We monitored variability of heat tolerance, osmotic water potential, and microclimate for seven alpine plant species at field sites and in response to a controlled in situ heat treatment ( 3 K) using infrared lamps. Mean and maximum heat tolerance differed significantly between species and were clearly related to growth form and moisture conditions in the preferred microhabitat. Diurnal heat tolerance changes greater than ±1.5 K occurred on 18% of summer days at a velocity of 0.4–2.2 K h⁻¹. The diurnal heat tolerance amplitude ranged from 4.8 to 9.5 K, exceeding even the seasonal amplitudes (5–8 K) previously reported for other higher plant species. Heat tolerance increased under warmer microsite conditions and in warmer years. Long-term artificial heating by 3 K led to a significant (P > 0.01) increase in heat tolerance by 0.6 K. Under drought stress, heat tolerance was reduced to minimum values, with the exception of xerophytic species. The plant species investigated appeared to be very well adapted to the temperature conditions of their microhabitat, with the exception of the cushion plant Minuartia recurva, which recurrently experienced heat damage during the investigation.

In Alaskan alpine tundra, grizzly bears excavate deep holes in search of ground squirrels, but few studies have tested the importance of grizzlies, or other large mammals, in maintaining plant community structure. We examined 43 bear digs, asking how they affect plant species richness and diversity, recolonization patterns, and plants with different clonal growth strategies. Bears remove most vegetation from digs, and recovering digs had lower species richness than adjacent mature tundra. Mature tundra alone, however, had significantly fewer species than mature tundra and bear digs combined, suggesting that bear digs contribute to the overall richness of tundra communities. Digs develop the highest plant richness and diversity at intermediate ages, but even in new digs the overall species composition is similar to adjacent tundra. Plants of different clonal growth forms reacted differently to bear digs. The two species significantly more common in digs than elsewhere have a nonspreading (phalanx) clonal habit, whereas five of six plant species significantly more common in mature tundra are capable of rapid, diffuse (guerrilla) clonal growth. Overall, bear digs cause less pronounced effects on community composition than mammalian diggings in some other systems, possibly because subarctic alpine tundra is already characterized by high levels of abiotic disturbance.

We assayed the amount and distribution of genetic variability in the clonal polyploid plant Carex scopulorum var. scopulorum. We used 5 polymorphic protein loci to sample 7 populations on the tundra in Rocky Mountain National Park, Colorado, U.S.A. Carex has levels of variability comparable to those found in nonclonal species. In the loci studied, transmission is disomic, as in diploid species. Of 148 tillers tested, 59.5% had different 5-locus genotypes. The distributions of genotypes fit Hardy-Weinberg expectations at 4 loci in most populations. These results suggest that there is significant cross-pollination and establishment by seed in this tundra plant. There were significant differences in allele frequencies between adjacent populations occupying wet versus moist environments at alcohol dehydrogenase, an enzyme known to be relevant to metabolism under oxygen-limiting conditions. There was no relationship between geographic distance and genetic distance at the scale we studied.

Both land use and expected climate change will probably cause range shifts of tree and shrub species in the European Alps. Attempts to predict the magnitude and direction of these processes will produce reliable results only if they consider both abiotic habitat conditions and biotic interactions. In this study we analyze recruitment patterns of Pinus mugo Turra in different grassland communities of the Northern Calcareous Alps, Austria. Pinus mugo is the most important invader of abandoned subalpine pastures in the area and the predominant woody plant at the current timberline. Results indicate strong dependence of colonization success on propagule pressure and differential invasibility of grassland types but only a marginal impact of local-scale site conditions, at least within the species' current altitudinal distribution limits. Because the grassland matrix at and above the current treeline is dominated by a particularly invasible grassland type, a possible climate change–driven upward movement of Pinus mugo shrublands may take place quite rapidly. In contrast, encroachment on abandoned subalpine pastures is frequently delayed by competition with vigorous grassland canopies.

