Print PageUnderstanding the interplay between environmental factors contributing to treeline formation and how these factors influence different life stages remains a major research challenge because detailed long-term observations of treeline patterns are rare. We asked the following research questions: 1. Can trees survive above the current treeline in the European Alps? 2. Are environmental factors that drive survival the same that drive height growth? 3. Does the relative importance of environmental variables measured at the time of tree establishment influencing survival and height growth change over the first thirty years after planting? We used an afforestation experiment including 92,000 trees to investigate the spatial and temporal dynamics of tree survival 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 a.s.l.) in 1975 and closely monitored during the following 30 years. We used general additive models to identify changes in survival and growth along gradients in elevation, snow duration, wind speed and microtopography and used decision-tree models to quantify interactions between the different variables.
Results/Conclusions
For all three species, snowmelt date and elevation were the most important environmental factors influencing survival and growth over the entire period studied. Individuals of all species survived at the highest point of the afforestation for more than 30 years, although survival declined strongly above 2160 m a.s.l., about 50 to 100 m above the current treeline position. Optimal conditions for height growth differed from those for survival in all three species: early snowmelt (ca. day 125-140) yielded highest survival rates but relatively later snowmelt (ca. day 145-150) yielded highest growth rates. Although snowmelt data and elevation were important throughout all life stages of the trees, variable importance of microtopography 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. Furthermore, the relative importance of different environmental variables for tree seedlings changes during the juvenile phase as they grow taller.
