Why do nitrogen pollute boreal forests?
Nitrogen supply normally limits plant growth in high latitude ecosystems, like the boreal forest. During recent decades the burning of fossil fuels and the increased use of industrial fertilizers in agriculture and forestry have however resulted in that nitrogen input to the biosphere has doubled. Biologically active nitrogen emitted to the atmosphere can be transported long distances and thus becomes a regional environmental problem. Nitrogen deposited from the atmosphere to sensitive natural ecosystems like heaths and 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. Performance of Vaccinium under high N supply is further impaired by the fact that Vaccinium growth declines from yearly repeated Valdensia heterodoxa infections. 

Main scopes of future research
A central objective of the project is to provide knowledge useful for decision makers who impact 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. In addition we will study processes of vegetation recovery following decreased nitrogen pollution. We aim to conclude our results in a dynamic model where the distribution of Vaccinium species in boreal forests under different nitrogen pollution scenarios can be modeled.

Selected publications

 

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

 

Nordin, A., Högberg, P. and 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. and Ericson, L. () Slow recovery of boreal forest ecosystem following decreased nitrogen input. Functional Ecology 15: 451-457.

 

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

 

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

 

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