Freezing processes were monitored at five sites within the Wolf Creek basin, Yukon, Canada during the winter of 1998–1999. Ground temperatures were measured using thermocouples in hillslopes that had frost status ranging from permanent to seasonal. The timing of freezing and ground thermal regimes varied among the five sites and was controlled by (1) the variation in surface soil temperature, (2) frost status (seasonal vs. permanent), (3) moisture content of the active layer, (4) properties of the soil profile, and (5) the presence/absence of subsurface drainage. On slopes with permafrost, cooling was rapid and two-sided freezing closed the active layer several months after the onset of freezing. On a slope with seasonal frost only, dry soil conditions allowed frost to penetrate to depth. In contrast, a slope with seasonal frost that had continuous drainage, frost depths were shallow due to heat advected from flowing water. A simple one-dimensional conduction model with latent heat was used to simulate freezing processes. Model performance varied among the slopes, and results indicate that (1) conduction is the predominant heat transfer mechanism during freezing, (2) latent heat is the principal factor controlling frost front descent, and (3) lateral flow significantly retards frost penetration because of heat advection. This information is valuable in assessing spatial variability within tile-based models and in predicting freezing, which defines an effective end-of-season on lateral hydrological processes.

The effects of ultraviolet radiation on plant frost hardiness were studied at two subarctic experimental sites, namely Abisko (northern Sweden, 68°N) and Sodankylä (northern Finland, 67°N). The Abisko treatment mimicked 15% ozone depletion and had been running for 7 yr. The investigated plant species was bilberry (Vaccinium myrtillus L.). The Sodankylä treatment consisted of lingonberry (V. vitis-idaea) and seedlings of mountain birch (Betula pubescens ssp. czerepanovii), which were exposed to both elevated UV-B and UV-A radiation in a modulated system corresponding to about 20% loss of the ozone layer. The frost hardiness tests in the Sodankylä experiment were carried out after UV exposure for one growing season. The frost hardiness of bilberry in Abisko was marginally decreased as a consequence of elevated UV-B exposure similarly to the frost hardiness of lingonberry and mountain birch in the Sodankylä experiment in August. The work provides evidence for rather than against a reducing effect of ultraviolet radiation on plant frost hardiness. The results suggest that elevated UV-A radiation probably reduces plant frost hardiness more efficiently than UV-B radiation.

We present here the 2001–2004 results of observational field studies aimed at quantifying tropical timberline climate and radial increment of Pinus hartwegii Lindl. trees on Nevado de Colima, in the middle of the North American Monsoon region. An automated weather station was installed at 3760 m a.s.l., 19°34.778′N latitude, 103°37.180′W longitude, within a forest where multi-century tree-ring records had been previously developed. At the same time, automated electronic sensors for recording tree growth at 30-min intervals were set up at two sites within a 1-km radius from the weather station. Meteorological observations recorded every 30 min were summarized on a daily basis. Time-series patterns are reported for atmospheric pressure, precipitation, incoming solar radiation, air and soil temperature, relative humidity, soil moisture, and wind speed and direction. Of particular interest is the sudden decrease in air temperature after the onset of the monsoon season, which determines very high relative humidity over the summer and results in air temperature having a spring maximum. Despite sub-freezing air temperatures in most months, soil temperatures never drop below 0°C. Dendrometer data show that the timberline growing season begins in March–April as temperature increases, then radial growth continues throughout the cool-wet summer monsoon, and ends in October–November. As an unexpected result, it was also possible to measure the progressive decline of Pinus hartwegii stem increment in response to an outbreak of roundheaded pine beetle (Dendroctonus adjunctus Blandford), which ultimately killed most trees at one of our two experimental sites.

Intraseasonal patterns of normalized difference vegetation index (NDVI), leaf area index (LAI), and phytomass were compared for four tundra vegetation types at Ivotuk, Alaska, during summer 1999. The vegetation types included moist acidic tundra (MAT), moist nonacidic tundra (MNT), mossy tussock tundra, and shrub tundra. The seasonal curves of NDVI were similar among the vegetation types but with varying magnitudes of the peak values. Peak NDVI in the shrub tundra (0.83) was significantly greater than in MAT (0.76), which was significantly greater than in MNT (0.71) and mossy tussock tundra (0.70). LAI and phytomass exhibited high temporal variability with distinct seasonality only in shrub tundra. Seasonal LAI and NDVI patterns were therefore correlated only in shrub tundra, which was attributed to the high quantity of deciduous shrub foliage present in this community and absent in the other vegetation types. Shrub tundra peak live above-ground phytomass (1256 ± 123 g m⁻²) was significantly greater than peak live above-ground phytomass for MAT, MNT, and mossy tussock tundra (722 ± 71, 773 ± 53, 703 ± 39 g m⁻² respectively, P < 0.05). Relative abundances of deciduous shrubs, mosses, and graminoids were revealed as key components controlling differences in NDVI, LAI, and phytomass among tundra vegetation types.

