Biomedical subjects

Stephan Hättenschwiler

Publications and source records attributed to Stephan Hättenschwiler.

5 recordsLinked to original sources

PubMed2005

Hemiparasite abundance in an alpine treeline ecotone increases in response to atmospheric CO(2) enrichment.

Populations of the annual hemiparasites Melampyrum pratense L. and Melampyrum sylvaticum L. were studied at the treeline in the Swiss Alps after 3 years of in situ CO(2) enrichment. The total density of Melampyrum doubled to an average of 44 individuals per square meter at elevated CO(2) compared to ambient CO(2). In response to elevated CO(2), the height of the more abundant and more evenly distributed M. pratense increased by 20%, the number of seeds per fruit by 21%, and the total seed dry mass per fruit by 27%, but the individual seed size did not change. These results suggest that rising atmospheric CO(2) may stimulate the reproductive output and increase the abundance of Melampyrum in the alpine treeline ecotone. Because hemiparasites can have important effects on community dynamics and ecosystem processes, notably the N cycle, changing Melampyrum abundance may potentially influence the functioning of alpine ecosystems in a future CO(2)-rich atmosphere.

Atmosphere↗

PubMed2005

Carbon flux and growth in mature deciduous forest trees exposed to elevated CO2.

Whether rising atmospheric carbon dioxide (CO2) concentrations will cause forests to grow faster and store more carbon is an open question. Using free air CO2 release in combination with a canopy crane, we found an immediate and sustained enhancement of carbon flux through 35-meter-tall temperate forest trees when exposed to elevated CO2. However, there was no overall stimulation in stem growth and leaf litter production after 4 years. Photosynthetic capacity was not reduced, leaf chemistry changes were minor, and tree species differed in their responses. Although growing vigorously, these trees did not accrete more biomass carbon in stems in response to elevated CO2, thus challenging projections of growth responses derived from tests with smaller trees.

Atmosphere↗

PubMed2005

Soil animals alter plant litter diversity effects on decomposition.

Most of the terrestrial net primary production enters the decomposer system as dead organic matter, and the subsequent recycling of C and nutrients are key processes for the functioning of ecosystems and the delivery of ecosystem goods and services. Although climatic and substrate quality controls are reasonably well understood, the functional role of biodiversity for biogeochemical cycles remains elusive. Here we ask how altering litter species diversity affects species-specific decomposition rates and whether large litter-feeding soil animals control the litter diversity-function relationship in a temperate forest ecosystem. We found that decomposition of a given litter species changed greatly in the presence of litters from other cooccurring species despite unaltered climatic conditions and litter chemistry. Most importantly, soil fauna determined the magnitude and direction of litter diversity effects. Our data show that litter species richness and soil fauna interactively determine rates of decomposition in a temperate forest, suggesting a combination of bottom-up and top-down controls of litter diversity effects on ecosystem C and nutrient cycling. These results provide evidence that, in ecosystems supporting a well developed soil macrofauna community, animal activity plays a fundamental role for altered decomposition in response to changing litter diversity, which in turn has important implications for biogeochemical cycles and the long-term functioning of ecosystems with ongoing biodiversity loss.

Animals↗

PubMed2004

Growth and reproduction of the alpine grasshopper Miramella alpina feeding on CO2-enriched dwarf shrubs at treeline.

The consequences for plant-insect interactions of atmospheric changes in alpine ecosystems are not well understood. Here, we tested the effects of elevated CO(2) on leaf quality in two dwarf shrub species (Vaccinium myrtillus and V. uliginosum) and the response of the alpine grasshopper (Miramella alpina) feeding on these plants in a field experiment at the alpine treeline (2,180 m a.s.l.) in Davos, Switzerland. Relative growth rates (RGR) of M. alpina nymphs were lower when they were feeding on V. myrtillus compared to V. uliginosum, and were affected by elevated CO(2) depending on plant species and nymph developmental stage. Changes in RGR correlated with CO(2)-induced changes in leaf water, nitrogen, and starch concentrations. Elevated CO(2) resulted in reduced female adult weight irrespective of plant species, and prolonged development time on V. uliginosum only, but there were no significant differences in nymphal mortality. Newly molted adults of M. alpina produced lighter eggs and less secretion (serving as egg protection) under elevated CO(2). When grasshoppers had a choice among four different plant species grown either under ambient or elevated CO(2), V. myrtillus and V. uliginosum consumption increased under elevated CO(2) in females while it decreased in males compared to ambient CO(2)-grown leaves. Our findings suggest that rising atmospheric CO(2) distinctly affects leaf chemistry in two important dwarf shrub species at the alpine treeline, leading to changes in feeding behavior, growth, and reproduction of the most important insect herbivore in this system. Changes in plant-grasshopper interactions might have significant long-term impacts on herbivore pressure, community dynamics and ecosystem stability in the alpine treeline ecotone.

Analysis of Variance↗

PubMed2002

Nitrogen and nature.

Anthropogenic changes to the global N cycle are important in part because added N alters the composition, productivity, and other properties of many natural ecosystems substantially. Why does added N have such a large impact? Why is N in short supply in so many natural ecosystems? Processes that slow the cycling of N relative to other elements and processes that control ecosystem-level inputs and outputs of N could cause N supply to limit the dynamics of ecosystems. We discuss stoichiometric differences between terrestrial plants and other organisms, the abundance of protein-precipitating plant defenses, and the nature of the C-N bond in soil organic matter as factors that can slow N cycling. For inputs, the energetic costs of N fixation and their consequences, the supply of nutrients other than N, and preferential grazing on N-fixers all could constrain the abundance and/or activity of biological N-fixers. Together these processes drive and sustain N limitation in many natural terrestrial ecosystems.

Animals↗