It has been used for thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs.[citation needed] In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients between the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, or wood chips or fiber. 

The Earth contains about 434 billion cubic meters of growing stock forest, 47% of which is commercial. As an abundant, carbon-neutral renewable resource, woody materials have been of intense interest as a source of renewable energy. In 1991, approximately 3.5 cubic kilometers of wood were harvested. Dominant uses were for furniture and building construction.

Answer the following questions:
1. How much wood was harvested in 1991?
2. Why?
3. How many meters of forest are there on our planet?
4. How much of that is available commercially?
5. Can this be renewed?
6. Is it rare?
7. How long has it been used in construction?
8. What is it's other main use?
9. What is it a composite of?
10. Are they weak?
1. 3.5 cubic kilometers
2. furniture and building construction.
3. 434 billion cubic meters
4. 47%
5. yes
6. no
7. thousands of years
8. fuel
9. cellulose fibers
10. no