The influence of land-use and climatic changes on mountain pine (Pinus mugo Turra spp. mugo) ecotone dynamics at its southern range margin on the Majella massif, Central Apennines

Mountain pine krummholz within the Majella National Park forms an isolated patch and is a relictual species, thought representing the potential natural vegetation of the subalpine belt in Central Apennines, Italy. Mountain pine grows above the timberline and has in the Majella massif its southernmost limit of the distributional range. This vegetation dominates the treeline ecotone, between the dense beech forest and the alpine prairies, and is highly sensitive even to minor environmental changes. For this reason, it is a major proxy for monitoring the effects of climate and land-use changes occurring in the Mediterranean Basin. Mediterranean climatic trend includes projected increasing temperature and a different distribution of total precipitation, which are increasing during winter and decreasing during summer and autumn, respectively. Moreover, in Central Apennines, subalpine shrublands have been deeply reduced from the past anthropic practices. The recent abandonment of marginal areas and grazing cessation are allowing a re-colonization of areas previously grazed. The main objective of this study was to assess the effects of land-use and climatic changes on mountain pine population dynamics in the Majella massif, using three different methodological approaches. Temporal and spatial distribution of mountain pine since 1954 was reconstructed and a multitemporal map was created by historical aerial ortophotos. Results documented an upward and downward expansion of mountain pine and an ingrowth into the formerly tree-free grassland in early 1900s, together with a decline of the local human population and livestock. The relative role of climatic variables on mountain pine growth was assessed by dendroclimatological analysis. Climate-growth relationship were assessed by correlation function (CF) analysis and moving CF (MCF), investigating the possible deviations in the response of mountain pine to recent climate change. Results demonstrated that climate variables affected mountain pine growth at the treeline ecotone in the Majella massif. Significant correlations were found with the spring maximum temperatures and summer precipitation. MCF showed that mountain pine is a sensitive species, in terms of tree ring growth, particularly at the beginning and at the end of the growing season. For better understand dendroclimatological results, wood anatomical analyses were conducted. Timings of mountain pine xylogenesis were assessed and the principal phenological phases (cambial division; enlargement; wall thickening; mature) were monitored during the entire growing season. At the same time timings of compression wood (CW) formation and possible differences with the opposite wood (OW) formation were investigated. Results indicated that CW is characterized by higher number of cells and thicker secondary wall, as well as longer xylogenesis compared with the OW. The growing season started simultaneously in both types of wood (on 20 May), and ended two week earlier in OW (on 22 September) than in CW (on 6 October). Increasing temperature of spring month would be favourable for the elongation of mountain pine growing season, which would probably start early, though decreasing precipitation could compromise wood formation during summer, mountain pine being highly sensitive to this climatic parameter. Growing above the limit of tree vegetation, mountain pine proved to be a promising species for monitoring climate- and land use-driven influences over tree-ring growth at high elevation. An excessive expansion of mountain pine krummholz, mainly at higher altitudes, may represent a threat for rare and endemic species growing in the alpine grasslands of the Majella National Park. However, conservation and preservation of mountain pine are required, in order to avoid a severe reduction of its distribution and then the risk of extinction of the last †œisland†� present in the Apennine.