Treeline Zone Research Articles

Impacts of Global Change on the Spatial Dynamics of Treeline in Venezuelan Andes

The treeline in the Andes is considered an essential ecotone between the Montane forest and Páramo. This treeline in the Venezuelan Andes corresponds with a transitional ecosystem defined as the Páramo forest. In this work, we identify and analyze the impact of climate warming and land transformation as agents altering the Páramo forest ecosystem’s spatial dynamics along the Venezuelan Andes’ altitudinal gradient. We carry out multitemporal studies of 57 years of the land transformation at different landscapes of the Cordillera de Mérida and made a detailed analysis to understand the replacement of the ecosystems potential distribution. We found that the main ecosystem transition is from Páramo to the Páramo forest and from Páramo to the Montane forest. Based on the difference between the current lower Páramo limit and the Forest upper limit for 1952, the treeline border’s displacement is 72.7 m in the 57 years of study, representing ∼12.8 m per decade. These changes are mainly driven by climate warming and are carried out through an ecological process of densification of the woody composition instead of the shrubland structure. We found that Páramo forest ecosystems practically have been replaced by the Pastures and fallow vegetation, and the Crops. We present a synthesis of the transition and displacement of the different ecosystems and vegetation types in the treeline zone. The impact of climate warming and deforestation on the Páramo forest as a representative ecosystem of the treeline shows us that this study is necessary for an integrated global change adaptation plan.

Estimating tree height from TanDEM-X data at the northwestern Canadian treeline

The circum-Arctic transitional zone between forest and tundra, i.e. the treeline zone, is shifting northward due to current Arctic warming and, therefore, requires systematic monitoring. Up to now, radar remote sensing was hardly possible in the treeline zone due to spatial resolution and/or temporal decorrelation constraints of preceding satellite missions. The unique constellation of the TanDEM-X satellites with its bistatic mode and very high spatial resolution opens up opportunities for monitoring small (≥0.01 km2) and isolated patches of very sparse forest which are typical for the transitional zone. We focused on an area at the northern edge of the treeline zone in the Northwest Territories, Canada, and evaluated the potential of TanDEM-X bistatic data to characterize the tree height in the forest patches in this region. TanDEM-X data were acquired during the TanDEM-X Science Phase in 2015, when the perpendicular baseline was large (corresponding to the height of ambiguity of approximately 14.6 m) and kept constant. We employed TanDEM-X backscatter, bistatic coherence, and interferometric height from the stack of seven multitemporal bistatic pairs and compared them to maximum vegetation height obtained from full-waveform airborne LiDAR data. We found strong linear relationships between all TanDEM-X metrics and LiDAR vegetation height within the forest patches with r = 0.67, r = −0.69, and r = 0.78 for the backscatter, coherence, and interferometric height, respectively. Furthermore, we extracted the position of individual trees from the LiDAR data and estimated tree density as the number of trees per unit area. The linear relationships between all TanDEM-X metrics and the tree density were very weak. The relationships between all TanDEM-X metrics and tree height differentiated for three tree density classes (low, medium, and high) remained strong. Random forests regression using all three TanDEM-X metrics predicted the tree height with a mean absolute error of 0.7 m (mean forest height in the study area was 2.5 m). CoSSC pairs were generally consistent with each other and the multitemporal averaging slightly improved the performance compared to single pairs. Taking into account the global coverage of bistatic TanDEM-X data acquired for the global digital elevation model, our results show a potential for quantifying the tree height in small forest patches along the circum-Arctic treeline zone.

