Abstract
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 Larix decidua and Pinus uncinata at treeline in the Swiss Alps. We monitored vegetative and reproductive phenology of Vaccinium myrtillus, Vaccinium gaultherioides and Empetrum hermaphroditum 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 V. myrtillus. Our results show that phenology of the three species was more influenced by snowmelt timing, and also by plot tree species (Larix or Pinus) in the case of V. myrtillus, than by soil warming. However, the warming treatment led to increased V. myrtillus 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 V. myrtillus under soil warming that was sustained over six years and was not caused by an extended growing period in early summer. In contrast, E. hermaphroditum only showed a positive shoot growth response to warming in 2011 (+21%), and V. gaultherioides showed no significant growth response. Our results indicate that V. myrtillus might have a competitive advantage over the less responsive co-occurring dwarf shrub species under future global warming.
Highlights
Alpine and Arctic ecosystems are predicted to be especially vulnerable to global warming [1,2] because plant growth and performance in these environments are strongly constrained by low temperature, short growing seasons and frequent freezing events during the snow-free period
Our aims were (i) to determine whether a positive growth response in Vaccinium myrtillus was maintained after six years of soil warming, and whether Vaccinium gaultherioides and Empetrum hermaphroditum showed any delayed increases in growth after this full experimental period
There were no differences in the date of snowmelt between soil warming treatments (F1,9 = 0.64, P = 0.447) or plot tree species (F1,18 = 1.04, P = 0.321)
Summary
Alpine and Arctic ecosystems are predicted to be especially vulnerable to global warming [1,2] because plant growth and performance in these environments are strongly constrained by low temperature, short growing seasons and frequent freezing events during the snow-free period. Community changes are of particular importance in alpine and Arctic ecosystems due to their potential effects on climate feedbacks through shifts in plant community composition and plant species cover [7,8]. Experimental studies have shown that responses of plants growing at high latitude and high elevation to warming are species-specific [9,10,11], demonstrating that understanding how plant community dynamics might change with higher temperatures can only be achieved through assessing responses of individual species. Species-specific studies lasting several years are crucial for understanding community responses to global warming
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