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Climate change could cause alpine treelines to shift in altitude or to change their spatial pattern, but little is known about the drivers of treeline dynamics and patterning. The position and patterns of tropical alpine treelines are generally attributed to land use, especially burning. Species interactions, in particular facilitation through shading, may also be important for treeline patterning and dynamics. We studied how fire in alpine vegetation and shade dependence of trees may affect the position and spatial pattern of tropical alpine treelines and their response to climatic warming, using a spatial minimal model of tree growth at treeline. Neighboring trees provided shade and protection from fire. The positive feedback that resulted from these neighbor interactions strongly affected the emergent treelines and always reduced the distance and speed of treeline advance after a temperature increase. Our model demonstrated that next to fire, shade dependence of trees can also lead to abrupt treelines and relatively low treeline positions. This implies that these patterns do not necessarily indicate human disturbance. Strong abruptness of a treeline may indicate that it will respond slowly to climatic changes.
We provide a map of lower and central Taylor Valley, Antarctica, that shows deposits from Taylor Glacier, local alpine glaciers, and grounded ice in the Ross Embayment. From our electronic database, which includes 153 sites from the coast 50 km upvalley to Pearse Valley, we show the distribution of permafrost type and soil subgroups according to Soil Taxonomy. Soils in eastern Taylor Valley are of late Pleistocene age, cryoturbated due to the presence of ground ice or ice-cemented permafrost within 70 cm of the surface, and classified as Glacic and Typic Haploturbels. In central Taylor Valley, soils are dominantly Typic Anhyorthels of mid-Pleistocene age that have dry-frozen permafrost within the upper 70 cm. Salt-enriched soils (Salic Anhyorthels and Petrosalic Anhyorthels) are of limited extent in Taylor Valley and occur primarily on drifts of early Pleistocene and Pliocene age. Soils are less developed in Taylor Valley than in nearby Wright Valley, because of lesser salt input from atmospheric deposition and salt weathering. Ice-cemented permafrost is ubiquitous on Ross Sea, pre–Ross Sea, and Bonney drifts that occur within 28 km of the McMurdo coast. In contrast, dry-frozen permafrost is prevalent on older (≥115 ky) surfaces to the west.
This study explores the physical, chemical and microclimatological properties of soils along a High Arctic glacier foreland and adjacent moraine in relation to the development of biological soil crusts. We examine various edaphic properties: soil temperature, volumetric water content, organic carbon content, and texture in surface samples (∼1 cm) with and without a cover of biological soil crust as well as changes in nitrogen, phosphorus, potassium, organic carbon, pH, volumetric water content, bulk density, and texture in crusted surfaces (<1 cm) and soil cores (5 cm) along a chronosequence following deglaciation. Soil crusts developed within four years of deglaciation and subsequent peaks in crust cover and thickness coincided with an accumulation of nitrogen and organic carbon in the crust. Crusted surfaces had significantly higher volumetric water content, organic carbon, a greater silt and clay fraction, and lower temperature compared to uncrusted soils. A steady supply of water from glacier melt promoted rapid development of biological soil crusts, creating an edaphic environment with enhanced moisture and nutrient properties which contributed to the high rate of vascular plant succession previously observed on this foreland. Results presented in this study are compared with edaphic conditions at other circumpolar sites and glacier forelands.
Melilotus alba and M. officinalis were introduced to Alaska in 1913 as potential forage crops. These species have become naturalized and are now invading large, exotic plant–free regions of Alaska. We determined distributions of M. alba and M. officinalis in Alaska from surveys conducted each summer from 2002 to 2005. Melilotus alba and M. officinalis occurred at 721 and 205 sites, respectively (39,756 total sites surveyed). The northward limit for M. alba and M. officinalis was 67.15°N and 64.87°N, respectively. Both species were strictly associated with soil disturbance. Melilotus alba extended no farther than 15 m from road edges except where M. alba on roadsides met river floodplains and dispersed downriver (Matanuska and Nenana Rivers). Melilotus has now reached the Tanana River, a tributary of the Yukon River. Populations on floodplains were most extensive on braided sections. On the Nenana River, soil characteristics did not differ between where M. alba was growing versus similar areas where it had not yet reached. The pH of river soils (7.9–8.3) was higher than highway soils (7.3). Upland taiga plant communities grow on acid soils which may protect them from invasion by Melilotus, which prefer alkaline soils; however, early succession communities on river floodplains are susceptible because soils are alkaline.