Variation in reproductive potential across a multi-species treeline

ABSTRACT Cone and seed production at the forest-tundra ecotone, or treeline, depend on species-specific tolerances to limiting abiotic and biotic factors. As range expansion via seed dispersal is needed to keep pace with climate change, reproductive limitations act as a bottleneck for treeline advance. The treeline in the Mealy Mountains, central Labrador, was comprised of four codominant species: black spruce (Picea mariana [Mill.] B.S.P.), white spruce (Picea glauca [Moench] Voss), eastern larch (Larix laricina [Du Roi] K. Koch), and balsam fir (Abies balsamea [L.] Mill.). Conifer stem surveys from three treeline zones (forest, forest-tundra transition, krummholz) were used to assess patterns of altitudinal distributions, tree densities, and cone production to provide insight into overall reproductive potential. The altitudinal limit of the spruce species was 39 m a.s.l. higher than the altitudinal limit of black spruce cone production. Black spruce had the highest densities of cone-bearing trees across treeline with eastern larch values being comparable in the forest-tundra transition zone, although overall cone production was low and highly variable in all species. Compared to the other treeline species, black spruce has the greatest reproductive potential for upslope advance.

Diversity and community structure of ectomycorrhizal fungi associated with Larix chinensis across the alpine treeline ecotone of Taibai Mountain.

Alpine treeline ecotones represent ecosystems that are vulnerable to climate change. We investigated the ectomycorrhizal (ECM) community, which has potential to stabilize alpine ecosystems. ECM communities associated with Larix chinensis were studied in four zones along a natural ecotone from a mixed forest stand over pure forest stands, the timberline, and eventually, the treeline (3050-3450m) in Tabai Mountain, China. Sixty operational taxonomic units (OTUs) of ECM fungi were identified by sequencing the rDNA internal transcribed spacer of ECM tips. The richness of ECM species increased with elevation. The soil C/N ratio was the most important factor explaining ECM species richness. The treeline zone harbored some unique ECM fungi whereas no unique genera were observed in the timberline and pure forest zone. Elevation and topography were equally important factors influencing ECM communities in the alpine region. We suggest that a higher diversity of the ECM fungal community associated with L. chinensis in the treeline zone could result from niche differentiation.

Complex Changes in Plant Communities across a Subarctic Alpine Tree Line in Labrador, Canada + Supplementary Appendix Table (See Article Tools)

As climate warms, abiotic factors, as well as plant community and biodiversity structure, may constrain or promote the movement of ecotone boundaries. Our study sought to examine how plant communities change across the tree-line ecotone of the Mealy Mountains in Labrador, Canada. We established eight transects (50–100 m in length) along an elevation gradient in three tree-line zones (forest, forest-tundra, and alpine-tundra) and recorded all species and cover of vegetation in contiguous 1 × 1 m quadrats. Companion abiotic parameters of nutrients and soil temperature were also measured. The absence of abrupt changes in important soil nutrients and growing season temperatures suggests that these factors do not limit tree species establishment beyond the current tree line. Vegetation cover and richness, however, were highly variable and in some cases changed non-linearly across the tree-line ecotone. Tree cover and species density generally decreased with elevation, while some field layer species (< 25 cm in height) increased; the latter change seems to be influenced by ground shrubs rather than herbaceous species. As expected, transects separated by the greatest difference in elevation were the least similar (higher beta diversity/species turnover); however, species turnover between the forest and forest-tundra transects was higher than it was between forest-tundra and alpine-tundra transects, even though the latter were separated by a greater elevation. Community structure and species turnover vary greatly across a tree line with the greatest differences between the forest and the forest-tundra, suggesting a biotic or abiotic barrier. While our ability to predict how the tree line will respond to continued climate change is complicated by these patterns in plant communities, the potential barriers investigated and others identified will be a useful focus for future studies.

