High-elevation Sites Research Articles

A millennium-long ‘Blue Ring’ chronology from the Spanish Pyrenees reveals severe ephemeral summer cooling after volcanic eruptions

‘Blue Rings’ (BRs) are distinct wood anatomical anomalies recently discovered in several tree species from different sites. While it is evident that they are associated with a cooling-induced lack of cell wall lignification, BRs have yet to be evaluated systematically in paleoclimate studies. Here, we present a continuous wood anatomical assessment of 31 living and relict pine samples from a high-elevation site in the central Spanish Pyrenees that span the period 1150–2017 CE at annual resolution. While most BR years coincide with cold summer temperatures and many BRs follow large volcanic eruptions, some were formed during overall warm summers. We also see a differential response between eruptions: the Samalas eruption is followed by 80% BRs in 1258, but only a modest signal is evident after the 1815 Tambora eruption, and there are no wood anatomical effects of the Laki eruption in 1783–1784. Apparently linked to a cluster of tropical eruptions in 1695 and 1696 CE, 85% BRs occurred in 1698. This new wood anatomical evidence is corroborated by the record of sulphur deposition in polar ice cores, and corresponds with catastrophic famine and unprecedented mortality in Scotland. The extremely rare occurrence of consecutive BRs in 1345 and 1346 marks the onset and spread of the Black Death, Europe’s most devastating plague pandemic. In their ability to capture severe ephemeral cold spells, as short as several days or weeks, BR chronologies can help to investigate and understand the impacts of volcanism on climate and society.

Hidden soils and their carbon stocks at high-elevation in the European Alps (North-West Italy)

Alpine soils store large quantities of total organic carbon (TOC). Given their high sensitivity to climate change, they may release large amounts of CO2 in a warming climate scenario. Thus, it is important to know their C stocks in order to estimate its possible release. While C stocks of forest, dwarf shrub and alpine grassland soils are well documented, little is known about soils and C stocks in high-elevated periglacial environments dominated by frost-driven processes. The object of this study is the periglacial environment of the Stolenberg Plateau (LTER site Istituto Mosso, 3030 m a.s.l.), at the foot of the Monte Rosa Massif (NW Italian Alps). The plateau is mostly covered by a thick stony layer, organized in periglacial blockfields and blockstreams. The plant cover reaches only 3–5%. During the construction of a chair lift, open trenches revealed, unexpectedly, well-developed soils under the active periglacial stone cover. In particular, thick (30–65 cm) and dark TOC-rich A horizons were observed. Below these Umbric horizons, Cambic Bw ones were developed but discontinuous. Despite the lack of vegetation, C stocks were remarkably high (up to ~ 5 kg m−2), comparable to vegetated soils at lower elevation. Non-invasive geophysical methods revealed that these hidden soils were widespread on the plateau under the stony cover, with a mean thickness of around 50 cm. These TOC-rich soils, without vegetation and covered by periglacial landforms, represent a unique pedoenvironment suggesting new perspectives on the actual C-stocks at high-elevation sites, which are probably underestimated.

Prospects for high-elevation radio detection of >100 PeV tau neutrinos

Tau neutrinos are expected to comprise roughly one third of both the astrophysical and cosmogenic neutrino flux, but currently the flavor ratio is poorly constrained and the expected flux at energies above 1017 eV is low. We present a detector concept aimed at measuring the diffuse flux of tau neutrinos in this energy range via a high-elevation mountaintop detector using the radio technique. The detector searches for radio signals from upgoing air showers generated by Earth-skimming tau neutrinos. Signals from several antennas in a compact array are coherently summed at the trigger level, permitting not only directional masking of anthropogenic backgrounds, but also a low trigger threshold. This design takes advantage of both the large viewing area available at high-elevation sites and the nearly full duty cycle available to radio instruments. We present trade studies that consider the station elevation, frequency band, number of antennas in the array, and the trigger threshold to develop a highly efficient station design. Such a mountaintop detector can achieve a factor of ten improvement in acceptance over existing instruments with 100 independent stations. With 1000 stations and three years of observation, it can achieve a sensitivity to an integrated ℰ−2 flux of <10−9 GeV cm−2 sr−1 s−1, in the range of the expected flux of all-flavor cosmogenic neutrinos assuming a pure iron cosmic-ray composition.

