Why do nitrogen pollute boreal forests?

Nitrogen supply normally limits plant growth in high latitude ecosystems, like the boreal forest. However, during recent decades the burning of fossil fuels and the increased use of industrial fertilizers in agriculture and forestry have resulted in deposition of nitrogen pollutants over natural ecosystems. Nitrogen deposited to sensitive natural ecosystems boreal forests will have large consequences for the structure and function of these ecosystems. Plant and bryophyte species proliferating from high nitrogen supply often exclude species that normally dominate the vegetation. We have found that nitrogen induced vegetation changes in boreal forests can persist for a considerable time (at least c. 50 years) even though nitrogen input to the ecosystem ceases.

 

Summary of results 
We have demonstrated that models of inter-specific plant species competition cannot alone predict nitrogen-induced changes in boreal forest understorey vegetation. This because plant pathogen and herbivore attacks can trigger vegetation changes. Plant biochemistry defines the substrate quality for plant pathogens and herbivores and proliferation of microorganisms and herbivores attacking plants are in many ecosystems limited by nitrogen. Plant susceptibility to attack from natural enemies thus increases if plant nitrogen concentrations increases. In boreal forests understorey vegetation dominated by Vaccinium myrtillus L. is common. In this type of forest we found that nitrogen addition caused glutamine concentrations of Vaccinium leaves to increase. This resulted in increased disease incidence of the common fungal leaf pathogen Valdensia heterodoxa. The fungus causes a brown spot disease on the leaves of its host, followed by premature leaf loss. In the Vaccinium cover this results in visible patches of leaf-less plants. In these patches of leaf-less Vaccinium the common grass, Deschampsia flexuosa (L.) Trin can proliferate due to the increased light penetration supporting grass establishment beneath the Vaccinium cover. Valdensia heterodoxa attack hence initiates the proliferation of Deschampsia in nitrogen exposed boreal forest. When established the rate of proliferation depends on both the amount and the form of the added nitrogen, as Deschampsia grows better on nitrate than on ammonium. In contrast, Vaccinium growth is not enhanced by nitrogen addition independent of whether nitrogen is added as ammonium or nitrate.

 

Main scopes of future research
A central objective of the project is to provide knowledge useful for decision makers who determine nitrogen emission rates over Europe. The data we produce should thus support the process determining critical loads of nitrogen to boreal forest ecosystems. (The critical load of nitrogen is the amount of nitrogen pollution the ecosystem can sustain without the occurrence of harmful effects.) In order to achieve this objective we will continue to explore mechanisms directing nitrogen induced vegetation changes. Our research will explore biochemical and ecological interactions between plants and their natural enemies as well as quantitative and qualitative aspects of soil – plant relations in N exposed boreal forests. Especially we are interested in how qualitative changes in the nitrogen supply can affect species distribution of vascular plants as well as bryophytes and lichens.

 

 

Nordin, A., Schmidt, I. K. & Shaver, G. R. . Nitrogen uptake by arctic soil microbes and plants in relation to soil nitrogen supply. Ecology 85: in press.

 

Strengbom, J., Nordin, A., Näsholm, T. & Ericson, L. . Parasitic fungus mediates vegetational changes in nitrogen exposed boreal forest. Journal of Ecology 90:61-67.

 

Nordin, A., Högberg, P. & Näsholm, T. . Soil N form and plant N uptake along a boreal forest productivity gradient. Oecologia 129: 125-132.

 

Strengbom, J., Nordin, A., Näsholm, T. & Ericson, L. . Slow recovery of boreal forest ecosystem following decreased nitrogen input. Functional Ecology15: 451-457.

 

Nordin, A., Uggla, C. & Näsholm, T. . Nitrogen forms in bark, wood and foliage of nitrogen fertilized Pinus sylvestris L. Tree Physiology 21: 59-64.

 

Nordin, A. & Gunnarsson, U. . Amino acid accumulation and growth of Sphagnum under different levels of N deposition. Ecoscience 7: 474-480.

 

Nordin, A., Näsholm, T. & Ericson, L. . Effects of simulated N deposition on understorey plants of a boreal coniferous forest. Functional Ecology 12: 691-699.