Detection of vegetation abundance change in the alpine tree line using multitemporal Landsat Thematic Mapper imagery

Detection of alpine tree line change using pixel-based approaches on medium spatial resolution imagery is challenging because of very slow tree sprawl without obvious boundaries. However, vegetation abundance or density in the tree line zones may change over time and such a change may be detected using subpixel-based approaches. In this research, a linear spectral mixture analysis (LSMA)-based approach was used to examine alpine tree line change in the Northern Tianshan Mountains located in Northwestern China. Landsat Thematic Mapper (TM) imagery was unmixed into three fraction images (i.e. green vegetation – GV, shade, and soil) using the LSMA approach. The GV and soil fractions at different years were used to examine vegetation abundance change based on samples in the alpine tree line. The results show that Picea schrenkiana abundance around the top of the forested area increased approximately by 18.6% between 1990 and 2010, but remained stable in the central forest region over this period. Juniperus sabina abundance around the top of the forested area, in the central scrub region, and at the top of the scrub region increased approximately by 19.3%, 8.2%, and 15.6%, respectively. The increased vegetation abundance and decreased soil abundance of both P. schrenkiana and J. sabina indicate vegetation sprawl in the alpine tree line between 1990 and 2010. This research will be valuable for better understanding the impacts of climate change on vegetation change in the alpine tree line of central Asia.

Role of terrestrial carbon in aquatic UV exposure and photoprotective pigmentation of meiofauna in subarctic lakes

Summary Aquatic organisms are adversely influenced by ultraviolet radiation (UV) and utilise photoprotective strategies, including pigmentation. We examined UV‐protective melanin pigmentation of aquatic meiofauna (Cladocera) in relation to the UV exposure across 25 tree line lakes in Finland to address the potential effects of increased UV and altered input of UV‐screening terrestrial dissolved organic carbon (DOC) on aquatic organisms. Bio‐optical parameters, including concentration of DOC, the coloured dissolved organic matter (CDOM) fraction, a suite of carbon quality indices and chlorophyll a, were analysed from lake water, and their role in controlling underwater UV environment (measured as diffuse UV attenuation coefficient Kd at 305 and 340 nm) was examined. Cladoceran (Alona affinis) carapaces were extracted from the surface sediments, and their melanisation was assessed with spectroscopic UV‐visible light absorbance measurements. DOC, CDOM and specific UV absorbance (SUVA) had strong positive relationships with the attenuation of UV in the lakes, suggesting that terrestrial organic carbon controls underwater UV exposure in the examined lakes. The absorbance measurements indicated the presence of melanin in the cladoceran carapaces, the degree of melanisation varying strongly among the lakes. Melanisation had significant relationships with SUVA and fluorescence index (FI). It was higher in lakes with low SUVA and high FI, indicating that cladocerans exhibit strong melanisation in lakes with low contribution of UV‐attenuating allochthonous DOC (i.e. high UV exposure). The results suggest that cladoceran meiofauna respond to UV by utilising photoprotective pigmentation and that the degree of pigmentation is affected by site‐specific underwater UV exposure, which is ultimately controlled by UV‐attenuating DOC of terrestrial origin. Although cladoceran meiobenthos are able to adapt to varying underwater UV doses, climate change with its multiple consequences on hydrology, limnology and catchment vegetation in the tree line zone may cause major changes in underwater UV environment for the organisms to adapt.

Insects can limit seed productivity at the treeline

Numerous factors contribute to the variability in treeline change; however, the potential role of insect predation in limiting seed productivity is not well documented. Conditions for seed germination, establishment, and survival are not limiting at the Mealy Mountains treeline (Labrador, Canada), but seedlings are rarely encountered, suggesting a seed-related bottleneck to recruitment. Mature cones were collected in 2008 from four tree species (black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.), balsam fir (Abies balsamea (L.) Mill.), eastern larch (Larix laricina (Du Roi) K. Koch), and white spruce (Picea glauca (Moench) Voss)) across three treeline zones (forest, transition, and krummholz) to assess potential seed limitation. During that year, an unexpectedly high diversity of insect larvae caused extensive reproductive loss and damaged the cones of ∼85% of trees sampled, confirming that treeline change models should include seed predation. Seed germination was low and variable in all treeline zones, although significantly higher in black spruce and eastern larch. Most reproductive quality measures decreased significantly with elevation, although no differences among zones or tree species in the percentages of seeds damaged by insects were found (mean ± standard deviation: 31% ± 23%). Based on tree density and seed production, black spruce is predicted to lead the treeline expansion in the Mealy Mountains. Although climate warming may create conditions conducive for increased seed production, predispersal seed predation may limit future treeline expansion.