Soil organic carbon pools controlled by climate and geochemistry in tropical volcanic regions

Understanding the factors that control the storage of soil organic carbon (SOC) is an urgent priority for mitigating global climate problems. The objective of this study was to determine the factors controlling SOC pools with differing stabilities. Surface soil samples were collected along an elevation gradient from four volcanic regions of Tanzania (two regions) and Indonesia (two regions) under largely-undisturbed vegetation (24 sites in total). A three-pool kinetic model was fitted to accumulative CO2 release curve produced over 343-day incubation to determine the sizes of the labile and intermediate SOC pools (CL and CI, respectively) and their mean residence times (1/KL and 1/KI, respectively), where the size of the stable SOC pool (CS) was measured as non-hydrolyzable carbon. Correlation and path analyses were performed using the results of soil fractionation and model fitting with climatic and geochemical properties.The intermediate pool comprised 50% of total SOC, was responsible for 58% of total accumulative CO2 release, and controlled total SOC stability. The content of nanocrystalline minerals (Alo + 1/2Feo: 5.5–110 g kg−1) was strongly correlated with CI and CS, suggesting that organo-mineral complexes is the essential factor that controls CI and CS rather than soil texture or pH. Temperature (12–26 °C) was weakly correlated with CI, CS, and strongly with CL, which was closely related to microbial biomass carbon. The low temperature at the high elevation sites retards the decomposition of the whole SOC. The significant correlations of excess precipitation with 1/KL and 1/KI represent the effect of moisture on the potential stabilities of the labile and intermediate SOC pools.Climatic factors primarily affect relatively labile SOC pools, whereas geochemical factors influence more stable pools and control total SOC. The findings have important implications for understanding the SOC stabilization mechanisms, which is an essential process of the carbon cycle, in tropical volcanic soils.

Sources of nitrous acid (HONO) in the upper boundary layer and lower free troposphere of the North China Plain: insights from the Mount Tai Observatory

Abstract. Nitrous acid (HONO) is a significant precursor of atmospheric “detergent” OH radicals and plays a vital role in tropospheric chemistry. The current knowledge about daytime HONO sources is incomplete, and its impact on the tropospheric radical chemistry has not been fully quantified. Existing observational studies of HONO were mostly conducted at the surface, with few efforts focusing on the high-elevation atmosphere. In order to better understand the characteristics and sources of HONO in the upper boundary layer and lower free troposphere, two intensive field observations were carried out at the summit of Mt. Tai (1534 m a.s.l.), the peak of the North China Plain (NCP), in winter 2017 and spring 2018. HONO showed moderate concentration levels (average ± standard deviation: 0.15±0.15 and 0.13±0.15 ppbv), with maximum values of 1.14 and 3.23 ppbv in winter and spring, respectively. Diurnal variation patterns with broad noontime maxima and lower nighttime concentrations were observed during both campaigns, which is distinct from most of the previous studies at the ground level. The Lagrangian particle dispersion model (LPDM, WRF-FLEXPART v3.3) simulations indicated the combined effects of the planetary boundary layer evolution and valley breeze on the daytime HONO peak. A photostationary state (PSS) analysis suggested a strong unknown daytime HONO source with production rates of 0.45±0.25 ppb h−1 in winter and 0.64±0.49 ppb h−1 in spring. Correlation analysis supported the important role of photo-enhanced heterogeneous conversion of NO2 to HONO on the aerosol surface at this high-elevation site. HONO photolysis is the predominant primary source of OH radical and plays a major role in the radical chemistry at Mt. Tai. The model only considering a homogenous HONO source predicted much lower levels of the HOx radicals and atmospheric oxidation capacity than the model constrained with measured HONO data. This study sheds light on the characteristics, sources, chemistry, and impacts of HONO in the upper boundary layer and lower free troposphere in the NCP region.

Long-term effects of 7-year warming experiment in the field on leaf hydraulic and economic traits of subtropical tree species.

