Abstract
Beside low temperatures, limited tree growth at the alpine treeline may also be attributed to a lack of available soil nutrients and competition with understory vegetation. Although intra-annual stem growth of Pinus cembra has been studied intensively at the alpine treeline, the responses of radial growth to soil warming, soil fertilization, and below ground competition awaits clarification. In this study we quantified the effects of nitrogen (N) fertilization, soil warming, and understory removal on stem radial growth of P. cembra at treeline. Soil warming was achieved by roofing the forest floor with a transparent polyvinyl skin, while understory competition was prevented by shading the forest floor with a non-transparent foil around six trees each. Six trees received N- fertilization and six other trees served as controls. Stem growth was monitored with band dendrometers during the growing seasons 2012–2014. Our 3 years experiment showed that soil warming had no considerable effect on radial growth. Though understory removal through shading was accompanied by root-zone cooling, understory removal as well as N fertilization led to a significant increase in radial growth. Hardly affected was tree root biomass, while N-fertilization and understory removal significantly increased in 100-needle surface area and 100-needle dry mass, implying a higher amount of N stored in needles. Overall, our results demonstrate that beside low temperatures, tree growth at cold-climate boundaries may also be limited by root competition for nutrients between trees and understory vegetation. We conclude that tree understory interactions may also control treeline dynamics in a future changing environment.
Highlights
The alpine treeline has attracted the interest of researchers for more than one century (Brockmann-Jerosch, 1919; Däniker, 1923; Wardle, 1974; Tranquillini, 1979; Körner, 1998, 2012; Holtmeier, 2003; Wieser and Tausz, 2007; Piper et al, 2016), the causes to treeline formation are still under debate
Results of this study indicated that P. cembra responded to soil warming with an increase and to soil cooling with a decline in stem radial growth, when compared to control trees with soil temperature left unmanipulated
The roofing prevented 33% of growing season precipitation to reach the soil, treatment differences in daily mean θ between control, warmed and understory removal blocks stayed within the typical variation of at the study site (Neuwinger, 1972) which confirmed that the employed roofing system did not prevent any shortage in soil water availability (Figure 1 and Table 2)
Summary
The alpine treeline has attracted the interest of researchers for more than one century (Brockmann-Jerosch, 1919; Däniker, 1923; Wardle, 1974; Tranquillini, 1979; Körner, 1998, 2012; Holtmeier, 2003; Wieser and Tausz, 2007; Piper et al, 2016), the causes to treeline formation are still under debate. A world-wide survey across natural high elevation treelines indicates that growing season mean root-zone temperature constrains tree growth in temperature limited ecosystems (Sveinbjörnsson, 2000; Körner and Hoch, 2006). A growth decline at the alpine treeline may be attributed to a lack of available soil nutrients (Tranquillini, 1979; Sveinbjörnsson et al, 1992), especially nitrogen (Hoch, 2013) and competition with understory vegetation (Elliott, 2011; Grau et al, 2012; Liang et al, 2016). Results of this study indicated that P. cembra responded to soil warming with an increase and to soil cooling with a decline in stem radial growth, when compared to control trees with soil temperature left unmanipulated. Differences in stem growth with respect to soil temperature may be attributed to varying soil nutrient contents with respect to micro topography (Anschlag et al, 2008)
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