Alpine treelines are expected to move upslope with a warming climate. However, so far treelines have responded inconsistently and future shifts remain difficult to predict since many factors unrelated to temperature, such as biotic interactions, affect responses at the local scale. Especially during the earliest regeneration stages, trees can be strongly influenced by alpine vegetation via both competition and facilitation. We aimed to understand the relative importance of these two types of interaction in different vegetation structures for treeline regeneration dynamics. Effects of herbaceous alpine vegetation on seedling emergence and first-year performance were studied in a field experiment in the French Alps (2100 m a.s.l.) with five important European treeline tree species: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia. Total emergence and locally-germinated seedling survival were not affected, but for seedlings planted at two months of age, negative vegetation impacts dominated for all response parameters: first-year survival, growth and carbohydrate accumulation. However, in the winter half-year, evergreen tree seedlings increased carbohydrate reserves under the protection of senescent herbs. Also, responses of locally-germinated seedlings suggest facilitative vegetation effects in the first two months after emergence. Thus, the interaction switched between competition and facilitation according to ontogenetic stage and seasons. Still, the net outcome after one year was negative, but species differed in their susceptibilities. Because initial establishment is the first bottleneck determining whether treelines remain stable or move upslope, understanding establishment, including site-, life-stage and species-specific processes, is essential for understanding observed treeline spatial patterns and dynamics. When developing predictive models of treeline dynamics, all these ‘local’ aspects should be incorporated in addition to more global drivers like changes in temperature.
Seedling emergence and survival_Loranger2017
Emergence and survival_Loranger2017.csv = Comma-delimited CSV. Emergence of tree seedlings from seeds and subsequent survival through the first growing season at alpine treeline elevation in the French Alps. Seeds were sown in autumn 2013 and emergence and survival/mortality were registered weekly throughout the growing season in 2014.
Date = date of monitoring
FreeDays = number of days after snowmelt
Block = experimental block
Treat = treatments: C = Control = bare-ground plots; Sh = shading = covered by shade roofs; V = full vegetation, V2 = intermediate vegetation.
Species: Lar = Larix decidua; Pic = Picea abies; Sor = Sorbus aucuparia; Pinc = Pinus cembra; Pinu = Pinus uncinata
Row = row in plot
Sown = number of seeds sown
Dead = number of dead seedlings
Alive = number of surviving seedlings
Germ_All = number of germinated seedlings (including subsequently dead ones)
Emergence and survival_Loranger2017.csv
Carbohydrates and Biomass_Loranger2017
Carbohydrates and Biomass_Loranger2017.txt = Tab-delimited text file. Non-structural carbohydrate concentrations and biomass of plant parts (roots, stems, leaves) of tree seedlings at alpine treeline elevation in the French Alps. Seedlings were planted in spring 2013 and harvested in autumn 2013 and spring 2014.
ID = Sample ID from carbohydrate extraction
Season = sampling season: S13 = September 2013, M14 = May 2014
Block = experimental block
Treat = treatments: C = Control = bare-ground plots; Sh = shading = covered by shade roofs; V = full vegetation, V2 = intermediate vegetation
Species: Lar = Larix decidua; Pic = Picea abies; Sor = Sorbus aucuparia; Pin = Pinus cembra; Pinu = Pinus uncinata
Ind = seedling number in the plot
Part = seedling part: L = leaves, S = stem, R = roots
MassSm = mass of extraction sample (mg)
MassPt = mass of seedling part (mg)
Glu = glucose concentration (mg/g)
Fru = fructose concentration (mg/g)
Sac = saccharose concentration (mg/g)
Mal = maltose concentration (mg/g)
Total_SC = total soluble sugar concentration (mg/g)
Starch = starch concentration (mg/g)
Total_NSC = total non-structural carb. Concentration (mg/g)
Mortality_Loranger2017_13-14
Mortality_Loranger2017_13-14.csv = Comma-delimited CSV. Seedling mortality in tree seedlings at alpine treeline elevation in the French Alps. Seedlings were planted in spring 2013 and mortality was registered in September 2013 and April-May 2014, after snowmelt. The first registration was April 16th in Block 1, in the other blocks the registration was later, as snow melted later.
Season = grouping of data collections into two seasons: May14 = Spring (16 April to late May) 2014, Sept 13 = 2 Sept 2013
Block = experimental block
Treat = treatments: C = Control = bare-ground plots; Sh = shading = covered by shade roofs; V = full vegetation, V2 = intermediate vegetation.
Species: Lar = Larix decidua; Pic = Picea abies; Sor = Sorbus aucuparia; Pin = Pinus cembra; Pinu = Pinus uncinata
Planted = number of seedlings planted
DeadOn = number of dead seedlings not counting those damaged by breakage or herbivory
DeadAl = number of dead seedlings