Georg Niedrist

Climate induced upslope migration of alpine plants is accompanied by a decline in atmospheric pressure, raising the question of whether plant performance at higher elevations is constrained by carbon availability and/or by water-related stress. Decreasing air pressure alters gas diffusivity, evaporative demand, and the partial pressure of CO₂, potentially shifting limitation from carbon acquisition towards plant water relations. However, in the field the impact of reduced air pressure is hardly to detect because it covaries with temperature, humidity, and radiation along natural elevational gradients.We thus addressed this question using ecotron experiments that isolate air-pressure effects from co-varying climatic factors. Mountain grassland species and model plants with contrasting functional strategies (Arabidopsis thaliana, Trifolium pratense, Hieracium pilosella, Brachypodium rupestre) were grown under atmospheric pressures corresponding to ~1,500–4,000 m a.s.l. (85–62 kPa), while temperature, radiation and air humidity were kept equal among the treatments. We quantified traits related to gas exchange (stomatal conductance, carbon isotope discrimination), carbon acquisition and allocation (photosynthetic efficiency, carbohydrate storage, biomass production), and nutrient status (leaf nitrogen and chlorophyll).Across species, reduced air pressure consistently lead towards higher photosynthetic energy-use efficiency and increased leaf carbohydrate pools (up to +40%), while aboveground biomass decreased. Gas-exchange revealed species-specific strategies: stomatal conductance increased or remained stable under low pressure in forb species, whereas grass responses depended on interactions with water availability. After 4 weeks results indicated a decreased carbon assimilation efficiency under low air pressure and a higher vulnerability to drought because of the higher Vapour-pressure deficit (VPD)These short- term results suggest that reduced air pressure is a relevant parameter for upwards-migrating mountain plants and may play an underestimated role in shaping composition and performance of alpine ecosystems.

Climate change is a major factor shaping the distribution of plant species. A well-documented response consequence is the upward shift of plant species to higher elevations as they track their thermal niches. However, plants migrating upward face complex environmental changes shaped by multiple interacting factors. Among these, reduced air pressure remains relatively understudied, its effects are often confounded with other covarying parameters. This study investigated the direct impact of reduced air pressure on the eco-physiological responses of two plant species (Hieracium pilosella L. and Trifolium pratensis L.). The plants were grown for 4 weeks in controlled climatic chambers under different air pressures (85, 75, and 62 kPa), while all other environmental parameters were kept constant. At the end of the experiment, photosynthesis, chlorophyll fluorescence, growth, carbohydrate content, carbon stable isotopes, and plant nitrogen concentrations were determined. Reduced air pressure decreased growth, carbon isotopic discrimination and chlorophyll content, but increased CO2 fixation efficiency and carbohydrate accumulation in the leaves. These results suggest that reduced air pressure impacts plant performance during upslope migration and may, in turn, contribute to shaping future distribution patterns in alpine ecosystems.

One of the primary challenges of climate change faced by viticulture is the rising frequency of extreme weather events, including heatwaves. The aim of this study was to test apical leaf removal as a mitigation strategy during heatwaves based on the hypothesis that reducing total leaf area translates to lower transpirational water losses, thereby improving the water status of young grapevine plants under multiple stress conditions. Two-year-old Sauvignon blanc vines were subjected to progressive severe drought stress in combination with a heatwave (max air temperature 40 °C), which were simulated in a controlled environment. Key physiological parameters, such as pre-dawn water potential, stomatal conductance, Fv/Fm and net assimilation were measured before (acclimation), during and after (recovery) the heatwave. Drought stressed plants reached a maximum pre-dawn water potential of –1.4 MPa. Drought stress negatively affected all physiological parameters, especially stomatal conductance, which was reduced to almost zero for the duration of the stress. As a consequence, leaf temperatures during the heatwave reached up to 48.8 °C in drought stressed (WS) plants in contrast to well-watered (WW) plants, which reached a recorded maximum of 43.2 °C. Leaf removal had contrasting effects depending on water availability: in WW vines, it reduced cumulative transpiration by 15 %, whereas under drought stress, it had no impact due to near-complete stomatal closure. However, leaf removal positively influenced recovery, since WS leaf removal vines regained stomatal conductance values comparable to WW vines three days after rewatering. By the end of the 5-day recovery period, all physiological parameters in WS plants, whether leaf removal or control, returned to levels comparable to WW vines. Overall, this work proved that the success of apical leaf removal is highly dependent on water stress level. The practical relevance of this research lies in the necessity of finding ways to mitigate the effects of extreme weather events, especially in a context where irrigation water may be not available and with young vines that are characterised by a still undeveloped root system.

