗传学指标,包括纤维素酶活性、放线菌相对丰度、与转录、防御、控制细胞周期相关的基因丰度、小分子碳降解基因丰度,均高于开阔区（P<0.05）。相反的,与微生物K-策略相关的指标,包括酸杆菌相对丰度、大分子碳降解基因相对丰度低于开阔区。揭示了树岛土壤微生物学特征,并从土壤微生物组学角度探讨了树岛形成的潜在机制,认为树岛内土壤养分增加并导致微生物群落r-策略倾向,这种变化反过来也可能促进树岛进一步扩大,进而影响林线动态。     

Strong topographic sheltering effects lead to spatially complex treeline advance and increased forest density in a subtropical mountain region

Altitudinal treelines are typically temperature limited such that increasing temperatures linked to global climate change are causing upslope shifts of treelines worldwide. While such elevational increases are readily predicted based on shifting isotherms, at the regional level the realized response is often much more complex, with topography and local environmental conditions playing an important modifying role. Here, we used repeated aerial photographs in combination with forest inventory data to investigate changes in treeline position in the Central Mountain Range of Taiwan over the last 60 years. A highly spatially variable upslope advance of treeline was identified in which topography is a major driver of both treeline form and advance. The changes in treeline position that we observed occurred alongside substantial increases in forest density, and lead to a large increase in overall forest area. These changes will have a significant impact on carbon stocking in the high altitude zone, while the concomitant decrease in alpine grassland area is likely to have negative implications for alpine species. The complex and spatially variable changes that we report highlight the necessity for considering local factors such as topography when attempting to predict species distributional responses to warming climate.     

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

Summary The importance of high winter winds and plant temperatures as causes of winter desiccation damage at the alpine treeline were studied in the Austrian Alps. Samples of 1- and 2-year twigs of Picea abies and Pinus cembra were collected from the valley bottom (1,000 m a.s.l.), forestline (1,940 m a.s.l.), kampfzone (2.090 m a.s.l.), wind-protected treeline (2,140 m a.s.l.), and wind-exposed treeline (2,140 m a.s.l.). Cuticular transpiration was measured at three different levels of wind speed (4, 10, and 15 ms^-1) and temperature (15°, 20°, and 25° C). At elevated wind speeds slight increases in water loss were observed, whereas at higher temperatures much greater increases occurred. Studies on winter water relations show a significant decline in the actual moisture content and osmotic potentials of twigs, especially in the kampfzone and at treeline. The roles of high winds and temperatures in depleting the winter water economy and causing desiccation damage in the alpine treeline environment are discussed.     

Snow distribution, soil temperature and late winter CO2 efflux from soils near the Arctic treeline in northwest Alaska

The Arctic treeline is advancing in many areas and changes in carbon (C) cycling are anticipated. Differences in CO₂ exchange between adjacent forest and tundra are not well known and contrasting conclusions have been drawn about the effects of forest advance on ecosystem C stocks. Measurements of CO₂ exchange in tundra and adjacent forest showed the forest was a greater C sink during the growing season in northern Canada. There is, however, reason to expect that forests lose more C than tundra during the wintertime, as forests may accumulate and retain more snow. Deeper snow insulates the soil and warmer soils should lead to greater rates of belowground respiration and CO₂ efflux. In this study, I tested the hypotheses that forests maintain a deeper snowpack, have warmer soils and lose more C during winter than adjacent tundra near the Arctic treeline in northwest Alaska. Measurements of snow depth, soil temperature and CO₂ efflux were made at five forest and two treeline sites in late winter of three consecutive years. Snow depth and soil temperature were greater in forest than treeline sites, particularly in years with higher snowfall. There was a close exponential correlation between soil temperature and CO₂ efflux across sites and years. The temperature-efflux model was driven using hourly soil temperatures from all the sites to provide a first approximation of the difference in winter C loss between treeline and forest sites. Results showed that greater wintertime C loss from forests could offset greater summertime C gain.     