On a gentle leeward slope in a snowy forest limit in northern Honshu Island, Japan, mechanical damage by snow settlement and creep on Abies mariesii trees buried below the snowpack was examined to detect signs of the snow-damage effect on future survival and crown development. Damage types were recorded based on direct observation of crowns in 1996, a year of high snow accumulation exceeding 4.5 m, and 1997, a year of moderate snow accumulation. Of 153 trees examined, 63% were damaged in 1996 and 15% were damaged in 1997. The most destructive damage type was breakage of stems ≥5 cm in diameter, which occurred on eight trees in 1996 and three in 1997, resulting in foliage loss and death of some trees. The prevalent damage type was branch tearing at branch-stem junctions primarily within a height range of 4–6 m, which occurred on 171 branches in 1996 and 5 in 1997. Under snowy and windy conditions, stem breakage and branch tearing, caused by forces active within restricted layers of the snowpack, may reduce the future survival and crown development of A. mariesii buried below the snowpack in years of heavy snowfall.

At 17 sites in the European Alps (2l70–3310 m), the population density and seed production of the high alpine cushion plant Eritrichium nanum were investigated. Recruitment in this non-clonal species relies exclusively on seeds. The population density rose significantly with increasing altitude. A mean number of 368 seeds per m² was observed. Whereas the quantity of the generative rosettes (metamers) was relatively constant, the annual seed production strongly varied and was found vulnerable, particularly in 1996, when it was suppressed by unfavorable weather conditions. In 1997 seed production again reached the usual level, when about the same amounts of seeds were recorded as in previous years, indicating the strong resistance of elevated E. nanum populations against the severe climate at high altitudes. With rising population density, E. nanum produces distinctly higher seed numbers. This increasing amount of seeds seems essential for the persistence of its uppermost populations. Our results indicate that it is caused by the rising number of rosettes/surface and not a higher reproduction by single rosettes. Compared to other alpine species, the seed weight of E. nanum (average 0.76 mg) is high. Most of its neighboring species are clonal, producing much lighter seeds. The strategy of producing heavy seeds favors successful recruitment and appears to be decisive for an obligatory seeder such as E. nanum.

Palsas in all stages of growth and decay were investigated for vegetation, snow depth, and thaw depth in summer in four bogs in northern Sweden. A field classification of successive palsa stages was developed based on palsa vegetation composition and morphology. The initial phase of the aggrading stage is characterized by low mound forms and vegetation with the same hydrophilous composition as in the bog, but somewhat dried out. In the stage of continued aggradation Betula nana colonizes from the surroundings and hydrophilous vegetation is poorly represented in the higher parts even though the mounds are still low. Stable palsas, which are usually the tallest mounds, have vegetation that tolerates drier conditions; Empetrum hermaphroditum is the vascular species with the highest coverage at this stage. Decay starts with block erosion and melting of the mound forms. Degrading palsas have taller shrub vegetation that causes significantly thicker snow reducing heat loss from the core in winter. Conversely, the shrub vegetation has a dampening effect on penetration of heat into the mounds in summer. The degraded stage bears no morphological resemblance to the mounds, but at this stage there is a higher diversity of species owing to additional moist and wet growing sites.

Roughly 70% of the tundra north of the Brooks Range, Alaska, can be classified as moist nonacidic (39%) and moist acidic tundra (31%). We investigated the differences in energy partitioning and carbon balance among these two important landscape types. Despite structural differences in plant growth forms, moss cover, and soil pH, the sensible and latent heat fluxes were quite similar. However, aerodynamic properties (i.e. roughness length), ground heat flux, and CO₂ flux were significantly different: aerodynamic roughness of moist acidic tundra was 2.1 times higher than, and ground heat flux was 36% lower than the values obtained from moist nonacidic tundra. Daily carbon balance showed 26% more net CO₂ uptake (with 34% greater ecosystem respiration and 30% greater gross primary production) in moist acidic tundra. The greater respiration rate in moist acidic tundra was explained by differences in surface soil temperatures, whereas the rate of gross primary production was only half of what was expected from observed differences in leaf area index. These differences suggest that understanding the controls of CO₂ exchange in nonacidic moist tundra vegetation will be critical for determining the carbon budget of the Low Arctic region.

