Tree and timberline shifts in the northern Romanian Carpathians during the Holocene and the responses to environmental changes

High altitude environments are experiencing more rapid changes in temperature than the global average with the risk of losing essential ecosystem services in mountain environments. The Carpathians Mountains are regarded as hosting Europe's most pristine mountain ecosystems, yet the paucity of past environmental records limits our understanding of their sensitivity to the various drivers of change. A multi-proxy palaeoecological approach (plant macro-remains, pollen, charcoal) applied to three Holocene sediment sequences (between 1540 and 1810 m a.s.l.) in the Rodna Mountains documents past treeline and timberline shifts in response to climate change and human impact to anticipate the likely future responses. Our results indicate that forest reacted sensitively to past climate conditions. The timberline had exceeded an elevation of 1540 m a.s.l. by 10,200 cal. yr BP, when summers were warmer than today. The treeline remained below 1810 m a.s.l. at this time and reached its maximum elevation after 8500 cal. yr BP, when winter temperatures became milder. Cool summer conditions probably caused a lowering of the timberline and an extension of the treeline ecotone from 4900 cal. yr BP, a process accentuated by human impact from the Bronze Age (3500 cal. yr BP) onwards. The anticipated upslope tree movements as a consequence ongoing global warming are not yet clearly visible in our records, but will more probably take place in abandoned agricultural areas and be counter-balanced by re-enforced anthropogenic pressure elsewhere. Pinus sylvestris was the dominant tree species in the timberline under a warm and dry climate, when fires were frequent, during the early Holocene (11,250-10,200 cal. yr BP), while Picea abies became dominant in the timberline and Pinus mugo in the treeline ecotone, respectively from 10,200 cal. yr BP to the present. Abies alba became a significant component of the timber over the last four millennia. The anticipated future warmer and moister climatic conditions will favour the persistence of P. abies as well as A. alba. However, A. alba is more sensitive to anthropogenic disturbance, which implies that in places with continuing farmland pressure, A. alba may be less prevalent than P. abies in the future. Anthropogenic pressure is expected to increase the proportion of tree species characteristic of more disturbed forests and consequently threaten biodiversity with important implications for mountain ecosystem services.