Growth limitation of trees and carbon balance of the vegetation in the treeline zone in north eastern Lapland

Growth limitation of trees and carbon balance of the vegetation in the treeline zone in north eastern Lapland

Balsam Poplar (<i>Populus balsamifera</i>; Salicaceae) Beyond the Tree Line in the Western Canadian Mainland Arctic (Northwest Territories)

Balsam poplar is the northernmost tree species in North America, with a reported range that extends generally to the tree line across the continent and beyond the tree line in Alaska, where extralimital stands growing in Arctic ecosystems on the North Slope have been documented and studied. Here we summarize existing information and report new data on extralimital stands of balsam poplar from the Arctic ecozone in the northeastern mainland Northwest Territories. These occurrences extend the geographical and ecological range of the species fully into the mainland Canadian Arctic. In this region, balsam poplar is known from four sites: two in Tuktut Nogait National Park and two along the Hornaday and Brock rivers just beyond the northwestern Park boundary. Balsam poplar was first reported from two of these sites more than 50 years ago, but those data have not been considered in most subsequent floristic and ecological work. A balsam poplar grove in Tuktut Nogait National Park consists of four discrete stands of shrubby plants growing on a low ridge adjacent to the Hornaday River; their tallest ramets measure 1.1 – 1.86 m. A larger grove along the edge of the lower Brock River consists of three large stands, the tallest ramets measuring 3.5 – 4 m. The boreal and subarctic regions of the Northwest Territories and Nunavut have large areas where balsam poplar has not been documented by herbarium specimens, including most of the forest-tundra and tree-line zones. Collections from these areas and other potential extralimital sites in the Canadian Arctic are urgently needed to document the current distribution of balsam poplar. Such data could serve as a baseline for assessing potential future alteration of the range of this species as a result of climate change.

The trade-off between cold resistance and growth determines the Nothofagus pumilio treeline

The upper and poleward limit of tree distribution are usually determined by abiotic factors such as low temperature and strong winds. Thus, cold resistance is a key element for survival in high altitudes and latitudes where conditions can reduce plant growth. A trade-off between resource allocation to cold resistance and growth could emerge in populations frequently exposed to low temperatures like those in the treeline zone. We studied annual height growth and ice nucleation temperature in Nothofagus pumilio (Nothofagaceae) populations growing in its extremes of altitudinal distribution and in 3 sites situated on a latitudinal gradient in the Chilean Andes. Additionally, gas exchange, water and nitrogen use efficiency and total soluble sugar (TSS) were also measured as possible mechanisms for survival in high altitudes. Individuals from the treeline populations showed lower annual height growth and lower ice nucleation temperatures compared with those from lower populations. In the same way, individuals from more poleward populations showed lower annual height growth and lower ice nucleation temperatures. Gas exchange, water and nitrogen use efficiency and TSS were also higher in the high altitude populations. The results obtained support the hypothesis of trade-off, because the upper and poleward populations showed more cold resistance but a lower height growth. Additionally, we show that cold resistance mechanisms do not impact the physiological performance, suggesting possible adaptation of the high altitude populations. Low temperatures may be affecting cellular growth instead of photosynthesis, creating a pool of carbohydrates that could participate in cold tolerance. Other abiotic and biotic factors should be also assessed to fully understand the distributional range of Nothofagus species.