Rising temperature associated with climate change may have substantial impacts on forest tree functions. We conducted a 7-year warming experiment in subtropical China by translocating important native forest tree species (Machilas breviflora, Syzygium rehderianum, Schima superba and Itea chinensis) from cooler high-elevation sites (600m) to 1-2°C warmer low-elevation sites (300 and 30m) to investigate warming effects on leaf hydraulic and economic traits. Here, we report data from the last 3years (Years 5-7) of the experiment. Warming increased leaf hydraulic conductance of S. superba to meet the higher evaporative demand. M. breviflora (300m), S. rehderianum, S. superba and I. chinensis (300 and 30m) exhibited higher area-based and mass-based maximum photosynthetic rates (Aa and Am , respectively) related to increasing stomatal conductance (gs ) and stomatal density in the wet season, which led to rapid growth; however, we observed decreased growth of M. breviflora at 30m due to lower stomatal density and decreased Aa in the wet season. Warming increased photosynthetic nitrogen-use efficiency and photosynthetic phosphorus-use efficiency, but reduced leaf dry mass per unit area due to lower leaf thickness, suggesting that these tree species allocated more resources into upregulating photosynthesis rather than into structural investment. Our findings highlight that there was trait variation in the capacity of trees to acclimate to warmer temperatures such that I. chinensis may benefit from warming, but S. superba may be negatively influenced by warming in future climates.

The positive effects of the alpine cushion plant Arenaria polytrichoides on insect dynamics are determined by both physical and biotic factors

Cushion plants' significant role for alpine biodiversity has been demonstrated in particular through their positive effects on plant diversity. However, their effects on higher trophic levels (e.g. insects) remain largely unclear. In this study, by field sampling in the Hengduan Mountains (HDM) in southwestern China, we evaluated the effects of an alpine gynodioecious cushion species, Arenaria polytrichoides (Carophyllaceae), on insect richness, abundance and population dynamics at two different elevations (4427 m vs. 4732 m) separately at two time periods (day vs. night) and in two growing seasons (early vs. late). The results showed that the total insect diversity decreases from low to high elevation sites. Some insect species were exclusively detected within A. polytrichoides cushions, leading to an increase in local insect richness from 7% to 35%. The positive effects of cushions on insect diversity could be attributed to unique biotic properties provided by cushions. Firstly, the effects were determined by the sexual dimorphism of the cushion with hermaphroditic cushions supporting higher insect diversity than female cushions. This could be because hermaphroditic cushions provide more resources, such as nectar and pollen grains, for insects than female cushions. Secondly, the cushions' associated beneficiary plants can also provide extra resources for attracting more insects, but this effect was mediated by the micro-environmental conditions. Finally, the magnitude of cushions' positive effects on insect dynamics were stronger under higher than under lower environmental stress. This study confirmed that facilitation by A. polytrichoides cushions in HDM plays an important role in constructing the alpine insect community and further regulating its dynamics. Moreover, the positive effects of cushions on insect dynamics increase with increasing environmental stress. Therefore, the distribution range of insects would quite possibly be expanded into higher elevation under future climate changes, which will induce new challenges for the local alpine ecosystems.

Impact of Recent Climate Change on Water-Use Efficiency Strategies of Larix sibirica in the Altai-Sayan Mountain Range

A strong increase in the mean annual air temperature during the past 50 years by up to 0.54 °C was recorded in the Altai region (45°–52° N; 84°–99° E) compared to the global value of 0.07 °C over the period 1901–2008. The impact of the climatic changes on the hydrology are complex in these mountainous forest ecosystems and not fully understood. We aim to reveal differences in the intrinsic water-use efficiencies (iWUE) strategy by larch (Larix sibirica Ledeb.) derived from stable carbon isotopes at contrasting sites, ranging from the steppe (Ersin, Chadan) to high-elevation (Mongun, Koksu) sites of the Altai over the past century. The iWUE trends increased rapidly for all study sites except Chadan, where a decreasing trend after 2010 has been observed. This decline can be related to increased amount of precipitation compared to increased drought at the other sites. In general, the iWUE is increased up to 14% (1985–2019 compared to 1919–1984), which is lower compared to other studies across the globe likely due to harsh climatic conditions. Vapor pressure deficit and maximal air temperature are impacting Siberian larch significantly and affecting their iWUE differently at the high-elevated and steppe sites of the Altai over the past century.

