A considerable number of boreal fungal, lichen, bryophyte and invertebrate species occur on aspen Populus tremula in the boreal woods of Scotland. The mature form of the fungal parasite Taphrina johanssonii occurs only on female flowers; it persists during the long periods between flowering in a yeast-like state on the shoots. The aspen hoverfly Hammerschmidtia ferruginea has larvae which feed on the rotting cambial layer of recently dead aspen, while the bristle-moss Orthotrichum gymnostomum is an epiphyte previously thought to be extinct in Scotland, but recently found distributed vertically down a number of aspen trunks. These are just three of the many important species associated with aspen in the Scottish Highlands, and the Highland Aspen Group (HAG) is making strenuous efforts to document and protect natural stands of this valuable tree (Cosgrove et al., 2005). Many of the associated plants and animals are dependent upon particular stages in the life cycle of aspen, so from the conservation point of view it is important to maintain adequate areas and genetic diversity of this clonal tree.
sedge Carex laevigata, pendulous sedge C. pendula, small teasel Dipsacus pilosus, wood horsetail Equisetum sylvaticum, herb paris Paris quadrifolia and small-leaved lime Tilia cordata provide a very strong indication of ancient woodland, while wood anemone Anemone nemorosa, wood-sorrel Oxalis acet-osella and yellow archangel Lamiastrum galeobdolon are amongst the species which occur to a rather greater extent in other habitats and so are weaker indicators.
As already mentioned most indicators of ancient woodlands do not readily colonize new sites so their presence tends to show that the community is long-lived, although there are regional differences. In Shropshire, for example, dog's mercury Mercurialis perennis is found in gardens and recent hedgerows as well as base-rich woodlands, but it is an ancient woodland vascular plant indicator (AWVP) in the more continental climate of eastern Britain (Rackham, 2003). The AWVP list for Shropshire gives both strong indicators and others which are either less strong or relatively weak (Whild, 2003). Species such as herb paris are found only within ancient woodlands in Shropshire, but in general a really good indication is provided by the presence of at least half a dozen AWVPs of which bluebell Hyacinthoides non-scripta, early-purple orchid Orchis mascula, moschatel Adoxa moschatellina, primrose Primula vulgaris, sanicle Sanicula europaea, sweet woodruff Galium odoratum, toothwort Lathraea squamaria, violet helleborine Epipactis purpurata, wild garlic Allium ursinum, wild service tree Sorbus torminalis, wood anemone, wood fescue Festuca altissima, wood melick Melica uniflora, wood sedge Carex sylva-tica, wood-sorrel, yellow archangel and yellow pimpernel Lysimachia nemorum are sound examples.
Rose (1999) points out that AD 1600, the defining date for ancient woods in Britain, was the time when reasonably accurate estate maps and also the first known widespread tree plantings began. He also makes a very strong case for the use of AWVPs in woodlands, pointing out that the flowering plants, native conifers, ferns and fern allies are by far the easiest of all groups, plant and animal, to observe and identify. In terms of biodiversity he emphasizes the old adage that the longer a habitat has been established the more species it is likely to contain. Of the 130 AWVPs he lists for four regions of southern Britain; a total of 23 are shown in Fig. 3.9, which illustrates the phenology of damp central European oak-hornbeam woods. These act in this way in all four regions including the south-east, while the grass wood barley Hordelymus europaeus is used only in central southern England and oxlip Primula elatior in the south-west (where Solomon's seal Polygonatum multiflorum is not used). Bird cherry Prunuspadus and yellow star of Bethlehem Gagea lutea are AWVPs in East Anglia, as is spindle Euonymus europaeus (also used in the south-west).
Similar associations of species with primary forest occur in North America, though in such a large area these vary from one area to another. In the densely wooded Harvard Forest of Massachusetts, Gerhardt (1993) found that whorled aster Aster acuminatus, pipsissewa Chimaphila umbellata, bluebead lily Clintonia borealis, bog fern Thelypteris simulata, painted trillium Trillium undulatum and hobblebush Viburnum alnifolium had significant positive associations with old woodland. Species with an affinity for secondary woods in this area include the clubmosses Lycopodium clavatum and L. obscurum, bracken Pteridium aquilinum, the bramble Rubus fragillaris, Canada mayflower Maianthemum canadense and the hay-scented fern Dennstaedtia punctiloba.
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