Subalpine Sites Research Articles

CO2 fertilization plays a minor role in long-term carbon accumulation patterns in temperate pine forests in the southwestern Pyrenees

Isolating the long-term fertilization effect of CO2 from other climate- and site-related effects on tree growth has been proven a challenging task. To isolate long-term effects of [CO2] on water use efficiency at ecosystem level, we used the FORECAST Climate forest model, calibrated for Scots pine (Pinus sylvestris L.) forests in the southwestern Pyrenees, growing at a Mediterranean montane site and at a continental subalpine site. Future climate scenarios (RCP 4.5 and RCP 8.5) were generated using a battery of six climate models to estimate daily values of temperature and precipitation in a 90-year series. A factorial experiment was designed to disentangle the importance on C pools of three growing limiting factors (nitrogen limitation, climate (temperature + precipitation) limitation and atmospheric CO2 concentration). The relative importance of each factor was quantified by comparing the scenario with the limitation of each individual factor turned on with the non-limitation scenario. Positive CO2 fertilization due to improvement in water use efficiency was detected by the model, but its quantitative impact improving tree growth was minimum: its average increase in ecosystem C pools ranged from 0.3 to 0.9%. At the site with cooler climate conditions (continental), the main limitation for tree growth was climate. Such limitation will be reduced under climate change and the ecosystem will store more carbon. At the site with milder climate conditions (Mediterranean), N availability was the main limiting factor albeit modulated by water availability. Such limitation could be reduced under climate change as N cycling could accelerate (higher litterfall production and decomposition rates) but also increase if droughts become more frequent and severe. In addition, the magnitude of the uncertainty related to climate model selection was much more important than CO2 fertilization, indicating that atmospheric processes are more important than tree physiological processes when defining how much carbon could be gained (or lost) in forests under climate change. In conclusion, due to the small changes in forest C pools caused by variation of atmospheric CO2 concentrations compared to changes caused by other growth limiting factors (nutrients, climate), reducing uncertainty related to climate projections seems a more efficient way to reduce uncertainty in tree growth projections than increasing forest model complexity.

Radon-based atmospheric stability classification in contrasting sub-Alpine and sub-Mediterranean environments

A recently-developed radon-based technique is used to investigate relative changes in summertime atmospheric stability at two sites in Slovenia with contrasting geographical settings. Although atmospheric stability for both sites (50 km apart) was shown to be governed by similar synoptic conditions, their contrasting settings caused differences in mixing conditions for each stability category. At the urban sub-Alpine site Ljubljana, situated within a topographic basin, wind speeds associated with the most stable conditions were 0.2–0.3 m s−1. By comparison, corresponding wind speeds for the near-coastal sub-Mediterranean site Ajdovščina, located at the foothills of the Trnovski gozd barrier, were 0–0.2 m s−1. The wind direction at Ljubljana under stable conditions (∼80°) was consistent with drainage flow into the basin along the Sava River valley. The corresponding wind direction at Ajdovščina was 20–40°, consistent with gentle katabatic drainage from the flanks of the Trnovski gozd barrier. After removing fetch effects on radon variability at each site, a large contrast in local contributions to the radon signal was noted: the diurnal amplitude of the local radon signal increased from ∼24 Bq m−3 at Ljubljana to ∼47 Bq m−3 at Ajdovščina. This difference was attributed to a greater nocturnal radon accumulation rate at Ajdovščina (3.5 Bq m−3 h−1 vs 2.1 Bq m−3 h−1) due to higher radon fluxes from flysch and carbonate rocks compared to the sea and lake sediments in the Ljubljana Basin. The ability of radon to consistently distinguish subtle changes in atmospheric mixing at sites with contrasting topographic settings indicates that it will be a powerful tool for characterising air quality in these complex environments. Specifically, diurnal radon cycles indicate that the capability of the atmosphere to dilute primary pollutants is considerably less in the basin environment.

