If a wine tastes like underbrush or "forest floor," where do those flavours come from? Does it mean the grapes came from a vineyard next to a forest?
Not at all. While there may be a forest nearby (there often is where vineyards are concerned), the trees most likely can't take credit for flavouring your wine. An aromatic suggestion of fallen leaves on damp earth happens to be pretty common in wine, and in my experience notably with reds from such places as Tuscany, Piedmont, Bordeaux. Many pinot noirs, too, can suggest that quality. The French call the essence sous bois. If nearby forests were in fact the source of sous bois, then surely white wines from such places would also carry similar aromatic overtones, but white wines almost never do.
We fall into a trap if we think about wine flavours too literally. A German riesling might suggest lime, among other things, yet lime trees don't grow in Germany. Similarly, the noble syrahs of France's Rhône Valley often suggest cracked pepper, yet there is no pepper in the wine. Fermented grapes, thanks largely to the influence of yeasts, can yield a huge variety of nuances.
In rare cases, grapes can directly transmit chemical compounds that waft onto their skins through the air. One example is smoke from forest fires, which contaminated some British Columbia wines from the 2003 vintage after blazes swept through the Kelowna area. In 2001, vineyards in Ontario's Niagara region were plagued by an infestation of Asian ladybugs, which were swept up in the harvest and imbued some of the wines with unusual vegetal flavours and even notes of stale peanuts. Some winemakers also believe that oils from eucalyptus trees growing near vines can drift onto grapes and come through in the wines. (On the other hand, I find eucalyptus characters common to Chilean reds that are produced nowhere near a eucalyptus tree.)
The Flavour Principle by Lucy Waverman and Beppi Crosariol (HarperCollins) won top prize for best general English cookbook at the 2014 Taste Canada Food Writing Awards.