Plant facilitation and soil microbiome modulate treeline advancement across the Apennines

Elevational treelines are expected to shift upwards in response to warming climate. Nevertheless, the global inconsistency in the upward shifts of treelines implies that factors such as topography, edaphic properties, disturbance, and the presence of competing vegetation may override the positive effect of cold stress alleviation. We report here the results of our research regarding the impact of climate warming and land abandonment on F. sylvatica at the treeline in Apennines. The underlying hypothesis was that in the Apennines, where historical human activities have significantly depressed the current altitudinal treeline position, shrubs act as nurse plants, promoting the upward shift of Fagus sylvatica. To test this hypothesis, nine treeline sites, along 500–km-long latitudinal gradient in Apennines, with different elevations, rock substrates, and physiognomic types including Juniperus communis, Pinus mugo, Vaccinium myrtillus shrublands, and grasslands were selected. In these sites, 68 transects were collected and analysed spatially fine-scale distribution data of F. sylvatica individuals with relation to both their age and their position associated or not to shrub. To evaluate the correlation between the above- and below-ground ecosystem components and to investigate the facilitation mechanisms more thoroughly, the soil microbiota was analysed within the different physiognomic types. The results showed that F. 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, a strong association between shrubs and F. sylvatica individuals was observed. Compared with the adjacent grassland, F. sylvatica regeneration was 58.3 times higher under P. mugo, 131.5 higher under J. communis and 102.4 higher under V. myrtillus. Results concerning soil microbial communities showed a high diversity between open grassland and shrubs and comparable between different shrub species. Age-structure of F. sylvatica population indicates that, in the last 50 years, recruitment under shrubs is continuous, while episodic in grassland. In conclusion our study reveals that above the existing treeline in the Apennines, the development of F. sylvatica individuals is contingent upon the presence of shrubs, which function 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.