Following the demise of corrie glacier activity at the end of the Lateglacial period there was a rapid warming of temperatures to levels probably greater than those of today (Atkinson et al. 1987). This climatic change, coupled with the development of soils, facilitated the spread of woodland across the existing open, herb- and shrub-dominated landscape. Mapped reconstructions portray the dominant tree types prior to major discernible human impacts c. 5000 BP (3780 cal BC) (McVean and Ratcliffe 1962; Bennett 1989; Tipping 1995; Edwards and Whittington 1997), although it is probable that a woodland mosaic existed in most areas. Research in peripheral areas suggests that they were wooded for much of the first half of the Holocene (cf. Wilkins 1984; Bohncke 1988; Bennett et al. 1990, 1992; Edwards 1990, 1996; Brayshay and Edwards 1996; Fossitt 1996). The density of the arboreal cover may be in question and the effects of long-distance transport of pollen can be significant (Tyldesley 1973; Donaldsonet al.2008; but see Brayshay et al. 2000).
Radiocarbon dating shows a marked time-transgressive nature to the spread of many woodland taxa (Birks 1989). For instance, tree birch (Betula spp.) was established over most of Scotland by 10000 BP (8050 cal BC); oak (Quercus), present in southern Scotland shortly after 8500 BP (7530 cal BC) did not reach Aberdeenshire and Skye until about 6000 BP (4870 cal BC); and the principal areas colonized by Pinus sylvestris (Scots pine) in Scotland may have come from multiple source areas at various times (Bennett 1984; Froyd and Bennett 2006).
A common feature of pollen diagrams is the prominence of Corylus avellana (hazel) representation and its maintenance from around 9000 BP (8030 cal BC). This phenomenon is sometimes ascribed to hunter-gatherer impacts and possible resource manipulation (e.g. coppicing or burning to enhance woody growth and enhanced hazel nut yields, which at the same time could increase flowering and pollen production [Smith 1970]). However, for Scotland, Edwards and Ralston (1985) noted the existence of high hazel values even for areas distant from likely Mesolithic activity, while a study of microscopic charcoal at a number of sites in Scotland (Edwards 1990) revealed no correspondence between enhanced fire incidence, as inferred from charcoal, and early maxima for hazel type pollen. Huntley (1993) explored a series of hypotheses concerning the spread of hazel and concluded that climate was likely to be the primary underlying cause. This in no sense denies the usefulness of hazel nuts and hazel wood products to Mesolithic peoples, nor of the utilization of hazel in a woodland management system.
Uncertainty also surrounds the role of humans in the rise and spread of alder (Alnus glutinosa). Following observations by McVean (1956a, b), Smith (1984) implicated Mesolithic people in the expansion of alder pollen. This was held to be subsequent to fire and woodland disturbance, and based on the supposition that such activity promoted catchment runoff and waterlogging in habitats favoured by Alnus. A number of Scottish pollen profiles do display an increase in microscopic charcoal as alder expands (Edwards 1990; Bunting 1994), but not all. Like the spread of many plants, that of Alnus is likely to have a number of contributory causes of which human activity can be one.
Many pollen diagrams display temporary and apparently small reductions in woodland of all species. These perturbations are sometimes accompanied by expansions in charcoal values and human agency may sometimes have been responsible — indeed, lithic artefacts are sometimes known from the pollen sites themselves or their vicinity (e.g. Knox 1954; Edwards et al. 1991; Tipping et al. 1993; Edwards and Mithen 1995). It remains difficult to separate natural from human causes and equifinality could apply. Woodland has always been subject to disease, death, windthrow, and lightning strikes which could create openings, while grazing activities could have maintained clearings for many hundreds of years (Buckland and Edwards 1984). By the same token, human communities, in using woodland resources for food and shelter, would have disturbed woodland.
Studies which demonstrate plausible impacts upon woodland come from island locations. Archaeological excavations at Kinloch, Rum have produced one of the earliest known Mesolithic occupation sites in Scotland, with dates on carbonized hazel nut shells extending back to 8590±95 BP (7700—7500 cal BC) (Wickham-Jones 1990). Palaeoecological studies from a site located 300 m from the excavation area reveal sharp and sustained changes in the pollen of alder, hazel, grasses, and willow, together with associated peaks in microscopic charcoal (Hirons and Edwards 1990). Although the interpretation of the patterns at Kinloch is very difficult, they do not seem to represent a natural vegetational succession and human involvement seems likely. At Loch an t-Sîl, South Uist, close sampling of Mesolithic age sediments reveals two phases of woodland removal, mainly involving birch and hazel, at c 8040 BP (7010 cal BC) and 7870 BP (6620 cal BC), lasting 130 and 70 radiocarbon years respectively (Edwards 1996a). These are associated with expansions in Poaceae, Calluna vulgaris (heather) and charcoal and reductions in ferns. The removal of birch and hazel may have an anthropogenic origin and the expansions in grass and heather could indicate their spread into cleared areas. Whether the extension of browse in order to attract grazing animals was the intention or a useful by-product of cropping woodland, remains unknown. The reduction of ferns is similar to features observed in the east Shetland pollen site of Dallican Water (Bennet et al. 1992). At Dallican Water this is taken to indicate possible grazing by red deer which may have been transported to Shetland by hunter-gatherers intent on introducing a valuable resource. In southern Shetland, a double shell midden of Mesolithic and early Neolithic age has been exposed by coastal erosion at West Voe, near Sumburgh (Melton and Nicholson 2004; Edwards et al. 2009c. 4200—3600 and 3500—3250 cal BC) and prior to this. Birch and hazel are both reduced in two phases from c. 6000 and 3910 cal BC, with concomitant increases in charcoal and mineral matter to the lake (the latter is inferred to be a consequence of soil erosion). Contrary to the situation of only a few years ago (Edwards 2009), the Outer Hebrides and Shetland have both furnished evidence, arguably, for a material Mesolithic presence (Gregory et al. 2005; Edwards et al. 2009) which extends beyond the data provided in pollen records. In both archipelagos, more Mesolithic finds are likely to be hidden beneath sea, sand or peat.
Given their speeds of occurrence, rising sea levels and the spread of peat are unlikely to have been greatly deleterious to Mesolithic lifestyles (cf. Edwards and Sugden 2003; Edwards 2004; 2009; Tipping 2008) — indeed they may have brought benefits in terms of increasing the variety of coastal habitats as new estuaries and islands formed and in the supply of peat as a fire and (albeit sub-optimal) grazing resource.
The sustained charcoal peaks found in the Western and Northern Isles, if anthropogenic (and this is an issue that has not been resolved; Edwards 1996; Tipping 1996), do not have to indicate woodland removal by fire or the driving of game, but may simply result from the burning of felled wood or peat for heating or cooking purposes, or the fire-related creation or maintenance of heaths as a grazing resource as has long been mooted for England (e.g. Dimbleby 1962; Simmons 1969; Caseldine and Hatton 1993). This process has also been conjectured for Callanish, Lewis (Bohncke 1988), and also for evidence from sites in South Uist (including Loch an t-Sîl), but only as a possibility (Edwards et al. 1995).
Hunter-gathering gave way wholly or in part to agriculture around the turn of the fourth millennium cal BC and many topics relevant to this period of transition can be dealt with in accounts which deal either with Mesolithic or Neolithic times. Relevant issues as pre-elm decline cereal pollen, the elm decline, simulation modelling, soil erosion, and climate change are discussed in the ScARF Neolithic report.