Early establishment of trees at the alpine treeline: idiosyncratic species responses to temperature-moisture interactions

Research Article Early establishment of trees at the alpine treeline: idiosyncratic species responses to temperature-moisture interactions Hannah Loranger,1 Gerhard Zotz1 and Maaike Y. Bader2* 1 Functional Ecology of Plants, Institute of Biology and Environmental Sciences, University of Oldenburg, D-26111 Oldenburg, Germany 2 Ecological Plant Geography, Faculty of Geography, University of Marburg, Deutschhausstraße 10, D-35032 Marburg, Germany Received: 18 November 2015; Accepted: 3 July 2016; Published: 10 July 2016 Associate Editor: Katharine J.M. Dickinson Citation: Loranger H, Zotz G, Bader MY. 2016. Early establishment of trees at the alpine treeline: idiosyncratic species responses to temperature-moisture interactions. AoB PLANTS 8: plw053; 10.1093/aobpla/plw053 Abstract. On a global scale, temperature is the main determinant of arctic and alpine treeline position. However on a local scale, treeline form and position vary considerably due to other climatic factors, tree species ecology and life-stage-dependent responses. For treelines to advance poleward or uphill, the first steps are germination and seedling establishment. These earliest life stages may be major bottlenecks for treeline tree populations and will depend differently on climatic conditions than adult trees. We investigated the effect of soil temperature and moisture on germination and early seedling survival in a field experiment in the French Alps near the local treeline (2100 m a.s.l.) using passive temperature manipulations and two watering regimes. Five European treeline tree species were studied: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia. In addition, we monitored the germination response of three of these species to low temperatures under controlled conditions in growth chambers. The early establishment of these trees at the alpine treeline was limited either by temperature or by moisture, the sensitivity to one factor often depending on the intensity of the other. The results showed that the relative importance of the two factors and the direction of the effects are highly species-specific, while both factors tend to have consistent effects on both germination and early seedling survival within each species. We show that temperature and water availability are both important contributors to establishment patterns of treeline trees and hence to species-specific forms and positions of alpine treelines. The observed idiosyncratic species responses highlight the need for studies including several species and life-stages to create predictive power concerning future treeline dynamics. Keywords: Alpine treelines; climate change; early seedling survival; germination; temperature–moisture interactions; time-to-event analysis. Introduction Treelines are conspicuous transition zones between two very different vegetation types. There is a growing concern about how global climate change may affect these systems, and as a consequence much attention has been drawn to both alpine and arctic treeline ecotones in recent years. Treelines could represent a distinct indicator of climate warming since temperature is recognized * Corresponding author’s e-mail address: C The Authors 2016. Published by Oxford University Press on behalf of the Annals of Botany Company. V This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. AoB PLANTS www.aobplants.oxfordjournals.org C The Authors 2016 V 100 Loranger et al. Early establishment of trees at the alpine treeline as the main determinant of treeline position on a global scale, roughly following a common isotherm of 5–7  C mean growing season temperature (Körner and Paulsen 2004). Many studies show recent advances of treelines poleward and to higher elevations, as well as increasing radial growth of the trees forming these ecotones (Rolland et al. 1998; Kullman 2007; Shiyatov et al. 2007; Qi et al. 2015). However, stable or receding treelines have been found (Harsch et al. 2009), and treeline position may vary considerably at a local scale (Holtmeier and Broll 2005; Case and Duncan 2014). Such local variations can be due to locally varying environmental conditions unrelated to temperature such as precipitation (Holtmeier and Broll 2005), tree species ecology (Körner and Paulsen 2004; Trant and Hermanutz 2014) and lifestage dependent environmental dependencies (Barbeito et al. 2012; Greenwood et al. 2015). These abiotic and biotic factors may also interact with temperature to determine the form and dynamics of a treeline at a specific site. For example, the consequences of moisture deficits—which can be positively linked to climate warming—have been shown to override positive temperature responses with respect to growth (Barber  lez de Andrés et al. 2015) and regeneret al. 2000; Gonza ation (Barton 1993; Daniels and Veblen 2004; Moyes et al. 2013). In such cases, treeline shifts may depend more on the interactions of temperature and water availability than on their absolute values (Ohse et al. 2012) and may differ between landscape positions accordingly (Elliott and Cowell 2015). It is commonly observed that tree cover is slow or unable to expand to its ultimate thermal boundary (Holtmeier 2009). The underlying mechanisms remain however difficult to disentangle and there is an urgent need for quantitative assessments of the specific environmental conditions and associated mechanisms preventing the establishment of different tree species beyond current treelines. Treelines represent distributional boundaries for an entire life-form—the tree. Consequently, ecosystems above the treeline differ fundamentally from those below, e.g. in regard to soils and microclimates (Sullivan and Sveinbjörnsson 2010; Thébault et al. 2014). This presents particular challenges for a successful tree regeneration and establishment in the treeline ecotone and beyond, as required for an upward distributional shift. Previous studies have shown that traits essential for regeneration such as the number of seed-bearing fruits or the number of viable seeds often decrease with increasing elevation, thereby reducing the probability of seedling establishment especially above treeline (Cuevas 2000; Kroiss and HilleRisLambers 2015). Although seed production and dispersal are critical prerequisites for tree regeneration, subsequent germination and seedling 002 establishment have also been widely recognized as potential life-history bottleneck of treeline tree populations (Stevens and Fox 1991; Germino et al. 2002; Smith et al. 2003; Johnson et al. 2011). Germination represents the earliest, critical life-stage transition and should thus be subject to strong natural selection (Baskin and Baskin 2001). Furthermore, the conditions duri (...truncated)