A thousand years of human impact in the northern Scandinavian mountain range: Long-lasting effects on forest lines and vegetation

The vegetation history has been examined by pollen, microscopic and macroscopic charcoal analyses in the vicinity of two alpine Stállo settlement sites and one forested ‘Reference area’ with no settlement remains in northwestern Sweden in the northern part of the Scandinavian mountain range. The acquired data provide indications of the effects of human activities on local vegetation during the settlement phase, and on ecological processes in both long- and short-term perspectives. The results show that one settlement site, Gieddeålge, was established just above the forest line in the Betula treeline zone, and the other, Varenodjukke, in the Betula forest. At Gieddeålge an increase in herbs and graminoids appears to have occurred c. 700 cal. BP (during the thirteenth century) suggesting that the vegetation was altered by nutrient addition, most likely related to human activities associated with the settlement. At Varenodjukke, the Betula trees around the settlement were cut down and the site developed into an alpine heath dominated by Betula nana, which still characterizes the site today. In the Reference area evidence of changes to the vegetation probably caused by human impact was also found from c. 3500 to 700 cal. BP. In conclusion, the vegetation currently present at Gieddeålge and Varenodjukke has been strongly influenced by previous human activities, which have clearly had profound effects, both short and long term, on the composition of the vegetation and the properties of the ecosystems in these areas. Further, when studying processes, such as anticipated global warming that will influence future patterns of vegetation in these forest-line areas, legacies of land use must be taken into consideration.

DIVERSIFICATION OF THE SOUTH AMERICAN AVIFAUNA: PATTERNS AND IMPLICATIONS FOR CONSERVATION IN THE ANDES1

By combining distributions and phylogenies for large groups of birds, it is now possible to disentangle the relative roles of contemporary ecology and history in explaining the distribution of biodiversity on earth. In South America, avian lineages, which represent radiations during the warm parts of the Tertiary, are best represented in the tropical lowlands and Andean forelands. During the upper Tertiary, diversification was most intense in the tropical Andes region, with recruitment back into the tropical lowlands and into South America's open biomes. Within the tropical Andes, endemism (mean inverse range size) and mean branch length (number of phylogenetic nodes on lineages) increase from the foothills up to the tree line and then decline again in the barren highlands, suggesting that the tree-line zone plays a special role in the diversification process. The resulting endemism is locally aggregated, often with marked peaks in areas immediately adjacent to ancient population centers. Thus, the process of evolution of new species is linked with local factors that, over a shorter time perspective, were also favorable for people. If we want to maintain the process of diversification, it becomes essential to supplement the traditional approach of preserving biodiversity in wilderness areas with few people with efforts to support sustainable development in populated areas.

Paleolimnological evidence of the response of the central Canadian treeline zone to radiative forcing and hemispheric patterns of temperature change over the past 2000 years

Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record, we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W. A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition (LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing, namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century. As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA. Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen and lower temperatures in the hypolimnion of the more stratification-prone lake.

A 2000 year midge-based paleotemperature reconstruction from the Canadian Arctic archipelago

A lake sediment core recovered from Lake V57 on Victoria Island, Nunavut, Canada, spanning the last 2000 years, was analyzed for sub-fossil midge remains and organic-matter content (estimated by loss-on-ignition (LOI)). Significant changes in midge community composition occurred during the last 2000 years, with a distinct midge community appearing after 1600 AD. The chironomid community between 0 and 1600 AD was dominated by Heterotrissocladius, Tanytarsus, Abiskomyia, and Paracladius. At approximately 1600 AD, Heterotrissocladius decreased in relative abundance and taxa such as Corynocera ambigua, C. oliveri, Psectrocladius sordidellus type, and Pentanneurini increased in relative abundance. Previously published midge-based inference models for average July air temperature (AJAT) and summer surface–water temperature (SSWT) were applied to the subfossil midge stratigraphy. The AJAT reconstruction indicates relatively cool conditions existed between 1100 and 1600 AD, with exceptional warming occuring after 1600 AD, as lake productivity inferred from organic-matter content increased concomitantly with midge-inferred AJAT and SSWT. The cooler conditions between 1200 and 1600 AD, and the pattern of warming over recent centuries inferred from Lake V57 is broadly consistent with temperature-sensitive biogenic silica records from other sites in the central Canadian Arctic and the treeline zone to the south suggesting a regionally synchronous response to climate forcing.