We studied the course of primary succession following deglaciation and the convergence/divergence of plant community development with respect to topographic factors at Koryto Glacier Valley on Kamchatka's Pacific coast. Vegetation changes over an ∼270-yr-old chronosequence were related to concurrent changes in substrate and soil properties. Ordination analyses showed that substrate texture and topography are the most important environmental factors influencing the course of succession. About 25 yrs after surface deglaciation, belowground stagnant ice melts, and moraine consolidation causes the successional communities to diverge. Species-poor communities, dominated by alder and grasses (Alnus fruticosa, Calamagrostis purpurea), occurred on the fine-grained substrate of moraine crests, while species-rich communities, dominated by legumes and forbs (Oxytropis kamtschatica, Saxifraga species), developed on the coarse-grained substrate of moraine flanks, and in depressions communities dominated by willows and sedges (Salix arctica, Juncus beringensis) developed. In depressions and plains adjacent to the ridges, succession toward Alnus stands is hindered by late-melting snow and flooding. Plant-species richness peaked at the 80-yr-old moraine, but thereafter decreased as the rapid growth of Alnus led to dense stands that dominated resources and inhibited colonization and growth of earlier, as well as later, successional species. Mat-forming capacity, high litter production, an extensive root system, and snow-pressure tolerance enable A. fruticosa to maintain dominance without replacement by Betula ermanii. This potential climax species remains scattered on rock terraces and elevated locations above the valley basin where it escapes snow avalanches and accumulation, a factor responsible for the inversion of vegetation zones in this maritime region.
Hourly temperature and humidity observations were obtained over 16 months from loggers ranging in elevation from 1890 to 5800 m a.s.l. up the southwestern slope of Kilimanjaro, Tanzania. The vertical gradient in mean air temperature is non-linear, with the treeline weakening the gradient and the snow-ice line enhancing it. On average, moisture availability (both relative humidity and absolute vapor pressure) decreases with elevation, but the seasonal and diurnal variability in relative humidity (RH) is enhanced toward the mountain summit. The strong diurnal cycle in humidity is shown to be an outcome of strong upslope moisture transport during the day, counterbalanced by downslope transport and drying at night. Cooling on the lower slopes during the months of June and July weakens the lapse rates and consequently convective activity. This is borne out by the reduction in cloud amounts (using a surrogate threshold of RH > 95%), toward the summit during these months. The lower slopes of Kilimanjaro are observed to be a major moisture source for the summit region, and implications of this for the mass balance of the summit glaciers are discussed.
We investigated vegetation structure and microenvironments on bare volcanic soil covered by scoria above the forest limit on Mt. Fuji, central Japan, to evaluate the effects of patches of a pioneer dwarf shrub (Salix reinii) on the establishment of early successional tree seedlings (Larix kaempferi). We analyzed species distribution patterns and the associations among them, and compared the performance (growth and survivorship) of Larix seedlings and the local environment (temperature, solar radiation, soil surface stability, soil moisture, and nitrogen) inside and outside Salix patches. Larix displayed significantly clumped distribution, and the clumping was apparently associated with the preferential occurrence of Larix in Salix patches. Salix patches moderated severe microenvironmental factors, such as drought, high temperature, and movement of the soil surface. Salix patches promoted increased height and decreased root∶shoot ratio, but not higher rate of biomass accumulation in Larix seedlings. Survival rate of L. kaempferi inside Salix patches was higher than that outside patches at the younger stage, but it was lower at the older stage after L. kaempferi emerged from the Salix crown. The results indicate S. reinii enhances seedling establishment and survival of young L. kaempferi, but may compete with it at later stages. The overall net effect of Salix patches on L. kaempferi is positive, and therefore S. reinii appears to accelerate succession from scoria bare land to pioneer woodland.
We investigated the seasonal variation in pools of water available to mature trees growing at high elevation in a tropical environment. The study focused on the dominant tree species (Pinus hartwegii) at about 3800 m a.s.l. on Nevado de Colima, Mexico, where climate is typical of the North American Monsoon System. Stable isotope ratios of hydrogen and oxygen in water extracted from soil, xylem, and leaves were measured through a cycle of two dry and two wet seasons in 2003–2004. Isotopic ratios were also measured in accumulated precipitation, a few single precipitation events, and in spring water over the two-year period. Based on evidence from water, stable isotopes in soil, and xylem samples, trees utilized water from relatively shallow soil depths, which are representative of current conditions, rather than tapping groundwater, which is more representative of long-term trends. While the stable isotope signature in environmental waters showed a slightly different pattern before and during the monsoon, the more pronounced differences in leaf water isotopes between the two seasons, due to drought stress, will lead to a clear seasonal isotopic signal in tree ring cellulose. This study represents a unique snapshot of water cycling in a tropical treeline ecosystem, where our understanding of eco-hydrological pathways is limited. This type of analysis is also useful for proper calibration of stable isotopic signals in tree ring records.
