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Corylus avellana (Hazel)
Corylus avellana was the first of the temperate deciduous forest trees to immigrate, establish itself and then become abundant in the postglacial period. Its sudden rise to very high pollen frequencies marks the start of the mesocratic phase of the present, Flandrian, interglacial (Birks 1986) and is the first stage in the development of dense, mixed deciduous forest. This Corylus pollen rise is an almost ubiquitous feature of Holocene pollen diagrams and reflects an extremely rapid increase in population (Bennett 1983), probably with no real competition from other taxa and no effective environmental constraints (Birks 1986). Radiocarbon dates for the Corylus rise usually fall between c.9,400 and c.9,000BP, although it occurs later in more environmentally marginal regions such as the uplands and on isolated islands such as Mull and Arran (Innes 1999, Boyd and Dickson 1986). Here the rational pollen limit (rise to abundance) of Corylus occurs more typically from c.8,800 to 8,500BP. Birks (1989) has presented an isochrone map for the Corylus pollen rise suggesting the earliest spread of hazel was through the Irish Sea area, including south and eastern Ireland. Similar conspicuous increases in Corylus pollen values occurred in previous interglacials but were less pronounced and occurred later in the interglacial cycle than in the Flandrian (West 1977, 1980). Huntley (1993) has considered the possible reasons for the rapid early Holocene expansion of Corylus. He concludes that the greater climatic tolerance of Corylus gave it an advantage over other thermophilous trees like Quercus and Ulmus in the more seasonally extreme climates of the early Holocene. Other factors such as the location of its glacial refugia (Deacon 1974), faster migration rates and the possible effects of Mesolithic human activity (Smith 1970) are judged less likely to be responsible. The early Holocene spread of Corylus avellana has been discussed in detail by Tallantire (2002). Corylus abundance was reduced after several centuries by the immigration and spread of Quercus and Ulmus. Hazel produces pollen in great quantities and also flowers at an early stage in the year giving it increased pollination advantage, resulting in over representation in pollen counts. However, under a heavy oak-elm tree canopy it flowers very little and then the bulk of hazel pollen may result from bushes growing at the woodland margin and in clearings. Moderate Corylus values in mid-Holocene pollen diagrams suggest it mainly grew as a shade-tolerant woodland understory shrub. Exceptionally, Corylus abundance continued in areas of favourable geology such as the limestones of parts of northern England (Bartley et al. 1976) or western Ireland (ref). Corylus is heliophytic and is favoured by forest opening. Very high Corylus pollen percentages are often recorded following the creation of woodland clearings. In the early and mid-Holocene these are often associated with charcoal, as hazel is more fire resistant than most other trees, and have been attributed to the effects of Mesolithic activity (Simmons 1996). Corylus pollen, wood, and fruits have been commonly identified in Late Quaternary deposits and the distinctive nuts of hazel are often recovered from British peat and alluvial sediments. They preserve well in waterlogged conditions, where they have been incorporated in such great quantities that hazel's former abundance seems certain. Sub-fossil nuts show teeth marks of rodents where they have been hoarded and some preserved nut distributions suggest former shoreline detritus of streams and ponds (Godwin, 1975). Corylus nuts remain viable in water for lengthy periods and Birks (1989) suggests that water currents may have been a main long-distance transport agent of Corylus in the early Holocene. At many prehistoric human settlements hazelnuts occur in large quantities, suggesting that they were collected for food. The distinctive microscopic anatomy of hazel wood allows definite identification of well-preserved specimens of wood and charcoal. The wood has been found in the Somerset peat levels, where Neolithic track-ways occur constructed of parallel, straight rods, 3-4m long and shown by annual rings to be 8-17 years old (Coles ref). This suggests early coppicing and hazel rods were used in the construction of many types of artifact, such as fence lines and fish traps (Coles ref). Hazel is palatable to sheep but not cattle and unlike other tree and shrub species hazel does not appear to have been adversely affected by the onset of Neolithic clearance and farming, after which its pollen frequencies generally increased.

 


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