Vegetation of the Continental Northwest Territories at 6 ka BP

Pollen records are used to reconstruct vegetation in the continental Northwest Territories at 6 ka (6000 14C yr BP). Picea glauca, P mariana, Larix laricina, Populus tremuloides, P. balsamifera, Alnus crispa and A. incana were present throughout their modern ranges in the Boreal and Subarctic Forest Zones by 6000 BP. Pinus banksiana, however, had not yet reached its present northern limits. Population densities of the dominant trees, Picea glauca and Picea mariana, were close to, or as high as, present. In the Mackenzie Delta region the range limit of Picea glauca was approximately 25 km north of its modern location just prior to 6000 BP. In contrast, the northern limits of the forest in central Canada were similar to present. The tundra vegetation close to the edge of the forest was similar to modern Low Arctic Tundra. Development of extensive Sphagnum peatlands had begun in the forested areas and the adjacent Low Arctic Tundra. Palaeoecological information regarding vegetation at 6000 BP remains lacking for the northeastern half of the study area. Therefore, the nature of the vegetation in much of the area now occupied by Low Arctic and Middle Arctic Tundra remains unknown. Important vegetation changes that occurred following 6 ka include : (1) the advance of Pinus banksiana to its present northern range limits, (2) the retreat of the northern range limits of Picea glauca in the Mackenzie Delta region between 6000 and 3500 BP and (3) the rapid and marked increase in the population density of Picea mariana in the treeline zone of the central Northwest Territories at 5000 BP followed by a decline at 4000 BP.

Treeline dynamics in relation to climatic variability in the central Tianshan Mountains, northwestern China

ABSTRACTAim Climate variability may be an important mediating agent of ecosystem dynamics in cold, arid regions such as the central Tianshan Mountains, north‐western China. Tree‐ring chronologies and the age structure of a Schrenk spruce (Picea schrenkiana) forest were developed to examine treeline dynamics in recent decades in relation to climatic variability. Of particular interest was whether tree‐ring growth and population recruitment patterns responded similarly to climate warming.Location The study was conducted in eight stands that ranged from 2500 m to 2750 m a.s.l. near the treeline in the Tianchi Nature Reserve (43°45′−43°59′ N, 88°00′−88°20′ E) in the central Xinjiang Uygur Autonomous Region, northwestern China.Methods Tree‐ring cores were collected and used to develop tree‐ring chronologies. The age of sampled trees was determined from basal cores sampled as close as possible to the ground. Population age structure and recruitment information were obtained using an age–d.b.h. (diameter at breast height) regression from the sampled cores and the d.b.h. measured on all trees in the plots. Ring‐width chronologies and tree age structure were both used to investigate the relationship between treeline dynamics and climate change.Results Comparisons with the climatic records showed that both the radial growth of trees and tree recruitment were influenced positively by temperature and precipitation in the cold high treeline zone, but the patterns of their responses differed. The annual variation in tree rings could be explained largely by the average monthly minimum temperatures during February and August of the current year and by the monthly precipitation of the previous August and January, which had a significant and positive effect on tree radial growth. P. schrenkiana recruitment was influenced mainly by consecutive years of high minimum summer temperatures and high precipitation during spring. Over the last several decades, the treeline did not show an obvious upward shift and new recruitment was rare. Some trees had established at the treeline at least 200 years ago. Recruitment increased until the early 20th century (1910s) but then decreased with poor recruitment over the past several decades (1950–2000).Main conclusions There were strong associations between climatic change and ring‐width patterns, and with recruitments in Schrenk spruce. Average minimum temperatures in February and August, and total precipitation in the previous August and January, had a positive effect on tree‐ring width, and several consecutive years of high minimum summer temperature and spring precipitation was a main factor favouring the establishment of P. schrenkiana following germination within the treeline ecotone. Both dendroclimatology and recruitment analysis were useful and compatible to understand and reconstruct treeline dynamics in the central Tianshan Mountains.

