Climate versus human-driven fire regimes in Mediterranean landscapes: the Holocene record of Lago dell’Accesa (Tuscany, Italy)

Abstract

A high-resolution sedimentary charcoal record from Lago dell'Accesa in southern Tuscany reveals numerous changes in fire regime over the last 11.6 kyr cal. BP and provides one of the longest gap-free series from Italy and the Mediterranean region. Charcoal analyses are coupled with gamma density measurements, organic-content analyses, and pollen counts to provide data about sedimentation and vegetation history. A comparison between fire frequency and lake-level reconstructions from the same site is used to address the centennial variability of fire regimes and its linkage to hydrological processes. Our data reveal strong relationships among climate, fire, vegetation, and land-use and attest to the paramount importance of fire in Mediterranean ecosystems. The mean fire interval (MFI) for the entire Holocene was estimated to be 150 yr, with a minimum around 80 yr and a maximum around 450 yr. Between 11.6 and 3.6 kyr cal. BP, up to eight high-frequency fire phases lasting 300-500 yr generally occurred during shifts towards low lake-level stands (ca 11,300, 10,700, 9500, 8700, 7600, 6200, 5300, 3400, 1800 and 1350 cal. yr BP). Therefore, we assume that most of these shifts were triggered by drier climatic conditions and especially a dry summer season that promoted ignition and biomass burning. At the beginning of the Holocene, high climate seasonality favoured fire expansion in this region, as in many other ecosystems of the northern and southern hemispheres. Human impact affected fire regimes and especially fire frequencies since the Neolithic (ca 8000-4000 cal. yr BP). Burning as a consequence of anthropogenic activities became more frequent after the onset of the Bronze Age (ca 3800-3600 cal. yr BP) and appear to be synchronous with the development of settlements in the region, slash-and-burn agriculture, animal husbandry, and mineral exploitation. The anthropogenic phases with maximum fire activity corresponded to greater sensitivity of the vegetation and triggered significant changes in vegetational communities (e.g. temporal declines of Quercus ilex forests and expansion of shrublands and macchia). The link between fire and climate persisted during the mid-and late Holocene, when human impact on vegetation and the fire regime was high. This finding suggests that climatic conditions were important for fire occurrence even under strongly humanised ecosystem conditions.

Holocene fire activity and vegetation response in South-Eastern Iberia

“…Southeastern Iberian forest dynamics agree with this sub-continental framework and, as illustrated in this paper, fire activity has fluctuated temporally but also spatially throughout the Holocene (Figs 3a, 3b and 4). This finding is in agreement with current fire activity in Mediterranean ecosystems, which has been demonstrated to be very erratic in time and space depending on land use, population density, biomass fragmentation and fuel availability (Pausas, 2004;Vannière et al, 2008).…”

“…If fire activity is reduced, mesophytes become superior competitors to Aleppo and Maritime pines, which are shade intolerant and obligate seeders (i.e., they germinate readily after fire and increase their population size in the open spaces generated by the fire). On the whole, a likely mid-holocene scenario in the SE Iberian Peninsula would imply the initial expansion of mesophytes and evergreen Quercus owing to warmer and wetter conditions as fire activity decreased due to shorter dry seasons produced by a less marked seasonal insolation.These results are coherent with the mid-holocene vegetation response found at similar latitudes in Southern Europe (Sadori and Giardiani, 2007;Tinner et al, 2009), where decreasing fire activity aided by growing moisture availability permitted the expansion of broadleaved forests (Vannière et al, 2008). Interestingly all sites, but Navarrés, show similarly negative FAA at 4 kyr cal BP, indicating an increment in fire activity compare to the previous Mid-Holocene phase, but still not as high as the fire activity after 3 Kyr cal BP (Fig 4).…”

“…The existence of long-term fire activity patterns in the Mediterranean Basin has been proved in different local and regional approaches (Wick et al, 2003;Sadori and Giardini, 2007;Colombaroli et al, 2007;Sadori et al, 2008;Turner et al, 2008;Tinner et al, 2009;Vannière et al, 2008). These approaches show in some cases synchronous trends but also many unexpected ecosystem responses due to particular physical settings, microclimatic aspects and different models of human agency.…”

“…6 kyr cal BP, being particularly strong from ca. 4 kyr BP onwards (Vannière et al, 2008), promoting the regional spread of Mediterranean and xerophytic taxa.…”

“…Hence the understanding of long-term relationships between vegetation, climate change and fluctuations in fire and human activity in the Mediterranean basin may help to assess the ecosystem's resilience to changing fire activities and human impact. Since conventional ecology is often limited when addressing issues within time frames beyond instrumental records, palaeoecological methods may serve as an adequate alternative, as proven by the abundant literature that has been produced across the European Mediterranean region (Colombaroli et al, 2007;Colombaroli et al, 2008;Sadori and Giardini, 2007;Tinner et al, 2009;Vannière et al, 2008). This is especially relevant under the current scenario of increasing temperatures in Southern Europe (Pausas, 2004), where fire management may be critical in the near future for biodiversity and landscape management.…”