Culturable fungi associated with wood decay of Picea abies in subalpine forest soils: a field-mesocosm case study

Abstract: Fungi are the principal wood decomposers in forest ecosystems and their activity provides wood necromass to other living organisms. However, the wood decay mechanisms and the associated microbial community are largely unknown, especially in Alpine areas. In this study, the culturable fraction of fungal communities associated with the decomposition of Norway spruce (Picea abies [L.] Karst) deadwood in subalpine forest soils were determined using microbiological methods coupled with molecular identification. Fungal communities were evaluated using in-field mesocosms after one year of exposition of P. abies wood blocks along an altitudinal gradient ranging from 1200 up to 2000 m a.s.l. comprising eight subalpine sites, four of them located at north- and other four at south-facing slopes. Although many saprotrophic species were isolated from the wood blocks, several white-rot species as the pathogenic fungi Armillaria cepistipes and Heterobasidion annosum, along with soft-rot fungi such as Lecytophora sp. were identified. Our results further indicated that the wood-inhabiting fungal community was mainly influenced by topographic features and by the chemical properties of the wood blocks, providing first insights into the effect of different slope exposure on the deadwood mycobiome in the subalpine forest ecosystem.

Colors of night: climate-morphology relationships of geometrid moths along spatial gradients in southwestern China.

Color lightness of insects is an important ecological trait affecting their performance through multiple functions such as thermoregulation, UV protection and disease resistance. The geographical pattern of color lightness in diurnal insects are relatively well understood and largely driven by thermal melanism through the enhancement of insect activity. In nocturnal insects, however, the ecological function of color lightness in response to climatic factors is poorly understood, particularly at small spatial scales. In this study, we investigated color lightness of nocturnal moth assemblages along environmental gradients. Using geometrid moths collected with comparable methodologies (light trapping), we examined assemblage-level changes in color lightness across elevational gradients and vertical strata (canopy vs understory) across three climatically different locations in Yunnan, China. The results showed that moths are darker in color at higher elevations. Such patterns are most apparent in canopy assemblages. In addition, the strength of the elevational pattern on color lightness varied across location, being most pronounced in the canopy of the subalpine site. These patterns are likely driven by UV protection and/or thermoregulation. Our study highlights the importance of abiotic factors such as temperature and solar radiation in structuring morphological patterns of nocturnal ectothermic assemblages along elevational gradients of climatically harsh environments.

Stem Circadian Phenology of Four Pine Species in Naturally Contrasting Climates from Sky-Island Forests of the Western USA

We identified intra-annual climatic drivers of stem water relationships and growth in foundation conifers at a subalpine site in the Great Basin Desert and at a montane site in the Mojave Desert (Nevada, USA). Sites were instrumented to represent naturally different levels of heat and drought stress as part of NevCAN (the Nevada Climate and ecohydrological Assessment Network). We analyzed three years (2013–2015) of sub-hourly dendrometer records for limber (Pinus flexilis) and bristlecone pine (Pinus longaeva) at the subalpine site, and for ponderosa (Pinus ponderosa) and pinyon pine (Pinus monophylla) at the montane site. Multiple logistic regression was used to identify relationships with environmental variables measured in-situ. At both sites, stem expansion occurred during the early morning and late afternoon, and irreversible stem increment was concentrated in the early morning hours. In subalpine species, stem growth started in late spring and continued until August, while at the montane site stem growth was episodic, peaking during summer monsoonal rainstorms. Circadian maximum and minimum stem size during the growing season were reversed during the dormant season at the colder/wetter subalpine site but not at the warmer/drier montane one. Knowledge of intra-annual tree-water relationships and stem growth helps understand how sky island forests grow under highly diverse climatic conditions, including severe drought and heat stress.

Observations and simulations of the seasonal evolution of snowpack cold content and its relation to snowmelt and the snowpack energy budget

Abstract. Cold content is a measure of a snowpack's energy deficit and is a linear function of snowpack mass and temperature. Positive energy fluxes into a snowpack must first satisfy the remaining energy deficit before snowmelt runoff begins, making cold content a key component of the snowpack energy budget. Nevertheless, uncertainty surrounds cold content development and its relationship to snowmelt, likely because of a lack of direct observations. This work clarifies the controls exerted by air temperature, precipitation, and negative energy fluxes on cold content development and quantifies the relationship between cold content and snowmelt timing and rate at daily to seasonal timescales. The analysis presented herein leverages a unique long-term snow pit record along with validated output from the SNOWPACK model forced with 23 water years (1991–2013) of quality controlled, infilled hourly meteorological data from an alpine and subalpine site in the Colorado Rocky Mountains. The results indicated that precipitation exerted the primary control on cold content development at our two sites with snowfall responsible for 84.4 and 73.0 % of simulated daily gains in the alpine and subalpine, respectively. A negative surface energy balance – primarily driven by sublimation and longwave radiation emission from the snowpack – during days without snowfall provided a secondary pathway for cold content development, and was responsible for the remaining 15.6 and 27.0 % of cold content additions. Non-zero cold content values were associated with reduced snowmelt rates and delayed snowmelt onset at daily to sub-seasonal timescales, while peak cold content magnitude had no significant relationship to seasonal snowmelt timing. These results suggest that the information provided by cold content observations and/or simulations is most relevant to snowmelt processes at shorter timescales, and may help water resource managers to better predict melt onset and rate.