We assessed the impact of human trampling in three different habitats on Marion Island (46°50′S, 37°50′E). The habitats were (1) mires with wet, peaty soils and grass- and bryophyte-dominated vegetation; (2) slopes with relatively dry mineral soils, dominated by small ferns and dwarf shrub; and (3) feldmark on dry mineral soils with an open vegetation of cushion dicots and bryophytes. We examined existing walking tracks on the island. Track width (25 to 800 cm) increased with soil moisture content. Trampling reduced vegetation height, total cover, and species richness in mires and feldmark and vegetation height and herb layer cover (but not bryophyte cover or species richness) in slopes. In mires, most species were negatively affected by trampling, but in slopes trampling increased the cover of 6 out of 9 significantly affected species. The total number of species in all trampled plots in mire and feldmark communities was ∼10% lower, but in slopes 28% higher, than in control plots. The impact of trampling differed between growth forms. Cushion dicot, shrub, and fern covers were reduced, whereas graminoid and pleurocarpous moss covers were unaffected or increased with trampling. Trampling reduced the cover of most bryophyte species, but it did increase the cover of some. In the slope habitat, destruction by trampling of the closed herb canopy allows increased light penetration and makes the habitat more favorable for small plants such as bryophytes. We attribute the differences in how the vegetation of different habitats responds to trampling to differences in the structure of the original vegetation as well as differences in soil characteristics, especially the soil's structural stability under pressure.

We compared ecophysiological characteristics of plant species that dominate during different stages of succession in fallow fields of a traditional agroecosystem in the Venezuelan High Andes. For each species we determined during the dry and wet seasons the photosynthetic light response and photosynthesis rate at light saturation (A_max), specific leaf area (SLA), stomatal conductance at light saturation (g_max), midday water potential (ψ), and intrinsic water use efficiency (WUE_i, photosynthesis rate per unit of stomatal conductance). The species studied were the forbs Rumex acetosella (early succession dominant) and Lupinus meridanus (intermediate stages), the shrubs Acaena elongata and Baccharis prunifolìa (late succession), the giant rosette Espeletia schultzii, and the shrub Hypericum laricifolium (mature ecosystem dominant). Clear ecophysiological trends were identified: early and intermediate successional species had higher A_max, g_max, and SLA but lower WUE_i. E. schultzii maintained a high water potential during the dry season and, together with B. prunifolia, was the only species with no significant differences in A_max between seasons. The results indicate that traits generally linked to fast growth (high A_max and SLA) are associated with dominance during early succession, while traits linked with drought resistance (e.g., high WUE_i and thick xeromorphic leaves) are associated with dominance during late succession in this tropical mountain environment.

To evaluate edge effects in subalpine coniferous forest, I quantified stand structure characteristics in a plot that extended into the forest from a road that had been constructed approximately 30 yr previously on Mt. Fuji, in central Japan. The basal area of standing dead trees and the density of saplings in the plot were greater near the road. However, cluster analysis indicated that stand structure near the road edge was similar to that of gaps in the interior of the stand. Since edge effects may result from interactions among several variables, analyzing data sets that include several variables rather than a single variable is a promising approach for evaluating edge effects. In a multiple regression analysis, sapling densities of Tsuga diversifolia and Abies veitchii were strongly affected by the basal area of standing dead trees that had apparently died as a result of the construction. A. veitchii is more shade intolerant than the other two main species in the study plot, but it responds quickly to increased light availability. Although the plot is currently composed of T. diversifolia and A. mariesii, successful regeneration of A. veitchii may eventually change the species composition of this plot.

Numerous exposures of Pleistocene sediments occur in the Noatak basin, which extends for 130 km along the Noatak River in northwestern Alaska. Nk-37, an extensive bluff exposure near the west end of the basin, contains a record of at least three glacial advances separated by interglacial and interstadial deposits. An ancient river-cut bluff and associated debris apron is exposed in profile through the central part of Nk-37. The debris apron contains a rich biotic record and represents part of an interglaciation that is probably assignable to marine-isotope stage 5. Pollen spectra from the lower part of the debris apron closely resemble modern samples taken from the Noatak floodplain in spruce gallery forest, and macrofossils of spruce are also present at this level. Fossil bark beetles and carpenter ants occur higher in the debris apron. Mutual Climatic Range (MCR) estimates from the fossil beetles suggest temperatures similar to or warmer than today. Together, these fossils indicate the presence of an interglacial spruce forest in the western part of the Noatak Basin, which lies about 80 km upstream of the modern limit of spruce forest.

