Twenty-two years of warming, fertilisation and shading of subarctic heath shrubs promote secondary growth and plasticity but not primary growth

Abstract

Most manipulation experiments simulating global change in tundra were short-term or did not measure plant growth directly. Here, we assessed the growth of three shrubs (Cassiope tetragona, Empetrum hermaphroditum and Betula nana) at a subarctic heath in Abisko (Northern Sweden) after 22 years of warming (passive greenhouses), fertilisation (nutrients addition) and shading (hessian fabric), and compare this to observations from the first decade of treatment. We assessed the growth rate of current-year leaves and apical stem (primary growth) and cambial growth (secondary growth), and integrated growth rates with morphological measurements and species coverage. Primary- and total growth of Cassiope and Empetrum were unaffected by manipulations, whereas growth was substantially reduced under fertilisation and shading (but not warming) for Betula. Overall, shrub height and length tended to increase under fertilisation and warming, whereas branching increased mostly in shaded Cassiope. Morphological changes were coupled to increased secondary growth under fertilisation. The species coverage showed a remarkable increase in graminoids in fertilised plots. Shrub response to fertilisation was positive in the short-term but changed over time, likely because of an increased competition with graminoids. More erected postures and large, canopies (requiring enhanced secondary growth for stem reinforcement) likely compensated for the increased light competition in Empetrum and Cassiope but did not avoid growth reduction in the shade intolerant Betula. The impact of warming and shading on shrub growth was more conservative. The lack of growth enhancement under warming suggests the absence of long-term acclimation for processes limiting biomass production. The lack of negative effects of shading on Cassiope was linked to morphological changes increasing the photosynthetic surface. Overall, tundra shrubs showed developmental plasticity over the longer term. However, such plasticity was associated clearly with growth rate trends only in fertilised plots.

Figure 1. Growth rate of subarctic shrubs in manipulated environment.

Growth rate (bars: total aboveground vegetative growth, indicated as ‘total’; white stacks: primary growth i.e. leaves plus apical stem, indicated as ‘prim.’; grey stacks: secondary growth i.e. stem diameter increment, indicated as ‘sec.’; mean+1SE; n = 5–6) of the shrubs Cassiope tetragona, Empetrum hermaphroditum and Betula nana in a subarctic heath in Abisko (Northern Sweden) subjected to 22 years of environmental manipulation: shading (S), warming (W), fertilisation (F), combined warming plus fertilisation (WF). The control is indicated by C. The environmental factors significantly affecting growth are reported on the top left corner of each panel (shad: shading; fert: fertilisation) with the symbols + and − indicating the direction of the response, positive and negative, respectively. Note the different scale between y-axes of panel A and panel B,C.

Figure 2. Shrub height, shrub length and apical (current year's stem) increment of subarctic shrubs in manipulated environment.

Values (mean+1SE; n = 5–6) of shrub height (left panels), apical increment (central panels) and shrub length (right panel) of three subarctic heath shrubs (Cassiope tetragona, Empetrum hermaphroditum and Betula nana) in Abisko (Northern Sweden) subjected to 22 years of environmental manipulation (shading S, warming W, fertilisation F, combined warming plus fertilisation WF) against the control (C). The environmental factors significantly affecting growth are reported on the upper part of each panel (shad: shading; warm: warming; fert: fertilisation, and warm×fert: warming×fertilisation) with the symbols + and − indicating the direction of the response (positive and negative, respectively).

Figure 3. Branch number and length (according to age) of subarctic shrubs in manipulated environment.

Number and length of the branches vs. branch age (means+1SE; n = 5–6) of the shrubs Cassiope tetragona, Empetrum hermaphroditum and Betula nana at a subarctic heath in Abisko (Northern Sweden) after 22 years of environmental manipulation. Text on the top left indicates the environmental factors with a significant impact (shad: shading; warm: warming; fert: fertilisation, and warm×fert: warming×fertilisation), their direction (+: positive, −: negative) and the age of the branches affected. Note the different scale between y-axes of panel A,B,D,E and panel C,F.

Figure 4. Species coverage in subarctic heath under environmental perturbations.

Coverage (mean+1SE; n = 6) of the shrubs Cassiope tetragona, Empetrum hermaphroditum and Betula nana, the graminoids (gram.) and of the total vascular species (total vasc.) at a subarctic heath in Abisko (Northern Sweden) after 22 years of environmental manipulation: shading (S), warming (W), fertilisation (F), combined warming plus fertilisation (WF). Text on the top left indicates the environmental factors with a significant impact on coverage (shad: shading; warm: warming; fert: fertilisation, and warm×fert: warming×fertilisation) and their direction (+: positive, −: negative).