Mountain ecosystems are particularly sensitive to global change and are therefore often used as early warning systems while providing essential ecosystem services. For this reason, researchers aim to understand how these ecosystems develop under climate change. While focus increased in the last decades, mainly due to manipulation experiments, our understanding of how these ecosystems respond to environmental changes remains fragmented. In this study we systematically reviewed 767 manipulation experiments on global change effects in mountain environments over the past three decades, analyzing 3082 ecological responses across different organizational levels. Temperature manipulation was the most common experiment type (45% of studies), showing strong effects on plant phenology, soil respiration, and nutrient cycling. Water availability significantly impacted productivity and carbon cycling, while nutrient manipulations altered community composition and biomass production. Plant responses dominated the research (71% of studies), showing species-specific adaptations to warming. Soil microbial communities exhibited significant responses to warming, affecting decomposition processes and nutrient availability. However, critical knowledge gaps remain. Experimental studies on adult trees in tropical and boreal regions are scarce, and animal responses, biotic interactions, and aquatic environments require more attention. Furthermore, most experiments (73%) were short-term, with a duration under 5 years and focused on single factors, limiting our understanding of long-term and interactive effects. A network of standardized experiments across mountain regions, combining different research methods and collaboration between research groups, could address these gaps.

Climate change is advancing at an unprecedented pace, impacting terrestrial ecosystems, particularly those in alpine regions. Consequently, there is a growing need to comprehend the associated impacts, underlying mechanisms, and implications. Long-term monitoring may face challenges in capturing the effects of accelerated climate change, and in-situ experiments in remote alpine areas often grapple with logistical constraints. Furthermore, attributing vegetative responses to specific manipulated variables proves challenging, especially under extreme alpine conditions such as low atmospheric pressure, low temperatures, or high radiation levels.Using a specially designed ecotron called 'TerraXcube' (Bozen, Italy), we investigated the feasibility of realistically reproducing harsh alpine conditions and explored the interactions among various parameters. For our measurements, we equipped the chamber with temperature and relative humidity probes, a spectrometer, barometer, and anemometer positioned at different heights within the chamber. We tested the spatial and temporal homogeneity of the variables— atmospheric pressure, temperature, relative humidity (RH), and radiation—independently, as well as their interactions over time and in space, by simulating various realistic alpine climatic scenarios.The measurements, conducted between -20°C and +25°C with relative humidity ranging from 10% to 95%, yielded satisfactory results. Over several hours, the largest difference at a specific position was 0.6°C and 4.3% RH, while the maximum difference between two sensors simultaneously was 1°C and 7% RH. At a height of 170 cm, the LED system emitted radiation at an intensity of 1,002 W/m² within the wavelength range of 280 to 900 nm; however, with a sharp decrease in intensity from the light source. The photosynthetically active radiation (PAR) at the chamber's center reached 1,883 μmol·m−2·s−1, achieving 77% of the potential annual maximum measured at 2,400 m a.s.l. This enables us to replicate the PAR level for 97% of the days throughout the year. Despite the high light intensity, the heating effect of the LED system was limited to a maximum of 2°C in the upper 40cm of the chamber. Pressure manipulation, with the highest technical demand, nonetheless resulted in high temporal homogeneity up to 4,000m a.s.l., corresponding to 618.9 mbar.In conclusion, the results emphasize the potential and utility of ecotrons in simulating a suitable climate for alpine ecological experiments. However, as in many ecotrons, it is crucial to acknowledge that minor island effects and irregularities are inevitable. Even more sophisticated parameters such as wind effects or pollinator function are currently not sufficiently addressed. A combined in- and outdoor usage of mobile field lysimeters might be a further step to bridge this gap between experimental results obtained in ecotrons and in the field.

