Responses of a subarctic dwarf shrub heath community to simulated environmental change

Abstract

Summary 0 A dwarf shrub heath in subarctic Sweden was subjected to factorial manipulation of air temperature\ water and nutrient supply for 4 years[ The responses of the vegetation to the perturbations were then assessed by point intercept "quadrat# analy! sis followed by determination of above!ground biomass[ 1 Nineteen vascular and 12 non!vascular species "or species groups# were recorded and the most dramatic response was that of the grass Calama`rostis lapponica to nutrient addition\ with abundance being stimulated by a factor of more than 07 compared with plots not subjected to nutrient addition[ Calama`rostis lapponica did not show any signi_cant responses to temperature or water alone but there was a synergistic interaction between all three variables[ 2 The abundance of the dominant dwarf shrubs "Empetrum hermaphroditum\ Vac! cinium vitis!idaea\ V[ myrtillus and V[ uli`inosum# was una}ected by the perturbations but elevated temperature stimulated the biomass of V[ uli`inosum by 014) and total shrub biomass by 05)[ The low ratio of current year|s growth to standing biomass may have concealed other responses of the dwarf shrub group to the perturbations[ 3 The response of the non!vascular~ora to the perturbations was either neutral or negative\ with lichens showing the most dramatic responses[ Lichen biomass on temperature! and nutrient!amended plots was 45) and 07)\ respectively\ of that on unperturbed plots[ Nutrients also exerted a negative e}ect on the biomass of bryophy! tes\ and the combined biomass of lichens and bryophytes on nutrient!treated plots was almost a third of that on plots which did not receive additional nutrients[ 4 Total above!ground biomass was not a}ected by the perturbations but the total number of interceptions determined by point quadrat analysis was greater on the elevated temperature and nutrient!treated plots[ Point quadrat analysis also revealed an accumulation of litter and standing dead material in response to the nutrient and temperature perturbations\ both singly and in combination\ suggesting a faster turnover of plant material[ 5 Both temperature and nutrients increased canopy height and also interacted syn! ergistically such that together they resulted in a mean canopy height of 03[8 cm compared with 7[9 cm in plots subjected to neither perturbation[ 6 Nutrient addition lowered species richness by 06[6)\ mainly through its impact on the mosses and lichens[ 7 In general\ nutrient addition elicited the greatest response\ followed by temperature\ with water exerting little measurable in~uence[ There were a number of important interactions that were often synergistic\ and some involved water[ 8 Species| responses were highly individualistic and changes in the community were mediated through the response of a small number of key species already present in the community\ with no invasion of new species[ In the short term at least\~oristic diversity may decline as understorey species become less abundant and immigration by new species is inhibited by the dominance of clonal a...

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.…”

“…Consequently, if these responses persist in the fellfield, the changes could lead to lower tissue quality for herbivores and reduced decomposition, because of higher recalcitrance of litter. Nutrient addition led to increased vascular plant biomass in several subarctic ecosystem types (Press et al 1998;Jonasson et al 1999), but plant biomass changes may be dampened due to immobilization of nutrients in microbial biomass ). These results suggest that the strong microbial nutrient sink in subarctic and alpine heaths may mitigate changes in the plant biomass even if warming increases the mineralization rate.…”

“…Similar greenhouses and open-top chambers (OTCs) have also been used in warming experiments near Abisko in mountain birch (Betula pubescens ssp. czerepanovii) forest understory (Press et al 1998;Sjögersten and Wookey 2009;Olsrud et al 2010), alpine fellfield heath , wet heath (Fig. 1b), higher altitude dry heat, meadow and tussock tundra (Björk et al 2007;Molau 2010) and a peat bog (Dorrepaal et al 2009) and typically lead to 1-2°C soil warming, and to 3-4°C air warming above ambient summer temperature.…”

“…In addition to warming treatments, nutrients have frequently been added to experimental plots (Press et al 1998;Jonasson et al 1999). In high doses, responses to such additions increase our understanding of how nutrients act as drivers in these, often nutrient-poor ecosystems.…”

Seasonal partitioning of resource use and constraints on the growth of soil microbes and a forage grass in a grazed Arctic salt-marsh

“…Additions of nitrogen and phosphorus to Arctic soils results in a marked increase in above-ground plant biomass, particularly when applied to graminoid communities indicating that these nutrients are in limited supply (Shaver and Chapin 1980;Chapin and Shaver 1985;Chapin et al 1986;Jonasson 1992;Press et al 1998). In contrast, Jonasson et al (1996) showed that soil microbial biomass failed to increase in response to additions of N, P and potassium, although microbes immobilized large amounts of the nutrients.…”