Abstract
Upper treeline ecotones are important life form boundaries and particularly sensitive to a warming climate. Changes in growth conditions at these ecotones have wide‐ranging implications for the provision of ecosystem services in densely populated mountain regions like the European Alps. We quantify climate effects on short‐ and long‐term tree growth responses, focusing on among‐tree variability and potential feedback effects. Although among‐tree variability is thought to be substantial, it has not been considered systematically yet in studies on growth–climate relationships. We compiled tree‐ring data including almost 600 trees of major treeline species (Larix decidua, Picea abies, Pinus cembra, and Pinus mugo) from three climate regions of the Swiss Alps. We further acquired tree size distribution data using unmanned aerial vehicles. To account for among‐tree variability, we employed information‐theoretic model selections based on linear mixed‐effects models (LMMs) with flexible choice of monthly temperature effects on growth. We isolated long‐term trends in ring‐width indices (RWI) in interaction with elevation. The LMMs revealed substantial amounts of previously unquantified among‐tree variability, indicating different strategies of single trees regarding when and to what extent to invest assimilates into growth. Furthermore, the LMMs indicated strongly positive temperature effects on growth during short summer periods across all species, and significant contributions of fall (L. decidua) and current year's spring (L. decidua, P. abies). In the longer term, all species showed consistently positive RWI trends at highest elevations, but different patterns with decreasing elevation. L. decidua exhibited even negative RWI trends compared to the highest treeline sites, whereas P. abies, P. cembra, and P. mugo showed steeper or flatter trends with decreasing elevation. This does not only reflect effects of ameliorated climate conditions on tree growth over time, but also reveals first signs of long‐suspected negative and positive feedback of climate change on stand dynamics at treeline.
Highlights
Investigating plant population dynamics at their distribution limits is a key approach for understanding global change impacts (Luckman, 1996; Motta & Nola, 2001)
L. decidua exhibited even negative ring-w idth indices (RWI) trends compared to the highest treeline sites, whereas P. abies, P. cembra, and P. mugo showed steeper or flatter trends with decreasing elevation
Temperature is widely recognized as the major large-scale driver determining upper treeline position around the globe (Brockmann-Jerosch, 1919; Holtmeier & Broll, 2005; Humboldt & Bonpland, 1807; Körner, 2012) some of the detailed processes are still subject to debate (e.g., Takahashi & Furuhata, 2016)
Summary
Investigating plant population dynamics at their distribution limits is a key approach for understanding global change impacts (Luckman, 1996; Motta & Nola, 2001). Changing stand density modifies canopy cover, thereby affecting tree-to-tree competition and root zone temperatures, which feed back to tree growth rates (Barbeito, Dawes, Rixen, Senn, & Bebi, 2012; Malanson et al, 2011; Wang et al, 2016) The interactions of these processes and the potential feedback effects make it challenging to predict tree population dynamics at the treeline ecotone in a changing climate, which in turn are expected to have consequences for trees at lower elevations (e.g., Holtmeier & Broll, 2005; Seidl, Schelhaas, Lindner, & Lexer, 2009). Tree-ring data are often available as long time series, which renders them an ideal measure for multi-decadal to centennial growth trend analyses, isolating the low-frequency variability caused by abiotic influencing factors remains challenging. We ask the following research questions: (i) Are the analyzed treeline sites limited by temperature and do they show signs of suspected upward movement? (ii) Which monthly temperature variables control the annual growth of trees at the treeline and how do their effects differ among climate regimes, species, and individual trees? (iii) Do long-term growth rates reflect the recently increasing temperatures due to climate change, and can we identify feedback effects on growth rates?
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