The winter woodland landscape may seem bare, but there is life stirring in the buds! One of the nicest things about seeing Holt Wood through the seasons is that we have begun to recognise trees during winter as a result of being able to closely observe the buds and bare twigs.  Perhaps we are fortunate to have trees that are relatively young (planted in 2005 or since then) and so their twigs are still low down so that we can readily see the winter buds. Some medicinal trees will stay smaller and shrublike (such as Cramp Bark, Elder and Witchhazel), others readily grow adventitious shoots on the main stem or trunk (such as Lime). Size, shape, colour and arrangement of buds are all useful to note. Buds can be terminal, on the end of the twig, or lateral, arranged in an opposite or alternate manner along the sides of twigs. Buds often have protective scales, if not they are considered ‘naked’. Other identification features on the twigs include the scars from previous leaves and the lenticels through which air travels in and out. Here are some examples of buds seen at Holt Wood in early January 2013.

Cramp Bark (Viburnum opulus) has opposite plump and pointy coppery-brown buds in January in Devon. Each bud has two pairs of bud scales although only one pair is visible in winter giving a smooth appearance. The bark of this tree can be readily lifted in spring for use as an antispasmodic in powder or tincture form.
Elder (Sambucus nigra) buds are purple and almost bursting open in January in Devon, little patches of green shoots already peeping out at the base of opposite twigs. Twigs are greyish with raised lenticels.  The leaves are no longer used in herbal medicine (they are fairly noxious so can be made into an insect repellent), but the flowers and fruit are valuable in colds and flu.
Horsechestnut (Aesculus hippocastanum) buds are huge with large and richly coloured brown scales. Apart from the tip, the other buds grow in pairs. The medicinal use of this tree is based on the preparations made from the fruit such as an ointment for varicose veins.
Lime (Tilia x europaea) has plump and rounded reddish-brown buds alternating on short shiny twigs which can pop out almost anywhere on the smooth trunk. Each bud has 2 scales, one larger than the other. The flowers will make a superb relaxing tea.
Witch Hazel (Hamamelis virginiana) still has tiny yellow flower clusters in January hanging on from the previous autumn, these are accompanied by furry loose hand-like buds. A North American tree or large shrub and not a native to the UK, this shrub will provide leafy twigs for distillation in late spring.
Some useful online tree identification sources for the UK:
http://www.nhm.ac.uk/nature-online/british-natural-history/urban-tree-survey/identify-trees/tree-key/index.html#tree-s1-2 This site provides a 28 page downloadable tree identification key which has interactive steps and includes ornamental trees as well as native trees. Includes leaves, flowers, fruit as well as buds.
http://paulkirtley.co.uk/2011/bark-buds-common-european-deciduous-trees-winter-identification/ This site has a ‘Bark and Buds’ page with superb photos of 12 native trees to help winter id.
http://www.bbc.co.uk/nature/animals/wildbritain/field_guides/tree_id.shtml A BBC page on ‘Trees in Winter’ which has some useful examples of buds.
http://www.hainaultforest.co.uk/3Winter%20twigs.htm Very useful page with common winter twig photos.
http://cabinetofcuriosities-greenfingers.blogspot.co.uk/2009/11/tree-spotters-guide-to-buds-part-1.html A naturalist site based on North-east England with glorious colour close up photos of buds. 
Since some of our medicinal trees are North American natives, some useful ID photos for North America include:

http://www.ccfpd.org/NaturalResources/WinterTreeIDpocketguide.pdf A US site with superb winter photos of key North American trees.
http://www.florabymax.net/FLORAbyMAX/WinterBuds.pdf Another US site with a wide range of bud photos.