It varies by species and other factors; however, it is often reported that live trees are approximately 50% water by weight and 50% carbon (oven-dried weight).
"Dry (moisture-free) wood is about 48-50% carbon, 38-42% oxygen, 6-7% hydrogen and a number of other elements, such as nitrogen and sulfur in very small percentages. These percentages are based on the weight of the elements as a percentage of dry wood mass.
Living trees, however, are very wet. In fact, although there can be great variation between tree species (and seasonally), a living tree may be made up of more than two thirds water by mass. Thus, a living tree is made up of 15-18% carbon, 9-10% hydrogen, and 65-75% oxygen by mass."
Source: Jeff Howe, PhD
Dovetail Partners, Inc.