Early to mid-Holocene climate change at Lago dell'Accesa (central Italy): climate signal or anthropogenic bias?

Despite the high potential of pollen records for climate reconstruction, pollen-climate relationships may be biased due to past and present human activities on the landscape. We use (i) transfer functions based on modern pollen-climate relationships to infer seasonal temperature and summer precipitation for the period 11 500-4500 cal. a BP and (ii) lake-level change records based on different sedimentary proxies in multiple cores that are mainly indicative for summer hydrology at Lago dell'Accesa (central Italy). Quantitative reconstructions indicate lowest summer precipitation during two phases (8500-7700 cal. a BP and after 6000 cal. a BP) and a gradual winter temperature increase from 11 500 to ca. 8000 cal. a BP. Lowest summer precipitation was reconstructed during these phases characterised by vegetation shifts from open forests dominated by summergreen oaks (Quercus) to forests dominated by evergreen oaks (Quercus ilex), which are at present most abundant where summer drought is stronger. Similarly, the lake-level record indicates two long-lasting low summer precipitation phases (8800-7700 and 6400-4400 cal. a BP) that were interrupted by short-term high summer precipitation events. Based on the broad agreement between the pollen-inferred summer precipitation and the low-frequency lake-level changes, we suggest that the duration of the high summer precipitation events may have been too short to maintain drought-sensitive trees, which may have been affected by high mortality rates when summer dry conditions returned. Although past and modern pollen-climate relationships may very likely have been affected by human activities since the Neolithic (i.e. when exploitation of the landscape started), we reject the hypothesis of a significant anthropogenic bias in the pollen-based climate reconstruction. In addition, we suggest that pollen-based and lake-level reconstructions may have different inherent abilities of capturing high- and low-frequency precipitation signals. Copyright (C) 2010 John Wiley & Sons, Ltd.