Long-Term Climate Regime Modulates the Impact of Short-Term Climate Variability on Decomposition in Alpine Grassland Soils

Decomposition of plant litter is an important process in the terrestrial carbon cycle and makes up approximately 70% of the global carbon flux from soils to the atmosphere. Climate change is expected to have significant direct and indirect effects on the litter decomposition processes at various timescales. Using the TeaBag Index, we investigated the impact on decomposition of short-term direct effects of temperature and precipitation by comparing temporal variability over years, versus long-term climate impacts that incorporate indirect effects mediated through environmental changes by comparing sites along climatic gradients. We measured the initial decomposition rate (k) and the stabilization factor (S; amount of labile litter stabilizing) across a climate grid combining three levels of summer temperature (6.5–10.5°C) with four levels of annual precipitation (600–2700 mm) in three summers with varying temperature and precipitation. Several (a)biotic factors were measured to characterize environmental differences between sites. Increased temperatures enhanced k, whereas increased precipitation decreased k across years and climatic regimes. In contrast, S showed diverse responses to annual changes in temperature and precipitation between climate regimes. Stabilization of labile litter fractions increased with temperature only in boreal and sub-alpine sites, while it decreased with increasing precipitation only in sub-alpine and alpine sites. Environmental factors such as soil pH, soil C/N, litter C/N, and plant diversity that are associated with long-term climate variation modulate the response of k and S. This highlights the importance of long-term climate in shaping the environmental conditions that influences the response of decomposition processes to climate change.

Patterns of Early Postfire Succession of Alpine, Subalpine and Lichen-Woodland Vegetation: 21 Years of Monitoring from Permanent Plots

Field observations using chronosequences are helpful to study vegetation succession. This method allows to establish comparisons based on soil composition, stand structure, micro- and macrofossil remains from sites of different ages but on similar edaphic and topographic conditions. In the boreal forest, post-fire succession through time is triggered by climate, disturbance history (insect epidemics, fire and logging), latitude and altitude. The main objective of this research is to identify the main patterns of early post-fire succession, including similarities and differences in vegetation composition and attributes, of three contrasted ecosystems distributed along an altitudinal gradient. To do so, we have monitored the successional development of the alpine, subalpine and boreal lichen-woodland sites during the first 21 years (1991 to 2011) of post-fire sequence in eastern Canada 1991 to 2011. Each site was characterized by a different functional group that became established following fire. A rapid resurgence of ericaceous shrubs and lichens was observed in the lichen woodland and subalpine sites. Bryophyte and lichen species were not an important component of vegetation communities during the earlier stages of post-fire succession. For all three sites monitored, lichens were the last functional group to establish in the chronosequences. Herbs and mosses characterized the post-fire succession in alpine areas, the latter functional group established late in the chronosequence to cover >25% of the site after 15 years. Post-fire chronosequences in the three contrasted environments indicate that plant succession is a repetitive process often involving similar resilient plant assemblages.