Pastoralism, plant conservation, and conflicts on proliferation of Himalayan knotweed in high altitude protected areas of the Western Himalaya, India

The conservation policy of banning pastoral grazing, and subsequently emerging conflicts between conservationists and pastoral communities regarding the proliferation of Himalayan knotweed (Polygonum polystachyum), was studied in the Valley of Flowers National Park, a high altitude protected area of the Western Himalaya, India. A total of 10 habitat types identified in the study area were sampled using quadrats along an altitudinal gradient between 3000 and 4500 m. Plant species richness decreases with altitude and also varies across habitat types. The highest density of P. polystachyum and its associated species, Impatiens sulcata, was found between 3300 and 3500 m in the disturbed habitat types, viz., bouldery areas, fragmented treeline zone, avalanche-prone areas and eroded slopes. Eradication of P. polystachyum from the national park by managers is not going to serve any meaningful purpose to the long-term conservation; rather it initiates soil erosion and instability, hindering the establishment of natural plant communities.

Holocene landscape development and climatic change in the low arctic, Northwest Territories, Canada

Holocene landscape development in the low arctic tundra zone was studied through fossil pollen analysis and sedimentary studies of a radiocarbon-dated core of lake sediment. The results indicate that Holocene landscape development in this area consists of three main stages. The first stage occurred between 8500 and 6300 14C yr B.P. when the local vegetation shifted from a herb and graminoid tundra to Betula shrub tundra, and finally to spruce forest tundra following the regional deglaciation. Soil erosion decreased greatly in response to vegetation development. The second stage is between 6300 and 3000 14C yr B.P. during which spruce forest tundra was developed and catchment soil erosion decreased due to increased vegetative cover. The chronological difference of several hundred years in the rise of black spruce population among the treeline sites may have been caused by edaphic conditions. During the third stage from 3000 14C yr B.P. to the present, vegetation degraded to the modern Betula-dominated shrub tundra. Peatlands expanded to its modern extent in the treeline zone from 1300 14C yr B.P. It seems treeline advanced northerly ca. 50 km beyond its present position between 6300 and 3000 14C yr B.P., reflecting likely a northward retreat of the polar front following the demise of the ice sheet in the middle Holocene. The retreat of the treeline at ca. 3000 14C yr B.P. may have been responded to an expansion of the north polar vortex or an intensification of the meridional airflow.

Stem water transport and freeze-thaw xylem embolism in conifers and angiosperms in a Tasmanian treeline heath.

The effect of freezing on stem xylem hydraulic conductivity and leaf chlorophyll a fluorescence was measured in 12 tree and shrub species from a treeline heath in Tasmania, Australia. Reduction in stem hydraulic conductivity after a single freeze-thaw cycle was minimal in conifers and the vessel-less angiosperm species Tasmannia lanceolata (Winteraceae), whereas mean loss of conductivity in vessel-forming angiosperms fell in the range 17-83%. A positive linear relationship was observed between percentage loss of hydraulic conductivity by freeze-thaw and the average conduit diameter across all 12 species. This supports the hypothesis that large-diameter vascular conduits have a greater likelihood of freeze-thaw cavitation because larger bubbles are produced, which are more likely to expand under tension. Leaf frost tolerances, as measured by a 50% loss of maximum PSII quantum yield, varied from -6 to -13°C, indicating that these species were more frost-sensitive than plants from northern hemisphere temperate forest and treeline communities. There was no evidence of a relationship between frost tolerance of leaves and the resilience of stem water transport to freezing, suggesting that low temperature survival and the resistance of stem water transport to freezing are independently evolving traits. The results of this study bear on the ecological importance of stem freezing in the southern hemisphere treeline zones.