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

Abstract

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.

Treelines—Approaches at Different Scales

“…Figure 1). The susceptibility of trees to different environmental factors may be different at different life stages [80]. Diseases, pathogens, insect calamities, and wildlife are factors that may lastingly affect trees and other vegetation at treeline at the regional and local scales and must not be disregarded [81].…”

“…The regionally varying response of the altitudinal and northern treelines to the warming periods from the 1920s to the 1940s and since the late 1980s (e.g., [5,39,118,[125][126][127]) and long-term dendrochronological studies (e.g., [80]) suggest that a more differentiating view is needed. In many areas, the treeline responds to climate warming by the transformation of suppressed existing growth forms rather than by the establishment of new trees at increasingly higher elevations (e.g., [6,35,36,45]).…”

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

“…In recent years, an increasing number of studies have investigated the response of treeline seedlings to changing environmental factors such as soil temperature, soil moisture, soil nutrients, light conditions, herbivory, and competition/facilitation (e.g. Maher and Germino, 2006;Anschlag et al, 2008;Batllori et al, 2009;Hofgaard et al, 2009;Kabeya, 2010;Munier et al, 2010); some studies have also explicitly addressed the relative importance of multiple factors (Barbeito et al, 2012;Zurbriggen et al, 2013).…”

Sustainable Land Use in Mountain Regions Under Global Change: Synthesis Across Scales and Disciplines

“…In addition, environmental factors other than temperature, such as nitrogen availability, exert a strong influence on their biomass production and allocation (Zurbriggen et al 2013). During the years 3 to 30 of their life, tree survival near the tree line is driven mainly by snow cover duration, although temperature is most important for growth (Barbeito et al 2012). This suggests that small changes in the duration of snow cover and other environmental factors can have strong impacts on tree population dynamics, which in turn will affect EGS provision, foremost in avalanche protection.…”

“…over the tree developmental stages because of the shifting relevance of environmental factors during the first 30 years of tree growth and the impact of variations in the duration of snow cover (Barbeito et al 2012). When the indirect effects of temperature increase on the duration of snow cover, disturbance regimes and, in particular, the positive feedback between avalanches and forest dynamics (Kulakowski et al 2011, Teich et al 2012a) are also taken into consideration, predicted forest cover and the corresponding EGS will change nonlinearly.…”