Factors driving mortality and growth at treeline: a 30-year experiment of 92 000 conifers

• Published in: Ecology (Durham) Vol. 93; no. 2; pp. 389 - 401
• Main Authors: Barbeito, Ignacio, Dawes, Melissa A, Rixen, Christian, Senn, Josef, Bebi, Peter
• Format: Journal Article
• Language: English
• Published: Washington, DC Ecological Society of America 01-02-2012
• Subjects: Afforestation; Altitude; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Coniferophyta - physiology; Ecosystem; elevation gradient; Environmental conditions; Forest ecology; Fundamental and applied biological sciences. Psychology; General aspects; height growth; Larix decidua; Mortality; Nonnative species; physiological threshold; Pinus cembra; Pinus mugo; Pinus mugo ssp. uncinata; Plant ecology; Plant growth; Population Dynamics; Seedlings; Snow; Snowmelt; snowmelt date; ssp; Swiss Alps; Temperature; Timberlines; Time Factors; Tree growth; treeline; Trees; uncinata; Alps; Switzerland, Alps Mts; physiological threshold; Growth; height growth; Larix decidua; Mortality; Afforestation; Pinus mugo; Swiss Alps; Pinus cembra; Snow melt; Treeline; Pinus mugo ssp; snowmelt date; Gymnospermae; elevation gradient; Spermatophyta; Coniferales; uncinata
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/11-0384.1

Understanding the interplay between environmental factors contributing to treeline formation and how these factors influence different life stages remains a major research challenge. We used an afforestation experiment including 92 000 trees to investigate the spatial and temporal dynamics of tree mortality and growth at treeline in the Swiss Alps. Seedlings of three high-elevation conifer species (
Larix decidua
,
Pinus mugo
ssp.
uncinata
, and
Pinus cembra
) were systematically planted along an altitudinal gradient at and above the current treeline (2075 to 2230 m above sea level [a.s.l.]) in 1975 and closely monitored during the following 30 years. We used decision-tree models and generalized additive models to identify patterns in mortality and growth along gradients in elevation, snow duration, wind speed, and solar radiation, and to quantify interactions between the different variables. For all three species, snowmelt date was always the most important environmental factor influencing mortality, and elevation was always the most important factor for growth over the entire period studied. Individuals of all species survived at the highest point of the afforestation for more than 30 years, although mortality was greater above 2160 m a.s.l., 50-100 m above the current treeline. Optimal conditions for height growth differed from those for survival in all three species: early snowmelt (ca. day of year 125-140 [where day 1 is 1 January]) yielded lowest mortality rates, but relatively later snowmelt (ca. day 145-150) yielded highest growth rates. Although snowmelt and elevation were important throughout all life stages of the trees, the importance of radiation decreased over time and that of wind speed increased. Our findings provide experimental evidence that tree survival and height growth require different environmental conditions and that even small changes in the duration of snow cover, in addition to changes in temperature, can strongly impact tree survival and growth patterns at treeline. Further, our results show that the relative importance of different environmental variables for tree seedlings changes during the juvenile phase as they grow taller.