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains

Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere     

长白山北坡林线岳桦种群空间分布格局

许多自然林线具有的趋同特征之一即乔木树种高生长受限,常演化为矮曲状或类似于灌木的形态(即树种的灌木型)占据高山植被带,因此研究林线树种乔木型与灌木型的结构、功能差异有助于进一步理解林线形成的原因。种群分布格局作为种群相对位置定量化描述的基本特征,可以表征物种对环境适应性选择的结果,反应生态过程的综合作用。利用点格局方法,研究长白山北坡林线岳桦种群各生活史阶段、两种生活型的分布格局,结果表明,长白山北坡林线岳桦树高生长受到限制,1.5—3.0m是一个关键的树高生长阶段;相比于老树、中树,幼苗和灌木型岳桦更为均匀,对空间的异质性选择更弱;林线岳桦发育过程中,存在一个生活型分离的重要阶段。此外,相对于老树,灌木型分布更为均匀,表明低矮、多枝这种相对紧凑的生活型更适宜在过渡带生存,乔木型岳桦和灌木型岳桦可能代表着不同的生存策略。     

Storage, patterns and controls of soil organic carbon in the Tibetan grasslands

The soils of the Qinghai-Tibetan Plateau store a large amount of organic carbon, but the magnitude, spatial patterns and environmental controls of the storage are little investigated. In this study, using data of soil organic carbon (SOC) in 405 profiles collected from 135 sites across the plateau and a satellite-based dataset of enhanced vegetation index (EVI) during 2001–2004, we estimated storage and spatial patterns of SOC in the alpine grasslands. We also explored the relationships between SOC density (soil carbon storage per area) and climatic variables and soil texture. Our results indicated that SOC storage in the top 1 m in the alpine grasslands was estimated at 7.4 Pg C (1 Pg=10¹⁵ g), with an average density of 6.5 kg m⁻². The density of SOC decreased from the southeastern to the northwestern areas, corresponding to the precipitation gradient. The SOC density increased significantly with soil moisture, clay and silt content, but weakly with mean annual temperature. These variables could together explain about 72% of total variation in SOC density, of which 54% was attributed to soil moisture, suggesting a key role of soil moisture in shaping spatial patterns of SOC density in the alpine grasslands.     

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

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

Carbon sink limitation and frost tolerance control performance of the tree <Emphasis Type="Italic">Kageneckia angustifolia</Emphasis> D. Don (Rosaceae) at the treeline in central Chile

The decrease in temperature with increasing elevation may determine the altitudinal tree distribution in different ways: affecting survival through freezing temperatures, by a negative carbon balance produced by lower photosynthetic rates, or by limiting growth activity. Here we assessed the relative importance of these direct and indirect effects of altitudinal decrease in temperature in determining the treeline in central Chile (33°S) dominated by Kageneckia angustifolia. We selected two altitudes (2000 and 2200 m a.s.l.) along the treeline ecotone. At each elevation, leaf non-structural carbohydrates (NSC) and gas exchange parameters were measured on five individuals during the growing season. We also determined the cold resistance of K.␣angustifolia, by measuring temperatures that cause 50% seedling mortality (LT₅₀) and ice nucleation (IN). No differences in net photosynthesis were found between altitudes. Although no differences were detected on NSC concentration on a dry matter basis between 2000 and 2200 m, when NSC concentration was expressed on a leaf area basis, higher contents were found at the higher elevation. Thus, carbon sink limitations may occur at the K. angustifolia’s upper altitudinal limit. For seedlings derived from seeds collected at the 2200 m, LT₅₀ of cold-acclimated and non-acclimated plants were −9.5 and −7 °C, respectively. However, temperatures as low as −10 °C can frequently occur at this altitude during the end of winter. Therefore, low temperature injury of seedlings seems also be involved in the treeline formation in this species. Hence, a confluence of global (carbon sink limitation) and regional (freezing tolerance) mechanisms explains the treeline formation in the Mediterranean-type climate zone of central Chile.