Optimization models of stomatal conductance (gs) provide a conceptual framework to understand stomatal responses to environmental changes in terms of tradeoffs between the benefits and costs of stomatal opening. Theoretically, stomata maximize the benefits (carbon gain, A) and minimize the costs (water loss through transpiration, E). However, during heat waves, there can be conflicts between the need to maintain a high E to control leaf temperature via evaporative cooling and avoid heat damage, and the decrease in gs in response to increased VPD and reduced soil water availability. Under these conditions, the balance between the gains and the associated costs remains unclear. We measured leaf gas exchange (A, E and gs) in potted grapevines, cv Sauvignon Blanc, before, during and after a simulated six-day heat wave (Tmax = 40 °C), in the morning (10 to 12) and the afternoon (15 to 17), using heated well-watered (HW), heated drought-stressed (HD), non-heated well-watered (CW) and non-heated dry (CD) vines. We also measured plant transpiration (Elys) and leaf temperature (Tleaf) continuously during the heat wave using lysimeters and infrared cameras. We test the hypothesis that under combined stress, in addition to the stomatal limitation to A, there is an additional non-stomatal cost for A caused by the heat damage in the photosystem. We explore whether this potential additional cost is captured by two gs optimization models that include soil-to-leaf hydraulic conductance (model 2) or not (model 1). We also hypothesize that gs in HW vines may not follow the optimal predictions because these models do not include stress-related risks such as heat damage due to higher-than-optimal Tleaf resulting from stomal closure and reduced E. There were no significant differences in gs in HD and CD vines, during the morning or the afternoon measurements. Consequently, there were no differences in measured E and A between HD and CD vines during the peak of the heat wave. This was probably due to the stronger effect of water stress (soil water potential from -400 to -800 kPa) than high VDP (5 kPa) on gs during the peak of the heat wave. Under well-watered conditions, measured gs, E and A in the morning were much higher in HW than in CW vines and the values decreased from the morning to the afternoon. Preliminary results suggest that optimal models including dynamic responses to soil water potential can correctly integrate plant responses to heat and drought stress. The incorporation of stress-related risks (such as heat damage to the photosystem) into these models will be discussed.

In this work, we present two datasets for specific areas located on the Alpine arc that can be exploited to monitor and understand water resource dynamics in mountain regions. The idea is to provide the reader with information about the different sources of water supply over five defined test areas over the South Tyrol (Italy) and Tyrol (Austria) areas in alpine environments. The snow cover fraction (SCF) and Soil Moisture Content (SMC) datasets are derived from machine learning algorithms based on remote sensing data. Both SCF and SMC products are characterized by a spatial resolution of 20 m and are provided for the period from October 2020 to May 2023 (SCF) and from October 2019 to September 2022 (SMC), respectively, covering winter seasons for SCF and spring–summer seasons for SMC. For SCF maps, the validation with very high-resolution images shows high correlation coefficients of around 0.9. The SMC products were originally produced with an algorithm validated at a global scale, but here, to obtain more insights into the specific alpine mountain environment, the values estimated from the maps are compared with ground measurements of automatic stations located at different altitudes and characterized by different aspects in the Val Mazia catchment in South Tyrol (Italy). In this case, an MAE between 0.05 and 0.08 and an unbiased RMSE between 0.05 and 0.09 m³·m⁻³ were achieved. The datasets presented can be used as input for hydrological models and to hydrologically characterize the study alpine area starting from different sources of information.

