Alpine ecology, plant biodiversity and photosynthetic performance of marker plants in a nitrogen gradient induced by Alnus bushes

Abstract

Background

Alpine alder vegetation acts upon the nearby grass and dwarf shrub vegetation by the nitrogen supply from the symbiotic bacteria Frankia alni of Alnus viridis. This has been studied in two transects concerning plant distribution, plant diversity, nitrate concentration in soil and photosynthetic performance of specific marker plants.
Results

Away from the alder stand, a band of some meters was dominated by Calamagrostis varia which then was followed by alpine dwarf shrub vegetation. Nitrate in the soil showed a concentration decrease away from the alder stand leading to values near the detection limit in the dwarf shrub zone. Within these three zones, plant species were distributed according to their N-index, given in the ecological literature. Three dominant species, Calamagrostis varia, Rhododendron ferrugineum and Vaccinium myrtillus were examined at sites of different N-availability in the horizontal transect for their photosynthetic performance, by measuring the prompt fluorescence, the OJIP named polyphasic rise of chlorophyll-a fluorescence. All three plant species showed signs of stress in the fluorescence rise kinetics at decreased nitrate availability. These are similar to other known stress effects such as faster reduction of the primary acceptor or an electron supply limitation on the donor site of photosystem II.
Conclusion

Prompt chlorophyll-a fluorescence data of the examined leaves in a natural vegetation system showed the effects of a decrease in the essential nutrient nitrogen and in a manner parallel to changes in plant diversity. The selected marker plants behaved differently towards decreasing nitrogen concentrations in soil.
Background

Nitrogen is one of the main nutrients for plant growth and is considered to be the limiting factor for net primary production in terrestrial ecosystems [34, 35]. For high productivity in agriculture, nitrogen is supplied as fertilizer in the form of nitrate or ammonium. In natural pristine environments nitrogen fixing bacteria, free living or as root symbionts, provide bound nitrogen in nitrogen-poor soil. In the alpine shrub and grass zones the concentration of available nitrogen is often low; although various nitrogen compounds may be imported through the atmosphere by wind or rain. Alnus viridis is an important component of the tall shrub vegetation in the subalpine and alpine areas. Due to root symbiosis with the nitrogen-fixing Actinobacterium Frankia alni, the soil at these sites is enriched in plant-available nitrogen and a remarkable accumulation of biomass may be seen nearby. This suggests the presence of a nitrogen flow away from the alder zone into the neighboring grass and shrub zones. Thus, a clear change in plant composition and diversity away from the border of the Alnus tall forbs is often observed even with the naked eye.