As a plant physiologist, I would automatically answer this type of
question on a dry mass basis.

The dry mass of a tree comes mainly from the air as CO2. Most of the
plant dry matter is carbohydrate, with the general composition ratio of
CH2O. The C and O come from CO2. The H comes from H2O. The atomic
weights of C, O and H, are 12, 16 and 1, respectively. The molecular
weight of CH2O is 30 so 2/30 x 100 =3D 6.7 % of its mass comes from H2O
and 93.3 % from CO2.

About 5% of the tree dry mass consists of mineral nutrients from the
soil although this can be higher in some species. Thus, about 88% of a
tree dry mass comes from CO2.

On a fresh weight basis, the percentage of a tree derived from the soil
will vary more widely and depend on the time of year, environmental
conditions, tree age and type of tree. For example, the study below
found new pine needles contained 85.8% water by weight in July and
about 58% in December.

http://dsp-psd.pwgsc.gc.ca/Collectio...-656-2002E.pdf

Fromm et al. (2001) found Norway spruce sapwood contained 180% water
and heartwood 50% water on a dry weight basis. English oak (Quercus
robur) sapwood and heartwood both contained 75% water on a dry weight
basis. On a fresh weight basis, English oak wood contained 43% water.
Thus, in a leafless condition, there are some trees where less than 50%
of the fresh weight came from the soil.

Fromm, J=F6rg H., Irina Sautter, Dietmar Matthies, Johannes Kremer,
Peter Schumacher, and Carl Ganter. 2001. Xylem Water Content and Wood
Density in Spruce and Oak Trees Detected by High-Resolution Computed
Tomography. Plant Physiol. 127: 416-425.
http://www.plantphysiol.org/cgi/content/full/127/2/416

David R. Hershey