The optimum temperatures of three species of snow algae were studied using four strains of Chloromonas (Cr.) rosae v. psychrophila, six strains of Cr. tughillensis, and one strain of Cr. chenangoensis. These axenic strains were from Upstate New York except for two of Cr. rosae v. psychrophila from the White Mountains, Arizona. Temperatures tested were from 2.5 to 20°C. The high elevation subalpine Cr. rosae v. psychrophila from New York and Arizona grew from 4 to 20°C and had the greatest cell counts at 4 to 15°C. In contrast, the subalpine to temperate low elevation strains of Cr. tughillensis grew from 2.5 to 10°C and optimally at 2.5 or 5°C, and Cr. chenangoensis grew from 2.5 to 7.5°C and optimally at 2.5 and 5°C. Chloromonas tughillensis and Cr. chenangoensis belong to a genetic subclade with low temperature optima, whereas Cr. rosae v. psychrophila belongs to a subclade with broad temperature optima. In acclimation experiments, there were no significant differences in cell counts when acclimating two Adirondack, New York, strains of Cr. rosae v. psychrophila for two weeks prior to experiments vs. using non-acclimated strains that were moved from 4°C directly to 4, 10, 15, or 20°C. For Cr. tughillensis, four of six strains had significantly higher cell counts when grown at 2.5°C after acclimation at 7.5°C for five months. These are the first reports of temperature optima of snow algae from eastern North America.
Numerous paleolimnological studies of Arctic lakes and ponds have shown marked shifts in both algal and invertebrate taxa within the past ∼150 years that are consistent with recent climatic warming. However, the magnitude and timing of changes are often non-uniform, with large, deep lakes frequently exhibiting muted assemblage shifts relative to smaller ponds. The hypothesis that duration and extent of ice cover exerts an overriding influence on habitat availability for biota has been commonly invoked to explain these differences, and many studies indicate that changes in ice cover are important drivers of recent biological changes. However, a detailed paleolimnological comparison of two lakes from the same region that have similar water chemistry but different ice cover regimes has not yet been attempted. Here we examine the influence of prolonged ice cover on the rate, magnitude, and direction of fossil diatom species shifts over time in two remarkably similar and adjacent Ellesmere Island lakes that mainly differ in their periods of ice cover. These two lakes exhibit strikingly different paleolimnological diatom profiles, despite their physical proximity, similar depths, and nearly identical water chemistry. In the lake characterized by prolonged ice cover, we find little evidence of diatom-inferred environmental change over its recent history, while diatom assemblages have undergone dramatic changes in the lake with the shorter duration of ice cover. This study supports the general hypothesis that changes in ice cover are a principle determinant of shifting diatom assemblages in High Arctic lakes.
We analyzed diversity patterns of alpine tundra ecosystems along environmental gradients. We hypothesized that alpine diversity is affected by climate at local and regional scales, nutrient availability, soil moisture, and disturbance related to herbivory. In all, 232 samples in 11 study areas in Troms and Finnmark counties were analyzed with regard to α- and β-diversity of vascular plants and lichens. Relationships between α-diversity and environmental variables were analyzed by regression trees. β-diversity defined as species turnover was investigated using indirect ordination methods. Sites with non-acidic soil parent material showed highest species densities. Lowest species numbers were typical for extreme topographic positions. Heavily grazed samples showed less species numbers and coverage percentage of vegetation. The number of graminoid species was found to be highest in areas of high grazing pressure. We concluded that α-diversity was controlled by growing season, snow cover, pH, soil moisture, disturbance, temperature, and precipitation, stressing the importance of multi-factorial approaches in diversity studies. Determinants of β-diversity were predominantly local environmental conditions, whereas regional conditions were less important.
We investigated changes in vascular plant species richness in nine summit floras in the central part of the Fennoscandian mountain range compared to historical data from 1950. We revisited the summits (defined as the top 50 altitudinal meters of each mountain) in 2002, and recorded all species. The changes in species richness were tested against both species and mountain characteristics. Species richness had declined on eight of the nine summits. Five of the species were new since the 1950s, while 17 species were lost from the summits. However, species turnover was even higher: 57 of our recorded species occurrences had established on at least one mountain since the 1950s, while we could not find 132 of the recorded occurrences in 1950 on one or more mountains. Temperature had increased since 1950 by about 1 °C and precipitation by 12%. The reindeer population has more than doubled. No correlations between plant responses, plant characteristics, and mountain characteristics were found, suggesting individualistic and mountain-specific responses. We conclude that climate changes may be responsible for an increased establishment and reindeer trampling for increased mortality of established individuals. However, the net result is a decline in species richness.