In naturally harsh environments, such as arctic-alpine regions, even low levels of anthropogenic pollution appear to have detrimental impacts on plants. We studied the symptoms induced by metal and acid rain treatments in the ultrastructure of Scots pine (Pinus sylvestris) needles in one of the northernmost pine stands in the world at the Kevo Subarctic Research Station, Finland. Adult Scots pines at their natural growing site were irrigated during 1991–1996 with (1) water, (2) acid (pH 3), (3) copper and nickel (below CuNi), (4) Cu and Ni combined with acid, or (5) left unirrigated. The two youngest needle age classes were sampled in September 1996 for microscopy and elemental analyses, and the three youngest needle age classes for an evaluation of visible injuries. The CuNi treatment increased significantly both Ni and Cu concentrations. The accumulation of Cu in C 1 needles was enhanced by acid addition. CuNi addition did not increase visible injuries, but significantly more ultrastructural dark accumulations and swollen thylakoids were seen in CuNi-treated C 1 needles. The results show that, even in a seemingly harsh environment, pines are able to resist acid and metal-induced stress for a few years. However, tree growth is likely to suffer in the long run because the injuries seen in older needles suggest that the total capacity of the photosynthetic machinery is likely to decrease.

A process geomorphological investigation was started in 1999 to study present denudation rates and the mutual relationship of chemical and mechanical fluvial denudation in periglacial environments. Latnjavagge (9 km²; 950–1440 m a.s.l.; 68°20′N, 18°30′E) was chosen as a representative drainage basin of the arctic-oceanic mountain area in northernmost Swedish Lapland. Atmospheric solute inputs, chemical denudation, and mechanical fluvial denudation were analyzed. During the arctic summer field seasons of 2000, 2001, and 2002 measurements of daily precipitation, solute concentrations in precipitation, and in melted snow cores, taken before snowmelt, were recorded. In addition, solute and suspended sediment concentrations in creeks were analyzed, and bedload tracer movements were registered during the entire summer seasons (end of May until beginning of September). Results show a mean annual chemical denudation net rate of 5.4 t km⁻² yr⁻¹ in the entire catchment. Chemical denudation in Latnjavagge is less than one third of chemical denudation rates reported for Kärkevagge (Swedish Lapland) but seems to be at a similar level as in a number of other subarctic, arctic, and alpine environments. Mechanical fluvial denudation is lower than chemical denudation. Most sediment transport in channels occurs in the early summer season during a few days with snowmelt generated runoff peaks. The main sediment sources in the drainage basin are mobilized channel bed pavements exposing fines, ice patches/fields, and material mobilized by slush flows. The calculated mean mechanical fluvial denudation rate is 2.3 t km⁻² yr⁻¹ at the inlet of lake Latnjajaure, situated in Latnjavagge close to the outlet of the valley. A very stable vegetation cover and rhyzosphere in this environment mainly explain the low value. The mean mechanical fluvial denudation rate at the outlet of the entire Latnjavagge drainage basin, below lake Latnjajaure, is only 0.8 t km⁻² yr⁻¹. Both chemical and mechanical fluvial denudation show low intensity. The results from Latnjavagge support the contention that chemical denudation is a somewhat important denudational process in periglacial environments.

The effect of environmental factors on the distribution of alpine plant community types has been extensively studied in Colorado. Much less attention has been paid to the effects of these factors within community types, however. Transects were placed in the alpine zone of Goliath Peak in the Front Range of the Colorado Rocky Mountains. Species presences were tallied in 1-m² rectangular plots. Fuzzy set ordination (FSO) was used to determine which environmental factors were responsible for changes in species composition. Most sites fell into the fellfield or dry meadow community types. Water availability, as indicated by elevation, estimated winter snow depth and, less strongly, soil texture, was a strong factor associated with changes in plant community composition, both within and among community types. Temperature, as indicated by aspect, was also associated with these changes. The lack of Kobresia myosuroides at Goliath Peak appears to be caused by snow depths too deep or too shallow for this species. FSO proved to be more effective when environmental factors were ordinated separately rather than sequentially, as originally suggested. This was confirmed by a Bray-Curtis ordination. A species-site biplot showed how species and sites can be displayed together on the same fuzzy set ordination.

