Appendix A. A figure showing the mean air temperature and the mean snowmelt date during the growing season.

A figure showing the mean air temperature and the mean snowmelt date during the growing season

Appendix C. Decision trees for mortality in the periods 1975–1985 and 1975–1995.

Decision trees for mortality in the periods 1975–1985 and 1975–1995

Appendix B. A figure showing the mean spatial variation of snowmelt date along the elevation gradient across Stillberg.

A figure showing the mean spatial variation of snowmelt date along the elevation gradient across Stillberg

Appendix D. Decision trees for height growth in the three 10-year periods studied.

Decision trees for height growth in the three 10-year periods studied

Shrub growth and plant diversity along an elevation gradient: Evidence of indirect effects of climate on alpine ecosystems

<div><p>Enhanced shrub growth and expansion are widespread responses to climate warming in many arctic and alpine ecosystems. Warmer temperatures and shrub expansion could cause major changes in plant community structure, affecting both species composition and diversity. To improve our understanding of the ongoing changes in plant communities in alpine tundra, we studied interrelations among climate, shrub growth, shrub cover and plant diversity, using an elevation gradient as a proxy for climate conditions. Specifically, we analyzed growth of bilberry (<i>Vaccinium myrtillus</i> L.) and its associated plant communities along an elevation gradient of ca. 600 vertical meters in the eastern European Alps. We assessed the ramet age, ring width and shoot length of <i>V</i>. <i>myrtillus</i>, and the shrub cover and plant diversity of the community. At higher elevation, ramets of <i>V</i>. <i>myrtillus</i> were younger, with shorter shoots and narrower growth rings. Shoot length was positively related to shrub cover, but shrub cover did not show a direct relationship with elevation. A greater shrub cover had a negative effect on species richness, also affecting species composition (beta-diversity), but these variables were not influenced by elevation. Our findings suggest that changes in plant diversity are driven directly by shrub cover and only indirectly by climate, here represented by changes in elevation.</p></div

Effect of elevation on <i>Vaccinium myrtillus</i> traits (a, b, c), of <i>V</i>. <i>myrtillus</i> shoot length on shrub cover (d), and of shrub cover on species richness (e) and beta-diversity (f).

<p>Plots show the results of general linear mixed-effects models (a-e) and regression on distance matrices (i.e. floristic dissimilarity vs differences in shrub cover among all the pairwise combinations of the plots) (f). Confidence intervals (95%) are also shown (a-e). In the regression on distance matrices (f), the density of paired plots is represented by the intensity of the background color (smoothed scatter plot).</p

Structural equation model diagram showing the hypothesized relationships among elevation (gray box) and <i>Vaccinium myrtillus</i> growth traits (a, purple boxes), shrub cover (b, yellow boxes) and species richness (c, blue boxes).

<p>Solid lines indicate significant relationships (p<0.05), whereas dashed lines indicate tested relationships that were not statistically significant. Standardized effect size (i.e. scaled by mean and variance) of significant variables and conditional coefficient of determination (r²_c) are also shown in the boxes.</p

Growth and Phenology of Three Dwarf Shrub Species in a Six-Year Soil Warming Experiment at the Alpine Treeline

<div><p>Global warming can have substantial impacts on the phenological and growth patterns of alpine and Arctic species, resulting in shifts in plant community composition and ecosystem dynamics. We evaluated the effects of a six-year experimental soil warming treatment (+4°C, 2007–2012) on the phenology and growth of three co-dominant dwarf shrub species growing in the understory of <i>Larix decidua</i> and <i>Pinus uncinata</i> at treeline in the Swiss Alps. We monitored vegetative and reproductive phenology of <i>Vaccinium myrtillus</i>, <i>Vaccinium gaultherioides</i> and <i>Empetrum hermaphroditum</i> throughout the early growing season of 2012 and, following a major harvest at peak season, we measured the biomass of above-ground ramet fractions. For all six years of soil warming we measured annual shoot growth of the three species and analyzed ramet age and xylem ring width of <i>V. myrtillus</i>. Our results show that phenology of the three species was more influenced by snowmelt timing, and also by plot tree species (<i>Larix</i> or <i>Pinus</i>) in the case of <i>V. myrtillus</i>, than by soil warming. However, the warming treatment led to increased <i>V. myrtillus</i> total above-ground ramet biomass (+36% in 2012), especially new shoot biomass (+63% in 2012), as well as increased new shoot increment length and xylem ring width (+22% and +41%, respectively; average for 2007–2012). These results indicate enhanced overall growth of <i>V. myrtillus</i> under soil warming that was sustained over six years and was not caused by an extended growing period in early summer. In contrast, <i>E. hermaphroditum</i> only showed a positive shoot growth response to warming in 2011 (+21%), and <i>V. gaultherioides</i> showed no significant growth response. Our results indicate that <i>V. myrtillus</i> might have a competitive advantage over the less responsive co-occurring dwarf shrub species under future global warming.</p></div

Vegetative and flowering phenology for the three dwarf shrub species studied.

<p>Vegetative and flowering phenology for the three dwarf shrub species studied. Circles represent the mean day of year (±1 SE) of each phenophase for the four different soil warming (open circles, unwarmed; closed circles, warmed) and plot tree species (larch or pine) combinations (n = 10). Crosses show significant differences between plot tree species (<i>P</i><0.05) and marginally significant differences (0.05≤<i>P</i><0.10) between soil warming treatments are given by asterisks in parentheses. Colons show treatment interactions. Note that in some cases symbols for different soil warming treatments overlap completely. Dotted lines show the date when the warming treatment started in 2012 and arrows on the x-axes show the mean snowmelt date for all plots with the same tree species.</p

Xylem ring widths of <i>Vaccinium myrtillus</i>.

<p>Xylem ring widths of <i>Vaccinium myrtillus</i> from 2007 to 2011 for warmed and unwarmed plots, pooled across plots containing a larch or pine tree (mean values ±1 SE, n = 20). Asterisks show significant differences between soil warming treatments (<i>P</i><0.05). Pre-warming values are shown in the shaded region (2005–2006).</p