Demography of the Rare Fern Botrychium pumicola (Pumice Moonwort) at Crater Lake National Park, Oregon

We investigated the demography (emergence, dormancy and recruitment) of the pumice moonwort, (Botrychium pumicola) (Ophioglossaceae), a rare fern endemic to the Cascade Range from central Oregon to northern California. Populations of B. pumicola have been declining in low elevation basin sites likely due to climate change. Determining the demographics of subalpine populations is important for management of the species. At three subalpine sites at Crater Lake National Park, Oregon, individual B. pumicola plants were tagged, mapped and monitored for six consecutive years. In sample plots, the population of B. pumicola was stable over the six-year period with recruitment rates approximating dormancy and death rates. The annual dormancy rate was 6.4% and annual recruitment rate was 9%. Plants that went dormant and new recruits were smaller in stature than those of the original tagged plants suggesting that these plants were experiencing factors affecting fitness. Reduced pre-dormancy sporophore production and plant stature was a predictor of population decline. Variations in snow depth did not affect plant emergence, recruitment, or dormancy. Our results show that B. pumicola does not have a large below-ground reservoir of dormant rhizomes and the aboveground population most accurately represents the total population of adult individuals. Tracking B. pumicola individuals provides valuable information for management of this elusive species.

Short-term carbon allocation dynamics in subalpine dwarf shrubs and their responses to experimental summer drought

Carbon (C) allocation is a key determinant of plant functioning and ecosystem processes. Its dynamic responses to environmental conditions and global change are still poorly understood. This holds particularly true for dwarf shrubs, which are an important component of high mountain vegetation. We studied the short-term C allocation dynamics in two dominant dwarf shrub species, Vaccinium myrtillus and Vaccinium uliginosum, at two subalpine sites, in the Pyrenees and in the Alps, and additionally performed an experimental rain exclusion to simulate summer drought at the site in the Alps. We compared net assimilation rates (Amax), stomatal conductance (gs) and shoot water potentials (ΨS) between the two species and carried out a 13C pulse labelling experiment to trace their C allocation dynamics, both under natural conditions and in the drought experiment. Under ambient conditions, V. uliginosum showed higher Amax and gs and used recent assimilates as a respiratory substrate more slowly than V. myrtillus. C allocation dynamics in current-year shoots differed significantly between the species, showing a progressive decrease after the pulse labelling in V. uliginosum shoots but a progressive increase in V. myrtillus shoots. In the drought experiment, a significant reduction of soil moisture caused predawn ΨS below −1.2MPa in both species, which is considered close to the turgor loss point. However, the stomatal conductance and the speed of transfer of newly assimilated C to belowground organs decreased in V. uliginosum only. We found a strong coupling in C allocation dynamics between xylem and phloem tissues of the rhizome, which was maintained under drought. Although both species were largely tolerant to the drought imposed, our results indicate a higher drought sensitivity of V. uliginosum compared to V. myrtillus, with potential implications for community composition and ecosystem C cycling in a future climate.

Increased complementarity in water‐limited environments in Scots pine and European beech mixtures under climate change

AbstractManagement of mixedwoods is advocated as an effective adaptation strategy to increase ecosystem resiliency in the context of climate change. Although mixedwoods have been shown to have greater resource use efficiency relative to pure stands, considerable uncertainty remains with respect to the underlying ecological processes. We explored species interactions in Scots pine/European beech mixedwoods with the process‐based model FORECAST Climate. The model was calibrated for two contrasting forests in the southwestern Pyrenees (northern Spain): a wet Mediterranean site at 625 m.a.s.l. and a subalpine site at 1335 m.a.s.l. Predicted mixedwood yield was higher than that for beech stands but lower than pine stands. When simulating climate change, mixedwood yield was reduced at the Mediterranean site (−33%) but increased at the subalpine site (+11%). Interaction effects were enhanced as stands developed. Complementarity dominated the Mediterranean stand but neutral or net competition dominated the subalpine stand, which had higher stand density and water availability. Reduced water demand and consumption, increased canopy interception, and improved water‐use efficiency in mixtures compared to beech stands, suggest a release of beech intraspecific competition. Beech also facilitated pine growth through better litter quality, nonsymbiotic nitrogen fixation, and above‐ and belowground stratification, leading to higher foliar nitrogen content and deeper canopies in pines. In conclusion, mixtures may improve water availability and use efficiency for beech and light interception for pine, the main limiting factors for each species, respectively. Encouraging pine–beech mixtures could be an effective adaptation to climate change in drought‐prone sites in the Mediterranean region.

