Mechanisms for Success after Long-term Nutrient Enrichment in a Boreal Forest Understory

Abstract

Global levels of reactive nitrogen are predicted to rise in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Many plant communities respond strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that was initiated in 1990 has been investigating the effects of long-term nutrient enrichment (fertilizer added annually) on a boreal forest understory community. We used this experiment to investigate why some species increase in abundance under nutrient enrichment whereas others decline. We focused on four species that differed in their responses to fertilization: Mertensia paniculata and Epilobium angustifolium increased in abundance, Achillea millefolium remained relatively constant and Festuca altaica declined. We hypothesized that the two species that were successful in the new high-nutrient, light-limited environment would be taller, have higher specific leaf area, change phenology by growing earlier in the season and be more morphologically plastic than their less successful counterparts. We compared plant height, specific leaf area, growth spurt date and allocation to leaves in plants grown in control and fertilized plots. We demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; M. paniculata has the highest specific leaf area of the four species whereas E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaves. These results indicate that rather than one strategy determining success when nutrients become available, a variety of traits and responses may contribute to a species' ability to persist in a nutrient-enriched boreal forest understory.

Nutrient loaded seedlings reduce the need for field fertilization and vegetation management on boreal forest reclamation sites

“…While some understory species can be particularly beneficial on reclamation sites, early successional and ruderal species often dominate restoration areas and can severely compete with the planted tree species (Macdonald et al 2015a). Generally, northern boreal forests are nutrient limited environments where nutrient amendments could favour nitrophilous grass and forb species which have rapid growth rates, large leaf areas and are significant competitors to both young trees and other native vegetation (Landhäusser and Lieffers 1998;Grainger and Turkington 2013;Hedwall et al 2013). As a result, field fertilization of boreal forest ecosystems is controversial as it can lead to a decline in plant species richness (Grainger and Turkington 2013;Gilliam 2007) and uncertain tree growth responses.…”

Effects of nitrogen fertilization on the ground vegetation cover and soil chemical properties in Scots pine and Norway spruce stands

“…However, a variable response to N fertilization has been observed among the grasses and herbs. Some species are favored, and other species suffer following fertilization (Turkington et al 1998), reflecting the different functional traits of the species (Grainger and Turkington 2013). The increased vascular plant cover we observed was less than the increase found after clear cutting, in response to increased nutrients and light (Palviainen et al 2005b).…”