Forests represent a barrier, especially one that is dark, mysterious and impenetrable. Woods, by contrast, convey a connotation of encounter and Transformation. If you do make it Into the Woods you may not return or, if you do, things will have changed. Forests have a grander and more ancient connotation. Not only are they bigger, and thus more of a barrier, but they have existed almost forever, and are thus more likely to contain creatures or spirits from the dawn of time, surviving while Thinning continues, as in The Not-World (1975) by Thomas Burnett Swann and in the Arafel sequence by C J Cherryh, particularly Ealdwood (1981; rev vt The Dreamstone 1983). The association of forests with the land of Faerie is strong – as in A Midsummer Night's Dream (performed circa 1595; 1600) by William Shakespeare – and occurs in many Folktales; other examples are Die Elfen (1811) by Johann Ludwig Tieck and "The Golden Key" (1867) by George MacDonald. In Arthurian legend (> Arthur) the enchanted Forest of Broceliande is where Merlin was imprisoned by Viviane; this forest was also the scene of a mighty battle between the forces of Heaven and Hell. This sense of the ancient nature of forests is crucial to the mythography of J R R Tolkien, who used forests to considerable effect; in The Lord of the Rings (1954-5) there is both the sinister enchantment of Mirkwood and the ancient world of the Old Forest, the home of Tom Bombadil. The power, governance and secrets of the forest are also utilized by Robert Holdstock in his Mythago Wood sequence, by Charles de Lint in various novels – especially Greenmantle (1988) – and by Diana Wynne Jones in Hexwood (1993).
There is another sense in which the forest, because of its vastness, takes on an innate sentience. This may derive from the ancient spirits in the wood, or from the very Trees themselves. This is particularly evident in the wild and uncivilized areas of the world where the powers of the Earth still prevail – the vast coniferous forests of northern Canada, as explored in stories by Algernon Blackwood, the depths of the African (or Indian) jungle (personified by Joseph Conrad as the Heart of Darkness and explored by writers like H Rider Haggard, Rudyard Kipling and Edgar Rice Burroughs), or the tropical rainforests, as depicted in Ferngully: The Last Rainforest (1992).
Forests do not have to be alien. They can be places of refuge, and if this also leads to an affinity with the spirits of the forest, the results can be powerful. The obvious example is the legend of Robin Hood and Sherwood Forest, which is closely associated with that of the Green Man. A thematic anthology is Enchanted Forests (anth 1995) ed Katharine Kerr and Martin H Greenberg. [MA]