Shrub facilitation promotes advancing of the Fagus sylvatica treeline across the Apennines (Italy)

Questions: Elevational treelines are expected to shift upwards in response to warming climate. However, worldwide upward shifts of treelines are inconsistent because local scale factors can affect the response to temperature. In this study, we explore the hypothesis that in the Apennines, where the current altitudinal treeline position is largely depressed because of past human activity, shrubs act as nurse plants promoting the upward migration of Fagus sylvatica. Location: A 500–km-long latitudinal gradient in the Apennines, Italy. Methods: We selected, along this gradient, nine treeline sites with different elevations, rock substrates, and physiognomic types including Juniperus communis, Pinus mugo, Vaccinium myrtillus shrublands, and grasslands. Here, in 68 transects we collected and analysed spatially fine-scale distribution data of F. sylvatica individuals in relation to both their age and their position, associated or not with shrubs. Results: Fagus sylvatica regeneration is rare in open secondary grasslands at 1,600–2,100 m a.s.l., highlighting a bottleneck in the regeneration phase of this species. On the contrary, we systematically observed a strong association between shrubs and F. sylvatica individuals. Compared with the adjacent grassland, F. sylvatica regeneration was 58.3 times higher under Pinus mugo, 131.5 timeshigher under Juniperus communis and 102.4 times higher under Vaccinium myrtillus. The age structure of F. sylvatica population indicates that, in the last 50 years, recruitment under shrubs is continuous, while in grassland it is episodic. Conclusions: Above the current treeline of the Apennines, F. sylvatica individuals develop only in the presence of shrubs, which act as nurse species. Shrubs are a necessary condition for F. sylvatica re-colonization of the high-altitude open areas affected, in the last centuries, by intense human land use.