Why does Whisky have Wood notes?

When analyzing Whisky as we nose and sip it to find flavour profiles present in our dram some whiskies have wood notes along with floral, fruit, cereal, spice, peat.  Generally speaking, wood contributes notes of spice, caramel and vanilla.  These can be displayed as toasted oak with cinnamon and leather typical of some bolder bourbons, or wood spices with notes of fudge, chocolate and raisins or even lighter hints of cedar or sandalwood with candied orange or lemon peel.  This is dependent on the wood used but generally American or European Oak.  The toasty oak character comes from the caramelized sugars in the layer of oak that is in contact with the whisky during the aging process.   Now as a rule, bourbons must be first filled in the charred American Oak barrels but these barrels can be reused in the aging of other whisky, spirits or wine.  As such barrels that have been used for wine or spirits can be used again to age whisky.  Sherry casks are a popular option as well as barrels used to age wine, rum or port can also be used.  But they are mainly used to add layers of additional flavouring through the finishing stage when a whisky is re-casked into secondary vessels for the end of the maturation stage.

Check out this interesting chart of flavour profiles noting the predecessor liquid in the barrel.

https://www.whisky.com/information/knowledge/production/background-knowledge/types-of-whisky-casks.html

Sources:

https://www.whiskyadvocate.com/taste-wood-flavors-in-whisky/

https://www.whisky.com/information/knowledge/production/background-knowledge/types-of-whisky-casks.html