Driving factors of a vegetation shift from Scots pine to pubescent oak in dry Alpine forests

作者：Andreas Rigling, Christof Bigler, Britta Eilmann, Elisabeth Feldmeyer‐Christe, Urs Gimmi, Christian Ginzler, Ulrich Graf, Philipp Mayer, Giorgio Vacchiano, Pascale Weber, Thomas Wohlgemuth, Roman Zweifel, Matthias Dobbertin · 发表于：Global Change Biology · 年份：2012 · DOI：10.1111/gcb.12038 · 被引用次数：366 · 研究领域：Plant Water Relations and Carbon Dynamics、Tree-ring climate responses、Ecology and Vegetation Dynamics Studies

An increasing number of studies have reported on forest declines and vegetation shifts triggered by drought. In the Swiss Rhone valley (Valais), one of the driest inner-Alpine regions, the species composition in low elevation forests is changing: The sub-boreal Scots pine (Pinus sylvestris L.) dominating the dry forests is showing high mortality rates. Concurrently the sub-Mediterranean pubescent oak (Quercus pubescens Willd.) has locally increased in abundance. However, it remains unclear whether this local change in species composition is part of a larger-scale vegetation shift. To study variability in mortality and regeneration in these dry forests we analysed data from the Swiss national forest inventory (NFI) on a regular grid between 1983 and 2003, and combined it with annual mortality data from a monitoring site. Pine mortality was found to be highest at low elevation (below 1000 m a.s.l.). Annual variation in pine mortality was correlated with a drought index computed for the summer months prior to observed tree death. A generalized linear mixed-effects model indicated for the NFI data increased pine mortality on dryer sites with high stand competition, particularly for small-diameter trees. Pine regeneration was low in comparison to its occurrence in the overstorey, whereas oak regeneration was comparably abundant. Although both species regenerated well at dry sites, pine regeneration was favoured at cooler sites at higher altitude and oak regeneration was more frequ...