The development and maintenance of several types of visually striking vegetation patterns are controlled by positive feedback between pattern and process. These patterns are particularly common at ecotones, where the influence of positive feedback may affect the position and dynamics of the boundary between the adjacent biotic communities. In this study, I use dendrochronology to examine the role of feedback between existing trees and the establishment and survival of seedlings in the advancement of linear, finger-like strips of subalpine forest in Glacier National Park, Montana. A general upslope, windward to leeward pattern of older trees followed by progressively younger trees was evident in all sample transects, although in some cases this pattern repeated several times along the length of a transect, with each repetition originating leeward of boulders. Overall advancement rates varied from 0.28 to 0.62 m yr⁻¹. The oldest trees established in the early to mid-1700s, but establishment and advancement increased rapidly after 1850, and peaked in the early 1900s. In addition, almost all seedlings established within 5 m downwind of existing trees between 1700 and 1850, while establishment beyond this distance was common after 1850. These patterns suggest that existing trees facilitate leeward seedling establishment and survival, by depositing wind-blown snow. These seedlings in turn modify their leeward environment, thus allowing forest advancement in a linear pattern. Feedback was critical for the survival of seedlings before 1800, and strongly controlled advancement between about 1800 and 1850, but appears to have had little effect on establishment patterns since that time. The importance of feedback between pattern and process may change over time and space as a result of changes in climatic conditions or biotic surroundings.

Antarctic Dry Valley ecosystems are among the most inhospitable soil ecosystems on earth with simple food webs and nearly undetectable fluxes of carbon (C) and nitrogen (N). Due to the lack of vascular plants, soil organic matter concentrations are extremely low, and it is unclear how much of the contemporary soil C budget is actively cycling or a legacy of paleolake production and sedimentation. While recent work indicates multiple sources of organic matter for dry valley soils, the composition and kinetics of organic pools remain poorly characterized. We examined soil organic matter pools and potential C and N turnover in soils from within six sites located across three hydrological basins of Taylor Valley, Antarctica that differed in surface age, microclimate and proximity to legacy (paleolake) sources of organic matter. We estimated potential C and N mineralization, and rate kinetics using gas exchange and repeated leaching techniques during 90-d incubations of surface soils collected from valley basin and valley slope positions in three basins of Taylor Valley. Soil organic C content was negatively correlated with the ages of underlying tills, supporting previous descriptions of legacy organic matter. Carbon and N mineralization generally followed 1st order kinetics and were well described by exponential models. Labile pools of C (90 d) were 10% of the total organic C in the upper 5 cm of the soil profile. Labile N was 50% of the total N in surface soils of Taylor Valley. These results show that a large proportion of soil C and particularly N are mineralizable under suitable conditions and suggest that a kinetically defined labile pool of organic matter is potentially active in the field during brief intervals of favorable microclimate. Climate variation changing the duration of these conditions may have potentially large effects on the small pools of C and N in these soils.

Nitrogen availability is considered limiting for plant growth at the forest-tundra ecotone, and it might modulate ecosystem response to climate warming. The aim of this research was to compare the impact of climate, vegetation cover, and soil organic matter (SOM) chemistry on N mineralization rates at the forest-tundra ecotone. We therefore estimated N mineralization in mountain birch (Betula pubescens Ehrh. ssp. czerepanovii) forest and tundra soil across a broad-scale latitudinal gradient in Fennoscandia, which incorporated 4 research sites (Dovrefjell, Vassijaure, Abisko, and Joatka). During the summer period, ammonium was the dominant form of mineralized nitrogen in forest soils, while nitrate mineralization rates were higher at tundra sites during the winter. A negative regression relationship between an index of climatic continentality and N mineralization was found. Further, summer NH₄ mineralization rates increased with total N content in soils, while NO₃⁻ mineralization seemed to be associated with C availability. Our study showed markedly contrasting inorganic N release in forest and tundra soil, and that, although mineralization rates differed between the summer and winter period, the winter activity was relatively high and should not be ignored. We conclude that a shift in the forest-tundra ecotone in response to climate warming will have stronger effects on nitrogen availability at these sites than the direct effects of warming.

This paper examines the behavior of perennial saline springs and their icings at Expedition Fiord in the Canadian High Arctic during the winter months when temperatures are below the eutectic point of the solution and during the early spring when temperatures are still below freezing but above the eutectic point. The spring outflow begins to freeze when it cools from the discharge temperature which is between −3.5°C and 6°C. As ice forms it remains mixed with the brine forming a salty, icy, slush which lines the sides of the flow channel. Networks of pipes and tunnels also allow the brine to flow under and through the icing before being frozen at the icing perimeter. In late winter complete freezing occurs several hundred meters from the springs' outlets. There appears to be incomplete fractionation of salt during the freezing process and the bulk ice contains 30 to 285 ppt salt. The icing reaches its maximum extent in late winter just before temperatures rise above the eutectic point. In April 2002 the icing had dimensions of 300 m by 700 m, an average thickness of 0.5 m and a total mass of approximately 2 ×10⁸ kg. This icing mass is consistent with the flow from the springs during the previous 6 mo.