Biotic interaction effects on seedling recruitment along bioclimatic gradients: testing the stress‐gradient hypothesis

AbstractQuestionsIs there a shift from positive to negative biotic interaction effects on seedling recruitment along two different stress gradients, temperature and precipitation (the stress‐gradient hypothesis); do such interaction effects differ between species with different bioclimatic affinities?LocationBoreal, sub‐alpine and alpine grassland in southern Norway.MethodsWe tested the stress‐gradient hypothesis by comparing seedling recruitment in bare‐ground gaps where vegetation has been removed vs in extant grassland vegetation in 12 boreal, sub‐alpine and alpine grassland sites along a precipitation gradient. This was tested in (1) a seed‐sowing experiment and (2) in naturally occurring recruitment of alpine, generalist and boreal species.ResultsEmergence of the sown alpine species was higher in the cold alpine than in the warmer sub‐alpine sites, with no effects of precipitation or vegetation removal. The sown generalists also decreased in emergence towards warmer sites, whereas there was no effect of temperature on the sown boreal species. Vegetation removal, interacting with precipitation, increased the emergence of the generalist and boreal species sown at intermediate precipitation levels. In contrast, interactions between temperature and vegetation removal regulated the emergence of all groups of naturally occurring seedlings. Alpine and generalist species emerged at the highest rate in alpine sites, whereas boreal species had highest emergence in the lowlands.ConclusionFor all species groups, strong effects of vegetation removal show that competition from the extant vegetation dominates in controlling seedling emergence across all study sites and species. In generalist and boreal species, positive interactions between vegetation removal and temperature show that competitive interactions affect seedling emergence more strongly towards warmer climates, in line with the stress‐gradient hypothesis. In contrast, alpine species show no such interactions. This suggests that species’ adaptations to climate, in combination with environmental forcing, control seedling emergence along the bioclimatic gradients. Our results have implications for nature conservation, as we propose that disturbance from grazing animals can be useful to release competition and thereby increase seedling recruitment and biodiversity in boreal and alpine grasslands in a warmer future.

Microbial Community Structure of Subalpine Snow in the Sierra Nevada, California

ABSTRCTMounting evidence suggests that Earth’s cryosphere harbors diverse and active microbial communities. However, our understanding of microbial composition and diversity in seasonal snowpack of montane ecosystems remains limited. We sequenced the 16S rRNA gene to determine microbial structure (composition and diversity) of snow at two depths (0–15 and 15–30 cm) of a subalpine site in the Southern Sierra Critical Zone Observatory, California, U.S.A. Proteobacteria dominated both depths (~72% of sequences), and this phylum was composed mostly of bacteria within the Rhodospirillales order. Cyanobacteria were almost exclusively present in the upper snow layer, while Actinobacteria and Firmicutes were more abundant in the deep snow layer. Many of the most abundant phylotypes were Acetobacteraceae. Phylotype NCR4874, which comprised 22%–32% of the sequences, was most closely related to the N2-fixing bacteria Asaia siamensis, suggesting that N2 fixation may be an important process within the Sierra snowpack. In addition, just under half (45%) of the numerically dominant phylotypes shared >98% similarity with sequences recovered from other cold environments. Our results suggest that microbial communities of subalpine Sierra Nevada snowpack are diverse, with both snow depths harboring distinct but overlapping communities consisting largely of cold-adapted bacteria.

Microorganisms associated with corrosion of structural steel in diverse atmospheres

The influence of atmospheric conditions on the corrosion of steel and its associated microbial community was studied. Surface analysis revealed greater localized corrosion in steel exposed to near-ocean atmospheres with high chloride deposition compared to inland and subalpine sites. High-throughput sequencing analysis of corrosion products showed that dissimilar microbial communities and dominant species were deposited on steel in the different atmospheres. Close to the ocean, Brevundimonas diminuta were predominant whereas Clostridium and Pseudomonas species dominated for inland sites with agricultural or forestry activities. Bacillus and Enterococcus were dominant for sites close to a fertilizer plant and a sewage treatment plant, respectively. Actinobacteria species dominated at sub-alpine conditions. Results from this study indicate that microbial communities on corroding steel exposed to atmospheric conditions are the result of deposition of locally-generated aerosols. Acid-producing activity and exopolysaccharide (EPS) production was widespread and rapidly detected in microbial cultures from all the exposure sites. Sulphate-reducing bacteria were not detected in this study. These results suggest that acid production and EPS synthesis can be important mechanisms for microbial corrosion of steel under atmospheric conditions.