Age, growth and survival rate in two populations of Darevskia derjugini (Nikolsky, 1898) from different altitudes (Squamata: Sauria: Lacertidae)

Abstract We investigated age structure, age at sexual maturity, lifespan, growth and survival rate and adult life expectancy (as demographic life-history traits) as well as body size of two Darevskia derjugini (Artvin lizard) populations from different altitudes, using skeletochronology. Our findings indicated that age upon attaining sexual maturity was two or three years in the low-altitude population (Fındıklı) while it was three years in the high-altitude population (Murgul) for both sexes. The maximum longevity was seven years in the high-elevation site while it was six years in the low-elevation site. As reported for many lizards, we found a significant positive relationship between age and body size within each sex of Artvin lizard at both altitudes. High- and low-altitude populations did not differ in age structure, survival rates, adult life expectancy and body size. Rather than the effect of altitude, which is hard to compare without replication of other low and high altitude populations, the fact that these two populations have similar growth rates and the similarity of local conditions (food availability and predator density) may indicate similarity between the two regions.

Growth-limiting factors and climate response variability in Norway spruce (Picea abies L.) along an elevation and precipitation gradients in Slovenia.

Norway spruce (Picea abies L.) is among the most sensitive coniferous species to ongoing climate change. However, previous studies on its growth response to increasing temperatures have yielded contrasting results (from stimulation to suppression), suggesting highly site-specific responses. Here, we present the first study that applies two independent approaches, i.e. the nonlinear, process-based Vaganov-Shashkin (VS) model and linear daily response functions. Data were collected at twelve sites in Slovenia differing in climate regimes and ranging elevation between 170 and 1300ma.s.l. VS model results revealed that drier Norway spruce sites at lower elevations are mostly moisture limited, while moist high-elevation sites are generally more temperature limited. Daily response functions match well the pattern of growth-limiting factors from the VS model and further explain the effect of climate on radial growth: prevailing growth-limiting factors correspond to the climate variable with higher correlations. Radial growth correlates negatively with rising summer temperature and positively with higher spring precipitation. The opposite response was observed for the wettest site at the highest elevation, which positively reacts to increased summer temperature and will most likely benefit from a warming climate. For all other sites, the future radial growth of Norway spruce largely depends on the balance between spring precipitation and summer temperature.

Temporal offset between precipitation and water uptake of Mediterranean pine trees varies with elevation and season

For climate models that use paleo-environment data to predict future climate change, tree-ring isotope variations are one important archive for the reconstruction of paleo-hydrological conditions. Due to the rather complicated pathway of water, starting from precipitation until its uptake by trees and the final incorporation of its components into tree-ring cellulose, a closer inspection of seasonal variations of tree water uptake is important. In this study, branch and needle samples of two pine species (Pinus pinaster and Pinus nigra subsp. laricio) and several water compartments (precipitation, creek, soil) were sampled over a two-year period and analyzed for the temporal variations of their oxygen and hydrogen stable isotope ratios (δ18O and δ2H) at five sites over an elevation gradient from sea level to around 1600 m a.s.l. on the Mediterranean island of Corsica (France). A new model was established to disentangle temporal relationships of source water uptake of trees. It uses a calculation method that incorporates the two processes mostly expected to affect source water composition: mixing of waters and evaporation. The model results showed that the temporal offset from precipitation to water uptake is not constant and varies with elevation and season. Overall, seasonal source water origin was shown to be dominated by precipitation from autumn and spring. While autumn precipitation was a more important water source for trees growing at mid- (~800–1000 m a.s.l) and high-elevation (~1600 m a.s.l.) sites, trees at coastal sites mostly took up water from late winter and spring. These findings show that predicted decreases in precipitation amounts during the wet season in the Mediterranean can have strong impacts on water availability for pine trees, especially at higher elevations.

Sphagnum spore banks in two montane peatlands at different elevations

Spore production of bryophytes is common in subalpine ecosystems, but how spore banks of bryophytes vary with elevations is unknown. We used peat cores from two Sphagnum dominated peatlands, a low- (780 m a.s.l.) and a high-elevation (1480 m a.s.l.) site to test the hypotheses that spore concentration, accumulation rate, germinability and longevity are lower at a high-elevation peatland than those at lower elevation due to the assumed climatic limitations at high elevation. Along the cores, Sphagnum spores were extracted, burial time of spores was dated and spore germinability was tested layer by layer. Sphagnum spore concentration, accumulation rate and germinability were greater at the high-elevation peatland than at the low-elevation site. Both peatlands had long-term persistent Sphagnum spore banks with a predicted longevity (the burial time when 1% spores are viable) of c. 390 and 140 years for the low- and high-elevation sites, respectively. Our study suggests that Sphagnum, the ecological engineer of peatlands, by regulating spore production and longevity can maintain relatively constant spore banks which may be of great importance in population regeneration and persistence in a changing world.

