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Tilia cordata (Small Leafed Lime)
Tilia is mostly insect-pollinated and so is considerably under-represented in the pollen rain. Percentage frequencies on pollen diagrams therefore generally underestimate true Tilia abundance. The Tilia pollen record for previous Quaternary interglacial stages has been considered by West (1980) and although pollen data are only available mainly from East Anglia, this location broadly corresponds with Tilia's most abundant Holocene distribution and is probably representative. Tilia records are mainly confined to the warmest, middle substages of past interglacials. For Oxygen Isotope Stage 5 (c.f. Ipswichian interglacial) Tilia percentages are low but for the preceding interglacials are much more significant, reaching up to 5% of tree pollen. Tilia must have formed an important constituent of the thermophilous, deciduous, mid-interglacial woodland in these past temperate phases, as during the present interglacial. The pollen record supports its status as the most thermophilous British forest tree. The great majority of pollen records for the Holocene and before are referable to Tilia cordata (Godwin 1975). There are many pollen records from which the Holocene palaeoecology of Tilia cordata may be reconstructed. Huntley & Birks (1983) have mapped its spread in a northwesterly direction across Europe. Birks (1986, 1989) has mapped its empirical limit (consistent presence) in south-east England to around 7,500 radiocarbon years ago, although as an under-represented pollen type it was probably present in low numbers substantially earlier than that. The recovery of Tilia cordata macrofossils by Kullman (1998) in the Scandes Mountains of central Sweden and AMS dated c.7,000BP, well to the north of its previously believed Holocene limit, suggests that T. cordata may have been present in Britain in low populations from much earlier in the Holocene than the pollen record suggests. Its more general spread through the forest to the north and west in Britain was slow because of its low rate of population expansion (Bennett 1986) and its need to immigrate into already established Quercus and Ulmus forest. Pollen data suggest (Pigott & Huntley 1980), however, that by about 5,500BP Tilia had reached the line between the southern Lake District and south Durham which climatic and edaphic factors determined as its northerly expansion limit (Pigott & Huntley 1978). The application of correction factors derived from modern pollen studies to its low postglacial pollen frequencies (Andersen 1970, Bradshaw 1981) suggests that Tilia cordata may well have been a very important, and perhaps even dominant, component of the undisturbed mid-Holocene mixed deciduous forest in lowland northern, central and southern England (Birks et al. 1975, Moore 1977, Greig 1982, Bennett 1989). Although generally pollen frequencies do not exceed c.15% of tree pollen in their most abundant phases, at some sites much higher values are recorded. These usually occur in very small peat sites or just after peat inception and probably reflect very local vegetation (Greig 1982). Waller (1994) has suggested that wetland basin size is critical in determining the pollen percentages of the poorly transported Tilia grains. Tilia populations were probably much larger than relatively low average Tilia pollen frequencies suggest. Tilia pollen is robust and can preserve well when other types are degraded, particularly in soils, so high frequencies may reflect differential preservation. Keatinge (1982) has shown that concentration of Tilia grains by stemflow may account for higher frequencies in these circumstances, indicating local woodland presence. Tilia pollen frequencies decline from about 5,000BP onwards, consistent with cooling climate after the mid-postglacial thermal optimum. The decline is often very sudden and forms one of the few pollen stratigraphic marker horizons in the later Holocene. Human influence was probably a more important factor than climate and Turner (1962) has shown that the Tilia decline was not a synchronous event but occurred at different times at different sites, usually in association with pollen of arable weeds and bracken spores, indicators of increased forest clearance and farming activity. In many cases this event correlates with Bronze Age forest clearance in the millennium before 3,000BP, but individual examples occur in Neolithic, Iron Age and Medieval times. Whenever it occurred, however, Tilia pollen values never recover their former levels and Tilia seems to have been unable to regenerate after clearance. The selection of the better loessic soils for arable farming would have particularly affected Tilia and the subsequent removal of those soils by erosion would have prevented its return. Prehistoric and later human communities would have found Tilia to be a valuable tree, providing leaf fodder, quality timber and stringy bark, or bast, useful for rope fibre (Godwin 1975). This would have resulted in a destructive impact on the tree's populations until its extensive coppicing in later periods.

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