Above- and belowground patterns in a subalpine grassland-shrub mosaic

Many mountain pastures consist of a mosaic of grassland and shrub communities. Ongoing changes in mountain agriculture have affected the balance between the two elements of the mosaic. In order to understand the consequences of these changes for ecosystem functioning, we studied patterns in vegetation, root structure and soil properties along transects of varying grassland-to-shrub proportions. Our hypothesis was that differences in the vegetation aboveground are accompanied by differences belowground, related to soil properties and depth. The research was conducted at a subalpine site in the Trentino region (South-eastern Alps), consisting of Nardus stricta grasslands alternating with shrub patches of Rhododendron ferrugineum. Our investigation showed that the composition of vegetation was mainly governed by R. ferrugineum cover and less by soil properties. Plant species richness peaked at low to intermediate degrees of shrub cover and composition between transects became more similar with increasing shrub cover. Where R. ferrugineum cover was higher, Hemicryptophytes caespitosae were replaced by Nano-phanerophytes with consequences for belowground structures. At increasing shrub cover, root length density decreased, especially in the top soil, while root weight density remained stable and C content increased insignificantly. We discuss that theses structural changes along the gradient of R. ferrugineum cover affect a number of ecosystem services. The presented evidence suggests that maintaining grasslands with a low cover of R. ferrugineum balances a number of services, namely plant species diversity, carbon stabilization in soil and the prevention of soil erosion.

Subalpine fires: the roles of vegetation, climate and, ultimately, land uses

Forest fires are controlled by local (vegetation, aspect, elevation, land uses) and regional (climate) factors. The role of these factors remains poorly assessed on centennial to millennial time scales. Here, we investigate fire and vegetation history in five subalpine sites over the past 8000 years to determine when, how, and why there were differences in fire regimes. We focus on time and aspect separately and account for vegetation composition. There are no significant differences in the fire return intervals among sites (~290 years between two fires). The rise of fire frequency from its lowest values (1.5 fires.1000 yr−1) at 8000–7000 years before present (yrBP) to its highest at 4000-3000 yrBP (5 fires-1000 yr−1) is well explained by the high temperatures and low precipitation of the Holocene, both leading to better conditions for fire ignition and spread. From 8000 yrBP, climate is likely the main factor driving the fire frequency on the north-facing sites, which contained mixed Larix decidua-Pinus cembra vegetation; whereas south-facing sites, dominated by the flammable P. cembra and broadleaved trees, appear to be driven by local features, including land use. A shift in the main drivers occurs at 4000 yrBP, when local processes start to control burning activity at all sites. Forest clearance for land use might have directly increased fire frequency, but has also helped to suppress fires through fuel limitation. During the last four millennia, land use likely altered the natural climate-vegetation-fire system mainly by converting forests to grassland by pastoral practices, thus leading to less fire and making the fire-vegetation-climate relationships more complex.

Grazing and the absence of fire promote the dominance of an unpalatable shrub in a patch mosaic cyclic successional system

We experimentally determined how an apparently unpalatable and fire-sensitive shrub, Richea acerosa (Lindley) F.Muell., responded to the single and interactive effects of grazing and burning over 3–24 years at two subalpine sites in Tasmania. At the Middlesex Plains site, a low-intensity burn killed most individuals of the species 3 years after fire and fencing. At the Lake Botsford site, total grazing exclusion for 24 years resulted in a strong reduction of the number of small shrubs of the species, and in a lower proportion of dead material in plants of any given size. An open shrub layer results from the absence of R. acerosa seedlings in the open centres of senescing plants and their preferential location along the margins of the foliage of mature and senescing plants. Thus, the persistence of R. acerosa appears to require an absence of frequent fire and the presence of sufficient grazing pressure from vertebrate herbivores to produce lawn between shrubs. Richea acerosa is, thus, an unpalatable shrub that depends on grazing.