Validation and calibration of soil δ2H and brGDGTs along (E-W) and strike (N-S) of the Himalayan climatic gradient

Reconstructing the timing of mountain range uplift and the evolution of high-altitude plateaus is important when attempting to understand potential feedbacks between tectonics and climate at geological timescales. This requires proxies that are able to accurately reconstruct elevation during different time periods in the past. Often, the sensitivity of climatic parameters to elevation gradients, recorded in geological archives such as soils, is used to estimate paleoelevations. However, most proxies reflect an indirect response to elevation change, adding uncertainties to reconstructions. In this study, we aim to identify those sources of uncertainty with respect to elevation reconstructions and test if the combined application of two such proxies, i.e., stable isotopes (δ2H) of plant waxes in modern soils and surface waters and bacterial membrane lipids (brGDGTs) in soils, which can potentially reduce uncertainties in the estimation of (paleo-) elevation. We performed this study in four Himalayan catchments (from west to east: Sutlej, Alaknanda, Khudi, and Arun), of which each individual catchment is subject to a unique precipitation regime, relative influences of moisture sources, and vegetation cover. In total, we analyzed 275 surface water samples, 9 precipitation samples, 131 xylem water samples, and 60 soil samples, which were collected between 2009 and 2014.The following key observations were made: Soil nC31-alkane δ2H values (δ2Hwax) in the Sutlej, Alaknanda, Khudi, and Arun generally record surface water δ2H values, confirming that the first-order control on the plant wax isotopic signature is precipitation δ2H and, therefore, the elevation in orogenic settings. We identified aridity as the factor that introduces scatter to this relationship. BrGDGT-derived Mean Annual Temperature (MAT) correlates in a statistically significant manner with sample site elevation and a 14-year annual average of remotely sensed land-surface temperature, showing that the main process influencing the brGDGT distribution is the adiabatic cooling of air.In an effort to combine these proxies to improve uncertainties in elevation reconstruction, elevations were inferred from both the δ2Hwax and brGDGT distributions. Arid, high elevation sites appear to underestimate actual sample site elevations using δ2H values while sites subject to high (>23–25 °C) annual temperatures overestimate the actual sample site elevation using brGDGT distributions. Elevations inferred from both proxies under such paleoclimatic conditions should be interpreted with caution. Elevations derived from the brGDGT distribution appear to most accurately reconstruct elevation. However, we show that the difference in elevation between the two proxies, described by the proposed ΔElevation parameter, can provide information on the hydrological conditions of the soil’s depositional environment. In conclusion, we emphasize that knowledge of the sample site’s climatic conditions are essential to reconstruct elevation from paleoarchives. In particular, knowledge of moisture availability and annual air temperatures are important, as these have been found to cause the largest scatter in the observed data.

Climate and growing season variability impacted the intensity and distribution of Fremont maize farmers during and after the Medieval Climate Anomaly based on a statistically downscaled climate model

The rise and decline of many complex, pre-European maize-farming cultures in the American Southwest coincides with the warm, climatically quiescent Medieval Climate Anomaly (MCA, ca. 850–1350 CE) and transition to the cool, hydrologically variable Little Ice Age (LIA, ca. 1350–1850 CE). The effects of drought on early subsistence agriculture in the Southwest is well studied, but the impact of temperature stress and variability on the growth of maize crops and which areas were most resilient to such stress remain open questions. We statistically downscaled outputs from a paleo-climate model experiment (CESM1 LME) to map changes to cumulative growing degree days for maize (GDD, 30/10 °C) and precipitation over Utah between 850 and 1449 CE, and downscaled GDD changes to local Fremont Culture archaeological site occupations from radiocarbon-dated contexts mapped as spatially discrete kernel density estimates of summed probability distributions (SPDs). We then analyzed correspondences between Fremont SPDs and GDD/precipitation between 850 and 1449 CE. In general, we found (1) high Fremont occupation intensity coincident with GDD that is less volatile than the long term average, and low occupation intensity coincident with, or following, periods of volatile GDD; (2) intensified occupation of high-elevation sites during the MCA, followed by a retreat to lower elevation sites coincident with a sudden rise in annual temperature volatility and increasing drought conditions; and (3) these occupation changes occurred in spite of the greater temperatures and variability in GDD at low-elevation sites. We found evidence that increased inter-annual variability of growing seasons prior to the onset of the LIA, was likely a determinant of Fremont subsistence strategy decision making, and high-elevation site occupation. The most resilient Fremont occupations in the face of these challenges were sited where growing season lengths were least variable.