Pollen, a regular component of tropical ice cores, has been shown to have great potential as a sensitive paleoenvironmental proxy in ice-core research. However, questions remain as to the modern dispersal and depositional patterns of pollen on high-alpine tropical ice caps. This information is vital to the accurate interpretation of the environmental reconstructions being derived from fossil pollen. In this study, 11 surface snow samples were collected around the caldera rim at the summit of Mt. Parinacota along the Bolivian-Chilean border. Results show that pollen concentration and assemblage are uniform in samples taken from the southwestern quadrant and the entire eastern half of the mountain. However, the pollen signatures are significantly different in the northwestern quadrant, probably due to long-distance transport of xerophytic Compositae shrub pollen from the prevailing winds. The sections of the mountain not directly impacted by the prevailing northwesterlies reflect a more locally influenced pollen assemblage dominated by grasses. These results are consistent with previous findings from the Quelccaya Ice Cap and confirm the importance of the prevailing winds in the dispersal and deposition of pollen on these high-alpine tropical ice caps.

We describe energy fluxes involved in melting ice in the proglacial lake Jökulsarlon and the transport of thermal energy into the lake from the atmosphere and the sea. Data from earlier fieldwork and campaigns have been used to estimate the net radiation balance, the turbulent fluxes, the heat provided by inflowing seawater, and the glacial meltwater flux. From aerial photographs, DGPS measurements, and mass balance measurements, we calculated a calving flux of 260 × 10⁶ m³ yr⁻¹ for the present. The total energy required to melt all the ice in the lake is approximately 160 W m⁻² assuming that all the calved ice is melted during 1 yr. The most important contribution is heat from seawater. Radiation provides approximately 70 W m⁻². The albedo depends on the ice-covered fraction of the lake and ranges from 22% in summer to 41% in winter. The turbulent fluxes are around 10 W m⁻². Difficulties occurred in finding an appropriate range for the roughness parameter z₀, but the most likely values are in the range of a few centimeters. We considered different future scenarios with respect to inflow of seawater and air temperature, albedo, and even inhibition of seawater intrusion, which would have a significant impact on ice cover in the lake.

The Hess altitude h on a valley glacier is the altitude of that contour that is most nearly straight. The Kurowsky altitude k is the average of the glacier's minimum and maximum elevations. The glaciation level g of a glacierized region is the average of the elevations of the lowest glacierized and the highest unglacierized summits. We study the relationships between these morphological variables in well-mapped areas in Axel Heiberg Island, Nunavut, and in northern British Columbia, and also in a set of more widely distributed glaciers with well-known equilibrium-line altitudes e. We confirm that the relationships are strong, although their dispersion is considerable both for single glaciers and at the regional scale. On average h is 130 m below the long-term mean equilibrium line, k is slightly below it, and g is 200 m above it. There is very little morphoclimatic information in h that is not also conveyed by k, and since the latter is faster to measure and is already tabulated for tens of thousands of glaciers, its use is preferable. The glaciation level is less concordant. We suggest that it records interactions between landscape and climate on a somewhat slower time scale. The linear dependences of h on k and g, and of e on each of the 3 morphological estimators, have slopes slightly but significantly less than 1. This result is not yet understood.

I replicated and analyzed six photographs taken in A.D. 1870 near the subalpine forest-alpine tundra ecotone in the northern Uinta Mountains to quantify changes in the distribution of vegetation. Three dramatic differences were noted. First, the historical photographs document a treeline 60 to 180 m (mean of ∼100 m) lower than at present, with greater depression on west-facing slopes. Given the modern lapse rate for mean July temperature (6.9°C km⁻¹), this difference corresponds to a temperature depression in A.D. 1870 of 0.4 to 1.2°C (mean of 0.7°C). Second, timberline forests in A.D. 1870 were significantly (P < 0.01) less dense, with tree densities approximately half those measured in the modern photographs. Third, the area of floodplain meadows decreased ∼75% from A.D. 1870 to the present. Because the original photographs were taken within a few decades of the end of the Little Ice Age, ca. A.D. 1850, I assumed that differences in vegetation distribution documented in the repeat photographs represent the biotic response to climate warming over the past ∼130 yr. This analysis provides an independent estimate of the magnitude of growing season temperature depression during the Little Ice Age.

