Air pressure effects on upwards shifting alpine ecosystems

For many organisms, climate change alters the maximum tolerable elevation and very often leads to an upward shift in occurrence. Montane organisms that migrate upwards to follow their thermal niche experience no temperature change, but have to cope with a reduced air pressure. This affects biologically relevant physical parameters such as vapor pressure deficit, CO2 partial pressure and gas diffusion, which in turn are involved in fundamental ecophysiological processes such as evapotranspiration, photosynthesis or respiration. These parameters have both direct effects on species and also indirect effects, as the way species and/or communities interact with each other might change. The effects of reduced atmospheric pressure on upward migrating plants and soil microorganisms and their novel associations are largely unknown, so that this hitherto neglected environmental issue urgently needs to be investigated. UPSHIFT will fill this knowledge gap by applying novel experimental approaches to study the effects of atmospheric pressure on ecophysiology and interactions between organisms. Our main goals are (i) to understand how upwards migrating soil microorganisms and plant species respond to lower atmospheric pressure, (ii) to assess how upwardly migrating soil microorganisms and plants interact with persistent plants and soil microorganisms and (iii) to assess the impact of lower air pressure on the water balance of the entire ecosystem. We will measure various ecophysiological, chemical and microbi al indicators to gain insight into fundamental processes such as water use, photosynthesis and growth. Our approach integrates a controlled environmental system of the next generation (terraXcube, Bozen) to simulate different alpine climatic conditions and to be able to investigate the effects of temperature and air pressure separately with pot and mesocosm (lysimeter) experiments. Finally, field experiments are carried out to evaluate the reliability of the results obtained. The UPSHIFT project is the first attempt known so far to determine the effects of atmospheric pressure on (micro)organisms in the context of climate change. This is of fundamental importance because of the upward migrating organisms and the profound consequences for the functioning of alpine ecosystems. UPSHIFT is based on a transnational cooperation between the Department of Microbiology at the University of Innsbruck (Paul Illmer) and the Institute for Alpine Environment at the Eurac Research, Bozen (Matteo Dainese).

Climate change is altering the upper elevational limits that many organisms can tolerate, frequently driving an upward shift in the distribution of montane species. Organisms migrating to higher elevations in order to track their thermal niche may experience no change in temperature; however, they are inevitably exposed to reduced atmospheric pressure. Lower pressure affects several biologically relevant physical parameters, including vapor pressure deficit, CO partial pressure, and gas diffusion rates. Despite their potential importance, the direct effects of reduced atmospheric pressure, as well as possible indirect effects, on upward-migrating plants and soil microorganisms have remained largely unexplored. Addressing this knowledge gap was the central objective of the UPSHIFT project. Beyond investigating individual species responses, UPSHIFT examined how reduced atmospheric pressure affects organismal interactions, particularly between plants and soil microorganisms in the rhizosphere. To address all these questions, the project combined field experiments conducted at an LTSER site in the Mazia Valley (South Tyrol, Italy; 1,500 m a.s.l.) with laboratory studies and, most importantly, experiments performed in the state-of-the-art TerraXCube climate-pressure chambers. These facilities enabled the investigation of effects of altered atmospheric pressure equivalent to elevations ranging from 250 to 4,000 m a.s.l. while maintaining all other abiotic factors-including temperature, soil moisture, and vapor pressure deficit-constant. The experiments focused on representative plant species of the genera Trifolium, Hieracium, and Brachypodium (clovers, hawkweeds, and false brome grasses), as well as the microorganisms inhabiting their rhizosphere. The results revealed significant and species-specific effects of reduced atmospheric pressure on both plant performance and microbial growth and activity. For example, microbial activity and microbial biomass in the rhizosphere of Trifolium species increased significantly with decreasing pressure (corresponding to increasing elevation), whereas a significant decline was observed in association with Hieracium species. Respective findings were independently confirmed using a range of bacterial and fungal pure cultures. The project therefore provided the first and clear evidence that reduced atmospheric pressure can exert direct effects on soil microorganisms. However, reduced pressure influenced not only the abundance and activity of soil microorganisms but we could also show, that the composition of the entire microbial community as well as the rate and success of microbial colonization of new soil habitats changes with altered pressure. Again, these findings are the first of their kind and are of particular significance in the context of climate change. In alpine regions, rising soil temperatures are driving plants and animals-and likely microorganisms-to shift their distributions towards higher elevations. The results obtained within this project suggest that such range shifts may give rise to novel interactions between plants and rhizosphere microorganisms, with potentially far-reaching consequences for the functioning and biodiversity of mountain ecosystems.