Effects of environmental perturbations on abundance of subarctic plants after three, seven and ten years of treatments

Abstract

Analyses of changes in vegetation were carried out after three, seven and ten years of fertilizer addition, warming and light attenuation in two subarctic, alpine dwarf shrub heaths. One site was just above the tree line, at ca 450 m a.s.l., and the other at a much colder fell‐field at ca 1150 m altitude. The aim was to investigate how the treatments affected the abundance of different species and growth forms over time, including examinations of transient changes. Grasses, which increased in abundance by fertilizer addition, and cryptogams, which, by contrast, decreased by fertilizer addition and warming, were the most sensitive functional groups to the treatments at both sites. Nutrient addition exerted a stronger and more consistent effect than both shading and warming. Warming at the fell‐field had slightly greater effect than at the warmer tree line with an increase in deciduous shrubs. The decreased abundance of mosses and lichens to fertilizer addition and/or warming was most likely an indirect treatment effect, caused by competition through increased abundance and overgrowth of grasses. Such changes in species composition are likely to alter decomposition rates and the water and energy exchange at the soil surface. We observed few, if any, transient effects of declining responses during the 10 yr of treatments. Instead, there were many cumulative effects of the treatments for all functional groups and many interactions between time and treatment, suggesting that once a change in community composition is triggered, it will continue with unchanged or accelerated rate for a long period of time.

Modelling Tundra Vegetation Response to Recent Arctic Warming

“…Plants invest part of their additional production in a higher leaf area and in horizontal and vertical canopy growth, increasing light interception and further augmenting production. The simulated productivity increase is consistent with results from tundra warming experiments (Michelsen et al 1996;Arft et al 1999;Graglia et al 2001;Walker et al 2006;Olsrud et al 2010), and productivity trends inferred from satellites (Bunn and Goetz 2006;Beck and Goetz 2011), while the increase in LAI is consistent with a positive trend in remotely sensed NDVI across the Arctic tundra biome over the same period ( Fig. 3; Bhatt et al 2010).…”

“…Despite numerous local exceptions, the weight of evidence from observational studies suggests that, in general, Arctic vegetation is responding to rising temperatures through increases in productivity, density, cover and stature of vegetation and, in many areas, an increase in woody biomass and the representation of trees and shrubs (Post et al 2009;Callaghan et al 2011;Elmendorf et al 2012). These findings are qualitatively consistent with expectations based on the results of tundra warming experiments (Chapin et al 1995;Michelsen et al 1996;Arft et al 1999;Graglia et al 2001;Walker et al 2006;Olsrud et al 2010), and simulations using vegetation models (Kaplan et al 2003;Smith et al 2008a;Wolf et al 2008a;Wramneby et al 2010).…”

Two Decades of Experimental Manipulations of Heaths and Forest Understory in the Subarctic

“…The experiments in the forest understory (Press et al 1998), the wet (Sorensen and Michelsen 2011) and the mesic (Graglia et al 2001) heaths and the high altitude fellfield have shown that both warming and, in particular, enhanced nutrient availability promote a higher biomass of dwarf shrubs, but that subcanopy mosses and lichens decline strongly as the plant canopy closes. Hence, plant species diversity is expected to be reduced in subarctic heaths following warming because of loss of cryptogams, while plant C accumulation is expected to increase as woody plants with large stem masses increase their cover.…”

Dwarf shrubs are stronger competitors than graminoid species at high nutrient supply in peat bogs

“…In a tundra ecosystem in Northern Sweden, Graglia et al (2001) showed in a 10-year fertilization experiment that grasses increased in abundance in response to fertilizer application. Similarly, Van Wijk et al (2003) reported that the biomass of deciduous shrub and graminoid plant types responded most strongly to nutrient addition on sites in Alaska and Northern Sweden during a long-term fertilization experiment ([3 year).…”

“…In environments with sufficient nutrients, plants with a high potential growth rate and a high leaf area ratio (the leaf area per unit total plant biomass) will have advantages over plants with lower potential growth rate (Berendse 1994;Lambers et al 1998). This might explain why graminoids often increase in abundance after the application of fertilizer (Graglia et al 2001;Van Wijk et al 2003;Bret-Harte et al 2004. In bog ecosystems, a shift in dominance from one plant functional group toward another might have great impact.…”