Hotter Drought as a Disturbance at Upper Treeline in the Southern Rocky Mountains

As we progress into the Anthropocene, rising temperatures have amplified evaporative demand and rendered heat-induced drought stress, or hotter drought, as the hallmark of climate change moving forward. It remains unknown, however, whether upper treeline environments have been affected. For this study, we grouped previously published and unpublished data from study sites within the southern Rocky Mountains by slope aspect to provide a possible baseline for what we expect for the Anthropocene. We returned to and resampled some of these study sites in the summer of 2019 after twelve years of sharply rising temperatures to measure patterns of seedling establishment. We also returned to a high-elevation site after seventeen years of warming to perform repeat photography in an attempt to capture visual evidence of threshold changes since 2002 in a location where little change had occurred during the twentieth century. Results from this research can be summarized into two main findings: (1) trends in recruitment over the past thirty years suggest that north-facing slopes are increasingly hospitable for successful seedling establishment compared to south-facing slopes at upper treeline and (2) spruce beetle–induced mortality is evident at upper treeline. Conceptually, this means that hotter drought could be progressively enveloping upper treeline along topoclimatic gradients. Unless ongoing trends in temperature deviate from expectations or unless precipitation increases considerably, there is little reason to justify the idea that upper treeline will continue to respond positively to hotter drought conditions during the Anthropocene, especially on south-facing slopes in the Northern Hemisphere.

Shotgun metagenomics reveal a diverse assemblage of protists in a model Antarctic soil ecosystem.

The soils of the McMurdo Dry Valleys (MDV) of Antarctica are established models for understanding fundamental processes in soil ecosystem functioning (e.g. ecological tipping points, community structuring and nutrient cycling) because the extreme physical environment drastically reduces biodiversity and ecological complexity. Understanding the functioning of MDV soils requires in-depth knowledge of the diversity of MDV soil species. Protists, which contribute significantly to soil ecosystem functioning worldwide, remain poorly characterized in the MDV. To better assess the diversity of MDV protists, we performed shotgun metagenomics on 18 sites representing a variety of landscape features and edaphic variables. Our results show MDV soil protists are diverse at both the genus (155 of 281 eukaryote genera) and family (120) levels, but comprise only 6% of eukaryotic reads. Protists are structured by moisture, total N and distance from the local coast and possess limited richness in arid (< 5% moisture) and at high elevation sites, known drivers of communities in the MDV. High relative diversity and broad distribution of protists in our study promotes these organisms as key members of MDV soil microbiomes and the MDV as a useful system for understanding the contribution of soil protists to the structure of soil microbiomes.

Climate sensitive single tree growth modeling using a hierarchical Bayes approach and integrated nested Laplace approximations (INLA) for a distributed lag model

A novel methodological framework is presented for climate-sensitive modeling of annual radial stem increments using tree-ring width time series. The approach is based on a hierarchical Bayes model together with a distributed time lag model that take into account the effects of a series of monthly temperature and precipitation values, as well as their interactions. By using a set of random walk priors, the hierarchical Bayes model allows both the detrending of the individual time series and the regression modeling to be performed simultaneously in a single model step. The approach was applied to comprehensive tree-ring width data from Austria collected on sample plots arranged in triplets representing different mixture types. Bayesian predictions revealed that European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) H. Karst.), and Scots pine (Pinus sylvestris L.) show positive climate-related growth trends throughout higher elevation sites in Tyrol, and these trends remain unchanged under a mixed-stand scenario. At the lower Austrian sites, Norway spruce was found to show a severely negative growth trend under both the pure- and mixed-stand scenario. The increment rates of European beech (Fagus sylvatica L.) were found to have a negative climate-related trend in pure stands, and the trend diminished through an admixture of spruce or larch. The trends of European larch and sessile oak (Quercus petraea (Matt.) Liebl.) showed stationary behavior, irrespective of the mixture scenario. Scots pine data showed a positive trend at the lower elevation sites under both the pure- and mixed-stand scenario. These findings indicate that species mixing does not lower the climate-related increment fluctuations of beech, oak, pine, and spruce at lower elevation sites.

