The Absence of Disturbances Promoted Late Holocene Expansion of Silver Fir (abies Alba) in the Bohemian Forest

Temperate forests in the Northern Hemisphere are strongly affected by increasing annual temperatures and natural disturbances such as droughts, fires, and pest outbreaks. In many regions, alternatives are explored by the forestry industry for previously commercialized tree species which are rapidly declining in areas outside their optimal climatic niches. However, as the current ranges of tree species have been mainly constrained by human activity, their true climatic and ecological niches are likely different from our observations. For example, little is known about environmental niches and population dynamics of tree species with limited dispersal ability, such as silver fir (Abies alba). Long-term (paleo) records of past landscape composition, past climate, and past disturbances can help to reveal the natural environmental niches of tree species. This study focuses on the Bohemian Forest (Sumava) in Czechia, where the human alteration of forests in higher elevations (>1000 m asl) has only been demonstrated from Medieval times onward. We present an interdisciplinary approach of geochemistry, pollen, charcoal, botanical and insect macro remains from a high-elevation peat record for the last seven millennia. Our multi-proxy study aimed to identify episodes of increased fire and other disturbances such as insect outbreaks, which could have influenced forest dynamics and vegetation succession in this montane region. However, the charcoal and macro-fossil records do not indicate any large local fires, insect outbreaks, browsing, or other disturbances (e.g., anthropogenic) for a major part of the record, suggesting that changes in the vegetation after 6500 cal yr BP were mainly caused by climatic fluctuations. Silver fir (Abies alba) expanded from 4300 cal yr BP, and likely became the dominant tree species at this locality for the next 2000 years, with extremely high pollen values up to 60%. During the establishment of silver fir around the peat bog around 5600 cal yr BP and its expansion around 4300 cal yr BP, the geochemical record and low counts of the testate amoeba Archerella flavum indicate drier conditions on the peat bog. Comparison with climatic simulations on a 1 x 1-km scale from the CHELSA database suggests that the local expansion of silver fir possibly coincided with a decrease in precipitation during both the wettest and warmest quarter of the year, related to a decreasing trend in seasonality of annual precipitation; this might confirm the species' higher tolerance to drought. Although silver fir is currently extremely rare in the Bohemian Forest, it has the potential for local expansion if logging, fire, and game browsing are kept to a minimum.