Abstract
Treelines are sensitive to environmental changes, but few studies provide a mechanistic approach to understand treeline dynamics based on field experiments. The aim of this study was to determine how changes in the abiotic and/or biotic conditions associated with global change affect the performance of tree seedlings (later saplings) at the treeline in a 10-year experiment. A fully factorial experiment in the Central Pyrenees was initiated in autumn 2006; 192 Pinus uncinata seedlings were transplanted into microplots with contrasting environmental conditions of (1) increased vs. ambient temperature, (2) increased nutrient availability vs. no increase, and (3) presence vs. absence of the dominant shrub Rhododendron ferrugineum. We assessed the performance of young pines on several occasions over 10 years. The pines were removed at the end of the experiment in autumn 2016 to characterize their morphology and to conduct chemical and isotopic analyses on their needles. Both the warming and the fertilization treatments increased seedling growth soon after the start of the experiment. R. ferrugineum facilitated the survival and development of pine seedlings during the early years and affected the chemical composition of the needles. Toward the end of the experiment, the transplanted P. uncinata individuals, by then saplings, competed with R. ferrugineum for light and nutrients; the presence of the shrub probably altered the strategy of P. uncinata for acquiring nutrients and buffered the effects of warming and fertilization. The pines were highly sensitive to all factors and their interactions throughout the entire experimental period. These findings indicated that the interactive effects of several key abiotic and biotic drivers associated with global change should be investigated simultaneously for understanding the contribution of young trees to treeline dynamics.
Highlights
Treeline ecotones are highly sensitive to climatic warming, because air and soil temperatures limit growth at high elevations and latitudes, where the growing season is generally short (Holtmeier, 2009; Körner, 2012)
Other factors may shape treeline dynamics: abiotic factors such as wind velocity, solar radiation, and duration of snow cover (Wipf et al, 2009); and increases in nitrogen (N) deposition (Holtmeier and Broll, 2005), atmospheric CO2 concentration (Hättenschwiler and Zumbrunn, 2002; Handa et al, 2006), and ozone concentration (Díaz-deQuijano et al, 2012; Huttunen and Manninen, 2013); and biotic factors such as plant–plant interactions (Germino et al, 2002; Grau et al, 2012; Liang et al, 2016; Lyu et al, 2016), dispersal patterns (Vetaas and Grytnes, 2002), damage caused by herbivory (Munier et al, 2010), and changes in land use (Hofgaard, 1997)
This study found that treeline dynamics were driven by complex environmental interactions amongst these factors and that facilitation, competition, herbivory, and environmental changes at the tree seedling stage acted as important filters in structuring the treeline ecotone
Summary
Treeline ecotones are highly sensitive to climatic warming, because air and soil temperatures limit growth at high elevations and latitudes, where the growing season is generally short (Holtmeier, 2009; Körner, 2012). Other factors may shape treeline dynamics: abiotic factors such as wind velocity, solar radiation, and duration of snow cover (Wipf et al, 2009); and increases in nitrogen (N) deposition (Holtmeier and Broll, 2005), atmospheric CO2 concentration (Hättenschwiler and Zumbrunn, 2002; Handa et al, 2006), and ozone concentration (Díaz-deQuijano et al, 2012; Huttunen and Manninen, 2013); and biotic factors such as plant–plant interactions (facilitation or competition for abiotic resources) (Germino et al, 2002; Grau et al, 2012; Liang et al, 2016; Lyu et al, 2016), dispersal patterns (Vetaas and Grytnes, 2002), damage caused by herbivory (Munier et al, 2010), and changes in land use (Hofgaard, 1997). The underlying factors that cause treeline shifts are not yet fully understood
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