Plants play an important role in many pre-Christian European religions.
To the Celts and many other peoples of the old world, certain trees held special significance as a fuel for heat, cooking, building materials and weaponry. In addition to this however, many woods also provided a powerful spiritual presence. The specific trees varied between different cultures and geographic locations, but those believed to be "sacred" shared certain traits.
Listed on this page are alder, apple, ash, birch, cedar, elm, fir, hazel, holly, oak, pine, willow, yew, etc.
LINK: http://wicca.com/celtic/celtic/sactrees.htm