Contrasting long-term alpine and subalpine precipitation trends in a mid-latitude North American mountain system, Colorado Front Range, USA

Background: Long-term climate trends in mountain systems often vary strongly with elevation.Aims: To evaluate elevation dependence in long-term precipitation trends in subalpine forest and alpine tundra zones of a mid-continental, mid-latitude North American mountain system and to relate such dependence to atmospheric circulation patterns.Methods: We contrasted 59-year (1952–2010) precipitation records of two high-elevation climate stations on Niwot Ridge, Colorado Front Range, Rocky Mountains, USA. The sites, one in forest (3022 m a.s.l.) and the other in alpine tundra (3739 m), are closely located (within 7 km horizontally, ca. 700 m vertically), but differ with respect to proximity to the mountain-system crest (the Continental Divide).Results: The sites exhibited significant differences in annual and seasonal precipitation trends, which depended strongly on their elevation and distance from the Continental Divide. Annual precipitation increased by 60 mm (+6%) per decade at the alpine site, with no significant change at the subalpine site. Seasonally, trends at the alpine site were dominated by increases in winter, which we suggest resulted from an increase in orographically generated precipitation over the Divide, driven by upper-air (700 hPa) north-westerly flow. Such a change was not evident at the subalpine site, which is less affected by orographic precipitation on north-westerly flow.Conclusions: Elevation dependence in precipitation trends appears to have arisen from a change in upper-air flow from predominantly south-westerly to north-westerly. Dependence of precipitation trends on topographic position and season has complex implications for the ecology and hydrology of Niwot Ridge and adjacent watersheds, involving interactions among physical processes (e.g. snowpack dynamics) and biotic responses (e.g. in phenologies and ecosystem productivity).

Soil geomorphology and frozen ground conditions at a subalpine talus slope having permafrost in the eastern Swiss Alps

At a subalpine site with permafrost below the timberline in the eastern Swiss Alps the complex interrelationship between different environmental factors was investigated by applying an integrated approach that combines soil mapping, 3D near-surface geophysics and surface and subsurface temperature monitoring. The objectives of this approach were to explore the 3D extension of isolated permafrost bodies and to relate this to surface parameters such as soils and humus forms and other extrinsic factors. Using this procedure, interdependencies between surface (soil properties and humus forms) and subsurface factors (ground thermal regime and frozen ground conditions) could be independently analysed. Although time-consuming, 3D ERI (electrical resistivity imaging) demonstrated its ability for precise permafrost mapping. Despite climate warming, super-cooling in winter along the investigated scree slopes is still efficient enough for establishing permafrost conditions. By comparing the spatial dimensions of the permafrost bodies with surface conditions, we found that thick moss or organic litter layers had an insulating effect and prevented the permafrost from melting. However, the comparison of soil units and soil thickness with the permafrost distribution did not always show an obvious relationship. This indicates that there is a complex evolution and behaviour of soils and permafrost at such sites.

Facilitation of Pinus albicaulis seedling regeneration by Vaccinium scoparium

Plants often facilitate each other in harsh environments, and facilitation appears to be important for conifer recruitment in subalpine forests and the formation of treelines. This study investigated the potential role of facilitation for the establishment of Pinus albicaulis (whitebark pine), a dominant Rocky Mountain tree species that is in decline due to an exotic blister rust disease (Cronartium ribicola), bark beetle outbreaks, and fire suppression. Surveys were conducted at five subalpine sites for spatial associations between naturally regenerating P. albicaulis seedlings and other plant species, and found that the cover of Vaccinium scoparium was higher in proximity to P. albicaulis seedlings. In transplant experiments, survival of P. albicaulis seedlings was higher when planted with V. scoparium and in shade cloth treatments, but neither survival nor growth rates were altered by clipping or rhizome severing of V. scoparium, suggesting a below-ground rather than above-ground facilitation mechanism. In contrast, Carex species reduced the survival and growth rates of P. albicaulis. In a fertilization experiment, no effect of nitrogen and/or phosphorous fertilization was found on Pinus albicaulis seedling growth, but seedlings growing with V. scoparium had higher growth rates and increased leaf phosphorus concentrations relative to seedlings growing in bare ground. In a microcosm experiment, P. albicaulis seedlings growing with V. scoparium had higher growth rates and leaf phosphorus concentrations than when grown alone, but only if P. albicaulis needles were added on the soil surface. My results suggest success of restoration plantings of greenhouse grown P. albicaulis seedlings may be improved by planting in areas where V. scoparium occurs, and avoiding areas dominated by Carex species. Removal of adjacent vegetation prior to planting seedlings may be ineffective in significantly altering either positive or negative effects of the neighboring plants. Selection of sites with adjacent shading elements, such as rocks, logs, stumps, or snags, may also improve seedling survival.