Chemical weathering in soils has not been studied extensively in high-latitude regions. Loess sequences with modern soils and paleosols are present in much of subarctic Alaska, and allow an assessment of present and past chemical weathering. Five sections were studied in detail in the Fairbanks, Alaska, area. Paleosols likely date to mid-Pleistocene interglacials, the last interglacial, and early-to-mid-Wisconsin interstadials. Ratios of mobile (Na, Ca, Mg, Si) to immobile (Ti or Zr) elements indicate that modern soils and most interstadial and interglacial paleosols are characterized by significant chemical weathering. Na2O/TiO2 is lower in modern soils and most paleosols compared to parent loess, indicating depletion of plagioclase. In the clay fraction, smectite is present in Tanana and Yukon River source sediments, but is absent or poorly expressed in modern soils and paleosols, indicating depletion of this mineral also. Loss of both plagioclase and smectite is well expressed in soils and paleosols as lower SiO2/TiO2. Carbonates are present in the river source sediments, but based on CaO/TiO2, they are depleted in soils and most paleosols (with one exception in the early-to-mid-Wisconsin period). Thus, most soil-forming intervals during past interglacial and interstadial periods in Alaska had climatic regimes that were at least as favorable to mineral weathering as today, and suggest boreal forest or acidic tundra vegetation.
Soil frost formation, snow distribution, and winter/spring/summer terminal infiltration rates (TIRs) were quantified in Icelandic Andisols with contrasting vegetation cover types (grassland, spruce and birch woodland, lupine, and sparsely vegetated lava site). TIRs (mm h−1; determined with double-ring infiltrometers) were generally higher in unfrozen than in frozen soils (102–369 vs. 9–306, respectively in sandy soils; 28–94 vs. 3–72 in finer-textured soils) and differed between land cover types, being consistently highest in birch woodlands. TIR was an inverse function of soil frost depth. Lowest TIRs were associated with deep and dense soil frost, which formed in spruce woodland and grassland communities where snow depth was shallow. Results suggest conditions conducive to erosion by water are most likely to occur during winter warm spells and in spring in vegetation types where snow cover is low or ephemeral. Threefold increases in TIRs occurred one year after livestock grazing was discontinued, suggesting Andisols are hydrologically resilient where vegetation cover is relatively continuous and soil organic carbon content is high.
We used the chamber method to measure growing season ecosystem carbon exchange and ecosystem respiration in Finnish alpine tundra. The average ecosystem respiration in the sites was 0.8–0.9 µmol m−2 s−1 and the daytime net ecosystem exchange (NEE) was around −0.4 to −0.5 µmol m−2 s−1. There were no detectable differences in cuvette-based net ecosystem exchange or ecosystem respiration between grazed fell areas and long term reindeer exclosure. Further analysis showed that net carbon exchange as well as ecosystem respiration were significantly correlated with the dwarf shrub cover, while the proportion of lichen cover (Cladina sp.) was not correlated with ecosystem carbon exchange. Clipping experiments showed that about half of the measured ecosystem respiration was heterotrophic. Plots that had been protected from reindeer grazing had almost two times higher above-ground plant biomass than grazed plots. The reason for this was 86% lower lichen biomass on the grazed side of the fell, while the biomass of Ericaceous dwarf shrubs did not differ even though there were changes in species composition. Surprisingly, the proportion of bare ground did not differ due to grazing pressure, but the reduction in biomass lead to a less stratified vegetation cover.
The depth and density of the Antarctic firn layer is modeled, using a combination of regional climate model output and a steady-state firn densification model. The modeled near-surface climate (temperature, wind speed, and accumulation) and the depth of two critical density levels (550 kg m−3 and 830 kg m−3) agree well with climate and firn density observations selected from >50 Antarctic coring sites (r = 0.90–0.99, p < 0.0001). The wide range of near-surface climate conditions in Antarctica forces a strong spatial variability in the depth and density of the Antarctic firn pack. In the calm, dry, and very cold interior, densification is slow and the firn-layer thickness exceeds 100 m and the firn age at pore close-off 2000 years. In the windier, wetter, and milder coastal zone, densification is more rapid and the firn layer shallower, typically 40–60 m, and younger, typically <50 years.
The altitudinal variation of precipitation, evapotranspiration, and runoff was quantified at 16 different grassland sites between 580 and 2550 m a.s.l. in the Austrian Alps. Along this altitudinal transect annual evapotranspiration decreased from roughly 690 mm at low elevation sites to 210–220 mm at the upper limit of the alpine grassland belt. A detailed analysis of the data showed that the observed reduction in the annual sum of evapotranspiration could be mainly explained by the altitudinal decline of the length of the snow-free period (i.e. the vegetation period). Daily mean sums of evapotranspiration showed no altitudinal trend and averaged 2.2 mm d−1 independent of elevation, although the leaf area index, growing season mean air temperature, and vapor pressure deficit declined with increasing altitude. As precipitation increased with elevation, evapotranspiration seems to be of secondary importance when compared to runoff. Inter-annual variability of evapotranspiration was fairly low across contrasting dry and wet years (coefficient of variation = 7%), indicating that even during dry years water availability was not limiting evapotranspiration.