In this work, we present two datasets for specific areas located in South Tyrol (Italy) and in Tyrol (Austria) that can be exploited to monitor and understand water resource dynamics in mountain regions. The idea is to provide the reader with information about the different sources of water supply over five defined test areas. The Snow Cover Fraction (SCF) and Soil Moisture Content (SMC) datasets are derived from machine learning algorithms based on remote sensing data. Both SCF and SMC products are characterized by a spatial resolution of 20 m and are provided for the period from October 2020 to May 2023 (SCF) and from October 2019 to September 2022 (SMC) respectively, each covering 3 seasons of interest, winter for SCF and spring-summer for SMC. For SCF maps, the validation with very high-resolution images shows high correlation coefficients of around 0.9. The SMC products were originally produced with an algorithm validated at global scale, but here, to obtain more insight in the specific alpine mountain environment, the values estimated from the maps are compared with ground measurements of automatic stations located at different altitudes and characterized by different aspects in the Val Mazia catchment in South Tyrol (Italy). In this case a MAE between 0.05 and 0.08 and an unbiased RMSE between 0.05 and 0.09 m3·m-3 were achieved. The datasets presented can be used as input for hydrological models as well as to hydrologically characterize the study alpine area starting from different sources of information.

The complex meso- and microclimatic heterogeneity inherent to mountainous regions, driven by both topographic and biotic factors, and the lack of observations, poses significant challenges to using climate models to predict and understand impacts at various scales. We present here a six-year dataset (2017–2022) of continuous climatic measurements collected at five elevations from 983 m to 2705 m above sea level in the Val Mazia - Matschertal valley in the Italian Alps. The measurements include the air temperature, relative humidity, wind speed and direction, solar radiation, soil properties, precipitation, and snow height. Collected within the European Long-Term Ecological Research program (LTER), this dataset is freely available in an open access repository. The time series may be valuable for the validation of regional climate models, atmospheric exchange modelling, and providing support for hydrological models and remote sensing products in mountain environments. Additionally, our data may be useful for research on the influence of elevation on ecological processes such as vegetation growth, plant composition, and soil biology. Beyond its utility in advancing such fundamental research, meteorological monitoring data contribute to informed socio-political decisions on climate adaptation strategies, land management, and water resource planning, enhancing the safety and resilience of mountain communities and biodiversity.

Background and aims: A better understanding of plant carbon assimilation, water status and photosystem performance responses to combined heat and drought stress would help to optimize grapevine management under such limiting conditions. Methods: Gas exchange and chlorophyll fluorescence parameters were measured in potted grapevines, cv Sauvignon Blanc, before, during and after simulated six-day heat (T_max = 40 °C) wave using heated well-watered (HW), heated drought-stressed (HD), non-heated well-watered (CW) and non-heated dry (CD) vines. Results: Photosynthesis and stomatal conductance in HW vines increased during the morning and dropped in the afternoon with respect to CW vines. Daily plant transpiration in HW almost doubled that of CW vines. When grapevines were already exposed to drought, the effects of the heat wave were negligible, with HD plants showing similar leaf photosynthesis and transpiration to their CD counterparts. Heat, but not drought stress, decreased the maximum (Fv/Fm) and effective photochemical quantum yield of PSII (φPSII), and also affected the use of absorbed energy. HW plants dissipated more radiative energy as heat, a protective mechanism of the photosystem, while HD vines increased the energy dissipated by non-regulated non-photochemical pathways, which might lead to photoinhibition damages. The different behavior could be due to the enhanced transpiration rate and consequent decrease in leaf temperature in HW as compared to HD vines. After the heat wave, only HW vines recovered the afternoon values of photosynthesis, stomatal conductance and φPSII to similar levels as those in CW vines. Conclusion: Drought had a more significant effect than heat stress on photosynthesis, stomatal conductance and transpiration. The combined heat and drought stress, however, increased the proportion of energy lost by the leaves through harmful non-regulated dissipative pathways. With adequate soil water availability, grapevines withstood the heat wave period through an increase in leaf transpiration, which decreased leaf temperature and protected the PSII from heat damage.