Nitrogen (N) deposition has been implicated in changes in surface water chemistry and algal composition in several dilute mountain lakes of the western United States. Lakes of the Snowy Range (Medicine Bow National Forest, Wyoming) appear to have low nitrate concentrations currently, and 2 Snowy Range lakes showed strong eutrophication responses to N or N phosphorus (P) additions in previous enclosure experiments. In this study, we explored the regional extent of phytoplankton N limitation by examining a nutrient ratio index (dissolved inorganic nitrogen:total phosphorus) and phytoplankton species-environment relationships across 15 Snowy Range lakes. Based on this index, we estimate that phytoplankton biomass in the study lakes is largely N limited or N P colimited. In addition, redundancy analysis demonstrated strong relationships between phytoplankton species composition and N gradients, with chrysophyte taxa favored in low-N lakes and cyanophytes and chlorophytes favored in higher-N lakes. We conclude that both phytoplankton biomass and community structure are sensitive indicators of N gradients in lakes of the Snowy Range, and that eutrophication responses to future increases in N loading could be widespread in these and other low-N lakes.

The physical and chemical limnological characteristics of 22 lakes and ponds in the remote region of Haughton Impact Crater, Devon Island, Nunavut, Canada, were explored. Our overall goals were to gather baseline information for use in climate and environmental change monitoring programs in the High Arctic as well as to compare these observations to other limnological surveys conducted in high-latitude regions. Study sites were alkaline (pH_mean = 8.3), ultraoligotrophic (TPU_mean = 3.7 μg L⁻¹), and phosphorus limited. Major and minor ionic concentrations of most sites are comparable to other previously surveyed high arctic sites. Lakes and ponds in close contact with the impact-generated carbonate melt rocks associated with Haughton Crater were distinguished from the larger data set due to their elevated Mg² , SO₄²⁻, Ba² , Sr² , and SiO₂ concentrations. Selenite (CaSO₄·2H₂O) formations commonly associated with the lower levels of the carbonate melt sheets were identified as a likely source for the elevated SO₄²⁻ concentrations in these sites. Principal Components Analysis separated sites along a conductivity and nutrient gradient on the primary and secondary axes, respectively. This study serves as a baseline for a long-term monitoring program in the Haughton Crater region.

Sediment cores from Pyramid Lake, an alpine tarn in the Cassiar Mountains of northwestern British Columbia, were investigated for changes in pollen, plant macrofossils, charcoal, and clastic sediment, which are used to infer changes in climate throughout the Holocene. Radiometric dating has yielded a chronology of high-magnitude rainstorm events and timberline migration for the Pyramid Lake basin since deglaciation at about 10600 B.P. Fifteen distinct minerogenic layers represent material delivered to the lake by runoff events. The frequency of minerogenic layer deposition, and by analogy of storms, has changed throughout the Holocene. Four large-magnitude rainstorm events occurred between 4400 and 5100 B.P. During this period white spruce (Picea cf. glauca) was likely present near the lake, although a closed forest stand did not develop around the lake at any point during the Holocene. The macrofossil record indicates that subalpine fir (Abies lasiocarpa) has been present, likely as krummholz, above the elevation of the lake since at least 9400 B.P. Pollen of western hemlock (Tsuga heterophylla) is represented from ca. 1500 B.P. to the present and may be a consequence of changes in regional air-mass circulation patterns.

We evaluate two approaches to spatially interpolating winter surface air-temperature fields over the terrestrial Arctic from available weather-station records. We then examine 30 yr (1961–1990) of winter air-temperature change over the terrestrial Arctic through a time-trend analysis of interpolated winter air-temperature fields. We used monthly average air temperatures from 4984 Arctic station records that were available for the period 1961–1990. The two spatial interpolation procedures employed were “traditional” interpolation and a method that makes use of spatially high-resolution digital-elevation information, called “DEM-assisted” (DAI). The Arctic average winter air temperature obtained from the traditionally interpolated 1961–1990 climatology is over 9°C colder than the mean winter station temperature, illustrating the considerable warm bias in Arctic weather station locations. The DAI-based average is 1°C colder, further emphasizing the importance of spatial interpolation prior to spatial averaging.

Over the 30 yr, increases in winter air temperature appear across western Canada and in parts of central Asia, with decreasing trends apparent over eastern Canada. Much of the Arctic exhibits no clear trend, with low explained variances. In western Canada, however, warming trends are on the order of 0.1 to 0.4°C yr⁻¹ when the fields analyzed were traditionally interpolated or interpolated using DAI. Explained variances (r²s) are higher where trends are largest: approximately 0.2 to 0.4 in western Canada and slightly higher (albeit spuriously) in an isolated area of central Asia. Over the entire terrestrial Arctic, mean winter air temperature has increased at a rate of about 0.05°C yr⁻¹ based on traditional interpolation and DAI.