From Alnus to Pinus: temperate peatland ecosystem transformation triggered by human-driven landscape change

Abstract

Peatlands are invaluable archives of palaeoenvironmental and climate dynamics, play a central role in the global carbon cycle and hydrological processes, preserve biological diversity, and act as climatic microrefugia. Over millennia, these ecosystems have been heavily modified by human land use, including drainage, overgrazing, and peat extraction, leading to large-scale degradation in many regions. Understanding the long-term dynamics of peatlands is crucial for determining their conservation and restoration needs, as well as for predicting their evolution, including responses to climate change, community changes, and carbon sequestration potential. In this study, we adopted an interdisciplinary approach to investigate the relationships between climate, vegetation, tree growth, hydrology, and human activities in a peatland ecosystem in one of the poorly explored regions of Central Europe, the Solska Forest in southeastern Poland. To reconstruct local ecosystem and landscape dynamics and assess possible climatic and anthropogenic impacts, we integrated various proxy data from natural and human archives: long-term meteorological data (1792–2020), tree ring data (1729–2022) from living peatland pines, palaeoecological data from peat sediment (pollen, plant macrofossils, testate amoebae, and charcoal), and archival written and cartographic sources. Our environmental reconstruction, spanning over 2300 years (ca. 330 BCE–2022 CE), identified three distinct periods in the peatland's history: ca. 330 BCE–1400 CE, ca. 1400–1830 CE, and ca. 1830–2022 CE. These phases are characterized by varying archival coverage, including a sedimentary hiatus between 1400 and 1830 CE, likely associated with a fire disturbance documented at the study site in the first half of the 19th century. However, the stratigraphic gap was largely bridged by integrating tree ring, climate, and historical data. This synthesis revealed considerable hydrological instability of the peatland and documented its complete transition from black alder bog forest to Scots pine bog forest. To our knowledge, this provides the first palaeoecological evidence of such a conversion in temperate Europe. This ecosystem shift was likely triggered by several interlinked factors, primarily anthropogenic land use change, including settlement development, deforestation, and the introduction of timber-oriented forest management. These drivers led to the landscape-scale expansion of pine forests and subsequent environmental acidification that facilitated Sphagnum encroachment. Furthermore, the roles of fire and drainage in this transition were clearly demonstrated. Our results, including the tree ring record of successful Scots pine establishment and historical data on settlement development throughout the 1700s, suggest that this environmental transformation occurred before the end of the period ca. 1400–1830 CE, between the 16th and 18th centuries. In addition, our findings indicate that both the transition period (> 400 years) and the current ecosystem type (ca. 200 years) represent a much shorter part of the peatland's history (> 600 years) than the preceding, substantially different ecosystem state (> 1700 years). We also address the methodological challenges of multi-proxy studies of landscape dynamics, such as reconciling inconsistent quantitative and qualitative data or managing periods of low archival resolution. Nevertheless, linking natural and human archives enabled us to gain a deeper understanding of a complex environmental system, with added value from combining different approaches. Moreover, by confirming the multifaceted interactions between different biotic and abiotic factors affecting both landscape and peatland ecosystems, we emphasized the continuing need for further research on peatland ecology, including past and current changes. Our evidence of a complete ecosystem shift from Alnus to Pinus and Sphagnum dominance under the synergistic influence of different factors, particularly the undeniable human impact, highlights the importance of a temporal perspective and long-term environmental data in conservation and land management, as such records are vital for assessing reference conditions and defining protection and restoration goals.