Physiological response of Swiss ecosystems to 2018 drought across plant types and elevation.

Using five eddy covariance flux sites (two forests and three grasslands), we investigated ecosystem physiological responses to the 2018 drought across elevational gradients in Switzerland. Flux measurements showed that at lower elevation sites (below 1000 m.a.s.l.; grassland and mixed forest) annual ecosystem productivity (GPP) declined by approximately 20% compared to the previous 2 years (2016 and 2017), which led to a reduced annual net ecosystem productivity (NEP). At the high elevation sites, however, GPP increased by approximately 14% and as a result NEP increased in the alpine and montane grasslands, but not in the subalpine coniferous forest. There, increased ecosystem respiration led to a reduced annual NEP, despite increased GPP and lengthening of the growing period. Among all ecosystems, the coniferous forest showed the most pronounced negative stomatal response to atmospheric dryness (i.e. vapour pressure deficit, VPD) that resulted in a decline in surface conductance and an increased water-use efficiency during drought. While increased temperature enhanced the water-use efficiency of both forests, de-coupling of GPP from evapotranspiration at the low-elevation grassland site negatively affected water-use efficiency due to non-stomatal reductions in photosynthesis. Our results show that hot droughts (such as in 2018) lead to different responses across plants types, and thus ecosystems. Particularly grasslands at lower elevations are the most vulnerable ecosystems to negative impacts of future drought in Switzerland. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Boundary Layer and Surface Verification of the High-Resolution Rapid Refresh, Version 3

AbstractWhile numerical weather prediction models have made considerable progress regarding forecast skill, less attention has been paid to the planetary boundary layer. This study leverages High-Resolution Rapid Refresh (HRRR) forecasts on native levels, 1-s radiosonde data, and (primarily airport) surface observations across the conterminous United States. We construct temporally and spatially averaged composites of wind speed and potential temperature in the lowest 1 km for selected months to identify systematic errors in both forecasts and observations in this critical layer. We find near-surface temperature and wind speed predictions to be skillful, although wind biases were negatively correlated with observed speed and temperature biases revealed a robust relationship with station elevation. Above ≈250 m above ground level, below which radiosonde wind data were apparently contaminated by processing, biases were small for wind speed and potential temperature at the analysis time (which incorporates sonde data) but became substantial by the 24-h forecast. Wind biases were positive through the layer for both 0000 and 1200 UTC, and morning potential temperature profiles were marked by excessively steep lapse rates that persisted across seasons and (again) exaggerated at higher elevation sites. While the source or cause of these systematic errors are not fully understood, this analysis highlights areas for potential model improvement and the need for a continued and accessible archive of the data that make analyses like this possible.

Source, timing and dynamics of ionic species mobility in the Svalbard annual snowpack

Nearly all ice core archives from the Arctic and middle latitudes (such as the Alps), apart from some very high elevation sites in Greenland and the North Pacific, are strongly influenced by melting processes. The increases in the average Arctic temperature has enhanced surface snow melting even of higher elevation ice caps, especially on the Svalbard Archipelago. The increase of the frequency and altitude of winter “rain on snow” events as well as the increase of the length of the melting season have had a direct impact on the chemical composition of the seasonal and permanent snow layers due to different migration processes of water-soluble species, such as inorganic ions. This re-allocation along the snowpack of ionic species could significantly modify the original chemical signal present in the annual snow. This paper aims to give a picture of the evolution of the seasonal snow strata with a daily time resolution to better understand: a) the processes that can influence deposition b) the distribution of ions in annual snow c) the impact of the presence of liquid water on chemical re-distribution within the annual snow pack. Specifically, the chemical composition of the first 100 cm of seasonal snow on the Austre Brøggerbreen Glacier (Spitsbergen, Svalbard Islands, Norway) was monitored daily from the 27th of March to the 31st of May 2015. The experimental period covered almost the entire Arctic spring until the melting season. This unique dataset gives us a daily picture of the snow pack composition, and helps us to understand the behaviour of cations (K+, Ca2+, Na+, Mg2+) and anions (Br−, I−, SO42−, NO3−, Cl−, MSA) in the Svalbard snow pack. We demonstrate that biologically related depositions occur only at the end of the snow season and that rain and melting events have different impacts on the snowpack chemistry.