Site-based long-term dendro-climatological monitoring of an alpine treeline ecotone in the Pirin Mountains (Bulgaria)

As treelines are among the key ecotones expected to undergo dramatic change as a result of global warming, it is vitally important that such locations be monitored to track their development. To this end, we set up a treeline monitoring system at a small spatial scale and medium-term temporal scale in the Pirin Mountains in south-eastern Europe. Rising over 400 m, the test slope features a closed Pinus heldreichii forest gradually leading to park-like scattered forest islands and then to open alpine meadows and some dwarf pine patches up to/above the treeline. Tree coverage, successional stages, tree species, tree growth, growth forms, age classes, seedling establishment and, in particular, survival rate, were mapped in relation to site conditions in the autumns of 2008 and 2023. The first and second baseline datasets documented in this report are used to describe the composition and structure of the current vegetation within the treeline ecotone. Overall, our data shows good vegetation development despite the harsh conditions in the high mountains. Almost no change was observed in the closed forest: the trees are ageing, and no seedlings were found. The greatest developmental dynamics were evident in the park-like, thinned-out middle section of the slope, where the forest transitions to the treeline. This is where the “future forest” is growing. The “battle zone” further up is changing extremely slowly, and an upwards shift in the forest/treeline is likely to take many decades.